Drainmod Modifications for Cold Conditions

The field hydrology model DRAINMOD was modified to include freezing and thawing, and snowmeltcomponents. Based on daily hydrologic predictions of the original model, the modified DRAINMOD numerically solvesthe heat flow equation to predict soil temperature. When freezing conditions are indicated by below zero temperatures, themodel calculates ice content in the soil profile and modifies soil hydraulic conductivity and infiltration rate accordingly.Recorded precipitation is separated as rain and snow when daily average air temperature is above or below a rain/snowdividing base temperature. Snow is predicted to accumulate on the ground until air temperature rises above a snowmeltbase temperature. Soil surface temperature is recalculated when snow cover exists. Daily snowmelt water is added torainfall, which may infiltrate or run off depending on soil freezing condition. The modified DRAINMOD predictions ofsoil temperature agreed well with field observations at Plymouth, North Carolina, Truro, Nova Scotia, and Lamberton,Minnesota. Assuming air temperature as the soil surface boundary condition increased the variability of soil temperaturepredictions at shallow depths, agreement with field measurements was still good. The method of using average airtemperature as an indicator to separate snow and rain worked very well for Carsamba, Turkey. At Truro, Nova Scotia,however, the method was not as successful, and several snow events were predicted as rainfall and vice versa. Comparedwith the original version of DRAINMOD, the modified version predicts fewer drainage flow events in winter monthsbecause of snow accumulation on the surface. Subsurface drainage and/or surface runoff resulting from snowmelt arepredicted when air temperature rises, the snow melts, and the soil begins to thaw.