Modeling phosphorus dynamics in rice irrigation systems: Integrating O-Fe-P coupling and regional water cycling

Abstract

The simulation of phosphorus (P) transport processes in rice irrigation areas plays a crucial role in managing eutrophication issues in downstream water bodies within the context of water conservation. Rice cultivation typically occurs in flat plains, where the soil and water environment of paddy fields undergo significant changes during the growth phase, particularly under water conservation practices. This study constructed a distributed model for the microenvironmental stratification of P transformation and transport in paddy fields, which was coupled with a hydrodynamic water quality model for river networks in irrigation areas. The model incorporated several crucial hydrological and water quality processes specific to rice irrigation areas, including water management within paddy fields, diffusion and coupled transformation processes of oxygen-iron-phosphorus in paddy soils, water partitioning-catchment processes in river networks, purification of P in rivers or drainage ditches, and other pertinent physical and biochemical processes related to P transport in irrigation areas. Application of the model in the Heping Irrigation District demonstrated that the simulation of water and P transport processes across various scales well matched the measured data. Both experimental and simulated results indicated that P loads in drainage ditches and rivers were primarily influenced by P discharge from upstream paddy fields, with the model effectively capturing the impact of hydrological fluctuations in paddy fields on P transformation and transport. Thus, the model proves highly suitable for assessing P loads in irrigation districts under varying water management practices.