SEASONALITY OF GROUNDWATER RECHARGE IN THE BASIN AND RANGE PROVINCE, WESTERN NORTH AMERICA

Groundwater recharge is the primary source of replenishment to aquifers. Aquifers are an important source of freshwater for human consumption and riparian area sustainability in semi-arid regions. It is critical to understand the current groundwater recharge regimes in groundwater basins throughout the Western U.S. and how those regimes might shift in the face of climate change, land use change and management manipulations that impact the availability and composition of groundwater resources. Watersheds in the Basin and Range Province are characterized by a bimodal precipitation regime of dry summers and wet winters. The horst-graben structure of these basins lends itself to orographic and continental precipitation effects that make mountain block and mountain front recharge critical components of annual recharge. The current assumption is that the relative contributions to groundwater recharge by summer and winter precipitation vary throughout the province, with winter precipitation dominating in the northern parts of the region, and summer monsoonal precipitation playing a more significant role in the south, where the North American Monsoon extends its influence. To test this hypothesis, stable water isotope data of groundwater and precipitation from sites in Sonora, Mexico and the U.S. states of California, Nevada, Utah, Arizona, Colorado, New Mexico, and Texas are examined to characterize and compare groundwater recharge regimes throughout the region. Preliminary stable water isotope results from a field site in the Rio San Miguel Basin in Sonora, Mexico indicate that groundwater is composed primarily of summer monsoon precipitation, in contrast to more northern basins where winter precipitation is the dominant source of basin groundwater.