DRAINMOD modifications for cold conditions

The field hydrology model DRAINMOD was modified to include freezing and tha wing, and snowmelt components. Based on daily hydrologic predictions of the original model, the modified DRAINMOD numerically solves the heat flow equ ation to predict soil temperature. When freezing conditions are indicated b y below zero temperatures, the model calculates ice content in the soil pro file and modifies soil hydraulic conductivity and infiltration rate accordi ngly Recorded precipitation is separated as rain and snow when daily averag e air temperature is above or below a rain/snow dividing base temperature. Snow is predicted to accumulate on the ground until air temperature rises a bove a snowmelt base temperature. Soil surface temperature is recalculated when snow cover exists. Daily snowmelt wafer is added to rainfall, which ma y infiltrate or run off depending on soil freezing condition. The modified DRAINMOD predictions of soil temperature agreed well with field observation s at Plymouth, North Carolina, Truro, Nova Scotia, and Lamberton, Minnesota . Assuming air temperature as the soil surface boundary condition increased the variability of soil temperature predictions at shallow depths, agreeme nt with field measurements was still good. The method of using average air temperature as an indicator to separate snow and rain worked very well for Carsamba, Turkey At Truro, Nova Scotia, however the method was not as succe ssful, and several snow events were predicted as rainfall and vice versa. C ompared with the original version of DRAINMOD, the modified version predict s fewer drainage flow events in winter months because of snow accumulation on the surface. Subsurface drainage and/or surface runoff resulting from sn owmelt are predicted when air temperature rises, the snow melts, and the so il begins to thaw.