Simulation and verification of water transformation of rice paddy and pond system based on system dynamics

Abstract: Ponds are widely distributed in Southern China. Considering the regulatory role of pond, Vensim software was used to build a system dynamic model to simulate water transformation in paddy and pond system, based on the system dynamic approach and water balance method. It was assumed that runoff from each land unit directly fed into drainage canal without flowing through other fields. The physical processes, such as irrigation, evapotranspiration, deep percolation, inflow and outflow of pond, were calculated in the model. In addition, various scenarios could be set to research the variation of the system with control factors. The proposed model was applied to simulate water transformation during the growing period of rice in Zhanghe irrigation district to examine its feasibility. The study area features a subtropical continental climate with an average annual precipitation of 947 mm, 85% of which occurs between April and October. There are 145 ponds with the storage capacity of 155 283 cubic meters, which are important for ensuring an adequate water supply for crops. In the simulation, all the ponds were integrated as a large pond. And water storage was obtained from the relation curve between water level and storage. The results showed a good agreement between the observed and simulated daily water level of pond: coefficient of determination of 0.90, coefficient of efficiency of 0.79 and root-mean-square error of 0.155 m/day. To estimate the significance of pond irrigation system for agriculture irrigation, 3 different pond operation modes were set and simulated: 1) pond supply with total operation, 2) pond supply with partial operation, and 3) pond closed irrigation system. The result showsed that pond operation could ensure full irrigation of rice, and water stress occurred for 72 days under partial operation, closing pond irrigation system was bad for the growth of rice. Therefore, the pond irrigation system had a significant effect on ensuring the normal growth of crop. Additionally, water transformation was simulated under 3 different irrigation modes. From the results, in the early growth period, pond storage was consistent under 3 irrigation modes. However, during the developmental stage, pond storage was the smallest under moist irrigation and produced surplus water under intermittent irrigation. Pond storage decreased significantly under the shallow water irrigation in the late growth stage. From the perspective of pond inflow, shallow water irrigation was the best, followed by intermittent irrigation and moist irrigation. For the water supply, shallow water irrigation was also the best, followed by moist irrigation and intermittent irrigation, and effective water use efficiencies were 84.7%, 80.9% and 67.7%, respectively. The proportions, calculated by the minimum pond storage dividing pond capacity, were 2.0%, 18.9% and 41.3% for shallow water irrigation, wet irrigation and intermittent irrigation，respectively. This shows that under intermittent irrigation mode, pond can maintain a certain amount of water for a long period of time, which provides a scope for aquaculture and drainage water treatment. This study provides valuable information for the rational use of water resources and irrigation management in paddy rice and pond system.