The Walnut Gulch Experimental Watershed (WGEW) is located in southeastern Arizona, primarily in a high foothill alluvial fan portion of the San Pedro River watershed underlain by very deep (>400 m) Cenozoic alluvium regional groundwater aquifer [Moran et al., 2008]. WGEW spans 150 km2 in area and 1220–1830 m in elevation. Depth to groundwater within the WGEW ranges from ∼50 m at the lower end to ∼145 m in the central portion of the watershed. The climate over the watershed is semiarid, with substantial interannual and intra-annual variations and an average annual precipitation amount of 324 mm.