Optimization of irrigation amounts to avoid a waste of irrigation water in order to minimize the risk of surface runoff -case study Menemen plain (Turkey)

The evaluation of the field water cycle under deficit irrigation plays an important role in studying mechanism of field water and water quality. In this study, the agro-hydrological Soil–Water–Atmosphere–Plant (SWAP) model for small irrigated area in Menemen Plain was applied to reveal im-portant terms of the water balance for evaluating the water quality in Gediz River. Specific areal data (meteorological, soil structure, water resources, and crop properties) were used as an input with water balance, crop growth and drainage modules. Simulations were performed for the period 2008-2011. Dif-ferent water balance models were obtained for each irrigation and water quality was evaluated for the under applying conditions. For this, various irrigation amounts (cotton1, cotton2, cotton3) were tried using SWAP model and optimum irrigation amount for this area was determined. Crop type was selected as cotton because it was main crop in plain. According to water balance for the applying conditions (cotton1 irrigation), GWL, drainage and run-off resulted from, rainfall and irrigation amount according to models. In irrigation season run-off and drainage level was determined as high and GWL was obtained as low. High run-off values were directly reasoned for high irrigation amount in the area. Optimum irrigation amount was determined as 350-400mm in terms of crops water need and run-off. According to water quality results, run-off originated from irrigation could be caused high TDS and EC values in irrigation season. Low TDS and EC values can be attributed to high rainfall, which causes significant dilution. The intensive agricultural activities in the area such as fertilizer caused for high concentration of nitrate, phos-phate especially in irrigation season and also high run-off level. High levels of run-off from agricultural land and leakage affect the water body. Consequently, water quality is resulted from rainfall and irrigation amount in wet and dry season respectively. In dry years, NO3-N and o-PO4 values were observed high. It can be caused by domestic and industrial wastewater discharge. It was concluded that the use of the SWAP model in a distributed way is a useful tool to analyze all the components of the water balance for a whole irrigation system. Due to Menemen Plain has intensive agricultural facilities, it has been continuously observed. Irrigation amount should be adjusted according to crop type and soil properties.