A conceptual and modeling framework to investigate groundwater recharge in arid/semi-arid environments

A conceptual and modeling framework to study groundwater recharge in arid/semi-arid flow systems is developed using a fully-integrated, physically-based, hydrologic code and GIS techniques. The framework is constructed by modeling recharge in a well-studied, large basin flow system (>20,000 km2), located in northeastern Arizona, using available unsaturated zone and integrated hydrologic flow codes and then conducting a sensitivity analysis. Eleven different mechanisms that describe recharge within the basin are identified and defined in this study as hydrogeomorphic units (HGU). The relative importance and dynamics of recharge in each of these HGUs is assessed using several years of high-resolution temporal climate input. Next, a fully integrated basin flow model is developed using 75 years of daily climate data (1920 to 1995) from NWS stations. Daily climate data are disaggregated to hourly distributions using available higher temporal resolution data to evaluate the spatial distribution of basin recharge. Initial simulations proved computationally inefficient and output was difficult to interpret. An important step in the proposed framework involves systematically developing increasingly complex sub-regional scale and partially-coupled flow process models to better understand the sensitivity of recharge to individual processes and parameters. Results of these sensitivity analyses indicate that good spatial distributions of saturated hydraulic conductivity and high temporal resolution of precipitation and potential evapotranspiration are essential for estimating realistic basin recharge rates. Predicted basin recharge rates (30 to 120 mm/year) vary spatially and temporally, but are similar to other reported rates for large semi-arid/arid basins (3.5 to 26.6% of annual precipitation). Simulated flows in HGUs, however, suggest that recharge in coarser regional integrated model grid cells is likely under-predicted where significant concentrated surface runoff, or ponding occurs. Results also suggest that traditional, empirical recharge methodology (like Maxey-Eakin) do not consider important parameters and can yield poor spatial and temporal basin recharge estimates.; Integrated hydrologic models offer tremendous potential in studying complex, integrated hydrologic conditions in arid/semi-arid systems, so that fundamental basin-scale mechanisms and processes that govern recharge can be better understood. However, several limitations were identified and must be considered in their development and application.