Changes in both temperature and the amount and timing of precipitation have the potential to profoundly impact water balance in mountain ecosystems. Although changes in the amount of precipitation and potential evapotranspiration are widely considered in climate change scenarios, less attention has been given to how changes in climate or land cover may affect hydrologic partitioning and plant available water. The focus of this presentation is on how spatial transitions in ecosystem structure and temporal transitions in climate affect the fraction of precipitation potentially available to vegetation. In most temperate mountain environments winter snows are a significant fraction of annual precipitation and understanding the partitioning of snow and snow melt is critical for predicting both ecosystem water availability and stream flow under future climate scenarios. Spatial variability in net snow water input is a function of the interaction of snowfall, wind, and solar radiation with topography and vegetation structure. Integrated over larger scales these interactions may result in between 0% and 40% sublimation of winter snowfall before melt, effectively excluding this water from growing season water balance. Once melt begins, variability in the partitioning of snowmelt is driven by the rate of melt, and somewhat less intuitively, by the timing of snow accumulation the previous fall. Early accumulating snowpacks insulate soils and minimize soil frost increasing infiltration of melt the following spring. In contrast, later snowfall results in colder soils, more soil frost, reduced infiltration, increased runoff during melt, and reduced plant available water during the following growing season. This change in hydrologic partitioning, mediated by the timing of snowpack accumulation, results in lower evapotranspiration (ET) and net ecosystem exchange (NEE) the following spring. These findings suggest that abiotic controls on the partitioning of precipitation may exacerbate or attenuate the effects of climate change on mountain water balance.
P.D. Brooks, M.E. Litvak, A.A. Harpold, N.P. Molotch, J.C. McIntosh, P.A. Troch, X. Zapata (2011): Non-linear feedbacks between climate change, hydrologic partitioning, plant available water, and carbon cycling in montane forests . AGU Fall Meeting Presentations (Invited) Abstract GC34A-08..