The study area represents a 1,400-km2 irrigated agricultural region in western Fresno County on the west side of the San Joaquin Valley (Fig. 1A) and includes three alluvial fans. The alluvial soils are derived from Coast Range alluvium and are generally fine-textured
(Fig. 1B). Irrigation water is managed by water districts for water distribution and drainage management. Details on the hydrogeologic
setting, soils, and history of irrigation are published elsewhere (6, 14, 15) and are summarized in Supporting Text and Fig. 5, which are published as supporting information
on the PNAS web site. Early irrigation in the valley, starting at the
end of the 19th century, was limited to gravity diversions from the San
Joaquin River and developed into intense groundwater pumping starting
in the 1920s, leading to an increase in irrigated acreage westwards and
upslope. After completion of the Central Valley Project and the State
Water Project in 1953 and 1967, respectively, the whole study area was
irrigated with high-quality imported water from the Sacramento Valley
conveyed by the Delta-Mendota Canal and the California Aqueduct. These
projects initially resulted in soil leaching of predevelopment salts.
However, increased deep percolation rates combined with a sharp
decrease in groundwater pumping resulted in a rise of the water table
over much of the area (16).
Since the mid-1980s the extent of saline-sodic soils has steadily
migrated to the west, generally following the expansion of the shallow
water table area [K. Arroues (2002), personal communication, Natural
Resources Control Service, Hanford, CA].
Figure 1Overview of the study area. (
A) Location of the study area in the western San Joaquin Valley that includes 13 water districts (W.D.). (
B) Soil texture map. (
C)
Soil gypsum contents. The main soil types are clay (52% of the study
area), clay loam (35%), loam (4%), and sandy loam (9%). The
finer-textured soils are found in the valley trough near the San
Joaquin River. These soils have clay contents from 40% to 60%. The clay
fraction is dominated by the montmorillonite mineral. Going from east
to west, the soils gradually become more coarsely textured. A distinct
feature is the sandy loam soils developed in stream deposits of Panoche
Creek. Organic matter contents are low. Gypsum is predominantly present
in the downslope soils. Soil data are from ref.
14.
The
salinity problem on the west side of the San Joaquin Valley is partly
attributed to the continuous presence of a low-permeability Corcoran
clay layer (6),
ranging in depths from ≈30 m near the San Joaquin River in the east to
a depth of ≈250 m in the west, thereby largely defining the regional
hydrology. To lower the water tables, subsurface drainage systems were
installed to intercept and collect the shallow groundwater. Yet, soon
thereafter it became eminently clear that drainage waters must be
disposed off in an environmentally safe manner. Specifically, the 1983
discovery of migratory bird deaths and deformities was linked to
elevated selenium levels in agricultural drainage water impounded in
Kesterson Reservoir (17, 18). This finding led to an intensive investigation carried out jointly by federal and state agencies through the San Joaquin
Valley Drainage Program (19).
Current solutions include increasing irrigation efficiency, growing
alternative salt-tolerant crops, drainage-water reuse, the collection
of drainage water in evaporation ponds, land retirement, and increased
groundwater pumping. However, for irrigated agriculture to remain
sustainable, a soil salt balance must be maintained that allows for
productive cropping systems.
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