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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Impact of Data Quality and Model Complexity on Prediction of Pesticide Leaching

^a Institute of Environmental Science and Research, PO Box 29-181, Christchurch, New Zealand
^b Landcare Research NZ Ltd, Private Bag 3127, Hamilton, New Zealand

^* Corresponding author (murray.close{at}esr.cri.nz)

Accurate input data for leaching models are expensive and difficult to obtain which may lead to the use of "general" non-site-specific input data. This study investigated the effect of using different quality data on model outputs. Three models of varying complexity, GLEAMS, LEACHM, and HYDRUS-2D, were used to simulate pesticide leaching at a field trial near Hamilton, New Zealand, on an allophanic silt loam using input data of varying quality. Each model was run for four different pesticides (hexazinone, procymidone, picloram and triclopyr); three different sets of pesticide sorption and degradation parameters (i.e., site optimized, laboratory derived, and sourced from the USDA Pesticide Properties Database); and three different sets of soil physical data of varying quality (i.e., site specific, regional database, and particle size distribution data). We found that the selection of site-optimized pesticide sorption (K_oc) and degradation parameters (half-life), compared to the use of more general database derived values, had significantly more impact than the quality of the soil input data used, but interestingly also more impact than the choice of the models. Models run with pesticide sorption and degradation parameters derived from observed solute concentrations data provided simulation outputs with goodness-of-fit values closest to optimum, followed by laboratory-derived parameters, with the USDA parameters providing the least accurate simulations. In general, when using pesticide sorption and degradation parameters optimized from site solute concentrations, the more complex models (LEACHM and HYDRUS-2D) were more accurate. However, when using USDA database derived parameters, all models performed about equally.

Abbreviations: CRM, coefficient of residual mass • GOF, goodness-of-fit • K_oc, organic carbon distribution coefficient • K_sat, saturated hydraulic conductivity • SS_res, sum of squares residual • T_1/2, half-life