The isotopic and hydrologic response of small, closed-basin lakes to climate forcing from predictive models: Application to paleoclimate studies in the upper Columbia River basin

Simulations conducted using a coupled lake-catchment, hydrologic and isotope mass-balance model indicate that small, closed-basin lakes in north-central Washington are isotopically sensitive to changes in precipitation, relative humidity, and temperature. Most notably, model simulations predicted inconsistent lake responses to precipitation changes due to differences in lake outseepage rates and surface area to volume (SA : V) ratios. Greater outseepage within model experiments resulted in increased sensitivity to changes in mean precipitation. Moreover, simulations suggest that, in lakes with appreciable outseepage, SA : V ratio changes resulting from lake-level variations control the direction of changes in lake water oxygen isotope composition (delta(18)O). Specifically, in lakes with a SA: V ratio that increases at higher lake levels, steady state delta(18)O values will increase in response to greater long-term average precipitation. These results suggest that closed-basin lakes with low outseepage rates will exhibit a transient isotopic response to stochastic variability in hydrologic forcing but will not strongly respond at steady state to variation in mean hydrologic conditions. Conversely, closed-basin lakes with appreciable outseepage will exhibit strong isotopic responses to both stochastic variability and variation in mean hydrologic conditions (i.e., mean precipitation, relative humidity, and temperature control of catchment hydrologic inputs to the lake). These relationships provide a mechanism for explaining inconsistencies in the isotopic responses of lakes within a given region to hydrologic forcing and demonstrate that semiquantitative models for describing the relationship between lake hydrologic and isotopic responses to climate variability are not appropriate for all closed-basin lakes.