WSP08775

• File Number: 8056
• Description: Drought Preparedness
• State: CO
• Basin: Statewide
• Date: 1/1/1979
• Author: USGS
• Title: Hydrologic and Human aspects of the 1976-77 Drought
• Water Supply Pro - Doc Type: Report/Study

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HYDROLOGIC AND HUMAN ASPECTS OF THE 1976-77 DROUGHT

available supplies as equitably as possible,
consistent with the legal doctrine in effect.
In some places, local water users arranged
exchanges among themselves. In Colorado,
downstream water users temporarily relin-
quished their rights to released flows in streams
and in turn received water from nearby irri-
gation ditches. The very high transit losses in
alluvial streams would have depleted the re-
leased flows excessively; thus reducing the
supply at the time it is needed.
Normally irrigation is not needed in Geor-
gia; but if the farmers could find irrigation
pipe and other accessories, they irrigated for
the first time since the 1950's. Soybean, pea-
nut, and other crops were not planted as early
as usual because moisture is needed to acti-
vate the chemicals in pesticides and weed
control agents.
Innovative farmers in California produced
nearly the normal value of crops on about 80
percent of. the usual water supplies (H. W.
Greydanus, oral commun.; 1977).

The Role of Forecasting

Weather -Forecasts of weather for water
supply are more complex than the day to day
forecasts of rain or sunshine. The reliability
of weather forecasts beyond a week or so
decreases rapidly, though some forecasters
will challenge this statement. The National
Weather Service publishes 30-day forecasts in
general terms; that is, the broad areas of the
United States where temperatures or precipi-
tation will be above or below normal. Fore-
casts are made twice a month. Some meteor-
. ologists in private practice make longer range
forecasts for their clients.
A man in Wisconsin maintains that a global
cooling is occurring that could extend the
typical 5-year drought cycle of the past to a
10-year one. Others contend that there is no
evidence of any "cycle" in the weather; where-
as, others have postulated cycles of various
lengths based on sun spots, tree rings, and
other evidence.
In this aecision-making process, the water
user considers the weather forecasts along
with other factors affecting his particular
endea vor such as economic, personnel,
environmental, or social constraints.
One problem with weather forecasts is the
wide range of predictions both by the profes-

sionals BJ1d by self-styled experts. Forecasts
for the" 1977-78 winter for the West Coast
States ranged from well below normal to well
above normal. It is the writer's opinion that
many decisions made by water managers were
based more on the chance that the drought
would continue beyond 1977 rather than on the
determination that a given weather forecast
was reliable or unreliable.
Weather forecasts are also an integral part
of weather modification activities. To seed or
not to seed is the question. The right kind of
clouds over the target area are essential; but
during a drought, the frequency of occurrence
a t the right place is markedly reduced. There-
fore, weather modification during a drought is
apt to produce the least amount of water when
it is needed the most.
Runoff-Several Federal and State
agencies have developed techniques to fore-
cast snowmelt runoff for specific seasons such
as the period April 1 to July 31, or October 1
to September 30 which is the water year, or
the calendar year. The techniques make use of
runoff, precipitation, water content and extent
of. the snow pack, and soil moisture conditions
up to the date of the forecasts. Several
forecasts are made usually for selected
amounts of precipitation during the remainder
of the forecast period and a range of tempera-
tures that might be expected.
Recently digital models have been deve-
loped to estimate snowmelt runoff. A major
advantage of such models is that they do not
require the field data related to the snowpack
that are expensive and sometimes hazardous to
collect. Also, forecasts can be made whenever
a set of precipitation data is obtained rather
than once a month. The models are calibrated
using low-elevation precipitation and runoff
data from previous years. Public utilities,
irrigators, and other water users rely heavily
on runoff forecasts to plan their management
of the water resource. An unreliable forecast,
whether too high or too low, can mean the loss
of considerable income by everyone involved
with water.
No matter which technique or model is
used, drought conditions usually make it neces-
sary to extrapolate beyond the calibration
data. Therefore, the reliability of the fore-
casts is reduced. Even so, most of the runoff
forecasts made in 1977 turned out to be good
estimates of the runoff that did occur. One