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Home My WebLink About6 - Report from Hydrosphere Resource Concultants, Simulation of Hypothetical Climate Change ScenarioSimulation of Hypothetical Climate Change Scenarios Using the Boulder Creek Watershed Model September 9, 2003 HYDROSPHERE Resource Consultants • ~ Sunulation of Hypothehcal Chmate Change Sccnauos September 9, 2003 , Usmg Uie Boulder C~eek Wateished Model Page 1 ~ ~ Summary ~ • This study evaluated 12 potential water supply/demand `futures' for the City of Boulder, ~ iepresenring a combmation of four alternative projccted future water demands aud three alternatrve hypothetical Uydrologic sccnarios, each depictmg a potential alternative futuie • climate regime + Assummg a contmuation of the climate reg~me of recent history (as evidenced by tree ' ring data from 1703-1987), Boulder s water supply system is capable of ineeting the ~ future water demands associated with Scenario 1, Scenario 2 or Scenar~o 3 as pro~ected ~ by the Jobs/Housmg pro~ect at an acceptable level of reliability and with a reasonable ~ margin of safety Achievmg this positroe ouYcome will require tmplementation of the cdy's comprehensrve watei conservation program and a caprtal development program • aimed at providing adequatc raw water conveyance and treatment capacihes. ~ However, if the population and employment growth associated with the Current Trends ' s watei supply system will not be able to meet scenario is allowed to materialize, Boulder ~ the future water demands associated with this scenario at an acceptable level of reliability ~ and with a reasonable margm oT safeCy unless additional water supplics are acquired oi . developed, or additional reductions m per caprta watei are achieved beyond the levels ' s comprehensive water conservation program anticipated m Boulder • ~ If climate change results in sigmFicantly reduced stream flows, Boulder's water supply • system may not be able to meet rts reliability criteria given cuirenily pro~ected per caprta water uses, particularly if the higher water demand scenarios embodied m Current ~ 1 rends, Scenario 1 or Scenario 2 are allowcd to materialize iJnder such combinations of ~ reduced stream flows and highei build-out demands, addrtional water supplies wonld • have to be acquired or developed, or additional reduct~ons m pei caprta water uses would have to be achieved. ` ~ Predicting how climate change may impact local water suppl~es is at best a lughly i uncertam and immature science. 'l he results of these modcl runs should not be • interpreted as a prcdiction of future events. 1'hc results ate mstiuctrve in that they ~ illustrate thc relationsl~ips between climatc, Bouldei's future watei demands, and the ' s water supply system ieliabilrty of Boulder ~ ` f ~ r r ~ ~ r ~ ~ I Iydrosphcrc Rcsourcc Comid[anh, Inc 1002 W alnn[ SVCCt, Sm[c 200, 6nuldcr, C O 80302 ~ ~ • Simulation of Hypothetical Climate Change Scenai ws Septcmbci 9, 2003 ~ Usmg the Bouldei Crcek Watershed Model Page 2 ~ ~ Introduction ~ ~ Based upon issues raised in Boulder's Jobs/Housing project, there was an identified need ~ to model a range of futuie water demand and supply scenanos for the crty. These , scenarios were defined by the combmation of two vaiiables Boulder's future water demands and the potential ~mpacts of global climate change on Bouldei's water supply l~ system. Wrth mput fi•om Uhhties staff, Hydiosphere developed and evaluated a matrix of + hypothetical scenarios based on these two variables This memo defines these scenarios • and the~r assumptions, and reporis on the resulis of the simulations ~ Scope and Objectives « The ob~ective of this work was to explore the potenhal impacts of climate change on Yhe ' reliability of Boulder s water supply under a iange of projected water demands based on ~ future population and employment assuinptions for Boulder's water supply scrvicc asca. + These scenarios were simulated usmg the Boulder Creek Watershed Model, developed and