Simulation of base-flow and tile-flow for storm events in a subsurface drained watershed

Quantifying base-flow and tile-flow to agricultural drainage ditches is essential to understand groundwater flow and nutrient dynamics in a subsurface (tile) drained watershed. This can give an insight into the contributions of different flow components to the total nutrient loads in a stream. MODFLOW (Groundwater Vistas) was used to simulate (steady-state) base-flow and tile-flow for storm events in the subsurface drained Big Ditch watershed in Champaign County, IL, USA. A stream section of approximately 200 m in an agricultural drainage ditch was selected for this study. Two tile drains were draining into the stream section from both sides (north and south banks, respectively). Two cut-throat flumes were installed at the upstream and downstream ends of the stream section to do a mass balance of flow volumes to determine base-flow. Dataloggers and stilling basins were connected to the flumes and tile drains to measure flow rates. Twelve wells were installed on both sides of the channel section to measure groundwater level for model calibration. Three storm periods were used to calibrate the model and another three storm periods to validate it. The simulated mean hydraulic conductivity of the study site was 5.52E-04 m s^-1. The mean conductance values of the tile drains flowing from north and south banks were 6.39E-04 m² s^-1 and 2.71E-03 m² s^-1, respectively. The simulated flow rates were within 20% of the measured rates. It may be concluded that the model simulated base-flow and tile-flow for the storm periods successfully.