IN-SITU AND GRACE-BASED GROUNDWATER OBSERVATIONS: SIMILARITIES, DISCREPANCIES, AND EVALUATION IN THE HIGH PLAINS AQUIFER IN KANSAS

Relating GRACE-derived terrestrial water storage (TWS) variations to in-situ groundwater data is important to understanding GRACE data utility. Here, GRACE-derived annual changes in TWS (ΔTWS) from NASA’s Jet Propulsion Laboratory (JPL) and Goddard Space Flight Center (GSFC) are compared to annual changes in saturated groundwater volumes (ΔGW) in the High Plains aquifer (HPA) and overlying alluvial aquifers in Kansas. The HPA is one of the largest and most heavily used aquifers in a semi-arid climate in the world. Long-term water resource viability and water availability in portions of the HPA, including in Kansas, are at risk due to groundwater pumping. Evaluation of linear trends suggests that ∆TWS is more closely related to ∆GW_alluvial than to ∆GW_HPA. Unlike alluvial aquifers, a thick vadose zone overlies the HPA. Changes in vadose zone water content (∆VZ) are estimated using GRACE products and shallow (0-2m) North American Land Data Assimilation System data. The estimates are similar in magnitude to ∆TWS, suggesting an unexpected dominance of ∆VZ given the quantity of groundwater extracted and declines in groundwater levels. Results indicate that even in regions with large, heavily used aquifers subject to substantial groundwater depletion, relatively small ΔVZ or well-constrained ΔVZ estimates may be needed in order for GRACE-derived estimates of ∆TWS to provide guidance for aquifer management.