The Effects of Climate Change on Lake Tahoe, and Implications for Design of Best Management Practices

Transcription

1 Proposal to: Exhibit A Section 205 UC Davis Sponsored Research Manual Pacific Southwest Research Station Tahoe Environmental Research Center, 3rd Floor 291 Country Club Drive, Incline Village, NV Submitting Organization: The Regents of the University of California Office of Research, Sponsored Programs University of California, Davis 1850 Research Park Drive, Suite 300 Davis, California Title of Proposed Research: The Effects of Climate Change on Lake Tahoe, and Implications for Design of Best Management Practices Total Amount Requested: Proposed Duration: Desired Starting Date: ~rinc.ipal Investigator: Department: Phone Number: John Reuter, Ph.D..IM I EITERC (530)

2 A. Project Title: THE EFECTS OF CLIMATE CHANGE ON LAKE TAHOE, AND IMPLICATIONS FOR DESIGN OF BEST MANAGEMENT PRACTICES B. Theme and Sub-theme: Climate Change; Climate Change and Application of Predictive Models C. Principal Investigator (PI) and Co-PI Dr. John Reuter, PI Dept. of Environmental Science & Policy University of California Davis, CA Phone (530) Dr. Robert Coats, Co-PI Hydroikos Ltd th St. Berkeley CA Phone Fax rncoats@ucdavis.edu D. Grants Contact Danielle Neff University of California, Davis, CA (530) dnneff@ucdavis.edu E. Total Funding Requested: $206,853 F. Total Value of In-kind Contribution: $26,651 1

3 II. Proposal Narrative A. Justification Statement In the Tahoe basin and the Northern Sierra, the documented impacts of climate change include 1) an increasing average temperature of Lake Tahoe, accompanied by increased thermal stability and resistance to mixing (Coats et al. 2006); 2) a shift toward earlier snowmelt (Dettinger and Cayan, 1995; Cayan et al, 2001; Stewart et al., 2005; Coats and Winder, 2006a); and 3) a shift in winter precipitation from snow to rain (Coats & Winder, 2006b). It has been suggested that these changes may interact to impact ongoing efforts to improve the clarity of Lake Tahoe. The shift from snow to rain may increase the transport of fine sediment and nutrients from watersheds to the lake. The shift in snowmelt timing may change the insertion depth at which streamflow enters the lake; and increased thermal stability may prolong the residence time of fine sediment near the lake surface. The Lahontan Regional Water Quality Control Board has undertaken a comprehensive program to develop Total Maximum Daily Load (TMDL) allocations for nutrients and sediment in the Tahoe basin. The core of this effort is the development of a set of management options including Best Management Practices (BMPs) that will reduce the load of nutrients and sediment to the Lake. The design and implementation of the BMP projects is closely linked to the existing precipitation and runoff regime in the basin, but with climate change, the frequency, depth and duration of runoff to the lake may change, and with it, the delivery of sediment and nutrients. Managers in the Basin need to answer the question: what are the implications of climate change for the design and selection of BMPs? B. Background/Problem Statement In order to analyze the likely future impacts of climate change and development on Lake Tahoe, four models (or suites of models) must be used together. First, a General Circulation Model (GCM) of global climate must be used to generate future scenarios of climate variables, at appropriate time and spatial scales. To be applied at the scale of the Tahoe basin, the model output must be downscaled using local records of temperature and precipitation. Second, a watershed model or empirical relationships must be used to model or predict stream discharge and loads of nutrients and sediment in response to long-term climate trends. For development of the Total Maximum Daily Load (TMDL) allocations for the Tahoe Basin, Tetra Tech (2007) developed the Load Simulation Program C++ (LSPC) model for basin hydrology, which uses local weather data as the forcing factor, together with watershed characteristics (including existing land use coverage, elevation, slope, and soils) and measured stream discharge and water quality to generate existing condition loads for ammonia, nitrate, organic nitrogen, dissolved phosphorus, and organic phosphorus (Roberts and Reuter, 2007). A second forcing factor in the watershed model is the amount, extent, and location of land cover in each of a set of land-use types. Figure 1 shows a conceptual process interaction diagram of the LSPC model. Third, the climate data and watershed outputs must be used to drive a lake hydrodynamic and clarity model. The UC Davis Dynamic Lake Model (DLM) coupled with the Water Quality Model (DLM-WQ) constitutes the Lake Clarity Model that was developed and used as part of the Total Maximum Daily Load (TMDL) program to meet regulatory water quality requirements (Sahoo et al., 2007). DLM-WQ is a complex system of sub-models including the hydrodynamic sub-model, ecological sub-model, water quality sub-model, particle sub-model and optical sub-model. The conceptual design of the lake clarity model is shown in Figure 2. Fourth, the implications of climate change for the design of water quality BMPs must be analyzed. For the Lake Tahoe TMDL, a model has also been developed to analyze the reduction in pollutant loads 1

