WATER TABLE CONTROL SYSTEMS

Transcription

1 CHAPTER 18 WATER TABLE CONTROL SYSTEMS James L. Fouss (USDA-ARS, Baton Rouge, Louisiana) Robert O. Evans (North Carolina State University, Raleigh, North Carolina) James E. Ayars (USDA-ARS,Parlier, California) Evan W. Christen (CSIRO Land and Water, Griffith, New South Wales, Australia) Abstract. Agricultural water management systems are designed and installed to (1) improve crop production by controlling the durations of excessive and deficient soilwater conditions in the root zone, and (2) improve the water quality of drainage discharge by controlling drainage flows to reduce agrochemical losses from farmland. Controlling subsurface drainage discharge to maintain a shallower water table and limit drainage outflow has been shown to reduce annual nitrate loss from cropland as much as 50%. The development of an integrated water table control system includes determination of site suitability, drain depth and spacing, preparation of a field installation plan, and incorporation of components for operating the system in the controlled drainage and subirrigation modes. The system design should permit control of the water table depth in the soil profile over the range needed for the cultural practices to be followed and the crops to be grown, and the operational requirements to reduce agrochemical losses. These objectives should also involve the efficient utilization of shallow groundwater supplied by rainfall or irrigation. Several methods are available for determining optimum design drain depth and spacing for water table management. The operational mode of the system, that is, whether it is in the conventional subsurface drainage, controlled drainage, or subirrigation mode, may vary from day to day, month to month, and from year to year. In most locations, it is not clear whether the greatest demands on the system design are to provide good drainage management under shallow water table conditions, or to provide sufficient subirrigation during the driest periods. Because of complicating design factors, a simulation modeling approach should be used to conduct a complete analysis and final design of the drainage water management or water table control system, and to predict its performance over a period of 20 to 30 years for the climatologic conditions at the specific site.

2 Design and Operation of Farm Irrigation Systems 685 Computer simulation models such as DRAINMOD can be used to evaluate various system design options for a specific site. A long-term simulation (20 to 30 years) can provide a good evaluation of the expected performance of the system. DRAINMOD- NII includes a routine to comprehensively evaluate the impact of system design and operational parameters on transport of various forms of nitrogen within the soil profile and losses in runoff and subsurface flow. This version of DRAINMOD is an important tool for designing drainage water management systems, along with a seasonal operation plan, to meet emerging water quality requirements for surface receiving streams and water bodies. The design for a water table management system that results in the optimum net profit, while minimizing the environmental impacts offsite, should be the best design and management strategy. The system may be technically feasible, but the final decision should be based on the feasibility of the system not only to pay for itself but to return a profit to the farmer for his investment, while minimizing offsite environmental impacts. Therefore, the final decision for a given or recommended design should be based on a thorough evaluation of economic and environmental impacts. Keywords. Agrochemical, Arid, Controlled drainage, Drainage management, DRAINMOD, Environmental impacts, Groundwater, Humid, Runoff, Salinity, Subirrigation, Subsurface drainage, Surface drainage, Water management, Water quality, Water table, Water table control INTRODUCTION Agricultural drainage and related water management systems to control water table depth have historically been installed to eliminate water-related factors that limit crop production, or to reduce those factors to acceptable levels. In recent years, research has shown that management of drainage water provides an opportunity to control the loss of agrochemicals (fertilizer nutrients and pesticides) from cropland, as these are carried in surface runoff and subsurface drainage discharge. For example, controlling subsurface drainage discharge to maintain a shallower water table and control (limit) drainage outflow can reduce annual nitrate loss in discharge by as much as 50%. Likewise, management of surface darainage systems has been shown to reduce phosphorus loss in runoff. Recent attention to agricultural drainage management systems 1 for water table control resulted from widely published nutrient (nitrate and phosphorus) contamination causing hypoxia in the northern Gulf of Mexico (see, for example, Rabalais et al., 1999). The primary source of the contamination is from drained agricultural lands in the Midwest and is transported through the Mississippi River system to the Gulf. In the irrigated western U.S., toxic elements such as selenium are transported by drainage flows to wildlife habitats, where these elements can bio-accumulate to toxic levels and eventually cause wildlife deaths. Thus, surface and subsurface drainage water man- 1 An interagency Agricultural Drainage Management Systems Task Force was formed in 2002 to promote and implement drainage water management (controlled drainage) practice in eight Midwest states to reduce nitrate loss from subsurface-drained cropland contributing to nutrient load in the Mississippi River system and hypoxia in the northern Gulf of Mexico; see Fouss et al. (2004) and web site: ~usdasdru/adms/admsindex.htm

3 686 Chapter 18 Water Table Control Systems agement systems are crucial for mitigating environmental impacts for both rainfed and irrigated croplands. In the irrigated West, water table control objectives are to improve the quality of drainage discharge waters (in terms of both agrochemicals and salinity) and improve irrigation water use efficiency. This is achieved by encouraging crop water use from the water table and reduction of bypass flows to drains, where these do not contribute to salt leaching and direct loss of irrigation water. Another important function of water table control is shifting a subsurface drainage system from a reclamation phase, when salt leaching from the root zone is the highest priority, to a maintenance phase, when a lesser level of drainage can maintain appropriate root-zone salinity. System design should permit control of the water table depth in the soil profile over the range needed for the cultural practices to be followed, the crops to be grown, and the operational requirements to reduce agrochemical losses. An integrated design for a water table control system includes determination of site suitability (including an adequate outlet); drain depth and spacing; drain conduit diameter; a drain layout and installation plan; an adequate water supply (if subirrigation is used); a method or structures for operating the system; and a monitoring and operational plan. An acceptable application of water table control depends on the costs of the required water management system in relation to the economic and environmental benefits expected. Such benefits vary from year to year with both weather and economic conditions and are difficult to quantify because of the complex interrelationships of crop production and environmental processes. As more is learned about plant growth and yield, machinery-soil interactions (e.g., trafficability), soil salination and reclamation processes, and agrochemical losses in drainage flows, it will be possible to simulate entire crop production processes, including crop selection, rotations, and fertilization. This will permit optimizing the water table management system design based on profit potential and environmental benefits or impacts. Lacking this knowledge at the present time, more intermediate or traditional objectives of water table management systems must be used. Such objectives are easier to quantify and generally form the basis for system selection and design MANAGEMENT OF SOIL WATER BY WATER TABLE CONTROL Application of Water Table Control Systems The performance criteria for water table management systems can be described in terms of several individual parameters, such as trafficability, control of excess and deficit soil-water conditions, salinity control, subirrigation efficiency, and off-site environmental impacts. 1. Traditional drainage parameters Parameters for trafficability and control of excess soil water were traditional parameters in rainfed areas. The recent focus on drainage water management (controlled drainage) to reduce nutrient (particularly nitrate) loss in subsurface drainage discharge has caused a shift in the performance parameters to include environmental or water quality impacts of subsurface drainage. In arid regions, salinity control has been a traditional primary parameter; in recent years parameters concerning environmental impacts have become very important. The major difference between current and traditional parameters is that operation in the controlled drainage and subirrigation modes may complicate or reduce the ability of the

