Costs Estimation of Sprinkler Irrigation Systems for Varying Lateral Pressure Loss in Agricultural Fields

Transcription

1 Advance in Agriculture and Biology E-ISSN: / P-ISSN: DOI: /PSCP.AAB Adv. Agric. Biol. 3 (4), 2015: PSCI Publications Costs Estimation of Sprinkler Irrigation Systems for Varying Lateral Pressure Loss in Agricultural Fields Sonia Zebardast 1,*, Ali Rahimi Khoob 2, Rouhollah Fatahi 3, Isaac Rahimi 4 1. PhD former student of Irrigation and Drainage, Water Engineering Department, University of Shahrekord, Iran 2. Associate Professor of Irrigation and Drainage, Water Engineering Department, College of Abouraihan, University of Tehran, Iran. 3. Associate Professor of Irrigation and Drainage, Water Engineering Department, University of Shahrekord, Iran. 4.Master science former student of water resources engineering, Water Engineering Department, Shahid Bahonar University of Kerman, Iran *Corresponding Author Sonia_Zebardast@yahoo.com Paper Information A B S T R A C T According to the design standards of sprinkler irrigation systems (such as Received: 4 July, 2015 wheel move, hand move, etc.), the allowable pressure variation along the lateral pipes is 20% of sprinkler normal operating pressure. This criterion Accepted: 24 November, 2015 has been used since the 1960 s, but the reason is ambiguous. Increasing the allowable pressure variations in the design of lateral pipes would lead to an Published: 20 December, 2015 increase in the pipe-pressure required and a lack of uniformity in water distribution, resulting in higher energy costs and greater water head losses. Citation This paper aims at addressing the effect of allowing pressure variations in the laterals of wheel move sprinkler irrigation systems on the costs of Zebardast S, Rahimi Khoob A, Rouhollah Fatahi R, Rahimi I Costs Estimation of Sprinkler Irrigation Systems for Varying Lateral Pressure Loss in Agricultural Fields. Advance in Agriculture and Biology, 3 (4), 135- pipes, energy supply and water in agricultural fields. In this study, alfalfa fields cultivated and sprinkler irrigation systems considering allowable pressure variations of 5, 10, 20, 25 and 30% were investigated in Khuzestan, Iran. Based on pressure variation and cost of water per m 3, the 142. Retrieved from (DOI: variable irrigation and total costs were estimated at interest rates of 7, /PSCP.AAB ) and 15%. The results showed that for all interest rates, the minimum and maximum water costs achieved in the fields with 15% and 30% pressure variations, respectively. Different interest rates had no effect on the economic pressure variation range, and the 15% pressure variation proved to be the best option for wheel move sprinkler irrigation systems in Khuzestan PSCI Publisher All rights reserved. Key words: allowable pressure variations, cost, interest rate, Khuzestan, pressurized irrigation system Introduction In many areas of the world, water resources are limited and irrigation is not economical (Valipour, 2013a). The absence of sufficient water resources and rainfall are the main restrictive factors in Iranian agriculture. Recently, with water scarcity, irrigated areas in these regions have been facing the challenge of improving irrigation water efficiencies. One way to achieve this has been recognized as the replacement of the obsolete open channel distribution networks by the on-demand pressurized networks. In pressurized irrigation, although irrigation efficiency is high but amount of required water is lower than surface irrigation but cost of pressurized method is very high (Valipour, 2013b). Anyway this change appears to be quite effective. Conveyance efficiencies are significantly improved from typical values of 60 70% for open channels to values close to 100% for pressurized networks (Rodríguez Díaz et al., 2009). Furthermore, these new systems allow farmers to use more efficient on-farm irrigation systems such as trickle or sprinkler where they receive water through hydrants at suitable pressures. It is reported that with pressurized networks energy costs can account for an average 25% of the total management, operation and maintenance (MOM) costs, but in some specific cases this can rise up to 50%. Total MOM costs move from average values of 0.02 m3 for open channel networks to more than 0.10 m3 for pressurized networks, their energy requirements, their higher maintenance, operation and amortization costs (Rodríguez Díaz et al., 2008). The water distribution pattern of the sprinkler and uniformity efficiency depends on the type of sprinkler, the number, and attitude of the nozzles, and the working pressure (Keller and Bliesner, 1990; Romero et al., 2006; Tarjuelo et al., 1999). Daccache et al (2010) described a methodology combining network design and performance analysis of a sprinkler network and applied to an irrigation distribution system operating at two different water demands using a case study in Italy. Four designs of the same sprinkler network were optimized at different upstream designing pressure and were evaluated at all the possible operating conditions of the system. The design objective was to obtain networks permitting efficient on-farm irrigation, leading to high crop yields at moderate investment and operational costs.

