CONTENTS Flood irrigation

Transcription

1 CONTENTS Flood irrigation 1 Introduction Description Definitions Types of flood irrigation Basin irrigation Background Functioning Layout Control equipment Crops Limitations Advantages Border irrigation Background Functioning Layout Control equipment Crops Limitations General advantages Furrow irrigation Background Functioning Layout Control equipment Crops Limitations Advantages Other types of flood irrigation systems Short furrow Furrow border Contour flooding Underground flooding Choices for a flood irrigation systems Control structures Sluices Sliding sluices Hinged sluices Valves Flood valves Hydrant valves Flow control valves Siphon pipes Distribution structures Measuring structures Drop structures Diversion structures Factors which influence flood irrigation Soil Soil type Crust formation and resistance to erosion

2 4.1.3 Limiting layers Soil water content Cultivation Vegetation Crops Weeds Water System Gradient Field preparation Stream size Contact time Shape of border, furrow or basin Construction Basin irrigation Staking out Cut and fill Border irrigation Staking out Cut and fill Furrow irrigation Staking out Cut and fill Laser equipment Evaluation General ifromation and test procedures Pressure readings Delivery tests Distribution tests Management and maintenance Basin irrigation Border irrigation Furrow irrigation Troubleshooting tables References

3 Flood irrigation Introduction The term flood or surface irrigation refers to a kind of irrigation where water flows over the soil surface under controlled conditions with the purpose of allowing the desired amount of water to infiltrate the soil. Although soil gradient is instrumental in setting the irrigation water in motion, the gradient of the water surface itself is in fact what causes the water to flow over level surfaces. This chapter contains mostly information contained in the Irrigation Design Manual of the ARC-Institute for Agricultural Engineering. Basin, border and furrow irrigation are discussed in full. Short-furrow, border-furrow, contour-flooding and subsurface flood irrigation are only referred to briefly. 1.1 Description Flood irrigation makes out more than 90% of all irrigation methods used worldwide. There is evidence that flood irrigation was in fact used about years ago in the Far East. In the Philippines, irrigation terraces built years ago are still in use to produce rice. There are many types of flood irrigation. These may be divided into three main categories, namely basin irrigation, furrow irrigation and border strip (or border) irrigation. Every flood irrigation system consists of a source, conveyance and distribution component. The efficiency of a flood irrigation systems is generally low. Better layout and design choices, improved soil preparation, less water conveyance losses and more effective management systems, however, will enhance its efficiency. A well-constructed flood irrigation system, properly planned and designed and operated correctly, can be as effective as sprinkler irrigation. In some instances the cost of soil preparation and conveyance systems may be a deterrent in selecting flood irrigation. Similarly, soil and crop limitations may render flood irrigation unpractical. The design and management of flood irrigation systems are usually more complex than for other systems. This may be because additional factors, such as soil irregularities and gradients, play a determining role with flood irrigation. The following interdependent factors are crucial when water is flowing over soil and infiltrating into the soil for irrigation purposes: Soil type Soil s tendency to crust formation and resistance to erosion Limiting layers in the topsoil Soil water level Flow resistance due to crops Weed density Quality of water Cultivation practices Soil finish Gradient Soil preparation Stream size Contact time between water and soil

4 11.2 Irrigation User s Manual The key factor that determines whether flood irrigation is effective and efficient, is the user and his labourer. If the producer ensures that his borders, furrows or basin are correctly constructed, the irrigation done correctly and faults corrected, then he is using his system to the optimum. Training will contribute largely to the success of the system. Without training the system is sure to fail. 1.2 Definitions Advance front The front of the irrigation stream moving down a border, basin or furrow. Advance time The time [min] it takes the advance front to move from the beginning of a border, furrow or basin to a certain point. Total advance time The time [min] it takes the advance front to move from the beginning of a border, furrow or basin to the end. Recession front The tail end of the irrigation stream moving down a border, basin or furrow. Recession time The time it takes the recession front to travel from the beginning of a border, furrow or basin to a certain point. Total recession time The time [min] it takes the recession front to travel from the beginning of a furrow, border or basin to the end. Contact time The time [min] of contact between the irrigation water and the soil surface. It is the time lapse between the advance front and recession front at a specific point which provides the opportunity for infiltration of water into the soil. Cut-off time This is the time [min] allowed for the water to flow into a basin, furrow or border from the top. Irrigation stream The amount of water that flows into a border, furrow or basin per unit of time [m³/h] for irrigation purposes.

