Chapter 3: Groundwater Resources, Pumping, and Sustained Yield

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3a. Spatial Distribution of Groundwater Pumping 3b: Impact of Drought on Goundwater Use 3c: Impact of Water Transfers on Goundwater Use 3d: Assessment of Sustained Yield 3e. Overdraft and Overdraft-Related Problems 3f. Summary

In this section, the groundwater resources under Yolo County are described and cases are examined for conditions where water use and supply deviate from the County-wide average presented in the previous section. The sustained yield of the County's groundwater resources, and the occurrence of overdraft conditions and their effects, are investigated to estimate whether current levels of groundwater use in the County will be sustainable in the future. Future sustainability is a key management concern for any water supply system, and must be addressed before appropriate objectives, strategies, and projects for conjunctive use management of the water supply system can be identified.

In contrast with the surface water resources listed in Table 3, groundwater resources are more difficult to quantify accurately, mainly because the data required to do so rarely exists. Consequently, the analysis and estimates presented in this section (see details in Appendix A and B) are somewhat crude; results here should be interpreted as representative of the correct direction and order of magnitude of effects, but not their exact values.

3a. Spatial Distribution of Groundwater Pumping

Yolo County has two distinct and hydraulically separate aquifers: the main aquifer (indicated in Figure 1), also called the shallow and intermediate depth aquifer (within 700 feet below ground surface); and the deep aquifer (below 1000 feet depth). In this investigation, all references to groundwater resources and the 'aquifer' are to the main aquifer. Because the deep aquifer has been found only under the southeastern portion of the County (i.e. in the vicinity of Davis), and has limited or no natural recharge (Scott, 1990), it is not considered as significant or reliable a supply source for the County as the main aquifer or other surface supplies (though it is used by the University of California, Davis and may become important for the city of Davis).

The six sub-basins of the main aquifer, shown in Figure 4 (Scott, et al., 1975; Bamgboye, et al.,1980), are hydraulically connected and exchange flows across their common boundaries. They are differentiated by their geologic characteristics and in some cases delineated by specific physical boundaries. Many technical reports and documents discuss the hydrogeologic characteristics of these sub-basins in great detail ( Olmstead, et al., 1961; Scott, et al., 1975; Woodward-Clyde, 1976; Environ, 1980; Wahler Associates, 1981; Hubbard, 1989; Yates, 1989; Dames and Moore, 1990; etc.) and provide information on their water resource potential in terms of storage capacity, transmissivities, recharge and geologic structure.

Groundwater pumping in the County is not uniformly distributed across the whole aquifer, nor within each of the sub-basins of Figure 4. Exact figures on current water use by sub-basin are difficult to know because the spatial distribution of irrigation practices, cropping patterns, and groundwater use, by sub-basin, have not been investigated since the Scott, et al. (1975) study. No formal process exists nor does any institution have a mandate, in the County, to carry out the data collection and analyses required for such a comprehensive study. For this project, estimates of water use in each sub-basin were calculated by adjusting Scott's 1975 projections for 1990, to reflect actual changes in total irrigated acreage and new water supplies (see Appendix B). The estimated average balance between surface water and groundwater use for each sub-basin is shown in Table 4. Sub-basins are ranked and listed in Table 4 and marked in Figure 4 starting with the one having the highest volume of water use. The degree of dependence on groundwater in different parts of the County varies considerably from an estimated 79.7 percent of water use in Lower Cache-Putah sub-basin to 29.2 percent of water use in East Yolo sub-basin. Groundwater withdrawals are most intensive in the Plainfield Ridge and Lower Cache-Putah sub-basins as seen in the number of feet of groundwater use per acre per year.

The Lower Cache-Putah sub-basin has the greatest dependence on groundwater for water supplies, and nearly the most intense water use per acre. An estimated 173,100 acre-feet of groundwater is pumped from this sub-basin in average wet year supply conditions (see Appendix B, Table B.2 ). Both Woodland and Davis are located in this sub-basin. However, their joint impact (at 1990 levels) accounts for only about 15 percent of the total amount pumped. Heavy agricultural withdrawals in the Lower Cache-Putah sub-basin account for the rest of the pumping, and are a consequence of the lack of distribution canals and other access to surface water resources for irrigation needs.

Colusa sub-basin is the next biggest groundwater withdrawer in the County, at an average estimated 71,700 acre-feet per year. The two water districts on the west side of the sub-basin, YZWD and DWD, account for nearly all of this quantity. In sharp contrast with Lower Cache-Putah sub-basin, surface water is accessible in Colusa sub-basin and much more intensively used, mainly along its eastern length.

Table 4. Balance of surface and groundwater use in sub-basins of Yolo County under average supply conditions (percent of sub-basin total water use, feet/acre-year, acre-feet/year)^a

Cache Creekb

Sub-basin	Gross land area (acres)	Water use (rank)	Surface water (%, ft/ac, and acre-feet)	Groundwater (%, ft/ac, and acre-feet)
Colusa	95,700	1	70.8 % 1.82 ft/acre 174,200 ac-ft	29.2 % 0.75 ft/ac 71,700 ac-ft
East Yolo	129,100	2	71.7 % 1.33 ft/ac 172,300 ac-ft	28.3 % 0.53 ft/ac 68,000 ac-ft
Lower Cache-Putah	97,300	3	20.3 % 0.45 ft/ac 44,200 ac-ft	79.7 % 1.77 ft/ac 173,100 ac-ft
Upper Cache-Putah	70,300	4	68.7 % 1.50 ft/ac 105,200 ac-ft	31.3 % 0.68 ft/ac 48,000 ac-ft
Cache Creekb	45,800	5	32.0 % 0.59 ft/ac 27,000 ac-ft	68.8 % 1.28 ft/ac 58,800 ac-ft
Plainfield Ridge	8,800	6	24.5 % 0.61 ft/ac 5,400 ac-ft	75.4 % 1.88 ft/ac 16,600 ac-ft
TOTAL	447,000		54.8 % 1.18 ft/ac 528,300 ac-ft	45.2 % 0.98 ft/ac 436,100 ac-ft

^a Values are from projections for 1990 made in 1975, and adjusted for changes in average surface water availability and for actual 1990 estimates of County water use as determined in this investigation

^b Due to recent YCFCWCD canal extensions into Hungry Hollow, the mix of supplies in this sub-basin has shifted towards a slightly greater use of surface water over groundwater than indicated here

Source: See Appendix B and Scott, et al., (1975)

Within Cache Creek, Upper Cache-Putah, and Plainfield Ridge sub-basins most farmers have access to surface water from Cache Creek through the YCFCWCD distribution system. The East Yolo sub-basin and the eastern length of the Colusa sub-basin use substantial quantities of Sacramento River water. In 1983 DWD began receiving 19,000 acre-feet of USBR surface water via the Tehama-Colusa Canal (Borcalli, et al., 1984) but YZWD still depends almost exclusively on groundwater pumping for irrigation.

The mix of surface and groundwater supply and the uses to which these supplies are presently allocated varies considerably among sub-basins. Adjusting this mix and matching available water resource supplies to the most appropriate uses to achieve a more efficient, effective, and reliable water supply system for all users in the County are basic features of a managed conjunctive use program. As indicated earlier, water agencies in the County have traditionally focused on the management of either only groundwater or only surface water supplies within their own districts. This greatly restricts any consideration or implementation of a whole set of conjunctive approaches to solving water management problems.