By R.T. Hanson, Zhen Li, and C.C. Faunt
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2004-5231
Sacramento, California 2004
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The Santa Clara Valley is a long, narrow trough extending
about 35 miles southeast from the southern end of San Francisco Bay where the
regional alluvial-aquifer system has been a major source of water. Intensive
agricultural and urban development throughout the 20th century and related ground-water
development resulted in ground-water-level declines of more than 200 feet and
land subsidence of as much as 12.7 feet between the early 1900s and the mid-1960s.
Since the 1960s, Santa Clara Valley Water District has imported surface water
to meet growing demands and reduce dependence on ground-water supplies. This
importation of water has resulted in a sustained recovery of the ground-water
flow system. To help support effective management of the ground-water resources,
a regional ground-water/surface-water flow model was developed. This model simulates
the flow of ground water and surface water, changes in ground-water storage,
and related effects such as land subsidence.
A numerical ground-water/surface-water flow model of the Santa Clara Valley
subbasin of the Santa Clara Valley was developed as part of a cooperative investigation
with the Santa Clara Valley Water District. The model better defines the geohydrologic
framework of the regional flow system and better delineates the supply and demand
components that affect the inflows to and outflows from the regional ground-water
flow system. Development of the model includes revisions to the previous ground-water
flow model that upgraded the temporal and spatial discretization, added source-specific
inflows and outflows, simulated additional flow features such as land subsidence
and multi-aquifer wellbore flow, and extended the period of simulation through
September 1999. The transient-state model was calibrated to historical surface-water
and ground-water data for the period 1970–99 and to historical subsidence
for the period 1983–99.
The regional ground-water flow system consists of multiple aquifers that are
grouped into upper- and lower-aquifer systems. Ground-water inflow occurs as
natural recharge in the form of streamflow infiltration and areal infiltration
of precipitation along stream channels, artificial recharge from infiltration
of imported water at recharge ponds and along selected stream channels, and
leakage along selected transmission pipelines. Ground-water outflow occurs as
evapotranspiration, stream base flow, discharge through pumpage from wells,
and subsurface flow to the San Francisco Bay.
The geohydrologic framework of the regional ground-water flow system was represented
as six model layers. The hydraulic properties were redefined on the basis of
cell-based lithologic properties that were delineated in terms of aggregate
thicknesses of coarse-grained, fine-grained, and mixed textural categories.
The regional aquifer systems also are dissected by several laterally extensive
faults that may form at least partial barriers to the lateral flow of ground
water. The spatial extent of the ground-water flow model was extended and refined
to cover the entire Santa Clara Valley, including the Evergreen subregion. The
temporal discretization was refined and the period of simulation was extended
to 1970–99.
The model was upgraded to MODFLOW-2000 (MF2K) and was calibrated to fit historical
ground-water levels, streamflow, and land subsidence for the period 1970–99.
The revised model slightly overestimates measured water levels with an root-mean-square error
of -7.34 feet. The streamflow generally shows a good match on gaged creeks and
rivers for flows greater than 1.2 cubic feet per second. The revised model also
fits the measured deformation at the borehole extensometer site located near
San Jose within 16 to 27 percent and the extensometer site near Sunnyvale within
3 percent of the maximum measured seasonal deformation for the deepest extensometers.
The total ground-water inflow and outflow of about 225,500 acre-feet per year (acre-ft/yr) for the period 1970–89 and of about 205,300 acre-feet per year for the period for the period 1970–99 is comparable with that of the previous model, 207,200 acre-ft/yr for the period 1970–89. Overall the simulated net change in storage increased by about 189,500 acre-ft/yr for the entire period of simulation, which represents about one and a half years of the 1970–99 average pumping. The changes in ground-water flow and storage generally reflect the major climate cycles and the additional importation of water by Santa Clara Valley Water District, with the basin in recovery since the drought of the late 1980s and early 1990s. The average total recharge rate, from natural and artificial recharge and from streamflow infiltration for the revised model for the entire simulation period 1970–99, was about 157,100 acre-ft/yr, which represents about 59 percent of the inflow to the ground-water flow system. The average rate of artificial recharge of about 77,600 acre-ft/yr represents about 30 percent of the inflow to the ground-water flow system. The average pumpage for the entire 29.75-year simulation period is about 133,400 acre-ft/yr and represents about 69 percent of the outflow from the ground-water flow system. Most of the simulated recharge infiltrates and flows through the uppermost layers (i.e. model layers 1 and 3) of the aquifer system. Most of the water that flows to the deeper model layers is occurring through wellbores, with wellbore flow representing 19 percent of the total ground-water inflow between model layers.
Abstract
Introduction
Purpose and Scope
Approach
Description of Study Area
Climate
Land and Water Use
Acknowledgments
Conceptual Model
Geohydrologic Framework
Simulation of Ground-Water Flow
Model Framework
Previous Models
Spatial Discretization
Temporal Discretization
Model Boundaries
Hydraulic Properties
Transmission Properties
Storage Properties
Flow Barriers
Simulated Inflows and Outflows
Valley-Floor Recharge
Artificial Recharge
Evapotranspiration
Streamflow Routing
Subsidence
Pumpage
Coastal Flow
Initial Conditions
Model Revisions
Model Calibration
Transient-State Calibration
Model Sensitivity and Uncertainty
Summary and Conclusions
References Cited
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