2 They represented that to achieve the objective, an appropriate and network layout (sprinkler spacing) designed to deliver the proper pressures to the sprinklers with minimum pipe cost is needed (Daccache et al., 2010). There is an optimum pressure range for sprinkler nozzle(s) that produces the best distribution uniformity. The working pressure has a particular effect on the energy costs. In addition, it can also have an influence on the investment costs, depending on the need to change the pipe diameter to fulfill the maximum difference in pressure in the subunit. Sprinkler pressure variation can cause a slight variation in irrigation uniformity (Romero et al., 2006). There is an optimal pressure for sprinklers that lead to high distribution efficiency which is determined by wind speed, arrangement and operation pressure of sprinklers. Hydraulically, the sprinklers are an aperture whose outlet flow rate is a function of the square root of their operation pressures. Therefore pressure variations are a cause of lack of uniformity in the distribution of water and the reduction of uniformity efficiency. The criteria set for optimum hydraulic performance of the subunit is that the maximum pressure variation in a subunit should not exceed 20% of the average working pressure of the sprinklers (Cuenca, 1989; Keller and Bliesner, 1990; Melby, 1995; Romero et al., 2006; Valiantzas and Decras, 2004). This criterion has been used to determine the length and diameter of laterals. However there is no scientific reason to explain this recommended allowable water pressure range variation. The allowable range of pressure variation is extremely important due to its effect on the required pressure, irrigation efficiency, and the length and diameter of laterals. Therefore, exceeding this range allows for the design of longer laterals with decreased diameters. Also the pipe costs and installation costs will be decreased. Conversely, increasing the variation of pressure increases irrigation inefficiency therefore water supply costs. Decreasing or increasing the allowable pressure variations affects the required pressure in the system. The required pressure is one of the inflections factors in the current costs of pressurized irrigation systems. Considering the above mentioned cases, the allowable range of pressure variations impacts costs and these impacts are dissimilar in different elements of sprinkler irrigation systems. It is usual to consider 20% allowable pressure variation along the lateral pipes to minimize the investment costs, irrigation water costs, power and other equipment related to sprinkler irrigation systems. Therefore it is important to study the effect of different allowable ranges of pressure variations in the sprinkler irrigation system costs. This study investigated the influence of pressure loss variations of 5% to 30% with 5% increased steppes and different interest rates. The six studied fields are located in Khuzestan, Iran equipped with wheel move sprinkler irrigation system. In each field, the total annual operation, maintenance and water costs were considered. The aim of this study is developing a procedure to determine the optimum pressure variation that can lead to the least cost of a pilot field equipped with wheel move sprinkler irrigation system in Khuzestsn. The study also evaluated whether different interest rates impacts the most economic pressure variation range option. Materials and Methods Data and study area The study area is in the province of Khuzestan which has vast fertilized plains and numerous irrigation networks. Most of agricultural lands are situated in the center of this province. The soil specification and climate of this region were considered in the design of the study fields. The soil texture in the fields is silt loam with maximum moisture retention of approximately 150 mm per meter of soil. The basic infiltration rate of the soil is 14 mm per hour (CECM, 2005). Karun River is the main agriculture water resource of the region with salinity about 1.5 millimhos per centimeter. Alfalfa is one of the main crops in this region and needs much more water than other crops. The maximum alfalfa root depth is 1.5 meters and the maximum allowable depletion is 0.5. Alfalfa is relatively sensitive to salinity and its maximum tolerated salinity is 3.4 millimhos per centimeter. The monthly meteorological parameters and the water requirements of alfalfa in the study region are shown in table 1. Table 1. Monthly meteorological parameters and water requirements of alfalfa in the study region Month January February March April May June July August September October November December Annual Maximum Water requirement (mm) Average wind speed (km/h) Average temperature (oc) According to NETWAT software (Iranian meteorological organization) the maximum water demand for alfalfa is attained in July, hence the field irrigation systems study was designed based on the July water demands and meteorological parameters (Alizadeh, 1998). Based on the water requirements of alfalfa and the soil specifications, the maximum irrigation cycle in July is ten days following MAD of 50%. Fields design The effect on the design of the lateral pipes, six types of allowable pressure variations investigated. It is evident that these variations affect the length and dimensions of the irrigation unit, irrigation efficiency, capacity of network pipes 136