4 Design and Operation of Farm Irrigation Systems 687 system to satisfy traditional drainage requirements and still meet current requirements for water quality protection in both rainfed and arid regions. 2. Deficit soil-water conditions While wetness is the primary concern to most landowners, yields on poorly drained soils with shallow water tables are sometimes substantially reduced by deficit soil-water conditions or drought stresses. Shallow groundwater conditions in the crop root zone often promote plant stresses caused by excessive soil-water conditions. However, the temporal and spatial variability of rainfall in many humid areas frequently results in excessive soil water and related reduced crop growth early in the growing season followed by deficit soil water conditions later in the growing season. Early excess water stress often aggravates deficit rainfall conditions later in the season because shallow root depths caused by high water tables early in the growing season may not be sufficiently deep to access deeper soil water needed by the crop later in the season. Simulation studies by Skaggs and Tabrizi (1983) found that drought stresses reduced corn yields on a poorly drained North Carolina soil by an average of 22% even though average annual rainfall exceeded 1300 mm. Drought stresses may be increased by the drainage systems that are required to farm these shallow water table soils (Doty et al., 1984, 1987; Skaggs and Tabrizi, 1983). The effects of drainage and associated water management systems on drought stresses and yields should be considered in the design and operation of those systems. 3. Controlled drainage The performance criteria for the controlled drainage mode of operation depends upon whether capabilities also exists to provide the subirrigation mode. Some modest benefits in terms of reducing deficit soil-water conditions can be achieved in some soils by maintaining, controlling, the water table level near the root-zone depth to insure that rainfall is retained in the soil profile for crop use. This controlled drainage mode may result in short-term excess soil-water conditions in the root zone immediately following rainfall for the benefit of less deficit soil water stress later. If the water table remains elevated for extended periods during early crop development, crop roots may not develop deep enough to prevent subsequent drought stress later in the growing season during extended periods with below normal rainfall. Thus, an operational plan must be developed that considers seasonal rainfall amount and frequenty to achieve a balance between excessive and deficit soil water conditions throughout the growing season. 4. Subirrigation The performance criteria for operation in the subirrigation mode includes the control of both deficit and excess soil-water conditions. That is, systems designed for subirrigation must also be designed to operate in the controlled drainage and conventional drainage modes to insure that rainfall events do not cause extreme fluctuations of water table depth into the root zone. Subirrigation may be the governing design performance parameter since a closer subsurface conduit spacing is typically required to supply adequate subirrigation water to the soil profile than the conduit spacing required to remove excess soil-water from the root zone and soil profile. In more humid regions, subsurface drainage following rainfall events that occur while subirrigating is often the governing design performance parameter. Even where subirrigation is economically feasible, not all farmers will install the drainage conduits at a close enough spacing for the future retrofit for subirrigation because of the higher initial cost for the installed system. Subirrigation design and operation should consider balancing the timing and magnitude of excessive and deficit soils water conditions similarly to controlled drainage.

5 688 Chapter 18 Water Table Control Systems 5. Off-site environmental impacts Drainage discharge water from agricultural cropland containing nitrate-nitrogen has caused undesirable off-site environmental impacts to receiving streams and water bodies. Rabalais et al. (2002) identified nitrate discharged in drainage waters from cropland in the Midwest and Upper Midwest states as the major source of nitrate carried down the Mississippi River System to the hypoxic zone (Rabalais et al., 1999) of the Northern Gulf of Mexico. To address these problems on a broad scale and to promote and implement solutions involving drainage water management, the Agricultural Drainage Management Systems Task Force was formed in 2002 (Fouss et al., 2004). Controlling subsurface drainage discharge to maintain a shallower water table and limit drainage outflow has been shown to reduce annual nitrate loss from cropland as much as 50% (Gilliam et al., 1979; Evans et al., 1995; Fausey aet al., 2004). The Task Force concluded that a suite of Best Management Practices, not just drainage water management, would be required to solve the water quality problems throughout the Mississippi River Basin and the Gulf (Fouss and Appelboom, 2006) Site Evaluation and Suitability Several soil properties and site parameters influence the design of a water table management system. Important properties include: lateral hydraulic conductivity, depth to a restrictive layer, soil-water characteristics, upward flux and drainable porosity as functions of water table depth, soil layering, infiltration, crop rooting depth, topography, and suitable drainage outlet. The design of the system is more sensitive to some parameters than others; some are more difficult to measure in the field than others; and some properties are more spatially varied than others. It is generally not practical to measure all of the important soil and site parameters in the field for every potential site. Properties that are generally practical to measure are hydraulic conductivity (field effective), soil-water characteristic, depth and thickness of soil horizons, and depth to the restrictive layer. Saturated lateral hydraulic conductivity is one of the most important factors influencing the design of the system (Skaggs, 1980, 1981) and is also one of the most spatially varied properties in the field (Tabrizi and Skaggs, 1983; Rogers and Fouss, 1989; and Rogers et al., 1991). Therefore, effort should be made to determine representative, field effective hydraulic conductivity values for each site. Drainable porosity can be calculated from soil-water characteristic data, when available. System design is also sensitive to upward flux as a function of water table depth (Skaggs, 1980); this relationship is usually estimated from steady-state solutions, which require the unsaturated hydraulic conductivity function. Drainable porosity and infiltration may also be estimated from the soil-water characteristic and unsaturated hydraulic conductivity function. The feasibility of subirrigation at a specific site depends on the source, cost, and reliability of a water supply. Evans et al. (1988a) discussed many types of water supplies and their associated reliability and cost for subirrigation. Potential seepage losses should also be identified. In poorly drained soils with a restricting barrier (layer) in the profile, vertical losses are normally small. Lateral seepage losses, on the other hand, may consume more than 25% of the pumping capacity in some cases. Lateral seepage losses can be minimized with good planning, system layout, and management. Whenever possible, supply canals should be located near the center of subirrigated fields rather than along field boundaries. Perimeter ditches and outlet canals should also be equipped with control structures. When seepage losses cannot be controlled, it is nec-

6 Design and Operation of Farm Irrigation Systems 689 essary to determine the length of the seepage boundary, the hydraulic gradient, and hydraulic conductivity along the boundary. Skaggs (1980) described methods for estimating seepage losses under steady state conditions, with specific examples on how to approximate the seepage losses to nearby drains or canals, adjacent undrained fields, and vertical or deep seepage. Often, an experienced engineer can determine the suitability of a potential site for water table management by a qualitative site investigation. The presence of six general site conditions (discussed below) will usually indicate whether or not water table management is feasible or practical. 1. Site drainage benefits For soils in the humid regions of the eastern U.S., water table management is practical only on those sites that will benefit from improved subsurface drainage. Where subsurface drainage is not needed under natural conditions, such as in soils lacking a shallow water table (hydric soil), irrigation needs are usually most efficiently satisfied by surface application systems (e.g., furrow, sprinkler, or drip irrigation). A soil survey report will indicate the natural drainage conditions for a given soil series. Soils that are classified as either somewhat poorly drained, poorly drained, or very poorly drained will usually benefit from artificial drainage and are candidates for water table management. 2. Topography Soils that support water table management systems are usually relatively flat. For example, poorly drained soils in the Southeastern Coastal Plains, the Midwest Great Lakes Region, the Florida Flatwoods, and the Mississippi Delta rarely occupy landscape positions on slopes greater than 2%. In fact, very few systems have been installed on slopes greater than 0.5%. As the slope approaches 1%, the number and cost of control structures to maintain a water table depth within a desired range for a specific crop may become economically prohibitive. Grain crops can usually tolerate a range in the water table depth of 0.30 m to 0.45 m, whereas shallowrooted vegetable crops can only tolerate a water table fluctuation of 0.15 m to 0.20 m without showing signs of water stress during dry periods. A control structure is then needed for each change in ground surface elevation corresponding to the desired water table range for that zone. From a physical standpoint, the maximum slope that can be tolerated tends to be site specific and related to hydraulic conductivity. At slopes above 2%, a uniform water table depth is difficult to maintain because of lateral seepage when the conductivity exceeds 0.5 m/day (0.7 in/h). By contrast, at lower conductivity, the cost for the closer drain spacing often becomes prohibitive. In general, the limiting factor with respect to slope will be economics rather than physical slope conditions. 3. Hydraulic conductivity Hydraulic conductivity is the single most important soil factor affecting the economic feasibility of a water table management system. For preliminary planning purposes, hydraulic conductivity can be estimated from values reported in soil survey reports. As with slope, prohibitive hydraulic conductivity values are a function of economics rather than physical limitations in the design and operation of the system. The limiting conductivity for a specific site will be dictated largely by the value of the increased crop obtained, or the potential value of obtaining consistently high yields year after year. 4. Restrictive layer or seasonal shallow water table Potential sites typically have a barrier at some depth in the soil profile, which prevents excessive vertical seepage losses. A restrictive layer is often encountered between 1.5 m and 10 m. The existence

