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<br /> <br />From its 1987 and 1988 research, Colorado Springs determined that the average recharge in <br />its study area was approximately 8,000 acre-ft per'year, and the average gross irrigation return was <br />approximately 37% of the total effective application (Gronning, 1989; Saletta and Kaufman, 1994). <br />In 1989, Colorado Springs filed a claim with the Water Court (89CW36) claiming the right to reuse <br />lawn irrigation return flows for all water derived ftom a reusable source. As part of these <br />proceedings, Colorado Springs was required to present its Iysimeter results in a Cottonwood-type <br />format using the same definitions of the x- and y-axes. It developed the "Colorado Springs <br />Polynomial Curve" (Fig. I) which they believed more accurately described the unique Colorado <br />Springs' conditions (such as soil and turf grass type, and irrigation practices of the town's people) <br />which are different from those represented in the Cottonwood study. The polynomial curve was' <br />accepted by the Court, and Colorado Springs is able to use this curve when determining return flows <br />(Saletta and Kaufman, 1994; Kaufman, 1994). Colorado Springs was "able to acquire about 3.5 cubic <br />feet per second of reusable water for municipal purposes. Future reusable irrigation return flows may <br />ultimately provide as much as 12,000 acre-feet of additional water per year" (Saletta and Kaufman, <br />1994). <br /> <br />Controversy Concerning the Cottonwood Curve and the Gronning Line <br /> <br />Although both the Cottonwood Curve and the Gronning Line are formatted based on <br />practical applications, there is some controversy a~sociated with their use. In the Cottonwood <br />Curve, water application appears in both the )[- and y-axes. This results in the following quadratic <br />relationship between water application (W A), con$umptive use (CU) and deep percolation (DP) in <br />the 0% to 160% WAICU range: <br /> <br />DP = (0.357 (WA)21 CU) - (19.6 WA) <br /> <br />(I) <br /> <br />Based on the large variability in the Iysimeter data,imany people involved in deep percolation <br />Iysimeter research consider a quadratic relationship between deep percolation and water application to <br />be more complex than required. It is suggested that a linear representation would be more <br />appropriate. <br /> <br />The Gronning Line, from which the Colorado Springs Polynomial is based, attempts to <br />remove this quadratic relationship and reports the r~sponse between water application and deep <br />percolation directly. However, the Gronning engin~ers modified both the water application and deep <br />percolation parameters. They defined water application, not as the total water application to the <br />Iysimeter, but as the irrigation water applied (I) to each Iysimeter (total water application minus <br />precipitation). Deep percolation is calculated as the total drainage from the Iysimeter minus the <br />corresponding percentage due to precipitation. That is, if 30% of the total water application was <br />precipitation, 30% of the drainage would be deducted from the drainage total (Kaufman, 1996). <br />Using this format, the Gronning Line equation for net drainage (ND) is (Gronning, 1989): <br /> <br />ND = (0.546 * I) - 0.019 <br /> <br />(2) <br /> <br />The accuracy of the Gronning format is questionable since the removal of the precipitation <br />component leads to a distortion of the results by significantly reducing the x-intercept, the application <br />rate at which deep percolation first begins to occur. ! Deep percolation response is based on the total <br /> <br />4 <br /> <br />