5 Flood irrigation 11.3 The advance and recession fronts may be depicted as follows: Figure 11.1: Ideal advance and recession time for flood irrigation 1.3 Types of flood irrigation systems Basin Irrigation Basins are square, rectangular or circular areas, without any gradient, bordered by ridges. For the sake of efficiency, the basin should be filled quickly. In South-Africa, basins are mostly used for orchard crops Background Basin irrigation is used all over the world and is the oldest form of irrigation. The basin is walled with earth ridges. Good examples of basin irrigation are rice paddies, round basins around fruit trees or the use of a lucerne field basin as large as 5 ha Functioning A determined amount of water is diverted into the irrigation basin by means of one or more sluices, openings in the ridge or pipes over the ridge. The quicker the basin is filled with water, the better is the application uniformity. A general rule is that the filling time should be a quarter or less of the infiltration time Layout of basins The bed of the basin should be level in all directions. The basin position should be selected in such a way as to require minimum earth moving. The basin size and construction must, however, be practical for tilling purposes and the optimal use of available soil. Provision should also be made for the drainage of surplus water, for instance stormwater. Square irrigation basins are ideal, but any shape may be used. Even relatively short furrows drawn on a contour may be regarded as basins.

6 11.4 Irrigation Design Manual Table 11.1: Minimum recommended flow rates [m³/h] according to basin sizes and soil types Basin area [m 2 ] Please note: Sand Loam Clay Recommended maximum flow rate per metre width of basin to prevent excess erosion: Sand 30 m³/h per metre Loam 50 m³/h per metre Clay 80 m³/h per metre Control equipment Sluices are normally used to let water in or out. With smaller basins, the ridges are opened or siphon pipes are placed over the ridges Crops Basin irrigation is suitable for the irrigation of all crops Limitations Proper field preparation, which may be costly, is a prerequisite for effective basin irrigation. Relatively flat topography requires less construction costs. There are examples of terraces built on steep gradients, but at very high cost. For effective basin irrigation it is important that the conveyance system should be able to carry a large enough stream. The efficiency of basin irrigation is largely dependant on the competency of the irrigator, for instance, knowing when to cut off the stream Advantages Energy costs are insignificant or nothing. Application uniformity may be 90% or higher with proper construction and management. Laser equipment may be used to create accurate, level soil surfaces in large irrigation basins. In the case of small basins, simple equipment, such as a level, may be used effectively. Measured against the advantages of improved uniformity brought about by the levelling, the cost of levelling the basins are relatively low in both cases. A well-planned basin layout which has been properly constructed usually has a longer lifespan and requires minimal maintenance. As soil compacts as a result of irrigation, application uniformity increases because the basin fills up more rapidly. A wide spectrum of crops, such as trees and broadcast and row crops, may be cultivated in irrigation basins. It is a simple system which easily operates with a high level of accuracy. The system easily lends itself to automisation. Leaching can be applied accurately and with ease. Soil conservation is almost inherent to the system.

7 Flood irrigation Border irrigation Borders are wide canals or strips with moderate gradients bordered by soil ridges. In South Africa grain and fodder crops are commonly planted in borders. In this chapter, a border implies a strip of soil with uniform gradient over the length of the border which is bordered by ridges. A border should not have any cross gradient. Moderate gradients are used in border irrigation. For effective functioning, the contact time should be the same across the length of the border. This irrigation system requires the use of canals and/or pipes, as well as borders, the finishing and condition of soil and the use of surface drainage Background Border irrigation is commonly used in South Africa. It is generally used for grain crops and is especially suited to grazing crops such as lucerne. Many areas are laid out with border irrigation systems, such as Vaalharts, Brits and the Fish River Valley Layout The irrigation stream, 50 to 500 m³/h, is diverted into the bed at the top end. Thereafter it flows in a wide front down to the bottom to allow the required amount of water to infiltrate the soil profile. Figure 11.2: Irrigation furrow ridge is broken to let water into the border The bottom part of the flood bed may be closed, open or partially closed. In the USA borders are usually open. The water is diverted into the border until the furthest end is well irrigated. Over application at the top end is then unavoidable. The water that flows out of the border needs to be drained off in waterways. Closed borders can be very effective provided that the irrigator cuts off the irrigation stream at the right moment. If the stream is cut off too soon, too little water will reach the bottom. If it is done too late, over-application will take place or the ridge will break, wasting water and causing flooding and/or waterlogging. The longitudinalridge of the border being irrigated can be broken in a controlled way, after which the end point of the following border is irrigated. After excess water has run out, the ridge must be repaired again, so that water from the incoming source does not push back into the already-irrigated border. A closed border is therefore created temporarily.