7 690 Chapter 18 Water Table Control Systems of the barrier becomes increasing difficult to locate with a hand auger when its depth exceeds 3 m. The presence of a seasonal shallow water table is usually sufficient evidence to indicate that a water table can be maintained on the site at an elevation suitable for subirrigation, whether or not the restrictive barrier can be located. In addition, the depth of the seasonal shallow groundwater is a good indicator of the natural drainage of the site. The depth of the seasonal shallow water table can be estimated from a soil survey or by inspection in the field. Gray mottles in the soil profile indicate the position of the seasonal water table under natural drainage conditions. When the gray mottles occur within 0.5 m of the soil surface, the site is a candidate for water table management. As the depth to the gray mottles increases to 1.0 m, the site is marginally suited and excessive seepage may be a problem. In this case, the actual depth to the restrictive barrier should be determined. Soils with gray mottles more than 1.0 m from the surface are naturally well drained. In these cases, benefits from subsurface drainage will be minimal and the cost of water table control would have to be justified on the basis of subirrigation benefits alone. Unless the natural drainage condition can be managed, excessive seepage will occur and the site is not well suited for water table management. 5. Drainage outlet requirements When evaluating the potential of any site for a water table management system, drainage is a primary consideration. A drainage outlet that will remove excessive surface and subsurface water within an acceptable interval of time (typically 24 h) must be available. When a gravity outlet to an existing stream or canal is available, the drainage outlet should be at least 1.2 m lower than the lowest land surface for the system. Where a gravity outlet is not available or possible, an outlet structure (e.g., a sump or pump station) can be constructed and drainage flow pumped to a higher elevation for discharge into an existing or constructed channel or for storage and later use during subirrigation. In some cases the sump outlet structure may be needed only for subsurface drainage, as a suitable gravity flow outlet is available for surface runoff. An example of a pumped subsurface drainage outlet sump is illustrated in Figure 18.1; this system is shown in the controlled drainage mode of operation. In cases where drainage water must be pumped, it is desirable to store as much of the drainage water as possible in a nearby on-farm pond or wetland reservoir. If stored, this water can be used later during the growing season to satisfy subirrigation requirements, often without the need to be pumped again. That is, a subsurface drainage-subirrigation system usually requires pumping in only one direction. Such a system can usually utilize gravity flow in the opposite mode. 6. Water supply requirements The reliability, location, quantity, and quality of irrigation water are critical considerations in evaluating a site for a subirrigation. The water source should be located as close as possible to minimize conveyance losses and costs. The quantity of water needed for subirrigation will vary depending upon the weather (rainfall and evapotranspiration), crop being irrigated, and water loss rate from the field by deep and lateral seepage. Water source requirements may range from 70 L/min/ha irrigated in the southeastern U.S. to 40 L/min/ha irrigated in the cooler Midwest and Canada. Once the physical suitability of a site for water table management has been determined, the cost of the system should be estimated and discussed with the landowner before any additional time is spent on design. Average cost for many of the compo-

8 Design and Operation of Farm Irrigation Systems 691 Figure Controlled-drainage and subirrigation modes of operation for water table management; outlet (sump) water level control based on feedback of field water table depth. nents of the system are usually available from the local Natural Resources Conservation Service (NRCS), Cooperative Extension Service, drainage pipe manufacturers, or drainage contractors in the area. The dominant costs of the system are usually drain tubing, system installation, outlet construction (if gravity outlet is not available), water supply costs, and system controller apparatus. The drain tube diameters for laterals and collector mains and lateral drain spacing must be estimated before the tubing and installation costs can be determined. At this stage of the design process, it is adequate to estimate the drain spacing from local rules of thumb or shortcut methods for either subsurface drainage (including controlled drainage) or subirrigation, depending upon which alternative is desired or deemed more important SYSTEM DESIGN AND OPERATION IN HUMID REGIONS System Design Objectives The design objectives of a water table control system include all the traditional objectives of drainage systems, plus objectives of conserving soil water and reducing deficit soil-water conditions, plus reducing agrochemical losses in drainage discharge. These objectives include: provide trafficable or workable soil conditions so that farming operations, such as seedbed preparation, tillage, and harvesting, can be conducted in a timely manner, and without damage to the soil structure; reduce plant stresses caused by excessive soil-water conditions; control soil salinity and alkalinity; reduce or eliminate plant stresses caused by deficit soil-water conditions; minimize harmful offsite environmental impacts from agrochemical losses;

9 692 Chapter 18 Water Table Control Systems conserve and efficiently utilize water supplied by precipitation, thus minimizing irrigation water requirements; and maintain a soil-water environment so that other practices, such as conservation tillage, post-harvest residue, or cover crops, are more effective and beneficial. The first three objectives are traditional ones for drainage system design. To incorporate controlled drainage or subirrigation, the others may be addressed. The relative importance of these objectives is case dependent, varying seasonally and from year to year with soil, crop, site, and climatological factors. For example, the drainage intensity required to provide workable conditions on the soil for spring planting may cause overdrainage and increase deficit soil-water conditions later in the growing season. This situation can be avoided by appropriate design and timely management, provided the system designer properly considers the objectives and factors controlling system performance, including the variable nature of rainfall. Weather forecast information (e.g. rainfall probability) may become an important input for real-time operation of water management systems, particularly the integrated management of water and agrochemicals to improve water quality. There may be several water management system design alternatives that satisfy the design objectives and environmental requirements. Whether or not a given system design will satisfy the objectives depends upon the location, crop, and soil properties. Of course, the objectives themselves may depend upon the individual farmer s management capabilities, equipment, and manpower available. For example, some farmers may be interested only in controlled drainage and do not feel the additional cost of subirrigation facilities can be justified for their farm enterprises. In system design, however, the future conversion of a controlled drainage system for operation in the subirrigation mode should be considered Design Methods Drain depth is usually dictated by the relative permeability of soil layers, depth of slow-draining layers, outlet depth, limitations of installation equipment, and local standards for drain grade and depth. When site and soil characteristics allow drain depth flexibility, a depth between 0.75 m and 1.5 m will usually be optimal. However, with the recent focus on water quality of subsurface drainage effluent, it has been recommended (ADMS-TF, 2005) that drain depths greater than 1.0 m (without outlet controls) be avoided, if possible. The shallower drains, even without outlet controls, remove less water from the soil profile and thus reduce the potential loss of applied agrochemicals. Several methods are available for determining optimum design spacing. On a fieldto-field basis, all of the methods will provide a better estimate of the required drain spacing if the saturated hydraulic conductivity and depth to the restrictive layer have been measured on the specific site rather than estimated from the soil survey or local drainage guide. The operation of the system, whether it is in the conventional subsurface drainage, controlled drainage, or subirrigation mode, varies from day to day and from year to year. This increases the complexity of designing controlled drainage and subirrigation systems. For most locations, it is not clear whether the greatest demands on the system design are to provide good drainage under shallow water table conditions, or to provide sufficient subirrigation to meet evapotranspiration (ET) demands during the driest periods. For these reasons, we recommend that a simulation model approach be used to conduct a complete analysis and final design of the water table

