The District's water supply system consists of Clear Lake, Indian Valley Reservoir, Cache Creek, and the groundwater basin within the District.

A. WATER RIGHTS

The District is the successor of numerous water and ditch companies. Thus, it has acquired numerous water rights. Additionally, it has appropriated water rights on its own behalf and has applications for appropriations in progress. The following is a summary:

Riparian Rights -- The District owns lands on Cache Creek and the North Fork of Cache Creek that have riparian rights. These rights are used for purposes of irrigation and hydroelectric power generation.

Pre-1914 Water Rights -- The District has an 1855 priority right to divert the natural flow of Cache Creek, and 1912 priority right to store waters in Clear Lake to elevation 7.56 feet Rumsey Gage for later release and beneficial use. These rights allow for the storage of 313,000 acre-feet in Clear Lake.

Post-1914 Water Rights --Permitted -- The District has permits for Indian Valley Reservoir which allow for the storage of 300,000 acre-feet during the winter for later release for irrigation and to generate hydroelectric power.

Applications in Process -- The District filed an application to appropriate up to 45,000 acre-feet of water from the Sacramento River, and up to 90,000 acre-feet from Cache Creek.

Groundwater -- To the extent the District imports water into an area that becomes part of the underlying groundwater, the District may claim a right to that water.

B. SURFACE WATER

1.  General

This section describes the surface water supply available to lands within the District.

The District's surface water supply consists of the Clear Lake-Indian Valley and Cache Creek system within the Cache Creek watershed, which encompasses approximately 950 square miles (Map 2). Virtually all precipitation in the Cache Creek watershed occurs as rainfall. The term "system" is used because it is truly the "system" that the District manages for its water users. As experienced in 1990, the District has and will continue to have years or periods where there is no surface water supply available for its water users.

The various components of the District's water supply system are described below:

Clear Lake -- Clear Lake is a large shallow natural body of water with an area of approximately 44,000 acres when full, and has a maximum depth of approximately 50 feet. The lake is operated under the terms of the "Solano Decree" (February 1978). This decree stipulates the amount and rate by which the District can withdraw water between the limits of zero and 7.54 feet on the Rumsey Gage, which is located on the lake at Lakeport. Zero on the Rumsey Gage is regarded as the natural rim of the lake. At zero, water ceases to flow into Cache Creek. Rumsey Gage 7.54 feet is considered a "full" lake with 313,000 acre-feet of storage. The District's allowable withdrawal from Clear Lake is determined by the stage of Clear Lake on May 1. The maximum withdrawal is 150,000 acre-feet. If the stage of Clear Lake is 3.22 feet or less on the Rumsey Gage on May 1, the District may not withdraw any water to deliver below the Cache Creek Dam that season.

Clear Lake provides no carryover storage. Therefore, the District attempts to use its full allowable withdrawal each year.

The District owns and operates Cache Creek Dam, a conservation structure constructed on Cache Creek approximately five miles downstream of Clear Lake. In 1986, the District completed construction of a hydroelectric project with a nominal capacity of 1,750 kW. Cache Creek Dam is located approximately 49 miles upstream from the District's Capay Diversion Dam.

Indian Valley Dam and Reservoir -- In 1975, the District completed construction of the Indian Valley Dam and Reservoir Project. The Indian Valley Dam and Reservoir are owned and operated by the District. The dam and reservoir are located on the North Fork Cache Creek approximately 54 miles from the Capay Diversion Dam.

When full, Indian Valley Reservoir has a surface area of 4,000 acres and a total storage capacity of 300,600 acre-feet. Forty thousand acre-feet of the reservoir storage is dedicated to flood control. Unlike Clear Lake, Indian Valley Reservoir provides carryover storage from one season to another.

In 1982, a hydroelectric project with a nominal capacity of 3,000 kW was retrofitted to the outlet works of the dam.

Cache Creek -- Downstream of Clear Lake and Indian Valley Dam and Reservoir, the most significant streams are Long Valley Creek, a tributary to the North Fork Cache Creek, and Bear Creek. As noted previously, all precipitation in the Cache Creek watershed occurs as rainfall. Thus, runoff tapers off sharply following winter and spring rainfall.

2. System Operation

The District's basic management objective of its water supply system is to utilize runoff in Cache Creek first. If the runoff in Cache Creek is not sufficient to meet irrigation demand, the District will withdraw from Clear Lake in accordance with the Solano Decree. Once the District compiles its "water orders" and estimates its seasonal demand, the District will then determine the amount of water required from Indian Valley Reservoir. Releases from Indian Valley Reservoir are made to augment releases from Clear Lake on as uniform a basis as possible.

In years when inadequate water supplies are available from Clear Lake, the District will withdraw water from Indian Valley Reservoir. Water supplies from Indian Valley Reservoir are used to meet current year demand. The facility is not operated to maximize carryover storage. Although Indian Valley Reservoir was designed to provide a firm yield of approximately 55,000 acre-feet, the District determined it was most efficient, from a water management standpoint, to operate to meet demand in a given year even though there may be no water available in subsequent years. This was the case in 1990, when the District had little or no water to deliver from Clear Lake or Indian Valley.

