Preparing for Uncertainty: Mitigating Climate Change Impacts in California s Central Valley

Transcription

1 Preparing for Uncertainty: Mitigating Climate Change Impacts in California s Central Valley Nathan T. VanRheenen, S.M. ASCE and Richard N. Palmer, M. ASCE 2 Abstract This paper presents the results of a two-year study that evaluates the potential impacts of climate change on water resources on California s Central Valley. A general circulation model is used to derive meteorological forcing functions that provide inputs for a macroscale hydrological model that simulates streamflows. These streamflows are used as inputs to a water management model that characterizes the performance of the coordinated State Water Project and Central Valley Project under altered climates. Results indicate that both short and long-term climate change will have important implications for future policy and openal decisions in the region. The changes in magnitude and timing of future streamflows under climate change conditions greatly increase vulnerability of the system, leading to an overall decrease in system storage and hydropower production, in addition to decreasing the reliability of environmental and flood control objectives. To address these concerns, a series of alternatives and strategies that mitigate future economic, hydropower, and environmental impacts on the system are developed. Strategies include recharacterization of reservoir-specific and system-wide operating rules and proactive demand management based on predicted future system inflows. Introduction Global climate change is expected to affect the performance of water resource systems significantly (Frederick and Major, 997; Gleick, 2000). In many parts of the world, including much of the United States, the demand for consumptive (water supply) and non-consumptive (navigation, hydroelectric power genen, instream flows) uses of fresh water is precariously balanced by the availability of sustainable surface and groundwater sources. Current indicators, including recent findings from the Intergovernmental Panel on Climate Change (IPCC, 200) strongly suggest that water resources will respond to global warming in ways that will negatively impact both overall supplies and long-term reliability. The ultimate impact of climate change on water resources will reflect the degree to which adaptive management can be effectively applied. In the case of large, complex, and heavily modified river systems, like those in the western U.S., the potential impacts of future climate change can only be understood in the context of the evolving stresses and conflicts that exist among the multiple uses of a basin s resources. When considered in the context of increasing demands and changing water policies, the implications of such variations are greater still. Among the western states, California is particularly susceptible to changes in water supply. Bulletin of the California Water Plan Update (Department of Water Resources, 998) estimates that, at 995 levels of development, water shortages of.6 million acre-ft (maf) occur in average water years. In drought years, shortages as large as 5. maf can be expected. By 2020, it is estimated that the shortages will be 2.4 maf in an average water year and 6.2 maf in drought years. A projected increase in population of nearly 50%, coupled with a 36% increase in urban water use are the drivers for this forecasted increase in water demands. These projected shortages do not, however, reflect any change in availability of water due to climate change. Inclusion of climate change effects would likely significantly impact the PhD Candidate and Professor 2. Department of Civil and Environmental Engineering, University of Washington, Box , Seattle, WA ; PH (206) ; FAX (206)