10 Design and Operation of Farm Irrigation Systems 693 management system, and to predict its performance over 20 to 30 years for the climatological conditions at the site. Steady-state solution approaches, such as the Hooghoudgt solution with the use of locally determined design drainage rates (Skaggs and Tabrizi, 1986), can be used to obtain a relatively good first estimate of the required drain spacing needed in the conventional subsurface drainage mode. When operated in the controlled drainage or subirrigation mode, steady-state solutions presented by Ernst (1975) can be used. The computer program SI-DESIGN (Belcher et al., 1993) also provides a rapid estimate of the required drain spacing for a design storm. These estimates will reduce the number of simulation trials needed to determine a optimum drain spacing. Otherwise, a greater range of drain spacings, from too narrow to too wide, will need to be tried in subsequent simulations to determine an optimum drain spacing DRAINMOD. The computer simulation model DRAINMOD (Skaggs, 1978, 1980; NRCS, 1994), provides one of the more comprehensive methods of relating water management system design to soil properties, climatological conditions, crop requirements (particularly corn and soybeans), and management alternatives (Fouss et al., 1987a). The model was developed specifically for subsurface drainage, controlled drainage and subirrigation system analysis, and considers the influence of rainfall infiltration, evapotranspiration, surface runoff, subsurface drainage or irrigation, and seepage on water table depth. The model avoids complex and timeconsuming numerical computational methods by using simplifying assumptions such as drained to equilibrium conditions directly above the water table. This allows personal or workstation computers to run simulations to evaluate various system design options. DRAINMOD was not intended, however, to predict or simulate transient changes in water table depth (or shape) that may occur with large or frequent changes in the drainage outlet water level as with controlled water table management systems (Fouss, 1985). The model can be satisfactorily used to design controlled systems provided outlet water level changes are infrequent, such as for manually controlled systems; or are small (e.g., m) where automated changes in outlet water level occur no more frequently than 3-day intervals. More complex simulation models are available for the designer to use in evaluating the performance of selected facilities or design options for fully automatic control of the outlet water level (particularly pumped drainage from sump outlet structures); see Fouss and Rogers (1992). The procedures for using DRAINMOD to conduct a series of simulations in a systematic manner to arrive at an optimum drain spacing and the best weir setting(s) for the outlet water level control during the growing season are documented by Nolte (1986), Evans and Skaggs (1989), and NRCS (1994). Thus, a detailed design example is not given here. This chapter includes a summary of the key points and additional simulation approaches specific for controlled water table systems. Evans and Skaggs (1989) suggest that the drain spacing calculated from one of the shortcut methods, such as the Ernst (1975) equation, can be used as a starting point. An approximate weir depth is chosen and simulations are conducted for a range of spacings, at least two less than and two greater than the first estimate. The relative yield is plotted to determine the spacing giving maximum yield. Additional simulations can be conducted for three or more weir settings for the controlled drainage and subirrigation period of operation. These simulations provide data to perform an economic evaluation (Evans et al., 1988b), and the drain spacing and weir setting that produces the highest projected

11 694 Chapter 18 Water Table Control Systems net profit is selected as the final design. For the final design selection, an additional evaluation step is suggested. This involves conducting simulations where the start-up time for subirrigation (i.e., raising the weir) is changed, e.g., starting irrigation pumping 2 to 3 weeks after planting for corn and varying the start time for 7 to 10 days until tasseling. The final design evaluation involves water usage and pumping cost for the different start-up times. DRAINMOD does not include economic or environmental impact analyses, so these must be performed externally (see Section ). The additional simulations to optimize drain spacing and outlet water level control depths (elevations) can be performed somewhat easier with the revised model, DRAINMOD with Feedback Control (Fouss, 1985; Fouss et al., 1989). This version of the model has a subroutine to simulate adjustments of the outlet water level (weir) at pre-selected (input) dates and levels, at different levels based on rainfall amount and monitored field water table depth (i.e., water table depth at the midpoint between drains), and for periodically adjusted outlet water levels based on feedback signals of monitored water table depth. This model has direct application in designing and selecting the final operational method for the water table management system; and, it is noted here because of its potential application in arriving at the final design drain spacing and outlet water level control guidelines. Fouss (1985) and Fouss et al. (1990) suggest conducting complementary simulations to evaluate performance of the selected system design (depth and spacing) for the wettest and the driest years during a 30-year period, and to compare the performances of different methods or systems for control of the outlet water level. For most systems, the outlet water level should be held relatively constant at some optimum elevation if the system performance and crop yields are acceptable. Such a simplified operational procedure minimizes the need for frequent adjustments or for more complex control (i.e., feedback automatic control) of the outlet water level, thus reducing the cost of the total system. DRAINMOD-NII (Youssef et al., 2005) allows the drainage design evaluation to include a comprehensive analysis of the impact of system design and operational parameters on transport of various forms of nitrogen within the soil profile and losses in runoff and subsurface drainage flow. This version of DRAINMOD is an important tool for designing drainage water management systems, along with their operational plans, to meet emerging water quality requirements. It does not have automated control or operational management options incorporated into the code, other than those operational changes often made manually (e.g., outlet weir depth) on specific calendar dates that could be input to the simulation model. A revision of DRAINMOD-NII is planned to include routines for operational management options, including automated control System Layout and Sizing Components For both controlled drainage and subirrigation, a relatively constant depth to the water table is needed during much of the growing season. To accomplish this, the field should be divided into zones where the surface elevation does not vary more than 0.3 to 0.45 m. Within each zone, regularly spaced lateral pipes carry water to collector pipes which deliver the water to the system outlet. For subirrigation, those same collector pipes deliver irrigation water to each zone. If full pipe flow without pressure is assumed, Manning s equation relates pipe roughness, flow area, hydraulic radius, and hydraulic grade line (HGL), and may be used as the basic equation for determining pipe diameter. For controlled drainage sys-

12 Design and Operation of Farm Irrigation Systems 695 tems, the collector pipes are sized for the maximum drainage rate needed for optimum performance of the system with the HGL taken as the bottom slope of the collector pipe. For subirrigation systems, the pipe often must be sized for both the maximum drainage rate with the HGL set equal to the bottom slope as well as the subirrigation rate with the HGL a function of the weir settings while subirrigating. A number of design aids are available to assist with sizing the collector pipes. These aids take the form of tables, nomographs, slide rules, computer spreadsheets, and computer programs. For examples, the MAIN module of SI-DESIGN (Belcher et al., 1993) can be used for determining the needed diameters of the submain and main collector pipes. The user describes the system layout by appropriate input data. The module calculates collector pipe design parameters such as drainage coefficient, subirrigation rate, subirrigation water table depth, and pipe depth. Results from the LSPACE and MAIN modules can be used in a COST module to estimate the cost for installing the system. An accurate and detailed topographic map is essential for properly designing a water table management system. During operation of the system in the subirrigation mode, it is important that the shallow groundwater be maintained at a relatively uniform depth below the soil surface. Unless the land is nearly flat, this requires dividing the field into control zones of nearly uniform surface elevations. Burnham and Belcher (1985) described a laser-plane type surveying system that aids in the design and layout of water management systems. The laser equipment components can be used for land surveying as well as automatic grade and depth control on machines installing subsurface drains or doing land grading (Kendrick- Peabody, 2004). For land surveying the laser-plane receiver unit is mounted onto a four-wheel drive vehicle and the receiver elevation outputs are automatically read and stored directly by an interfaced portable PC as each preselected grid point in the field is passed by the vehicle. Each grid crossing is determined by a ground travel measurement unit connected to the computer logic circuit as the vehicle travels along preselected and marked lines in the field, e.g., about every 30 m. Typically 32 hectares (80 acres) can be covered in two hours. After the fieldwork, data are downloaded from the portable PC and commercially available software is used to plot topographic-contour maps and three-dimensional views of the field. An electronic interface between design and installation is in the development phase. It is anticipated that a laser control system mounted on installation equipment will be capable of coordinating and guiding the machine during installation, controlling depth and grade by using data downloaded from the design computer. Currently, commercially available global position systems (GPS) are utilized to obtain field survey information and during drain pipe installation. There are other specialized software packages available to aid the engineer in developing topographic maps, laying out subsurface drainage and water table control systems, compiling a list of materials needed, and estimating materials and installation costs. Examples include SUBDRAIN (Bottcher et al., 1984), from Cornell University, for subsurface drainage design, and LANDRAIN (Sands and Gaddis, 1985), a computer-aided design (CAD) program for subsurface drainage systems, with automatic drainpipe sizing routines. In both cases, the drain spacing must be predetermined by other methods. A companion program, LANDIMPROVE (Sands and Gaddis, 1985), is available for land leveling and grading design. And finally, there are now commercial

