Introduction

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Water development in California is among the most extensive in the world, with water shifted from one basin to another over distances of hundreds of kilometres in order to satisfy water demands. The main river systems are the Sacramento and San Joaquin Rivers that converge in a region known as the Delta that then flows into San Francisco Bay - the combined area is referred to as the San Francisco Bay Watershed (SFBW). Below Fresno, the Central Valley is in fact a closed basin associated with what was once the Tulare Lake, although the lakebed itself has been reclaimed for irrigated agriculture through the impoundment and regulation of the rivers entering that portion of the valley (Fig. 11.1). Most water management issues relate to ever increasing urban demands competing with agricultural and ecosystem needs. Primary among the sources of water available in California is the Sacramento River, which not only supplies demands within the basin and the critical agricultural area of the Central Valley, but also supplies municipal and industrial demands on the Southern California Coastal Plain between Los Angeles and San Diego (Table 11.1). As such, the water resource situation in the Sacramento Basin cannot be discussed in isolation from the situation statewide.

It is anticipated that metropolitan regions in the Central Valley, such as Sacramento, will continue to grow dramatically in the future as the large coastal metropolitan areas, such as the San Francisco Bay area and Los Angeles, become increasingly crowded. This urban expansion is taking place beside large-scale agriculture, therefore competing with both land and water resources.

The Sacramento River and its tributaries convey 31% of California's average annual runoff, a water resource that has supported the development of over 850,000 ha of irrigated agriculture in the basin, as well as expansive irrigation development in © CAB International 2004. Climate Change in Contrasting River Basins

Fig. 11.1. Relief map of the State of California, with the location of the Sacramento River Basin.

other parts of the state. The principal crops grown in the Sacramento Basin include rice, olives, orchard fruits and nuts, maize, lucerne, tomatoes and vegetables. For many of these commodities, the basin is a globally important production centre.

In addition to urban and agricultural use, the waters of the Sacramento Basin also support several important ecosystems. Not surprisingly, the development of irrigated agriculture has dramatically changed the natural landscape in the basin. Today only 5% of historic wetlands in the Sacramento Basin remain, and these are contained largely within the 13,000 ha of the Sacramento National Wildlife Refuge Complex. In addition, only 5% of the original 200,000 ha of riparian forest along the river and its tributaries remains. While the Sacramento River and its tributaries continue to support some of the southern-most runs of Pacific salmonids, the continued viability of these runs is threatened by extensive water development. Three ecosystem components are of particular interest and are presented here, although many other important ecosystem services are provided. The first ecosystem component of note is the anadromous fishery, and most notably the Chinook salmon fishery that spends a portion of its life cycle in the Sacramento Basin. The second is the waterfowl migrating along the Pacific Flyway that rely upon wetlands in the Sacramento Basin during their north-south migration. The final ecosystem component of note is the riparian cottonwood and willow forests that shelter many birds and mammals in the Sacramento Basin.

The climate in the Sacramento Basin, as in much of California, is Mediterranean in character, typified by wet winters and dry summers. Most precipitation occurs during the period between November and April, with little or no precipitation falling

Table 11.1. Approximate Sacramento Basin water budget.

Annual volume (million m3)

Sacramento Basin runoff A 27,630

Import from Trinity River system B 1,087

Total Sacramento Basin supply C: A+B 28,717

Export to southern California D 1,938

Export to Tulare Lake Basin E 3,020

Export to San Joaquin Valley F 1,806

Export to San Francisco Bay area G 113

Unexported Sacramento Basin runoff H: (A+B)—(D + E + F+G) 21,840

Sacramento Basin urban water use I 871

Sacramento Basin agricultural water use J 9,948

Unallocated Sacramento Basin runoff K: H—(I+J) 11,021

As a percentage of Sacramento Basin runoff L: K/AX100 40%

OCT NCW DEC JAN FES MAR APR MAY JUN JUL AUG SEP Month ■ Eureka □ Los Angeles! Fig. 11.2. Current north-south variation in monthly precipitation.

between May and October. In addition to the north-south variation, there is also an east-west variation in precipitation that is controlled largely by the orographic effect of mountains on Pacific storms (Fig. 11.2).

Given the climate conditions common to California, it is not surprising that the surface water hydrology of the Sacramento Basin is dominated by winter snowfall and subsequent spring runoff. Prior to the initiation of large-scale water development in the basin, this climate pattern resulted in flow maxima in the Sacramento River main stem and its principal tributaries - the Feather, Yuba and American Rivers - during the late winter through spring period. Water development in the basin, primarily the construction of major reservoirs on all of the major tributaries, has dramatically altered the surface water hydrology in the basin. The operation of these reservoirs

Month Month

Fig. 11.3. Monthly flow volumes in the Sacramento River below Shasta dam as (A) an estimate of the full natural flow and (B) the observed flow.

generally creates peak flow conditions earlier in the winter as operators manipulate reservoir storage as part of flood control operations in advance of the main runoff season. Spring flows are typically reduced as operators attempt to capture reservoir inflow for later release as part of water supply operations. As a result, summer flows are significantly higher than under natural conditions as operators release water downstream to meet summer irrigation demands (Fig. 11.3B).

Currently, there are three distinct perspectives on the state of water management in the Sacramento Basin. The first is that the balance between water for food and water for the environment has been destructively tipped in favour of irrigated agriculture and that the only possible future is one based on constant efforts to roll back the irrigated area in the basin. The second is that the Sacramento Basin is too valuable as an agricultural resource to be constrained by environmental considerations and that issues of water for the environment should be dealt with in other, less valuable, areas. Both of these views are increasingly giving way to a third perspective that seeks to balance the complex trade-offs and interactions between water for food and water for the environment in the basin. Establishing this balance is a work in progress, and the prospect of climate change offers the real possibility that the emerging balance will be upset and that further adaptation will be required.

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