Impact Assessment of Operating Agricultural Management Scenarios on Quality and Quantity of Surface Water Resources Using Semi-Distributed Model and Checking the Vulnerability of the Aquifer (Case study: Hashtgerd Plain)

The source of many qualitative and quantitative problems of water resources is the activities that are done in a watershed. Therefore, reviewing the planning strategies of watershed can lead to the reduction of these challenges therefore it is necessary to assess the effects of operating of these strategies before implementing them.

This research is carried out in the Hashtgerd basin is located in the central part of Alborz province. After making the semi distributed hydrological model (SWAT), this model has been developed to convert quantity and quality of surface water resources. For this purpose, the statistics of hydrometric stations Fashand, Dehsomeh and Najmabad and qualitative data of nitrate of the index station were calibrated and validated as control points. After model sensitivity analysis, its calibration process was conducted using SWAT-CUP software with SUFI2 algorithm and 500 iteration and the best values of selected parameters were obtained. Then, impact of implementing management scenarios are required such as modifying cropping pattern and improving irrigation methods to enhance the agricultural efficiency for improving agricultural state and reducing the contaminations. At last assessing the vulnerability of the Hastgerd aquifer using seven DRASTIC parameters by overlaying layers, ranking and weighing was assessed.

The results of the monthly surface runoff simulation were tested by Nash-Sutcliffe indicators and coefficient of determination. The results above 0.7 which showed that the simulation was done with good precision showed that simulation was done with proper precision(results over 0.7). The results of the Nitrate calibration and validation show that the NS was 0.83 and 0.7 and the R2 was estimated .87 and 0.89. These values indicate the proper performance of the model and approve high correlation between the observed and simulated data. After determining the vulnerability map of the Hashtgerd aquifer along the density of wells correcting the weights of the model layers, the central part of the plain is the most vulnerable.

Two management scenarios were modeled including the change in the cropping pattern and improvement of the irrigation efficiency to assess the quality and quantity variation in surface resource. It was indicated, by modifying the cropping pattern and reducing water exploitation, the amount of surface runoff was increased. However, the density of Nitrate was reduced by 5 to 20 percent in different months. With implementation of scenarios, the water consumption in the agriculture sector will be reduced from the 85.3 MCM to 59.8 MCM. The final vulnerability index of the aquifer varied between 44 to 90 by using DRASTIC method.