GRACE satellites and in-situ well data reveals that specific yield declines with depleting groundwater levels

Specific yield (S_y) is defined as the ratio of the volume of water that saturated rock or soil yields by gravity to the total volume of the rock or soil. S_y is often taken as a constant that when multiplied to groundwater level change provides water volume change, hence it is crucial in validating Gravity Recovery And Climate Experiment (GRACE)-derived groundwater changes, providing estimates of available groundwater resources, and in modelling groundwater aquifers. In this study, we used GRACE data and available quality-controlled in-situ well data to estimate S_y instead. Our hope was that it would match the available S_y, however we observed a time-varying S_y. Upon further investigation we found a negative correlation between water level and S_y. Hence the time-evolution of S_y was due to changes in the water level. We processed available well data and GRACE(-FO) in India, the United States, Europe, and Australia, spanning from January 2004 to December 2022. We also developed a general law/empirical relationship between S_y and groundwater level. All regional-specific empirical relationships exhibit a decrease in S_y as the average groundwater level depth drops, but the decay rate of S_y is notably faster in India (0.17 ± 0.04 m^-1) compared to the United States (0.03 ± 0.01 m^-1), Australia (0.06 ± 0.02 m^-1), and European countries (0.04 ± 0.03 m^-1). This empirical expression allows for the estimation of S_y based on readily available groundwater level data, thus supporting large-scale groundwater assessments and modelling efforts. At the global scale, a 50% decrease in S_y results in a groundwater level depletion of ~17 meters. However, in India, due to a faster decay rate, the same 50% reduction in S_y causes a groundwater level depletion of ~4 meters. This relationship can be utilized by hydrologists, water resource managers, and policymakers to predict S_y and assess changes in groundwater levels over time, aiding in more effective and sustainable water resource management.