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
system’s facilities and/or operations both for multiple year drought and long-term average
conditions. A Parsons/CH2MHill report stated, “[HH/LSM] can also be used to assess benefits
and impacts to SFPUC regional water system long-term delivery reliability based on…changing
hydrologic conditions.” The model itself, however, does not use climatological variables directly
as inputs; instead, watershed runoffs are a model input to HH/LSM. Integrating climate models
would require runoff analyses outside of the HH/LSM platform to estimate climate change
effects on system facilities and operations using HH/LSM for utility planning efforts.
Spatial scale: Each of the three watershed areas under consideration: the Tuolumne River
system, the Alameda Creek system and the Peninsula watershed system. Hetch Hetchy
Reservoir, part of the Tuolumne River system is fed by a 459-mi 2 (1,190-km 2 ) watershed. This is
the largest watershed in the system. Alameda Creek system watersheds total approximately
175 mi 2 (for two reservoirs). Peninsula system watersheds total approximately 33 mi 2 (for three
reservoirs).
Time scale: All variables have a monthly timestep.
Input variables: Unimpaired runoff at all reservoirs and some other locations, accumulated
precipitation at Hetch Hetchy, empirically calculated monthly evaporation rates based on
reservoir surface area without using temperature values.
Output variables: Water in reservoir storage, releases and stream flows, water deliveries, and
other parameters associated with the system’s reservoirs, conveyance facilities, and treatment
plants. The model provides information representing monthly volumes of water, although certain
parameters have been converted to flow rates.
A.5 Seattle Public Utilities
Seattle Forecast Model: SEAFM is a hydrology (rainfall-runoff) model used in operational
forecasting and operations planning (also making it an operational model). This model uses
meteorological data as inputs, and could therefore use GCM outputs. SPU uses other tools, such
as spreadsheet models, to support operational planning as well. This model has been updated to
HFAM and is known as HFAMII; this model has more, smaller land segments compared to
SEAFM.
Spatial scale: The largest area that could be modeled is 203 km 2 , as this represents the total area
draining to Masonry Dam on the Cedar River. Depending on which version of this model is used,
the total drainage area is divided into approximately 30 or 300 land segments of differing shapes
and sizes. The land segments range in size from 27 km 2 to 0.021 km 2 .
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