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[1] Water scarcity is a critical issue in semiarid regions; however, regional groundwater monitoring is extremely limited. This study evaluates the ability of the GRACE satellites to monitor groundwater storage in the semiarid High Plains aquifer, United States (450,000 km 2 area), which is subjected to intense irrigation. GRACE-derived terrestrial water storage (TWS) is highly correlated with the sum of soil moisture (SM) and groundwater storage (GWS) (R = 0.96 for in situ measured SM from 78 stations and R = 0.95 for simulated SM with the Noah land surface model with root-mean-square difference of 38 mm and 36 mm, respectively). Correlation between seasonal GWS changes calculated from GRACE TWS minus SM and measured GWS ($1000 wells per season) is also high (R = 0.73 for in situ SM and R = 0.72 for simulated SM). Variability in SM is mostly restricted to the upper 2 m of the soil. Monitored SM compared favorably with simulated SM (R = 0.82). Study results show the potential for using GRACE gravity measurements to monitor TWS and GWS over large semiarid regions subjected to intense irrigation.

[1] This study presents the first comparison of seasonal groundwater storage (GWS) variations derived from GRACE satellite data with groundwater-level measurements in the High Plains Aquifer, USA (450,000 km 2 ). Correlation between seasonal GRACE terrestrial water storage (TWS) and the sum of GWS estimated from field measurements (2,700 wells) and soil moisture (SM) simulated by a land surface model is high (R = 0.82). Correlation between GRACE-derived and measured GWS is also significant (R = 0.58). Seasonal GRACE-derived TWS and GWS changes were detectable (! uncertainty) in 7 and 5 out of 9 monitored periods respectively whereas maximum changes (between winter/spring and summer/fall) in TWS and GWS were detectable in all 5 monitored periods. These results show the potential for GRACE to monitor GWS changes in semiarid regions where irrigation pumpage causes large seasonal GWS variations. Citation: Strassberg, G., B. R.Scanlon, and M. Rodell (2007), Comparison of seasonal terrestrial water storage variations from GRACE with groundwater-level measurements from the High Plains Aquifer (USA), Geophys. Res. Lett., 34, L14402,

[1] Groundwater recharge is critical in semiarid regions where aquifers are currently being mined for intensive irrigation. Land use management related to agriculture can be used to control partitioning of water near the land surface and to potentially manage water resources. The purpose of this study was to quantify impacts of deep plowing in rainfed (nonirrigated) agriculture in a semiarid region on groundwater recharge, which had not been previously evaluated. Deep (0.7 m) plowing was conducted once in 1971 to remove low-permeability soil layers (0.15-0.70 m deep) in Pullman clay loam (20,000 km 2 area) in a bench terrace system in the semiarid High Plains in Texas (USA). Deep plowing was followed by conventional tillage. Boreholes were drilled in deep plowed cropland (three boreholes) and also beneath conventionally tilled cropland (four) and natural ecosystems (three) to provide baseline controls. Soil samples were analyzed for water content, chloride concentrations, and matric potentials to quantify subsurface water movement. Bulges of chloride that accumulated beneath natural ecosystems during the past 9,000-14,000 years (Holocene period) provided a marker to quantify timeintegrated response of subsurface drainage to hydrologic forcing in deep-plowed cropland. Displacement of chloride bulges to depths of 10.7, 12.3, and 13.7 m beneath deep-plowed cropland indicate minimum drainage rates of 58, 60, and 81 mm/a, whereas drainage beneath conventionally tilled cropland ranged from 0 (nonterraced) to 9 and 14 mm/a (bench terraced). Deep plowing slightly increased crop yield during wet years by reducing waterlogging. If deep plowing were applied to 10% of the Pullman soils, it could increase regional volumetric recharge by 0.1 km 3 /a, which is similar to the current regional volumetric recharge. Low-permeability soil layers are widespread in cropland areas globally, and deep plowing could greatly enhance groundwater recharge in such areas, which is critical in semiarid regions where recharge is negligible.

The Arc Hydro ground water data model is a geographic data model for representing spatial and temporal ground water information within a geographic information system (GIS). The data model is a standardized representation of ground water systems within a spatial database that provides a public domain template for GIS users to store, document, and analyze commonly used spatial and temporal ground water data sets. This paper describes the data model framework, a simplified version of the complete ground water data model that includes two-dimensional and three-dimensional (3D) object classes for representing aquifers, wells, and borehole data, and the 3D geospatial context in which these data exist. The framework data model also includes tabular objects for representing temporal information such as water levels and water quality samples that are related with spatial features.