8 11.6 Irrigation Design Manual The cut-off time of the following border will therefore necessarily be shorter, depending on how large the already-wetted area at the end of the border is. The greatest benefit of the practice is that under-irrigation does not occur at the end of the border and the cut-off time is not as critical Layout The following table shows the ballpark figures for border irrigation. Table 11.2: Ballpark figures for gradients, flow rates and border dimensions Soil Length [m] Width [m] Flow rate series [m³/h per metre width] Acceptable gradient (vert:hor) Sand :150-1:300 Loam :250-1:600 Sandy loam :750 1:1000 Clay :500-1:1 000 Please note: During the construction of border ridges with a bank block (2 dishes that ridge-up the wall) a small ditch is made inevitable between the land level and the bank. Because of practical problems (e.g. unnecessary wheel traffic and compacting of wet soil), it is almost always impossible to level this small ditch mechanically before irrigation. Small banks across the flow direction are therefore necessary to collect the water in this small ditch to ensure uniform water distribution. At 1:1 000 gradients, almost no cross banks are necessary in the bed to stop the advance front. At 1:650 gradients, cross banks are needed each ±30 m to stop the advance front. The ideal is that only a longitudinal gradient and no cross gradient should occur. The type of crop influences the application. Row crops, e.g. maize and cotton, need a flatter gradient than wheat or lucerne. It is however impractical to seasonally change the gradient. It is cost effective to select an average gradient for a semi-permanent and a permanent crop such as lucerne or vineyard. Border widths must correspond with implement widths. The minimum width is established according to practical and financial considerations and the maximum width depends of the available topsoil and water distribution. Gradients steeper than 1:150 often cause erosion. Gradients flatter than 1:600 must be constructed extremely accurately because level differences are so small (e.g. 15 cm over 100 m). The minimum flow rate is based on acceptable water distribution and the maximum on the prevention of erosion.. Short borders increase the cost of the system and allow only small applications. Long borders may cause uneven distribution over the length and especially over-irrigation at the top end. Borders should preferably not be longer than 400 m. Special care should be taken with long borders to ensure proper distribution. Guidelines for optimal application depth, based on 50% of readily available soil water, are: Sand mm/m Loam mm/m Clay mm/m

9 Flood irrigation 11.7 Borders should preferably not be constructed over more than one soil type. A constant gradient is recommended. If more than one gradient should occur over the length of a border, the steeper one should be first and the flatter one last, and never the other way round Control equipment With traditional border irrigation systems the irrigation stream is diverted into the border by opening the earth supply furrow while the furrow is being filled. It is difficult to maintain a high degree of efficiency with this kind of system, because the irrigation stream cannot be cut off in time. Sluices can be used effectively to immediately cut off an irrigation stream. Sluices are common in concrete canals, but may also be used in earth canals if it has an anchoring structure, such as concrete. Siphon pipes are easy to use and are adaptable. The flow rate may be managed or changed accurately by the irrigator during the process of irrigation. The supply canal should be at least 0,2 m higher than the borders for the siphon to work properly. A constant flow rate in the supply canal is necessary so that the same number of siphon pipes per irrigation can be used. The result of varying supply streams is flooding of the supply canal s banks Crops There is no limitation on the crops that may be cultivated in border irrigation systems. Broadcast crops are especially suited to border irrigation. Semi-permanent crops, such as grazing, may be served with a border irrigation system without hampering normal farm traffic. Crops with a natural root depth of at least 1 m should be considered. Because of the inclination to crust forming on the surface, flood irrigation can be a problem in warmer areas where fine seeds are planted in dry soil and are then irrigated for germination. The surface hardens even before the plants have penetrated the soil surface Limitations Costly field preparation is often necessary to properly construct the borders. Deviations from the ideal bed level of more than 50 mm are unacceptable because it causes uneven water distribution. Topsoil depth is also limiting because only half of it may be cut away, especially if there are steep cross gradients. For effective irrigation the irrigation stream must be constant, controlled and manageable. An irrigation stream from a dam varies according to the level of water and makes the task of the irrigator difficult. The choice in irrigation applications is limited, compared to those of pressure systems, and small applications are almost impossible. This limitation may be easily overcome by implementing a relatively long cycle time ( 7 days) and by cultivating crops with deep root systems Advantages The system requires little or no energy costs.

10 11.8 Irrigation Design Manual With proper construction and management the application uniformity (AU) in border irrigation is 80% or better. Border gradients can be constructed accurately with the help of laser equipment without much extra cost and within a shorter time than with conventional equipment. A good border system has a long lifespan if basic maintenance is kept up. Rain is utilised optimally and surplus rainwater is drained off safely. Erosion is inherently combated by the system in this way because the drainage of rain from where it lands on the ground is controlled. The system readily lends itself to automation Furrow irrigation A furrow is a V-shaped earth canal with a moderate gradient. Sugar-cane, vineyard, vegetables and row crops are usually irrigated by means of furrows. Furrow irrigation consists of small earth canals constructed at a constant gradient. This irrigation system lends itself especially to the use of equipment such as siphon pipes which simplifies its functioning. However, care should always be taken with the design of furrow irrigation to ensure that irrigation is sufficient and uniform Background Furrow irrigation may be used in many applications, from highly sophisticated to minimum technology level. It basically consists of small parallel earth canals into which the irrigation water is diverted Functioning Irrigation furrows are usually made by means of a tractor and implements. The spacing between furrows varies in the vicinity of one meter and has to comply with the spacing of the tractor wheels. Furrow irrigation only partially wets the soil surface. Water infiltrates vertically and horizontally into the soil. A group of furrows may receive water simultaneously from a canal or pipe, which then flows down the furrows. The bottom ends of the furrows may be closed, but rainwater has to be drained off safely to avoid storm water damage to the furrows Layout Furrows need to have a uniform moderate gradient in the flow direction. If these requirements could be met with straight furrows, parallel to field edges, it will simplify cultivation. Contour ridges are unfortunately seldom parallel to each other, but with innovation the best may be made of the situation by for instance forfeiting a little on the consistency of gradients and furrow lengths. Furrows may be laid out according to the natural topography. None or little field preparation is necessary for such a layout. Since furrows are now lying in curves, tilling must be adapted accordingly. Contour walls can be used as guides to simplify tilling. Contour walls are unfortunately seldom parallel to each other. But with innovation the best can be made from any situation by sometimes forfeiting constancy of gradients and furrow lengths.

