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<br />For <br />For <br /> <br />55 < WAlCU < 160, DP/WA = (0.357. WAlCU) - 19.6 <br />W A/CU > 160, DP/W A = 100 (W A/CU - 100) / (W A/CU) <br /> <br />(5) <br />(6) <br /> <br /> <br />As shown in Fig. 5, the CSU regression line and the linear part of the Cottonwood curve are <br />very close to each other with very similar slopes and identical x-intercept points. The x-intercept <br />point is significant since it indicates the amount of water applied, as percentage of consumptive use, <br />below which no drainage will occur. The x-intercept point f('f both the CSU research data and the <br />Cottonwood Curve is 55 percent. <br /> <br />Evaluation of the Gronning Line <br /> <br />Figure 6 presents four years' lysimeter data using a format based on total water application <br />(W A) (irrigation plus precipitation) and total drainage. The CSU research results are compared with <br />the Gronning Line (Gronning, 1989). The equation of the CSU regression line is, <br /> <br />DP = (0.793 · WA) - 0.113 <br /> <br />(7) <br /> <br />The R' value for the regression is 0.57. As mentioned, the City of Colorado Springs' study defined <br />water application, not as the total water application, but as the irrigation water applied (total water <br />application minus precipitation). Also, the deep percolation was calculated as the total drainage from <br />the Iysimeter minus the corresponding percentage due to precipitation. For these reasons, the <br />Gronning Line equation (Eq. 2) is not comparable to CSU equation (Eq. 7). <br /> <br />The CSU research data was modified to make it comparable to the Gronning format by <br />removing precipitation and corresponding drainage component. The results and the Gronning Line <br />are shown in Fig. 7. The equation of the modified CSU regression line is, <br /> <br />ND = (0.676 · 1) - 0.054 <br /> <br />(8) <br /> <br />which is essentially similar to the Gronning equation (Eq. 2). Similar to the Cottonwood Curve, the <br />Gronning format is based on practical considerations since it enables the City of Colorado Springs to <br />determine deep percolation solely as a function of irrigation water application. However, the <br />accuracy of the Gronning format is questionable since removal of the precipitation component leads <br />to a distortion of the results. This distortion is observed in the large difference in the x-intercept <br />values for the CSU regression lines using the two water applications formats. <br /> <br />13 <br />