3 and investment costs. So in this study, six types of fields were designed as outlined below. According to Consulting engineers and based on average land ownership, farmers cooperatives and cultivation patterns, the best net area for sprinkler irrigated fields is 50 hectares (CECM, 2005; Monserrat, 2009). This base net area was used in the sample study fields with six types of pressure variation in lateral pipes. Figure 1 shows the configuration of the elements of the sample fields irrigated with wheel move sprinkler systems. Figure 1. General chart of the sample fields The pumping station is usually designed to meet the peak irrigation discharge which is strongly variable during the irrigation season but is limited to only a few days. This means that the pumping station is oversized during most of the irrigation season, i.e. during the off-peak periods the required pressure to inlet at the upstream end of the distribution network is much less than that provided by the pumping station (Lamaddalena and Khila, 2011). The lateral pipe branches in the direction of the contours of the sides of the sub main pipe, so the length of each subunit is twice that of the lateral pipe (Kale et al., 2008). The sub-main pipe determined the width of the land. The number of hydrants in the sub-main pipe was calculated so that at peak consumption, a lateral pipe could irrigate the field between two irrigation periods. Therefore, the subunit areas varied in terms of pressure variation and field area was approximately 50 hectares. Among the many factors that affect the optimization of water use (Ortega et al., 2004a), this study will focus on irrigation uniformity and its economic implications. Water Application efficiency depends mainly on irrigation uniformity,drift and evaporation losses (Martinez et al., 2004). To study the effect of pressure variation on the uniformity efficiency of subunits, water distribution uniformity and uniformity efficiency were calculated based on Keller and Bliesner (1990). Therefore the irrigation gross depth was obtained from the irrigation net depth divided by the irrigation efficiency in each field. The wheel move system design was made based on wind speed and the basic soil infiltration rate. The system was moved two times per day when applies water to the maximum consumption rate in order to prevent runoff. A sprinkler with mm 2 nozzle and configuration of m 2 was chosen (Keller and Bliesner, 1990). The normal pressure of sprinklers was 350 kpa and flow rates of 0.62 Lps. According to Keller and Bliesner (1990), its uniformity coefficient (CU) in the field with wind speed of 9.42 kilometers per hour was 85%. The required time of water distribution during the maximum application period was estimated at 11 hours. Therefore the laterals were displaced twice a day in this period. The laterals were placed in the direction of the contours of the land according to the flat and smooth central and southern plains of Khuzestan. Water pressure variation is a result of hydraulic load loss in the length of the lateral therefore to determine the pressure at lateral inlet, equation 1 was used (Keller and Bliesner, 1990): HO Ha 0.75H f Hr (1) Here, H o is the pressure at lateral inlet (meter), H a is the average functional pressure of the sprinkler (meter), H f is pressure loss as a result of friction (meter) and here is the riser height (meter). In order to calculate pressure loss the following equation was used which was given for smooth pipes (Keller and Bliesner, 1990): L 1.75 H f Q D F (2) Where L is length of lateral (meter), Q is inlet flow rate for lateral (liter per second), D is internal diameter of lateral (millimeter) and F is Christiansen coefficient for pipes with different outlets. F was calculated from relationships identified by Keller and Bliesner (1990). 137

