|
MILLER, DALE, BRUSH, VICUNA, KADIR, DOGRUL, AND CHUNG
<br />to rivers decline 23% in a slight drought, 32% in a
<br />moderate drought, and 68% in a severe drought
<br />(Table 8). The reduction in ground -water discharge to
<br />rivers limits the decline of ground -water levels during
<br />droughts, and also contributes to streamflow reduc-
<br />tion beyond the reduction in valley -rim inflow.
<br />Changes in Aquifer Storage
<br />The change in aquifer storage over time is the sum
<br />of aquifer withdrawals, including ground -water
<br />pumping and discharges to streams, minus the aqui-
<br />fer inflows, including stream inflows and irrigation
<br />recharge. Changes in boundary flows have an addi-
<br />tional, but very minor, impact on storage levels. Dur-
<br />ing the base period (a mix of normal and above
<br />normal rainfall years), Central Valley storage
<br />increases by 0.03 m/year (0.10 ft/year). During the
<br />drought scenarios, Central Valley aquifer storage
<br />declines by 0.08 m/year (0.26 ft/year) in the slight
<br />drought scenario to 0.19 m/year (0.57 ft/year) in the
<br />severe drought scenario (Table 9).
<br />Ground -Water Levels
<br />Central Valley ground -water levels adjust to
<br />changes in storage, rising during the base period and
<br />falling during the drought scenarios. During the base
<br />period, average Central Valley ground -water levels
<br />rise 0.09 m/year (0.29 ft/year), with average levels in
<br />the Sacramento and San Joaquin Basins increasing
<br />by 0.08 m/year (0.24 ft/year) and 0.22 m/year
<br />(0.66 ft/year), respectively, and the Tulare Basin
<br />increasing by only 0.02 m/year (0.07 ft/year). Aver-
<br />age Central Valley ground -water levels decline
<br />0.29 m/year (0.88 ft/year) under the slight drought
<br />scenario and 0.77 m/year (2.33 ft/year), respectively,
<br />during the light and severe drought scenarios, with
<br />substantial variation shown by region (Table 10).
<br />TABLE 9. Impact of Drought on Aquifer Storage.
<br />TABLE 10. Impact of Drought on Ground -Water Levels.
<br />Change From Base Period
<br />Base
<br />Severe
<br />Moderate
<br />Slight
<br />Hydrologic
<br />Period
<br />Drought
<br />Drought
<br />Drought
<br />Region
<br />(m/year)
<br />(m/year)
<br />(m/year)
<br />(m/year)
<br />Sacramento
<br />0.069
<br />-0.12
<br />-0.10
<br />-0.073
<br />Eastside
<br />0.027
<br />-0.12
<br />-0.10
<br />-0.078
<br />San Joaquin
<br />0.057
<br />-0.23
<br />-0.15
<br />-0.096
<br />Tulare
<br />-0.007
<br />-0.25
<br />-0.20
<br />-0.095
<br />Central Valley
<br />0.034
<br />-0.19
<br />-0.14
<br />-0.084
<br />TABLE 10. Impact of Drought on Ground -Water Levels.
<br />Ground -Water Decline and Recovery
<br />The average ground -water level of the Central Val-
<br />ley falls 46 m (140 ft) by the end of the severe 60-
<br />year drought scenario, 33 m (101 ft) by the end of the
<br />moderate drought scenario, and 17 m (53 ft) by the
<br />end of the slight drought scenario (Table 11). Ground-
<br />water levels drop more in the San Joaquin and
<br />Tulare Basins than the other regions due primarily
<br />to the larger increase in pumping for these regions.
<br />The Tulare Basin experiences the largest decline,
<br />ranging from 35 m (105 ft) in the slight drought sce-
<br />nario to 84 m (258 ft) in the severe drought scenario.
<br />The simulations included a 30 -year "recovery per-
<br />iod" to explore how Central Valley aquifers respond
<br />to a return to normal rainfall and irrigation condi-
<br />tions. After the severe 60 -year drought, the average
<br />ground -water level in the Central Valley rises 8 m
<br />(26 ft) over the 10 -year recovery period (Table 11), a
<br />recovery of only 18 %. The average Central Valley
<br />ground -water level recovers 15% after a moderate 60-
<br />year drought and 21% after a light 60 -year drought.
