Physical Controls on Irrigation Return Flow Contributions to Stream Flow in Irrigated Alluvial Valleys

Abstract

Irrigation can be a significant source of groundwater recharge in many agricultural regions, particularly in arid and semi-arid climates. Once infiltrated, irrigation recharge can travel via subsurface flowpaths that return to the river system in a lagged manner, supplementing natural streamflow weeks, months, or even years from when the irrigation was applied. In regions that experience low flows during summer and early fall, return flows can be a significant source of supplementary streamflow. Many water planning and operations models either ignore return flows or roughly approximate them with analytical solutions. Thus, return flows represent an important but often overlooked component of the hydrological exchange and overall water balance in agricultural regions. This study uses groundwater models to explore a wide range of factors that control irrigation return flow timing in irrigated alluvial valleys. A sensitivity analysis approach is used to assess how factors such as the extent of irrigated land adjacent to a stream, irrigation recharge rate, aquifer hydraulic conductivity, aquifer thickness, water table configuration, and seasonal fluctuations in stream stage control the timing of subsurface return flows. Modeling is conducted using MODFLOW models representing an irrigated alluvial valley adjacent to a stream. While a simplification of the full complexity in real systems, the models are a significant advancement from the analytical solution and provide new insight into the timescales of return flows over a broad range of possible conditions. To contextualize our modeling results, they are compared to an analytical solution commonly used for approximating return flows to evaluate its performance. Our findings show what factors and conditions influence return flow timing and control whether they contribute to streamflow over short term (months) or longer term (seasonal) time scales.

Beneficial ‘inefficiencies’ of western ranching: Flood-irrigated hay production sustains wetland systems by mimicking historic hydrologic processes

“…Since Euro-American settlement, ranching has relied on riparian ecosystems for fertile floodplain soils and irrigation supporting grass-hay production needed for livestock (Talbert et al, 2007). Flood-irrigation sustaining this practice results in seasonally flooded grasslands linked to contemporary ES through local studies that identify associated groundwater recharge as a key component of riparian ecosystem resilience that maintains wetland function, cools water temperatures, and bolsters late summer flows through in-stream groundwater discharge (Alger et al, 2021; Essaid and Caldwell, 2017; Ferencz and Tidwell, 2022; Gordon et al, 2020; Welsh et al, 2013). Integrated over large basins, return flows from flood-irrigation (generated through groundwater recharge and subsequent in-stream discharge) can provide substantial quantities of additional in-stream baseflow.…”

Managed aquifer recharge as a strategy to redistribute excess surface flow to baseflow in snowmelt hydrologic regimes

“…Our results demonstrate how an existing GW model can be an effective screening tool for initial MAR for enhanced baseflow suitability. Some key benefits of using a regional GW model to evaluate the baseflow response over more conceptual approaches, such as analytical solutions (Knight et al, 2005) or simplified 2D models (Szabó et al, 2023) is that the numerical model is able to account for (1) anthropogenic effects (pumping, additional recharge from SW irrigation) that can alter the baseflow response, (2) geological heterogeneity that may control local hydraulic connection to rivers or boundaries, and (3) the effects of transient hydrologic conditions such as river stage, periodic well pumping, irrigation, and changing water table elevations that can alter the baseflow response (Kendy and Bredehoeft, 2006;Ferencz and Tidwell, 2022). This study benefitted from the regional model being designed in MODFLOW 6, which is compatible with FloPy.…”

Assessing impacts of irrigation on flows frequency downstream of an irrigated agricultural system by the SWAT model

“…This approach gives a practical solution to estimate the water flow needed to preserve ecosystem services. However, local conditions such as irrigation or land use are not taken into account, even though they could have a significant effect on return flows (Ferencz and Tidwell, 2022).…”