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I I 'c., (~1 ro 00 I f\) CO I I I I I I I I I I. I I Memorandum to Steven J. Witte April 30, 1998 Page 2 Consumptive Irriaation Reauirements The consumptive irrigation requirements were calculated using the modified Blaney- Criddle method with temperature and precipitation data for Las Animas and the crop distribution shown in Table 1. The values were calculated for 1938 through 1993 but only the post-compact years, 1950 throu9h 1993, are summarized. The resulting consumptive irrigation requirements in inches are shown in Table 2. The consumptive irrigation requirements for 2,998.7 acres averaged 7,104 acre-feet annually and ranged from 6,183 acre-feet in 1958 to 8,284 acre-feet in 1963 as shown in Table 3. Diversion Reauirement The theoretical diversion requirement for 2,998.7 acres was calculated by dividing the monthly consumptive irrigation requirements by the maximum fann efficiency, 65 percent, and by the conveyance efficiency, 86.5 percent. The conveyance efficiency is the result of canal losses of 10 percent and off-fann lateral losses of 3.5 percent. These factors are based on parameters used in the H-I Model for Arkansas River ditches. As shown in Table 4, the diversion requirements averaged 12,635 acre-feet annually and ranged from 10,997 acre-feet in 1958 to 14,733 acre-feet in 1963. Consumptive Use Consumptive use has the following two components: 1) crop evapotranspiration, and 2) secondary evaporation and transpiration (SEV) of water lost through canal and lateral seepage and farm tailwater. In 1998, LAWMA will continue to operate the entire length of the canal so the canal loss portion of the SEV will not be available. The SEV was calculated by multiplying the monthly SEV rate by the sum of the off-fann lateral seepage, on-fann lateral seepage, and fann tailwater. As shown on Table 5, the total consumptive use averaged 7,793 acre-feet annually and ranged from 6,763 acre-feet in 1958 to 9,099 acre-feet in 1952. Annually, the consumptive use waS' 61.7 percent of the average annual diversion requirement for 2,998.7 acres. We propose to establish a maximum annual limit of 9,099 acre-feet of stream credits for 3,569 shares and a running 10-year volume of 77,930 acre-feet. For 1998, tAWMA will-own 3,382 shares, so the annual limit should be 8,622 acre-feet (9,099 x 3,382/3,569). When LAWMA begins accounting for its water by use of a river gage near the Highland diversian, the maximum annual and 10-year total limitations will be adjusted ta account for SEV from canal loss. Stream DeDletion Factors Manthly stream credit factors were calculated using H-I Model parameters as described in our August 8, 1997 letter which is enclosed as Attachment 1. Those stream credit factors were displayed as a percentage of river diversions. In 1998, LAWMA will deliver its pra rata share of the water back ta the Purgatoire River at Wasteway No. 3 as it did last year. Accordingly, the monthly factors were adjusted to reflect continued operation of the canal so that credit is not taken for secondary evaporation attributable to canal losses. The calculation of the depletion factors for 1998 are shown in Table 6. We understand that you would like us ta derive factors for calculation of delayed winter return flow obligations. The winter return flow obligations total 3.4 percent of the total annual credits. The winter return flow factors are calculated as 3.4 percent of values shown in row 18 of Table 6. Net credit is the difference E.,Jo~~r\- S (eo.!)