<br />In general, ground -water levels recover fairly rapidly
<br />in the Sacramento Basin, Eastside Drainage, and
<br />San Joaquin Basin, and very slowly in the Tulare
<br />Basin (Figure 2). A large portion of the Tulare Basin
<br />has experienced chronic overdraft as ground -water
<br />withdrawals have often exceeded recharge (CDWR,
<br />2003). The simulated recovery rates suggest that the
<br />Tulare Basin would not achieve pre - drought ground-
<br />water levels for at least 30 years, if ever. The other
<br />regions experience more rapid rates of ground -water
<br />recovery, and simulation results suggest these
<br />regions would likely achieve pre - drought ground-
<br />water levels relatively rapidly after a drought.
<br />CONCLUSIONS
<br />Drought simulations for a set of specified scenarios
<br />were performed by constructing reservoir releases
<br />JAWRA 864 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
<br />Change From Base Period
<br />Base
<br />Severe
<br />Moderate
<br />Slight
<br />Hydrologic
<br />Period
<br />Drought
<br />Drought
<br />Drought
<br />Region
<br />(m/year)
<br />( m/year)
<br />( m/year)
<br />( m/year)
<br />Sacramento
<br />0.079
<br />-0.17
<br />-0.12
<br />-0.055
<br />Eastside
<br />0.295
<br />-0.33
<br />-0.11
<br />-0.078
<br />Delta
<br />-0.003
<br />-0.43
<br />-0.34
<br />-0.258
<br />San Joaquin
<br />0.216
<br />-0.58
<br />-0.25
<br />-0.134
<br />Tulare
<br />0.023
<br />-1.41
<br />-1.12
<br />-0.575
<br />Central Valley
<br />0.094
<br />-0.77
<br />-0.55
<br />-0.288
<br />Ground -Water Decline and Recovery
<br />The average ground -water level of the Central Val-
<br />ley falls 46 m (140 ft) by the end of the severe 60-
<br />year drought scenario, 33 m (101 ft) by the end of the
<br />moderate drought scenario, and 17 m (53 ft) by the
<br />end of the slight drought scenario (Table 11). Ground-
<br />water levels drop more in the San Joaquin and
<br />Tulare Basins than the other regions due primarily
<br />to the larger increase in pumping for these regions.
<br />The Tulare Basin experiences the largest decline,
<br />ranging from 35 m (105 ft) in the slight drought sce-
<br />nario to 84 m (258 ft) in the severe drought scenario.
<br />The simulations included a 30 -year "recovery per-
<br />iod" to explore how Central Valley aquifers respond
<br />to a return to normal rainfall and irrigation condi-
<br />tions. After the severe 60 -year drought, the average
<br />ground -water level in the Central Valley rises 8 m
<br />(26 ft) over the 10 -year recovery period (Table 11), a
<br />recovery of only 18 %. The average Central Valley
<br />ground -water level recovers 15% after a moderate 60-
<br />year drought and 21% after a light 60 -year drought.
<br />In general, ground -water levels recover fairly rapidly
<br />in the Sacramento Basin, Eastside Drainage, and
<br />San Joaquin Basin, and very slowly in the Tulare
<br />Basin (Figure 2). A large portion of the Tulare Basin
<br />has experienced chronic overdraft as ground -water
<br />withdrawals have often exceeded recharge (CDWR,
<br />2003). The simulated recovery rates suggest that the
<br />Tulare Basin would not achieve pre - drought ground-
<br />water levels for at least 30 years, if ever. The other
<br />regions experience more rapid rates of ground -water
<br />recovery, and simulation results suggest these
<br />regions would likely achieve pre - drought ground-
<br />water levels relatively rapidly after a drought.
<br />CONCLUSIONS
<br />Drought simulations for a set of specified scenarios
<br />were performed by constructing reservoir releases
<br />JAWRA 864 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION
<br />
|