Groundwater recharge estimation in arid hardrock-alluvium aquifers using combined water-table fluctuation and groundwater balance approaches

This paper proposes an approach to estimate groundwater recharge using an optimization-based water-table fluctuation method combined with a groundwater balance model in an arid hardrock-alluvium region, located at the Oman-United Arab Emirates (UAE) border. We introduce an “effective hardrock thickness” term to identify the percentage of the considered hardrock thickness in which effective groundwater flow takes place. The proposed method is based upon a Thiessen polygon zoning approach. The method includes sub-polygons to represent specific geologic units and to enhance the confidence of the estimated groundwater recharge. Two linear and one nonlinear sub-models were developed to evaluate the model components for the calibration (Oct. 1996 to Sep. 2008) and validation (Oct. 2008 to Sep. 2013) periods. Long-term annual groundwater recharge from rainfall and return flow over the model domain are estimated as 24.62 and 5.71 Mm3, respectively, while the effective groundwater flow circulation is found to occur in the upper 7% of the known hardrock thickness (42 m), confirming conclusions of previous field studies. Considering a total difference in groundwater levels between eastern and western points of the study area of the order of 220 m and a 12-year monthly calibration period, a weighted root mean squared error in predicted groundwater elevation of 2.75 m is considered quite reasonable for the study area characterized by remarkable geological and hydrogeological diversity. The proposed approach provides an efficient and robust method to estimate groundwater recharge in regions with a complex geological setting in which interaction between fractured and porous media cannot be easily assessed.