Options for Improving Climate Modeling to Assist Water Utility ...

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Water Utility Climate Alliance White Paper Options for Improving Climate Modeling to Assist Water Utility Planning for Climate Change Executive Summary This report, which was commissioned by the Water Utility Climate Alliance (WUCA), concerns how investments in the science of climate change, and in particular climate modeling, can best be directed to help improve the quality of science so that it may be more useful to water utilities and other possible users in adapting to climate change. The main focus of this report is the identification of investments in the science of climate change that, in the opinion of the authors, can best improve the science to support adaptation. How utilities are studying climate change and information needs A number of WUCA members including Denver Water, the New York City Department of Environmental Protection, the Portland Water Bureau, and Seattle Public Utilities have examined climate change impacts within their water management systems. Many have used the “scenario approach,” meaning they have selected at least three global climate models (GCMs), often under different greenhouse gas emissions scenarios, in conjunction with statistical and dynamic downscaling methods to capture a wide range of potential changes in climate in specific locations. Reflecting current confidence in the state of the science, most utilities did not examine probabilities of changes in climate at the watershed scale or changes in extreme events. In general, WUCA members prefer to have climate model projections at the same space and time scales as their system models to best capture the physical processes and operations associated with their supplies. Table S.1 summarizes the scale information from the utilities. These spatial scales range from a few kilometers to thousands of kilometers. Most of the utility models for which we obtained information run on daily time steps. Even if scales aligned, many utilities would need hydrologic models to convert temperature and precipitation projections from the models to estimates of runoff and other attributes of water resources. The science of climate modeling GCMs model, as their title implies, global climate. They use computer codes that solve mathematical equations based on scientific understanding of the processes which govern the Earth’s climate and model the atmosphere, the oceans, the land surface, and sea ice. Because of the complexity of modeling, GCMs divide the world into grid cells that are typically one hundred to a few hundred kilometers across. With such large grids, important climate processes such as thunderstorms are unresolved and modeled via simplified process models. However, such processes are incorporated into GCM downscaling tools.
Water Utility Climate Alliance White Paper Options for Improving Climate Modeling to Assist Water Utility Planning for Climate Change Table S.1. Summary of relevant scale information from WUCA utilities Utility Primary utility model Denver Water PACSM 2.6 km 2 New York City Department of Environmental Protection Portland Water Bureau San Francisco Public Utilities Commission Seattle Public Utilities Southern Nevada Water Authority GWLF, VSLF, CEQUAL-W2, UFI 1-D reservoir eutrophication, OASIS Geographic scale (min) (470 unequally spaced model nodes) 25 km 2 (for water quality modeling) Geographic scale (max) 26,000 km 2 (entire modeled region) 5,100 km 2 (entire modeled region) DHSVM 150 m grid boxes 370 km 2 HH/LSM SEAFM/ HFAMII CRSS 4 mi 2 (Pilarcitos reservoir watershed in Peninsula) < 1 km 2 (unequal model nodes) Unknown, but probably specific hydrographic basins (watershed) 1,200 km 2 (Hetch Hetchy Reservoir watershed) 203 km 2 (Masonry Dam watershed on Cedar River) Entire Colorado River basin Time scale (input) Daily (diversions, streamflow, demand, etc.) Daily and hourly Daily (temperature and precipitation, solar radiation, wind speed and direction, humidity) Daily (temperature, precipitation, and demand) Monthly (runoff) Daily minimum/ maximum for temperature and total for precipitation Daily and monthly (temperature, precipitation, and wind speed) Time scale (output) Daily, monthly, and annual (streamflow) (streamflow, nutrients and sediment loads, dissolved particulates, turbidity, phytoplankton, reservoir levels and system status) Daily (streamflow) Monthly (reservoir levels, etc.) Hourly/daily (streamflow, reservoir levels, etc.) Monthly and annual (streamflow and evaporative loss) CRSS = Colorado River Simulation System; DHSVM = Distributed Hydrology, Soil-Vegetation Model; GWLF = Generalized Watershed Loading Function model; HH/LSM = Hetch Hetchy/Local Simulation Model; OASIS = a proprietary model developed by HydroLogics; PACSM = Platte and Colorado Supply Model; SEAFM/HFAMII = Seattle Forecast Model/Hydrocomp Forecast and Analysis Modeling System II; VSLF = Variable Source Loading Function. Page S-2
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