Modification and testing of DRAINMOD for freezing, thawing and snowmelt

The field hydrology model DRAINMOD was modified to include freezing and thawing, and snowmelt components. Based on daily hydrological predictions of the original model, the modified DRAINMOD numerically solves the heat flow equation to predict soil temperature. When freezing conditions are indicated by below zero temperatures, the model 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/snow dividing base temperature. Snow is predicted to accumulate on the surface until air temperature rises above a snowmelt base temperature. Daily snowmelt is added to rainfall, which may infiltrate or run off depending on soil freezing condition.; The modified DRAINMOD predictions of soil temperature agreed well with field observations at Plymouth, NC, Truro, Nova Scotia and Lamberton, MN. 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 successful and several snow events were predicted as rainfall and vice versa. Compared with the original version of DRAINMOD, the modified version predicts fewer drainage flow events in winter months because of snow accumulation on the surface. Subsurface drainage and/or surface runoff resulting from snowmelt are predicted when air temperature rises and the snow melts and the soil begins to thaw.; Field observations of water table depth and subsurface drainage from Carsamba, Turkey; Truro, Nova Scotia and Lamberton, Minnesota were used to test the modified DRAINMOD. At Carsamba, Turkey, the modified DRAINMOD correctly predicted the timing and magnitude of drainage events results from snowfall accumulation and subsequent snow melt. Although the variable weather pattern and nature of rainfall/snowfall mixture at the Nova Scotia site made it difficult to describe hydrology during winter months, DRAINMOD predicted the timing and magnitude of most flow events. Continuous long-term model simulations at Lamberton, Minnesota were generally in good agreement with drainage measurements during spring and summer months.; Soil temperature predictions from the modified DRAINMOD were introduced into DRAINMOD_N to replace the analytical solution for daily soil temperature. Compared with the analytical solution, the improved soil temperature predictions resulted in generally higher predicted net mineralization and N loss in subsurface drainage at a site near Plymouth, NC.; Based on hydrological prediction of the modified DRAINMOD for Lamberton, MN, DRAINMOD_N was used to simulated nitrate nitrogen concentration at the site. Model predictions of NO₃-N concentration in subsurface drainage are generally in agreement with field measurements. A long term simulation of N loss in subsurface drainage at the Lamberton site indicate that rainfall pattern is an important factor in causing large amount of nitrogen loss from agricultural fields to the receiving waters. Wet years following dry years result in significant nitrogen loss due to the build-up of nitrogen concentration in the soils in dry years. Reducing nitrogen fertilizer application rate and drainage intensity may significantly reduce N output from fields.