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Home My WebLink About6 - Information Item, Transmittal of Report on Climate Change Scenarios Effect on Water Supplyt~ ~' CITY Or BOULD~R WAT~R RESOURCES ADVISORY BOARD INFORMATION IT~M MEETING DATE: October 20, 2003 SUBJECT: Transmittal of Report on Climate Change Scenarios Effect on Water Supply REQUESTING DEPARTM~NT: Carol Ellinghouse, Water Resources Coordmator Beverly Johnson, Planner II BOARD ACTION REQUESTED: None, mformalion item only. FISCAL IMPACT: None Summarv A study of the potenrial effects on Boulder's water supply if global warrtuqg resulted m lowered streamflows is attached and will also be transmrtted to the City of Boulder Plannmg Board. The study evaluated the ability of the crty's presently-owned watei supply to saCisfy needs both for the potential population and employment scenarios under considerat~on for Boulder's future in the crty's dobs/Housing pro~ect and for the Current Trends scenarto that is not intended to occur. The conclusion of the study is that, if severe decreases in streamflow due to global warming occurred, the city would be unable to meet the adopted reliability criteria foi the water supply under any proposed scenario or for Current Trends. If less drastic decreases m streamtlow occurred, future water needs could be met foi all of the pioposed scenarios m accordance with the reliability criteria However, m this instance and, also if no streamflow changes occurred, the city could not meet the water needs of the Current Trend scenario without either ignormg safety maigms or reducmg the estabhshed rehabihty criteria. Study Purpose and Design As a result of the 2002 drought and of the evahiations of scenaiios for Boulder's future population and employment presently occurring through the Jobs/Housmg project, there have been many questions about the city's ability to meet its ultimate water needs. In addrtion to ~ ~ questions about water system reliabilrty if future hydrology is similar to the past, concerns have been raised about the possibility that global warmmg may reduce future water yields In order to address these questions, the attached study, "Simulation of Hypothetical Chmate Change Scenarios Using the Boulder Creek Water shed Model," was prepared for the city by Hydrosphere Resource Consultants It is imporiant to understand that there is presently no mformahon to support a claim that global warming will cause streamflows in Boulder Creek to decrease or that it will cause the city's water yields to drop Many scienhsts agree that global temperatures are increasing, but existrng climate models are not sufficiently complex to evaluate climate changes at a regional or local level. Various models have reached differmg conclusions on possible mcreases or decreases m streamflow m the northern Front Range. Some hmrted research has suggested that climate change may result m earlier spring runoff and lower late summer streamflows m the river basins supplying Boulder's water. Given the configuration of the city's water rights portfolio, this development might possibly mciease Yhe ciYy's water supphes. Given the high level of uncertainty about future hydrologic condit~ons, a study of possible future warst-case scenarios was prudent. The Hydrosphere study evaluates a baseline condition that assumes future hydrology similar to the past and two hypothehcal future conditions with reduced streamflows. The hypothetical scenanos were devised to depict the reasonable limits of possible future streamflow declmes due to global warming, but there is no way to guarantee that this is the case. ReliabrlitV Ciiteria for the City's Water Supply In 1989, City Council estaUhshed reliability criteria for the water supply. It is expected that the city will provide sufficient water to meet aIl municipal water needs up to a drought severe enough to occur only once every 20 years on average. Water for landscaping needs may be restricted for droughts that occur more often than every 20 years. Water restrictions severe enough to cause loss of trees and shrubs should not occur more often than once per hundred years. Interruption of water meeting essential mdoor needs should not occur unless facing a drought year of a severity seen only once m every 1000 years These reliabihty criteria were translated into drought response triggers for the city's four Drought Alert Stages. Based on the 285 years of modeled hydrology in the city's Boulder Creek Watershed Model, it would be expected that up to 14 years with either voluntary or mandatory water supply restrichons would occur in the 285 years wrthout violatmg the rehabilrty criteria. Three of these 14 years could have restrictions so severe that lawns and gardens were permanently damaged. Conclusions If global wazming were to cause sevcre reductions m streamflow in the basms feedmg Boulder's water supply, it would impair the city's ability to meet future water needs to the level 2 A r., 4 of reliability presently anticipated. A 15% reduction m streamflows would gieatly mcrease the number of years when reductions in Boulder's water demand were needed for all anticipated future population/employment scenarios to the point that reliability crrteria could not be met. In addrtion, for all scenarios, there would be several years when the city's water supplies could not meet demand even with the use reduchons called for m the Drought Plan If global warrmng were to cause moderate reductions m streamflow, Boulder should be able to meet its water demands in all years by followmg the guidehnes for implementing water use reductions m the Diought Plan. However, there would be more years when use reductions were called for than are presently anticipated under the city's adopted water system reliability criteria If there are no changes m fuWre hydrology as compared to past hydrology, Boulder would be able to supply water in conformance with both the reliability criteria and the Drought Plan for all proposed future scenaiios. For the Current Trends scenario, water needs would always be met by foliowing the Drought Plan guidelines, howevei, there would be more years of voluntary and moderate mandatory water use restrictions than anticipated in the rehability criteria. 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