Simulating the fate of molinate in rice paddies using the RICEWQ model

The contamination of drainage channels and creeks with pesticides used in the production of rice is of concern in south eastern Australia. Of major concern is the molinate that at times has been detected in over 25% of water samples from surface drains and creeks. The objective of this study was to evaluate the rice pesticide model RICEWQ version 1.7.2 for its applicability in simulating the fate of the pesticide molinate in paddy rice floodwater in south eastern Australia. The model was successfully calibrated using water depths and molinate concentrations obtained from a rice bay. By using the calibrated model, the effects of the different application methods and the rates of molinate on water quality were investigated.The molinate, which was applied directly onto ponded water, led to higher maximum concentrations in the ponded water than for application onto a dry bay, which was subsequently filled. However, the concentrations in water declined more rapidly for the application onto a ponded bay than those for the application onto a dry bay. The simulation results suggest that water and chemical management has great effects on chemical concentration in the runoff water. Overall, the RICEWQ model accurately predicted the molinate concentrations in floodwater and this model can be used to develop best management practices in rice farming.