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The potential of winter cover crops to reduce nitrate nitrogen (nitrate-N) loading from artificial subsurface drainage systems is being researched in the Northern Corn Belt. A soil-plantatmosphere simulation model, RyeGro, was developed to investigate the efficacy of winter rye over the long term. This paper describes the soil water balance and subsurface drainage component of RyeGro. The soil water component provides an estimate of infiltration, average soil moisture content, and subsurface drainage over the fall-spring drainage season. The model was purposely kept simple, with the soil represented as a series of tanks. Holtans infiltration scheme was used to divide precipitation into runoff and infiltration components. Model calibration was performed with data from a field study in southwestern Minnesota. Validation was executed using data from previous work done on the same field plots. The model predicted drainage volumes within one mm (1%) and 39 mm (23%) of the measured values for the validation years. The simulated N losses for the two validation years were within 7 kg N/ha (-22%) and 1 kg N/ha (4%) of measured values. Difficulty was experienced matching the timing of drainage events in the late winter / early spring period. A more rigorous soil temperature/freeze component may be needed to improve these predictions. The model uses basic soil properties as inputs and has few parameters to calibrate, facilitating ease of use and application to other locations.