Influence of water management on the water cycle in a small watershed irrigation system based on a distributed hydrologic model

•Rice Irrigation System Soil and Water Assessment Tool (RIS-SWAT) is developed.•Impact of water management on water cycle was evaluated using RIS-SWAT model.•Compared with the current situation, a 20% reduction in canal water supply would not obviously reduce the rice yield.•Temporal and spatial distribution of irrigation water in different subbasins has little impact on the water cycle in small watershed scale.•20–40% reduction in canal water supply accompanied with much more ponds water supply would not reduce the rice yield. Based on the hydrological characteristics of a rice-based irrigation system in Southern China, the SWAT (Soil and Water Assessment Tool) model was modified in order to develop the Rice Irrigation System-Soil and Water Assessment Tool (RIS-SWAT). The irrigation water movement, water balance module, and rice yield calculations of the paddy field were improved within the SWAT model. In addition, the seepage of the irrigation canal and the pond’s effect on the irrigation modules were added to the RIS-SWAT. The impact of water management on the water cycle of the small watershed irrigation system (SWIS) was evaluated by conducting a scenario analysis with RIS-SWAT. The results indicated that an increase in the maximum water ponding depth of a paddy field could decrease irrigation water and rainfall loss. The canal water supply was reduced by 20%, resulting in an increase in irrigation water productivity without a reduction in the rice yield. The temporal and spatial distributions of the irrigation water in different subbasins did not significantly affect the water cycle within the SWIS. Furthermore, when the canal water supply was reduced by 20–40% and the pond water supply was increased, the rice yield was not affected. Thus, the water productivity of irrigation systems could be improved through the implementation of water management policies and practices that reduce field outflows and reuse the return flows captured by ponds.