11 Flood irrigation 11.9 Table 11.3: Typical cross-cut dimensions, lengths and gradients of furrows Soil types Recommended gradient (vert:hor) Length W [mm] Y [mm] Sand 1:150-1: Loam 1:200-1: Clay 1:200-1: Gradients steeper than 1:150 create a danger of erosion. Furrows may have gradients as flat as possible, as long as construction is still accurate. Maximum lengths are determined by the distribution uniformity requirements and minimum lengths are there to ensure enough contact time and cost-efficiency. The furrow shape is usually determined by practical construction considerations and to allow for a larger contact area where infiltration rates are low, and less contact area where infiltration rates are high. Flow rates for furrows vary between 10 m³/h and 30 m³/h and the flow is usually cut off when the advance front approaches the bottom end Control equipment Furrows are often only supplied with irrigation water by opening the sidewalls of an earth supply channel. It is almost impossible to divide the water equally with this system. Short pipes (diameter 150 mm) with plugs may lead out of the supply furrow, which may be either lined or not. The supply furrow is then dammed up and the plugs are removed to irrigate the required amount of furrows. Two pipes for each furrow will improve the control because it makes flow reduction possible. Sluice pipes with electric or hydraulic controls are used on a limited scale in the USA because of high capital costs.the concept of cablegation, also developed in the USA, consists of a pipe with outlets near the top, and a plug that moves downstream under water pressure, but movement is controlled by a cable inside the pipe. The effect is that water is distributed at a decreasing rate through the outlets to the furrow as the plug moves further down the pipe. The high cost of sophisticated control systems such as this one rarely justifies the advantages they offer Crops Usually row crops are irrigated by means of furrows. Crops of which the leaves should not be wetted are especially suited to furrow irrigation. Crops are commonly planted along the side of the furrow, but in very dry areas they may be planted in the bottom and in wet areas on top of the ridge Limitations Management has to be of high quality to achieve high levels of efficiency because the application is sensitive to application time, gradient, furrow length and flow rates. Farm traffic is severely hampered by furrows. If irrigation or rain-water flows over the edge of a furrow or furrows, it may break the ridges and cause excessive damage. It is difficult to divide water equally between furrows during irrigation.

12 11.10 Irrigation Design Manual The accumulation of salts between furrows may cause the soil to become brackish. The leaching of built-up salts is difficult Advantages Furrows could be constructed according to the natural topography, thus leaving no need for expensive field preparation. Because partial wetting is applied, irrigation water is used effectively. The furrows themselves form an excellent surface drainage system for excess rain - if provision is made for the safe drainage of water at the bottom-ends of the furrows. Low or no energy levels are required to operate the system. Relatively small applications are usually given with furrow irrigation Other types of flood irrigation systems Short furrow A variation on furrow irrigation is to divide furrows into shorter parts by means of cross canals in order to create blocks of short furrows. The short furrows of each block are filled individually until the whole land is irrigated. It operates on similar principles as basin irrigation. Although it requires more labour than long furrows, this system is more adaptable and less sensitive for variations in gradient and stream volume Furrow border ( Potch system) Furrow borders are bordered by one or more furrows instead of ridges. The furrow is dammed up at regular intervals to allow the water to flow over the borders. It is virtually impossible to achieve an even water distribution with this system Contour flooding Contour flooding is an irrigation system whereby water is directed down a contour ridge and then dammed up to overflow the contour wall in order to irrigate pastures by means of this flooding Underground flooding This system is used in Tanzania. Locally produced clay pipe sections of about 100 mm in diameter and 0,5 m long are buried roughly 300 mm deep. The pipe is porous and releases water into the root zone. The pipes are filled with water via a duct above the ground.

