Modelling Tile Drainage Discharge Using a Linear Reservoir Model.

More than 50 % of Danish agricultural areas are expected to be artificial drained. Transport of water and nutrients through the drain system to the aquatic environment is expected to be significant. Drainage filter technologies might be an effective tool to mitigate site-specific losses of nutrients from agricultural areas. However, for the dimensioning of the filter systems, knowledge of the water load coming from the tile drainage into the filter systems is essential. In this work we aim at predicting drain water discharge in order to obtain knowledge of site specific parameters controlling the discharge. A simple linear reservoir model was used for the drain water discharge modeling. The model consisted of three outlets where the lower one represented groundwater discharge and the two upper ones represented the drain water discharge. Discharge from thirteen small drain catchments were modeled ranging in size from one to ten hectares. The model was calibrated using a variant of the Gauss-Marquardt-Levenberg method of nonlinear parameter estimation integrated into the PEST software package. Calibration was performed according both to the daily discharge, weekly volumes, and flow exceedence probabilities. Values of calibrated model parameters was compared to site specific parameters such as soil type, catchment topography, catchment area, depth to groundwater table, etc. The results open up for the understanding of the dynamics of drain water discharge in smaller tile drained catchments.