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

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.