Field-scale simulation of runoff and water quality in a paddy field: A study of novel RWQSPad model

A Runoff & Water Quality Simulation model for Paddy fields (RWQSPad) based on water balance and nutrients mass balance was developed to simulate paddy hydrological process, nutrients (nitrogen and phosphorous) and chlorophyll-a dynamics at field scale. A paddy field was separated into flooded water layer and soil layer, and a plant growth component of EPIC model was applied to simulate crop interception and evapotranspiration. Parameters named minimum suitable water depth (Hd), maximum suitable water depth (Hu) and minimum ridge height (Ht) were introduced to control the processes of irrigation and runoff. Besides that, all the important nutrients transformation processes that take place in flooded rice field were considered by the first-order reaction kinetics equations. The effect of limitations of light, temperature and nutrients to floating-algae dynamics indicated by chlorophyll-a was taken into account. The application of RWQSPad model in a paddy field plot of the Shangzhuan Town, Beijing, China showed that the model can simulate well the hydrological, nitrogen and phosphorous transformation processes and chlorophyll-a dynamics in surface water in the paddy field. Simultaneously, it also demonstrated a good level of runoff simulation which can fit quite well with the observed data and a great potential in simulating rainfall-runoff-pollution. RWQSPad model gives more reasonable results of the runoff amount and nutrient losses, and shows a better performance on the modelling of drainage processes.