4 In wheel move systems, sprinklers are installed 12 meters away from the laterals and the inlet flow rate to the laterals depends on the number and the flow rate of the sprinklers. The length of the lateral was determined then that it conformed to the dimensions of the subunit, and the loss of pressure was less than the allowable pressure loss. The allowable pressure loss depends on water pressure variations in lateral length and is given by the following equation (Tafazoli, 2004): H H (3) a a Where ΔH a is the allowable pressure loss along the lateral length (meters) and α is percentage of pressure variations. Six variations were used in this research and for each variation the following were calculated: flow rate, maximum lateral length, and pressure loss at the beginning and at the end of lateral. The main and sub-main pipe diameters were determined by Keller and Bliesner (1990). An economic chart was plotted according to field flow rate in irrigation projects, purchase and operation costs of pipes, pipe-life time expectancy, interest rate, power consumption cost, and rate of increase of energy. The main and sub-main pipes are constructed of polyethylene which has a lifetime expectancy of approximately 30 years. Interest rates of 7, 10 and 15% were used. The tariffs announced by the power ministry, priced power at per kilowatt per hour. These were the determinants used to discern the most economically efficient diameters (CECM, 2007). An economic chart was developed with diameters of the main and sub-main pipes for each of the six different fields. Daccache et al (2010) illustrated the expensive large pipe size diameter design presented the best performance and the highest reliability at a wide range of hydrant pressure while the small pipe size designs have the tendency to fail during the peak water demand period as a result of low hydrant pressure. The required pressure in the pumping station was obtained as the sum of the pressures required at the beginning of the subunit and the water pressure in the main pipe (from pumping station to furthest subunit). At the beginning of the subunit, the required pressure was determined by the sum of the pressures at lateral inlet and the pressure loss in the main pipe. The loss allowed in the main and sub-main pipe lines were estimated by Keller and Bliesner (1990). The size of the pump and electromotor was chosen based on the required flow rate during the maximum consumption period. The pressure required in the fields and the type of equipments was chosen based on local experience. The power consumption of the pumping station was estimated by the following equation (Cuenca, 1989): PO Q H E (4) Where P O is the power consumption of pumping station (kilowatt), Q is capacity of field design (liter per second), H is the pressure required for pumping station (meter), E is electromotor and pump efficiencies (percentage) and electromotor and pump efficiencies were determined from their brochures. Annual electricity consumption was calculated by multiplying the annual work hours in each field by power consumption (Valiantzas and Decras, 2004). Annual working hours were also obtained from dividing the gross water volume required for the field by the flow rate of the pumping station. Costs of irrigated fields The cost of the sprinkler irrigation system depended on the equipment and its design (Romero et al., 2006). Variable irrigation and investment costs in each field were calculated according to the items required and equipment size. Investment costs are including the price of pipes, junction installation, irrigation equipment, electrical tools, pump and power branch. Purchasing costs were determined by manufacturing and implementation costs based on a price menu in In order to calculate all costs and compare different fields, fixed investment costs were changed into annual costs and added to the annual variable irrigation costs (Cuenca, 1989). According to local experiences with 7, 10 and 15% interest rates, the life time of a sprinkler irrigation project was estimated at 30 years (CECM, 2005). The cost of each component in the field sprinkler irrigation system which its lifetime expectancy was less than the project lifetime expectancy has been corrected. The lifetime expectancy of the irrigation equipment and pumping station was considered to be years, therefore during the system s lifetime expectancy the equipment and pumping station were renewed twice and once, respectively. Annual variable irrigation costs included electricity, operation maintenance and labor. The percentage of operation and maintenance costs were evaluated from the annual costs, so that the percentage for sprinkler irrigation equipment, pipes, junctions, electrical equipment and mechanical plants of the pumping station was suggested as 1% and 3% respectively (Keller and Bliesner, 1990). Labor costs were calculated based on the labor, number of annual irrigation events and cost per operating hour. The labor per hectare in the wheel move sprinkler irrigation system was suggested as 1.73 persons per irrigation (Keller and Bliesner, 1990; Tafazoli, 2004). In order to understand the influence of these factors on annual water costs it was necessary to consider the layout and spacing, number of laterals and the sprinklers per lateral (in the irrigation subunit), working pressure, the average application rate of the system and efficiency (Romero et al., 2006). So, to compare field economies and to choose the best allowable pressure variation in the calculation of investment costs the economic value of water per cubic meter (m3) was considered. The costs of water supply in different designs and regions are varied because the costs of water sources, and transport networks vary. According to studies in Khuzestan in 2008, water supply costs from the source of the field were estimated at In this study, the costs and interest rates were converted to annual values resulting in an average of 138

