Process-Based Modeling of Paddy Water Dynamics in SWAT Through Crop-Model Algorithm Integration

Paddy rice is a major irrigated crop in Asia and plays a critical role in regional food security, particularly in countries such as India. It is also highly water-intensive, accounting for roughly 40% of global agricultural irrigation withdrawals. Flooded paddy systems exhibit a unique water balance characterized by continuous ponding, high evapotranspiration, seepage and percolation losses, return flows, and controlled drainage, leading to substantial seasonal water requirements. However, most basin-scale hydrological models are originally developed for upland crops and rainfall–runoff systems, and therefore have limited capability to represent irrigated, ponded systems.The Soil and Water Assessment Tool (SWAT) is widely used for watershed-scale hydrological assessments but lacks explicit representation of flooded rice cultivation. Existing approaches in SWAT including the curve number (CN) method (treating paddy as upland) and pothole routines do not fully capture paddy-specific irrigation management or field-level water balance components. In contrast, field-scale crop models such as ORYZA and CERES-Rice provide more advanced representations of paddy water dynamics, but are not intended for watershed-scale analysis. To address this gap, this study develops a new process-based paddy module (SWAT-PADDY) by integrating soil water routing and irrigation management algorithms adapted from crop model frameworks into SWAT. The module accounts for key management practices, including transplanting, puddling, irrigation and drainage scheduling, and bunded field hydraulics. Soil water routing was reformulated to couple the ponded layer with the soil profile, enabling realistic simulation of infiltration, percolation, overflow, and return flows. The enhanced model was evaluated at ten paddy fields in South India over two cropping seasons using observed water levels, and key water balance components were assessed through cross-model comparisons with ORYZA, CERES-Rice, and a numerical soil water flow model (HYDRUS-1D) to assess consistency and process representation.Results show that SWAT-PADDY realistically simulates ponded water levels and major water balance components, including evapotranspiration, infiltration, percolation, overflow, and soil water storage. The enhanced model demonstrated good statistical performance for observed water levels (NSE > 0.5) and achieved zero water balance closure error at field scale. Cross-model comparisons showed strong agreement with field-scale crop and numerical model simulations. The improved process representation broadens the applicability of SWAT for regions dominated by irrigated rice cultivation and enables basin-scale assessment of water use under diverse climatic, soil, and management conditions.Keywords: Flooded rice systems, SWAT model development, Process-based modelling, hydrology, Water balance simulation