13 696 Chapter 18 Water Table Control Systems laser-plane and GPS-based systems available to assist the designer or contractor in this regard (Grandia, 2002; Welch, 2002) System Operational Objectives The overall design and operational objectives for water table management systems were outlined in the previous sections. The specific operational objectives for a water table management system can vary with crop, soil, climate, and topographic conditions, but the following objectives apply to most systems designed for humid regions: provide trafficable soil conditions required for conducting timely farm field operations, without damaging soil structure (note that drainage of the soil profile to a greater depth may be necessary during the time required for the field operations). minimize the frequency and duration of excess soil water in the root zone caused by rainfall as well as deficit soil-water conditions during droughty periods; prevent over-drainage of the soil profile, thus maintaining the water table at a depth shallow enough to provide a soil-water supply for crop roots (and also reducing agrochemical losses in any drainage discharge); and minimize the need for pumping subirrigation water by efficient use of infiltrated rainfall. For topographic conditions that dictate a pumped subsurface drainage outlet (e.g., a gravity outlet is not available or possible), minimizing pumping (energy) requirements for the subsurface drain effluent may also be an important objective. As noted earlier, the initiative to promote and implement drainage water management technology (controlled drainage) to improve drainage effluent water quality may be only the beginning of changes in drainage and water management design objectives. In the future, more precise integrated management of water, fertilizers, and pesticides may be required to achieve and sustain acceptable surfacewater and groundwater quality Selection and Design of Operational Methods This section outlines several methods that have been developed to operate dualpurpose controlled-drainage and subirrigation systems for water table management. For both manual and automatic operation of water table management systems, basic principals of operation must be understood and followed in order to meet the operational objectives Manual operation. Many dual-purpose systems in humid regions are operated manually. Proper manual operation requires skill of the farmer or manager and attention to system responses and performance. Frequent visits to the field are required during the growing season to monitor water table depth. 3 Drainage rate is commonly controlled by a manually adjusted weir at the drain outlet for each zone or in the main outlet ditch, in conduit mains or submains, or perhaps in individual lateral drainlines. Depending upon the size and length of an individual outlet ditch controlling a zone, the response of the water table depth in the field may 2 Trade and company names are included in this monograph for the benefit of the reader and do not imply endorsement or preferential treatment of the product listed by USDA or cooperators. 3 Manual water table measurements are commonly made in observation wells fabricated from smalldiameter, perforated, plastic pipe installed vertically in the ground midway between drainlines. In some soils a filter sock and sand backfill around the well are required. A blowtube is typically used to measure the water table depth.

14 Design and Operation of Farm Irrigation Systems 697 be relatively slow (several hours) because of the large amount of water that must to be removed (drainage) or added (subirrigation) to change the water level in the ditch, Figure Examples of flow-limiting devices for subsurface submains or mains are illustrated in Figures 18.3 (a) & (b); the field water table response with this type of outlet water level control is more rapid than where an outlet ditch is used. When excessive rainfall occurs, the water level at the weir or outlet may be lowered to drain depth to allow the system to function in a conventional subsurface drainage mode to lower the water table more quickly. For manually operated systems, the lowering of the weir or adjusting of a flow-limiting device to full open is often delayed until after the storm, when excessive soil-water conditions already exist. A major decision for the farmer is when to raise or reset the outlet water level. Delaying too long may lead to overdrainage of the soil profile, thereby reducing potential agrochemical losses in soil-water discharges and accelerating the need to irrigate if subsequent rainfall does not reestablish the desired soil water in the root-zone. If the farmer waits until the observed (measured) water table midway between drains recedes to pre-storm depth, overdrainage of the soil profile may occur, especially for fine-textured soils. During periods when subirrigation is needed, the outlet weir is raised (manually) and irrigation water may be pumped into the outlet weir structure periodically as needed during selected hours each day. A float-activated valve on the irrigation water line may be used to hold the water level nearly constant just below the weir s overflow elevation. If an irrigation water supply is not available, the outlet control weir is raised to control subsurface drainage and to capture rainfall in the soil profile up to the elevation of the weir. Any excess soil water from rainfall overflows the preset weir and the field water table slowly recedes to the elevation of the weir Automated operation. Dual-purpose system automation provides an alternative to labor-intensive system management methods. Automatic control of the system can take on many different options or modes. Fully automatic or semi- automatic control of the outlet water level, with or without feedback of the monitored Figure Flashboard riser type structure used in open ditch. Considerable time may be required to raise or lower the water level in the ditch due to the ditch volume.

15 698 Chapter 18 Water Table Control Systems (a) (b) Figure (a) Weir-type flow-limiting device within a drainline or submain riser pipe; (b) Float-activated flow-limiting valve within a drainline or submain riser pipe. water table depth in the field, allows the field water table to be maintained within predetermined depth limits. For purposes of discussion here, it is assumed that the drainlines are connected into a sump structure for control of the outlet water level. The water level in the sumps during drainage cycles can be controlled by pumping from the sump structure into a surface drainage channel (see Figure 18.1). During subirrigation, the water level in the sump outlet control structure is maintained within a predetermined range by pumping from an external source such as a well. Automatic control of the sump water level (SWL) to regulate subsurface drainage discharge (i.e., controlled drainage) and subirrigation flow into the soil profile may additionally include options with or without feedback of the field-monitored water table depth (WTD) between drainlines. Automatic control with feedback is much preferred. A crosssectional schematic of the sump structure and WTD sensor 4 in the controlled-drainage 4 Fouss and Rogers (1992, 1998) and Fouss et al. (1999) provide detailed discussions on the various types of water table depth (WTD) sensors that can be used in automated control systems.

16 Design and Operation of Farm Irrigation Systems 699 mode of operation is shown in Figure The mode of operation is automatically switched by the system controller from controlled-drainage to subirrigation, and vice versa, as needed to maintain the SWL between the maximum and minimum water level elevations specified. Brief descriptions of various methods of control are given below. It should be emphasized here that over-aggressive drainage to maintain or lower a water table that fluctuates quickly to a shallow depth during rainfall events may result in excessive loss of agrochemicals in the drainage discharge. Thus, an operational balance needs to be followed between controlled subsurface drainage and subirrigation modes to prevent excessive short-term agrochemical losses in drainage discharge (ADMS-TF, 2005). Float-switch activated control. A combination of four float-activated electrical switches can be configured to operate the drainage and irrigation pumps in an outlet sump structure. A single-float activated switch can be used at the maximum SWL elevation to change the system operation from subirrigation to controlled drainage. Another single-float switch can be used at the minimum SWL elevation to change operation from controlled drainage to subirrigation. A double-float activated switch can be used to operate the drainage pump between high-drainage (HD) and low-drainage (LD) sump water levels. Another double-float switch is used to operate the irrigation pump (or valve) between the low-irrigation (LI) and high-irrigation (HI) sump water levels. This type of control mode does not involve feedback of the WTD in the field. The elevations of the float switches typically require manual repositioning to adjust the SWL control threshold elevations, that is, the MIN, LD, HD, LI, HI, and MAX water level elevations in the sump. The subsurface drainage and subirrigation pumps (or valves) can also be operated by manual electrical switches, if desired; this is commonly done to periodically check the functioning of the drainage pumps and irrigation pumps or valves. Microprocessor-controlled system. This is a fully automated control system. In the controlled-drainage mode, without feedback of the field WTD, the SWL is main- Figure Controlled-drainage mode of sump-controlled water table management. WTD = water table depth, HI = high-irrigation, LI = low-irrigation, HD = high-drainage, and LD = low-drainage sump water elevations.