5 0.047 (CECM, 2008; Cuenca, 1989). The basic interest rate for developmental designs is 7%, in Iran, but this rate does not fixed, the costs were analyzed by interest rates of 7, 10 and 15%. Results and Discussion The results of the lateral patterns in a different percentage pressure variation of length are presented in table 2. Table 2. Lateral specifications for different percentages of lateral pressure loss Variations of pressure loss of lateral (%) Length of lateral (m) Flow rate of lateral (l/s) Pressure at lateral inlet (m) Pressure at the end of lateral (m) Pressure loss of lateral (m) As seen, an increase in pressure variation leads to an increase in the required pressure at lateral inlet pipe and a decrease in the required pressure at the end. As it was expected, increasing pressure variation will increase the length of the lateral but increasing the lateral length will reduce pressure variations rate of 5-10%. Increasing pressure variation adds 29% to pipe length but in 25-30% pressure variation, only 5% will be added to the lateral length. By increasing the lateral length, lateral flow rate will increase. Following pressure variation increases, increasing the lateral length enhances the required pressure at lateral inlet pipe, but it does not have a uniform trend. With regard to water pressure variations of 25-30% and 20-25%, the lowest pressure increase percentages at lateral inlet pipe was 2.3% and 4%, respectively. In designing, according to 5-30% pressure variations, the pressure difference is 6.55 meters. The pressure of choosing sprinkler was 350 kpa while the pressure at lateral inlet in the designed system with 5% and 30% pressure variation, were measured 3.25 meters and 9.8 meters higher than, respectively. Pressure differences from functional pressure of the sprinkler, causes non uniform water distribution, so uniformity efficiency is reduced. The minimum pressure at the end of the lateral (34.22 meters) is related to the 30% pressure variation. A 0.78 meters pressure difference in functional sprinkler pressure affects water distribution uniformity. Field and subunit specifications for different pressure variation percentages are given in table 3. Table 3. Specifications for different pressure variation percentages in lateral pipes Variations of pressure loss of lateral (%) Width of field (m) Length of field (m) Field area (ha) Number of irrigation subunits Subunit area (ha) Number of working lateral Uniformity efficiency (%) Pressure loss of sub main pipe (m) Pressure loss of main pipe (m) Pressure required for the field (m) Uniformity coefficient of system (%) Total application efficiency (%) Flow rate required for the field (l/s.ha) Annual volume of water required for the field (1000m 3 / ha) Annual electrical consumption (MW/ha) The greatest pressure loss in the length of the sub-main lateral was related to a 15% pressure loss because of this pressure loss, the economical diameter of mentioned pipe was between two ranges of allowable diameters and a smaller diameter was chosen to reduce costs. Therefore with a reduced lateral length, the irrigated area during pressure loss is low and the number of parts in the field was increased so that the main pipe would be longer to irrigate the parts. An important point to note is the effect of allowing pressure variation on irrigation efficiency and water head losses. Also Provenzano et al (2005) illustrated in drip irrigation, Proper hydraulic design of drip laterals usually requires the accurate evaluation of the total head losses, represented by friction losses along the pipe and the emitters, and local losses due to the emitter connections. We have obtained clear results on the economic coexistence of designing and installing sprinkler irrigation systems on plots with high uniformity coefficients. It is necessary to obtain good application efficiency with suitable management to ensure high water use efficiency. Byelich et al (2013) estimated application efficiency 60-85% for portable hand line such as hand move and wheel move that confirm the results of this study. Also environmental aspects coupled with the irrigation water use guarantee this conclusion (Ortega et al., 2004b). The results obtained from irrigation efficiency and gross volume for different pressure variations are presented in table 3. The more pressure variations in lateral pipes the greater the reduction in uniformity efficiency. Uniformity efficiency reduction is followed by water application efficiency reduction. Due to the different shapes of fields, different lengths of laterals and numbers of sprinklers, the required flow rate for the field is 1.79 liters per second. Water flow rate entering the field is not the same. 139

