A Method of Estimating Groundwater Return Flow to Rivers from Riparian Irrigation Districts

An improved understanding of the water budget components in floodplain aquifers is necessary to implement and support water accounting requirements for use of river water. Of critical importance is the groundwater return flow as a result of recharge from irrigated water in a typical irrigation district near a river, which is the most uncertain and difficult component to be estimated. The objectives of this study are to (1) develop an effective but simple approach to estimate the amount of groundwater return flow to the river due to irrigation recharge, and (2) conduct uncertainty and sensitivity analyses of groundwater return flow to various recharge scenarios. The approach takes advantage of comprehensive drainage network typical in irrigation districts. The approach is evaluated and tested against numerical model results. It is found that the approach is accurate in calculating groundwater return flow for practical purposes with less than 10% error. We further examine how groundwater return flow may change in response to variations in recharge distributions using the proposed approach. As long as information about river stage, drain stage, and distributions of irrigation water is available, the approach could forecast changes in the amount of return flows.