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Estimation of groundwater recharge by Chloride Mass Balance (CMB) in the southwestern United States
Proceedings of the 2019 Mississippi Water Resources Conference

Year: 2019 Authors: Mukherjee A., Holt R.M., O'Reilly A.M.

Groundwater recharge is defined as water flux across the water table from the unsaturated to saturated zone. Recharge is a critical factor in groundwater resource management in arid and semiarid regions and often the most difficult component of the hydrologic cycle to quantify. Recharge is very small in arid and semiarid regions and thus needs to be carefully and accurately estimated. The objective of this study is to calculate recharge in the arid and semiarid areas of the southwestern United States using the chloride mass balance (CMB) method based on numerical simulation. Estimation of recharge using a natural tracer such as chloride (Cl) has the significant advantage of increased sensitivity at lower recharge rates. The HYDRUS 1D computer code was used to simulate water flow and solute transport for four sites: High Plains (HP), Eagle Flat (EF), Hueco Bolson (HB), and Amargosa Desert (AD). The entire unsaturated zone, extending from land surface to water table, was simulated at each site in this simplified modeling approach. Assigned conditions are assumed to be representative of long-term average conditions, circumventing the diurnal and seasonal variation of complex water flux dynamics, especially in the upper part of the domain near the surface. All Cl profiles are bulge shaped with low concentrations near land surface, increasing to peak concentrations at depth assumed to be root zones, and then decreasing to a more or less constant concentration with depth. Recharge fluxes were calculated based on Cl concentrations beneath the bulges. Recharge rates calculated using the CMB method from the four sites are 3.67 mm/yr for HP, 0.03 mm/yr for EF, 0.1 mm/yr for HB, and 8.64 mm/yr for AD, which represent <1%, <0.01 %, <0.05 %, and 8% of mean annual precipitation of each respective site.

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