Modeling irrigation return flow for the return flow reuse system in paddy fields

Abstract

This research is to construct a water balance model to estimate the amount of return flow in an irrigation system. A simple computation framework for the model was established to include various irrigation applications in cropping seasons. The model was able to estimate evapotranspiration, deep percolation into groundwater aquifer, and return flow. Return flow can be split into two parts, which are surface and subsurface return flows. The water balance model was then applied at the irrigation system (rotational block No. 11-2 of five paddy field units) which is operated by the Taoyuan Irrigation Association in Taiwan as an example. Two study cases were simulated, in which one was for using return flow and the other one was for using no return flow. The study period for the model simulations is the first rice cropping term in 2010 which was from February 16 to July 10. As a result, return flows calculated by the model were 27, 27, 34, and 39% of outflows for sandy loam, sandy clay, clay loam, and light clay soil, respectively. Irrigation water at the downstream field unit with use of return flow was supplemented by the upstream field units, and the amount is 5–8% of irrigation water for using no return flow. Furthermore, it can be seen from the simulations that increases in irrigation water provide increases of return flow. Increases of irrigation water result in slight increases of subsurface return flow, while increases of irrigation water cause nearly none of change in deep percolation.