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HomeMy WebLinkAbout6 - Information Item: Cooperative study on climate change and Boulder's water supply CITY OF BOULDER WATER RESOURCES ADVISORY BOARD AGENDA ITEM MEETING DATE: November 19, 2007 AGENDA TITLE: Information Item - Cooperative Study on Climate Change and Boulder's Water Supply PRESENTER/S: Ned Williams, Director of Public Work for Utilities Carol Ellinghouse, Water Resources Coordinator Joel Smith and KC Hallett, Stratus Consulting Kenneth Strzepek, University of Colorado Lee Rozaklis, Hydrosphere Resource Consultants EXECUTIVE SUMMARY: City staff is working in cooperation with Stratus Consulting and Hydrosphere Resource Consultants on a study of the potential effects of future climate change on water supply availability for the city. Stratus was awarded a grant in September 2005 from the National Oceanic and Atmospheric Administration (NOAA) for the study. Stratus has generated streamflow data under varying future climate conditions for the Boulder Creek basin based on global climate models and local tree-ring based historic climate information. Hydrosphere has completed runs of the city's water system model using the generated streamflow data. This computer model simulates operation of the city water system as it meets a specified demand level and evaluates the ability of the city water rights portfolio to meet that demand without shortages. Work on the study is nearing completion. A one-day workshop is planned to present the result of the analysis to other water managers along the Front Range. Key Issue Identification: One use of the city water system model is to evaluate the adequacy of the city water rights portfolio to meet buildout water needs. These runs use the most current buildout water demand levels available at the time. The water demand projection is based on expected water needs when areas within the city water service area are fully developed according to the Boulder Valley Comprehensive Plan (BVCP). The most recent major update to the BVCP occurred in 2005 and the most recent comprehensive water demand projections and model runs were done for the 2003 Drought Plan. The Drought Plan model runs were based on a buildout demand of 28,600 acre-feet per year. It is estimated that the buildout demand under the 2005 BVCP Update conditions is approximately 25,000 acre-feet per year. AGENDA ITEM 9 PAGE 1 If future climate change occurs in a manner that leads to no change in future streamflow as compared to past streamflow (such as a scenario where increased precipitation offsets higher evaporative losses due to higher temperatures), the city's current water rights portfolio would be sufficient to supply water in conformance with the Council-adopted water system reliability criteria. However, if global warming were to cause severe reductions in streamflow or detrimental changes in streamflow patterns in the basins feeding Boulder's water supply, it could impair the city's ability to meet future water needs. In an effort to reduce the high level of unknowns about how future hydrologic changes caused by global warming might affect Boulder's water yields, city staff pursued a cooperative study with Stratus, NOAA and Hydrosphere to determine the range of outcomes predicted for this region by existing global climate models and then to determine the range of potential effects these scenarios might have on Boulder's future water yields. STAFF RECOMMENDATION: None-Information item only. ANALYSIS: Over the past decade, the city's water resources consultant, Hydrosphere, has developed for the city an extensive computer model of the Boulder Creek basin and its interaction with Boulder's water system. The model is based on diversion records for all water rights within the basin and historic streamflow records for both Boulder Creek and the western slope basins feeding the Colorado-Big Thompson (CBT) system. Several hundred years of historic streamflow data were developed by melding stream gage records with streamflow reconstructions from tree ring data produced by scientists with the NOAA Paleo-climatology Program. The water system model is used to evaluate the expected performance of Boulder's water system and water rights portfolio under various water demand and hydrologic conditions. The 2003 Drought Plan water demand projection for Boulder's service area at buildout was that 28,600 acre-feet per year would be needed to meet all municipal treated water needs in non-drought years. This value corresponded to the buildout population and employment projections at that time, as shown in the table below. This level of demand Population and Employment Values (Current and Projected Buildout) Drought Plan 2005 BVCP Update 2004 Actual Buildout Projection Buildout Projection (2003) Population 111,500 140,500 125,560 Employment 101,100 164,600 171,970 Annual Water 19,400 28,600 25,000 Demand (acre-feet) (avg. 2003-2006) (preliminary estimate) All values are for the BVCP designated Service Area (Areas I and II). AGENDA ITEM 9 PAGE 2 includes implementation of the Council-adopted Comprehensive Water Conservation Program (which includes a goal of a 10% reduction in per capita water use at