Dynamic specific yield explains accelerated groundwater loss

Specific yield is an important parameter capturing sub-surface characteristics, such as grain size, shape, and pore distribution. Specific yield is used to estimate change in groundwater storage from groundwater levels. Hence, it is critical for estimating changes in groundwater availability, a critical resource for ensuring socioeconomic prosperity. At present the norm is to consider specific yield as a constant in time. In the present study, using the Gravity Recovery and Climate Experiment (GRACE) data based total water storage anomalies and quality controlled well observations at global scale (the United States, Europe, Australia, India, and China), we show that specific yield is not constant in time and varies with groundwater level. Further, we establish an exponential relation between groundwater levels and specific yield. The parameters  (α = specific yield at zero groundwater level, β =  rate of groundwater level decay) of the best fit exponential function  are found to be the same across the United States (α= 0.17 ± 0.02, β= 0.02 ± 0.01 m^-1), Europe (α= 0.11 ± 0.01, β= 0.03 ± 0.01 m^-1), Australia (α= 0.12 ± 0.03, β= 0.02 ± 0.02 m^-1), China (α= 0.11 ± 0.03, β= 0.05 ± 0.02 m^-1), and India (α= 0.07 ± 0.02, β= 0.03 ± 0.03 m^-1), within the uncertainty of the exponent. The methodology is validated using literature based specific yield values across the United States. In addition, improvement in modelling groundwater levels using AMBHAS-1D is observed when using a varying specific yield instead of a constant (NSE= 0.92, RMSE= 5.23 m). Hence, we conclude that with increased groundwater exploitation, its availability will drop faster than expected. Most of the regions investigated experienced a decline in specific yield over the last two decades, and the perceived groundwater availability for some locations is 80% less than that estimated using constant specific yield.