Historical adaptations to climate variability

Water managers in California have long had to cope with the challenges posed by the state's variable hydrology The earliest adaptation strategy in response to this variability was to develop simple irrigation systems that allowed for the capture of the base flow available in rivers and streams. The next major adaptation was in response to heavy precipitation and high flow during the 1860s. The extensive flooding during this period led to the creation of the State Reclamation Board, which was charged with the construction of levees to protect growing cities and areas with a high concentration of agricultural production.

With time the population expanded, as did the demand for food. A responsive adaptation strategy was the construction of reservoirs that could carry over winter rainfall and runoff into the summer irrigation season. Protection of growing communities from the risk of flooding also increased in importance. Two additional adaptation strategies were the development of flood control operating rules for large reservoirs and the construction of numerous flood bypasses in the Central Valley.

All of these adaptations, driven by the desire to expand irrigated agriculture in the Central Valley and to reduce the risk of flooding, dramatically altered the hydrology of the Central Valley and the ecosystems that had developed in response to the natural hydrologic regime. Beginning in the 1960s, there began a series of adaptations designed to limit the impact on these important ecosystems. Early adaptations included the establishment of minimum instream flow requirements at important points in the system. More recently the physical rehabilitation of riverine ecosystems has taken on increased importance. Planners now realize that the extensive levies in the Central Valley limit the amount of wetland and riparian habitat available. Levy set-back adaptations are now being considered alongside the concept that flow-bypass structures can be managed as wetland complexes. Already a portion of one bypass in the Central Valley has been converted to a national wildlife refuge. There is also a growing recognition that assuring the proper volume of flow for ecosystems is necessary but not sufficient. Other factors, such as the temperature and quality of the water in rivers, are also important. Recent adaptations with regards to water temperature include the construction of temperature control devices in large dams which allow for the controlled management of cold and warm water pools that generally develop when large reservoirs stratify. Water quality adaptations include the development of discharge permitting requirements. These have been limited to date to point discharges, but are now being contemplated for non-point sources as well.

In summary, the Sacramento Basin has in place a number of adaptations to address natural climate variability in maintaining food and environmental security, as shown in Table 11.3. There has been a clear historical trend towards placing higher priority on environmental security, as people in the basin have come to value the role of ecosystems. Each of the adaptation strategies discussed assumes that several of the existing strategies are essentially maintained, particularly that no reservoirs would be built or destroyed, and there would be no relaxation of water quality constraints.

Table 11.3. Existing adaptation types.

Adaptation type

Food security

Environmental security

Irrigation

X

Reservoirs

X

X

Levees

X

Flood bypasses

X

X

Instream flow requirements

X

Conversion of bypasses to wildlife refuges

X

Temperature control devices

X

Water quality permits

X

X

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