buildout) plus a safety factor of 10% to allow for unknown factors. The 2005 BVCP Update projects about 15,000 fewer residents and about 7,400 more jobs in Boulder than were used to determine the projected buildout water demand for the 2003 Drought Plan. An updated comprehensive evaluation of water demands corresponding to the 2005 BVCP Update has not yet been done, but is planned. It is estimated that the revised water demand will be very close to 25,000 acre-feet/year, assuming the same 10% reduction due to water conservation by buildout and adding a 10% safety factor. The water system performance is judged based on reliability criteria for the water supply which were established by City Council in 1989. It is expected that the city will provide sufficient water to meet all 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 less frequently than every 20 years. Water restrictions severe enough to cause permanent loss of some trees and shrubs should not occur more often than once every 100 years. Interruption of essential indoor water needs should not occur unless facing a drought year of a severity seen only once in every 1000 years. These reliability criteria became the basis for developing the city's Drought Plan which delineated drought response triggers for the city's four Drought Alert Stages. Modeling of 285 years of the city's water system operations was completed in 2003 for the Drought Plan. The modeling shows that it would be expected that up to 14 years water supply restrictions would occur in the 285 modeled years. Three of these modeled 14 years had restrictions so severe that lawns and gardens were permanently damaged. This level of water reduction in drought years is within the reliability criteria. Recent research on climate variability has indicated that the inter-mountain western United States is experiencing a warming trend that is very likely to persist and intensify in the next fifty to 100 years. While the research is less certain about whether the future precipitation in the Boulder Creek basin will be wetter or drier or whether Boulder Creek streamflows will increase or decrease on an annual volumetric basis, it is likely that mountain snowmelt will occur earlier in the year and that late summer flows will be significantly lower. The resultant effects for Boulder's water system could be anything from an increased average water yield to a decreased yield depending on the timing of seasonal streamflow changes and which of the city's water rights are in priority. The climate study that is nearing completion identified the range and likelihood of occurrence of long-term mean changes in climate predicted for the Boulder area by different existing global climate models. The initial work performed by Stratus and a University of Colorado researcher evaluated 17 different climate models. The models are in general agreement that climate change will result in higher temperatures, but the range of precipitation changes predicted by the various models range from wetter than current conditions to drier. Three models that were believed to best represent the potential climate near Boulder under wetter, mid-range or drier future conditions were selected. These three models were evaluated under three different greenhouse gas (GHG) emission AGENDA ITEM 9 PAGE 3 scenarios representing the range of conditions that might exist in the years 2030 and 2070. The resultant temperature and precipitation changes were then imposed on the historic streamflow record, as extended back to 1566 through tree-ring analysis, to combine the effects of climate change with the previously experienced long-term variability in local climate. Hydrosphere then used the resulting streamflow sequences and climate variations as input into the city's water system model to determine the capacity of the city's water system to meet demands. The results describe Boulder's ability to continue meeting the established water system reliability criteria should the climate change scenarios predicted by the global models occur. For the eight GHG emission scenarios in the year 2030, three showed an increase in occurrence of minor 1-in-20 year droughts such that the city water system did not meet reliability criteria. However, an increase in minor drought effects can occur when reservoir storage levels drop year after year from several mildly dry years occurring one after another. All eight Year 2030 GHG emission scenarios satisfied the reliability criteria for 1-in-100 year and 1-in-1000 year droughts. By 2070, three of the eight GHG emission scenarios fail to meet the 1-in-20 year reliability criteria. One of these GHG emission scenarios is the same as one failing the same criteria in 2030, but the other two are different. In Year 2070, one of the GHG emission scenarios that fails the 1-in-20 year criteria also fails both the 1-in-100 year and 1-in-1000 year criteria. One of the other GHG emission scenarios that fails the 1-in-20 year criteria also fails the 1-in-1000 year criteria, but not the 1-in-100 year criteria. These results are, in part, a reflection of the ability of the city's reservoir storage space to smooth the effects from the uneven pattern of drought years and recovery