WHERE DOES GROUNDWATER IN MOUNTAINOUS TERRAIN COME FROM?: THE IMPORTANCE OF HIGH-ELEVATION PRECIPITATION AND SNOW AS CONTRIBUTORS TO MOUNTAIN GROUNDWATER

Groundwater is a significant resource in the arid West, and its sources must be understood to allow proper management. Mountain ranges in the West are known to be major contributors to alluvial basin aquifers. Recent work shows that in many cases, mountains can store and transmit significant quantities of water; sub-water-table transfer from ranges to adjacent basin aquifers can be a significant part of the mountain-to-basin contribution. Groundwater baseflow often makes up a large portion of streamflow, so even stream infiltration at the mountain front can be related to mountain groundwater.

In the Chiricahua Mts. (AZ), recharge is controlled by a combination of geology, precipitation amount, and precipitation type. Much of the recharge in the range takes place above 2300 m ASL. Because snowpack only develops above 2300 m, it appears that snowpack accumulation significantly enhances recharge. The isotopic similarity of low-elevation recharge (δ¹⁸O≈-9.0‰,δD≈-65.5‰) and high-elevation precipitation (δ¹⁸O≈-10.6‰, δD≈-67.0‰), show that “re-recharge” is an important process.

Three other sites in Arizona and New Mexico show many of the same characteristics: 40-70% of groundwater recharge derived from snow (snowfall makes up 25-49% of average annual precipitation), and high-elevation precipitation playing a major role. In conjunction with the work of Szecsody et al (1983) and Winograd et al (1998) in the Carson and Spring ranges (NV), our research suggests that snowmelt may be responsible for the majority of recharge in many Western ranges, and may often make up a significantly higher percentage of recharge than suggested by the proportion of snow in average annual precipitation. In many of the ranges studied, the highest zones contribute more recharge than the lower zones. Although high zones have less surface area than lower zones, factors such as decreased ET, higher amounts of precipitation, and higher proportions of snow allow higher recharge per unit area.