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DROUGHT RESILIENCE OF THE CALIFORNIA CENTRAL VALLEY SURFACE - GROUND - WATER - CONVEYANCE SYSTEM
<br />TABLE 1. Drought Scenario Notation.
<br />Specified
<br />Scenarios 10 Years 20 Years 30 Years 60 Years
<br />30% reduction 30_10 30_20 30_30 30_60
<br />50% reduction 50_10 50_20 50_30 50_60
<br />70% reduction 70_10 70_20 70_30 70_60
<br />TABLE 2. Drought Scenario Reductions in
<br />Precipitation, Releases, and Deliveries.
<br />Percentage Reduction in:
<br />Scenario Precipitation ( %) Releases M Deliveries ( %)
<br />30_10
<br />26
<br />40
<br />26
<br />30_60
<br />25
<br />41
<br />27
<br />50_10
<br />34
<br />50
<br />41
<br />50_60
<br />27
<br />54
<br />46
<br />70_10
<br />39
<br />61
<br />53
<br />70_60
<br />39
<br />59
<br />51
<br />Valley (model subregions 1 -7; 14,927 km the San
<br />Joaquin Basin is in the center of the Central Valley
<br />(model subregions 10 -13; 9,950 km the Tulare
<br />Basin in the southern end of the Central Valley
<br />(subregions 14 -21; 19,958 km the Sacramento -
<br />San Joaquin Delta (subregion 9; 2,936 km and the
<br />Eastside Streams to the east of the Sacramento -San
<br />Joaquin Delta (subregion 8; 3,624 km The impacts
<br />of the simulated droughts are discussed for the Cen-
<br />tral Valley, and for the Sacramento Basin, Eastside
<br />Drainage, San Joaquin Basin, and Tulare Basin, with
<br />a detailed focus on three drought scenarios, the 30-
<br />year moderate drought, the 60 -year slight drought,
<br />and the 60 -year severe drought. Simulated river flows
<br />in the Sacramento -San Joaquin Delta region are dom-
<br />inated by surface water transfers, and drought
<br />impacts on this region were therefore omitted from
<br />this study. To compare impacts across the four hydro-
<br />logic regions, all flow rates were normalized against
<br />the region area, transforming volume per area to
<br />depth; normalizing flow rates against crop area would
<br />yield similar results as normalizing against regional
<br />area, as the regional water budgets are dominated by
<br />agricultural water use.
<br />In response to drought- induced reductions in sur-
<br />face water availability, combined with static demands
<br />based on a fixed land use and population, the IWFM
<br />application automatically increases ground -water
<br />pumping to exactly meet the specified agricultural
<br />and urban water demands. The reduced surface
<br />water flows and precipitation and increased ground-
<br />water pumping induce changes in water table
<br />altitude, ground -water volumetric storage, and
<br />stream to ground -water flow. Ground -water recharge
<br />is also reduced owing to both reduced precipitation at
<br />the land surface and reduced recoverable losses (i.e.,
<br />canal leakage) from surface water diversions. The 30-
<br />year recovery period and fixed land use and demands
<br />were required to isolate the impacts associated with
<br />surface water flow reductions alone. Future studies
<br />planned as part of this work include sequential and
<br />combined changes in both the land use types and
<br />demands.
<br />Surface Water Diversions
<br />The 12 simulated droughts all begin with the same
<br />initial conditions, and spin -up for 10 years, during
<br />which surface diversions across the Central Valley
<br />average 13.4 km /year (10.9 maf/year). Reservoir
<br />releases and surface water diversions were simulated
<br />by CALSIM II in response to specified reservoir
<br />inflows and constant 2003 -level demands for each of
<br />the three levels of drought. Surface water diversions
<br />were lower than base period diversions in all months
<br />(Table 3), except for December diversions under the
<br />slight drought scenario, which were elevated due to
<br />the shift of the runoff hydrograph to increased winter
<br />runoff. After the 10 -year spin -up period, surface
<br />water diversions in the Central Valley fall 39% dur-
<br />ing the severe drought scenario, 22% during the mod-
<br />erate drought scenario, and 13% during the slight
<br />drought scenario (Table 4). Each scenario concludes
<br />with a 30 -year recovery period.
<br />It is apparent from the simulation results that
<br />drought scenario impacts are concentrated in the San
<br />Joaquin and Tulare Basins (Figure 2). In the severe
<br />60 -year drought scenario, these basins experience
<br />average annual declines of 0.15 and 0.13 m (0.46 and
<br />0.41 ft), respectively, in surface deliveries compared
<br />with the base period (Table 5), representing a 43%
<br />decline in the San Joaquin Basin and a 70% decline
<br />in the Tulare Basin. The Sacramento Basin and East -
<br />side Drainage experience declines of 27 and 60 %,
<br />respectively. In the moderate 30 -year drought, the
<br />Sacramento Basin, Eastside Drainage, San Joaquin
<br />Basin, and Tulare Basin experience declines of 5, 40,
<br />19, and 62% respectively. In the light 60 -year
<br />drought scenario, average annual surface water deliv-
<br />eries increase by 7% in the Sacramento Basin (due to
<br />higher winter flows), and decline by 43, 17 and 46%
<br />for the Eastside Drainage, San Joaquin Basin, and
<br />Tulare Basin, respectively.
<br />Ground -Water Pumping
<br />Farmers in the Central Valley have historically
<br />increased ground -water pumping during drought
<br />JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 861 JAWRA
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