On transient qanat discharge in an unconfined aquifer underlain by a fractured aquifer

Abstract

Fractured aquifers sometimes underlie unconfined aquifers to form unconfined-fractured aquifer systems. The discharges of qanats constructed in such aquifer systems depend not only on the hydraulic parameters of the overlying aquifer but also on the hydraulic parameters of the underlying aquifer, where qanats can be approximated as constant-head slightly inclined horizontal wells. The available analytical solutions of the transient discharge variation of a qanat usually consider a single-layer unconfined aquifer and cannot be used to simulate the transient discharge variation of a qanat in an unconfined-fractured aquifer system. Thus, the goal of this work is to develop such an analytical model. The solution of the drawdown due to a point sink and the solution of the groundwater mound due to an areal recharge in an unconfined-fractured aquifer system are obtained using Laplace and Fourier transforms. These solutions are utilized to obtain the discharge variation of qanat in an unconfined-fractured aquifer system. By removing the inter-porosity flow term from the underlying fractured aquifer, the solution can be used to simulate the discharge variation of qanat in a two-layer aquifer or an aquifer-aquitard system. The effects of the hydraulic and geometric parameters of the underlying fractured aquifer on the discharge variation of qanat in an overlying, unconfined aquifer are explored. The solution can be utilized to predict the transient discharge of qanat due to rainfall or constant head horizontal well in an unconfined-fractured aquifer system; to estimate the hydraulic parameters of the underlying fractured aquifer using the discharge data of qanat or a horizontal constant-head well; to evaluate the response of qanat to areal recharge due to rainfall among other applications, thereby demonstrating the practical relevance and potential impact of our research on real-world hydrological problems.