13 Flood irrigation Choice of a flood irrigation system Choosing a flood irrigation system is not simple. There are considerations regarding capital layout, operating costs, complexity, influence on farm workers, vandalism, soil type, crop type and topography. Table 11.4 may enable one to consider all factors together in order to choose the optimum system for the farmer and the farm. Table 11.4: Factors which may influence the choice in flood irrigation system Basin Border Furrow Short furrow Field preparation or capital costs Low with moderate gradients Low with moderate gradients Usually low Low Energy costs No or little No or little No or little No or little Application efficiency High with proper layout and management Good with proper layout and management Reasonably good with proper layout and management Good with practical layout and management Uniformity of distribution Very good with proper construction Good with proper construction and management Reasonably good with proper construction and management Good with proper use Topography of available field Preferably flat because gradients limit soil depth and increase costs Moderate and uniform gradients preferred Reasonable gradients with moderate variations may be handled Reasonable gradients which vary may be handled Critical soil characteristics All types except for course sand preferably deeper than 1 m Preferably loam deeper than 1 m Loam to clay soil deeper than 0,6 m System adaptable to all soil types, but conveyance losses should be limited Crop type General crops General crops Row crops preferred Row crops, ideal for vegetables Maintenance requirements Annually Annually Annually, intensive Annually, little Management Important for high efficiency Critical Critical Simple. High efficiency easily achieved Farm traffic and marketing Manageable Manageable with limitations Hampering Hampering Labour Moderately intensive Moderately intensive Intensive Very intensive

14 11.12 Irrigation Design Manual 3 Control structures Control structures ensure that irrigation water is utilised effectively, while it also important for efficient flood irrigation. 3.1 Sluices Diversion structures in canals have a similar function as valves in pipes. Sluices make it possible to quickly open or cut the irrigation stream. Without these effective irrigation would not be possible Sliding sluices Sliding sluices are used in canals to cut off or control the water flow. The sliding plate and groove are usually made of steel, while the rest of the structure is constructed with concrete. It is difficult to make this type of sluice watertight, because the cut-off surface is also the sliding surface. If a sluice is out of use for a couple of months, it becomes rusted. Grease will combat rusting and improve the sealing properties. Figure 11.3: Steel sluice structure with concrete works to prevent erosion of an earth wall Hinged sluices Hinged sluices are hinged at the top to enable them to close tightly against a rubber seal at the closing surface. These sluices close tightly as long as the rubber seal remains in place and the shut-off plate makes good contact with the whole closing surface. Although these sluices are also made of steel, rusting will not effect their working. It is not possible to control the stream size with a hinged sluice. It is also difficult to open or close the sluice against water flow.

15 Flood irrigation Valves Flood valve This valve was developed locally for use in border irrigation, but may be used for any kind of flood irrigation. Only 200 mm, 250 mm and 300 mm valves are available commercially. A farmer with a basic workshop should be able to produce his own flood valves in the sizes that he requires. Flood control by means of valves is also possible by only partially covering the opening with the plate. Figure 11.4: A typical flood valve Hydrant valves These are commercial valves similar to the hydrant valves used in sprinkler irrigation systems. The high purchase cost of hydrant valves limits their application possibilities. It can only be effectively used where water is under pressure in a pipeline system. By spacing hydrants e.g. 30 m apart and using e.g. 3 hydrant valves fitted with e.g. 10 m movable soft pipe (fire fighting pipe), quite a number of borders can be served from one hydrant. There must always be a spare valve and pipe that can be moved to the next border, before the stream to be closed, is closed off.

16 11.14 Irrigation Design Manual Figuur 11.5 : Hydrant valve with movable pipe for irrigation of more than one border Flow control valves Diaphragm valves may be used to great advantage for flood irrigation because they ensure constant flow, while at the same time allowing the supply level to be varied. They may also be used to maintain a constant water level in canals, for instance. Another advantage of this valve is that it may be used as a shut-off valve. If the farmer constructs this flow-control valve himself it will be cheaper than a standard shut-off valve. A self-build manual for this valve is available at the ARC-Institute for Agricultural Engineering. These valves are also available commercially Siphon pipes The use of siphon pipes offers the irrigator a wide range of options, especially regarding flow rate. Soil ridges need not to be broken, there are no sluices and the simple working and handling of the siphon pipe makes it easy to irrigate on a time schedule. Siphon pipes are usually manufactured from upvc, but any light flexible material may be used, as long as the bent shape is retained. Typical sizes are 50 mm and 75 mm. Pipes eventually become brittle in the sun and are exposed to damage. It is reasonably labour intensive since pipes are usually used per supply stream and has to be moved from border to border. The number of pipes in use and the stream size must be synchronised, else the water in the pipes will stop running or the supply stream will flood. Logistically, the moving of pipes between fields and the storage area is often problematic, because such a great number of pipes must be moved by means of a vehicle or trailer.

17 Flood irrigation Figure 11.6: Siphon pipes in use Table 11.5: Theoretic flow rates (m³/h) of siphon pipes (SCS National Engineering Handbook) Diameter [mm] Pressure head of siphon [mm] ,9 1,3 1,6 1,8 32 1,5 2,2 2,7 3,1 40 2,4 3,5 4,2 4,8 50 4, ,9 9, ,6 16,4 20,0 22,7 Please note: Length of tubes = 1,5 m and tubes are made of aluminium. 3.3 Division structures Division structures may be used to great advantage where there is a need to divide the flow of a canal proportionally. It seems practical not to make the division ratio smaller than 1 to 5, while a one-to-one division is deemed to be ideal. Division structures may either have a permanent division ratio, or an adjustable one. Measurement and division may often be done simultaneously, rendering the structure extremely useful to the farmer or a group of users.

