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HomeMy WebLinkAbout6 - Handout - Boulder Creek Water Rights Computer Model City of Boulder Drought Plan Volume 2-Drought Plan Technical Information and Analysis February 20, 2003--revised Page 35 DROUGHT RECOGNITION AND RESPONSE DROUGHT ALERT STAGES AND WATER USE REDUCTION GOALS Droughts vary considerably and Boulder's response must be suited to the severity of a given drought. In developing a drought response plan it is useful to categorize Boulder's demand reduction responses into specific stages that are associated with various levels of drought. The range of droughts that Boulder might have to respond to, along with Boulder's demand reduction goals, can be categorized into drought alert stages as shown in Table 3. The range of drought associated with each alert stage was developed by modeling the operation of Boulder's water supply system through a wide range of wet and dry periods, as described in the following section. The details of Boulder's water use reduction goals for each of the alert stages are presented in Volume 1 of this Drought Plan. Table 3: Drought alert stages Drought Alert Stage Drought Description I Moderate II Serious III Severe IV Extreme DROUGHT RECOGNITION CONSIDERATIONS While water supply conditions and the potential for drought are both continuously monitored by Boulder's water managers and consultants, there are several factors that can be used to influence how and when Boulder recognizes and reacts to droughts. Other climate indicators that are relevant in other parts of the country have little predictive value in Colorado. The Colorado Front Range is a location where four different major climate systems converge. This makes weather prediction for the area notoriously difficult. The following factors are known about the Boulder area. ♦ Snowpack accumulation becomes a relatively reliable indicator of runoff by May 1 of each year (see Figure 21 below.) 4 To date, global factors (El Nino Southern Oscillation, Pacific Decadal Oscillation, etc.) basically have no predictive skill with respect to droughts in Boulder Creek or the Upper Colorado/CBT drainage area. City of Boulder Drought Plan Volume 2-Drought Plan Technical Information and Analysis February 20, 2003--revised Page 36 ♦ Multi-year persistence of droughts is evident only in the years following extremely low flow (<65% of average) years. ♦ Modeling studies suggest that, under historical and tree ring-based climate regime, Boulder's water supply would be very infrequently impacted by droughts, only 1-in-50 year droughts or worse. ♦ May 1 is the time when Boulder has the most foreknowledge about its water supply system because virtually all of the snowpack has usually developed by this time and Boulder's final CBT quota for the year is known. May 1 st is also early enough for Boulder to influence almost all outdoor uses for the ensuing irrigation season. 100% - - - - - - 90% > C 80% co LL :5 E c 70% H U) y d N Y N 60% L 0 3 a~ 50% 3 C Z O 0 40% a 30% 20% - 10% - 0% - 1-Feb 1-Mar 1-Apr 1-May Date of Snowpack Reading Figure 24: Seasonal value of Boulder Creek snowpack in predicting Boulder Creek runoff volume DROUGHT RESPONSE TRIGGERS Based on the factors listed above, drought response triggers were developed for use on May 1 of each year. The triggers incorporated three quantitative factors: City of Boulder Drought Plan Volume 2-Drought Plan Technical Information and Analysis February 20, 2003--revised Page 37 ♦ Boulder's projected mountain storage during the ensuing May-June period based on snowpack measurements and the projected resulting streamflows during the spring runoff period. ♦ Boulder's portion of water projected to be available in CBT reservoirs during the ensuing May-June period. Boulder's unrestrained water demand. Perhaps the most important factor in evaluating the drought stage is how full the city's Boulder Creek basin reservoirs are likely to get during the following runoff, because mountain storage water is likely to be Boulder's main supply in meeting its demands in a severe drought year. The interpretation of the factors listed above should be modified as necessary based on other appropriate data and operating experience. The next most important factor is Boulder's portion of the total amount of water that is likely to be in storage in the CBT project during the following runoff. Boulder owns a 7% share of the contract delivery rights for the CBT project, which corresponds to about 50,000 acre-feet of the usable storage capacity in the project. Boulder does not own this storage space directly and cannot draw this entire volume of water from the project in a single year due to CBT quota policies and capacity limits at the Boulder Reservoir treatment plant. However, Boulder does have the right to expect water delivery in accordance with the operating policies of the project and this represents Boulder's main 'bank account' during severe, multiple-year droughts. The two factors relating to storage levels in the upper and the lower part of Boulder's municipal water supply system are equivalent in value as components of an overall drought response trigger. If the portion of CBT storage associated with Boulder's CBT units is at or near its maximum value, this could somewhat compensate for Boulder's mountain reservoirs being abnormally low., Finally, the size of Boulder's unrestrained annual water demand should also be factored into Boulder's drought response triggers. As Boulder's water demand increases, more storage would be needed if unrestrained demands were to be met during a given drought. Given the potential environmental and economic effects of providing more storage, it is more cost- effective to plan for restraints on demand during significant drought periods. Based on detailed modeling of the operation of Boulder's water supply system and the CBT and Windy Gap projects over 300 years of stream flows evidenced by tree rings, the drought response triggers in Table 4 appear to be the most appropriate. These triggers mathematically incorporate all three of the factors listed above. However, Boulder's portion of CBT storage is discounted by 40% because of the multi-year carryover function of this supply. City of Boulder Drought Plan Volume 2-Drought Plan Technical Information and Analysis February 20, 2003--revised Page 38 Table 4: Suggested drought response triggers for May 1 • • Drought Alert Storage Greater than 0.85 None Between 0.85 and 0.7 1 Between 0.7 and 0.55 II Between 0.55 and 0.4 III Less than 0.4 IV (1) Projected storage index = (projected usable Boulder mountain storage + 40% of Boulder's portion of projected CBT storage) / Boulder's unrestrained water demand in non-drought years. Projected usable Boulder