2 Department of Water Resource s (DWR) water availability projections. These impacts are to be addressed in the next Water Plan Update, scheduled for The State Water Project (SWP) and the Central Valley Project (CVP) coordinate opens of a system of 20 major dams and reservoirs with a combined storage capacity of 2 maf, 3 major power plants, 500 miles of major canals and aqueducts, and various related facilities. The combined SWP/CVP system is one of the largest water storage and conveyance systems in the world. The SWP/CVP system is operated in accordance with their respective water rights permits and licenses, which are administered by the State Water Resources Control Board (SWRCB). The SWP and CVP are required to meet water quality standards and the demands of senior water rights holders while minimizing the likelihood of jeopardizing endangered and threatened fish species (US Bureau of Reclamation, 999). It can be expected that difficulty in reliably meeting California s energy needs, as well as reliably meeting other demands in the system, will become more significant as the impacts of global warming are increasingly felt. The Advanced Climate Prediction Initiative (ACPI) is a project initiated by the U.S. Department of Energy designed to utilize advanced climate prediction capabilities for better assessment of the vulnerabilities of western U.S. water resources systems to climate change. As part of ACPI, the University of Washington (UW) is using the Variable Infiltn Capacity (VIC) macroscale hydrological model, coupled with a water management model, to assess the implications of climate change for the Central Valley of California. Results from this study indicate that both short and long-term climate change have important implications to future policy and openal decisions in the region. Of particular interest are the effects of both the current and future alternative dynamic openal strategies on hydropower genen, water supply, fish endangerment, and flood control. This paper specifically addresses these alternative management strategies. Climate Prediction and Inflow Development A Brief Overview The impacts of climate variability on water resources in the Central Valley are studied using a suite of models that predict future precipitation and temperature, watershed hydrology, and reservoir system performance. The NCAR-DOE Parallel Climate Model (PCM) provides global surface and atmospheric fields, including monthly total precipitation and monthly average temperature, for model integns of length 20 to 300 years. The PCM output is adjusted statistically to remove regional bias and the climate scenario anomalies are downscaled for use as forcings at the finer resolution of the Variable Infiltn Capacity (VIC) macroscale physical hydrology model. The resulting streamflow time series produced by the hydrologic model is used as inflow to the water resources system, Central Valley Model (CVmod). CVmod is a monthly timestep reservoir model developed by the University of Washington that incorporates the major projects and openal features of the Sacramento-San Joaquin River basin and the Sacramento-San Joaquin Delta, and evaluates the performance of each hydrologic scenario with respect to a comprehensive set of system objectives. The analysis of transient climate change effects on land surface hydrology and water resources management is complicated by the natural variability associated with climate. Although most climate change impact assessments to date use transient GCM output that produces a single future climate scenario, such deterministic approaches could well lead to misleading results. To address this issue ensembles of PCM scenarios are used, each giving rise to a different statistical realization of the surface hydrologic forcings and system performance corresponding to a given emissions scenario. Simulated streamflows based on PCM-predicted climate in the Central Valley from have been generated at 8 locations in the region. Details of the PCM model development, bias-correction procedure, and methodology of downscaling to the VIC model have been well-documented (for bias correction, see e.g., Watson et al., 996; Wilby et al., 999; for streamflow scenario development by VIC, see, e.g., Hamlet and Lettenmaier, 999; Lohmann et al., 998a; 998b; Maurer et al., 200; Nijssen et al., 2000). 2

3 Reservoir Opens CVmod is a monthly timestep water resources model that incorporates the major projects and openal features of the Sacramento-San Joaquin basin and simulates the movement and storage of water within the basin given current openal policies. The model domain ranges from Clair Engle Lake and Lake Shasta, near the headwaters of the Trinity and Sacramento Rivers, respectively, to Millerton Reservoir, near the headwaters of the San Joaquin River. It includes many of the major tributaries: the American, Feather, Cosumnes, Mokelumne, Stanislaus, Tuolumne, and Merced River systems. Also included are the Sacramento-San Joaquin Delta and related water supply reservoirs and canals, which most notably include the San Luis Reservoir, Delta-Mendota Canal, and the California Aqueduct. The primary hydrologic input to CVmod is monthly streamflow, which can be taken either from observed natural or unregulated flows (for studies of past climate) or from the hydrology model driven by output from a climate model (for assessment of future climate). CVmod can thus be used to explore system performance and reliability given various operating policies and alternative climate and operating scenarios. The outputs of the model are reservoir levels and releases, from which the predicted performance of the system with respect to such operating criteria as water quality, flood control, hydropower production, agricultural and municipal diversions, navigation, and instream flows for fish is calculated. Model Results and Discussion Future System Inflows Figures and 2 show regulated inflows from the Sacramento and San Joaquin Rivers into the Sacramento-San Joaquin Delta using both the control and climate-change ensemble inflows and given the current operating practices in use by the Bureau of Reclamation (CVP facilities) and California Department of Water Resources (DWR, operating SWP facilities). For those flows entering the Delta from the Sacramento River, yearly volumes of the 20-year monthly mean flows to 2098 are significantly different from the control flow volumes. Generally, spring runoff volumes decrease by as much as 25% from the control. In winter and late spring, this overall decrease in flows relative to the control transitions to a nearly 00% increase in flows from y-ember thousand acre-feet Ctrl mean thousand acre-feet Ctrl mean Figure. Predicted flows at the mouth of the Sacramento River just north of the Sacramento-San Joaquin Delta. These flows are comprised of water that has been released from reservoirs on the Trinity, Sacramento, American, and Feather Rivers and subject to CVP and SWP demands. Unimpaired inflows into the system were developed by the VIC model. 0 Figure 2. Predicted flows at the mouth of the San Joaquin River just south of the Sacramento-San Joaquin Delta. These flows are comprised of water that has been released from reservoirs on the San Joaquin, Mokelumne, Calaveras, Stanislaus, Merced, and Tuolumne Rivers and subject to CVP and SWP demands. Unimpaired inflows into the system were developed by the VIC model. 3