17 700 Chapter 18 Water Table Control Systems tained between preset HD and LD elevations in the sump (Figure 18.4) by pumping water from the sump. These desired HD and LD elevations (on/off electrical switch levels for the drainage pump) may be stored as software values in a microprocessor controller system. When subsurface drainage flow ceases and the SWL recedes to the minimum elevation (via subirrigation flow from the sump), the system operation is switched to the subirrigation mode. With feedback of the field WTD, the microprocessor controller system will operate the drainage pump as described above for controlled drainage without feedback, except if the monitored field WTD is outside of a desired depth range (WTD min WTD WTD max ) during the previous 24-h period (midnight to midnight). If the WTD is out of range, the SWL control threshold elevations (MAX, HI, LI, HD, LD, and MIN) are automatically adjusted upward or downward, as appropriate, by an amount Y stored in the software in the microprocessor controller program, for the next 24-h period. Once the field WTD returns to the desired range, the SWL control thresholds are returned to the predetermined standard elevations (MAXS, HIS, LIS, HDS, LDS, MINS) by the controller unit the following midnight. The system operation is switched to subirrigation anytime the SWL falls below the minimum elevation. In the subirrigation mode, without feedback control, the SWL is maintained between the low-irrigation and high-irrigation elevations by pumping into the sump from a nearby well. A microprocessor controller system, as described above for the controlled-drainage mode, can be used to operate the irrigation well pump. As gravityflow subirrigation from the sump lowers the SWL to the low-irrigation elevation, the irrigation pump is operated to raise the SWL to the high-irrigation elevation. The field WTD is not monitored in this mode. With feedback control, the sump operation is the same as subirrigation without feedback, except the SWL control threshold elevations are adjusted as described above for controlled drainage with feedback, whenever the monitored field WTD is outside the desired range for a midnight-to-midnight 24-h period. The same feedback adjustment parameter Y as used for the controlled-drainage mode may be used for the subirrigation mode, or a different adjustment parameter Z can be selected, if desired. The value of Y or Z will depend on the system design, soil characteristics, water table response, etc., but typically is set to a value of about 10% to 15% of the drain depth. If rainfall occurs when the system is operating in the subirrigation mode, and the cumulative amount exceeds a threshold in a given period of time, the system operation may be switched to the controlled-drainage mode. If the rainfall does not exceed this threshold (which is based on experience at the site), but infiltration is sufficient to cause subsurface drainage into the sump that raises the SWL to the maximum elevation, the system operation will be switched to the controlleddrainage mode. An example of fully automated operation of a controlled drainage and subirrigation system for water table control was reported by Fouss and Rogers (1998) and Fouss et al. (1995, 1999), and full details on the design and objectives of the research project were reported by Willis et al. (1991). Semi-automatic float valve for gravity-flow systems. Where a gravity-flow outlet is possible, an alternative method of SWL control in the subsurface drainage mode can involve a dual-float activated valve as illustrated in Figure 18.3 (b). The larger float provides the force to lift the outlet pipe-plug at a preset water level, and the smaller float provides sufficient power to hold the pipe-plug and prevent it from closing again

18 Design and Operation of Farm Irrigation Systems 701 until the water level in the sump recedes to some preset lower level. Such a valve mechanism allows fast subsurface drainage during peak flow events, functioning similar to a two-stage weir described by Fouss et al. (1987b). The overflow pipe in the sump allows sufficient drainage to control the SWL during smaller rainfall events when rapid drainage is not required because excess soil-water conditions last only for short periods of time. Automated operation for gravity-flow systems. In humid areas, dual-purpose systems often have multiple field zones with each zone outletting to a main and the main outletting by gravity into an open channel. Automation of such systems requires slight modification of the methods described above for pumped-outlet systems. A computerbased prototype automation system has been developed at Michigan State University (Belcher and Fehr, 1990). This system is capable of minimizing the fluctuation of the water table above or below the desired elevation, alerting the operator to problems (such as a pump not working), providing a visual display in the office (via modem and PC) of what is happening in the field, providing the user the ability to input the desired water table elevation in each zone as a function of time, and storing useful operation data throughout the growing season (water table depths, pumping duration, etc.). The system uses rainfall, discharge of underground pipes, and feedback of monitored water table level to control the on/off cycles of the irrigation pump. When it rains, the system shuts off the irrigation pump and adjusts the flow restriction device in each zone as needed to hold the water table near the desired elevation. When the water table falls below the desired elevation, the system restricts drainage flow and activates the irrigation pump. The system allows the subirrigation system operator to monitor and modify the subirrigation operation parameters from a remote office via PC and modem. Figure 18.5 is a schematic of the automation system for a single water Sensor/Controller Irrigation Pump Rainfall Sensor Water Table Water Table Depth Sensor Submain Flow Limiting Device Figure Schematic of gravity discharge subirrigation automation feedback control.

19 702 Chapter 18 Water Table Control Systems table management zone. A prototype system tested in Michigan has the capability to handle up to 16 zones (Belcher and Fehr, 1990). Two types of flow restriction devices have been used in this automated control system. The first consists of a diaphragm valve placed in-line in the submain. The controller restricts flow by inflating the valve and allows flow by deflating the valve. The second device is a modification of the weir control structure shown in Figure 18.3 (a), in which a small DC motor is added to raise or lower the bottom weir plate to block or allow subsurface drainage flow. This type of weir control structure is commercially available; e.g., the Smart Drainage System. 5 Remote control. Three general types of remote control of water table management systems are currently available: one- or two-way radio control, electronic communications from a cell phone, and digital communications from PCs and modem equipment via telephone lines. Communications via satellite are economically feasible only for larger projects at great distances from the control decision center. A typical application of remote control would be to change the mode of operation at one or more outlet (sump) structures; for example, to stop pumping subirrigation water and/or to permit drainage of the system in advance of predicted heavy rainfall (see below on use of weather forecasts). Two-way radio communication permits confirmation that the operational change was actually made at the remote site. Electronic communications via a PC and modem to a microprocessor-controller system for sump structures permits more sophisticated remote control, or provides a means to override an automated control system. Such remote control or control override can permit the manager to change the sump water trigger levels for controlled-drainage, subirrigation, or other mode switch levels, by merely changing stored values in the microprocessor program. The microprocessor system also permits monitoring of field site parameters, such as water table depth, rainfall amounts, or mode of operation from the PC in the control center. Seasonal modes of operational control. In the humid region of the U.S. and Canada, removing excess soil water from the field is the system s most important role. Drought conditions in soils of this region are generally temporary in nature, and thus the system functions in the controlled drainage or conventional subsurface drainage mode the majority of the time. When rapid drainage is required, the outlet water-level control structure should be set to the depth of the subsurface drainage conduit to provide the maximum drainage rate; it needs to be recognized that this may cause excessive losses of applied agrochemicals from the soil profile while rapid drainage is permitted. When trafficable field operations are required, the outlet water level should be maintained near or slightly above the drain depth until after the crop has been planted. The recommended operational practice during the winter (non-cropping) months is to keep the system in the controlled drainage mode to maintain the water table shallow in the soil profile, which reduces nitrate losses because drain outflow is decreased and a denitrification zone can develop in the upper soil layers (Gilliam et al., 1979, 1999). During the growing season, the outlet water level should be controlled (by one of the methods described above) to maintain the field water table depth within the desired range relative to the crop root zone to provide a steady supply of water to plant roots by upward flux. A week to ten days prior to crop harvest, the outlet water level should 5 Agri Drain, Inc., th St., Adair, IA 5002: info@agridrain.com.

20 Design and Operation of Farm Irrigation Systems 703 again be lowered to near or slightly above drain depth to provide good trafficable conditions for harvesting operations, and to reduce damage to the soil structure by equipment traffic. Following harvest, the operational-control annual cycle described above is repeated. Use of weather forecasts to aid in operational management. In many areas of the humid region of the U.S., the predicted probability of rainfall and the estimated daily amount of rainfall in National Weather Service forecasts (e.g., the daily, three- and seven-day forecasts) are becoming sufficiently accurate to permit adjustment of day-to-day operation of water table management systems, and to aid in the timing of fertilizer and pesticide applications (Fouss and Willis, 1994; Schneider and Garbrecht, 2006). Principal management objectives may be to reduce the occurrences of severe excess soil-water events, and the duration of deficit soil-water conditions, improve the efficiency of utilizing rainfall received, thus minimizing the need for pumping irrigation water, and increase the effectiveness of fertilizers and pesticides applied and reduce the potential of losses. For example, if large rainfall amounts are predicted, the system operation may be switched from subirrigation to controlled-drainage or conventional subsurface drainage several hours to a day in advance of the predicted rain. Also, following the rainfall and the subsequent recession of the water table to the desired depth, the restarting of subirrigation may be delayed if significant rainfall is predicted to occur again within the next two to three days. The minimum percent probability of rain for which these actions may be taken can differ in various geographic regions because of the regional accuracy of the forecasts (Schneider and Garbrecht, 2006). Expert advice or experience will be needed to determine the minimum percent probability that best applies in a given area. Use of weather forecasts to aid in operational management is best justified for high-value crops, such as vegetables, and crops that are most affected by wet soil conditions SYSTEM DESIGN AND OPERATION IN ARID REGIONS Characterizing Climate Conditions In humid regions the source of water used in water table control systems is either rainfall percolating through the soil, which is captured by controls on the drainage system, or water pumped into the drainage system. The random nature of rainfall makes it difficult, if not impossible, to determine the availability of rainwater in a water table control system. When the drainage system is being used for subirrigation and a water source is available, it is possible to pump water into the drainage system, as needed, to maintain the water level. In this case the water supply is a stream or river, or ponded water. In arid and semi-arid areas the source of excess water being used in a water table control system is generally deep percolation from irrigation on the subject field or lateral flow from an adjacent field. Rainfall amounts in these areas are generally limited and add little water to shallow groundwater. For example, the west side of the San Joaquin Valley is semi-arid, averaging 150 mm of rainfall each year, generally during the winter months. The average rainfall event is 5 mm or less and is not effective in providing water for either winter crop use or deep percolation. Rainfall can be more important in other regions, such as southeast Australia, leading to shallow water tables after winter or sporadic heavy rainfall. These make irrigation management more diffi-