4 associated with specific BMPs and sets of BMPs (nhc and GeoSyntec, 200. It can be used to compare the effectiveness of a given BMP design with and without the increased magnitude and frequency of runoff that may result from climate change. Figure 3 is a flow chart showing the flow of information between the models that we propose to use in this project. C. Goals, Objectives and Hypotheses The goals of this project are (1) to determine how climate change will affect the future clarity of Lake Tahoe and (2) determine how climate change will affect BMP effectiveness. These will be accomplished by applying four models: a climate change model (or model output), a watershed hydrologic model, a project-scale BMP model, and a lake hydrodynamic-water quality model. The hypotheses to be tested are: 1. The magnitude and frequency of hydraulic loading to BMPs will increase as precipitation shifts from snow to rain, and rainfall intensity increases. 2. Increasing thermal stability will suppress deep mixing of the lake, and increase the input and persistence of fine sediment and nutrients near the surface. 3. The shift of precipitation from snow to rain, and increased frequency of rain-on-snow events will increase the input of fine sediment to the lake. 4. The changing lake temperature combined with the shift in snowmelt timing will change the spring insertion depth, that is, the depth at which stream water enters the lake during spring runoff. 5. Without application of BMPs to control nutrient and sediment input to the lake, climate change will act to reduce the clarity of Lake Tahoe. D. Approach, Methodology and Geographic Location of Research 1. The Climate Change Models Two GCMs will be used in this project: the Parallel Climate Model (PCM; Barnett et al., 2004, and ) and the Geophysical Fluid Dynamics Laboratory Model (GFDL2.1) (see: The PCM is a relatively conservative model, and generally predicts a lower warming rate than some of the other models in use, whereas the CM2.1 tends to predict a higher rate (Cayan et al, in press). By using results from both models, we will bracket the most likely actual outcome. For each model, we will run two scenarios: the A2 ( Business as Usual ) with accelerating GHG emissions, and the more optimistic B1, in which GHG emissions level off by 2100 (scenario definitions are from the 4 th Assessment Report of the Intergovernmental Panel on Climate Change (2007; see ) Dr. Michael Dettinger (a collaborator on this project) and colleagues at Scripps are producing daily fields of precipitation totals, maximum temperatures, and minimum temperatures, from , downscaled to a 12 km grid over the conterminous US, for the PCM and GFDL2.1) and climate models for two greenhouse-gas-plus-sulfate-aerosols emissions scenarios each (Hidalgo et al., 2007; see Figure 4). Down-scaling will also include solar radiation, specific humidity and wind on a 10-km grid (Figure 4). Further downscaling will be done by adjusting model predictions for the 1990s to match measured meteorological data from SNOTEL sites in the basin (Dettinger et al., 2004). The hydrology model we will use (LSPC, see below) needs hourly temperature and precipitation. The former will be interpolated from maximum and minimum temperature; the latter can be approximated from a statistical analysis of hourly event records. The modeling results will have application to other problems, such a vegetation change and increasing fire risk, and will be made available to other investigators. 2

