ENARTECH inc. Censuitln'g Engineers and August a, 1995 Mr Phil Overeynder Utility Directot City of Aspen Water Department - 130 S. Galena Street - - Aspen, CO 81611- RE: Assessment of Water Demand Castle Creek Reservoir and Maroon Creek Reservoir Dear Phil: . This letter report summarizes the results of a reconnaissance level assessment of the potential demand for reservoir storage in the Castle Creek and Maroon Creek watei sheds The primary objective of this assessment was to determine the City of Aspen? near-term demand for the Castle Creek Reservoir and the Maroon Creek Reservoir. 1' 1111' As identified in our October 1994 Evaluation of Raw Water'Availability, it is estimated that the City of Aspen may not have suf?cient water supplies from Castle Creek and Maroon Cteek to satisfy existing potable and nonwpotable demands during critically dry years. As growth witlu?n the City' service area continues, potential dry yea1 shortages will morease We understand that the City is currently evaluating several alternatives to alleviate the . potential raw water shot tages These alternatives include the increased use of local groundwater sourcesreduce existing delivery system leaks. An additional - alternative involves the construction of water storage facilities upstream of City intake facilities 011 Castle Creek and Maroon Creek We have simulated the potential demand for upstream storage with a hydrologic simulatiOn model developed for the City This model simulates the operation of the City's integrated water supply system on a daily basis for a 22 year historic pet 10d Municipal water demands associated With Seenat 1o 6 of the October 1994 study have been utilized. This demand scenario assumes a growth rate of 3. 8% per year for the next 16 years. It is dif?cult to forecast specific future water demands. Funii'e water demands are in?uenced . by many complex variables and may vary significantly depending upon the rate and timing of g1 owth within the City setvice area, the success of a leak detection program, and the availability of water supplies from alternate supply sources such as ground water. However, 302 Eighth Street, Suite 325 RC. Drawer 160 Glenwood Springs. Colorado-81602 (303) 945-2236 Mr. Phil Overeynder Page 2 based on our recent hydrologic simulation, we estimate that a dry year demand for water storage upstream of the City intake facilities could exceed 6,000 acre feet by the end of the 16 year planning period. Long-term storage demands could significantly exceed this amount. The conditional water rights for the City?s Castle Creek and Maroon Creek reservoirs are relatively junior, and they may not be able to fill during dry years. It may be prudent to construct storage capacity that is greater than the dry year demand. This will allow the City to utilize carryover Storage from previous years. Also, if a permanent recreation pool is desired, the reservoir size should be increased. In this regard, the actual capacity of-any constructed reservoir may be significantly greater than the anticipated water demand. A summary of our study results are outlined below. Mills The City of Aspen has conditional water rights decrees for the Castle Creek "Reservoir and the Maroon Creek Reservoir. Beth reservoirs were decreed in 1971. The Castle Creek Reservoir was decreed 9,062 acre feet for municipal, irrigation and other uses. This proposed reservoir is located about 6 miles upstream of the City?s Castle Creek diversion structure, and about 2 miles of Ashcroft. Based on historic ?ling maps, the reservoir would have a maximum water depth of 165 feet, and a maximum surface area of about 120 acres The Maroon Creek Reservoir was decreed 4,567 acre feet for municipal, irrigation and other uses. This proposed reservoir is located about 4 miles upstream of the City's Maroon Creek diversion structure, immediately of the con?uence of East Maroon Creek and West Maroon Creek Based on historic tiling maps, the reservoir would have a maximum water depth of 150 feet, and a maximum surface area of about 85 acres. The potential demand for upstream reservoir storage was assessed with a daily simulation model of the City?s integrated water supply facilities. This simulation model, and the primary simulation assumptions, are described in detail in our October 1994 report. Important assumptions utilized in this study include: - a Wate1 demand for the 16 year planning period will increase as outlined' 1n Scenario 6 of the October 1994 study. . - Mr. Phil Overeynder Page - 3 The City policy of maintaining instream ?ow conditions will continue. This policy curtails diversions by the .City in order to maintain stream .?ow at or above the instream flow rates recommended by, and decreed to, the Colorado "Water Conservation Board. . Ground water supplies will not be available to supplement surface water diversions dut 111g critically dry pelibds. This assumption re?ects the possibility that ground . water depletions would reduce stream ?ow of the Roaring Fork River at times that 'instreatn flow ?ghts are not satisfied. The City is currently evaluating ground water conditions to determine the effect of well pumping on local stream flow. in] ir I) Upstream storage on Castle Creek or Maroon Creek could be used to supply anticipated de?cits in potable water, as well as anticipated deficits in untreated irrigation supplies. Releases of water to maintain instream ?ow conditions may also be a desired use. An oVerview of potential storage demands occurs below. - Deficits