Bulletin 1.pdf - California Department of Water Resources - State of ...

48 WATER RESOURCES OF CALIFORNIA
depth of precipitation were summed, to obtain mean weighted volume of
precipitation for all subareas and Areas of the State.
RUNOFF
Estimates of runoff given in this bulletin generally include only the
portion of precipitation that 1I0ws from mountain and foothill lands. For
the Sacramento Valley, however, runoff from precipitation on the valley
lIoor also is included in estimated runoff.
Runoff data previously published by the Division of Water
Resources, principally in Bulletins Nos. 5, 26, and 29, have been extended
or revised to conform to later information.
The 53-year period from 1894-95 to 1946-47 was selected for determining
mean seasonal runoff, in order to include dry years between 1894
and 1904 in the southern part of the State. The period from 1894 to 1904
was the driest ofrecord in that region, whereas the driest period of record
in the north was from 1923 to 1934.
All estimates of runoff in this bulletin are for naturalllow as defined
earlier in this chapter. In computing runoff data, measured runoff was
adjusted to natural 1I0w by adding known upstream diversions and
quantities stored in reservoirs, and subtracting importations and reservoir
releases. Evaporation from reservoirs and lakes was also added to
measured runoff where large enough to be significant. In the absence of
records at a reservoir or lake, evaporation was estimated from records at
comparable locations. Where no records were available and water was
known to have been stored, storage and release were estimated either
from records of operation in other seasons, or from records of operation
of other reservoirs in the same watershed, taking into account differences
in runoff between seasons with available records and seasons for which
estimates were made. The foregoing method of computing natural runoff
was used only for periods for which records of measured runoff were
available.
One of the three methods described in the following paragraphs was
used to estimate runoff for periods of incomplete records, although the
first and second methods were used to only a limited extent.
The first method involved use of the C Cindex of seasonal wetness,"
which is defined as precipitation for the season expressed as a percentage
of the long-time seasonal mean. In pursuing this method, indexes of wetness
developed in Division of Engineering and Irrigation Bulletin No.5,
"Flow in California Streams," were used. In this previous study the
State was divided into 26 "precipitation divisions" and indexes of seasonal
wetness were determined for each division. A c, curve of probable
runoff" was then drawn for the stream being studied, by plotting runoff
for seasons for which records were available against corresponding
indexes of seasonal wetness. The curves were extended to give runoff for
indexes as great as 200. Runoff for seasons lacking stream 1I0w records
was then taken from the appropriate curve of indexes of seasonal wetness.
In the second method, runoff for seasons of record was plotted
against precipitation, and runoff for seasons lacking records was then
taken from the curve. This method was followed where good coverage of
precipitation records was available over the drainage basin.