WHERE, WHEN, AND HOW MUCH WILL CLIMATE CHANGE AFFECT TEMPERATURES IN GROUNDWATER-FED ECOSYSTEMS?

Changes in groundwater temperature resulting from climate-driven boundary conditions (recharge and land surface temperature) can be evaluated using new analytic solutions of the groundwater heat transport equation. These steady-state solutions account for land-surface boundary conditions, hydrology, and geothermal heating, and can be used to identify the key physical processes that control thermal responses of groundwater-fed ecosystems to climate change, in particular (1) groundwater recharge rate and recharge temperature and (2) vadose zone heat conduction controlled by land surface temperature and vadose zone thermal properties. The relative contribution of each physical process can be quantified as a function of system geometry and groundwater recharge rate and temperature. In addition to these steady-state solutions, transient solutions of thermal response are used to estimate the time required for new thermal signals to arrive at groundwater-dependent ecosystems. The Medicine Lake Highlands, California, USA, and associated springs complexes are used to demonstrate the methods, providing quantitative estimates of the magnitude and timing of spring temperature changes as a function of position within the hydrologic system. High elevation springs will be dominated by recharge conditions, with most of the thermal response occurring within the first 50 years. Low elevation spring temperature changes will be a combination of recharge signal and the vadose zone conductive signal, with the fastest rate of spring temperature change from the vadose heat conduction occurring during the first 30 years, and the fastest rate of spring temperature change from advective transport occurring during the period 25-100 years.