This operational strategy maximizes storage in the groundwater basin, which is the most efficient reservoir available to lands within the District. If Indian Valley was operated on a firm yield basis, the frequency and magnitude of flood spills would be greater than under current operations. Water "dumped" as a flood spill is essentially lost to the system. The efficiency of the District's operational strategy is illustrated using the District's operations model and varying the annual demand on the system (Table 1).

TABLE 1

YOLO COUNTY FLOOD CONTROL

WATER CONSERVATION DISTRICT

WATER MANAGEMENT PLAN

INDIAN VALLEY FLOOD SPILLS IN

RELATION TO ANNUAL SYSTEM DEMAND¹

Item	Scenarios
Annual System Demand, acre-feet	150,000	170,000	190,000
Indian Valley-Average Annual Flood Spill, acre-feet	31,100	21,200	13,600

¹Based upon simulated system operations, 1922-1992.

Additionally, the District owns and operates a small community drinking water system at the Indian Valley Reservoir for the District's resident employee and the campground facilities. The system complies with Title 22 Standards with respect to operation, testing, and reporting.

3.  Water Quality

With respect to water quality, the District monitors the boron concentration at various locations throughout its water supply system. The locations and an example of the range in boron concentration for a September and January sampling, are presented on Table 2.

C. GROUNDWATER

1.  General

Yolo County is underlain with a substantial amount of fresh groundwater. Clendenon (1976) estimated 13,200,000 acre-feet of water in storage between 20 and 420 feet. Roughly 50 percent underlies the District.

More important than the amount of water within the groundwater basin, however, is the amount of water that can be used without adversely impacting beneficial users of the groundwater basin.

Yolo County, to a greater extent than many areas, has an extensive network of wells that are used for monitoring groundwater levels. Presented on Map 3 are the locations of groundwater monitoring wells in Yolo County and the northern part of Solano County. In addition to this extensive network, numerous wells have records dating back more than 40 years. The majority of the well readings are made twice a year, in the spring and fall. The intent of the measurements is to observe the basin in the spring before pumping for

TABLE 2

YOLO COUNTY FLOOD CONTROL

WATER CONSERVATION DISTRICT

WATER MANAGEMENT PLAN

CACHE CREEK SYSTEM BORON CONCENTRATION

	Boron Concentration, ppm
Location	September 1995	January 1996
Clear Lake at Lakeport	0.84	1.10
Clear Lake Dam Outflow	0.98	0.96
Indian Valley Reservoir	0.45	0.48
Indian Valley Dam Outflow	0.35	0.49
Bear Creek at Cache Creek	15.0	8.4
Cache Creek at Bear Creek	0.86	1.2
Capay Diversion Dam	1.0	2.2
Cache Creek at Moore Crossing	1.71	3.0

irrigation commences and in the fall following the irrigation season. Presented on Figures 1, 2, 3, and 4, are hydrographs for selected wells within the District and outside the District. The general location of the respective monitoring wells is presented on Map 3. These hydrographs reflect the seasonal behavior of the groundwater basin and its behavior over time. The hydrograph presented on Figure 3 reflects the overdraft that was occurring during the 1950 to 1976 period prior to the District's construction of Indian Valley Dam and Reservoir.

A monitoring program of this nature provides good information on the behavior of the basin in years of "average" or greater precipitation. In drier than "average" years, the spring measurement may not reflect the full extent of basin recovery or recharge because irrigation may have commenced earlier. Monthly well readings are helpful in this regard.

The general groundwater gradients as shown on Map 4 for the Spring 1996, are typically in an east to southeasterly direction across the District.

To provide some dimension on recharge capability and utilization of the groundwater basin, selected analyses were performed. These analyses are addressed below.

a.  Seasonal Groundwater Recharge

To illustrate an upper limit to the amount of recharge likely to occur in a fall to spring season, the change in groundwater storage from Fall 1977 to Spring 1978, was selected. Generally, the groundwater basin was stressed to its greatest extent during the 1976/1977 drought. The change in groundwater levels through this period is shown on Map 5. The magnitude of recharge within the District is on the order of 250,000 acre-feet, or approximately 1.25 acre-feet/acre.

Figure 1

Figure 2

Figure 3

Figure 4

b.  Groundwater Basin Depletion During Drought

To put the groundwater basin capacity into perspective, two situations are examined below. The first situation is the estimated amount of groundwater depletion during the most recent drought from Spring 1986 to Fall 1990. The second situation is the estimated groundwater storage capacity within the range to which the basin has been stressed. The level in Spring 1986 to the level in Fall 1977, is used for this purpose.

The most severe drought experienced since Indian Valley Dam and Reservoir became fully operational began in the winter of 1986/1987. The overall groundwater basin depletion within the District from Spring 1986 through Fall 1990 (Map 6), was approximately 460,000 acre-feet, or approximately 2.09 acre-feet/acre.

