Simulation of phosphate transfer cycle in Zanjanrood River using SWAT model

The availability of water for agriculture is of great importance, and despite the water crisis that is becoming more severe every year, both quantitatively and qualitatively, this issue should be seriously considered. Water resources include surface and groundwater, which are qualitatively more at risk, therefore, in order to preserve them, the sources of pollution must be known and appropriate solutions must be provided to prevent or eliminate these pollutions.

In this research, phosphate transfer cycle in Zanjanrood Watershed has been simulated using SWAT model. For calibration and validation, SWAT-CUP software and measured values ​​of average monthly current intensity at Sarcham hydrometric station between (1996-2013) were used and 26 sensitive parameters were selected for sensitivity analysis. There are three options for irrigation method, three options for fertilizer application and two combined options. In order to analyze the uncertainty of the indicators p-factor and r-factor and to analyze the quality of the model results, two indices of coefficient of determination (R2) and nash-sutcliffe coefficient (NS) have been used.

In the monthly runoff calibration stage, at the output of the field, the coefficients of r-factor, p-factor, R2, NS were 0.27, 0.11, 0.83 and 0.53, respectively, and in the validation stage were 0.60, 0.18, 0.73 and 0.53, respectively. The results showed that with increasing the level of pressurized irrigation, the amount of phosphate contamination at the outlet of the basin did not change significantly. Regarding the amount of fertilizer, the 50% reduction in the consumption of phosphate fertilizers has reduced the amount of phosphate entering the Zanjanrud River by 19.2%. On the other hand, a 50% increase in the use of fertilizers has increased the input phosphate by 17.7%.

The results showed the proper performance of the SWAT model and its ability in the mentioned simulation. Also, by changing the surface irrigation method to subsurface and increasing the irrigation efficiency, there is no significant change in the average amount of phosphate output from the basin. On the other hand, by reducing the amount of fertilizer and preventing improper fertilization by farmers, pollution of surface and groundwater resources can be greatly prevented.