21 704 Chapter 18 Water Table Control Systems cult; water table control can assist in making better use of these rainfall contributions. Generally, as rainfall is not a significant contributor to the total water supply, it is possible to estimate the shallow groundwater availability based on the water requirements of the crop rotation and the irrigation system efficiency and practices Characterizing Shallow Groundwater Conditions Shallow groundwater is affected by the geology of the area and the irrigation practices on adjacent fields, in addition to the irrigation practices on the field itself. In areas with soils developed from alluvial deposits, there are often sand stringers or zones of soil with high hydraulic conductivities passing from one field to an adjacent field at depths less than 2 m. In these instances, deep percolation in adjacent fields is transmitted to the field of interest and water table control becomes significantly more difficult. In humid areas, shallow groundwater quality conditions are characterized by the presence of manmade chemicals such as nitrate, other fertilizer elements, and pesticides. In arid areas shallow groundwater may contain salt and trace elements, in addition to man-made contaminants, making it more difficult to characterize its quality. The remainder of this section will discuss the impact of salt and trace elements in the groundwater on shallow groundwater management. Many of these topics are discussed in more detail elsewhere in this monograph. However, basics concepts are reviewed here to provide a basis for the discussion of design and management of drainage systems used for water table management in arid areas Soil salinity. Sources of salinity include irrigation water, fertilizers, and natural sources such as salt deposited from the ocean during the formation of the soil. Regardless of its purity, all irrigation water contains salt. Irrigation water with an electrical conductivity of 0.3 ds m -1 is considered suitable for use on all crops; however, even this contains approximately 200 mg L -1 of salt, which accumulates in the soil as crops remove water. In arid and semi-arid areas, it is quite common for soils to contain salt of geologic origin. The Mancos shale, the parent material of the soils in the Grand Valley of Colorado, contains lenses of salt deposited while this area was being formed under the ocean (Walker et al., 1979). The soils on the west side of the San Joaquin Valley of California were derived from the Coast Range, also marine sediment, and as a result contain high concentrations of salt and elements such as boron, selenium, arsenic, and other trace elements (Deverel and Fio, 1990). In southeast Australia the huge stores of salt found in the landscape were deposited over geologic time by dust and rainfall transport (Herczeg et al., 2001; Ladaney-Bell and Acworth, 2002). The salt concentration in the soil profile generally increases with depth in arid and semi-arid areas of irrigated agriculture. Soils in the irrigated regions of semi-arid southeast Australia often have a 10-fold increase in salinity between 0.1 and 2 m (Hornbuckle and Christen, 1999). The surface soils are leached and the salts are moved deeper into the profile, so the salt concentration in the surface soil is low enough that neither seed germination nor early plant growth is adversely affected. If the water table is close to the soil surface, it is possible for evaporation to move water and salt up from shallow saline groundwater and accumulate salt on the soil surface. When a crop uses significant quantities of water from shallow saline groundwater the salt moves with the water and is deposited in the root zone. Either situation creates an inverse salinity profile in which the soil salinity decreases with depth. This type of profile should be managed to prevent deleterious effects on seed germination

22 Design and Operation of Farm Irrigation Systems 705 and plant growth. Pre-plant irrigation during a fallow period is one method routinely employed in the San Joaquin Valley of California to leach salts and restore the salinity profile prior to planting Leaching. Leaching is required to both maintain a salt balance in the root zone and to prevent inverted salinity profiles. Assuming piston displacement and no contribution of salinity by groundwater, the minimum fraction of applied water needed to maintain a salt balance is referred to as the leaching requirement (L r ). This is established based on the salt input and the crop salinity tolerance and is the minimum volume of water that must pass through the soil profile to maintain the salt balance. The leaching requirement is in addition to the crop water demand. The requirement can be estimated from L r = D * * d /D a = c a /c d (18.1) where D d is the equivalent depth passing below the root zone, D a is the equivalent depth of application (irrigation plus rain) (Hoffman, 1990), c a is the weighted mean concentration of the applied water and c * d is the salt concentration in the water passing below the root zone. The required values versus the actual are designated by asterisks. When the water table is being managed to provide part of the crop water requirement, c a needs to be modified to include the salinity of the groundwater and the percentage of water used by the crop (Fouss et al., 1990). The leaching fraction (L f ) is the actual amount of water passing through the soil rather than the required amount. The leaching requirement is often met through irrigation inefficiency and/or of the irrigation system distribution uniformity, or even in some cases by winter rainfall, thus additional water is not often required for leaching. This is particularly true when a surface irrigation method, such as flood/border or furrow, is used. Water passing through the root zone moves to the shallow groundwater where it is either collected and discharged for disposal through a subsurface drainage system or percolates to the regional groundwater, or if the water table remains shallow the groundwater can be evaporated leading to salt accumulation in the surface layers Crop salt tolerance. The crop salt tolerance, soil salinity, applied water quality, groundwater quality, and stage of growth will establish the potential of shallow groundwater for meeting crop water requirements. Crop salt tolerance has been evaluated through experimentation as ranging from sensitive to tolerant. The tolerance is characterized mathematically using the Maas-Hoffman equation (Maas and Hoffman, 1977), which sets a salinity threshold at which yields begin to decline. The rate of decline varies with a crop and its salt tolerance. The studies used to characterize plant salt tolerance were done under conditions of uniform root-zone salinity, a condition not generally found in the field. Other research has demonstrated that the average root-zone salinity (Shalhevet, 1994) is the most important factor to consider when characterizing the tolerance and the distribution of salt in the soil profile. Crops will selectively extract water from the least saline portions of the root zone. As the soil profile becomes more salinized, there will be less water available for crop use and potential yield losses will be realized. Where crops use water from a shallow water table, it has been found that the crops apparently use water at higher salinity levels than predicted with the Maas and Hoffman parameters without yield decline. The reasons for this are not well understood; since crops generally only receive part of their water requirement from a water table

