Rainfall and Snowmelt Runoff from Intermediate Elevation Watersheds

Many low and intermediate elevation watersheds in the Pacific Northwest receive both rain and snow precipitation during the winter months. This results in winter runoff events that are damaging to agricultural land and can also decrease the snow water storage limiting water availability in late spring and early summer. The winter and spring runoff from forested lands was studied at a snow research site where melt from the rain and snow events could be measured. Predictable lags occurred between time of melt and time of peak in stream hydrographs. The Kentucky Watershed Model was also used to evaluate the effects of precipitation type and amount on runoff patterns. Runoff in the early winter was not satisfactorily modeled. An agricultural watershed was used to study runoff and erosion resulting from rain on snow and rain on frozen ground. Soil loss was found to depend mainly on amounts of precipitation since runoff began, volume of runoff, change in snow cover and soil moisture conditions. Rain on frozen ground produced less erosion than com parable amounts of rain on bare unfrozen soil. A study of the water balance showed that precipitation was distributed approximately 27 percent to runoff, 57 percent to evapotranspiration and 16 percent to deep percolation. It also showed the Penman method of computing evapotranspiration was satisfactory when compared to soil moisture changes