years. A single severe drought year followed by more typical years may significantly lower reservoirs for a year, but severe droughts occur so infrequently that there is time for reservoir recovery before the next severe drought occurs. Due to the ability of one part of Boulder's water system to offset moderate shortage in other parts for a time, the sequence in which below average years occur can determine if reservoir drawdown amounts will accumulate over several years and result in more frequent minor water shortages. Boulder's water supply system appears to be sufficiently robust to meet reliability criteria in almost all of the future possible climate conditions, although three out of eight emission scenarios show an increased likelihood of minor droughts that may require more years of slight water use reduction for the city's water customers than at present. Alternatively, the city could expand water supplies and/or reservoir storage to prevent an increase in occurrence of effects from minor droughts. Two of the eight GHG emission scenarios result in violation of reliability criteria for the more severe droughts by 2070, but these effects do not materialize by 2030. ATTACHMENTS: Attachment A: Summary of Model Results - Reliability Criteria Attachment B: Background Information on Climate Change AGENDA ITEM 9 PAGE 4 Attachment A: Summary of Model Results - Reliability Criteria 1-in-20 1-in-100 1-in-1000 year year year Emission Model criterion criterion criterion Scenario Type Year met? met? met? Drought Plan (300 years) yes yes yes BASE CASE yes yes es B1 Wet 2030 yes yes yes B1 Mid 2030 yes yes yes B1 Dr 2030 no es es Al B Wet 2030 yes yes yes AI B Mid 2030 yes yes yes Al B Dr 2030 no es es A2 Mid 2030 yes yes yes A2 Dr 2030 no es es B1 Wet 2070 yes yes yes B1 Mid 2070 yes yes yes B1 Dr 2070 es es es Al B Wet 2070 yes yes yes Al B Mid 2070 yes es yes Al B Dry 2070 no yes no A2 Mid 2070 no yes yes A2 Dry 2070 no no no AGENDA ITEM 9 PAGE 5 Attachment B: Background Information on Climate Change AGENDA ITEM 9 PAGE 6 Attachment B Climate Change and Water Resources: A Primer for Municipal Water Providers Authors: Kathleen Miller and David Yates National Center for Atmospheric Research* 3450 Mitchell Lane Boulder, CO 80301 With assistance from: Conrad Roesch and D. Jan Stewart National Center for Atmospheric Research* 3450 Mitchell Lane Boulder, CO 80301 Jointly sponsored by: Awwa Research Foundation 6666 West Quincy Ave. Denver, CO 80235-3098 and University Corporation for Atmospheric Research (UCAR) PO Box 3000 Boulder, CO 80307 Awwa .-:I4V Research UCAR Foundation '111 1 Advancing the Science of Water- *The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation and managed by the University Corporation for Atmospheric Research (UCAR). v Attachment B Hydrologic Implications for Water Utilities Global climate change will likely alter the hydrologic cycle in ways that may cause substantial impacts on water resource availability and changes in water quality. For example, the amount, intensity, and temporal distribution of precipitation are likely to change. Warmer temperatures will affect the proportion of winter precipitation falling as rain or snow, how much is stored as snow and ice, and when it melts. Long- term climatic trends could trigger vegetation changes that would alter a region's water balance. In forested areas, the combination of warmer temperatures and drying soils caused by earlier snowmelt or longer drought periods The amount, Znten~Zty, could cause wildfires to become more frequent and temporal distribution of and extensive. Where that occurs, land cover precipitation are likely and watershed runoff characteristics may change quickly and dramatically as wildfires to change. reduce forest cover, thereby altering the runoff response. Less dramatic but equally important changes in runoff could arise from the fact that the amount of water transpired by plants will change with changes in soil moisture availability, and plant responses to elevated CO2 concentrations. In addition, changes in the quantity of water percolating to groundwater storage will result in changes in aquifer levels, in base flows entering surface streams, and in seepage losses from surface water bodies to the groundwater system. The scientific literature is rich with studies describing the potential influence of climate change on both individual water cycle components and the overall hydrologic cycle. This section of the Primer provides a brief summary of potential impacts on the most important elements. Precipitation amount A change that appears most likely is that global average precipitation will increase as global temperatures rise. Evaporation potential will increase with warming because a warmer atmosphere can hold more moisture. This capacity is governed by the exponential Clausius-Claperyon equation, which states that for a one-degree Celsius increase in air temperature, the water-holding capacity of the atmosphere is increased by about seven percent. A simple-minded explanation for the resulting intensification of the hydrologic cycle is that "what goes up, must come down." Of course, it really is not that simple, but the overall scientific consensus