Estimation of the groundwater recharge coefficient by minimizing the sum total error of a regional water balance

The sustainable use of groundwater resources requires that the components of a regional water balance are understood with a high degree of accuracy, especially in arid and semi-arid regions. Although groundwater recharge is one of the most important components of a groundwater balance, its rate is one of the most uncertain components. Despite the simplicity and widespread use of water balance method (WB) for estimating groundwater recharge in arid and semi-arid regions, the accuracy of the groundwater recharge estimation depends on the accuracy of the other components in this method, so a reduction in errors that are associated with measuring these can improve the accuracy of recharge estimation. Therefore, the main objective of the current paper was to present a method for estimating groundwater recharge based on minimization of the sum total error of the system water and groundwater balance equations simultaneously. A set of correction coefficients that reflect the error in estimation of each component of balance equations, were applied to different components in annual scale. Reasonable ranges, obtained from error analysis, were considered for the correction coefficients and the sum of absolute errors in the overall system water balance and groundwater balance equations was minimized for the period of study. The proposed method was used to estimate groundwater recharge in Mahvelat basin in Khorasan Razavi province of Iran, as a case study. The minimization process used in this research reduced the error of system water and groundwater balance equations by 55% and 65%, respectively. Moreover, as the results of optimization process on the correction coefficients, the recharge coefficients due to precipitation and irrigation return flow were estimated to be 2% and 16.5%, respectively.