3e. 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 Associates, 1981).

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 some crops.

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.

F. SUMMARY

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.

1. 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.
2. 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.
3. 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.
4. 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.
5. Unplanned transfers of surface water out of the County, like those that occurred in 1991, threaten to further distort the groundwater balance during droughts.