is that globally the Earth will be warmer with higher globally averaged precipitation. Exactly how much global average precipitation will increase is less certain. On average, current climate models suggest an increase 37 Attachment B Change Climate for Water Utilities of about 1-2 percent per degree Celsius due to warming forced by CO2 (Allen and Ingram 2002). An increase in global average precipitation does not mean that it will get wetter everywhere and in all seasons. In fact, all climate model simulations show complex patterns of precipitation change, with some regions receiving less and others more precipitation than they do now. The local balance between changes in precipitation and changes in actual evaporation will determine the net change in river flows and groundwater recharge. In general, the models agree in projecting precipitation increases over high-latitude land areas, much smaller and less certain increases over the equatorial regions, and decreases over some subtropical areas. Elsewhere, precipitation changes are more variable across models (Carter et at. 1999; IPCC 2001). Wigley (2004) has developed a statistical summary of the spatial distribution of precipitation change seen in scenarios generated by various climate models. Figure 21 displays these results in the form of normalized signal- to-noise ratios, where noise represents scatter among model projections. In other words, at the red end of the spectrum, the models tend to agree on increased precipitation. At the opposite end, where the map is shaded in blue tones, they tend to agree on reduced precipitation. However, in the middle of the color spectrum (corresponding to the green and yellow-green tones), the various projections give differing results regarding whether annual precipitation will increase or decrease. This suggests that mid-latitude areas such as the continental U.S. and much of Europe and Asia face an especially uncertain future regarding changes in average annual precipitation. The major difficulty is that although different model simulations are fairly consistent in regional temperature changes, they often display very different regional precipitation 90 w 70-- 30- 10- _10- -30- ` s -50-' - -70 -:180 -1 0 D 6 110 LongAude_pr -2 1 0 1 2 3 -1.5 -0.5 0.5 1.5 2.5 3.5 Figure 21. Inter-model signal-to-noise ratios for annaeal-mean precipitation (paean precipitation change per I oC global-paean waraning, averaged over 17 AOGC111s, divided by the inter-model standard deviation). This is a measure of both the sign and strength of the expected precipitation change and the level of agreement between models. Valuer between -1 and + 1 indicate considerable uncertainty in the expecedted change. 38 Attachment B Change Climate for Water Utilities patterns. To understand why this occurs and what it implies for the usefulness of climate Alid-latitude areas model projections, it is helpful to begin with face an especially an explanation of what climate models are and how they are used to simulate present and future uncertain future regarding climates. changes in average annual Coupled Atmosphere-Ocean General prec-ipatataon. Circulation Models (AOGCMs) are currently the primary tool used to analyze the potential impacts of increased greenhouse gases, aerosols and other factors on global climate. To be useful for the analysis of climate change, the atmospheric model must be coupled to models of other components of the climate system, such as the oceans, the sea ice, and the land surface. The major climate models include tens of vertical layers in the atmosphere and the oceans, dynamic sea-ice sub-models, and effects of changes in vegetation and other land surface characteristics (Gates et al. 1996; Washington 1996). The atmospheric part of a climate model is a mathematical representation of the behavior of the atmosphere based upon the fundamental, non-linear equations of classical physics. A three-dimensional horizontal and vertical grid structure (as depicted in Figure 22) is used to track the movement of air parcels and the exchange of energy and moisture between parcels. Despite tremendous technological advances in computing capability, it is still very time-consuming and costly to use these models to simulate future climates. One of the most important choices for achieving model results in a reasonable amount of time is to increase the model's IN THE ATMOSPHERIC horizontal resolution. This COLUMN limitation means that it is bel wPCn levci prohibitively costly to run full C d, pt- t~ T m E a coupled-climate models at a spatial resolution that would A- THE SORFACE accurately depict the effects of - pc t ~ ~ Hor zonal ex CM1~n~y gY »n m s mountains and other complex r surface features on regional A climates. The problem with such a coarse horizontal resolution is that important processes that occur at finer scales are not i well resolved. Topography, for example, is very important Ti-n e. step-30 minutes Grltl sparn.