Hydrologic Storage Functions of an Alpine Talus: Linking Field Observations with Numerical Simulations

Abstract:

Alpine watersheds source major rivers in many regions of the world and thereby supply essential water for irrigation, human consumption, and hydroelectricity. Coarse depositional units in alpine catchments, such as taluses and proglacial moraines, are thought to store and transmit significant volumes of groundwater and thus buffer flooding during snowmelt and augment stream discharge during the dry season when water supply is critical. This study focuses on the hydrologic functions of an alpine talus unit within the Lake O’Hara watershed in the Canadian Rockies of British Columbia. Previous field investigations indicate that the talus exhibits very high hydraulic conductivity, low storage capacity, and a fast hydrograph recession with an exponential decay of approximately 1 d^-1. Despite the low storage and flashy response to snowmelt input, the isotopic signature of the stream fed by the talus indicates that the discharging water is predominantly pre-event water.

Herein we investigate internal processes controlling the bulk hydrologic functioning of this talus unit using a finite element model of coupled subsurface water flow and transport. The hydrologic parameters of the model are calibrated to achieve congruence between the simulated and observed response (lag time) to the snowmelt input. The transport equation is employed to simulate the age of discharging water and investigate how the mean transit time is influenced by hydraulic conductivity, boundary conditions, and talus geometry (e.g., step features in the bedrock plane beneath the talus). The hydraulic properties and talus boundaries are adjusted within the model to see how other talus units with differing geologic composition and geometry may store and transmit water and thus attenuate flood stage or enhance baseflow in other alpine catchments. Also, earlier snowmelt infiltration is considered to examine how a warming climate may influence the timing and magnitude of talus discharge to alpine streams.