WMOD00597

• Project Name: Sierra Ecology Project
• Title: The Sierra Ecology Project - Volume II - Workshop IV, Workshop V
• Date: 7/1/1980
• Weather Modification - Doc Type: Report

water budget of the basin; and (3) modification of outputs from the watershed, again in terms of
particulate matter, macronutrients, heavy metals, and other constituents.
From this standpoint, the role of water is considered partially as a vehicle of transport for the
primary constituents, particulate and dissolved, which could act as pollutants within the system.
Thus, the section on watershed hydrology links the inputs and outputs discussions by describing
the framework through which the nutrients, particulates, and heavy metals move. Several
scenarios on the anticipated effects of extreme precipitation events and the consequences of fire,
logging, and grazing are depicted. These are not doomsday forecasts, but rather a discussion of
effects likely to result under extreme natural or artificial circumstances but within the context of
suspension criteria developed for the SCPP. For example, it is assumed that cloud seeding will
not continue during periods of potential flooding. The final section addresses research and en-
vironmental monitoring needs recommended by the panel for future assessment of possible
environmental impacts.

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BACKGROUND INFORMATION

General Climatic Conditions

Most precipitation in the American River Basin occurs during the fall and winter seasons and is
associated with the passage of cyclonic storms with occluded frontal systems which move into the
area from the Pacific. Precipitation in the basin increases quite rapidly with increasing elevation
from the valley to about the 1680- to 1830-m (5500- to 6000-ft) level. From that elevation to the
Sierra crest, there is little or no additional increase in precipitation with elevation. In most years, a
general decrease in precipitation occurs as one moves from north to south within the watershed.
Average annual precipitation in the American River watershed varies from less than 89 cm (35
in) near the mouth of the basin to over 203 cm (80 in) at some of the higher elevations in the
northern portion of the watershed.
Snowline elevation may vary from below 300 m (1000 ft) to over 2740 m (9000 ft) under extreme
conditions. Some snow can be expected every year in the 910- to 1220-m (3000- to 4000-ft) eleva-
tion range and, in most years, snowpack persists through the end of March at elevations above
about 1680 m (5500 ft).
Three main cloud types produce most of the precipitation in the Sierra Nevada:

Orographic Clouds.- These clouds form as moist air rises over the mountain barrier. These
clouds are quite stable and have very little vertical development. Precipitation from orographic
clouds is generally light but persists for long periods of time (frequently 18 hours or more).

Convective Bands.-Lines of organized convective activity may occur with, or in advance of,
surface frontal systems. Bands move across the valley and foothill areas and into higher eleva-
tions in the watershed.

Cells.-Isolated or embedded convection cells occur most frequently during postfrontal condi-
tions. Cells have a relatively short life span (frequently 1 hour).
Surface wind flow in the Central Valley during prefrontal conditions is generally from the
southeast while winds at higher altitudes are most frequently from the southwest. As the front
and upper-level low-pressure trough passes through the area, surface winds will typically veer to a
more southwesterly direction, while the upper flow becomes more westerly.
Temperature inversions are common during the winter in the Central Valley. These inversions
trap much of the urban pollution in the valley. The temperature inversion is frequently broken as
frontal systems, or stronger convective bands, pass through the area.

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IV-4