Development of DRAIN–WARMF model to simulate flow and nitrogen transport in a tile-drained agricultural watershed in Eastern Canada

A new watershed model, DRAIN–WARMF, was developed to simulate the hydrologic processes and the nitrogen fate and transport that occur in small, predominantly subsurface-drained, agricultural watersheds that experience periodic freezing and thawing conditions. In this modeling approach, surface flow is simulated using a watershed scale model, WARMF, and subsurface flow is estimated using a field-scale model for subsurface-drained shallow water table fields, DRAINMOD 5.1. For subsurface flow calculations, the watershed is subdivided into uniform cells, and DRAINMOD is run on each cell with inputs based on the individual hydrologic characteristics of the cell. The coupling results in a distributed parameter model that calculates the total flow at the outlet of a watershed as well as the nitrogen losses. The model was evaluated for the St. Esprit watershed, located approximately 50 km northeast of Montreal. Simulations were carried out from 1994 to 1996; data from 1994 and 1995 was used for model calibration and data from 1996 was used for model validation. The new model was able to adequately simulate the hydrologic response and nitrate losses at the outlet of the watershed. Comparing the observed daily flow/monthly nitrogen with the model's outputs over the validation period returned an R2 value of 0.74/0.86 and modeling efficiency of 0.72/0.83. This clearly demonstrates the model's ability to simulate hydrology and nitrogen losses occurring in small agricultural watersheds in cold climates.

Research highlights▶ Development of a new distributed watershed hydrologic/water quality model for simulating the hydrologic processes and the nitrogen fate and transport in tile-drained watersheds. ▶ The model is capable of simulating flow in watersheds which experience periodic freezing and thawing conditions. ▶ The new model uses a distributed parameter approach by subdividing the watershed into cells to calculate tile-flow at each cell. ▶ The new model provides a method for simulating the hydrology of tile-drained watersheds where subsurface drainage systems are the main mechanisms for removing excess water from the root zone.