Hydrologic modeling of reach scale hydrologic responses to flood irrigation through merging of geophysical and hydrological data

Flood irrigation is globally one of the most common applied irrigation methods. Not all applied irrigation water is consumed by crops and a portion of the applied water can flow back to adjacent riparian zones, wetlands or surface water bodies as return flow. Identifying flow paths and quantifying subsurface flow contributions is critical to reconcile freshwater demands with limited resources and answer questions about return flow timing and the fluxes of applied water returning to riparian zones and adjacent streams. The sparse availability and coarse resolution of return flow ecohydrological datasets combined with limited information about subsurface structure make it difficult to provide detailed estimates of return flow and restrict quantitative assessments to larger spatio-temporal scales. In this study we assess the impact of flood irrigation on a local water balance by quantifying the volume and timing of applied irrigation water leaving irrigated fields through modeling of surface and subsurface processes and identify the drivers or processes that cause heterogeneity in return flow from these fields. We used time-lapse geophysical data to derive hydrologic changes and flow path dynamics in the subsurface, in combination with information about the heterogeneous structure of the soil profile at field scale. These datasets were merged with daily measured values of evapotranspiration and atmospheric conditions together with flood irrigation water application estimates as boundary conditions in a vadose zone transport model. We were able to quantify the hydrologic response of four flood-irrigated fields and at the reach-scale their contributing fluxes to riparian zones and adjacent streams. The majority of applied irrigation water left the fields through subsurface flow paths, while only 13% of applied irrigation water was consumed by crops. The dominant subsurface flow process contributing to fluxes out of the fields was subsurface runoff, governed by subsurface structural characteristics.