Evaluation of the root zone water quality model using field-measured data from a sandy soil

Testing of simulation models against field-measured data is an important st ep that must be taken before a model can be used as a management tool. Fiel d soil water contents, intensively measured by time-domain reflectometry (T DR) on a Zimmerman fine sand soil (Argic Udipsamments) in the Minnesota Man agement System Evaluation Area (MSEA), were used to evaluate the Root Zone Water Quality Model (RZWQM) over two rotated crop growing seasons in 1992 ( corn, Zea mays L,) and 1993 [soybean, Glycine max (L.) Merr,], The model wa s evaluated based on laboratory-measured soil hydraulic properties, Reasona ble agreement exists between the daily TDR-measured and RZWQM-predicted wat er contents for the upper soil depths. However, the model overestimated the lower soil profile, leading to a consistent overestimation of the total wa ter depth (TWD) in the entire root zone (0 to 150 cm) during both the 1992 and 1993 growing seasons. The maximum errors in TWD for the entire root zon e were 2.47 and 2.77 cm, respectively, in 1992 and 1993, Predictions of sol ute transport by RZWQM were examined for three herbicides: atrazine [6-chlo ro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine], alachlor [2-chlo ro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide], and metribuzin [4-ami no-6-(1,1-dimethylethyl)-3(methylthio)-1,2,4-triazin-5(4H)-one]. Comparison between measured and predicted soil herbicide 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 th at all pesticides were somewhat more persistent over time than the model pr edicted.