Assessing the Sensitivity of a Volcanic Aquifer-Spring System to Climate Change

Abstract

The Fall River Spring system, located in northeastern California, is home to some of the largest first order springs in the U.S., with an average discharge of 34m3/s, and are a valuable water resource economically and ecologically. The goal of this thesis is to assess the possibility that climate change (warming and transition from snow to rain) will impact system recharge and to enhance the conceptual understanding of the aquifer-spring system. Spring discharge sampling for isotopic analysis of δ18O and δD supplements past work and provides insight to the location of the recharge area. Chlorofluorocarbon sampling indicates an apparent groundwater age of 33 – 46 years. Calibration of a steady state MODFLOW model suggests a bulk hydraulic conductivity on the order of 10^2 m/d. Output from the Basin Characterization Model for a projection of climate change reveals a decrease in average groundwater recharge from 44.1 cm/yr to 39.0 cm/yr across the watershed from the historic to end of century time periods. This response reflects an overall decrease in precipitation, increase in the proportion of precipitation that becomes actual evapotranspiration and decrease in snowfall.