return flow

Return flow is water that moves out of an irrigated field after water has been deliberately applied for crop growth. It includes both surface runoff that travels across the ground and subsurface seepage that percolates downward and laterally through soil layers. The quantity and timing of return flow depends on irrigation method, soil type, field slope, and how much water was applied relative to what the crop and soil can hold.

Surface return flow occurs when irrigation water, rainfall, or snowmelt exceeds the soil's infiltration capacity. On a sloped field receiving flood or furrow irrigation, water moves downhill toward field edges or drainage ditches. On level fields with sprinkler or drip systems, puddling may occur, but less return flow is typical. Subsurface return flow develops when water moves down past the root zone and encounters less permeable layers, then migrates horizontally toward lower ground or drainage systems.

Composition and water quality concerns

Return flow carries dissolved salts, nutrients, pesticides, and sediment picked up from the soil and applied chemicals. In arid and semi-arid regions, accumulated salts in return flow can degrade downstream water quality and accumulate in soil of fields receiving that water. Nutrient loading from nitrogen and phosphorus in return flow contributes to eutrophication in receiving waters. These water quality impacts make return flow measurement and management important for irrigation districts and environmental agencies.

Return flow is distinguished from consumptive use, the portion of applied water that is transpired by plants or evaporated from soil and water surfaces. In most irrigation schemes, return flow represents 30 to 60 percent of applied water, though this varies widely. Irrigation designers and water rights managers calculate available return flow to estimate how much water can be reused downstream, and farmers structure fields and practices to minimize return flow loss of nutrients and chemicals.