VARIABILITY OF GROUNDWATER RECHARGE IN VARIABLE CLIMATES: A GROUNDWATER FLOW MODEL FOR THE SOUTHERN GREAT BASIN

Pumping of groundwater for agricultural, industrial and residential uses has caused groundwater levels to drop across much of the United States in the last half century. In the modern southern Great Basin, low precipitation and high temperature limits modern groundwater recharge. However, since the Last Glacial Maximum (21,000 years ago), precipitation has been much higher at times and temperature was much lower with warming toward the present. To assess the impact of past climate on groundwater levels we develop a first-order groundwater flow model coupled to closed-basin lake area to assess the response of groundwater recharge rate to climate variability in a closed basin. We use an energy mass balance framework constrained by lake area to control the rate of evaporation. We calculate the rate of groundwater flow by calculating local groundwater velocity.

Using our model we test the sensitivity of an idealized basin to temperature and precipitation forcings. We calculate the time for water table stabilization after a forcing in dry and wet basins, hot and cold basins, small and large basins, and steep and shallow basins. We determine the proportion of recharge rate variability that can be explained by precipitation change versus temperature change. Colder and wetter than modern conditions during the Last Glacial Maximum lead to faster-than-modern groundwater recharge. Finally, we model groundwater recharge for the southern Great Basin under these conditions and compare our idealized results of the maximum historic recharge rate to modern rates of groundwater extraction.