Enhancement of the SWAT+ model for simulating paddy rice cultivation and irrigation management in agricultural watersheds

Paddy cultivation accounts for over two-thirds of global rice production and 21 % of agricultural irrigation. While SWAT+ shows potential for simulating hydrology in paddy-dominant watersheds, improvements are needed. This study enhances SWAT+ by introducing a process module to simulate paddy hydrology and irrigation management. Unlike conventional hydrologic settings for Hydrologic Response Units (HRUs), the paddy module developed in SWAT+ introduces hydrologic mass balance for standing water in paddy HRUs. To implement paddy management, including transplanting, puddling, paddy irrigation, and fertilizer application, new paddy-specific conditions and actions are incorporated into SWAT+ decision tables. Case studies in South Korea and Spain demonstrate significant improvements in streamflow prediction and irrigation volume estimation. The Potential EvapoTranspiration COefficient (PETCO) was the most sensitive parameter in both watersheds, while the PERcolation Coefficient (PERCO) was more influential in non-paddy areas with high percolation rates. The study highlights distinct water balance differences, with paddy fields exhibiting higher evapotranspiration (>75 %) and surface runoff (>175 %) than other land uses. Compared to the curve number method, the paddy module improved streamflow simulation, achieving NSE values of 0.7–0.84 and PBIAS within ±10 %, particularly capturing high flows during the growing season. These enhancements strengthen SWAT+’s applicability for paddy-dominant watersheds, offering valuable insights for agricultural hydrology research.