Drought Resilience JAWRA Article

Home Browse Search

DROUGHT RESILIENCE OF THE CALIFORNIA CENTRAL VALLEY SURFACE GROUND SYSTEM TABLE 5. Surface Water Diversions in Base and Drought Periods. Change From Base Period TABLE 6. Impact of Drought on Ground -Water Pumping. Change From Base Period Base Base Severe Moderate Slight Hydrologic Periofd Drought Drought Drought Region (m/year) (m/year) (m/year) (m/year) Sacramento 0.34 -0.09 -0.02 0.02 Eastside 0.005 -0.003 -0.002 -0.002 San Joaquin 0.35 -0.15 -0.07 -0.06 Tulare 0.19 -0.13 -0.12 -0.09 Central Valley 0.28 -0.11 -0.06 -0.03 Change( %) -0.06 -38% -22% -12% TABLE 6. Impact of Drought on Ground -Water Pumping. Change From Base Period Base Severe Moderate Slight Hydrologic Period Drought Drought Drought Region (m/year) (m/year) (m/year) (m/year) Sacramento 0.06 0.04 0.02 0.00 Eastside 0.15 0.05 0.04 0.02 San Joaquin 0.13 0.17 0.10 0.07 Tulare 0.29 0.17 0.14 0.07 Central Valley 0.16 0.12 0.08 0.04 Change (%) -0.03 71% 49% 27% Aquifer Recharge In a normal year, the Central Valley aquifers are recharged with excess from surface irrigation deliver- ies and rainwater percolation. For the Central Valley as a whole, this aquifer recharge generally exceeds ground -water withdrawals, although withdrawals exceed recharge in local areas of persistent ground- water overdraft (CDWR, 2003). In the base period, for example, the Central Valley ground -water recharge is 0.21 m/year (0.63 ft/year) (Table 7) com- pared to ground -water pumping of 0.16 m/year (0.50 ft/year) (Table 6). Excess ground -water storage derived from recharge in normal years helps to main- tain ground -water storage levels during short -dura- tion droughts when there is a dramatic decline in recharge. Average recharge across the Central Valley drops 14 %, during the slight drought scenario, to as much as 42 %, during the severe drought scenario (Table 7). Annual rainfall rates are highest in the northern Sacramento Basin and lowest in the southern Tulare Basin. Under the simulated drought scenarios, recharge varies across regions in proportion to changes in both surface water deliveries and rainfall. In the severe drought scenario for example, the Sacramento, San Joaquin, and Tulare Basins experience large declines in both precipitation and surface water deliv- eries and register large declines in aquifer recharge. The regional variation in rainfall helps to explain the regional variation in recharge not explained by regio- nal differences in surface water deliveries. Stream - Aquifer Flows In normal years, the rivers in the Sacramento and San Joaquin Basins are net "gaining rivers," meaning that their flow is increased by a net movement of water from aquifers that are adjacent to rivers (Table 8). Alternatively, in normal years, the rivers in the Eastside Drainage and Tulare Basin are "los- ing rivers," with a net movement of water out of riv- ers and into adjacent aquifers. Stream - aquifer flows in the Sacramento and San Joaquin Basins are larger than those in the Eastside Drainage and Tulare Basin, and tend to dominate the average stream - aquifer flow in the Central Valley, which experiences a net flow of water from aquifers to rivers in normal years. The net flow of water from ground water to rivers decreases during droughts as regional ground- water levels decline in response to reduced recharge and increased withdrawals. In addition, flows from rivers to aquifers decrease because there is less water available in the rivers. Net ground -water discharges TABLE 7. Impact of Drought on Aquifer Recharge. TABLE 8. Impact of Drought on Stream - Aquifer Flows Hydrologic Region Base Period (- /year) Change From Base Period Moderate Drought (m/year) Base Severe Moderate Slight Hydrologic Period Drought Drought Drought Region (m /year) ( m/year) (m/year) (m/year) Sacramento 0.21 -0.13 -0.08 -0.05 Eastside 0.05 -0.03 -0.03 -0.01 San Joaquin 0.24 -0.11 -0.06 -0.03 Tulare 0.21 -0.07 -0.06 -0.02 Central Valley 0.21 -0.09 -0.06 -0.03 Change( %) -42% -28% -4% TABLE 8. Impact of Drought on Stream - Aquifer Flows Hydrologic Region Base Period (- /year) Severe Drought (m/year) Moderate Drought (m/year) Slight Drought ( m/year) Sacramento -0.131 -0.036 -0.084 -0.104 Eastside 0.037 0.018 0.018 0.023 San Joaquin -0.076 0.006 -0.036 -0.040 Tulare 0.015 0.011 0.013 0.016 Central Valley -0.078 -0.036 -0.058 -0.064 Change (%) -68% -32% -23% Note: Positive values are from the stream to the aquifer, and negative values are from the aquifer to the stream. JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 863 JAWRA