Irrigation return flow is defined as the part of artificially applied water that is not consumed by evapotranspiration and that either drains to the water table or runs off to a surface-water body. Water that drains to the water table will eventually either discharge to a surface-water body, such as a lake, stream, or Puget Sound, or be pumped from a well. The partitioning of aquifer recharge and direct runoff to surface-water bodies depends on the irrigation rates and properties of the irrigated soils.

In WRIA 1 soils in the irrigated agricultural areas are mostly highly permeable, so that return flows by surface runoff are negligible (Cox and Kahle, 1999; and H. Bierlink, Whatcom County Conservation District, oral commun., 1999). Instead, drainage from irrigation locally increases the height of the water table, which in turn increases discharge to surface-water bodies. Because most of the irrigation water in the WRIA 1 study area is ground water pumped from wells, a cycle of ground-water pumpage and recharge of part of the pumped water is created. Water is lost from this cycle because it escapes into the atmosphere as evapotranspiration and a smaller amount becomes crop biomass.

Irrigated agriculture constitutes a significant portion of land use in the lowlands of WRIA 1. It includes hay, grasses, and forage crops to support dairy operations, orchards, and row crops such as berries, potatoes, and vegetables (Historical Annual Crop Production and Livestock Numbers in Whatcom County and Average Annual Water Demands by Crop and Livestock and Historical Estimates of Irrigation by Land Area and Number of Farms in Whatcom County). During a typical irrigation season (May through September), fields are irrigated because precipitation is insufficient to prevent a deficit in soil moisture. The timing and amounts of irrigation vary from year to year, depending on the weather. In WRIA 1, the source of water for irrigation is about 80 percent from ground water and 20 percent from surface water (H. Bierlink, Whatcom County Conservation District, oral commun., 1999).

Two types of irrigation are used in WRIA 1. They are drip (or micro) irrigation for berries, and sprinkler irrigation for all other crops (H. Bierlink, Whatcom County Conservation District, oral commun., 1999). No flood irrigation occurs and water ways in the WRIA 1 study area are for ground-water drainage rather than delivery of irrigation water. These drainage systems were installed over time to allow naturally wet lowlands to drain so they could be used for agriculture.

Reference Cited

Cox, S.E., and Kahle, S.C., 1999, Hydrogeology, Ground-water quality, and sources of nitrate in lowland glacial aquifers of Whatcom County, Washington, and British Columbia, Canada: U.S. Geological Survey Water-Resources Investigations Report 98-4195, 5 plates, 251 p.