in potable water supply have been simulated to occur? in 11 of the 22' years that were studied (Attachment 1). During critically dry periods. the annual potable supply deficits are simulated to as great as 1,650 acre feet. This de?cit could be supplied by releases from a reservoir located on either Castle Creek or Matoon Creek. Assuming a 5% . transit loss from the reservoir to the City' intake facilities, releases of approximately 1 ,725 acre feet could be required in a dry period such as the 1977 and 1978 years (Table 1). The City. uses non?potable water from both Castle Creek and Maroon Creek for irrigation purposes. Estimated deficits in these non-potable. supplies are summarized in Attachments 2 and 3 Considering potential transit losses d1y year reservoh releases to supply non?potable uses are estimated to be 675 acre feet for Castle Creek and 300 acre feet fot Matoon Creek (Table 1). InstreamIIDJLMalenaM Reservoir releases fo1 potable and non-potable purposes will increase stream flow during dry periods and Could enhance instream flow conditions of both Castle and Maroon creeks. If the City desires to maintain stream flow at the decreed Colorado Water Conservation Board instream ?ow amounts (12 for Castle Creek and 14 for Maroon Creek), additional reservoir releases wouldbe required. These releases would occur during critically dry periods and would elevate natural stream ?ow to the recommended instream flow levels. We estimate that during the 1977 and 1978 years, additional reservoir releases of 400 acre feet to Castle Creek and 1,100 acre feet to Maroon Creek would be required to maintain recommended instream flow conditions (Table 1). If potable releases Mr. Phil Overeynder Page - 4 were not made to a specific stream such as Maroon Creek all potable releases were made from Castle Creek Reservoir), additional reservoir releases would be required to maintain instream flow conditions on Maroon Creek. The reservoirs will use water for evaporation and seepage. Evaporation and seepage estimates are outlined on Table 1, and assume 2 feet of evaporation annually, and a. seepage rate of 0.25 efs. TABLE 1 DRY YEAR RESERVOIR DEMAND Castle Creek Maroon Creek .U-se I ReserVoir Reservoir Potable Supply I l?i?Z?I AF 17251 AP Non-Potable Irrigation Supply I 675 AF 300 AF Instream Flow Maintenance 400 AF 1,100 AF Eraneta?ndandieenag. M25 . Total I 3,225 As I 3,500 AF I I This represents the total simulated shortage in potable supplies. Releases to satisfy this demand couldl?oe provided by either reservoir but would not he required from each reservoir. Total?enmni Total potential annual demand for Castle Creek is 3,225 acre feet, and 3,500 acre feet for. Maroon Creek (Table The cumulative demand for both streams exceeds 6,000 acre feet per year. These demands are associated with a critically. dry year, and in many years reservoir release demands would be significantly less. Please note that these demands are based on a relatively short planning period, and longer~term reservoir demands may be significantly greater than these estimates." Mr. Phil Overeynder Page - 5 . . oi Reservoirs are often sized to provide carryover storage. In many instances, reSer'voirs are not able to store water and till to capacity during the dry years when the maximum demand for releases occurs. With carryover storage, water stored during a previous year can be released - in subsequent dry periods. If carryover storage is required, the capacity of a reservoir is often increased. The water rights for the Castle Creek and Maroon Creek reservoirs are relatively junior and will be subject to a water right call from the. senior Cameo. Demand located on the Colorado River near Grand Junction. During dry years, these junior reservoir rights may only be in? priority for approximately one month, or less, during spring runoff. A review of stream ?ow data for Castle Creek and Maroon Creek indicate that these reservoirs may not be able to fill during dry years. In 1977, estimated storable in?ow to Castle Creek Reservoir was potentially less than 3,000 acre feet. Storable in?ow to the Maroon Creek Reservoir was patentially less than 4,000 acre feet in this dry year. Given the small amount of storable in?ow in dry years, it may be prudent to plan for carryover storage. In this regard, the reservoirs. wouldbe constructed to a capacity greater than the potential annual reservoir demand, and water stored in average and wetter than ayerage years could be released in subsequent dry periods. If permanent recreation pools are - desired for either storage facility, the total reservoir capacity should also be increased; We hope this reconnaissance level study is of assistance.- If you require any additional information atthis time, please do not hesitate to contact us. Sincerely, ENARYECH, INC. Krill I Kerry I). undeen H?ologi Enclosure cc: Cindy Covell Kara-as ATTACHMENT 1 CITY OF ASPEN . . SUMMARY OF SEMULATED SHORTAGES POTABLE SUPPLY (ACRE FEET) Water . Year October November December January February March April May June Jury August September Annuai 1970 1971 1972 1973 1974' 1975 1976 1977 1978 1979 1980' 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 ?1991 1992 '558 1486 160 C) DOC) C) 21 DOUG (I) 000000 (30000 C) O) In 00 c3:3 cat; :1 ca 0 000000009 F. t: d:c: canTr'r" 00 {0 {0 00000000000090 (3) (0 10 38 151 234 396 cat: :3 c3<3 Cat: cat: c:c3 c313 ca?3 98 151 . 314 125 cat: cat: CDID c3 cicnun has: C3t3 CJCD cnko c3<3 cacato czc: 00 ur- CD V- Average Maximum Minimum1466 CO 0