GRACE satellite data are widely used to estimate groundwater storage (GWS) changes in aquifers globally; however, comparisons with GW monitoring and modeling data are limited. Here we compared GWS changes from GRACE over 15 yr (2002-2017) in 14 major U.S. aquifers with groundwater-level (GWL) monitoring data in~23,000 wells and with regional and global hydrologic and land surface models. Results show declining GWS trends from GRACE data in the six southwestern and south-central U.S. aquifers, totaling −90 km 3 over 15 yr, related to long-term (5-15 yr) droughts, and exceeding Lake Mead volume by~2.5×. GWS trends in most remaining aquifers were stable or slightly rising. GRACE-derived GWS changes agree with GWL monitoring data in most aquifers (correlation coefficients, R = 0.52-0.95), showing that GRACE satellites capture groundwater (GW) dynamics. Regional GW models (eight models) generally show similar or greater GWS trends than those from GRACE. Large discrepancies in the Mississippi Embayment aquifer, with modeled GWS decline approximately four times that of GRACE, may reflect uncertainties in model storage parameters, stream capture, pumpage, and/or recharge rates. Global hydrologic models (2003-2014), which include GW pumping, generally overestimate GRACE GWS depletion (total: approximately −172 to −186 km 3) in heavily exploited aquifers in southwestern and south-central U.S. by~2.4× (GRACE: −74 km 3), underscoring needed modeling improvements relative to anthropogenic impacts. Global land surface models tend to track GRACE GWS dynamics better than global hydrologic models. Intercomparing remote sensing, monitoring, and modeling data underscores the importance of considering all data sources to constrain GWS uncertainties. Plain Language Summary The major U.S. aquifers provide an ideal system to assess GRACE (Gravity Recovery and Climate Experiment) satellite data. We compared GRACE groundwater storage anomalies (GWSAs) with groundwater level anomalies (GWLAs) from~23,000 wells and with groundwater storage (GWS) from regional and global models in 14 major U.S. aquifers. Results show large GWS declines from GRACE in southwestern (Central Valley and Arizona Alluvial Basins) and south-central (Central and Southern High Plains and Texas) aquifers from multiyear droughts (5-15 yr). In contrast, GWS trends in aquifers throughout the rest of the U.S. showed mostly stable or rising values. Time series of GRACE GWSAs compared favorably with GWLAs from most aquifers, suggesting that GRACE data track groundwater (GW) dynamics. Regional GW models show similar or greater declines in GWS compared with GRACE data, with the largest discrepancy of a factor of four times greater modeled depletion in the ©2020. American Geophysical Union. All Rights Reserved.

Understanding the controls of agriculture and climate change on recharge rates is critically important to develop appropriate sustainable management plans for groundwater resources and coupled irrigated agricultural systems. In this study, several physical (total potential (ψ T) time series) and chemical tracer and dating (3 H, Cl − , Br − , CFCs, SF 6 , and 3 H/ 3 He) methods were used to quantify diffuse recharge rates beneath two rangeland sites and irrigation recharge rates beneath two irrigated corn sites along an east-west (wet-dry) transect of the northern High Plains aquifer, Platte River Basin, central Nebraska. The field-based recharge estimates and historical climate were used to calibrate site-specific Hydrus-1D models, and irrigation requirements were estimated using the Crops Simulation Model (CROPSIM). Future model simulations were driven by an ensemble of 16 global climate models and two global warming scenarios to project a 2050 climate relative to the historical baseline 1990 climate, and simulate changes in precipitation, irrigation, evapotranspiration, and diffuse and irrigation recharge rates. Although results indicate statistical differences between the historical variables at the eastern and western sites and rangeland and irrigated sites, the low warming scenario (+1.0°C) simulations indicate no statistical differences between 2050 and 1990. However, the high warming scenarios (+2.4°C) indicate a 25% and 15% increase in median annual evapotranspiration and irrigation demand, and decreases in future diffuse recharge by 53% and 98% and irrigation recharge by 47% and 29% at the eastern and western sites, respectively. These results indicate an important threshold between the low and high warming scenarios that if exceeded could trigger a significant bidirectional shift in 2050 hydroclimatology and recharge gradients. The bidirectional shift is that future northern High Plains temperatures will resemble present central High Plains temperatures and future recharge rates in the east will resemble present recharge rates in the western part of the northern High Plains aquifer. The reductions in recharge rates could accelerate declining water levels if irrigation demand and other management strategies are not implemented. Findings here have important implications for future management of irrigation practices and to slow groundwater depletion in this important agricultural region.

