In many agricultural regions, irrigation can be a significant source of groundwater recharge, particularly in arid and semi-arid climates. Once infiltrated, irrigation recharge can travel via subsurface flowpaths and return to the river system in a lagged manor. Return flows are a potentially important component of hydrological exchange and overall water balance in agricultural regions. Importantly, in snowpack runoff dominated systems in the mountainous regions of the Western US, return flows generated from early season irrigation provide a critical supplement to stream flows during the late summer and early fall. However, many water planning and operations models either ignore return flows or coarsely approximate them. We present a generalized modeling study of controls on return flow timing followed by an analysis of how assumptions about return flow timing influence estimates of basin-scale water availability. Groundwater modeling of 2D cross-sections of irrigated land adjacent to a stream is used to perform a sensitivity analysis of how factors such as extent of irrigated land, aquifer hydraulic conductivity, aquifer thickness, water table configuration, and stream stage control the timing of subsurface return flows. To lend real-world context, we then present a sensitivity analysis of how return flow lag parameterizations influence water availability in the State of Colorado’s StateMod model for a subbasin of in Upper Colorado River Basin.

This research was supported by the Office of Science of the US Department of Energy as part of 623 research in the Multi-Sector Dynamics, Earth and Environmental System Modeling Program. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE‐NA‐0003525.