mountain storage is the maximum amount of projected storage in Barker Reservoir plus the Silver Lake Watershed following the peak of the spring runoff during the ensuing May-June period, minus 1,000 acre-feet for dead storage allowance. This value will be based on estimates of final snowpack levels and resulting streamflows to occur following May 1. Presently, the maximum potential projected usable mountain storage, if all reservoirs were full, less 1000 acre- feet of dead storage is 18,500 acre-feet. Boulder's portion of projected CBT storage is the maximum amount of projected available water stored in Granby, Carter and Horsetooth Reservoirs during the ensuing May-June period, times the ratio of (Boulder's CBT units divided by the total number of CBT units in the CBT Project), plus the net amount of any CBT water that Boulder is carrying over in the CBT system. Presently, Boulder Utilities owns 21,015 units of CBT and there are 310,000 units, so the ratio would be 6.78%. Boulder's unrestrained demand is the average of Boulder's annual treated water demand (May 1 to April 30) for the last two years in which there were no drought response-related demand reduction programs in place. For example, in the drought year of 2002, the average demand for the period of May 1, 2000 to April 30, 2002 was used for the calculation. If a drought continues for an extended period, it may be desirable to adjust the information for unrestrained year demand to account for increases in population. It should be noted that these drought response triggers are intended to be used only as a guideline and in conjunction with other appropriate data and operating experience. The response triggers should be modified as necessary to account for the unique characteristics City of Boulder Drought Plan Volume 2--Drought Plan Technical Information and Analysis February 20, 2003--revised Page 39 of the drought period in progress. Modifications should be made as necessary based on actual measurements of snowpack and streamflow, observations of the operational response of Boulder's municipal water supply system to the drought, and the recommendations of Boulder's water system managers. While Boulder's drought response can be decided upon and communicated to the public in early May, Boulder should continue to monitor trends in snowpack, runoff and reservoir filling through May and June, and should be ready to modify its drought response plan accordingly, if needed. WATER SUPPLY RELIABILITY ASSESSMENT SUMMARY OF APPROACH i Hydrosphere used the Boulder Watershed Model to assess the ability of Boulder's water supply system to meet its projected build-out demands in accordance with Boulder's water supply reliability criteria. The model was originally developed to simulate all significant aspects of hydrology, water rights, water storage and diversion facilities and water uses in the Boulder Creek basin. The model utilized a 1950-1994 period of hydrologic record under the assumption that the historical drought of 1953-1956 was a sufficiently severe drought event against which to test the city's water supply system. In developing this plan, the model was expanded to include a simplified sub-model of the CBT Project and Windy Gap projects. This allowed for simulating operation of the two projects during periods prior to their construction. The model was modified to run against a 300-year period of record that reflects the results of tree ring-based reconstructions of natural flows for Boulder Creek and the Colorado River for the years 1703-1987 (actual historical data were used for 1988-2002). This expanded data set allowed for a more robust assessment of the reliability of the city's water supply system, including the performance of the CBT and Windy Gap projects. The model was also refined to include the drought response triggers and associated demand reductions shown in Table 4 above. The response triggers were evaluated on May 1 of each year in the model. The response triggers were a function of the forecasted degree of fill of Boulder's upper Boulder Creek water system storage and CBT system storage over the following two-month period. The demand reductions were applied to the subsequent 12- month period. The model also simulated Boulder's invoking of drought reservations associated with its instream flow program and its raw water delivery obligations. KEY ASSUMPTIONS In the modeling analyses done in this reliability assessment, several key assumptions were made. Each assumption is treated as a 'given', including two capital expenditure items that City of Boulder Drought Plan Volume 2-Drought Plan Technical Information and Analysis February 20, 2003--revised Page 42 Table 5. This equates to some level of demand reduction once every 30 years on average and no demand reduction great enough to cause significant permanent damage to landscaping. Table 5: Results of reliability assessment Number Drought Alert Stage i • Years of Occurrence Full demand satisfied 290 All years but those listed below Level 1 5 1842, 1848, 1852, 1885, 189 Level 11 3 1851, 188 1889? Level III 2 1888, 2002 Level IV 0 None CAVEATS AND AREAS OF UNCERTAINTY It should be noted that this finding of adequacy is contingent upon the assumptions listed above. Several of these assume significant capital expenditures by the city. Sensitivity analyses show that if Boulder's modeled build-out demand is increased by an additional 10% to 31,500 acre-feet (while still maintaining a 10% safety factor), the number of years with required demand reductions would rise from 10 to 25 years out of 300, equivalent to demand reductions once every 12 years on average. The 25 reductions would include seven years with Level 2 reductions, five years with a Level 3 reduction, and one year with a Level 4 reduction as shown in Figure 26. This level of performance would not meet the city's reliability criteria and still maintain a 10% safety factor. The 10% safety factor should not be considered discretionary as this safety factor addresses uncertainties related to potential increases in water-intensive industries and 'real world vs. modeled' operational factors of the water supply system. A 10% increase in the city's build-out demand could result from failure to implement the Comprehensive Water Conservation Program included in the Water Conservation Futures Study. An 11% increase could result from allowing the Jobs and Population Project's 'Current Trends' scenario to materialize. Failure to implement the Comprehensive Water Conservation Program or allowing the Current Trends scenario to materialize would place water demand levels beyond the margin of error for assurance of a reliable water supply. A comparison of supply and demand under various scenarios is shown in Figure 27.