6 Therefore a field with 30% pressure variation consumes 1300 cubic meters of water more than the field with 5% pressure variation. The amount of water for 50 hectares of land is 65,000 m3. Therefore the accuracy of the allowable pressure variation controls water head loss. An annual increase of gross water volume and different flow rates in each field impacts on the working hours and annual electricity in hectare per unit area. The difference between fields with 5% and 30% pressure variations is 0.98 megawatts per hectare. The field with 30% pressure variation had electricity consumption greater than 50 megawatts in 50 hectares. The results showed that an increase in the allowable pressure variation increased water head losses and electrical consumption. If the choice of pressure variation is only based on water head loss reduction and electrical consumption, the lowest pressure variation should be considered in the design. However its effect on initial costs should be studied. Initial investment costs for different parts of the field in different percentages of pressure variations are presented in table 4. Interest rate (%) Table 4. Initial investment costs for different parts of the field in different percentages of pressure variation ( per hectare) Variations of pressure loss of lateral (%) Pipe and junctions Electricity branch Irrigation equipment Mechanical equipment Electrical equipment Implementation cost Water price Total investment cost per unit area As mentioned above, the step increase of pressure variations on head loss and fields required energy met ascending trend, but this is not the case with pressure variation increases in irrigated field costs. There is no significant trend in terms of costs except with regard to a decrease in the cost of purchasing irrigation equipment with an increase in pressure variation. The highest cost items are related to electrical and implementation equipment. The wide variety of design alternatives make it necessary to identify the lowest total cost, including investment and operation costs (Romero et al., 2006). The annual cost of water supply is one of the major investment costs and 35-45% of total investment annuity costs are allocated to it at different pressure losses. Therefore investment costs could be decreased by correct water supply management and transmission from the source to the field. The highest investment costs of 21-24% are allocated to implementation costs and the lowest investment costs of 2% are allocated to the purchase of electricity. The maximum investment cost occurred at the 15% interest rate and investment cost increasing with increased interest rates. The minimum and maximum difference of investment costs at all interest rates suggests a 10% increase in costs. The least annual investment at all interest rates belonged to the fields with 30% pressure variations. An increase in interest rates has no effect on pressure variation steps related to minimum and maximum costs. Values of annual variable irrigation cost in different parts of the irrigated fields at differing percentages of pressure loss and water prices are presented in table 5. Results showed the costs of labor were 6-8%, operation and maintenance were 10-13%, and electrical consumption was 79-83%. The lowest operation and maintenance costs belonged to the fields with 15-25% pressure variations. This highest operation and maintenance costs occurred in the fields with 30% pressure variation. The maximum increase in variable irrigation costs and different interest rates was 18%. Electrical consumption costs were relatively high. The minimum electrical consumption costs of per hectare and the minimum annual variable irrigation costs at all of the interest rates were in the fields with 5% pressure variations and the maximum were in the fields with 30% pressure variations. Increasing the interest rates decreases the costs of renewing equipment thereby having a slight effect on the operation, maintenance, and annual variable irrigation costs. Increasing the interest rates decreased the annual total variable irrigation costs. Fig 2 shows the annual costs per unit area of irrigated fields at different percentages of pressure loss and interest rates. 140