--3"x in determining the location Figure 22. The strrlctilre of an atmospheric GCM. of precipitation. As moist Soarce: Henderson-Sellers and McGieffae 1987. air rises over mountains or 39 Attachment B Change Climate for Water Utilities hills, the moisture condenses, producing clouds and if conditions are right, precipitation occurs. Although there has been marked improvement over the last three decades, the coarse horizontal resolution of typical climate models still tends to smooth out important landscape features that affect atmospheric processes. At the resolution of most AOGCMs, the models see the mountains of the western United States as a large set of ridges and do not resolve finer-scale features that influence regional climate. Clearly, that spatial resolution is too coarse to reproduce the effects of topography on the region's precipitation and runoff patterns (Grotch and MacCracken 1991; Giorgi and Mearns 1991; Pan et at. 2004). For example, coarse-resolution models would see the Great Basin area as being located on an upslope. They would therefore predict it to be wet, when it is actually a desert. The global-scale models cannot adequately capture the actual rain- shadowing effect of the Sierra Nevada Mountain Range. In short, raw AOGCM output will put the precipitation in the wrong places and perhaps at the wrong time. Recognition of limits imposed by the relatively coarse horizontal scales of AOGCMs has led to the application of "downscaling" as a means of trying to understand how local-scale processes, of greater interest to eater utilities, might respond to larger-scale weather and climate changes (Wilby, et at., 2004). Downscaling includes statistical methods and the use of regional climate models run at a relatively high resolution over a limited area with boundary conditions (and sometimes interior domain information as well) prescribed from the lower resolution AOGCM. Like global climate models, regional climate models will vary in their precipitation projections depending on the Downscaling method, the model specifications, and the AOGCM scenario that is downscaled. While it is possible for a downscaled model to resolve some limitations of general circulation models for a specific region, they are still limited in their capabilities to give reliable projections for future precipitation. Downscaling can produce more sub-regional detail but not necessarily more information. The section entitled "Climate Change Information in Utility Planning" provides a further discussion of the usefulness of downscaled models in generating plausible scenarios for use by water utilities. Precipitation frequency and intensity In addition to changes in global average precipitation, some have argued there could be more pronounced changes in the characteristics of regional and local precipitation due to global -arming. For example, Trenberth et al. (2003) hypothesized that, on average, precipitation will tend to be less frequent, but more intense when it does occur, implying greater incidence of extreme floods and droughts, with resulting consequences for water storage. Their arguments are based on the premise that local and regional rainfall rates greatly exceed evaporation rates and thus depend on the convergence of regional to continental scale moisture sources. They surmise that rainfall intensity should increase at about the same rate as the increase in atmospheric moisture, namely 7 percent per degree Celsius with warming. This means that the changes in rain rates, when it 40 Attachment B Change Climate for Water Utilities rains, are at odds with the 1-2 percent per degree Celsius model estimates for total rainfall amounts as discussed previously. The implication is that there must be a decrease in light and moderate rains, and/or a decrease in the frequency of rain events, as found by Hennessey et at. (1997). Thus, the prospect may be for fewer but more intense rainfall - or snowfall - events. Evaporation and transpiration Evaporation from the land surface includes evaporation from open water, soil, shallow groundwater, and water stored on vegetation, along with transpiration through plants. The combined effect, commonly referred to as evapotranspiration, has a substantial influence on basin water budgets, runoff, and groundwater recharge. There is an enormous hydrologic literature regarding the nature, response, and controls of evapotranspiration under current and future climate conditions, but the interplay between atmospheric energy, moisture, and turbulence, and plant water use efficiency under different water, energy, nutrient, and CO2 levels is complex and not yet fully understood. A consistent prediction of climate models is that global warming will increase total evaporation. Increases in surface temperature and higher wind speeds promote potential evaporation, while the greatest change will likely result from an increase in the water- holding capacity of the atmosphere. While potential evaporation will almost certainly increase with temperature, its impact on precipitation in specific regions remains uncertain. There are many balances and counter-balances in the atmosphere that aren't fully understood. For example, atmospheric moisture originating from actual evaporation over oceans may help offset, and possibly even lessen, potential evaporative pressures over land. Likewise, there are regional controls on evaporation. In humid regions where water is not limiting and actual and