5 2. The Hydrology model (LSPC) The Tahoe Watershed Model provides a process-based numerical representation of key watershed boundary conditions including flow, total fine sediment, and nutrients. The model has been calibrated and verified using actual stream discharge and water quality data, at daily to annual time scales. The model subdivides the basin into 184 subwatersheds, and uses a digital elevation model to calculate local temperature based on a basin-wide lapse rate and hourly data from 8 SNOTEL sites around the basin. Input data, which will be generated by the climate change models, include hourly values of precipitation, air temperature, wind speed, dew point, and cloud cover; evapotranspiration and solar radiation are calculated from primary data. Outputs include daily stream discharge and concentrations of suspended sediment, total N, and total P. Fine sediment and nutrient species (e.g. NO 3 -N) are estimated from streamwater concentration ratios, using field data. Application of the Lake Tahoe Watershed Model will be the responsibility of John Riverson. The work will be done through the Tetra Tech Water Resources Center in Fairfax, VA and South Lake Tahoe, CA. 3. The Pollutant Load Reduction Model Northwest Hydraulic Consultants (nhc) is leading the development of the Pollutant Load Reduction Model. The beta-version release of the model (December 2008) will estimate water quality performance of Best Management Practices (BMPs) implemented to retrofit existing urban areas in the Tahoe Basin with storm water controls. The model is applied to the spatial scale of typical storm water quality improvement projects (roughly 10 to 100 acres), which are designed and constructed by local Tahoe Basin agencies as part of the Environmental Improvement Program. Three primary components to an urban storm water project are evaluated by the model: (1) hydrology, including the effects of hydrologic source controls in reducing runoff; (2) pollutant load generation, including the effects of pollutant source controls; and (3) storm water treatment, focusing on centralized BMPs that remove pollutant loads from storm water runoff. Using computational procedures focused on long-term continuous simulations of hydrology, the overall pollutant load reduction of a storm water quality improvement project is estimated as the net result of hydrologic source controls, pollutant source controls, and storm water treatment. The Pollutant Load Reduction Model will be used in this effort to assess potential changes in the performance of storm water quality improvement projects, and certain types of controls (private property BMPs, detention basins, etc.), under varying climate change scenarios. The assessment will use meteorological data developed from the climate change model(s) to run the Pollutant Load Reduction Model against a standard representation of controls typically implemented for storm water quality improvement projects. Results from model simulations will be summarized and interpreted to assess how climate change may affect BMP effectiveness and overall pollutant load reduction performance. 4. The Lake Clarity Model Once a historical climate sequence has been defined and modified to include the effect of long-term changes in climate (from the climate change models), and the LSPC run to define the inputs to the lake, the DLM-WQ model will be run with both the future watershed outputs and modified climate conditions as input. The lake parameters that will be generated as output from the lake model include: 3

6 Water quality. This will include the concentrations and depth distributions of phytoplankton, fine sediments, and nutrient concentrations over the entire simulation period, as well as a time series of monthly average Secchi depth. Lake stratification and mixing. The warming of the lake over the last three decades has been shown to suppress deep mixing, and the reduced mixing has important implications for lake clarity and trophic status. The predicted maximum mixing depth and thermal stability will be plotted for future decades. Timing and delivery of stream loading. The depth of insertion of each of the streamflows into Lake Tahoe is a complex process governed by the density (temperature) of each stream, the stratification of the lake, the streamflow and the geometry of the streambed and alluvial fan. A stream inflow that plunges into the hypolimnion of the lake results in different ecological consequences than when the stream inflow is inserted closer to the water surface. The seasonal pattern of Secchi depth will be affected. Stream temperature is estimated by regression with shortwave radiation and air temperature. The model will report the average insertion depth for streams entering the lake, and the changes in this variable over time will be plotted. Currently the lake clarity model uses regression-based estimates of fine sediment. Suspended sediment concentrations (SSC) are estimated by regression with daily discharge, and fine sediment is estimated as a fraction of SSC, for each watershed. Based on extensive field measurements of the particle size distribution of fine sediments (< 63 µm) in Tahoe s tributaries (Rabidoux, 2005), UC Davis researchers have developed a converter that estimates the binned categories of fine particle number (i.e µm, 1-2 µm, 2-4 µm, 4-8, 8-16 µm, µm and µm) based on the projected fine sediment loads (Sahoo, unpublished). Lake level. Some climate change scenarios predict a decrease in total precipitation, and increase in evapotranspiration. These factors may operate together to reduce annual lake levels. If lake level falls below the natural rim, then outflow from the Truckee River ceases. The effect of the climate change on monthly lake level will be examined by comparing modeled lake level under different scenarios, and modeled levels will be compared with the excellent record of historic monthly lake levels. Application of the DLM-WQ model will be the responsibility of Dr. Goloka Sahoo and Dr Geoffrey Schladow. Sarah Null, currently finishing a Ph.D. in Geography at U.C. Davis, will analyze the effect of climate change on lake level. Most of this work will be done at the UC Davis, Tahoe Environmental Research Center. 5. Statistical analysis of trends and relationships Hydrologic and water quality data for Lake Tahoe and basin watersheds must form the basis for any modeling effort aimed at projecting the impacts of climate change and land development. Data are used both to calibrate and verify the models, and to examine historic trends in climate and water quality. This phase of the project will include: (1) updating streamflow, water quality, weather and lake data through water year 2006; (2) analyzing and testing apparent historic and future time trends in snowmelt timing, precipitation form, stream temperature, and lake temperature, stability, clarity, and elevation; and (3) developing and testing empirical relationships for use in the lake clarity model, e.g. the relationship between discharge, suspended sediment concentration (SSC) and fine sediment; Dr. Robert Coats will have primary responsibility for these tasks; the work will be mostly carried out at Berkeley CA. 4

