Evaluating the Impact of Agricultural Drainage on the Quantity and Quality of Flow Into Shadegan Wetland By WEAP Model(Case Study: Maroon-Jarahi Basin)

Background and purpose: Development of agricultural lands, the occurrence of droughts, and climate change have made the Maroon-Jarahi basin to become one of the most stressful basins both quantitatively and qualitatively. Due to the existence of numerous irrigation and drainage networks in this basin, it is very important to investigate the effect of returned drainages on the river. Due to having suitable water potential, the study basin has always been suitable for the development of irrigation and drainage networks, also Shadegan Wetland at the end of the basin has increased the importance of quantitative and qualitative water resources management. Therefore, in this study, the effect of drainage from irrigation and drainage networks and traditional agriculture on the quantity and quality of rivers, and more importantly, the endpoint of the Jarahi River and the entrance to Shadegan wetland was investigated. Quantitative and qualitative simulation of the basin for a period of 60-year was performed by the WEAP model. Considering that none of the studies conducted in the Maroon-Jarahi basin has comprehensively and accurately examined all agricultural drains and critical points of the basin from a quantitative and qualitative perspective, so the study was conducted in this regard.
Materials and methods: In this study, the WEAP model was used as a comprehensive tool for quantitative and qualitative Modeling and their effect on downstream. First, the WEAP model was calibrated and validated in a period of 5-year (2012-2016) using the observed data. In order to evaluate the results of calibration and validation of the quantitative model, the statistical indices of root mean square error (RMSE), squared correlation coefficient (R2), and NASH model efficiency coefficient were used. The model was implemented for a 60-year simulation period (2017 to 2077) under the control management scenarios of network drainage in terms of quantity and quality to determine the effect of each drainage on the river system of the basin.
Results: The results of the model showed that the amount of salinity in the reference scenario is 3.6 (dS m-1) which in comparison with other scenarios only in Matbag drainage transmission scenario with a decrease of 34% (1.2 (dS m-1)) salinity compared to the reference scenario. This rate of salinity improvement is very important at the end of the Jarahi River and the entrance of the Shadegan Wetland and will have an excessive effect on the environment. Also, according to the reliability values of the environmental node of the Jarahi River, the difference between the reliability in the two references and the Matbag drainage transmission scenarios is 0.4%, which shows the minor effect of the Matbag drainage on the flow of the Jarahi River and also the reliability of other scenarios is equal to the reference scenario.
Conclusion: According to the results of quantitative and qualitative modeling in the Maroon-Jarahi basin, the Matbag drainage transmission scenario (left drainage network of Ramshir) has the maximum effect in terms of quality compared to other scenarios. If this drain is transferred, the salinity at the end of the basin during the simulation period will decrease by 1.2 (dS m-1) (34%).