Chapter 2: Water Use and Supplies

The most fundamental characteristics of the existing water supply system in Yolo County are the balance between agricultural and municipal/domestic and industrial (M&I) water demands, and the mix of surface and groundwater resources available for these water uses. An inventory of current water uses in Yolo County and their sources of supply is presented in this section. Appendix A contains details of the data, assumptions and analysis used by the author to arrive at the estimates for this inventory. The values represent average conditions for the County.

A. COUNTY SETTING
The principal features of the County's water system are shown on the map in Figure 1. These consist of the surface water resources, the portion of the County lying over groundwater, and the major M&I and agricultural water purveyors.

Surface water resources available to the County come from three natural water courses: the Cache Creek watershed, the Sacramento River watershed, and Putah Creek watershed. Some water from Cache Creek is supplied from Clear Lake and Indian Valley Reservoir. Sacramento River water is supplied directly from the river, and under Bureau of Reclamation contracts through the Tehama-Colusa Canal, and indirectly via the Colusa Drain in the form of return flow from irrigation in Colusa and Yolo Counties. Shaded areas on the map in Figure 2 are served by these surface water supplies. Lake Berryessa (Putah Creek) water is supplied almost exclusively to Solano County. Groundwater underlies all of the County to the east of the aquifer boundary delineated in Figure 1. About 68 percent of the County, or 447,000 acres, has accessible groundwater resources. Located within this groundwater zone are all of the County's irrigated agricultural lands and urban areas.

M&I water use is located in the four cities of Davis, Woodland, West Sacramento and Winters. The University of California at Davis is an important M&I user in the County, managing its own water supply system. Rural residents and small towns account for minor amounts of domestic water use (which by convention is included in the M&I sum).

Agricultural water use is controlled by both private farm enterprises and by the irrigation districts. The largest irrigation district is the Yolo County Flood Control and Water Conservation District (YCFCWCD). Other districts are the Dunnigan Water District (DWD), the Yolo-Zamora Water District (YZWD), and the 2047 (Colusa) Drain Users Association.. The YCFCWCD operates an extensive network of irrigation canals in the western half of the County to distribute water from the Cache Creek System to farmers. DWD, YZWD and the 2047 Drain Users are all smaller and located in the northeast central portion of the County and receive most of their water from the Tehama-Colusa Canal, groundwater, and the Colusa Drain respectively. A number of other smaller reclamation districts have been formed in the eastern part of the County.

While conjunctive use of surface and groundwater occurs in Yolo County, conjunctive use management of these two supply sources has not yet happened. Even within each of the four main irrigation districts where both supplies are used (albeit each to different degrees), the districts themselves are involved only in the management, distribution and sale of surface water supplies. Groundwater remains a unmanaged resource, and its use is largely determined by the private decisions of farmers.

County-wide water planning has been very limited both in theory and practice. One important reason is that there is no agency, entity or body in the County to do it. The initial effort at County-wide planning occurred in 1984 to develop a water plan (Borcalli, et al., 1984). The plan was never implemented, apparently for lack of interest and an organizational structure. Recently, new activity including an update for the 1984 Plan (Borcalli and Associates, 1992a) and a search for supplemental water supplies (Borcalli and Associates, 1992b) has been initiated by two new interagency coordinating groups -- the update by the Interagency Water Management Coordinating Group (ICOR), and the joint search by the Yolo County Water Group (YCWG) in collaboration with the Solano County Water Authority. In both cases, the YCFCWCD, the largest water agency in the County, was the initiator. The voluntary formation of these two groups during the last several years indicates a growing awareness in the County of the need to coordinate water planning. However, so far these efforts have not been fully representative nor comprehensive, but rather are single-focus activities. Thus, on the whole, water planning and management activities continue to be made independently of each other by over a dozen local entities and hundreds of individual farmers.

B. 1990 WATER USE ESTIMATES
Based upon an analysis of data for 1990, estimated total water use in Yolo County is 964,400 acre-feet/year (Appendix A). This value represents all water uses in the County except for about 9,000 acre-feet /year of M&I use in the city of West Sacramento (see Table A.1). With an independently operated and separate Sacramento River water supply system, and because data was not readily available to the author, West Sacramento was left out of this investigation. Thus, all quantities reported in this document reflect this omission of West Sacramento M&I supplies and demands.

