Preliminary Modeling of Pesticide Fate in Drainage Channels Using the RIVWQ Model

© MODSIM 2005 - International Congress on Modelling and Simulation: Advances and Applications for Management and Decision Making, Proceedings. All rights reserved. Contamination of drainage channels and creeks with pesticides used in agriculture is of major concern in south eastern Australia. In this study the stream pesticide model RIVWQ version 2.02 was assessed for its applicability to simulate pesticide fate in drainage channels. The model was successfully calibrated against field data collected on flows and pesticide concentrations for a drainage channel from a small catchment of five farms in the Murrumbidgee Irrigation Area of south western New South Wales. Using the calibrated model the effects of different pesticide loading scenarios from farm fields on channel water quality were analysed. Drainage channel and stream water quality in agricultural areas is directly related to the land use, water and pesticide management practices. The contamination of drainage channels by the pesticide molinate is a great concern in the rice growing irrigation areas of south east Australia. Therefore, molinate concentration variations with different management practices were simulated in this study. The same farm fields as used in the model calibration were used for the management simulations. In order to assess the sensitivity of the model to the various input parameters a series of simulations were conducted using the calibrated input file and varying significant parameter values by ±20% of their original values. The volatilization coefficient is the most sensitive followed by the sediment bulk density, water/sediment partitioning coefficient and degradation rate in water. Parameters such as solubility and dispersion coefficient showed zero sensitivity. The model was calibrated against molinate concentrations in a drainage channel that serviced 5 farms using data from an upstream monitoring point and a downstream monitoring point which were 1.9 km apart (Thomas et al., 1998). The RIVWQ model can be used in conjunction with surface runoff models to simulate the effects of land use, water management and pesticide management. However, observed field data are rare, therefore, only the effects of rice crop area on molinate concentrations in the drainage channel were investigated in this study. The rice crop area assumed for model simulation were 420 ha, 210 ha and 105 ha out of a total 900 ha area. Both the surface drainage flow rate and molinate concentration at the upstream point changed due to the rice crop area change. The results of the model simulations suggest that the RIVWQ model can be effectively used for predicting pesticide fate in the drainage channels and exposure assessment in the agricultural environment. Thus, the model, in conjunction with surface runoff models, could be used to develop guidelines for the management of pesticide contaminated waters.