Bulletin 160-93, The California Water Plan Update, October 1994

Introduction to Water Use

This part of Bulletin 160-93 covers urban, agricultural, environmental, and recreational water use. Certain key concepts, defined below, are important to understand before reading the following chapters because they are employed in analyzing water use and presenting results of planning studies.

Applied Water Demand: The amount of water from any source needed to meet the demand of the user. It is the quantity of water delivered to any of the following locations:

• the intake to a city water system or factory
• the farm headgate
• a marsh or wetland, either directly or by incidental drainage flows; this is water for wildlife areas

For existing instream use, applied water demand is the portion of the stream flow dedicated to: instream use (or reserved under the federal or State Wild and Scenic Rivers acts); repelling salinity; or maintaining flows in the San Francisco Bay/Delta under State Water Resources Control Board's standards.

Net Water Demand: The amount of water needed in a water service area to meet all requirements. It is the sum of evapotranspiration of applied water, ETAW, in an area; the irrecoverable losses from the distribution system; and agricultural return flow or treated municipal outflow leaving the area.

Irrecoverable Losses: The water lost to a salt sink or lost by evaporation or evapotranspiration from a conveyance facility, drainage canal, or in fringe areas.

Depletion: The water consumed within a service area and no longer available as a source of supply. For agriculture and wetlands, it is ETAW (and ET of flooded wetlands) plus irrecoverable losses. For urban water use, it is ETAW (water applied to landscaping or home gardens), sewage effluent that flows to a salt sink, and incidental evapotranspiration losses. For instream use, it is the amount of dedicated flow that proceeds to a salt sink.

Figures III-A through III-C show examples of how applied water, net water use, and depletion amounts are derived in three different cases. Figure III-A shows how outflow in an inland area is reusable; Figure III-B shows how outflow to a salt sink is not reusable; and Figure III-C shows how outflow in an inland area is reusable when agricultural water use is more efficient.


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