Water Rights to Return Flow from Urban Landscape Irrigation

This paper presents our research on the issue of water rights to return flow from irrigation of urban landscapes. Municipalities in Colorado's Front Range, with rights to transmountain water and other "use to extinction" water rights, have begun to examine lawn irrigation as a possible source to augment their supplies. They claim that a significant percentage of water applied to lawns is not used by the turf grass, and eventually returns to the streams and ground-water systems. In accordance with their water rights, this deep percolation water can be reused by the municipalities. This paper reviews the methodologies used by various cities to estimate deep percolation and return flow as a function of applied water. It then analyzes how these findings were evaluated by the Water Courts in their decisions concerning credits for return flow. To provide an independent analysis of the irrigation-return flow relationships, research was recently conducted at the Colorado State University (CSU) (1992-96). The CSU research results indicate that the small lysimeters used by various cities are of acceptable accuracy compared to a large lysimeter and standard evapotranspiration equations for estimating consumptive use. For estimating deep percolation, results of this research support the findings of the previous studies conducted for the city of Colorado Springs and for the Cottonwood Water and Sanitation District in Denver.