Assessment of changes in surface water and groundwater interactions in the Leningrad Sub-basin

In this study, water exchange in normal and severe drought conditions was also investigated. Based on the historical time series of zayanderood stream flow in Sadtanzimi, the water years 1373-1374 and 1379-1380 were selected as normal year and severe drought condition, respectively. By derived equations, outflows of three reaches were obtained under normal and drought condition. Based on the results, in normal condition net exchange of surface water was negative in three reaches, and only a small amount of 5 million cubic meters per year has been added to surface water in the second period. The results in normal year showed that surface water was fed by groundwater in the first reach and in the other two reaches fed groundwater. Compared to the normal water year, significant changes were observed in the exchange of surface water with groundwater and atmosphere in severe drought conditions. In the first reach, surface water gain by groundwater was observed, which this increase is 17% of outflow of this reach during drought conditions. During the drought, only the first period has not faced with shortage. The flows and water exchange in this situation have been such that the observed outflow of the three studied reaches is reduced by half the inflows. Surface water of the first reach has net gain in drought conditions. While under normal conditions, this interval was purely water withdrawal. Duo to the lack of accurate data on withdrawals and return flow, these components of the surface water balance were considered as annual average. However, the obtained results and their comparison with the data and information in the previous studies showed that the proposed method is a suitable toll for assessing the exchange of surface water with groundwater. Therefore, in future studies it is suggested studying this method with more accurate data on exploitation and return flow. Keywords: Surface water and Groundwater Interaction, Streamflow, Mass balance, River reac