maintained by Hydrosphere for the city The model simulates all significant aspects ` of hydrology, water riglrts, water storage and diversion facilities, watei uses and return ~ flows in the Boulder Creek basm. The model also simulates the operahon of the ` Colorado-Big Thompson (CBT) and Wmdy Gap pro~ects, from which Boulder obtams a " sigmi icant porCion of its water supply. The model can be run on two alternative sets of ~ hydrologic mput data. historical data far the years 1950-1998, or 300 yeais of data • representing the years 1703-2002 For this latter data set, hydrology for 1703-1987 was ' reconstructed using tree ring data, while historical data were used foi 1988-2002. ~ Thrce variations of the 285-year tree-ring-based portion of the 300-year data set were ~ used m this analysis to poitray alYernative chmate regimes. By combmmg three i variahons of this hydrology wrth four alternativc pro~ections of future water demand, we created twelvc differcnt sccnarios for evaluation ~ ~ The results of these model runs were evaluated agamst the Crty's water supply reliabiltty + criteria, as adopted by the City and described in the Crty's Raw Water and Trcated Water Master Plans and rts recently completed Drought Plan ~ ~ Model Assumptions ~ Hvdroloev: ~ . Three variations of hydrology, iepresentmg alternahve chmate rcgimes as defined below, ` were used as mpuYs to the Boulder Geek Watershed Model It shoLild be noted that these alternative hydrologtes were arbiYrarily selected foi illustration puiposes only While + there ~s a broad consensus within the scientific community Yhat tlie earth is warmmg, . primarily due to human activity, there is very lrttle certamty about implications to water • resouices at a scale relevant to Boulder's water supply system. The ieader is referred to ~ ~ ~ I lydrosphcrc Resouroc Concullanlc, Inc 1002 Wnlnn[ Strcet, Sm[c 20Q Bouldci, G~lorado 80302 ~ ~ r s ~ ~ ,~ ~ ~ ~ ~ ~ ~ ~ a ~ ~ ~ ~ ~ ~ r ~ ~ ~ ~ ~ i ~ i ~ ` a ~ ~ r ~ r ~ r i ~ . • Simulation of 1lypothchcal Clunate Change Scenarios Usmg the Qoulder C~ecl< Watershed Model Septembe~ 9, 2003 Yagc 3 Volumc 2 of Boulder's Drought Plan foi a summaty of tlie potential effects of climate change upon Boulder's water supply system and associated uncertamties Existing treo-ring hydrology, which assumes no climate change compared to what occurred over the past 300 ycars (baseline hydrology). 2. Baselme hydrology modified to rellect a ieduction m mean flow of 15% (15% reduced flow hydrology) 3 Baseline hydrology, but with an increase m annual variability eqmvalent to 0 25 standard deviations (25% increased variation hydrology) Developmg three sets of hydiologic data involved exammation of scveral key aspects of the model's hydrology and demand data. These areas included nahve (virgm) flows m the Boulder Creek basm, pro~ected yields from Boulder's portion oFthe Colorado-Big Thompson and Wmdy Gap projects, competmg M&I and iriigahon dcmands withm the Boulder Creek basm, and calls fiom downstream water iights m thc South Platle. Boulder Creek Watersheci Native Flows: • Por the 15% reduced ffow hydrology, all mdividual quarter-monthly tnflow values at all locations m the model were reduced by 15% Thus, annual ilow volumes werc reduced by 15%, but seasonal stream flow atp terns remamed the same While somc research has suggestcd that climate change may iesult m earlier runoff and lowei late summer stream flows due to moie ram and less snow, we did not attempt to redistnbute seasonal stream flows tn this scenario This decision was made for the sake of simplicrty and conservattsm The degree of shifl m seasonal runoff patterns has not been suggestcd by research to date Eailiei runoff is l~kely to mcreasc the yield of Boulder's water supply because Boulder's reservoits would be able to store more watei befoie the onset of the irrigation season Boulder's ma~or direct flow rights are sidfimently senior m priority and have suffic~ently large physical supplies that Yheu yields would probably not bc