2012-12-26_HYDROLOGY - M2003091 - Laserfiche WebLink

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Dave Bailey, Esq. November 12, 2010 Page 4 Irrigation Guide values for reasonableness. The Colorado Irrigation Guide values were found to be only slightly less, 1.25 feet for silage corn and 2.00 feet for alfalfa, which may in part be due to the different study period analyzed. Based on the average cropping pattern described above of fifty percent alfalfa and fifty percent silage corn, the average CIR was calculated to be 19.94 inches, or 1.66 ac -ft per acre per year. The yearly variation in CIR was multiplied by the number of irrigated acres to determine a volumetric CIR, which has been summarized in Table 5, and averaged 100.4 ac -ft/yr for the study period. In other words, if crop demands were always satisfied, 100.4 ac -ft/yr of the 163.4 ac -ft /yr delivered to the farm would have been consumed. Water Budget Based on the 1911 through 1998 farm headgate deliveries, the annual on -site depletions attributed to the two Rural Ditch shares were calculated to average approximately 90.1 ac -ft /yr, as shown in Table 6. The annual consumptive irrigation requirement is met in some months without diversions or insufficient diversions with the use of the soil moisture reservoir. The difference between the potential CIR and the CIR met is approximately 10.3 ac -ft /yr (100.4 ac -ft /yr minus 90.1 ac -ft /yr) and reflects the periodic shortages throughout the study period. Depletions A historic stream depletion analysis was run for a single year based on the study period average of diversions using the SEO's Glover ground water lagging program, and is summarized in Table 7. We assumed a return flow breakdown of 35% surface runoff and 65% deep percolation, based upon discussion with Mr. Adams and knowledge from other historic irrigation analyses. A transmissivity of 35,000 gpd /ft and a storage coefficient of 0.1 was obtained from a nearby groundwater pump test as published in Groundwater Circular No. 11 data. Average year stream depletions and return flows have been calculated, and can also be found in Table 7. As the table shows, the average annual depletion associated with use of the two Rural Ditch shares was 90.1 ac- ft /yr. Of this amount, an average of 100.9 ac -ft of depletions occurred during the irrigation season (May through September) and 10.8 ac -ft of return flow back to the stream during the other months. Based upon the results of the historic water budget analysis for Lafarge's two Rural Ditch shares, a set of historic monthly depletion and return flow factors was derived, as shown in Table 8. These factors may be used to determine the future depletion credits, or return flow obligations assessed to the shares. The depletion factors are shown for May through September and equal the average historic depletion in each month, divided by the average annual farm headgate delivery. The depletion factor is greatest in May and equals 72.1 percent. In other words, 72.1 percent of the water historically delivered to the farm in May depleted the stream. The return flow factors for May through September are simply equal to one minus the depletion factor, and therefore represent the percentage of the historic farm delivery which returned to the stream in a given month. For example, the May return flow factor is 27.9 percent, which means on average, the return flow to the stream in May was equal to 27.9 percent of the farm headgate delivery. During the October through April period, the return flow factor is equal to the average historic monthly return flow, divided by the average annual farm headgate delivery. For example, the December return flow factor is 1.1 percent. This means, that on average, 1.1 percent of the annual farm headgate delivery returned to the stream in December.