In the Fall 1977, the driest year of record, the overall groundwater basin was drawn down approximately 20 feet lower than in the Fall 1990. The estimated depletion or storage capacity represented by the difference in groundwater levels between Spring 1986 and Fall 1977, was about 700,000 acre-feet or about 3.5 acre-feet/acre.

c.  Groundwater Basin Storage Enhancement

The District's construction of Indian Valley Dam and Reservoir in 1975, clearly enhanced groundwater storage within the District. The impact is very graphic in the well hydrographs. Agricultural and municipal users of groundwater, directly and indirectly, have benefitted significantly from the water supply contribution made by the District's operation of Indian Valley Dam and Reservoir. In 1987, DWR reported "the large recovery (in groundwater levels) in Yolo County is partly due to new surface water supplies from Indian Valley Reservoir."

2.  Water Quality

The District does not routinely monitor the quality of groundwater. Outside the cities, the monitoring of groundwater quality is limited and intermittent, thus data for the detection of trends or changes in groundwater quality is not available.

Boron in groundwater in the lower Cache Creek area ranges from 2 to 3 ppm. The source of boron was determined by the USGS to be from Cache Creek as opposed to upwelling from deeper stratum.

Detailed water quality testing is performed by the cities and UCD. Woodland has experienced nitrate contamination in certain wells. The City of Davis has experienced selenium contamination.

In January 1999, the Central Valley Regional Water Quality Control Board reported several sites in Davis, Woodland, West Sacramento, and Dunnigan with MTBE contamination. The District exercises no regulatory authority for handling groundwater contamination. This is handled by the state and county.

Although DWR performs some water quality tests outside the cities, the quality of groundwater is not well documented.

3.  Intrusion of Saline Water

The intrusion of saline or brackish water into what was historically freshwater is generally thought to be associated with coastal areas (e.g., the Salinas Valley). However, the intrusion of saline or brackish water could occur in the Sacramento Valley, including eastern Yolo County.

As shown on Map 7, the base of freshwater (less than 2,000 mg/l dissolved solids) is at an elevation of -2400 to -2800 feet mean sea level. New wells for agriculture within the District are generally being developed to depths of 500-600 feet. The City of Davis is developing wells for municipal supply to depths of 1,400 and 1,000 feet. UCD has also developed wells to depths of 1,200 to 1,400 feet.

As a result of water supplies developed or acquired by special districts and the private sector, Yolo County has been able to meet its water demands without significant depletion or lowering of its groundwater basin. To the extent the groundwater basin is not stressed beyond the limits already experienced, the probability of groundwater supplies being contaminated from upwelling of saline water is small. To what extent groundwater levels would have to be lowered to initiate upwelling of saline water is not known. Evidence of this type of occurrence, however, is illustrated through cross sections developed for South Sacramento County. In this area, the persistent lowering of the groundwater basin has allowed saline water to upwell significantly. This information is presented in Appendix A.

4.  Groundwater Recharge

Groundwater recharge within the District occurs from percolation of rainfall, applied irrigation water, water flowing in Cache Creek, and water flowing in Putah Creek. To the extent the pumping of groundwater by the cities of Woodland and Davis create a pumping depression, recharge occurs from the east Yolo Bypass area also.

Relative contributions of each are presented on Table 3.

The information on Table 3 is presented to merely reflect relative orders of magnitude as it may be helpful in assessing priorities when directing attention to protecting the groundwater basin water quality and augmenting quantity as well.

Table 3

Areas within the District where groundwater conditions can be enhanced are limited. To identify areas where potential may exist for groundwater level enhancement, the depth to groundwater was mapped for Spring 1996. Areas where the depth to ground- water are 20 feet or more are highlighted on Map 8. From inspection of Map 8, it appears the areas where groundwater conditions could be enhanced are in the general vicinity of Davis, Woodland, and the Hungry Hollow within the District. Outside the District, the areas of Yolo-Zamora and Dunnigan appear to have potential for groundwater level enhancement.

Although areas along the margin of Putah Creek are highlighted, there is no opportunity for effective groundwater level enhancement. Raising groundwater levels along Putah Creek will result in water draining to Putah Creek, and groundwater flow to Solano County would be increased.

5.  Well Construction and Abandonment

The District exercises no regulatory authority in the construction and abandonment of wells. The construction and abandonment of wells in Yolo County is regulated by Yolo County.

6.  Subsidence

Land subsidence, due to groundwater extraction, is documented along the east side of Yolo County from Davis to an area east of Zamora. Subsidence between Zamora and Knights Landing is reportedly to be nearly five feet and in the vicinity of Davis and Woodland, two to three feet. There are two extensometers installed in Yolo County. One extensometer is located east of Zamora and the other is east of Woodland near the west levee of the Yolo Bypass. The latter was installed as part of the monitoring program negotiated as a condition of water transfers during the DWR water bank.

More recently, interested agencies of the WRA, including the District, formed a subsidence monitoring group. The purpose of the group is to develop a network of monuments throughout the valley portion of Yolo County and, using GPS technology, establish the elevation, latitude, and longitude for each monument and horizontal as well and vertical relationships between monuments. The data will be stored in the UCD computer and made available to the public and other agencies. The network will be monitored as deemed appropriate by the group to document subsidence. This network includes the extensometers mentioned earlier.