7 6. Error and Sensitivity Analysis Most of the uncertainty in our approach will result from the uncertainty in future GHG emissions and the uncertainty inherent in the climate modeling. By using two climate models and two emission scenarios in each, we should bracket most likely range of outcomes. The central estimate for temperature and precipitation change compiled from Cayan et al. (2006) and Dettinger (2005) shows a Central Projection of a 2 C warming and a 10 percent decrease in total precipitation by To test the sensitivity of the model to variation in climate change projection, one standard deviation above and below the central projection for temperature and precipitation was used to generate a matrix of variability (Tetra Tech, 2007). This resulted in a suite of 11 sets of weather forcing files for the watershed model that represents different combinations of temperature and precipitation change within the standard deviation brackets, including a no-change set for baseline comparison. This analysis characterizes the uncertainty in the hydrology model resulting from uncertainty in the climate models. Additional uncertainty will enter from the downscaling of the climate model results to the basin scale, since this will include adjustment of variables by regression with SNOTEL data. The confidence limits on the regression parameters will be used to set upper and lower bounds for the downscaled estimates of climate variables, and the resulting values will be used as input for runs of the lake model. E. Strategy for Engaging with Managers To solicit input and comments from managers in the basin, and to make them aware of our work, we will hold a one-day workshop in the basin, during the second month of the project. The draft report will be circulated to a broad list (including managers), and feedback will be solicited. In addition, as results are developed, they will be posted on the web site of the Tahoe Environmental Research Center. Dr. John Reuter, PI, is in virtually daily contact with managers and decision makers in the basin, and is well aware of their needs and concerns. Results will be shared with the TMDL agencies and the Tahoe Regional Planning Agency (TRPA) as these and others embark and progress with the TMDL and the Regional Plan that defines the approach for implementation of water quality improvement efforts over the next 20 years. F. Deliverables/Products 1. A detailed work-plan, showing the flow of information in the modeling effort. 2. Quarterly progress reports, describing the climate change and development scenarios chosen for analysis, and preliminary results. 3. A Final Report, describing the methods and results in detail. 4. A public presentation, by poster or talk, in an appropriate conference. 5. A press release for the UC Davis News Service to distribute, highlighting the results of the study. 5

8 G. Schedule of Milestones & Deliverables THE EFECTS OF CLIMATE CHANGE ON LAKE TAHOE, AND IMPLICATIONS FOR DESIGN OF BMPs II-G. Project Schedule Month from Start of Project TASK Start-up meeting and workplan 2. Ageny workshop 3. Downscale climate model results using SNOTEL data 4. Generate input files for Hydrology Model 5. Finalize Load Reduction model 1st Quartlyr Prog. Rep. * 6. Create input files for Load Reduction Model 7. Run Hydrology Model; create input for Lake Model 2nd Quarterly Prog. Rep * 8. Run Lake Model with hydrology and climate change scenarios 3rd Quarterly Prog. Rep. * 9. Run Load Reduction Model with and w/o climate change scenarios 10. Team Meeting First year Prog. Rep. * 11. Analyze model sensitivity and uncertainty Additional Quarterly Reports * * * 12. Write project report; present results in meeting 6

