Exploring How Excess Irrigation Becomes Streamflow in Mountainous Watersheds
Irrigation from stream diversions is a major source of water in the western United States. Because no irrigation method is 100 percent efficient, some of the water not used by crops recharges groundwater and can eventually return to the stream weeks, months, or even years later. Water management models designed to evaluate surface water availability for permitting, planning, and impact assessment often either ignore irrigation return flows or roughly approximate them with analytical solutions. This study uses groundwater models to explore hundreds of possible scenarios for irrigation return flows and compares the results to the approximations often made in water management models.
These results provide new insight into how local, site-specific factors influence return flow behavior. This is the first study to evaluate return flow dynamics over a broad range of conditions. Researchers found that return flows are strongly influenced by irrigation recharge, seasonal stream flow variations, and aquifer properties. The results show the risk of over-generalizing return flows over large model areas by not accounting for local conditions, potentially affecting monthly water availability estimates. This work highlights the need for improved methods of assigning return flow parameters in water management models.
Irrigation can be a significant fraction of total groundwater recharge in agricultural regions, particularly in arid and semi-arid climates. In regions that experience low stream flows during the summer and early fall, including snowmelt-dominated regimes in the western United States, return flows can be a significant source of supplementary streamflow. Return flows represent an important, but often overlooked, component of the hydrological exchange and overall water balance in agricultural regions. A sensitivity analysis approach was 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 the stream stage control the timing of subsurface return flows. The United States Geological Survey groundwater modular finite-difference flow model (MODFLOW) was used to create models representing an irrigated alluvial valley adjacent to a stream. In total, 360 11-year simulations were evaluated. The modeling results were compared to an analytical solution commonly used for approximating return flows for performance evaluation. The modeling results showed a wide range in magnitudes and seasonal variability of return flows, depending on the specific combination of recharge pattern, river fluctuation, and subsurface properties used. These have implications for both surface water availability and water quality. Wider valleys with finer sediments result in more attenuated return flows that provide more constant year-round flow, while narrower aquifers with coarser sediments generate more seasonally variable return flows concentrated during the summer and early fall.