23 706 Chapter 18 Water Table Control Systems (saline) and the bulk of their water requirement from irrigation (non-saline) and the overall volume-weighted salinity of the water used by the crop may be tolerable. This can be demonstrated using data from Ayars and Schoneman (1984) for a cotton crop. The estimated crop evapotranspiration was 647 mm with 429 mm applied low salinity (2 ds m -1 ) and 167 mm of upflow of saline (7 ds m -1 ) water. The weighted average of the water supplied by these two sources is 3.13 ds m -1. This value is less than the threshold value of soil salinity that would result in a yield reduction. While this value is less than the threshold, the time of uptake of water from shallow groundwater also has an effect since much of the extraction occurs late in the growth cycle when the plant is most salt tolerant. There may also be some uncertainty in the actual salinity of the groundwater and hence the water that the crop is taking up. This can be due to the spatial variation in groundwater salinity across a field and also the temporal variation as water tables rise and fall. There is also variability introduced by sampling groundwater at depths below that at which the groundwater is interacting with the root zone (Northey et al., 2006) Drainwater quality. Drainwater quality is generally not an accurate description of the shallow groundwater quality being used in a water table control system. The shallow groundwater below the root zone is a mixture of the salts in equilibrium with the soil salinity at that depth and the deep percolate from the applied irrigation water. Drainwater from a subsurface drain is a mixture of the shallow groundwater and deeper groundwater. In many arid areas the water quality becomes progressively poorer as the depth in the soil profile increases. Grismer (1990) demonstrated that as a drain spacing gets wider or the drain depth gets deeper, proportionately more water is taken from deeper in the profile, which results in a poorer water quality than expected from sampling shallow groundwater. Christen and Skehan (1999) also demonstrated that drainwater salinity increases as the depth to the water table increases and hence flow paths to the drains become deeper. They found a 50% increase in drain flow salinity from when the mid-drain water table was at 1 m (8 ds m -1 ) and at 1.6 m (11.5 ds m -1 ). Water quality sampling of shallow groundwater using wells installed at the appropriate depths will provide a better characterization of the shallow groundwater quality than will drainwater samples Depth to groundwater. In irrigated areas the source of shallow groundwater is oftentimes deep percolation from inefficient irrigation and, as a result, the groundwater fluctuation responds to the irrigation management in the area. Early in the season irrigations are difficult to schedule and match to the restricted root zone of young plants and often even pre-plant irrigation is practiced. These early-season irrigations lead to large volumes of drainage past the root zone so that groundwater is often closest to the soil surface at the beginning of the irrigation season. As the irrigation season progresses, the depth to water table increases through a combination of lateral and vertical flow in the groundwater system and plant uptake. If the irrigation system is inefficient, it is possible to have a condition where the depth to groundwater decreases until the end of the irrigation season at which time it gradually increases. For example, Figure 18.6 illustrates groundwater depth data showing an increase in water table elevation under furrow-irrigated tomato plots in a field without subsurface drains. The depth to groundwater affects the total uptake by the plants and the time the uptake begins (Ayars et al., 2006).

24 Design and Operation of Farm Irrigation Systems 707 Figure Water table response under two furrow irrigated tomato plots (NCFA, NCFB) in a field without subsurface drainage Irrigation System Selection, Management, and Operation Irrigation system selection will depend on the soil, crop, and financial conditions of the enterprise and the preferences of the manager. The available systems can be subdivided into either pressurized or non-pressurized and within each category there are many options. Pressurized systems include sprinklers (hand-move), center-pivot, linear-move, drip systems, and microsprays. Non-pressurized systems (surface irrigation methods) include level basins, furrows with surges, and/or gated pipe systems. The design of these systems is discussed elsewhere in this monograph. The important aspects of irrigation system design for this section are the potential irrigation efficiency, uniformity, and management. Non-pressurized irrigation systems are characterized as the least efficient of systems with irrigation efficiency and distribution uniformity in the range of 50% to 80%, resulting in excess deep percolation. The infiltration rate of the soil surface limits the potential irrigation efficiency and distribution uniformity because of the variability of the infiltration with time and space. It is very difficult to apply small depths of water with surface irrigation with the exception being level-basin systems that apply as little as 55 mm in an application (Dedrick et al., 1982). Surface systems work best on soils, such as clays and clay loams, because of the low infiltration rates compared to sands. Because pressurized systems have the potential for high uniformity of application, high distribution uniformities, and small depths of application, they enable better control of deep-percolation losses. The advantage of pressurized over non-pressurized systems is that infiltration is controlled by the application rate of the sprinkler rather than the soil surface. With pressurized systems it is possible to apply irrigations as small as 2 to 4 mm, three to four times daily, as compared to surface systems that are limited to 50 mm as the minimum application. Irrigation scheduling determines the time and depth of application. In shallow groundwater conditions the timing and the depth of application can be altered by the crop water use from shallow groundwater. The timing is altered because a portion of the water requirement is met from the groundwater, not the stored soil water, and traditional volume-balance calculations assume all water use is from stored soil water (Ayars and Hutmacher, 1994). Thus, groundwater contribution extends the interval

25 708 Chapter 18 Water Table Control Systems between irrigations when timing is based on soil water depletion. If saline groundwater is being used by plants, salts are being moved into the root zone and the potential exists for an increase in soil water osmotic potential stress, which is added to the matric potential. The plant will respond to the combined stresses and indicate an earlier irrigation than would matric stress alone. In this case less water will be removed from stored soil water than indicated by matric stress alone. Alternative methods have been developed to determine plant stress levels that can be used as indicators for irrigation timing in shallow groundwater areas. Leaf water potential, which reflects osmotic and matric stress, has been used successfully to schedule irrigation in cotton (Ayars and Schoneman, 1984; Kite and Hanson, 1984). This is a valuable technique and is applicable for crops having the stress values needed for irrigation scheduling. The crop water stress index has also been shown to reflect both matric and osmotic stress using infrared thermometry to determine plant temperature. The temperature data are used with vapor pressure data to determine the index (Howell et al., 1984). After reaching a critical value, irrigation is indicated. This technique is limited to the time after full canopy cover is reached to prevent background soil temperature readings from affecting the results. At present its use is limited because of the lack of data needed to establish the critical index values needed for irrigation scheduling. After the time to irrigate is established, the depth of application is determined. Currently this is done by gravimetric analysis or using neutron attenuation to establish the water depletion in the interval between irrigations. New devices to determine soil water content which offer promise include time-domain reflectometry and capacitance systems. Ayars and Hutmacher (1994) demonstrated a modified cotton crop coefficient that accounted for the groundwater contribution to crop water use (Figure 18.7) as a function of groundwater salinity and depth enabling a volume-balance calculation for soil water depletion by the crop. Figure Basal cotton crop coefficient and regressions of modified crop coefficients derived from lysimeter data for five groundwater qualities and at various depths.

26 Design and Operation of Farm Irrigation Systems Subsurface Drainage System Design for Shallow Groundwater Management Design possibilities for both existing and new systems will be considered in this section. There are millions of hectares of drained irrigated land which might be considered for shallow groundwater management. The ability to control flow from the drains and to maintain the water table at a desirable level over a significant portion of the field is the major factor in determining the suitability for the field for modifications to control shallow groundwater. Also, the cost to modify the system and the potential disruption of cultural operations in the field should be considered in the design. Previous drainage system modifications have included use of weir type flow restricting valves (Fig a, b) installed on drainage laterals (Lord, 1987) to control the water table. For example, an existing drainage system on a 65-ha field on the west side of the semi-arid San Joaquin Valley was modified by installing butterfly valves on each of the seven drainage laterals (Ayars, 1996) to test possible control structures. In addition, weir structures were installed along the submain to provide regional control of the water table. The schematic of the system is given in Figure In this system the drainage laterals were installed perpendicular to the direction of cultivation in the field. Examples of alternative water table control structures are shown in Figures 18.9 and The structure in Figure 18.9 was used on an individual lateral in a vineyard Figure Schematic layout of drain system and control structures on the shallow groundwater management study site on a field located on the west side of the San Joaquin Valley of California.

27 710 Chapter 18 Water Table Control Systems in Australia. The weir structure in Figure was used on the submain of the drainage system depicted in Figure Figures 18.11a,b give the water table depth between a set of three subsurface drainage laterals in the experimental field for two dates during the production of a tomato crop. The water table control system resulted in less applied irrigation water and improved crop quality in the areas with the water table closest to the soil surface (Ayars, 1996; Ayars et al., 2000). Figure Individual lateral control structure used to control the water table in a vineyard in Australia. Figure Control structure used to measure drainage flow and control the water table in California.

28 Design and Operation of Farm Irrigation Systems 711 Figure Depth to groundwater after closing (A) or opening (B) lateral control valves on the field diagrammed in A study by Christen and Skehan (2001) in a vineyard in semi-arid southeast Australia compared no management (continuous flow) with active management of subsurface drains, 2 m deep and 20 m apart. The management of the system prevented drainage once the water table reached 1.2 m deep or when an irrigation event was occurring. Without management, the drains flowed continuously during the two irrigation seasons with a salinity of around 11 ds m -1. This resulted in a drainage salt load over