18 11.16 Irrigation Design Manual 3.4 Measuring structures Measuring of irrigation water is vital for proper irrigation management and development. Parshall flumes, Crump and sharp crested measuring structures may be used for measurement purposes. In the case of already existing canals, a Crump measuring structure will possibly cause the least disruption with installation and utilization 3.5 Drop structures Drop structures are used to safely cope with height differences due to variations in the canal gradient and the soil gradient, or due to the presence of a terrace. These structures are also often used to maintain the required water level in a canal to make irrigation possible. A drop structure may be constructed of either concrete, bricks or stones. 3.6 Diversion structures Figure 11.7: A simple drop structure These are sluice structures designed to divert water from a canal. The description of sluices in Section 3.1 also applies to diversion structures.

19 Flood irrigation Factors which influence flood irrigation Factors which may influence flood irrigation are grouped under soil, water and system. These factors operate separately and are mutually dependant on each other. 4.1 Soil With all types of flood irrigation, water flows over the soil surface while it infiltrates. However, a significant percentage irrigation water infiltrates under ponded conditions Soil type Soil has to be well drained and at least 1 m deep. Surface drainage will, however, have to be effective to prevent waterlogging of soils with a shallow limiting layer. Soil texture is a critical flood irrigation factor because the size and quantity of pores in the soil have the highest influence on infiltration rate. The infiltration rate is usually low in soils with high clay content, and high in sandy soils. A very fine sandy soil, on the other hand, may have low infiltration rates because of a compact structure. Clay soils which form deep cracks cause uncontrolled infiltration conditions until the cracks close up due to swelling of the clay. Soils with low infiltration rates are best irrigated by basin irrigation methods Crust formation and resistance to erosion Many South African soils form crusts after rain or the first irrigation. The infiltration rate of water through soils with crusts is much lower than in those without. Crusts on steep inclinations may erode with the following irrigation. This will increase the infiltration rate, but it causes soil loss Limiting layers In the case of long border strips, limiting layers may increase the speed of the advance front because little infiltration occurs in the upper part of the border or furrow after saturation point is reached. Less irrigation water will infiltrate in parts of a field where limiting layers occur Soil water content A high soil water content usually slows down the infiltration rate. A low soil water content usually increases the infiltration rate.

20 11.18 Irrigation Design Manual Cultivation The infiltration rate and water applied is significantly increased on freshly cultivated soils. The soil surface is hardened and smoothed with each irrigation. The infiltration rate would therefore decrease with each successive irrigation, with the greatest difference between the first en second irrigation. 4.2 Vegetation With flood irrigation, water usually flows through vegetation. The influence of vegetation on infiltration will vary according to the growth stage, plant density, and sudden changes such as cutting Crops Dense crops offer greater resistance to flowing water, thus slowing down the advance front and increasing the contact time, causing greater infiltration depth during irrigation. Flow resistance, and therefore also infiltration depth, is greater if the water flows across plant rows, as compared to a flow direction parallel to plant rows. Roots and other organic material in the soil will usually increase the infiltration rate since it creates flow openings for water, particularly when it starts to decompose. Crop resistance to flow increases with plant growth and is decreased when crops are cut Weeds In the case of row crops, weeds may seriously retard the advance front, while the crop will offer relatively little resistance when it is planted in the direction of flow. Hoeing of weeds will increase the infiltration rate due to the disturbance of the soil surface. 4.3 Water Flood irrigation is particularly suited for the use of occasional irrigation because the infrastructure can be provided at low cost. The quality of the water may also have an influence on the infiltration rate. High ph values, for instance, may cause some soil types to block or seal. 4.4 System The irrigation system consists of constructed borders, basins, furrows and canals or pipes to convey and distribute the water according to the users requirements. The operation and management functions are integral elements of the system.

21 Flood irrigation Gradient The flow rate of water increases and thus contact time decrease with steeper gradients, if the water is not pended. Because gradients are relatively flat, small irregularities, such as humps, depressions or cross gradients drastically affects water distribution Field preparation In the case of flood irrigation, field preparation may be seen as the equivalent to the laterals and emitters of sprinkler and micro-systems. The soil surface serves as the distribution medium for the irrigation water. Distribution uniformity is therefore directly dependant on the standard of the field preparation. Water drainage systems should form an integral part of field preparation to prevent damage to borders, basins and furrows. Accurately construction of systems, according to a proper design, is required for uniform water distribution and infiltration depth. Accurate construction not only improves distribution uniformity, it also increase the utilisation of rain because the water flow is controlled, allowing more opportunity for infiltration. The use of laser equipment to construct flood irrigation borders within 20 mm of the desired level is affordable and effective. If the gradient is 1:300 or less, it is possible to recover capital input and construction costs within a couple of seasons, because of better yields and water saving because of more effective application. Figure 11.8: The same irrigation stream and inflow time. Borders on the right were finished with laser and those on the left with traditional methods.