4 Flows entering the Delta from the San Joaquin River are subject to both a volume shift and a slight temporal shift with respect to the control flows. Predicted peak flow shifts from -e to il-, and show a decrease from the control of as much as 50% in the driest 20-year period. The period from ust-ruary shows a less dramatic overall decrease, however, the control mean monthly flow is typically higher than the ensemble flows. Alternatives to Current Rules Flood Rule Manipulation for Timing and Volume Shifts The impact that such substantial changes in flow volume and timing have on the opens of the 6000 Ctrl mean Central Valley system are significant. Figure 3 shows the mean monthly system storage, spill volume, and hydropower produced from in twenty-year groupings. With few exceptions, the lower future 3000 streamflows produce significant decreases from the 2000 control during each twenty-year monthly mean. The 000 flood rule curves that ensure ample space to protect vulnerable areas downstream are poorly designed to handle long periods of decreased inflows. Generally, the current rules do not allow the reservoirs to both 00 meet downstream demands and maintain ample 80 hydraulic head to generate the maximum energy possible for that month. 60 To better accommodate the inflows generated by the VIC model, several flood rule curve-based alternatives that would address these issues were generated. The first of these alternatives attempts to conserve previously spilled water by allowing the reservoirs to refill -2 months earlier. This alternative achieves two goals (Figure 4). First, it increases the mean monthly storage over the control by as much as 7% from , and by nearly 9% from , and allows for an overall decrease in yearly spilling. The second goal achieved is an increase in hydropower production from the control of nearly 20,000 MW-hr/year. The large decrease in power production from the control during ruary and ch can be attributed to the Lake Oroville Complex and its many flow-through power-producing stations. The increased storage it experiences with an earlier refill results in decreasing the releases necessary to produce the bulk of its hydropower. mega watt-hours thousand acre-feet thousand acre-feet Figure 3. Predicted Central Valley total system storage (a), spills (b), and hydropower (c) given current operating rules. The second alternative also augments the flood control rules. In this case, however, the flood rule of each reservoir is shifted either as per alternative one, or by the of the future inflow to the control (historic) inflow at 0 probability (by cumulative distribution function) for each month. The determination of the best fit of either the timing shift (0,, or 2 months) or the probability-based shift for each reservoir was determined by inspection of storage volumes, spill volumes, hydropower production, and reliability. The of storage, spill, and hydropower production to the control (spill is control/spill) for this alternative is shown in Figure 4. The timing + volume shift alternative further improves upon the timing only alternative in every regard. a) b) c) 4

5 ..05. a) b).05 ra tio.9.6 De c.8 Ja n M c).6 d).4 ar De c Ja n.2 e) f) un J g Au Se p.. De c, 2-month shift Timing + Volume shift Shifts + Delivery Reduction No Alternative Figure 4. Ratios of mean monthly Central Valley storage (a-b), spills (c-d), and hydropower production (e-f) of ensemble to control climate change scenarios for various alternatives not subject to demand growth. Figures on the left show s from Figures on the right show s from Alternatives to Current Rules Water Delivery Modification Several scenarios for water delivery modification and reduction have been considered. Figure 4 shows the resulting storage, spill volume, and hydropower production for an alternative considering a moderate water conservative scenario based upon yearly inflows into the system. Using DWR and Bureau of Reclamation guidelines for delivery reduction, CVP agricultural deliveries are reduced by up to 50% and CVP municipal deliveries by 25% in critically dry years (Tier 2 delivery reduction per ision 3406 (b)(2), Bureau of Reclamation, 2000) and SWP agricultural deliveries are reduced by up to 30% in critically dry years (DWR, 998). As expected, the increased availability of water due to the decrease in water deliveries downstream allows the reservoirs to refill to higher-than-control levels and, as a result, increase overall hydropower production. Managing for the Impacts of Changes in Agricultural, Municipal, and Environmental Needs It is unrealistic, however, to expect that the proposed combination of timing + volume shifts and delivery reductions would so easily address the water supply problems faced by the Central Valley. Expected population growth will greatly impact future water supply needs. This alternative addresses the municipal and agricultural demand growth predicted by the State of California (DWR, 998). Yearly demand growth is given in Table. Because total system demands are nearly 70% agricultural, the increase in 5