Table 1. Estimated monthly average water use in Yolo County for 1990

Month	M&I (acre-feet/mo)	Irrigated Agriculture (acre-feet/mo)
January	1,721	0
February	1,577	0
March	1,945	14,679
April	3,219	78,365
May	4,428	118,999
June	4,730	180,357
July	4,842	239,064
August	4,692	162,949
September	3,936	108,762
October	3,153	23,713
November	1,758	0
December	1,539	0
TOTAL (year)	37,536	926,888

      This includes municipal, industrial and domestic water uses

Source: See Appendix A for data, assumptions and analysis used to calculate these values

Only 3.9 percent of all water used in the County goes to M&I uses. The remaining 96.1 percent is used for irrigated agriculture. The cities of Davis, Woodland and Winters account for 71 percent of the M&I total. Monthly average quantities of water for M&I and agricultural use are plotted in Figure 3. The plotted values are listed in Table 1.

Agricultural water use was estimated using irrigated acreage and cropping pattern data for 1989 (Yolo County Agricultural Department, 1990), average historical evapotranspiration and precipitation conditions, and assumptions about current irrigation practices. M&I water use was calculated using 1990 population figures, one per capita rate for Davis, and another for the rest of the County. Figure 3 reveals a highly seasonal pattern of activity where 95 percent of annual water use occurs during the dry season (April to September). Water use is comparatively very low during the rest of the year and attributable mainly to the M&I demands. In July usage peaks for both agricultural and M&I activities, at 239,100 acre-feet per month and 4,842 acre-feet per month respectively. Note also in Figure 3 that M&I demands are much more constant throughout the year than agricultural water demands.

C. FUTURE WATER USE PROJECTIONS
When predicting future needs for water, two levels of analysis can be used: an aggregate analysis at the County-wide level or an analysis at the level of each local purveyor or agency concerned with water supply. In Yolo County irrigated acreage has stabilized at or very near the maximum limit of available designated land (Borcalli, et al., 1984; Scalmanini, 1991), while urban areas and population will continue growing at two to three percent a year (Yolo County Planning Department, 1991; see Table A.1). The effect of projected urban growth on County-wide water demands depends on whether: 1) M&I water use is greater or less than irrigated agricultural use, on an acre-for-acre basis; and 2) marginal lands in the western foothills and Dunnigan Hills area are developed for irrigated agriculture, as residential areas, or remain unchanged as natural shrub vegetation.

At the current (1990) rate of 12,600 acre-feet of water per year, the amount of water per acre used in the city of Davis is 2.3 feet (assuming 5,527 acres of incorporated area). On the other hand, taking average annual agricultural water use(i.e. applied water) and gross irrigated acreage in the County, irrigation uses approximately 2.6 feet per acre (Scott, et al., 1975). These numbers suggest that growth in urban water demand should be offset roughly by reductions in irrigated agricultural water use as urban areas expand into adjacent irrigated agricultural land. Rural residential expansion in areas of the County such as Dunnigan Hills, where land is not currently irrigated, will impose a small added water demand on the County. Considering that the accuracy in estimating agricultural water use is probably between 5 and 10 percent, or 46,000 to 93,000 acre-feet per year, and that actual use fluctuates each year as cropping patterns are adjusted, present levels of M&I water use and their increases, are hardly significant in the total water balance for the County. Therefore, from a County-wide perspective, total County water use has stabilized and potential future net increases will be very minimal.

Nonetheless, at the local agency level, pressure for more water supplies is growing substantially in urban areas, such as Davis, Woodland and Winters. These communities are pumping more groundwater to meet growing demands. Davis, Woodland and Winters are unable to make use of any surface water resources without major investments in new transfer and treatment facilities, structural modifications to their distribution networks, and acquisition to rights to surface water. Consequently, any surface water supplies freed-up by the converting of irrigated agricultural land to urban development cannot be directly transferred to urban uses without major investment and construction projects. In cases where new urban development will occupy agricultural land supplied by surface water, these communities face the question of how to develop more water supplies. (West Sacramento, for the moment, has a excess water supply which will enable it to meet some levels of future urban growth.)