22 11.20 Irrigation Design Manual Stream size Infiltration depth would be less for larger streams since the contact area between water and soil surface is less per volume of water than in case of a smaller stream. The contact time is also shorter for the same volume of water Contact time The inflow time, as chosen by the irrigator, will to a great extent determine the efficiency of the irrigation. If the right amount of water is let through to the border, furrow or basin of a well-designed and constructed system, the distribution uniformity will be high. In the case of basins, closed beds and furrows, the cut-off time is crucial. If too little time is allowed, the water will not reach the far end. If it is too long, it may either cause excessive pending at the end, or it may cause the ridges to collapse, which in turn may cause other fields to become waterlogged or eroded. With open borders or furrows the irrigator will cut-off the stream the moment sufficient infiltration depth is reached at the furthest end. Provision should therefore be made for the beneficial use or safe disposal of excess water. The inflow time should be adjusted throughout the season to ensure an acceptable level of efficiency Shape of border, furrow or basin The dimensions of a basin, border or furrow determine the movement of the advance front and the amount of water that will infiltrate, as well as the uniformity of the infiltration. The process of flood irrigation is clearly depicted below by means of graphic illustrations of the advance and recession fronts. Advance time is longer because of one of the following reasons: Smaller supply stream Flatter gradient Higher infiltration rate Higher surface roughness/resistance Uneven gradient Figure 11.9: Factors which prolong the advance time

23 Flood irrigation Advance time is shorter because of one or more of the following reasons: Larger supply stream Steeper gradient Lower inflitration rate Reduced surface roughness/resistance possibly due to hardened surface after irrigation or the crop have been cut. Figure 11.10: Factors which reduce the advance time 5 Construction 5.1 Basin irrigation Staking out The designer should indicate the distances between basins and canals to existing beacons on the plan to rule out further calculations. This means that the designer should know the area well and know exactly where the irrigation system should be layed out Cut and fill To ensure uniform, level basins, the soil has to be cut and filled first. With a small basin (< 10 m²) it is possible to smooth out the bottom with a level without surveys or calculations. Height differences may be determined by letting water into the basin and identifying the high and low areas. With large basins (> 10 m²) the volume of soil which is cut away has to balance with the volume that should be filled. Basins should be divided into rectangular blocks and spot heights must be determined from the middle of each block. Spot heights need not be closer than 2 m and not further than 20 m. Yardsticks should be inserted at each height measuring point (in the center of the block) to indicate the filling level or cutting depth for construction purposes. 1,1 V V s v 1,4 (11.1) where: V s = volume soil cut away [m 3 ] V v = volume soil filled up [m 3 ] According to Equation 11.1 more soil has to be cut away than what is needed to fill up. This ratio is commonly accepted and based on practical experience. In any case it makes soil available for ridges

24 11.22 Irrigation Design Manual and roads if any is left over after the cutting and filling operation. 5.2 Border irrigation Staking out The final plan should be drafted out in such a way, containing all the necessary information, that it will be possible to use the plan for staking out the field Cutting and filling The cutting and filling process is meant to minimise the cross gradients, while making the gradients over the length uniform. Determining the cutting depth and filling level may be done as explained in the section for basin irrigation, but unfortunately it is rather cumbersome to do by hand. Computer programmes may simplify the task considerably. The same principles apply to the cutting and filling process in border and basin irrigation, with the exception of constant gradients in the case of border irrigation. Laser equipment is almost essential for border irrigation, especially where gradients of 1:400 and flatter occur. The capital input or contractor s fee to construct borders quickly and accurately is usually recovered within a year or two. 5.3 Furrow irrigation Staking out Figuur 11.11: Final laser levelling Staking out the contour furrows are difficult because of the varying curves. The same staking out method for contour furrows may be used as for contour intervals. Controls should be built in to ensure that furrows are placed correctly. Obvious layout adjustments which become apparent during construction should be made and indicated on the plan Cutting and filling Little or no cutting and filling take place while creating contour furrows. Where artificial gradients are created in furrows, cutting and filling take place as for border irrigation.

