The isotopic and hydrologic response of small, closed-basin lakes to climate forcing from predictive models: Simulations of stochastic and mean-state precipitation variations

A hydrologic and isotope mass-balance model is applied to two small, closed-basin lakes, Castor and Scanlon, in north-central Washington to describe the influence of hydroclimatic forcing on lake hydrologic and isotopic evolution. Simulations of lake responses to the combined effects of stochastic variability (i.e., random interannual fluctuations) and long-term (i.e., multidecade to century), mean-state changes in precipitation were conducted using 300 yr of randomly generated precipitation data as model inputs. Simulation results demonstrate that average (long-term) closed-basin, lake-water, oxygen-isotope values are dependent largely upon lake-water outseepage (i.e., subsurface outflow) rates, with lower (higher) outseepage resulting in decreased (increased) isotopic sensitivity to long term precipitation changes, and that the lake basin surface-area-to-volume (SA : V) ratio changes with depth influence the direction of the isotopic response. Simulation results also suggest that, as average lake volume decreases as a consequence of decreasing mean-state precipitation amounts, interannual lake-water isotopic variability in response to stochastic forcing will increase. Conversely, as the average lake volume increases in response to increasing mean-state precipitation amounts, interannual lake-water isotopic variations associated with stochastic forcing will decrease. Additional model experiments demonstrate that increased (decreased) variance in precipitation leads to increased (decreased) water volume within lakes over the long term, which (if stochastic variance changes are large enough) could result in decreased (increased) lake-water oxygen-isotopic sensitivity to stochastic precipitation.