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

Heavy releases of stored water in 1976 masked
the effects of the drought, and the low runoff
in 1977 resulted in all-time low levels in most
reservoirs in the fall of 1977. This condition
reduced hydroelectric power generation in
1977 to 30 to 40 percent of average.
Ground-water levels declined as much as 30
feet, about 40,000 wells were drilled, and the
increased withdrawals of ground water caused
the resumption of land subsidence in the San
Joaquin Valley,
Saltwater encroached farther upstream in
the Sacramento-San Joaquin Delta, chloride
concentrations increased in wells near the
Pajaro River, and restrictions were imposed on
the dischange of wastes to maintain water
quality.
Water rationing was widespread in northern
California, and temporary exchanges of water
were arranged to provide additional water
where it was needed.
Early newspapers have accounts of
droughts in 1827-29, 1856-57, and 1863-64,
The latter was probably the driest of all
recorded in Southern California and reduced
the beef cattle industry from a major one to a
minor one (California Department of Water
Resources, 1976), The single driest year since
records began in the early 1890's was 1924, and
the 6- or 7-year drought ending in 1934 was
the driest period recorded, The probability of
occurrence of such a drought has been esti-
mated as between 0.01 and 0.003 or on the
average about once in 200 years (California
Department of Water Resources, 1976),

Precipitation and Runoff

The precipitation in California for the 9-
year period between the 1958-66 dry period
and the 1976-77 drought averaged roughly 105
percent of normal, Only 1971-72 was very dry,
being less than 75 percent of average, and two
dry years did not occur in succession.
The rainy season in California is between
October 1 and April 30, During that period in
1975-76, precipitation ranged from 30 percent
to 90 percent of normal over most of the
State. Only relatively small areas had more
than 100 percent, and these are desert areas
where local precipitation is a minor factor in
the water supply, The largest area with
precipitation less than 50 percent of normal
encompasses the coast from Arroyo Grande

north to the mouth of the Russian River north
of San Francisco, the San Joaquin Valley north
of Madera, most of the Sacramento Valley
south of Red Bluff, and the western slope of
the Sierra Nevada, Seasonal precipitation at
several locations ranked within the lowest four
years of record,
The significance of the areal distribution of
the much below normal precipitation is that
most of the rich agricultural land in the
Central Valley was affected; therefore, the
demands for irrigation water increased. Also,
much of the higher elevations of the Sierra
Nevada was in the deficient precipitation area,
and the snowpack that would normally furnish
water for irrigation later in the season was
well below average,
Shelton (1977) selected the precipitation
records at seven cities throughout the State to
provide a representative index of statewide
precipitation in 1976 and 1977. The cumu-
lative precipitation for the seven locations in
1976 was 58 percent of the cumulative aver-
ages at the same sites and that in 1977 was 54
percent, The precipitation in 1924 at these
locations was 52 percent of average. At four
of the seven cities, precipitation in either 1976
or 1977 was less than that in 1924; and at two
cities, precipitation was less during both 1976
and 1977 than it was in 1924. Shelton con-
cluded that the drought in 1976 was more
severe and more widespread than the drought
in 1924 in northern California and that the
1977 drought was more severe and more wide-
spread than either the 1924 or 1976 drought.
Based on information published by the Cali-
fornia Department of Water Resources (1976),
the water content of the snowpack in the
Sierra Nevada on April 1, 1976 was the lowest
on record at one-third of the snow courses, By
May 1, 1976, 68 of the 222 snow courses
visited then were bare, and the water content
was only 30 percent of normal in the Sacra-
mento Valley watershed and 25 percent of
normal in the San Joaquin Valley watershed.
Satellite imagery showed that the snow cover
in the San Joaquin River basin and the basins
to the south was 2,000 mi 2 on May 1, 1976
compared to 3,650 mi2 on the same date in
1975,
The water content of the snowpack, ex-
pressed as a percentage of the April 1 average,
for the Sacramento Valley and San Joaquin
Valley areas is plotted in figure 24. The data