بررسي تلفات آب و نيترات در كودآبياري جويچه‌اي با هيدروگراف‌هاي ورودي موجي و ثابت

Evaluation of water and nitrate losses in furrow fertigation with surge and fixed input hydrographs

تعيين شاخص‌هاي صحيح كودآبياري در روش‌هاي جديد آبياري سطحي از جمله آبياري موجي امري ضروري به نظر مي‌رسد. در اين پژوهش مقدار تلفات آب و كود نيترات در كودآبياري جويچه‌اي با جريان موجي و پيوسته مقايسه شد. بدين منظور آزمايش‌هاي مزرعه‌اي در يك خاك با بافت لوم رسي در مزرعه تحقيقاتي پرديس كشاورزي و منابع طبيعي دانشگاه تهران انجام شد. مديريت‌هاي كودآبياري مختلف با جريان موجي شامل تزريق كود در تمامي موج‌هاي مرحله پيشروي، مرحله پس از پيشروي و تركيب موج انتهايي فاز پيشروي و ابتداي مرحله پس از پيشروي آزمايش شد. براي جريان پيوسته نيز تزريق كود در نيمه دوم مرحله پيشروي و پس از مرحله پيشروي اعمال شد. تلفات رواناب براي آزمايش‌هاي با جريان موجي و پيوسته به‌ترتيب در محدوده 8/13 تا 4/33 و 1/9 تا 0/15 درصد و تلفات نفوذ عمقي آب به‌ترتيب در محدوده 1/9 تا 0/17 و 5/24 تا 34 درصد به دست آمد. كمترين و بيشترين مقدار تلفات نيترات از طريق رواناب در آزمايش‌هاي با جريان موجي به‌ترتيب 1/7 و 6/58 و در جريان پيوسته 1/4 و 5/28 درصد محاسبه شد. همچنين تلفات نفوذ عمقي نيترات در تمامي آزمايش‌ها ناچيز و خطر آبشويي نيترات وجود نداشت. نتايج نشان داد كه مناسب‌ترين گزينه مديريت كودآبياري براي كاهش تلفات رواناب نيترات، تزريق كود در تمامي موج‌هاي مرحله پيشروي و نيمه دوم مرحله پيشروي به‌ترتيب در جويچه‌هاي با جريان موجي و پيوسته است.

Surge irrigation is one of the most advanced methods of surface irrigation that has developed in recent decades. Surge irrigation, introduced by Stringham and Keller (1979), is defined as the intermittent application of water to furrow, rills or borders in a series of relatively short on- and off-time periods of constant or variable time spans. The main advantages of this method compared to irrigation with continuous flow (ordinary) are increasing water use efficiency and uniformity of water distribution. On the other hand, fertigation is the effective method for application of fertilizer and reduce nitrate losses. Fertilizers are widely applied to agricultural fields using surface fertigation. However, there are still no adequate guidelines for the proper design and management of surface fertigation. The proper management of surface irrigation fertigation is important because of the low uniformity of distribution of water in surface irrigation. The efficient application and distribution of water by furrow irrigation is highly dependent on parameters such as inflow rate and inflow hydrograph shape. Determination of fertigation correct indicators in new surface irrigation methods like surge irrigation is imperative. In this research, water and nitrate losses in furrow fertigation with surge and continues flow were evaluated and compared. For this purpose, field experiments were conducted on a soil with clay loam texture at the experimental station of the College of Agriculture and Natural Resources, University of Tehran. Three fertigation treatments with the surge flow included injection fertilizer during all advance cycles, wetting phase and last of the advance cycle plus start of wetting phase tested. Also for continues flow, two treatments included injection fertilizer in the second half of advance phase and wetting phase was applied. Each treatment contained three furrows of which the central furrow was as main furrows and the others as guard furrows. Furrows had 0.75 m distance and 150 m length. The longitudinal slope of furrows varied as it was 0.9% in the first 60m and 0.4% until the end of the furrow. Inflow and outflow were measured by WSC flumes. Six irrigations were performed every seven days, and the third and sixth irrigations were with fertilizers (fertigation experiments). For estimating nitrate losses through surface runoff, water samples were collected from outflow water in five minutes intervals until the end of outflow. After that, water samples from inflow and outflow were analyzed by spectra photometer to estimate the nitrate concentration. Runoff losses of water for first and second fertigation experiments with surge flow amounted to 15.8-21.8% and 13.8-33.4% and for experiments with continuous flow amounted to 9.1- 9.4% and 13.0-15.1%, respectively. Deep percolation losses of water for first and second fertigation experiments with the surge flow were obtained 9.1-14.3% and 14.2-17.0% and for experiments with continuous flow were obtained 33.2-34% and 24.5-25.5%, respectively. Ratio of flow advance rate in first and second surge experiments to continuous was calculated 1.75 and 1.2, respectively. Runoff losses of nitrate for first and second fertigation experiments with the surge flow amounted to 5.1-37.7% and 5.4-47.1% and for experiments with continuous flow amounted to 3.7- 20.2% and 6.0-27.2%, respectively. Deep percolation losses of nitrate were negligible in all experiments. Outflow and inflow nitrate concentration had a time lag of about 15 to 20 minutes. In experiments that fertilizer had injected in wetting phase, this time lag was less than injection in advance phase. Nitrate runoff losses in first fertigation experiments with continuous flow were low compared to surge flow, but there was no significant difference between them in the second fertigation experiments. Decreasing of surge effect and soil consolidation were the main reasons for this result. Conversely, infiltrated nitrate in fertigation experiments with the surge flow was low compared to continuous flow in both experiments. Nitrate runoff losses in experiments that, fertilizer was injected in advance phase was less than that injected in wetting phase. According to average soil depth of field, water and nitrate deep percolation losses were calculated up to 80 cm soil depth for all experiments. Nitrate deep percolation losses were negligible and there was no risk of nitrate leaching in all experiments. Results indicated that the best fertigation management for decrease nitrate runoff losses is injection fertilizer during all advance cycles and second half of advance phase in furrows with surge and continues flow, respectively. As conclusions, nitrate losses were dependent on both the shape of inflow hydrograph and fertigate injection management. It is recommended that the tests conducted on soils with sandy and sandy loam texture.