sigmficantly dimimshed by reduced late summer stieam flows Thus, maintairung curient seasonal stream flow patterns under the 15% reduced Ilow scenaiio is a conservat~ve assumption. For the `25% mcieased vanahon' hydrology, all annual milows were re-computed by inereasing their annual departure from the mcan annual value based on an inereased standard deviation assumption (1 25 times the original value) Once the annual volumes were scaled appropiiately, mcreases or decreases in annual volumes were distributed among the mdividual quarter-months according to their relat~ve coirixibution to ann~ial volumes, as computed from the ongmal data. The milows modified foi thesc runs mcluded only those occurring upstieam of thc City of Boulder• Boulder Creek above the mouth of Boulder Canyon, South Bouldei Cieek at Eldoiado Sprmgs, and Bcar Creek Lower elevation tributary mflows and mflows representmg ii>>gation return flows wcre not modificd foi this analysis I IyArosphcrc Itctiuurcc Lonsultnnls, Ino 1002 Walnut Slreul, Swtc 200, 13ouldti, Cnlomdo 80102 • ~ Simulatton of Hypothehcal CLmate Change Scenanos September 9, 2003 • Usmg the Boulder Creek Waleished Model Page 4 ~ ` Colorado-13ie Thomnson/Windy Gan Yields: ~ Any chmate change scenano sufficient to alter the hydrology of Boulder Creek would probably also change ihe hydrology ofthe Coloiado River Basm and would affectthe + yields of the Colorado-Big Thompson (CBT) and Windy Gap pro~ects, From which , Boulder obYains a significant portion of its water supply ~stimating the localized • impacts of global climate change upon Coloiado River Basin stream flows is tricky at best, due to the complexities of the Colorado River and Upper Colorado Basin Compacts. + I'or this analysis, we s~mplired the pioblem by ignormg potenual interstate Coloiado M River Compact issues That is, we assumed that the yields of the CBT and Windy Gap i pro~ects would contmue to be a funchon of pro~ect inflows and bypass requ~rements caused by compeYmg water nghts wrthm Colorado alone i ~ For the CBT pio~ect we assumed that the project's yield would continue to be a fLmction ~ of mflows to Lake Gianby atid Willow Creek Reservoir and local bypass requirements ' " ' 1 project s 52,000 AF below those reservoirs. Impl~cri m this assumption is that the CB i replaccment pool in Green Mountam Reservoir would continuc to be sufficient to allow . the CBT project to divert all inflows in excess of local bypass requirements Tree-ring- . based inffows to Lake Granby and Willow Creek Reservoii were origmally derived fiom a correlation wdh tree-rmg-based hydrology for Middle Boulder Creek at Nederland. ~ '1'heref'ore, we simply substrtuted the new Ncderland native t]ow data for the 15% . reduced flow hydrology and 25% increased vanation hydrology mto the model and • recomputed Bouldei's porhon of CBT project yiclds far the climate change scenatio runs. ~ In the model, the Wmdy Gap pro~ecYs potential annual diversions under treo-ring-based ~ hydrology were estimated by correlatmg the pro~ecYs potential annual divers~ons for the , 1950-1989 historical pcriod of record wrth annual flows for corresponding years at the Nederland gage, and applymg that correlation to tree nng-based Nedcrland flows We ~ applied Yhis same corxelation equahon to ad~ usYed flows at the Nederland gage to obtam ` estimates of annual Wmdy Gap potential diversions yields under the 15% reduced flow ` and 25% mereased variation hydrologies V/e distributed annual diversion volumes mto ' s averagc seasonal diversion patterns quarler-monthly portions based upon Wmdy Gap . . Comnetine Boulder Creek Demands: ~ We made no changes to thc baselme model mputs for competing irrigatron and mumcipal ; deniands wrthm the Bouldei Creek basm. Climate change iesearch does not suggest that i drier or wetter annual precipitation trends in the uppei elevations of Boulder Creek and ~ the Upper Colorado River can be correlated wrih drier or wettei summer prempriation m the lower elevations, which would