Estimating century-long changes in groundwater storage using a one-parameter ET model

Long-term information on regional groundwater resources is important for water resources management. This information is unfortunately also scarce in most places. Even in regions with active groundwater monitoring programs, data cover only a few decades and there is considerable uncertainty about the reliability of regional extrapolations from point data. Long-term water budget analyses, thus, are often confounded by the need to estimate both the change in water storage and evapotranspiration (ET) simultaneously, i.e., the 'one equation - two unknown-variables' problem. To address this we used a simple monthly water budget approach using a combination of Gravity Recovery and Climate Experiment (GRACE) satellite data, ground-based measures of stream discharge and precipitation, and a simple ET model. This approach allowed us to estimate ET for several large basins in the U.S. over the past decade. These estimates were used to calibrate the one-parameter ET-storage model of Tuttle and Salvucci (2012, WRR W05556). Using this calibrated ET model in conjunction with precipitation and stream discharge records, we then estimated annual changes in water storage over the entire 20th century. These are corroborated against estimates made using long-term groundwater and weather datasets. It is anticipated that the method used here will be especially useful for water resources management in regions with poor or few historical data.