Computing aquitard depletion, which is often overlooked, is of great significance for the assessment of groundwater resources and land subsidence. The issue is viewed as troublesome because of the additional computational burden, the poorly known hydrogeological parameters of the aquitard, and the lack of drawdown history in pumped aquifers. In this study, an analytical solution is derived to describe the drawdown variation in a nonlinear-consolidated aquitard under the condition of variable drawdowns in adjacent aquifers. Based on the analytical solution, we study the characteristics of groundwater dynamics and water balance under the conditions of linearly increasing drawdown of aquifers in adjacent aquifers. In addition, we put forward a method to calculate the depletion and hydrogeological parameters of an aquitard corresponding to variable drawdowns in adjacent aquifers, applicable even when historical drawdown data are lacking. The accuracy of the method is generally very good, but results improve when the drawdown history of pumped aquifers is divided into more periods for estimation. Under the condition of linear drawdown in adjacent aquifers, groundwater depletion and maximum water release rate of the aquitard increases with increasing compression index, coefficient of consolidation, aquitard thickness, rate of drawdown change in the adjacent aquifer, while decreasing with initial void ratio, and initial effective stress. The proposed approach is demonstrated at a field site in Shanghai City of China, and it would help for the effective management of groundwater resources and estimation of the global transfer from groundwater to surface water.

Groundwater is generally overexploited over the world due to the increasing demand for water resources, and it has brought a series of environmental problems, including the decline of groundwater levels, land subsidence (Li et al., 2021;Shi et al., 2007Shi et al., , 2008, and sea level rise (Konikow & Kendy, 2005). A multilayered aquifer system in the sedimentary plain area, such as the Dakota aquifer system in the United States and the Yangtze Delta in China (Guo & Li, 2015;Ye et al., 2016), usually consists of multiple aquifers with alternating aquitards in between (Zhuang et al., 2015). Water stored in aquitards is a significant source of water from pumped aquifers, and aquitards tend to have higher specific storage than confined aquifers (Liu et al., 2022;Zhang, He, et al., 2020). Meanwhile, aquitard storage is difficult to recover and could often be the primary source of groundwater released from the storage of aquifer systems (Shi et al., 2008). Consequently, the accurate calculation of groundwater depletion in aquitards is essential for the effective management of groundwater resources, and it would help to estimate the global transfer of groundwater to surface water (Konikow & Neuzil, 2007).Due to low hydraulic conductivity and nonnegligible specific storage, water release from aquitards and its deformation always lag behind the drawdown in adjacent confined aquifers (Bakr, 2015;Liu et al., 2022;Ye et al., 2016). Zhou et al. (2013) studied the groundwater dynamics and water balance of an aquitard, while the drawdown was a constant amount in an adjacent confined aquifer. The hydraulic head in aquifers lying above or under the aquitard usually decreased with increasing groundwater extraction (Custodio, 2002), Neuman and Gardner (1989) presented convolution integrals for calculating the drawdown in the aquitard under the condition of water table fluctuation. A widely applicable method was proposed to estimate the groundwater depletion of the aquitard in the entire or limited period of exploitation history, especially when the data on drawdown history is sufficient (Li & Zhou, 2015;Li et al., 2017). Konikow and Neuzil (2007) presented a simplified method to estimate the groundwater depletion from the confining layers in response to withdrawals from adjacent aquifers. Alternatively, given the same information, a well-calibrated, numerical simulation model (i.e., three-dimensional