impact these demands. We feel that these variables , are at best very difficult to quantify under climate change scenarios, and doing so may ~ dampen or exaggerate the impacts of the two primary variables of mteiest m this study r' South Platte Calls: ~ ' Water rights m the Boulder Creek basin aie occasionally callcd out by downstream water nghts on the South Platte. South Platte calls affcctmg Boulder Creek generally occur ~ ~ • I lydrotiphcic Rccoumc Consid[anis, Inc 1002 Walnul Shcet, Swtc 20Q I~ouldcr, Colomdo R0302 ~ ~ ~ ~ ~ ~ r ~ i ~ ~ r r ~ ~ ~ ~ i ~ ~ ~ ~ ~ ~ M ~ ~ ` ~ ~ r ~ ~ s ~ ~ ~ ~ ~ ~ ~ ~ • ~ Sunulation of h[ypothet~cal Chmate Change Sccnarios Usmg the Boulder Creek WaCCrshed Model Seplembe~ 9, 2003 Page 5 moie often in dry years, and m years immediately followmg dry years when downstream South Platte reservo~rs aie refilhng after being heavily drawn down. The model includes a repiesentation of South Plattc calls that is based upon historical call records but is `mdexed' to iree rmg-based hydiology In the model, five different South Platte call patterns are defined based on the ielationship between historical calls and general 17ow conditions within the Boulder Creek basin during the current and preceding hydrologic years (e g, very wet, wet, average, diy, or very diy) Generally spcaking, South Platte calls werc; more extensrve dunng and immediately followmg diy and very dry years We used the modePs current defirution of this relationshtp to estimate a new set of South Plattc calls under the chmate change scenai ios VJe used the threshold runoff volumes from the baselme tree rmg hydrology to classify each year undei the cl~mate change scenarios. For the 15% ieduced flow hydrology, tbis resulted in a skewmg of the overall class~fication toward drier years (i c, there were significantly moie "very dry" years, and very few "very wet" years Por the 25% increased vanation scenano, this resulted in fewer "aveiage" values and mereased numbers of both "very weP' and "very dry" years Citv of Boulder Demands Boulder's future water demands for the model runs were based on foui alteinative projections of build-out population and employment for Boulder's water supply service area (which corresponds to Areas I and II of the Boulder Valley Comprehensive Plan) as reported m the iable entitled `Summary of Scenarios and Current Trends', dated 9/24/02 from the Jobs/Housmg Project These pro~ections reflect tl~e Current Trends scenario and Scenarios 1, 2 and 3 as shown in the cited summary. The population, household and~ob estimates fiom the Jobs/Housing Project werc used as mputs to a water demand projection model (developed by Aquacrafl for Boulder's Water Conservation Futures Study) to project watei demands foi each ~obs/population scenario The population, housmg and ~obs assumptions and the resulting water demands based on these estimates are sliown m Table 1 Whilc thc inodeled water demands reffect the demaud reductions that are expected to occur from implemcntation of Boulder's Comprehensive Conservahon Program, they also reflect an adddional 10% safety factor to address uncertamties ielated to pote~lrial mcreases m water-mtensive industrics aud `rcal world vs. modeled' operational factors of the water supply system Table 1. City of Boulder Water llemands for Modeled Jobs/Housing Scenarios. Boulder Water Supply Service Area Areas I and II Pro ected Water Demand, AFIYear With Water Modeled Wdhout ConservaUon, Water House- Water Wilh Water Plus 10% Demand, Scenario Po ulation holds Jobs Conservat~on Conservation Safet Factor AF/ ear Current Trends 126,300 53,900 216,700 31,730 28,780 31,650 31,700 Scenarro 1 132,300 56,500 162,600 28,770 26,020 28,620 28,600 Scenario2 138,500 59,500 145,000 28,340 25,620 28,180 28,200 Scenano 3 131,200 56,200 130,400 26,450 23,840 26,230 26,200 ^yJrovphcrc ltcsomce Consultanls, Inc 1002 Walnul Strcet, Suuc 20Q I3nulder C olorudo 80302 • a ~ ~ ~ ~ ~ ~ ~ ~ ~ r ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ i ~ r ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ • Simulation of Hypothedcal Chmatc Change Scenarios Usmg the Boulde~ Creek Wateished Model Facilitv Canacities, Water Ri¢hts and Drou~ht Resnonse September 9, 2003 Page 6 The model used the same assumptions iegarding Pacility capacities, water itghts and drought responses that were presented in Volume 2 of Boulder's Drought Plan, namely • All existmg raw water storage, diveision and conveyance faciliries which Boulder uses m good working order and capable of operating up to their full capacities • The reliable capacrt~es of the Baxkei Gravrty Lme and the Lakewood Pipelme were assumed to be 26 MGD and 20 MGD, respectively "I'he Betasso plant was assumed to be capable of treatmg these pipelmes' combmed maximum inflows of 46 MGD • The reliable year-round capamty of the Boulder Reseivoir plant was assumed to be 15 MGD yeai-round, includmg when the Boulder Feeder Canal is not operating • Boulder is able to use all of its water rights accordmg to their decrees. • Boulder's raw water dclivery obl~gations to the Silver Lake Ditch, Caribou Ranch and Valmont Reservoir operate accordmg to their respective contractual agreements with iespect to droughts and diought reservatrons. • Boulder's annual leases of water to agricultural users are discontmued durmg drought years so that the watei is available for delivery mto the municipal system • 1'he drought mterruption clause wrthm the donation agrecments to the Colorado Water Conservation Roard ~s mvoked dunng severe drought periods and use of the donated water for mstream Ilow purposes is temporauly suspended ~ Boulder's drought response trtggers and demand reductions, developed as part of Boulder's Drought Plan and summartzed m 1'able 2 below, were mcorporated mto the model. The model simulaYed the evaluation of drought response triggers against modeled Boulder Creek and CBT storage levels and reduced the crty's modeled water demands m accordance the watei use reduction goals. Table 2: Boulder's Drought Response Triggers and Demand Reductions Pro~ected Stora e Index 1 Drought Alert Sta e Total Annual Water Use Reduction Goal Irrigation Season Water Use Reduction Goal Greater than 0 85 None 0% 0% Between 0 85 and 0 7 I 8% 10% Between 0 7 and 0 55 II 14% 20% Between 0 55 and 0 4 III 22% 30% Less than 0 4 IV 40% 55% (1 J Pro~ected storage mdex =(pro~ected usab/e Boulder mountam storage + 40% of Boulder's portion of pro~ected CBT storage)/Boulder's unrestra~ned demand Hydrosphcrc Resourcc ConSUllants, Ino 1002 Walnul Stroof, Swtc 20Q Hoiddc~, ColoraAo 80302 Simulat~on of Hypothetical Chmate Change Scenarws September 9, 2003 Usmg the Bouldei Creek Watershed Model Page 7 Model Results Results from the simulahon runs are presented m summary below. Tables 3-5 show the number of years (out of 285 yeais per simulation) m which different levels of drought response would be necessary Table 3. Frequency and Level of Drought Response Based on 13aseline Hydrology and Alternative Demand Scenarios. Baseline Hydrology Drought Response Level Current Trends Scenario 1 Scenario 2 Scenario 3 0 251 276 278 282 1 22 5 3 2 2 9 3 3 1 3 2 1 1 0 4 1 0 0 0 Table 4. Frequency and Level of Drought Response Based on 15% Reduced Flow Hydrology and Alternative Demand Scenarios. 15% Reduced Flow Hvdrologv Drought Response Level Current Trends Scenario 1 Scenario 2 Scenario 3 0 156 217 224 248 1 48 29 26 22 2 46 22 22 7 3 22 11 8 5 4 13 6 5 3 Table 5. Frequency and Lcvel of Drought Response Based on 25% Incrcased Variation Hydrology and Alternative Demand Scenarios. 