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<br /> <br />DETERMINATIONS OF RETURN FLOW BY MUNICIPALITIES <br /> <br />In order to obtain return flow credit, municipalities must first quantify the components of the <br />irrigation-return flow system. First, they must determine the amount or percentage of the application <br />water which is not consumptively used by the turf grass and passes through the turf grass root zone <br />(deep percolation water). Then, they must determine the amount or percentage of the deep percolation <br />water that will eventually return to the stream and groundwater systems (return flow). The return flow <br />is less than the deep percolation since trees, shrubs, and other landscape vegetation can consume deep <br />percolation water, as well as additional consumption taking place during transit to the stream or <br />groundwater system. <br /> <br />The Cottonwood District Study -- the Cottonwood Curve <br /> <br />In 1983, the Cottonwood Water and Sanitation District retained consulting engineers W.W. <br />Wheeler and Associates, and began a lysimeter study to quantify the amount of deep percolation from <br />lawn irrigation. This was done as part of its augmentation plan filed in the Colorado Water Court <br />(Case No. 81CWI42). By 1984, forty, 30.5 cm diameter, weighing type Iysimeters were installed in <br />Cherry Creek and Denver's southeast metropolitan area. The "Cottonwood Curve" was developed <br />from these data which demonstrates a relationship between water application, deep percolation, and <br />potential consumptive use of turf grass using smalllysimeters (Fig. 1). <br /> <br />Wheeler first developed a curve that it felt best represented the relationship between deep <br />percolation (DP), water applied (W A), and potential consumptive use (CU) using all the data points <br />and giving special consideration to the controlled lysimeters and the shape of the curve expected by <br />physical reality (Wheeler, 1987). This "best fit" curve was linearized for W A/CU ratio less than 160, <br />and slightly lowered (from 22% to 16% at 100% W A/CU) to form the Cottonwood Curve (Wheeler, <br />1987; Walter, 1996). Continued Iysimeter studies by Wheeler in 1984-1986 reinforced the opinion <br />that the Cottonwood Curve underestimated the amount of deep percolation from lawn irrigation <br />(Wheeler, 1987). The Water Court accepted the results of the Cottonwood Curve, and other <br />municipalities began using it when requesting return flow credits (Castle Rock, Case Number <br />84CW656; Centennial Water and Sanitation District, 85CW415; Westminster, 86CW397; and, <br />Greeley, 87CW329)(Walter et al., 1991). <br /> <br />Observing these Water Court proceedings, Colorado's Office of the State Engineer began <br />accepting municipalities' requests for return flow credits of 15% of the water application. This <br />number was arrived at by using the Cottonwood Curve and assuming: <br /> <br />. that water was applied to lawns at a rate equal to the rate of potential consumptive use (100% <br />on the x-axis), and <br /> <br />. a reduction for consumptive use by trees and shrubs (tree canopy consumption). If <br />municipalities requested a return flow credit greater than 15%, they were required to provide <br />the engineering reports to document the validity of this request (Wolfe, 1996). <br /> <br />2 <br />