6 municipal demands is overshadowed by the small decrease in agricultural needs. As a result, the State predicts an overall decrease in total water demand from 995 to 2020 in the Central Valley. This is primarily due to conservation through crop and irrigation management. (Though not discussed in this paper, we are also considering alternatives where agricultural demands do not increase as expected.) Table. Predicted growth rates for agricultural, municipal, and environmental water demands in the Sacramento and San Joaquin River Basins. Rates extrapolated from 2020 predicted water usage in the Water Plan Update (DWR, 998). Demand Type Yearly Growth Rate (%) 995 Total (taf) 2020 Projected (taf) Sacramento Agriculture ,065 7,939 Municipal ,39 Environmental ,833 5,839 Total 4,660 4,920 San Joaquin Agriculture ,027 6,450 Municipal Environmental ,396 3,4 Total,000 0,820 It would be expected that the future decreasing total demands impact positively the mean monthly storage volumes, thus decreasing hydropower production when compared to the no alternative scenario. This is precisely what occurs for most of the reservoirs in the system. However, for those reservoirs that support high municipal demands relative to agricultural demands, the opposite effect occurs. In the Central Valley, the linked SWP reservoirs of Lake Oroville and San Luis Reservoir show such an impact. The mean monthly storage curves with projected demand growth calculated in Figures 5a and 5b show just how significant the impact of increasing municipal demands is on Oroville and San Luis. The decrease in the system storage s to below the no alternative storage curve are primarily due to Oroville and San Luis. The mitigation alternatives, however, progressively increase the storage, spilling, and hydropower s to nearly the same level as without projected demand growth. As was the previously the case, the temporal + volume + delivery reduction alternative achieved the best performance, and succeeded in regaining nearly 200,000 megawatt-hours of total power that would have otherwise been lost. Assessing Tradeoffs The impacts of potential increases in environmental demands (fish flow targets) and reductions in water delivery on hydropower production are addressed in a series of tradeoff curves. Figure 6 illustrates the impact that increasing fish flow targets by 5%, 0%, and 20% has on CVP hydropower production (given the timing + volume shift alternative). In general, the system shows good resiliency with little change in production with increasing fish targets. ust and ober show the greatest difference in production during both time periods. Agricultural and municipal demand volumes have a greater impact on production, as shown in Figure 7. It is of no surprise that the greatest reduction in deliveries results in the highest levels of hydropower produced, since the power-producing reservoirs are better able to maintain higher heads. 6

7 ..05 a) b) v c) d).6 No Ju l No v Ja n e) f) ul J Projected Demand Growth Dem Growth + Rule Adj Dem Growth + Rule Adj + Del Red No Alternative Figure 5. Ratios of mean monthly Central Valley storage (a-b), spills (c-d), and hydropower production (e-f) of ensemble to control climate change scenarios for various alternatives subject to projected demand growth (see Table ). Figures on the left show s from Figures on the right show s from a) b) Figure 6. Ratios of mean monthly CVP hydropower production to control production given gradually increasing fish flow target volumes. 6a shows from b shows from % Increase 0% Increase 20% Increase Timing + Volume Shift 7