9 REFERENCES Cayan, D. R., S. Kammerdiener, M. Dettinger, J. Caprio, and D. Peterson, Changes in the onset of spring in Western United States. Bull. Amer. Meteor. Soc. 82: Cayan, D.R., Maurer, E.P., Dettinger, M.D., Tyree, M., and Hayhoe, K., in press, Climate change scenarios for the California region: Climatic Change, 39 p. Coats, R. N., and Monika Winder, Climate Change Impacts in the Tahoe Basin: Snowmelt Timing, Lake Thermal Structure, and Phytoplankton Dynamics. Poster for the 3rd Biennial Conference on Tahoe Environmental Concerns. Tahoe Center for the Environmental Sciences, Incline Village. Oct Coats, R. N., and Monika Winder, Climate Change Impacts in the Tahoe Basin: Snowmelt Timing, Precipitation, and Lake Warming. Poster for the 11 th Biennial Conference of the Watershed Management Council. Walla Walla, WA. Oct Coats, R. N., J. Perez-Losada, G. Schladow, R. Richards, and C. R. Goldman, The Warming of Lake Tahoe. Climatic Change 76: Dettinger, M. D., and D. R. Cayan, Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California. Jour. Climate 8: Dettinger, M. D., From climate-change spaghetti to climate change distributions for 21st century California. San Francisco Estuary and Watershed Science 3: Dettinger, M.D., Cayan, D.R., Meyer, M.K., and Jeton, A.E., Simulated hydrologic responses to climate variations and change in the Merced, Carson, and American River basins, Sierra Nevada, California, : Climatic Change, 62, Hidalgo, H.G., Dettinger, M.D., and Cayan, D.R., submitted, Downscaling daily US temperature and precipitation fields using constructed analogues: J. Climate, 24 p. Intergovernmental Panel on Climate Change (IPCC), Summary for Policymakers of the Synthesis Report of the IPCC Fourth Assessment Report. World Meteorological Organization and United Nations Environmental Program. New York. 23 pp. Northwest Hydraulic Consultants (nhc) and GeoSyntec Consultants Methodology to Estimate Pollutant Load Reductions. Prepared for California - Lahontan Water Board. South Lake Tahoe, CA. Roberts, D. and J. Reuter, Lake Tahoe Total Maximum Daily Load Technical Report California and Nevada. California Regional Water Quality Control Board, Lahontan Region and Nevada Division of Environmental Protection. South Lake Tahoe, CA and Carson City, NV. Sahoo, G.B., S.G. Schladow and J.E. Reuter Technical support document for the Lake Tahoe Clarity Model. Tahoe Environmental Research Center, John Muir Institute of the Environment, University of California, Davis. 56 p. Stewart, I. T., D. R. Cayan, and M. Dettinger, Changes toward earlier streamflow timing across western North America. Jour. Clim. 18: Tetra Tech, Inc Hydrologic Modeling and Sediment and Nutrient Loading Estimation for the Lake Tahoe Total Maximum Daily Load. For the Lahontan Regional Water Quality Control Board. Fairfax, VA. Westerling, A. L., H. G. Hidalgo, D. R. Cayan, and T. Swetnam Warming and earlier spring increases western U.S. forest wildfire activity. Scienceexpress /science :

10 III. Figures Figure 1. Lake Tahoe Watershed Model conceptual process interaction diagram. Land Use Atmospheric Deposition Atmospheric Deposition Land Use Tributaries Climate, Precipitation Secchi Depth Abiotic Particles Light Algal Growth Lake N, P, Si Scattering & Absorption Loss Loss Groundwater One- Dimensional Hydrodynamics Tributaries Climate, Precipitation Figure 2. Schematic diagram of the lake hydrodynamic and clarity models. 8

11 Parallel Climate Model Princeton Model Climate Change Scenarios Pollutant Load Reduction Model Tahoe Watershed Model (LSPC) BMP Project- Scale Responses Watershed Scale Responses Lake Clarity Model Lake Responses Figure 3. Flow chart showing the relationship of the models that will be used in this project. Rectangles represent models; ovals represent output, which may become input to the next model. 9

12 Figure 4. Example of 21 st Century trends in downscaled (top panels) and global-climate-model (bottom panels) precipitation and maximum daily temperatures, from Geophysical Fluid Dynamics Laboratory CM2.1 climate model under an aggressive (A2) emissions scenario. 10

13 Figure 5. Example of down-scaled wind speed, from the GFDL model. 11