Chapter 3. WINTER HYDROLOGY

The winter hydrology component of the WEPP computer model is designed to simulate snow accumulation and density, snowmelt, and soil frost and thaw, all on a hourly basis. The snow accumulation routine predicts whether the hourly falling precipitation is rain or snow, as well as changes in snow depth and density. 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. Winter hydrologic processes are an important part of the hydrology of watersheds located mainly north of 40 degrees latitude in the Northern hemisphere and south of 40 degrees latitude in the Southern hemisphere. Soil freezing and thawing influence the soil physical properties such as hydraulic conductivity, erodibility and soil water holding capacity. Freezing modifies the physical characteristics of soil, changing its ability to transmit or retain water (Benoit and Bornstein, 1970; Benoit and Mostaghimi, 1985; Campbell et al., 1970; Loch and Kay, 1978), its structural stability (Benoit, 1973; Mostaghimi et al., 1988), and its erodibility (Bisal and Nielsen, 1967). 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 takes 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 (Benoit et al., 1986).