Abstract
To assess the interaction between basin-scale hydrological cycles and conditions of paddies in hilly areas, a distributed water circulation model, which accounts for heterogeneous land uses and paddy cultivation status in a grid-cell, was developed. The model was applied to three experimental watersheds, each of which is dominated by cultivated paddies, abandoned paddies and forest, respectively, namely: Cultivated Paddy-dominant Watershed, Abandoned Paddy-dominant Watershed and Forest Watershed, each of approximately 1km2 in area. To calculate the short-term runoff, first, long-term rainfall runoff calculations at 1-day intervals were carried out for each experimental watershed. Then, model state variables were extracted for rainfalls that exceeded the predefined threshold, which is daily precipitation of 20 mm and rainfall intensity of 5 mm/h. Lastly, the extracted state variables were expediently used as initial conditions for short-term runoff calculations at 10-min intervals in the Cultivated and Abandoned Paddy-dominant Watersheds. The calculated short-term discharges closely agreed with the observed discharges and represented the differences in short-term runoff characteristics between the watersheds. Following the model assessment, the differences in calculated state variables were compared between the experimental watersheds. The comparison revealed that the storage volumes of saturated zones in the grid-cells where the abandoned paddies exist are higher than those with the cultivated paddies, suggesting higher amount of saturated excess overland flow from the grid-cells with abandoned paddies. In addition, model experiments implied that those discrepancies may affect the short-term runoff characteristics of small watersheds.