25 Flood irrigation Laser equipment Apart from the common use for creating an average flat surface in a field, laser equipment may also be used to accurately stake out gradients. 6 Evaluation of flood irrigation systems As with other types of irrigation systems, a flood irrigation system must also be evaluated before the efficiency thereof can be determined. Furrow and border irrigation systems are evaluated in the same way. Flow measurement can be done by means of weirs (e.g. the Cipoletti or V-notch) or where siphon pipes are used, by means of siphon pipe calibration. 6.1 General information and test procedures The tests and measurements done during an evaluation are described in the following paragraphs. The evaluation must be done while normal irrigation practices are being used. 6.2 Pressure readings Pressure readings are only done in flood systems where a surface pipe distribution system is used. Test if the operating pressure is in accordance with the design parameters. It will differ from one design to another. 6.3 Delivery tests The delivery of border irrigation systems can be determined by means of measuring weirs (e.g. the Cipoletti or V-notch). Use a Cipoletti notch with a full length of 457 mm, which is install at the inlet of the border or furrow (Figure 11.12). Allow for a reasonably stilling basin in front of the weir, a temporary water retaining structure can be built upstream. Other methods of measuring, such as the V- notch (Figure 11.13), rectangular notch or volumetric method can also be used. If the flow is lower than 100 m 3 /h, the V-notch can be used. With a certain depth of flow over the crown of the Cipolettiweir, the flow rate (Q) can be read from Table Figure 11.12: Cipoletti notch Table11.6: Flow rate values (m 3 /h) for a specific overflow height (mm) for the Cipoletti notch

26 11.24 Irrigation Design Manual H (mm) Q (m 3 /h) H (mm) Q (m 3 /h) H (mm) Q (m 3 /h) 13,8 5 73, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,0 230 Figure 11.13: V-notch for flow measuring Other methods of flow measuring are discussed in Chapter 5: Water of this manual. 6.4 Distribution tests Flood irrigation is the application of a certain amount of water to the soil where the soil is wetted as the water flows over and infiltrates the soil. The evaluation of flood irrigation systems is done by monitoring the advance and recession times over a certain distance, in order to determine the contact time of the water over the length of the furrow or border. The furrow or border must be measured and steel or wooden pegs must be driven in on the prescribed intervals on the side of the bed (Figure 11.4). The stream is deflected on the upper end of the border and the strength of the stream and the time is recorded. As the water flows down in the border and the advance front reaches the pegs, the time is recorded. As soon as the water supply at the upper end is cut off, the time is recorded (cut-off time). The difference between this time (cut-off time) and the inlet time, the strength of the stream and the size of the border or furrow (m) can be used to determine the gross application (mm) applied to the border.

27 Flood irrigation Figure 11.14: Pegs driven into a bed for measurement of the advance front As the last water flows down the border, the border begins to dry out (recession front). The time is recorded when the area is approximately 80% dried out on both sides of a peg (Figure 11.15). This evaluation is refined by experience. At uneven gradients, it may occur that water dams up at certain points, which means that there is still standing water when the recession front has passed. Figure 11.15: Drying-off area The contact time, which is the difference between the advance front and the recession front, is the time that the water is in contact with the soil and therefore an indication of the time the water has to infiltrate the soil. By drawing this fronts on graph paper, the contact time and distribution will be seen. The contact time must be constant to deliver as uniform an irrigation over the length of the border or furrow as possible. To manage the contact time, the irrigator must know which factor has an influence on it - the gradient of the furrow or border, the infiltration rate of the soil and the soil/water conditions, as well as the flow rate of water in the border or furrow. The gradient of the border must be determined by measurement. The volume of water supplied to each border or furrow, is determined by the cut-off time.

28 11.26 Irrigation Design Manual The actual application is calculated by means of Equation11.2. yw = 1000QT A (11.2) where y w = average gross application [mm] Q = flow rate [m 3 /h] T = time difference between inlet and cut-off time [hours] A = border size [m m] The average gross application is the actual application and can be used by the producer to verify his planned scheduled application. The deviation of the average application at each point is determined and from these figures, the Christiansen s uniformity coefficient (CU) is calculated (Stimie, 2002). The following formula is used: CU = 1 n i= 1 T T T c c n ci 100 (11.3) where CU = Christiansen s uniformity coefficient [%] T c = average contact time [mm] = n Tci /n i=1 T ci = contact time at peg no i [min] n = number of pegs or measuring points Typical guidelines for CU-values is as follows (Reinders, 1986): Excellent : CU 90% Good : 80% CU 89% Poor : CU 79% If the CU < 80%, an infiltration test for the soil must be done and the CU of the application must be determined. This is normally done by using a ring-infiltration meter. The method is described in detail in the Irrigation Evaluation Manual of the ARC-Institute for Agricultural Engineering (Breedt & Koegelenberg, 2002). The ring is forced into the soil and the infiltration rate is calculated by the time the water level in he ring takes to drop a certain height (mm). The ring infiltration method is however not very accurate because the movement of the soil water through the ring is influenced, especially in very dry soil. Example 11.1 An irrigation border that was evaluated was 165 m long and 11 m wide. Pegs were driven in each 10 m along the length of the border. The crop on the field was cauliflower and the evaluation was done during a normal irrigation cycle. The farmer planned to apply approximately 75 mm of water. The stream flow was measured with a portable Cipoletti-weir at 150 m³/h. The farmer chose a cut-off time of 51 minutes according to previous experience. The contact time was calculated by measuring the time difference between the advance front and the recession front and is as follows: