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Groundwater recharge (GR) controls vegetation, geomorphology, groundwater, wetlands and surface flow, and ultimately, the ecology and economics of semi-arid regions. Therefore, it is critical to assess hydroclimate model scenarios and the uncertainty in future GR to force regional groundwater models. We use basic statistics of downscaled Global Circulation Model (GCM)-projected cumulative potential GR ( GR p ) for selecting representative projections. Cumulative GR p is the net recharge (difference between precipitation ( P ) and evapotranspiration ( ET ) rates) over the projection period. The approach is illustrated with an example in the Nebraska Sand Hills (NSH), the largest dune region in the Western Hemisphere, where sandy soils are not conducive to overland flow. Changes in decadal-average GR p at 1/8° (∼12-km) scale were estimated from spatially downscaled, bias-corrected temperature and P output from 16 commonly used GCMs for years 2010 to 2099. These changes accounted for three greenhouse gas emissions scenarios, and projections were subsequently used as input to the Variable Infiltration Capacity (VIC) land surface hydrology model. For each of the 48 GCM/VIC hydroclimate projections, cumulative GR p was calculated and averaged over the study area. Three projections (those with cumulative average GR p nearest the median and ±1 standard deviation) were selected as representing Median, Wet and Dry conditions. These projections allow for rapid screening of the sensitivity of regional groundwater models, using readily available downscaled GCM-projected climate changes, thereby optimizing modeling efforts. Future GR p was calculated for the NSH using the selected GR projections by adjusting the 2000–2009 baseline GR p estimates at 1-km scale. The latter was inferred from a previously calibrated groundwater model, with ET based on remote sensing (MODIS) temperature data, and matching regional baseflows. In the NSH by 2099, the Median projection indicates an increase in GR p of 3 mm/yr (+5%) relative to the 2000–2009 baseline of 52.6 mm/yr. The Wet projection has an average increase of 22 mm/yr (+42%), and the Dry projection shows an average decrease of 15 mm/yr (−29%), relative to the baseline. Effects of projection period duration and time-step averaging on selection of GR projections with this approach are discussed. The new detailed GR p projections clarify varying trends of past large-scale analyses of the Northern High Plains region and indicate the possibility for substantial future changes in the NSH hydrologic system. This approach can be extended to other arid-to-humid regions with available GCM hydroclimate projections. [ABSTRACT FROM AUTHOR]

Summary Deep drainage of water below plant root zones (potential groundwater recharge) will become groundwater recharge ( GR ) after a delay (or lag time) in which soil moisture traverses the vadose zone before reaching the water table. Depending on the thickness of the vadose zone, the magnitude of deep drainage, and soil hydraulic properties, lag times will vary broadly, exceeding decades to centuries in semi-arid and arid environments. Yet, studies of future climate change impacts to GR have typically avoided focusing on impacts beyond 100 years and often neglect to consider lag effects caused by the vadose zone. We investigate the effects of vadose zone lag time and potential 21st century climate change on the spatial distribution and timing of GR throughout the semi-arid Nebraska Sand Hills (NSH) region (∼50,000 km 2 ). We propose a simple and rapid quantitative approach for assessment of the groundwater system response time to changes in climate. Understanding of such effects is needed for groundwater modeling, analysis of climate change impacts on groundwater, and the effective management and sustainability of future water resources. Lag time estimates are made using the pressure-based vertical velocity of soil moisture changes, equivalent to a kinematic wave approximation of Richards’ equation. The underlying assumptions (unit hydraulic gradient and relatively slow changes in climate) are supported by observations in the High Plains aquifer region, encompassing the NSH. The analysis relies on four sources of input data, including: Spatially distributed high resolution (1.1-km) GR rates (as the difference in 2000–2009 mean precipitation and evapotranspiration—estimated using a novel MODIS-based approach); thickness of the vadose zone—based on 30-m digital elevation models of the land surface and water table elevations; statistically downscaled estimates of future (2010–2099) potential GR rates from two hydroclimate model projections (WRCP CMIP3) with opposing GR trends; and estimates of hydraulic conductivity and vertical velocity of the vadose zone pressure changes using the van Genuchten–Mualem equation. We found that vadose zone lag times (with a mean GR rate of 78 mm/yr for 2000–2009) have an estimated spatial mean of 5.28 yrs (vertical velocity of 5.68 m/yr). End-of-century lag times (with mean potential GR rates of 128 and 61 mm/yr for 2090–2099) have estimated spatial means of 3.78 and 6.55 yrs (vertical velocities of 8.13 and 4.69 m/yr) for wet and dry scenarios, respectively. Findings suggest that the highly conductive materials (primarily sand deposits) and high potential GR rates in the NSH lead to relatively quick drainage of vadose zone water on the order of several years (less than a decade) for the entire 21st century. However, considerable spatial variations of lag times exist due to gradients in potential GR rates and the highly variable vadose zone thickness in the NSH. [ABSTRACT FROM AUTHOR]

Potentiomanometers (PMs) are commonly used to determine flux directions across interfaces between surface waters and aquifers. We describe a complementary function: estimating small-scale hydraulic conductivity ( K) in a lakebed, using the constant-head injection test (CHIT) by Cardenas and Zlotnik (2003) with the PM designed by Winter et al. (1988). A piezometer with a small screen is inserted into the lakebed. Local head potential is obtained by measuring the head difference between the test point and the aquifer interface. The piezometer is then used for water injection. This technique is illustrated by measurements taken from Alkali Lake in the Sand Hills, Nebraska, United States. Lakebed K and seepage fluxes ranged from 0.037 to 0.090 m/d and Darcy velocities ranged from 0.004 to 0.027 m/d. Results were consistent with the supplementary data gathered using a modified CHIT and a cone penetrometer. The compact size of the device and the small volumes used for injection enable this method to estimate lakebed K values as low as 0.01 to 0.1 m/d, a range seldom explored in lake-aquifer interface systems. [ABSTRACT FROM AUTHOR]

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A permeameter was designed and constructed, and its theory of operation was developed; it has a subsurface probe for internal permeability measurements in poorly consolidated sediments. It was used to measure the permeability of sand at five depths in the upper 1.3 m (4.3 ft) of structural features on a large vegetated composite crescentic dune in the Nebraska Sand Hills. Permeability measurements were taken at 34 locations on the dune's steep leeward face, its long windward slope, fans below the leeward face, blowouts, peaks, crests, transverse ridges, benches, and the heads of gullies. Correlation of the permeabilities with the morphology of the dune is illustrated by a plot of the depth-averaged permeabilities and dune profile along a transect perpendicular to the lee face. Some of the permeabilities do not match the values anticipated on the basis of the theory of sedimentation (and experimental results) for active crescentic dunes. Although the large-scale features of the dune studied were evidently because of the original eolian sedimentation, the permeability was found to be strongly influenced by dune degradation processes involving running water and animal activity. [ABSTRACT FROM AUTHOR]