The snowmelt and frozen soil component of the USDA-Water Erosion Prediction Project (WEPP) computer model is designed to simulate snow accumulations and density, snow drift, snowmelt, soil frost and thaw on an hourly basis. The snow accumulation routine predicts whether the hourly falling precipitation is rain or snow. The snowdrift component estimates the depth, density, and distribution of snow cover over a hillslope and watershed. The melt component estimates the amount of snowmelt occurring for any given hour during the day. The frost component estimates the extent of frost development and thawing over the winter period, as well as changes in soil water content and infiltration capacity of the soil during the winter period. Soil freezing and thawing influences the soil physical properties such as hydraulic conductivity, erodibility and soil water holding capacity. Freezing modifies the physical characteristics of soil, changing soil's ability to transmit or retain water, structural stability, and erodibility. The development of soil frost is the result of complex interactions of several primary factors, including soil characteristics, type of tillage and residue management, surface roughness, type of vegetative cover, duration and extent of freezing temperatures, and the extent and timing of snow cover. The freezing process itself modifies those soil physical properties that, along with temperature, determine the depth and duration of soil frost. The magnitude of soil changes that take place as a result of soil freezing depends on freezing temperature, soil water content at freezing, initial size of soil aggregates, and the number of freeze-thaw cycles that take place over winter. As a result, tillage-residue management combined with over winter frost action determines a soil's erodibility during winter thaw periods and from spring snowmelt to planting. The winter hydrology routine works on an hourly basis, however, the WEPP climate input file provides daily values for precipitation or breakpoint, maximum and minimum temperature, dew point temperature and incoming radiation. Therefore, hourly precipitation, temperature and radiation need to be calculated before simulating snow accumulation or melt and frozen soil. In addition, the time of day when precipitation is occurring is needed in order to differentiate between snow fall and rainfall, given hourly temperature.

M. R. Savabi, Hydrologist
USDA-ARS, NSERL & Purdue University
1196 Soil Building
West Lafayette, IN 47907
Phone: (317) 494-5051
Fax: (317) 494-5948
E-Mail: savabi@ecn.purdue.edu