بررسي و ارزيابي فني برخي سامانه‌هاي آبياري باراني كلاسيك ثابت دشت كوهدشت

Investigation and technical evaluation of solid set sprinkler irrigation systems in Koohdasht plain

هدف از اين پژوهش، ارزيابي و چگونگي طراحي و بهره‌برداري برخي سامانه‌هاي آبياري باراني دشت كوهدشت در استان لرستان است. در اين پژوهش 10 سيستم آبياري باراني كلاسيك ثابت با آبپاش متحرك در دشت كوهدشت به عنوان نمونه ارزيابي شد. از معيارهاي ضريب يكنواختي كريستيانسن (CU)، يكنواختي توزيع (DU)، راندمان پتانسيل كاربرد ربع پايين (PELQ)، راندمان كاربرد ربع پايين (AELQ)، تلفات تبخير و باد (WDEL)، تلفات نفوذ عمقي (DP) و كفايت آبياري (ADirr) استفاده شد. مقادير متوسط اين شاخص‌ها براي 10 مزرعه ارزيابي شده به‌ ترتيب 84/70، 64/43، 85/46، 01/38، 77/8، 02/31 و 66/64 درصد به ‌دست آمد. كليه سيستم‌ها داراي راندمان كاربرد پايين بودند و يكنواختي توزيع در آن‌ها كمتر از مقادير توصيه شده كلر و مريام است. در 4 سيستم از 10 سيستم ارزيابي شده، آبياري كامل و در بقيه مزارع به دليل كم‌آبياري، راندمان واقعي و راندمان پتانسيل كاربرد برابر بوده است. طراحي و اجراي نامناسب سيستم‌هاي مورد بررسي مهم‌ترين دليل پايين بودن راندمان پتانسيل كاربرد تشخيص داده شد. از نامناسب بودن فشار مي‌توان به عنوان مهم‌ترين اين عوامل نام برد. همچنين، استفاده هم‌زمان از تعداد آبپاش‌ زياد و استفاده بيش از يك آبپاش روي لوله فرعي آبياري، دليل اصلي پايين بودن ضريب يكنواختي و يكنواختي توزيع در اين سيستم‌ها بوده است. به‌طور كلي نتايج اين پژوهش نشان داد كه اگرچه در بسياري از موارد مشكلات طراحي و اجرايي وجود داشته است اما سهم بزرگي از دليل‌هاي پايين بودن عملكرد سيستم‌هاي آبياري باراني كلاسيك ثابت دشت كوهدشت، مديريت و بهره‌برداري ضعيف از اين سيستم‌ها است.

Today, irrigation systems management with efforts to maximize their efficiency under water shortage conditions is an important and critical challenge for agricultural scientists and irrigation engineers. Evaluation of irrigation systems is an analysis of irrigation systems based on the module in real terms during the normal work of the farm system. Evaluation of the performance of irrigation systems can illuminate weaknesses of different aspects of the system and make constructive suggestions for improving the studied systemʹs performance presented in terms of management that can ultimately lead to increased irrigation efficiency. In this study, ten solid-set sprinkler irrigation systems were evaluated in Koohdasht plain located in western Iran. In the evaluation, information regarding topography, profile sources of water supply, pumping system, main pipe arrangement, irrigation interval and sprinkler type was gathered and analyzed. Then, on each of the farms, on the days of irrigation and before irrigation, undisturbed samples of various soil layers of the depth of 0-25, 25-50, 50-75 cm were taken to determine bulk density and field capacity (FC). In addition, disturbed soil samples were taken to determine specific weight and other physical and chemical properties of the soil. Before testing, the water uniformity distribution and soil moisture testing were conducted to determine soil moisture deficit (SMD). Moreover, in all the fields, hydraulic variables such as pressure and sprinkler flow were determined and pressure in sprinkler was measured using a barometer and Pitot tube. Furthermore, sprinkler flow was calculated using a stopwatch and a 20-liter gallon based on volumetric method. Taking into account the topography of the region, an area was chosen for conducting tests where medium pressure systems occurred. After choosing an appropriate test area, the area between two sprinklers by means of a tape measure and wooden nails was determined with 3 × 3 m grid spacing. After that, cans with a height of 12cm and an internal diameter of 9.6 cm were used to collect water and were placed at grid points. Next, sprinklers were set to begin work and after working for at least an hour, the water inside the scaled cylinder was measured and recorded. Water samples of each of the fieldsʹ irrigation systems were taken and transported to laboratories to determine their chemical properties. Results from field tests of soil and water quality assessment showed that all the soil fields contain normal ECe and SAR. Water supply for all systems came from well shafts and based on Soil Salinity Laboratory and Americaʹs classification were without limitation. Almost all physical and chemical properties of soil and water evaluated on the farms have been within the limits set and have no limitation of usage for agricultural purposes. The average pressure in sprinkler output in only three systems was within the limited allowable range (4 bar) and the average pressure of other systems was more or less than the allowable limited range. The normal range of pressure changes should be less than 20 percent and exist only in two systems. Existence of dramatic pressure changes is due to a lack of suitable pumping system design and system leakage. Some performance indicators were used, including Christiansen’s coefficient of uniformity (CU), distribution uniformity (DU), potential application efficiency of low-quarter (PELQ), application efficiency of low-quarter (AELQ), wind drift and evaporation losses (WDEL), deep percolation losses (DP) and adequacy of irrigation (ADirr). The mean values of these parameters in the order mentioned were as follows: 70.84, 43.64, 46.85, 38.01, 8.77, 31.02 and 64.66%, respectively. The results showed that the coefficient of uniformity and distribution uniformity in the all of systems is less than the range recommended by Merriam and Keller (81%?CU?87% , 67%?DU?80% ). DU is a good indicator of the extent of the probable uniformity problem. If small amounts of DU, in all parts take adequate irrigation, it is a sign of deep water percolation losses. Although small amounts of DU are relative, generally less than 67 percent is unacceptable. In 4 out of 10 systems, complete irrigation took place. In the remaining systems, due to the phenomenon of deficit irrigation occurring AELQ values were equal to PELQ. Unsuitable design and implementation of the irrigation systems under investigation were recognized as the most important reason for low PELQ. Unsuitable pressure was recognized as one of the most important causes. Furthermore, simultaneous use of many sprinklers and using more than one sprinkler on laterals were considered as the most important reason for low water distribution. Water and soil quality of Koohdasht plain lacked any problems. Physical and chemical properties of soil and water in all fields had a normal range and for irrigation did not set any limitations. The results indicated that solid-set sprinkler irrigation systems have low application efficiency and their uniformity of distribution is less than the amounts recommended by Keller and Merriam. In general, the results of this study showed that in many cases design and operation problems existed, but the major share of the problem of low performance of sprinkler irrigation systems in Koohdasht plain lay with weak management and exploitation of these systems.