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(b) Another way to assess the impact of future rainfall is
for a given basin, it ultimately depends upon the capabilities
to test several different future rainfall scenarios. For example,
and resources of local users. Complex models requiring a
the forecaster can examine the effect of 25 mm (1 in.) of
high level of support might be appropriate in cases where
additional rain in the next hour, 50 mm (2 in.) in the next
local skills and resources can handle it. However, the same
hour, 76 mm (3 in.) in the next hour, etc., on forecast flows
model may be entirely inappropriate in situations with lower
by applying these rainfall scenarios to the hydrologic model.
levels of local hydrologic skill and resources.
The sensitivity of the forecast to the different rainfall
scenarios can provide insight into the types of warnings that
(c) To summarize proper selection, one must choose:
might be necessary.
A model that is within the capabilities of the local
(c) If the model indicates that the current hydrologic
user to understand, operate, and maintain,
conditions are extremely sensitive to small amounts of addi-
tional rainfall, a set of aggressive warning and preparedness
A model that is appropriate for the local hydrologic
activities might be called for. If the models indicate that large
variations of future rainfall will not cause significant increases
in forecast flows, emergency officials may choose to watch
A model that will provide the best estimate of the
the situation closely rather than activate emergency plans.
rising limb, the time to peak, and the flood peak.
4-3. Flood Warning and Dissemination
(3) Model Selection. How to choose the "appropriate"
hydrologic model is often debated. A widely cited study by
the World Meteorological Organization (WMO 1975) indi-
A key element of any flood warning system is how users and
cated that the API technique, the Sacramento model, and the
local officials are informed that dangerous conditions may be
Streamflow Synthesis and Reservoir Regulation (SSARR)
developing. Audio alarms and flashing video displays alert
(USACE 1991) model all gave about the same results in
the microcomputer operator that alarm conditions have been
humid climates. However, explicit soil moisture accounting
detected. Voice dial-out systems are turned on to "call" key
models like SSARR and the Sacramento model were clearly
officials and provide a synthesized voice warning via tele-
superior to the API model for arid and semiarid climates. In
phone. Beeper systems can be activated to alert key person-
humid environments, soil moisture conditions are less
nel. Dial-out and beeper systems are especially valuable if
variable than in arid or semiarid climates. The added
alarms are detected during the night or during the weekend
complexity of the explicit soil moisture accounting models to
when the computer system may be unattended. Wall size
handle wide ranging conditions does not contribute
displays can be used as back drops for television broadcasts.
significantly to model performance when conditions are
Television quality video output can also be generated by the
relatively stable. However, when conditions are rapidly
microcomputer for direct distribution over broadcast or cable
changing, some researchers have found that explicit soil
moisture accounting models offer a significant performance
advantage (Kitanidis and Bras 1980).
a. Once emergency personnel have been activated,
(a) When reviewing studies comparing the complex
television can be effective warning tools, especially during
explicit soil moisture accounting models with simpler index
regular broadcast hours. National Oceanic and Atmospheric
approaches, an important insight was noted (Sittner 1985).
Administration weather radios can also be effective dissem-
While the simpler models performed well statistically, com-
ination tools for local warnings if the local agency has an
pared with the explicit soil moisture accounting models,
effective working relationship with, and the necessary com-
significant deviations occurred at key points. These devia-
munication links to, the National Weather Service.
tions, while significant, were rare and tended not to affect the
overall statistics. However, the deviations were frequently
observed when extreme hydrologic conditions existed. The
there is enough time without unnecessarily endangering
complex models could manage the extremes where the
emergency personnel. Sirens and public address systems can
simpler approaches were not capable. These rare events are
also be used to get the word out.
precisely the events that offer the greatest potential for
c. One must be cautious when using sirens. Some-
times siren systems are used for multiple purposes. Different
(b) The choice of models in specific situations remains
siren codes are used to alert for different emergencies. In the
difficult. After the analysis of which model performs the best
Midwestern United States, sirens are commonly used for