Landscapes with Tiles and Potholes

SWAT (Soil and Water Assessment Tool) is a watershed model that has been incorporated into USEPA’s modeling framework called BASINS used for total maximum daily load (TMDL) analysis. It is thus important that SWAT realistically simulate tile flow and pothole landscapes that are common in much of the Corn Belt and Great Lakes states. In this study, SWAT was modified to simulate water table dynamics and linked with a simple tile flow equation. Algorithms were also added to SWAT to include simulation of potholes (closed depressions), surface tile inlets, and aeration stress on plants. Flow interaction between HRUs was introduced in order to simulate pothole water. The modified SWAT model (SWAT-M) was evaluated using eight years of measured flow data from Walnut Creek watershed (WCW), an intensively tile-drained watershed in central Iowa. The model was calibrated during the period of 1992 to 1995 and validated during the period of 1996 to 1999. In addition, comparisons between the enhanced version (SWAT-M) and older version (SWAT2000) of SWAT were conducted. For assessing overall performance of the SWAT models, predictions were compared to data measured at stream sites approximately at the midpoint of the watershed (site 310) and at the outlet (site 330). Nash-Sutcliffe E values for the simulated monthly flows during the calibration/validation periods were 0.88/0.82 and 0.84/0.72 at sites 310 and 330, respectively. The relative mean errors (RME) of the simulated monthly flows during the calibration/validation periods were −2% / −1% and −18% / 10%, respectively, for the same two sites. These statistical values indicate that SWAT-M estimated both pattern and amount of the monthly flows reasonably well for the large flat landscape of WCW containing tile drains and potholes. SWAT-M needs to improve in its daily prediction because of its lower E values (−0.11 to 0.55), compared to the monthly results. In applying the model to a third site (site 210) that was predominantly influenced by tile drainage, it was concluded that the pattern and amount of simulated monthly subsurface flows (E values of 0.61 and 0.70 and RME values of 10% and −9% for the calibration and validation periods, respectively) were relatively close to the measured values. Nevertheless, SWAT-M simulation of daily subsurface flows was less accurate than monthly results. In general, the pattern and amount of monthly flow and subsurface tile drainage predicted by SWAT-M has been greatly improved as compared to SWAT2000.