Ephemeral-streamflow Induced Focused Recharge in the Desert Southwest (US)

Abstract:

Multiyear studies examined meteorologic and hydrogeologic controls on ephemeral streamflow and focused groundwater recharge at 8 sites across arid and semiarid southwestern regions of the US. Intensive data collection targeted the Great Basin, Mojave Desert, Sonoran Desert, Rio Grande Rift, and Colorado Plateau physiographic areas (USGS Prof. Pap. 1703-C). During the study period (1997–2002), the region went from wetter than normal conditions associated with a strong El Niño climatic pattern (1997–1998) to drier than normal conditions associated with a La Niña climatic pattern. The 1997–1998 El Niño, the strongest in the modern instrumental record, roughly doubled precipitation at the Great Basin, Mojave Desert, and Colorado Plateau study sites. Precipitation at all sites thereafter trended sharply lower, producing moderate- to severe-drought conditions by the end of the study. Streamflow in regional rivers indicated diminishing groundwater recharge, with annual-flow volumes declining to levels of only 10–46% of their respective long-term averages by 2002. Local streamflows showed higher variability, reflecting smaller scales of integration (in time and space) within study-site watersheds. Toward the end of study, extended periods (9–15 months) of zero or negligible flow were observed at half the sites. Summer monsoonal rains generated the majority of streamflow and associated recharge in the Sonoran Desert sites and the more southerly Rio Grande Rift site, whereas winter storms and spring snowmelt dominated the northern and westernmost sites. Proximity to moisture sources and meteorologic fluctuations, in concert with orography, largely control the generation of focused groundwater recharge from ephemeral streamflow, although other aspects (geology, soil, and vegetation) were also identified as factors. Estimates of annual focused infiltration for the research sites ranged from about 10⁵–10⁷ cubic meters from contributing areas that ranged from 26–2,260 square kilometers.