Simulating Return Flow Of Paddy Rice Based On System Dynamics Approach

Because there is standing water in the paddy field and irrigation return flow or other runoff enter other water bodies via channel, it is thought that irrigation water is wasted. However, at a larger scale, most discharged water has been reused. Although there are many studies on return flow, but most of them are based on measured data to estimate return flow. How to quantify return flow from paddy rice field using numerical simulation to better understanding and use it more efficiently has been one of the focuses of agricultural water management and regional water management.Previous related studies have been reviewed. A Vensim based system dynamics water balance model for three typical fields (upper, middle and lower) was developed to simulate return flow, and then use the model to simulate the return flow of the paddy rice field at field scale. On the basis of model of field, according to different kinds of the land use the model at the medium scale. We divided land uses into three kinds: rice, upper land and others; meanwhile assumed that the different land use is concentrated. Using the data of the three kinds of soil, daily input irrigation, rainfall and ETo,we simulate the irrigated, drained off water in succession. Summarizing the daily discharge of different land uses we got the daily whole medium-sized discharge. Finally we applied the model to simulate the return flow of middle scale in the paddy rice field.We applied the model to Zhanghe irrigation scheme. The results show the simulated variation trend of the surface flow is consistent with the observed and the model can reflect the transform processing of water balance at the field scale. The dynamic simulated results of the field scale indicates surface return flow changing with inflow but subsurface return flow changing little; the daily return flow increases with the increasing of inflow and the cumulated return flow also increased with the cumulated inflow. The amount underground of return flow depends mainly on local natural conditions, such as the topography, land situation, while the slope has great impact influence on it.The sensitive analysis of the return flow proves the surface return flow decreased with the increasing of effective height of the border, but can neglect the influence of the subsurface return flow; the return flow of the border increased with the length of the terrain and obviously decreased with the increasing of the width of the terrain; the return flow of the border and farm increased with the increasing of hydraulic conductivity; the subsurface return flow increased with increasing of the slope; the surface return flow and the returnflow of the border and the farm decreased with the percentage of the upper land increasing; the effective return flow of the long and narrow combination is larger than the wide and narrow one .while the recycling factor is less than the other one.After calculating evaluation indicator of return flow, it indicates return flow factor of irrigation can reflect the runoff of the field and the use of water better than return flow factor of irrigation and rainfall; the return flow factor of irrigation and return flow factor of irrigation and rainfall of middle scale is about twice of the factors of the field; the recycling factor of the middle scale is low, while the effective return flow factor is high, which proves that can offer so much return flow for the lower fields; the recycling factor of the droughty year is bigger than the median flow year.