COVER:Collage showing multiple photographic images of surface nuclear magnetic resonance and aquifer-test data collection.Center top, a broad view showing trailer-mounted hydraulic pump, used in constant-discharge aquifer test. Project staff shown for general scale.Lower left, hydraulic head recording instrumentation in the observation wells, with wire reels (also shown in large photograph) approximately 25 centimeters in diameter.Lower right, surface nuclear magnetic resonance instrument. Boxes contain electronic equipment and are about 60 centimeters by 60 centimeters in plan view. For an overview of USGS information products, including maps, imagery, and publications, visit http://www.usgs.gov/pubprodTo order this and other USGS information products, visit

Uncertainty about the effects of land use and climate on water movement in the unsaturated zone and on groundwater recharge rates can lead to uncertainty in water budgets used for groundwater-flow models. To better understand these effects, a cooperative study between the U.S. Geological Survey and the Central Platte Natural Resources District was initiated in 2007 to determine field-based estimates of recharge rates in selected land-use areas of the Central Platte Natural Resources District in Nebraska. Measured total water potential and unsaturated-zone profiles of tritium, chloride, nitrate as nitrogen, and bromide, along with groundwater-age dates, were used to evaluate water movement in the unsaturated zone and groundwater recharge rates in the central Platte River study area. Eight study sites represented an east-west precipitation contrast across the study area-four beneath groundwater-irrigated cropland (sites 2, 5, and 6 were irrigated corn and site 7 was irrigated alfalfa/corn rotation), three beneath rangeland (sites 1, 4, and 8), and one beneath nonirrigated cropland, or dryland (site 3). Measurements of transient vertical gradients in total water potential indicated that periodic wetting fronts reached greater mean maximum depths beneath the irrigated sites than beneath the rangeland sites, in part, because of the presence of greater and constant antecedent moisture. Beneath the rangeland sites, greater temporal variation in antecedent moisture and total water potential existed and was, in part, likely a result of local precipitation and evapotranspiration. Moreover, greater variability was noticed in the total water potential profiles beneath the western sites than the corresponding eastern sites, which was attributed to less mean annual precipitation in the west. The depth of the peak post-bomb tritium concentration or the interface between the pre-bomb/post-bomb tritium, along with a tritium mass balance, within sampled soil profiles were used to estimate water fluxes in the unsaturated zone at three of the eight study sites: site 2 (irrigated), site 3 (dryland), and site 8 (rangeland). Estimates for recharge were about 68 millimeters per year [(mm/yr), post-bomb Groundwater dates from the shallowest monitoring wells were used to determine recharge rates at the water table. These rates generally were similar to recharge rates determined from tritium and chloride mass-balance methods. Groundwater recharge rates generally increased with well depth, and the deeper monitoring wells likely do not represent local recharge conditions but recharge from a regional flow system that receives recharge from distant sources. Overall, these data generally indicate that water movement within the unsaturated zone primarily is affected by spatial contrasts in mean annual precipitation and by the land use or land cover. The eight unsaturated-zone sites each generated unique, valuable datasets that likely will improve the understanding of water movement and recharge rates in the central Platte River valley.

Repeat sampling and age tracers were used to examine trends in nitrate, arsenic, and uranium concentrations in groundwater beneath irrigated cropland. Much higher nitrate concentrations in shallow modern groundwater were observed at both the Columbia Plateau and High Plains sites (median values of 10.2 and 15.4 mg/L as N, respectively) than in groundwater that recharged prior to the onset of intensive irrigation (median values of <1 and <4 mg/L as N, respectively). Repeat sampling of these well networks indicates that high nitrate concentrations in modern, shallow groundwater have been sustained for decades, posing a future risk to older, deeper groundwater used for drinking water. In fact, nitrate concentrations in older modern water (30−60 years since recharge) at the High Plains site have increased in the past decade. Groundwater irrigated areas in the Columbia Plateau tend to have higher nitrate concentrations in groundwater than continued...