25% Increased Variation Hvdrologv Drought Response Level Current Trends Scenario 1 Scenario 2 Scenario 3 0 250 267 268 280 1 18 14 14 4 2 16 4 3 1 3 1 0 0 0 4 0 0 0 0 HyArosphece Rcsoume Consultants, Inc 1002 Walnut Street, Smte 200, C~ouldcr, Cnlomdo ft0302 Simulation of~ Flypothetical Climate Change Scenarios September 9, 2003 Using the Boulder G~eck Watershed Model Page 8 Of these l 2 sccnario siil~ulaiions, only those under the 15~% reduced Ilow hycirology actu~lly resulted in modclcd water shortages lo I3ould~r. Shortages are deli~iecl as deliveries lcss than those spccified under the city's drought response water use reduction goals. "f~h~; fre,quency, avera~;e annual shorta~;e, and maximum annual shorla~e simulated under thesc scen~~rios is shown in I'abla 6. Table G. Shortagcs Under the 1S°/- Reduccd Flow Hydrology (Frequency, Average Annual Shortage, and Maxirnum Annual Shorta~;e). urren Trends Scenario 1 Scenario 2 Scena~io 3 Number of Years With Shortage 18 3 2 3 Average Annual Shortage Volume (AF) 791 989 1,341 528 aximum nnua Shortage Volume (AF} 3,200 1,641 1,382 997 1~igures 1-12 show annual demand, supply, and shortages for the entire 285-year simulation pcriad. 'fhe entire area shaded itl tliese graphs represents Boulder's total dcmand. Variations in demands are due to drought responses as per f3oulder's Drought Plan. Figures 1-4 and 9-12 show no shortages, hence supply equals demand in these charts. For I~igures 5-8, tlie red ai•eas represent shortages in supply. i2e1'er to '1'able 5 1or a summary of these shortages. 35,OD0 30,000 25,000 ~ ¢ T 20,000 a a ~ ~ ro ~ 15,000 c a ~o,aoo 5,000 rigure 1. Demands cXi Supplies: Bascline Hydrolo~;y, Currcnt Trends Scenario (demand = 31,700 AF/year). I iydruspherc KcsuGUCC C'onsullanls. Inc. I l)U2 Wulnul Slrecl, Suitc 200, lioul~l~r. C'ulor~id~, 8030~ 0 1703 1723 1743 1765 1783 1803 1825 1843 1863 1883 7903 1823 19A3 1963 1983 Simulation of I ly~othetical Climate Chaiige Scenarios September 9, 2003 Usin~; the Eioulder Creek Wa~crshed Model Page 9 _ _ ____ __ 35,000 _ _ - _ __ _ .... _ _ ~ _ ~ ~ ~ ~ I 30,D00 zs.oon ~ a ;, 20,000 Q a ~ ~ ~ 15,000 c a 10,000 5,000 I~I I ~ Figure 2. Dem~nds and Supplies: [iascline Hydrology, Scenario 1 (clemand = 28,G00 AF/ye~r). 35,000 i _ - _ _ _ i _ _ 30,000 25,000 ~ ¢ ~, 20,000 n a ~ ~ C 16,000 c Q 10,00a 5,000 [+'igurc 3. I)em~nds and Sup~lics: I3aseline Hydrology, Scenario 2 (dcm~ind = 28,20[1 AF/year). f lydruspli~ro Rcsuurcc ['unsullants. Inc. Ib02 Waluul S1rect, Suitc 200. 13ouldcr. C~uluradu 8f13Q2 0 1703 1723 1743 1765 1783 1803 1823 1843 1863 1883 1903 1923 1943 1963 1983 ~ i~rx.u~-~ne~ .nr:a~a~~ ip~~~ermr~+xnn.~~ry~;:a~[.'~ 1703 1723 1743 1763 1783 1803 1623 1843 1663 1883 1903 1923 1943 1963 1983 Simulation ~~f ! Cypothetical Climate Changc Seenarios Usi~ig th~ [3oulder Creek W~itershed Madcl 35,000 i -~ 30,000 25,000 LL Q ~, 20,000 a a - in ~ 15,000 _ a 10,000 5,000 0 AF ~_,.:::._ __~~ shortaqRS = 0 ~ . _..a... Respninse Level # Years 0 282 1 2 2 1 3 0 a o September 9, 2003 Pa~c 10 O X '~" ~'Si~~RIC'~'ERiE=!'~~':,~I'.:~ _ ' '.~ +~~i. 1703 1723 1743 7769 1763 1803 1823 1843 1863 78f33 1903 1923 1943 1J83 1983 1~ igure 4. Demands & Supplies: I3asel'rne Hydrology, Scenario 3 (dtmand = 2$,200 A1+/year). 35~000 I _ _ ~------~----T---.~-_.~.._..__.-^_~_~ ~0,000 25.000 ~ Q y, 2D,OOD n. n. :~ ~ ~ 15,D00 c Q 10,000 5,000 Figurc 5. Demands & Supplies: 15'% Reduced Nlow 1-lydrology, Current Trends ~cenx~•io (deni~nd - 31,70t) A[+lye~r). I IVdrns~it~cru Itcsuurce C~insuh~m~s. InC 10(12 bV~ilnul ,SlrccL Suile 2Ub, 13oulder, ('nlurailu Kl)10? ~ _ .r.i 1703 1723 1743 1763 1783 1803 1823 1843 1863 1883 1903 1923 1~343 1963 19II3 Simul~llion of I lypothctical Climate Change Scenaric~s lJsing ihe lioiilder Creel< Watershc~ Modcl 35,000 ~ I -~-i --~ 30,OQ0 25,000 I I ~~ ~I ~ Q ~, 20,000 a a ~ m m ~ 15,000 c ¢ lil II ~ Septer7iber 9, 2003 I'age 1 1 ~ Shortaqe C7 Supply Demand equal supply when shortages = 0 ~ p k, ~,ught 10,000 ?; Number ot years wi4h shortage = 3 ~a~;sponse i, Average annual shortage= 989 AF i I.::vei # Years : Maximum annual shortage ~ 0 217 5,000 Year:1889 ~ 1 29 Domand: 22,047 AP I 2 22 5hortage: 1,641 AF i ~ 17 A B ~ ~ ~ _ ~ ~ ~-.. .