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INTRODUCTION Specific Water Problem The western United States is experiencing rapid population growth which has placed increased demands on the limited natural resources of the region, most notably, water. It has become crucial for both agricultural and urban water users to protect their water rights and resources by careful accounting and planning processes. Many Colorado Front Range cities obtain a significant portion of their water supplies from transmountian sources, and as provided by Colorado water law, transmountain water can be "used to extinction." Colorado Springs, for example, acquires about 75% of its water supply from transmountain sources (Saletta and Kaufman, 1994). Municipalities with large transmountain water sources and other "use to extinction" water rights have examined landscape irrigation as a possible area to augment their water supplies. They believed that a percentage of the lawn irrigation water was not used by the turf grass, deep percolated through the turf grass root zone, and eventually became return flow to the stream and ground water systems. In accordance with their water rights, this deep percolation water can be reused by the municipalities (Wheeler, 1987; Gronning, 1989). To quantify return flows several cities have used smalllysimeters to estimate turf grass consumptive use and return flows. The accuracy of smalllysimeters is not well known and the practice has been questioned by some who believe that smalllysimeters predict inaccurate consumptive use and return flows. In response to these concerns, this research was sponsored by the Office of the State Engineer of Colorado, the City of Colorado Springs, and the Colorado Water Resources Research Institute. Research Objectives The overall goal of the research project was to evaluate methodologies used by various municipalities in Colorado for estimating deep percolation from urban lawn water use. The methodologies have used smalllysimeters whose accuracy in estimating consumptive use and deep percolation was not well established. The other major objective was to cbeck the validity of irrigation-return flow relationships developed by municipalities for claiming return flow credits. In this context, the specific research concerns were as follows. . Accuracy of smalllysimeters in estimating turf grass consumptive use, . Amount of deep percolation as influenced by the amount of water applied, . Amount of deep percolation as influenced by other factors such as the frequency of water applications and the soil type. The research findings reported in this paper are based on the analysis oHour-years data (1992-1995). The CSU results of deep percolation using smalllysimeters are compared to two previous studies conducted for the Cottonwood Water and Sanitation District in Denver (Wheeler, 1987) and for the City of Colorado Springs (Gronning, 1989). Accuracy of smalllysimeters in estimating consumptive use is evaluated by comparing evapotranspiration results obtained from the smalllysimeters to those obtained from a standard large Iysimeter and from the 1963 Penman Equation (Jensen et aI., 1990).