Pesticide fate at watershed scale: A new framework integrating multimedia behavior with hydrological processes.

Pesticide pollution has been one serious ecological and environmental issue due to its wide application, high toxicity, and complex environmental behavior. The fugacity model has been widely used to quantify biogeochemical cycles of pesticides due to its clear compartments, simple structure, and easy-accessible data. However, the lack of detailed hydrological processes limits its application for large and heterogeneous watershed. In present study, a new framework was proposed through integration of hydrological processes of SWAT and pesticide fate of fugacity model, and was applied into a typical watershed in the Three Gorges Reservoir Area, China. The results showed that surface runoff, soil erosion, and percolation varied spatiotemporally, which highlighted the importance of considering regional and seasonal heterogeneity of pesticide transport variables in the fugacity model. The amount of dichlorvos (DDV) and chlorpyrifos (CHP) in air, water, soil, and sediment phase were estimated as 0.26 kg, 19.77 kg, 1.06 × 104 kg, and 0.55 kg, respectively. Spatiotemporally, pesticide concentrations in water phase peaked in summer, while the middle and southwest regions of the watershed were identified as the hotspots for pesticide pollution. Compared with the classical model, the new framework provided technical support for the pesticide assessment at watershed scale with heterogeneous hydrological conditions, which can be easily extended to other watersheds, and integrated with other models for comprehensive agricultural management.

New framework for managing the water environmental capacity integrating the watershed model and stochastic algorithm.

Integrated calculations of pollution load and water environmental capacity (WEC) are essential for effective water quality management. However, few studies have focused on the dynamic WEC and pollution load in a nonpoint source pollution (NPS)-dominant temperate monsoon watershed under changing rainfall conditions. In this study, a new framework based on the watershed model and WEC calculation with stochastic rainfall input (SR-WEC), was proposed to reveal the dynamic WEC and pollution load under changing rainfall conditions. Stochastic rainfall series was generated by a first-order Markov chain and gamma distribution, and further input into the Soil and Water Assessment Tool (SWAT) to explore the dynamic response of water quality to rainfall. The framework was applied to the Daning River watershed, Three Georges Reservoir Region, China. The results suggested that compared with the new SR-WEC, the traditional return period method with limited observed rainfall input would result in an underestimation of ideal WEC and NPS pollution load by 23% and 48% for TN and 48% and 51% for TP, respectively. Approximately 46% of the annual TN reduction and 51% of the annual TP reduction were concentrated from April to June in a relatively small area. The regression relationships between rainfall and the ideal WEC, pollution load and remnant WEC obtained by the SR-WEC were superior to those of the traditional method, with R2 values increasing from 0.005-0.797 to 0.718-0.989. Specific threshold (120 mm/month for the study area) was observed for the effect of rainfall on water quality, beyond which the remnant WEC of organic N would change from decreasing to increasing. The new framework proposed identifies the key periods and areas with consideration of uncertainty of rainfall on water quality, and provides basis for NPS pollution management.