potential evaporation are nearly equal, evaporation is constrained by the water-holding capacity of air above the surface, so an increase in this capacity due to warming may have a large evaporative effect. In dry regions, other factors such as surface water availability, surface temperature and wind are more important determinants of actual evaporation. A reduction in summer soil water, for example, could lead to a reduction in the rate of actual evaporative demands from a catchment despite an increase in potential evaporation. Arnett (1996) estimated for a sample of UK catchments that the rate of actual evaporation would increase by a smaller percentage than the atmospheric demand for evaporation, with the greatest difference between actual and potential evaporation occurring in the "driest" catchments, where water limitations are greatest (IPCC 2001). In their 1993 study on the Colorado River Basin, Nash and Gleick demonstrate the importance of evapotranspiration in determining water availability. The study modeled the impact of several climate change scenarios on runoff. Hypothetical precipitation changes ranged from a 20-percent increase to a 20-percent decrease, and the study considered temperature increases of 2°C and 4°C. Their study showed that changes in precipitation would cause proportional changes in runoff, if all else remained constant. 41 Attachment B Change Climate for Water Utilities Therefore, an increase or decrease in precipitation of 20 percent would result in runoff changing by approximately 20 percent. However, the impact of temperature on runoff was also substantial, due to evapotranspiration. The study found that, with no change in precipitation, a 2°C increase in temperature would reduce mean annual runoff by 4 to 12 percent. The change in runoff for a 40C increase would be between 9 and 21 percent. Therefore, if temperature increased by 4°C, precipitation would need to increase by nearly 20 percent to maintain runoff at historical levels. Changes in average annual runoff Runoff changes will depend on changes in temperatures and precipitation, among other variables. A study by Arnell (2003) used several climate models to simulate future climate under differing emissions scenarios. The study linked these climate simulations to a large-scale hydrological model to examine changes in annual average surface runoff by 2050 (Figure 23). The striking thing about this figure is the fact that all simulations yield a global average increase in precipitation (not shown), but likewise exhibit substantial areas where there are large decreases in runoff. Thus, the global message of increased precipitation clearly does not readily translate into regional increases in water availability. In addition, the fact that these different simulations produce quite different Change in average annual runoff: 2050s A2 HadCM3 (A2a) ECHAM4/OPYC vie CGCM2 GFDL_R30 % change compared to 1961-1990 '-3010.20[7-70to00 Fjo0t020,>30 30 Change less than one standard deviation shown in grey Figure 23. Percentage changes in average annual runojj projected by four climate models under IPCC Scenario A2 (Source: Courtesy of Nigel Arnell). 42 Attachment B Change Climate for Water Utilities regional impacts demonstrates the uncertainty related to climate projections. In North America, for example, some models project much larger areas with reduced runoff than do other models. What about natural variability? It is very important to understand that natural variability will not go away. Any projected change in average annual runoff will occur "on top" of ongoing natural variability. In many cases, natural variability can be quite large compared to the changes projected from global warming. Furthermore, relatively short instrumental records may not provide an adequate picture of the full range of natural climatic variability. The work of several researchers who have developed proxy records for precipitation and streamflow based on tree rings and geological evidence provides a longer-term view. Figure 24 provides examples of such proxy records for the reconstructed streamflow of the Colorado River at Lee's Ferry and for the Four Rivers Index in northern California (Sacramento, American, Yuba, and Feather). These 20-year moving averages indicate that both regions have experienced extended periods of drought as well as periods of sustained above-average flow. While there is a very weak positive correlation between annual floes in the two regions, there is no consistent pattern of association for the longer-term fluctuations between wet and dry conditions. For example, northern California experienced an extended dry period from 1918-37, during which time the Reconstructed Flow, 20 Year Mean 22 20- 18- Sacramento R. 16 -Colorado R. 14 12 10 1520 1560 1600 1640 1680 1720 1760 1800 1840 1880 1920 1960 Figure 24. Time series plots of 20-year running nneans of reconstructed flows for Me Colorado River at Lee's Ferry (lower line) and for the Four Rivers Index, northern California (upper line). Data Sources: Stockton and Jacoby 1976, 2004, Meko et al. 2001a, b - available at: http:/ ze u-a: ncde. noaa.govlpaleol recons, htntl. 