Aspect control of water movement on hillslopes near the rain-snow transition of the Colorado Front Range

In the Colorado Front Range, forested catchments near the rain-snow transition are likely to experience changes in snowmelt delivery and subsurface water transport with climate warming and associated shifts in precipitation patterns. Snowpack dynamics are strongly affected by aspect: Lodgepole pine forested north-facing slopes develop a seasonal snowpack, whereas Ponderosa pine-dotted south-facing slopes experience intermittent snow accumulation throughout winter and spring. We tested the degree to which these contrasting water input patterns cause different near-surface hydrologic response on north-facing and south-facing hillslopes during the snowmelt period. During spring snowmelt, we applied lithium bromide (LiBr) tracer to instrumented plots along a north-south catchment transect. Bromide broke through immediately at 10- and 30-cm depths on the north-facing slope and was transported out of soil waters within 40days. On the south-facing slope, Br- was transported to significant depths only during spring storms and remained above the detection limit throughout the study. Modelling of unsaturated zone hydrologic response using Hydrus-1D corroborated these aspect-driven differences in subsurface transport. Our multiple lines of evidence suggest that north-facing slopes are dominated by connected flow through the soil matrix, whereas south-facing slope soils experience brief periods of rapid vertical transport following snowmelt events and are drier overall than north-facing slopes. These differences in hydrologic response were largely a function of energy-driven differences in water supply, emphasizing the importance of aspect and climate forcing when considering contributions of water and solutes to streamflow in catchments near the snow line. Copyright (c) 2012 John Wiley & Sons, Ltd.