EVALUATION OF SWAT IN SIMULATING NITRATE NITROGEN AND ATRAZINE FATES IN A WATERSHED WITH TILES AND POTHOLES

Authors: DU, B - TARLETON STATE UNIVERSITY; SALEH, A - TARLETON STATE UNIVERSITY; Jaynes, Dan; Arnold, Jeffrey

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/31/2006
Publication Date: 8/8/2006
Citation: Du, B., Saleh, A., Jaynes, D.B., Arnold, J.G. 2006. Evaluation of SWAT in Simulating Nitrate Nitrogen and Atrazine Fates in a Watershed with Tiles and Potholes. Transactions of the ASABE. 49(4):949-959.

Interpretive Summary: Land use and farm management practices can greatly affect water outflow, sediment, nutrient, and pesticide losses from agricultural watersheds. There is an increasing demand by decision makers and planners for information regarding the magnitude of these effects and how water quality parameters may change due to adoption of various agricultural practices. Because of the complexity of watershed systems and the large number of possible management practices to be considered, the development and use of computer simulation models is required to achieve this task. We modified the ARS Soil and Water Assessment Tool (SWAT) model to better simulate the fate and transport of nitrate and the common herbicide atrazine within intensively tile drained landscapes commonly found in much of the U.S. corn-belt. We demonstrated that the modified model could successfully simulate not only water flow but also nitrate and atrazine concentrations within the Walnut Creek watershed of central Iowa – a watershed that has been extensively tile drained and has been intensively studied by ARS since 1990. The improved model better predicts water and agrichemical transport in the Midwest and will allow water quality researchers to better determine the impacts of existing and future management practices on water quality in this important agronomic area of the country. These findings and the improved watershed model will be of interest and use to a wide range of scientists, regulators, and decision makers and will greatly improve the outcome of the Conservation Effects Assessment Project (CEAP).

Technical Abstract: An increasing concern about chemical pollution from farming practices has led to an effort to develop models to seek solutions to this problem. We evaluated a version of the Soil Water Assessment Tool (SWAT-M) model that was modified to more accurately simulate tile drainage and water flow in a landscape dominated by closed surface depressions or potholes at a watershed scale using ten years of measured nitrate-nitrogen (NO3-N) and atrazine data in stream discharge in the Walnut Creek watershed (WCW). The model was calibrated during the period of 1992 to 1995, and validated during the period of 1996 to 2001. Stream sites in the middle and outlet of the WCW were selected to assess overall performance of the model while one drainage district drain was used for investigating chemical loads in subsurface flows. With the introduction of an independent tile drain lag time parameter, the performance of SWAT-M for daily flow simulation was improved. In comparison to our previous results, the Nash-Sutcliffe E values for the calibrated daily flow at the mid-watershed and outlet simulated by the enhanced SWAT model rose from 0.55 to 0.69 and from 0.51 to 0.63 respectively. Of special note, the E value for calibrated flow rose from -0.23 to 0.40 for the drainage district drain, which was dominated by tile and subsurface flow. Both the predicted corn yields and N uptake by corn were very similar to the measured data. The predicted yield and N uptake by soybean were relatively lower than the measured ones. The monthly NO3-N loads in stream discharges at the center and outlet of the WCW were accurately predicted with good Nash-Sutcliffe E values of 0.91/0.80 and 0.85/0.67 in calibration/validation, respectively. Nevertheless, the model’s simulation of the daily NO3-N loads was not as good as the monthly simulation, for the E values of daily NO3-N loads at the mid-watershed and outlet of the WCW were only 0.41 and 0.26 respectively, during the validation period. The good agreement between the simulated and measured monthly NO3-N loads from drainage district site (E values=0.73/0.71 in calibration/validation, and RME values=13%/-17%), leads us to conclude that SWAT can reasonably simulate tile flow from pothole dominated landscapes although the model needs to be improved in the simulation of daily subsurface NO3-N fluxes (E values = 0.25/0.42 in calibration/validation). The enhanced SWAT-M model simulated the NO3-N loads in a watershed with intensive tile drainage systems much more accurately than the original SWAT2000 version. A second pesticide degradation half-life in soil was added for SWAT-M, which greatly improved the model performance for predicting atrazine losses from the watershed. Overall SWAT-M is capable of simulating atrazine loads in the stream discharge of the WCW (E values for the predicted monthly atrazine at the mid-watershed, outlet, and drainage district sites were 0.73/0.58, 0.50/0.53 and 0.92/0.31 in calibration/validation, respectively). But the simulation of daily atrazine loads by SWAT-M needs to be improved, as the E values in validation at all sites were less than 0.50. Nevertheless, SWAT-M is a much improved tool over SWAT2000 for predicting both daily and monthly atrazine losses in nearly level, tile drained watersheds.