~~ ~'~I r 1703 1723 1743 1763 1783 1803 7823 7843 1863 1883 1903 1923 1943 1963 19ki3 Figure 6. Demands & Supplics: 15'%, Rcduced Fluw Hydrology, Scenario 1(demand = 28,G00 AF/ycsu•). ss,ooo i _ _-T-_ 30,000 25,000 I ~ I ~~ `~ ~ I~ LL Q ~ 20~~00 a a ~ ~ ~ 16,000 c a Demand equal Supply when Shortages = 0 ' Number of years with ahortage = 2 10,OD0 4; Average annual shortage= 1,341 AF ~: Max annual ahortac~e ~ Year:1889 £ Demand: Z1,736 AF b,ODO `' Shortage: 7,382 AF ; il Shortefle ~j C1Supp~Y ~I # Years 0 224 7 26 2 22 3 B 4 6 0 1703 1723 1743 1763 1783 1803 1823 1843 1863 1683 1909 7923 1943 1963 1983 Figure 7. Demands & Supplies: 15'%~ Rcduced rlow Hydrology, Scenario 2(dernand = 28,200 AF/year). I Iyilrospherc Iteso~u~cc C'onsullnnls, Inc. 1011? Walnut titrcc~. Suilu 1,00, 13oulder. ('ulur~~do R(130' Simulatian of I-Iypothetical Climate Change Scenarios Septcmber 9, 2003 LJsing the [3oulder Creek Watershed Moclel Pa~;e 12 36,000 - r , ~ 30,000 ~ ~ ~ ~ ~ I 26,000 LL Q ~ 20~QQd a a ' ~ m ~ 15,000 c a 10,000 5,000 Fi~;ure 8. Demands & Supplies: 15%, Reduced ~low Hydrology, Scenario 3(demand = 26,200 At~/year). 36,000 30,000 25,000 LL Q ~, Za.O~~ a a ~ ~ ~ 76,000 C a 70,000 6,000 Figure 9. Uemands & Supplies: 25'%~ Increased Variation 1lyclrolc-gy, Current Trends Scenario (dem:~nd = 31,7(i0 AF/year). I lydrosphcre Rcsource ('uutiu~~auls. Inc. I U02 W~~Inul Slrect, tiuilc 2U0, Hotd~lcr. C'uluridu 8U102 O ~r.x~.arrt-.rvaraexa,ux~:rw.wFm.r~a~unna~:~.~ _ ~ .~o~~r_~v:e~_?•h:~~i 1703 1723 1743 1763 1783 18U3 1ti23 1843 1863 1883 1903 1923 1843 1963 1983 0 1703 172~ 1%4a 1763 178a 180~ 1823 1843 1863 1883 1903 1923 1943 1963 1983 ~imulation of f-lypothetical Climatc Changt Scenarios September 9, 2003 Usin~ the Bouldcr Crcek Watershed Modcl Page 13 36,000 30,000 25,000 LL Q T 20i~~~ a a a ~ m e 15,000 c Q 10,000 5,000 Figure 10. Ucmands & Supplies: 25'%- Increased Variatic-n Hydrology, Scenario 1 (demand = 28,600 Ar/ycar). 35,000 30,000 25,000 ~ a ~, 20,000 a a ~ m m ~ 16,000 c 4 10,000 5,000 Figurc 11. Demands & Supplies: 25%, Increased Variation H,ydrolo~y, Scenario 2 (demancl = 28,200 AP/ycar). Iiydn~tiphurc ItcsuurCC C'on~ultanls, -nc. I(10~ Walnul tilrcol. Suilc 2110, 13nuldcr, ('olnr~~do 8U302 Q 1703 7723 1743 1763 1783 1809 1823 1843 1869 1883 7903 1923 1843 1963 1983 0 1703 1723 1743 1763 1783 1803 1823 1843 1863 7883 180a 1823 1843 1963 1883 Simulation o1~llypothetical Climate Change Scenaric~s September 9, 2003 Usin~, the F3ouldcr G~cek W~itcrshcd Moclcl Pa~e 14 36,000 30,000 25.000 ~ a ~, 20,000 a a ' ~ ~ ~ 76,000 C a 10,000 b,000 Figure 12. Demands & Supplies: 2_5°/- lncreased Vari:~tion f-Iydrology, Scenario 3 (demand = 26,200 AE+'/ycar). F.valuation of Results Based on d1e drou~ht r•csponse levels devel~ped i~~ Boulder Di•ought Plan ~ncl F3oulder's water ~u~~ply reliahility criteri~~, rel~ttic~nships can he Iormulated between the city's ccliability crilcria, the City's adopted drou~;ht response levels, and thc~ nutnbcr of allowable uccuri-ences c~ver a 285-year period ol~analysi4, as showri in "I~al~le 7 below. Table 7. Rcliability Criteria A~~plic~l to ,a 285-Year Simulation Period. ~ Corresponding Apprnximatc numbcr of Iteli:~bilit,y Drougl~t Response Draufil~t Recurrence allowable events in 285 Criterion Levcl (D-4) Intcrval year simulation period Full Supply U <20 ycars N/A. Any recluction in ~~~r grcatcr 20 ycars or grcater 14 suPp~Y Continucd Viability of Lawns 3 or grcatcr 100 ycars or grcater 3 and Gardcus Uses essential to basic health, satety 4, plus short~~c I,(100 ycars 0 & welf~re I lydrusph~rc Rcsuurcc; ~'umullanl~. Inc. I(IU2 Walnul SU~ccl. Suitc 20U, ISuuldoi, C'ulnrudu R0302 0 1703 1723 1743 1763 1783 1803 1823 1843 1863 1863 1903 7923 7943 1963 1983 M • ~ ~ ~ ~ ~ ~ ~ ~ ~ s ~ ~ ~ ~ ~ ` ~ ~ ~ ~ s ~ ~ ~ ~ ~ a ~ ~ , ~ ~ ~ Stmulat~on oFHypothehcal Chmate Chenge Scenauos Usmg the Bouldei Creek Watershed Model Septembei 9, 2003 Page 15 By comparing this information with the results of Tables 3 through 6, an mdicat~on can be derived of the ability of Boulder's water supply system to meet its water supply rehabilrty crrteria under the various possible future demand and water supply scenarios Under the baseline hydrology and the 25% mcieased variation hydrology (F~gures 1-4 