Irrigation return flows to the Salton Sea originating from on-farm and conveyance losses are potential water sources that could be used by the Imperial Irrigation District (IID) to increase the irrigated area in the Imperial Valley or by other California users of Colorado River water. An on-farm irrigation study conducted by USDA-ARS and IID between 1977 and 1981 provided estimates of on-farm return flows originating from runoff and drainage, as well as crop water and leaching requirements. Recovery of all runoff water without any change in cropping practices would yield about 0.4 km³/y. All but 0.1 km³/y of this runoff water would be needed on-farm, if irrigation practices were changed so that crop water and leaching requirements were fully met without recycling drainage water for irrigation. If drainage water were recycled, only 0.1 km³/y would be needed, leaving 0.3 km³/y which could be used elsewhere. Combining these estimates of recoverable water with those for recoverable conveyance losses from IID operations of 0.3 km³/y results in a total which ranges from 0.4 to 0.6 km³/y. This brackets the maximum reduction in Colorado River water available to California after full implementation of the Central Arizona Project.