A Simulation Model for Predicting Pesticide Concentrations in Paddy Water and Surface Soil. I. Model Development

Abstract

A simulation model for predicting pesticide concentrations in paddy water and surface soil (PCPF-1) was developed. The PCPF-1 model simulates the fate and transport of pesticide in paddy water and 1cm deep oxidative paddy surface soil layer, or the pesticide source layer (PSL). The model considers the pesticide fate and transport processes in paddy water such as dissolution of pesticide in paddy water compartment, pesticide transfer by desorption from the PSL, dilution by precipitation and irrigation, concentration from evaporation and transpiration, and dissipation by biochemical and photochemical degradation. In the PSL compartment, adsorption in soil, pesticide transport though percolation of paddy water, and pesticide dissipation by biochemical degradation are considered. The model program was coded using Visual Basic for Applications as a Macro in Microsoft Excel that includes data sheets of daily water balance and daily UV-B radiation received on paddy water. The model behavior and sensitivity analysis was conducted by using field data of daily water balance and UV-B radiation obtained at the experimental paddy rice field at National Institute of Agro-Environmental Sciences (NIAES), Ibaraki, Japan in 1988. Simulated pesticide concentrations in paddy water and PSL decreased exponentially with time. PCPF-1 successfully simulated oscillating pesticide concentrations in paddy water that affected by abrupt dilution from irrigation and precipitation and pesticide transfer from PSL. PCPF-1 indicated potentials to be a tool for investigating the pesticide fate and transport processes. The model validation using experimental data will be discussed in the concurrent paper.