D. AVERAGE WATER SUPPLIES
The average mix of surface and groundwater used each year to supply M&I and agricultural activities is shown in

Table 2. Notice that the M&I sector is singularly supplied by, and dependent on groundwater. Under average supply conditions ('normal' year hydrologic conditions) groundwater provides approximately 44-45 percent of the County's annual water requirements (436,100 acre-feet), while surface water is used for the remaining 55-56 percent (528,300 acre-feet) (Scott, et al., 1975; Borcalli, et al., 1984; and Appendix A).

Estimates of surface water supplies available under average hydrologic conditions are listed by source and matched to their destination/use in Table 3. As indicated in Table 2, these supplies are currently only of use for agricultural activities.

Table 2. Summary of estimated water use in 1990 and average supply mix for Yolo County

	Total use (acre-feet)	Groundwater supply (acre-feet)	Surface water supply (acre-feet)
M&I	37,500 (3.9)	37,500 (8.6)	0 (0)
Agriculture	926,900 (96.1)	398,600 (91.4)	528,300 (100)
Total	964,400 (100)	436,100 (100)	528,300 (100)

The numbers in parentheses are the percent of the column total

Source: See Appendix A

Table 3. Annual average surface water supplies in Yolo County (used exclusively for irrigated agriculture)

Source	Institution/User	Quantity (acre-feet/year)
Cache Creek	YCFCWCD	120,000a
Sacramento River	Individual CVPb contractors	372,892
Sacramento River	Private lands with riparian & appropriative water rights	63,968c
Tehama-Colusa Canal	DWD	19,000
Tehama-Colusa Canal	Colusa Water District	3,120c
Colusa Drain 2047	Drain Users	(24,960)d
Putah Creek	UC Davis	4,000
Delta Channels	North Delta Water Agency	?
	TOTAL	+/- 582,980

a) 150,000 less 20 percent conveyance losses (Frederiksen, Kamine, and Associates, 1978)

b) Central Valley Project operated by the Bureau of Reclamation (USBR)

c) Estimated based on gross land area x 2.6 ft (Scott, et al., 1975, p.65).

d) This water is irrigation return flow and is accounted for in the reuse rate.

Source: Borcalli, et al., (1984), Scalmanini (1978), YCFCWCD(1986)

E. SUMMARY
Groundwater under Yolo County is the single largest water supply source for the County, providing in average years 45 percent of the nearly one million acre-feet of water used each year. Groundwater is also crucial for M&I activities, which depend exclusively (with the exception of West Sacramento), on this resource for their water uses. In the big County picture, M&I water use is very small, representing only about 4 percent of the total use in the County and only about 9 percent of the groundwater pumped in average years (see Table 2). The cities of Davis, Woodland, and Winters account for about 71 percent of the M&I total. Agriculture, on the other hand, accounts for 96 percent of total water use and gets 57 percent of its supplies, in average years, from surface water and 43 percent from groundwater. The 9 percent of groundwater use in the M&I sector is almost all measured and controlled by public agencies while the remaining 91 percent of groundwater use, in agriculture, is unmeasured and privately controlled by a multitude of individual farm enterprises.

The second most important water supply source is surface water from the Sacramento River watershed, which provides an estimated 41 percent of the County's supplies under average year conditions. In practice, this water is imported into the County at points along the Sacramento River and through the Tehama-Colusa Canal under entitlements from Central Valley Project (CVP) contracts, riparian water rights, or appropriative water rights. As is typical for surface water, the actual quantities available during any one year are subject to large hydrologi fluctuations. The conjunctive nature of agriculture's supply system, and its vast size, relative to M&I's, are two very important features of the County's present water system. These two features offer substantial flexibility and potential opportunities for within-County, economical solutions to present and future water problems in the County. Such solutions will require, however, taking a more comprehensive and holistic perspective for planning and management of water activities in the County than has thus far been taken.