7 Table 5. Components of annual variable irrigation costs of irrigated fields for different percentages of pressure loss in laterals ( per hectare) Interest rate (%) Annual variable Variations of pressure Electricity Maintenance Labor cost irrigation cost per loss of lateral (%) consumption cost cost unit area Figure 2. Costs of field irrigated with sprinkler irrigation for different percentages of pressure loss of laterals at different interest rates The greatest total annual cost of interest rates from 7-15% was obtained in the fields with 30% of pressure variation, and the lowest total annual cost of interest rates was acquired in the fields with 15% of pressure variation. Investment cost accounted for 56-71% of total costs and variable irrigation costs accounted for 29-44%. Total costs per unit area at 20% pressure loss and interest rates of 7-15% were , and per hectare. At interest rates of 7 and 10% and pressure variations of 5, 10, 15 and 25%, the total costs were 2.2 to 6.4%. At interest rates of 15% and pressure variations of 5-15%, the costs were 1.2-3% less than the cost obtained for the 20% pressure variation. As seen, investment annuity costs included most of the total annual costs. The total annual and variable irrigation costs in fields with 15 and 25% pressure variations were low. Water cost per cubic meter at different percentages of pressure losses and interest rates are shown in Fig 3. Figure 3. Final cost of water per cubic meter in fields under sprinkler irrigation for different percentages of pressure losses of laterals at different interest rates The minimum and maximum obtained water prices at 7 to 15% interest rates are related to the fields with 15 and 30% pressure variations respectively. Water cost at interest rates of 7 and 10% are 5, 15 and 25% in pressure variations, and also at an interest rate of 15% the pressure variations are 15 and 25% which are less than the interest rate of pressure variation of 20%. At all interest rates, the water prices do not vary greatly in the 15 and 25% pressure variations and also the difference between the maximum and minimum of water price at interest rates of 7 to 15% is to per m3 which is considerable in large fields and for crops with high water demands. The higher the interest rate becomes, the higher the water price will be. In general and with regard to interest rates, the least water price was computed in the fields with pressure variations of 15, 25, 5, 20, 10, and 30% respectively. Interest rates have an effect on water prices in pressure 141

8 variation steps and its least effective on pressure variation has been 20%. The water price at 15% pressure variation has the least increase at all interest rates. Conclusion This study investigated of allowed pressure variation effect along lateral pipes of wheel move sprinkler irrigation system. The six studied fields with allowed pressure variation 5, 10, 15, 20, 25 and 30 percent and interest rates 7, 10 and 15 % for alfalfa culture in Khuzestan, and based on these designing and economic value per cubic meter of water in this region, investment cost, current and total cost were estimated. The results of this study show that the maximum total costs for studied wheel move sprinkler irrigation systems at all interest rates were about 1.14 of minimum cos. The obtained cost differences might be explained as a result of field shapes and relate to pressure variation steps. This is due to the fact that different pressure variations change the length and width of the field so that the length of the main and sub-main pipes and other components and equipment will change. The length of laterals and number of sprinklers placed on them, flow rate, volume water required for the field, and electricity consumption vary according to pressure variations. At different interest rates, there is a 2% reduction in total costs for 10 to 15% pressure variations. A 10% increase in total costs was achieved for the same pressure variations range. The minimum finished cost of water per unit area has been achieved at 15% pressure loss and the values of that at interest rates of 7-15% were 0.017, and per cubic meter. Therefore in the lateral, a 15% pressure variation could be a suitable option for designing a wheel move sprinkler irrigation system in Khuzestan. Considering the increasing total costs at 20% pressure variations and more, it is concluded that pressure variations of 20% and more are not affordable and so not recommendable. Increasing interest rates lead to an increase in annual costs and total costs, but different interest rates had no effect on the best pressure variation option. This study dealt with alfalfa growth in Khuzestan and wheel move sprinkler irrigation systems. It is concluded that in order to improve irrigation system design, pressure variations should be optimized through an economic comparison of laterals, irrigation systems, different regions for different cultivation patterns and crops. In the present study, 5-30% pressure variations were also studied and further research is needed to ascertain the lowest cost pressure variation. It is suggested that in all irrigation designs, different interest rates and water costs, and pressure variation percentages should be accurately determined in agricultural fields. 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