Quantifying Stream Return Flow of Agricultural Water Using SWAT-MODFLOW-PADDY Model in Korean Paddy Fields

Abstract

In many countries, the irrigation return flow focuses only on surface and subsurface flows. In contrast, South Korea adopts a broader approach, defining the stream return flow as encompassing both quick and delayed return flows, which include subsurface flow and deep percolation. This study proposes redefining the stream return flow to include only the subsurface return flow, excluding deep percolation. We quantified the subsurface return flow and deep percolation using the SWAT-MODFLOW-PADDY (SMP) model, confirming that the current definition overestimated the stream return flow in Korea. The results show that the subsurface return flow accounted for 20% to 60% of the total infiltration, with the remaining 40% to 80% contributing to deep percolation and groundwater recharge. These findings reveal significant regional variations in the subsurface return flow rates, underscoring the limitations of applying a uniform stream return flow rate. We propose that allocated management water, subsurface, and quick return flows should be the primary components considered in stream return flow calculations, as the current practice of including delayed return flow leads to overestimated results. This study highlights the challenges in monitoring the subsurface return flow and the need for region-specific models that account for local conditions such as topography, soil characteristics, and climate. Our findings provide a more accurate approach to estimating the subsurface return flow, which is crucial for improving the efficiency and sustainability of agricultural water management in Korea.