Groundwater response to climate variability and land cover change is important for sustainable management of water resources in the Southwest US. Global Climate Models (GCM) project that the region will dry in the 21st century and the transition to a more arid climate may be under way. In semiarid Basin and Range systems, this impact is likely to be most pronounced in Mountain System Recharge (MSR), a process which constitutes a significant component of recharge in these basins. Despite the importance of MSR the physical processes that control MSR, and hence the climate change impacts, have not been fully investigated because of the complexity of recharge processes in mountainous catchments and limited observations. In this study, methodologies were developed to provide process-based understanding of MSR based on empirical and data-driven approaches. For the empirical approach, a hydrologically-based seasonal ratio the Normalized Seasonal Wetness Index (NSWI) was developed. It incorporates seasonal precipitation variability and temperature regimes to seasonal MSR estimation using existing empirical equations. Stable isotopic data was used to verify recharge partitioning. Using the NSWI and statistically downscaled monthly GCM precipitation and temperature data, climate change impacts on seasonal MSR are evaluated. Second, a novel data-based approach was developed to quantify mountain block recharge based on the catchment storage-discharge (S-Q) relationships and informed by isotopic data. Development of S-Q relationships across the Sabino Creek catchment, Arizona, allowed understanding of MBR dynamics across scale.Two ArcGIS desktop applications were developed for ArcGIS 9.2 to enhance recharge and evapotranspiration (ET) estimation: Arc-Recharge and RIPGIS-NET. Arc-Recharge was developed to quantify and distribute recharge along MODFLOW cells using spatially explicit precipitation data and a digital elevation model. RIPGIS-NET was developed to provide parameters for the RIP-ET package and to visualize MODFLOW results. RIP-ET is an improved MODFLOW ET module for simulating ET. RIPGIS-NET improves alluvial recharge estimation by providing spatially explicit riparian ET estimates. Using such tools and the above methods improves recharge and ET estimation in groundwater models by incorporating temporally and spatially explicit data and hence the assessment of climate variability and land cover change on groundwater resources can be improved.

<div> <p>Despite its location in the rain shadow of the southern Sierra Nevada, the Panamint Range within Death Valley National Park, CA hosts a complex aquifer system that supports numerous springs. These springs, in turn, support unique groundwater-dependent ecological communities. Spring emergences range in elevation from 2434 m above sea level (within the mountain block) to 77 m below sea level (in the adjacent basins). Waters were collected from representative Panamint Range springs and analyzed for environmental isotopes and geochemical tracers to address the following questions: 1) What is the primary source of recharge for the springs? How much recharge occurs on the Panamint Range? 2) What groundwater flowpaths and geologic units support springflow generation? and 3) What are the residence times of the springs? The stable isotopic composition (δ¹⁸O and δ²H) of spring water and precipitation indicate that localized high-elevation snowmelt is the dominant source of recharge to these perennial springs, though recharge from rainfall is not wholly insignificant. Geochemical evolution was evaluated using principle component analysis to compare the concentrations of all major spring cations and anions in a multidimensional space and group them according to dominant geochemical signatures. These resulting geochemical groups are controlled primarily by topography. The Noonday Dolomite and other carbonate units in the range are identified as the water-bearing units in the mountain block based on the ⁸⁷Sr/⁸⁶Sr of spring waters and rock samples. These units also offer higher hydraulic conductivities than other formations and are chemically similar. Radiocarbon- and ³H derived residence times of these spring waters range from modern to approximately 1840 years, with the shortest residence times at higher altitudes and Hanaupah Canyon and increasing residence times with decreasing altitude. This residence time-altitude relationship is likewise likely topography-driven though there are significant disparities in mountain block storage between the various canyons of the range resulting in variable residence times between drainages. Lower Warm Springs A and B, however, are the exceptions to this trend as they emerge at lower altitudes (750m above sea level) and are likely driven by the transport of groundwater to the surface along faults which increases both the temperature and groundwater residence times of waters from these springs. Benthic macroinvertebrates and benthic and planktonic microbes were also sampled for each spring studied. BMI and microbial community structure in the Panamint Range is likewise topography-controlled with more tolerant communities at lower elevations (within more chemically evolved waters) and less tolerant species in the unevolved waters at higher elevations.</p></div>

Direct mountain front recharge (MFR), water table recharge at the base of the mountain front, was evaluated on the arid (<250 mm/yr precipitation) Silver Island Mountains by comparing mountain precipitation to groundwater response. Direct MFR contributions were assessed on two catchments, one bedrock (i.e., mountain block) dominated and the other alluvial fan (i.e., mountain front) dominated. Catchment precipitation and shallow groundwater levels at each catchment outlet were measured for a 24 month period beginning October 2005. This time period captured one complete hydrologic cycle (December 2005-February 2007) for which annual and seasonal direct MFR rates were calculated. Annual direct MFR was calculated using a modified version of the water table fluctuation (WTF) method as 0.015-0.016% of precipitation on both catchments, with seasonal variations of 0% in summer up to 0.023% in winter, spring and fall. Seasonal direct MFR contributions are similar on the bedrock and the alluvial fan dominated catchments, with a notable exception during fall 2006 when direct MFR was twice as effective on the bedrock dominated system than on the alluvial fan dominated system (0.022% and 0.011% of precipitation, respectively). Darcy's law calculations show similarly low annual direct MFR contributions (0.013-0.032% of precipitation) as those calculated by the WTF method. Calculated direct MFR is 10% or less than typical calculated combined MFR (near surface recharge and deep underflow from the mountain block) for similar terrains and climates, and is only 3.5% of the combined MFR for the Silver Island Mountains as calculated by the Maxey-Eakin model. However, based on total recharge to the adjacent playa, it is apparent that the Maxey-Eakin model overestimates combined MFR, and the small calculated direct MFR is at least 50% of combined MFR. Despite some uncertainty in the numerical results, several patterns are evident in the data. The data show that direct MFR occurs in response to small rainfall events throughout much of the year, and that snowmelt is not necessary to produce direct MFR. The data also show that direct MFR responds more quickly and flushes through the system faster on the alluvial fan catchment than on the bedrock catchment.