University of California Water Resources Center

A field and model investigation has been performed to assess the potential of

subsurface temperature (T) measurements for tracing groundwater flow in montane areas.

Field measurements included monitoring of soil T at numerous sites and measurement of

T-profiles in many wells and boreholes. Field data collection for thermal tracing of

groundwater flow is inexpensive and easy to perform relative to standard geophysical

techniques for groundwater flow characterization, given wells that are accessible for Tprofile

measurement. Modeling investigations include development of a novel,

conceptually simple ‘black box’ subsurface thermal energy balance approach. This robust

model approach can be used together with T-profile data to bracket the rate of mountainfront

groundwater recharge.

Results of the soil T monitoring investigation show a wide range in summer and

mean annual soil T between different soil sites. Factors governing the observed inter-site

differences in soil T were identified. A model for spatial variability in mean soil T was

developed, showing the feasibility of creating maps of mean surface T in the Sierra

Nevada using standard GIS land surface attributes. Accurate knowledge of surface T is

needed in modeling subsurface heat and water flow, to distinguish spatial changes in T

due to groundwater flow from those due to heterogeneity in surface T. T-profiles

measured in the Tahoe Basin region indicate that there are substantial areal differences in

the rate of high elevation deep bedrock groundwater recharge, and in valley bedrock

discharge. Surface T and T-profile measurements together are shown to be useful in

defining valley basin-fill aquifer recharge sources. Subsurface heat flow patterns are

altered in distinct ways, as reflected in T-profiles, by recharge into the top (soil

percolation down to water table), sides (mountain-front recharge) and bottom (mountainblock

recharge) of montane valley basin-fill aquifers.