Evaluation of Thiobencarb runoff from rice farming practices in a California watershed using an integrated RiceWQ-AnnAGNPS system

Authors: WANG, RUOYU - University Of California; Bingner, Ronald - Ron; YUAN, YONGPING - Us Environmental Protection Agency (EPA); Locke, Martin; Herring, Glenn; DENTON, DEBRA - Us Environmental Protection Agency (EPA); ZHANG, MINGHUA - University Of California

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2020
Publication Date: 5/1/2021
Citation: Wang, R., Bingner, R.L., Yuan, Y., Locke, M.A., Herring, G.E., Denton, D., Zhang, M. 2021. Evaluation of Thiobencarb runoff from rice farming practices in a California watershed using an integrated RiceWQ-AnnAGNPS system. Science of the Total Environment. https://doi.org/10.1016/j.scitotenv.2020.144898.
DOI: https://doi.org/10.1016/j.scitotenv.2020.144898

Interpretive Summary: Pesticide application in rice paddies has the potential to be harmful to ecological health. Thiobencarb is a commonly used herbicide in Northern California rice fields that may pose ecological risks to non-targeted organisms. Currently available models do not adequately represent the fate and transport of rice pesticides at watershed or basin scales. For this study, rice pesticide transport technology at the field-scale was integrated with watershed-scale technology to follow the fate of pesticides throughout the watershed system. The integrated technology successfully extends field level simulations to watershed scales while considering the impact of mixed land uses on downstream loads. The new technology was evaluated by investigating the fate and transport of thiobencarb residues from paddy fields in the Colusa Basin, California as a case study. Thiobencarb concentrations in both water and sediment phases were accurately captured by the calibrated model at the edge of field. The integrated system successfully reflected both the seasonal pattern of surface runoff and the timing of monthly thiobencarb loadings downstream as well. This integrated modeling system provides a framework for evaluating rice pesticide load impacts as part of a basin level management approach to improve water quality, and can be extended to other paddy applied agrochemicals, or other areas with fine-scale spatial information of pesticide applications.

Technical Abstract: The development of modeling technology to adequately simulate water and pesticide movement within the rice paddy environment faces several challenges. These include: (1) adequately representing the processes of ponded conditions for rice paddies; (2) the collection/implementation of temporal and spatial pesticide application data at field scales; and (3) the integration of various simulation schemes for mixed landuse conditions. Currently available models do not fully consider these challenges and results may not be sufficiently accurate to adequately represent fate and transport of rice pesticides at watershed or basin scales. Therefore, in this study, an integrated simulation system, “RiceWQ-AnnAGNPS”, was developed to fully address these modeling challenges and is illustrated using a case study in California watersheds utilizing rice farming practices. The integrated system successfully extends field level simulations to watershed scales while considering the impact of mixed land uses on downstream loads. The system also maintains the fine-scale spatial information of pesticide applications and handles areas of varying treated rice and other landuse via the “split and adjust” approach. The new system was evaluated by investigating the fate and transport of thiobencarb residues from paddy fields in the Colusa Basin, California as a case study. Thiobencarb concentrations in both water and sediment phases were accurately captured by the calibrated RiceWQ model at the edge of field. After upscaling the pesticide runoff loadings from the field to the watershed level, the integrated system successfully reflected both the seasonal pattern of surface runoff and the timing of monthly thiobencarb loadings. Incorporating future enhancements can further improve model performance by including more detailed water drainage schedules and management practices, improving the accuracy of summer runoff estimations, and incorporating a more sophisticated in-stream process module. This integrated modeling system provides a framework for evaluating rice pesticide load impacts as part of a basin level management approach to improve water quality, which can be extended to other paddy applied agrochemicals, or other areas with fine-scale spatial information of pesticide applications.