and 9-12 and Tables 3 and 5), the city's water supply system would be capable (or veiy nearly capable) of ineetmg rts rehability criteria under all demand scenarios except the Current Trends scenario While the numbet of dt ought responses occurnng under Scenarios 1 and 2 combined wrth the 25% mcreased vanation hydrology shghtly exceeds total number allowed under the city's rcliability critena, most of the occurrences would be Level 1 responses, which are voluntary and minor in nature. Under the basclme and the 25% mcreased variatron hydrology, the Cunent Trcnds scenario results m much higher occurrences of drought response levels 1 and 2 than aie allowable under thc city's reliability cntena If the 10% safety factor were ignored, the city's system would be able to meet its reliability crrteria under the Current Trends Scenaiio. However, we recommend agamst ehmmation of a safety factor m the city's water supply planning If the Curient Trends scenario is allowed to materiahze, Boulder would have to acquire or develop additional water supplies, oi reduce per capita water uses beyond the levels anticipated m ds comprehensive water conservaYion program, in order to aclueve its rehabiliry cnteria. Under the 15% reduced ilow hydrology, the crty's water supply system would be unable to meet rts iehability criteria under any demand scenario, unless addittonal water supplies are acquired or developed, or addihonal ieductions m per capita water are achieved beyond the levels anticipated in Boulder's comprehensive water conservation program. I'rom a broader perspective this analysis suggests that, given the chmatc iegime of recent h~story (as evidenced by t~ee ring data from 1703-1987), the city's water supply system would be capable of ineetmg the likely range of future build-out water demand scenarios aY acceptable levels of rehabilrty and with a reasonable margm of safety. Achievmg this posrtive outcome will iequire implementation of the city's comprehensive water conservation piogram and a capital development program aimed at providing adequate raw water conveyance and treatment capacitics The likehhood that climate change will result m sigruficantly ieduced stream ffows m the Boulder Creek basm and the Upper Colorado Rivei Basm is unknown at this time Current iesearch suggests that mcreased stream flows may be ~ust as likely to occui as reduced stream Ilows, although it is probable that peak runoff will occur slightly earlier in the spr~ng and tUat late summer sYream Ilows will be sliglltly lower m the future ~ If climate change does result m sigmficantly reduced stream flows, the city's water * supply system may not be able to meet its reliability enteria given cuirenily projected per caprta water uses, particulaily if the higher water detnand scenanos embodied m Cm rent ~ Trends, Scenario 1 oi Scenauo 2 ate allowed to mateual~za Undei such combmations ~ of sigmficantly reduced stream flows and higher build-out demands, addttional water ~ ~ ~ I Iydrosphcrc Rcsource (.oneullnntti, Inc 1002 W nlnul Shccl, Swtc 200, 13ouldcr, Colorndn R0302 • Simulation of Hypothehcal Chmate Change Scenarios September 9, 2003 Usmg the Boulder Creek Watershed Modcl Page 16 supphes would have to be acquired oi developed, or addihonal reductions m per capita water uses would have to be acl~ieved The most hkely supply-side expansion options would mclude acquisition of addrtional CBT shares, building additional storage for firmmg of Boulder's Wmdy Gap supplies, and fully developmg Boulder's potenttal to reuse its Wmdy Gap watei by exchange. Addittonal reductions in per capita water uses beyond those antictpated in Boulder's comprehensivc water conservation program would probably involve more extens~ve use of xeriscaping Because outdoor use is proport~onately greater withm the resident~al sector, most of the demand ieductious to be achieved by xeriscaping would hkcly occur m that sector HyArotiphorc Rcsource Concnltznls, Ino 1002 Walnut Strwt, Suitc 200, L3oulAcr, Colomdo R0302