8 a) b).05 Typical (Up to 50% Ag, Up to 25% MI) Moderate (U p to 75% Ag, Up to 35% MI) Severe (Up to 95% Ag, Up to 50% MI) Timing + Volume Shift Figure 7. Ratios of mean monthly CVP hydropower production to control production given gradually decreasing levels of agricultural and municipal delivery volumes and predicted growth of demands required. 7a shows from b shows from Summary and Conclusions This paper presents results of a study that evaluates the impacts of a global climate change on the Sacramento and San Joaquin River watersheds. These results were generated by integrating the outputs from global circulation models and physically based hydrologic models into a water resource simulation model. The findings indicate that, for climate conditions predicted to occur through 2098, impacts on performance of the CVP and SWP facilities in the Central Valley would be significant. These impacts result from a decrease in total annual runoff and a temporal shift in peak flows. The mean annual system storage decreases more than 7% and annual energy produced decreases over 2%. The impact of such a decrease on power production translates to a total annual decrease of over million megawatt-hours of power. Predicted increases in water demands increase the annual loss in hydropower generated to. million megawatt-hours. Mean annual system storage decreases by nearly 3% given projected demand growth. To mitigate these predicted impacts, a suite of alternatives are developed that address the temporal and volumetric differences in future system inflows and incorporate the impacts of more proactive demand management. While it is impossible to restore hydropower production to current (or control) levels, it is possible to recover nearly 200,000 megawatt-hours of power that would have been lost given projected demand growth and current management strategies while still meeting environmental requirements and scheduled release rules. Through more stringent demand management, an additional 50,000 megawatt-hours could be recovered, as well. It is expected that additional recovery could occur with better crop management strategies. Investigations are underway to evaluate the economic impacts that crop management strategies have on hydropower production in the system. Currently, hydropower production comprises approximately 20% of the total power produced in California. As additional power sources come on-line in the Central Valley, it is expected that the contribution of hydropower to the whole will change. This contribution may be based on the relative tradeoff between the revenue generated by agriculture and the revenue generated through hydropower. Determining the point at which the value of hydropower production surpasses the value of crop production may help to better understand future prioritization and management of future water supplies in the region. Finally, the Central Valley system is appreciably resilient. This, in part, is due to its complexity, inter-connectedness, and over-built infrastructure. Over the years, as new facilities and plants have come on-line and demands have changed spatially and volumetrically, the capabilities of CVP and SWP operators have been pushed to their limits to achieve a modicum of success. Issues of yearly demand management are difficult to conceptualize, at best. In any other system, deliveries of 45% of entitlement 8

9 would be disastrous for agricultural customers. In California, such a delivery level is common (and is the case, as of uary, 2002). Even so, this delivery can change before the year is over. Our intent with this project is to demonstrate the potential for use of linked climate, hydrology, and reservoir opens models to improve understanding of the implications of climate variability and change on water resources in the western United States. The upcoming California Water Plan Update will address the issues of climate change impacts on the future open of the state s water supplies. We believe that the lessons learned in this study are well-suited for the Central Valley, and should serve as building blocks to adapt the current rules to rules better adapted for the future. References Department of Water Resources. (998). California Water Plan Update. Bulletin California Department of Water Resources, Sacramento, CA, ember, 998. Frederick, K. D., and Major, D. C. (997). Climate change and water resources. Climatic Change, 37, Gleick, P. H. (2000). Water: The potential consequences of climate variability and change for the water resources of the United States, Report of the Water Sector Assessment Team of the National Assessment of the Potential Consequences of Climate Variability and Change, Pacific Institute, Berkeley, CA. Hamlet, A. F, and Lettenmaier, D. P. (999). Columbia River streamflow forecasting based on ENSO and PDO climate signals. Journal of Water Resources Planning and Management, 25(6), Lohmann, D., Raschke, E., Nijssen, B., and Lettenmaier, D. P. (998a). Regional scale hydrology I: formulation of the VIC-2L model coupled to a routing model. Hydrological Sciences Journal, 43(), 3-4. Lohmann, D., Raschke, E., Nijssen, B., and Lettenmaier, D. P. (998b). Regional scale hydrology II: application of the VIC-2L model to the Weser River, Germany. Hydrological Sciences Journal, 43(), Maurer, E. P., O'Donnell, G. M., Lettenmaier, D.P., and Roads, J.O. (200). Evaluation of the land surface water budget in NCEP/NCAR and NCEP/DOE AMIP-II reanalyses using an off-line hydrologic model. J. Geophys. Res., 06(D6), 7,84-7,862. Nijssen, B. N., Schnur, R., and Lettenmaier, D. P. (200). Global retrospective estimation of soil moisture using the VIC land surface model, J. Clim, 4, U.S. Bureau of Reclamation. (999). Central Valley Project Improvement Act: final programmatic environmental impact statement. U.S. Department of the Interior, Sacramento, CA, ober, 999. U.S. Bureau of Reclamation. (2000). ision 3406(b)(2). U.S. Department of the Interior, Sacramento, CA, ober, 999. Watson, R. T., Zinyowera, M. C., and Moss. R. H. (996). Climate Change: Impacts, adaptations, and mitigation of climate change, 995, 879 p., Cambridge University Press. 9

10 Wilby, R. L., Hay, L. E., and Leavesley, G. H. (999). A comparison of downscaled and raw GCM output: implications for climate change scenarios in the San Juan River basin, Colorado. J. Hydrol., 225,