Overdraft and Overdraft-Related Problems
During the period 1950 through 1975, the water table steadily
declined in much of the Cache Creek sub-basin, in the Lower Cache-Putah
sub-basin and on the western side of the Colusa sub-basin (Scott, et al., 1975; DWR, 1961).
Heavy pumping, coincident with the lowering of the Cache Creek
thalweg from aggregate mining, was responsible for declines in
the Hungry Hollow area, in the Yolo/Woodland area and in the
areas on both sides of the Creek from Esparto Bridge to Yolo
(Wahler Associates, 1981).
Intensification of irrigated agriculture using groundwater also
was responsible for declines in the Upper and Lower Cache-Putah
sub-basins and Plainfield Ridge, although the effect on Upper
Cache-Putah sub-basin has since been mitigated with delivery
of adequate surface supplies from YCFCWCD's Indian Valley Reservoir.
Since 1975, with the ordinance on in-stream mining to protect
the thalweg, and introduction of supplemental Indian Valley surface
supplies in the Hungry Hollow area, declining water levels have
been reversed, or stabilized in the part of the Cache Creek sub-basin
west of the Plainfield Ridge. Though much of the Cache Creek
sub-basin in this area is full in normal years, it is estimated
to have lost approximately 10 feet of groundwater storage elevation
as a result of the lowered thalweg (Wahler
In normal years most parts of Upper Cache-Putah, Cache Creek
west of the Plainfield Ridge, East Yolo, and eastern Colusa sub-basins
have water tables that are stabilized or at their upper limits.
These are all areas that receive good surface water supplies
for irrigation, and therefore normally have low groundwater usage.
The water tables in some areas in Upper Cache-Putah, Colusa and
East Yolo basins may actually be so high, at levels within 10
feet of the ground surface, that they threaten to interfere with
agricultural activities. These levels result in direct evaporation
losses and may cause water logging and drainage problems for
The areas of continuing overdraft in the County cover most
of the Lower Cache-Putah and western Colusa sub-basins. To some
extent improvements from reduced pumping have occurred in the
DWD area of Colusa sub-basin. Particular overdraft spots include
the Yolo-Woodland area on both sides of Cache Creek, areas between
Zamora and Yolo, and areas to the northeast of Davis. These areas
coincide with places where subsidence and/or deteriorating groundwater
quality have been measured. Boron water quality problems exist
on both sides of lower Cache Creek, east of Stephens Bridge and
Plainfield ridge (Scott, et al.,
1975; Borcalli, et al., 1984).
The highest levels of subsidence, up to 4 feet, have occurred
in the YZWD area of Colusa sub-basin (Borcalli,
et al., 1984). Evidence of one or more feet of subsidence
exists along the area starting northwest of Davis and extending
north to northwest of Woodland (Brown
and Caldwell, 1989) and up to Zamora (Borcalli,
et al., 1984). Rising TDS (total dissolved solids) levels
and elevated selenium levels in the Davis area (Brown and Caldwell,
1989; City of Davis, 1990)
could indicate overdraft-induced deteriorating water quality.
Despite, the appearance of a near balanced groundwater budget
in average years (Table 5),
there are clearly localized groundwater problems where 'capture'
is not adequate to cover pumping. The results are declining water
tables, and their related impacts on the groundwater resource.
The cumulative impacts of overdraft pumping during the period
from 1944-1974 represent a substantial loss in stored groundwater
of over 500,000 acre-feet (Scott,
et al., 1975, p.48). In 1974 vacant groundwater storage capacity,
in the zone from 20 to 420 feet below ground surface, was over
700,000 acre-feet (Scott, et al.,
1975; Table B.1). Water tables appear to have risen in localized
areas of Cache Creek, Upper Cache-Putah, and Colusa sub-basins
or stabilized since then, but in other sub-basin areas they have
continued to decline. There is no evidence to suggest that on
an overall basis this groundwater storage deficit has been reversed
since 1975, and this study's analysis of the current groundwater
situation indicates that further cumulative depletion of stored
water may be occurring.
This Chapter has explored in more detail various features
of Yolo County's groundwater resources and the essential role
they play in the County's water system. The analysis shows the
dependence on groundwater for meeting water demands for both
M&I and agricultural sectors. In droughts, or under any situation
of reduced surface water supplies (e.g. water transfers out of
the County), sustaining the County's agriculturally based economy
depends even more critically on having adequate groundwater resources
to draw from. However, unplanned and excessive groundwater withdrawals
in some areas of the County maybe causing a number of irreversible
environmental problems, adversely affecting storage levels, and
threatening to undermine the reliability of groundwater resources
and the sustainability of the County's water system. If the County
continues to ignore the importance of managing its groundwater
resources, current practices may lead to serious problems for
the County's water future.
On the other hand, management of Yolo County's groundwater
basin can help to avoid or control many of the groundwater problems
described in this chapter. It can also serve to protect and enhance
groundwater's role as the most reliable buffer for the County
in times of surface water shortages. Groundwater management cannot,
however, be conducted in isolation from the larger context of
the complete water supply and demand situation in the County.
The need to conjunctively manage the groundwater basin with the
planning and management of surface water resources in Yolo County
is supported by the following points which summarize the principle
features of the County's water system that have emerged thus
far in this investigation.
- The sustained yield of the basin, and overdraft conditions,
are intimately related to natural and human-induced recharge
quantities, pumping levels, and their spatial and temporal distributions.
- There are many, many individual users, both in the M&I
and agricultural sectors of the County, who make decisions about
pumping that affect groundwater conditions and groundwater uses
throughout the basin.
- A large portion of the agricultural users in the County have
the option of using either or both ground and surface water supplies,
when available. However, all M&I users in the County (except
West Sacramento) are limited to only groundwater.
- During drought periods of surface water shortage, water users
in the County historically have been able to rely on groundwater
resources from within the County to maintain their normal activities.
- Unplanned transfers of surface water out of the County, like
those that occurred in 1991, threaten to further distort the
groundwater balance during droughts.