CHARACTERIZATION OF TERRESTRIAL WATER STORAGE USING A REGIONAL INTEGRATED HYDROLOGIC MODEL TO EVALUATE WATER STORAGE ESTIMATES FROM GRACE SATELLITE IN THE NORTHERN HIGH PLAINS

Since its launch in 2002, the Gravity Recovery and Climate Experiment (GRACE) satellite has provided estimates of terrestrial water storage (TWS) by measuring Earth’s gravity variation. Several published studies used a standard approach to estimate water storage components (e.g. groundwater) from GRACE by disaggregating GRACE-derived TWS using parameters (e.g. soil moisture) from independent products (e.g. land surface models), and finally compare the result with in-situ data (e.g. groundwater level measurements) which induces potentially large and unknown bias on the estimated product. The novel approach used in this study characterizes TWS from in-situ data using a regional, integrated hydrologic model (MIKE SHE) and water balance approach to evaluate GRACE-measured TWS and estimate its uncertainty in the Northern High Plains. MIKE SHE, a deterministic, physics-based, and spatially distributed model code, simulates hydrological processes in the land phase of the hydrological cycle, such as the variations in hydraulic heads, flows and water storage on the ground surface, and in the unsaturated and saturated subsurface zones. Model input data such as climate (obtained from NOAA NCDC), landuse (USGS), soil (CONUS-Soil), hydrologic, and aquifer properties data (USGS and local agencies) were used to set up and calibrate the model. The result from the model - spatially distributed TWS - was used to calibrate the GRACE-derived water storage product.

Preliminary model calibration results for the period 2000-2009 shows an average increase in TWS of ~2.3 cm/year which is consistent with GRACE-estimated TWS. GRACE results from the NHP indicate the area is losing TWS with an average water height of ~2 cm/year from 2002-2009. However, the area started gaining TWS afterwards with a peak (~86 cm) in 2010 and declined thereafter. Cumulative TWS estimated by a mass balance approach using data (precipitation, evaporation, and runoff) from NLDAS for the period 2002-2013 is ~131 cm which is consistent with the estimation from GRACE (127 cm). The integrated hydrologic model, which provides spatially continuous estimates of the entire terrestrial water storage, allows a holistic assessment of GRACE TWS as aggregate and individual water storage components (e.g. groundwater) with calculated uncertainties.