Assessing Variation of Runoff and Nitrate Concentration in Management Scenarios Using SWAT Model, (Case Study: Hashtgerd Plain)

Watershed management strategies can be considered in mitigating water pollution issues. In this study, using SWAT as a semi-distributive hydrological model, runoff and nitrate amounts have been investigated according to management scenarios in Hashtgerd plain, Iran. After runoff simulation, three hydrometric stations were used for calibration in the period (1990-2013) and validation in the period (2014-2018). The results of R2 and NS statistical indices (greater than 0.6) show that the simulation results in this catchment have been done with good accuracy. The results also indicate that applying the first scenario (changing the crop pattern) and the second scenario (increasing irrigation efficiency) reduced the water usage in the agricultural sector from 85.3 MCM to 59.8 MCM in the present condition, which also will improve the fertilizing efficiency. It was demonstrated that changing the crop pattern can help reduce the deep percolation. On the other hand, using drip irrigation, the leaching requirement, and irrigation water loss will be reduced. Comparing the amounts of observed nitrate in the hydrometric station in the base period with the simulation results in the first scenario revealed that, by applying the first scenario, the amount of nitrate simulated has relatively a decreasing trend except in winter and early spring. The results of the second scenario show that by changing the irrigation method, the amount of nitrate leaching has decreased significantly compared to the base period. In general, the second scenario is more effective in reducing nitrate leaching and is recommended.