The Influence of Precipitation Phase on Hydrograph Form

Mountainous watersheds are characterized by large relief, non-uniform elevation distribution and exhibit basin wide variations in vegetative cover and land use. This topographic complexity leads to diverse atmospheric conditions and differences in precipitation magnitude and phase over small spatial scales, resulting in non-uniform hydrologic fluxes. To help elucidate these complexities and to discern how differences in precipitation phase influence hydrograph form and streamflow patterns, we characterize these parameters in twelve tributaries to the Salmon River, Idaho. Study watersheds are bounded within distinct elevation zones allowing classification as either rain dominated, mixed rain and snow or snow dominated. Rain dominated watershed bounded within low elevations receive 473 mm of precipitation annually; on average 80% of precipitation falls as rain and 20% as snow. Snow accumulation is minimal and of brief in these watersheds. Streamflow patterns for the 2009 and 2010 water years exhibited low peak, median and baseflow yields compared to mixed rain and snow and snow dominated watersheds. In addition, rainfall and snowmelt events occurred earlier in the year and were of short duration and low magnitude. Mixed rain and snow watersheds bounded within mid-elevations typically receive 862 mm of annual precipitation; 40% falls as rain and 60% falls as snow. These watersheds accumulate substantial snowpack and exhibit the greatest variability precipitation phase. Streamflow in mixed rain and snow watersheds exhibited greater peak, median and baseflow yields than rain dominated watersheds. Snowmelt runoff in this region exhibited the greatest complexity because of numerous of rain-on-snow and temperature-driven snowmelt events throughout the spring and early summer. Snow dominated watersheds are entirely within high elevations and receive a mean annual precipitation of 984mm; 30% falls as rain and 70% as snow. Higher total precipitation in the snow dominated region results in it having the greatest snow accumulation as well as the highest peak, median and baseflow yields. Snowmelt events were of longer duration, higher magnitude and occurred later in the year than in mixed or rain dominated watersheds. Observations from this study demonstrate that the overall streamflow patterns and the timing, duration, and magnitude of hydrologic events are sensitive to precipitation phase within mountainous watersheds. Future changes in air temperature will increase the snowline elevation, increase the probability of rain-on-snow events in mid and high elevations (complicating flood forecasting) and altering hydrograph form in all domains. By understanding the differences between the three precipitation domains described above, we can better anticipate how changes in climate will alter hydrologic, geomorphic and ecological processes occurring near the boundaries between precipitation domains.