California’s Hydrologic Cycle

The hydrologic, or water cycle is the movement of water
on, above, below, and through the earth’s surface. Processes of the hydrologic cycle include precipitation, runoff, evaporation/evapotranspiration, and condensation.
Processes occurring within the hydrologic cycle are important for both human and ecological systems. For most of California, humans and other living things rely on the annual accumulation and melt‐off of the Sierra Nevada snowpack. The Sierra snowpack acts as a free reservoir feeding
traditional reservoirs and streams. Ecosystems of the Sierra rely on the snowpack as a means of water supply, such as fish populations who rely on snowmelt for migration upstream to spawning grounds. The mechanisms that drive the hydrologic cycle determine whether precipitation falls as rain or snow, which drives how quickly precipitation
flows into streams or infiltrates the groundwater, which dictates water availability for plants and reservoirs, which determines types of vegetation and water supply for humans.
An explanation of the hydrologic cycle as it occurs in California:
Water vapor can be added to an air mass by evaporation from the ocean, fresh water bodies, wet land surfaces, or by transpiration from plants. The Pacific Ocean provides a large reservoir of water for evaporation to occur.
Once water vapor has evaporated off the earth’s surface, the resultant air mass must undergo cooling in order to condense.
Condensation is the change of the physical phase of water from the gas to the liquid phase and generally refers to the formation of clouds in the atmosphere, but can also occur in the formation of
dew and frost.
Once the dew point has been reached, water vapor will then condense and form water droplets and, if this process of condensation continues, precipitation results. Precipitation is the general name given for any form of condensed water that
falls to the earth’s surface. Some common examples of precipitation include rain, snow, sleet, and hail.
The majority of the precipitation California experiences on an annual basis occurs during the winter and spring. Winter storms form over the Pacific Ocean and travel eastward towards the
coast and into the Central Valley typically in the form of liquid rainfall. As storms move east out of the valley, and encounters a mountain range, the air mass is forced to rise.
As the air mass rises, it expands and cools, called the orographic effect. This effect plays a major role in the hydrologic cycle in the Sierra Nevada, resulting in large unloading of precipitation onto
the Sierra Nevada. As storms are forced further up in elevation, the air mass cools further and precipitation falls as snow. The geographic area that this change occurs is referred to as the rain‐
snow transition. Below the rain‐snow transition, the land is dominated by precipitation in the form of rain. Above the transition, snow dominates the precipitation phase.
A mixture of rain and snow account for much of the precipitation at the rain‐snow transition. Snow dominated areas serve as a major reservoir for California water supplies. Whether you live in The
San Francisco Bay Area, greater Los Angeles area, or the Central Valley of California, your drinking water comes, either directly or indirectly, from the Sierra Nevada snow pack.
The following are additional process occurring within California’s hydrologic cycle: Runoff is the movement of water over the land
surface. When runoff reaches a stream, it is called stream flow, or the flow of surface water contained in a stream channel.
Water that does not flow to the stream channel as overland flow goes into the ground as infiltration. This water is referred to as groundwater. Groundwater plays a very important role in
recharging streams in the Sierra Nevada, providing water to plants, as a storage reservoir, and recharging streams and aquifers in the Central Valley.
The movement of water from the liquid phase at the earth’s surface to the gas phase in the atmosphere is accomplished through two main pathways: 1) Water evaporates from surface water bodies (evaporation); and, 2) Water is transpired
from plants (transpiration). Together, this process is called evapotranspiration.
Sublimation is the process by which snow evaporates directly into the atmosphere without ever melting.

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