Investigation of groudwater level decreasing effects on Qazvin aquifer hydrodynamic characteristics

Increasing groundwater consumption and water table level leads to changes in hydrodynamic properties of the aquifer especially the parameters of hydraulic conductivity and specific yield. In this study, that was conducted in 2021 using GMS model version. 10 in Qazvin aquifer, the coefficients of hydraulic conductivity and specific yield of aquifer in three census periods of 2002, 2009 and 2018 after calibration and validation of the model in two cases steady and unsteady were determined and compared. The root mean square errors in the steady state  for the proposed periods were 0.86, 0.73 and 0.84 m, respectively, and in the unsteady state were 1.7, 0.93 and 1.06 m, respectively. That shows high accuracy of the model simulation. The average hydraulic conductivity in the aquifer was estimated for the years of 2002, 2009 and 2018 which was 19.13, 16.85 and 14.06 meters per day and the average specific yield was 0.078, 0.071 and 0.064, respectively. The results showed that over time and by decreasing the groundwater level, the parameters of hydraulic conductivity and specific yield decrease. Since 2015 due to the measures taken for balancing and artificial recharge plan, the slope rate of hydraulic conductivity has decreased from 0.38 to 0.31. Also the slope rate of special discharge has decreased from 0.00117 to 0.00078. The main reason for this decreasing trend can be the destruction of soil structure and effective reduction of porosity indicating the irreversibility of the aquifer to its previous state. Therefore, it is necessary to pay attention to these changes regarding the recharge of aquifers and return water from irrigation systems.