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<br /> <br />ABSTRACT <br /> <br /> <br />This is a report of our research on issues related to return flow from irrigation of urban <br />landscapes. Municipalities in Colorado with rights to transmountain water and other "use to <br />extinction" water rights have examined lawn irrigation as a possible source to augment their supplies. <br />They claim that a significant percentage of water ilPplied to lawns is not used by the turf grass, and <br />eventually returns to the streams and ground watet systems. In accordance with their water rights, <br />this deep percolation water can be reused by the municipalities. Return flow credits, therefore, <br />involve significant amounts of additional water supply and financial benefits to municipalities. <br /> <br />To quantify irrigation-deep percolation relationship, municipalities have used smalllysimeters <br />whose accuracy in estimating turf grass consumptive use and deep percolation was not well <br />established. To provide an independent analysis, research was conducted at the Colorado State <br />University, CSU (1992-96). This paper presents the research findings on the accuracy of <br />methodologies used by various cities to estimate deep percolation as a function of applied water. It <br />also analyzes how these methodologies were evaluated by the Water Courts in their decisions <br />concerning credits for return flow. <br /> <br />The CSU research results indicate that the smalllysimeters used by various cities are of <br />acceptable accuracy compared to a large Iysimeterand standard evapotranspiration equations for <br />estimating consumptive use. Also, there is no significant difference between the two types of small <br />Iysimeters used by municipalities -- drainage and weighing type smalllysimeters -- to estimate deep <br />percolation. For estimating deep percolation, results of this research support the findings of the <br />previous studies conducted for the City of Colorado Spring (Gronning Line) and for the Cottonwood <br />Water and Sanitation District in Denver (Cottonwood Curve). <br /> <br />ii <br />