Submitted to: Agronomy Abstracts
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/1994
Publication Date: N/A
Interpretive Summary: There is much public concern about the impact of agricultural chemicals on water quality, but it is difficult for producers or regulators to choose management practices to minimize these impacts without tools that allow them to predict potential effects. Computer models can be very useful for this, if they have been shown to be sufficiently accurate and reliable. The goal of this research was to test such a model, known as the Root Zone Water Quality Model (RZWQM). As part of a larger effort involving scientists in several states, we specifically evaluated the performance of the model in predicting water flow in sandy soils. The model was run with two years of weather data for a field site in Princeton, Minnesota that had been extensively instrumented to measure both the input variables (weather data and soil properties) needed to run the model and the output data (soil water contents) necessary to test it. Soil water contents were measured automatically on an hourly basis at 9 depths over the entire two year period, which included one year of corn production and one year of soybean production. Soil samples were also taken periodically to measure pesticide concentrations. We found that the model did a reasonable job of predicting general seasonal trends in soil moisture, but systematically over predicted water movement to deeper depths following rainfall or irrigation. This may have been partly due to the presence of very thin soil layers of higher clay content, known as lamellae, which can impede water flow. This information will be used to improve the model, which will then be used to provide information to producers on management practices to reduce or eliminate contamination of ground and surface waters.
Technical Abstract: Field soil-water dynamics, intensively measured by time-domain reflectometry (TDR) on a Zimmerman fine sand soil in the Minnesota Management System Evaluation Area (MSEA), were used to test the Root Zone Water Quality Model (RZWQM) over two rotated crop growing seasons in 1992 (corn) and 1993 (soybean). The model was evaluated based on the in-situ measured hydraulic properties (MHP). Reasonable agreement exists between the daily TDR-measured and RZWQM-predicted water contents for the upper soil depths. The model however overestimated the lower soil profile, leading to a consistent overestimation of the total water depth (TWD) in the entire root zone (0 to 150 cm) during both 1992 and 1993 growing seasons. Predictions of solute transport by RZWQM were examined for three herbicides: atrazine (2-chloro-4-ethylamino-6-isopropylamino-1, 3, 5-triazine), alachlor [2-chloro-N(2, 6-diethylphenyl)-N-(methoxymethyl) acetamide], and metribuzin(4-amino-6(1,1-dimethylethyl)-3-(methylthio) -1,2,4-triazin-5(4H)). Comparison between measured and predicted soil concentrations averaged over the top 15 cm layer showed that the model matched the temporal changes of the field measurements reasonably well, although the measurements showed that all pesticides were somewhat more persistent over time than the model predicted.