43 Attachment B Change Climate for Water Utilities Four Rivers Index dropped to 13.55 million acre-feet (Mat) from its long-term mean of 17.4 Maf. At the same time, conditions in the Upper Colorado River were much wetter than the long term mean of 13.5 Maf. On the other hand, the most severe extended drought in the Upper Colorado River Basin occurred during the period 1579-98, when average annual flow was only 10.95 Maf. That same period was among the driest in the northern California tree-ring record (Meko et al. 2001). Paleo-environmental records also indicate Water supplies can that there were some "mega-droughts" in the pre- change dramatically, and instrumental period that were far more severe than any experienced within recent history (Woodhouse for extender' periods. even and Overpeck 1998). without anthropogenic These records suggest that water supplies can change dramatically, and for extended periods, even climate change. without anthropogenic climate change. Where they are available, such reconstructions of past variability could be useful for examining the vulnerability of a water system to conditions outside of the range of recent experience. Temperature, snowpack and runoff There is a high level of confidence in projections of warmer temperatures over most land surfaces. Unlike their projections of precipitation change, climate models are fairly consistent in predictions of regional surface temperature. Because temperature is central in determining the accumulation and melting of snow and ice, these scenarios are especially relevant to regions where snowpack or glacial runoff dominate the hydrology. In a warmer climate, it is very likely that a greater portion of winter precipitation will fall as rain rather than snow, especially in areas where winter temperatures are now only slightly below freezing. An increase in rain events would increase winter runoff but result in smaller total snowpack accumulations. Temperature also determines the timing of melt-off, and a warmer climate will likely result in an earlier melt season. Many regions are likely to see an increase in winter or spring flows and reduced summer flows. In fact, there is evidence that this is already occurring. Studies by Cayan et al. (2001) and Stewart et al. (2004) document the fact that the peak in spring runoff has been arriving earlier in the last few decades (Figure 25). Warmer temperatures could increase the number of rain or snow events in some river basins, increasing the risk of winter and spring floods (Lettenmaier and Gan 1990; Hughes et al. 1993). In currently glaciated basins, declining glacier reservoir capacity may eventually lead to an earlier peak of seasonal runoff and reduced late- summer streamflows. In some cases, increased melting of glacial ice can sustain summer streamflows in the near term but will deplete this source in the long run (Pelto 1993). The loss of snow mass from sublimation (sublimation is the change of ice to water vapor, bypassing the liquid phase) is a critical part of basin-scale water budgets in 44 Attachment B Change Climate for Water Utilities Trends in Cente s ofilV~ass° (1948-2002) 'F w OR ° d ce 9• 0 > 20d earlier^ o • 15-20d earlier r' O 10-15d earlier "a . 4K s O 5-10d earlier <5d O 5-10d later o 40N O 10-15d later ♦„7 ~ • 15-20d later ~'t r 20d later ,0 t ~ 160W 140W 120W 100W Figure 25. Centers of mass of yearly streamflow hydrographs in rivers throughout western North America, based on US Geological Survey streamflow gaging stations in the United States and and an equivalent Canadian streamflow network. Large circles indicate sites with trends that differ sigificantly from zero at a 90 % confidence level,: small circler are not confidently identified. (Courtesy of Al ichael Dettinger, based on Stewart et al. 2005.) snow--dominated regions, but is an understudied topic. In exposed landcover regions such as prairie and tundra, Pomeroy and Gray (1995) estimated sublimation loss of blowing snow to be 15-41 percent of annual snowfall. Sublimation attributed to radiative energy tends to be greater in areas with less cloud cover (e.g., the Southwestern US), as sublimation is enhanced under direct sunlight, since photons of solar energy add the energy necessary for solid ice molecules to escape, On the eastern slopes of the Rocky Mountains, the warm and dry Chinook or "snow-eater" winds will quickly sublimate a snowpack, leading to unexpected reductions in basin water budgets. It is unclear how climate change could affect sublimation dynamics, since all three forces that contribute to sublimation (solar forcing, wind, and blowing snow) could change under anthropogenic warming. Coastal zones The IPCC Working Group 11 (2001) Third Assessment Report identifies sea level rise as one of the most important coastal impacts of global warming, and identifies several key impacts. A number of these are particularly relevant for water utilities located in coastal areas, including: 1) lowland inundation and wetland displacement; 2) altered tidal range in rivers and bays; 3) changes in sedimentation patterns; 4) severe storm- surge flooding; 5) saltwater intrusion into estuaries and freshwater aquifers; and 6) increased wind and rainfall damage in regions prone to tropical cyclones. 45