Chapter 4: Conjunctive Use Management of the Groundwater Basin

The coordinated and integrated management of the County's surface and groundwater resources, under a conjunctive use management program, would aim to optimize the joint use of all water resources in the County. This approach to water planning would require a comprehensive consideration of County-wide water objectives and the matching of the characteristics of different supplies (such as quality, availability, cost, and reliability) to the requirements of different water demands as conditions change inside and outside the County. In general, greater benefits from the conjunctive management of all water supplies together can be achieved over the isolated management of each individual supply system.

This Chapter provides an exploration of potential conjunctive use management opportunities for improving Yolo County's water supply situation now and in the future, for addressing some of the overdraft-related groundwater problems identified in the previous Chapter, and for managing droughts. Opportunities introduced in this Chapter include 1) various ideas for artificially enhancing groundwater recharge using excess surface water; 2) ways of adjusting the mix and match of supplies and demands within the County through the exchange, redistribution or re-allocation of water supplies; and 3) examples of how planned water transfers outside the County might be designed to contribute positively under a conjunctive use management approach to the County's overall water situation.

Before presenting specific conjunctive management strategies, a brief comparative review of the characteristics of surface and groundwater is provided in the first section. The different roles these two types of water can play in conjunctively operated water systems result directly from these characteristics. Next, unmanaged flows in Cache Creek, and other surface supplies are examined as possible sources of water for artificial recharge schemes in the County. Then some specific concepts for artificial recharge, redistribution of supplies, and planned water transfers in Yolo County are described and assessed for their potential contributions to the County's water system.

A. COMPARISON OF GROUNDWATER AND SURFACE WATER CHARACTERISTICS

Characteristics of ground and surface water supplies, relevant to conjunctive management, are compared below.

A.1. Availability and storage
The groundwater aquifer under the County provides a natural ready-made long-term water storage reservoir for the County. Unlike most surface water reservoirs, the aquifer can store water for longer, even unlimited periods of time, making it available when needed in drought years. The groundwater basin under Yolo County in the depth range of 20 to 420 feet below the ground surface can store 14,038,000 acre-feet of water (Scott, et al., 1975, p.48). In contrast, Clear Lake and Indian Valley reservoir in the Cache Creek watershed contain only 320,000 and 260,000 acre-feet of storage respectively (YCFCWCD, 1986). As mentioned in Chapter 3, about 700,000 acre-feet of empty storage space existed in 1975 in Yolo's aquifer according to Scott's study (1975). The current drought (beginning in 1987) had removed an additional one million acre-feet of groundwater from storage by fall 1990 (Borcalli and Associates, 1992, p.26). Thus, ample free storage space is available in the County for conjunctive use activities and artificial groundwater recharge.

Reliable direct use of surface water requires surface water storage reservoirs because the timing of natural streamflow does not match the times when water is required (compare monthly flow in Cache Creek, Figure 7 and monthly water use in Yolo County, Figures 3) in California. No new reservoirs for storing Cache Creek water are anticipated for Yolo County. Clear Lake and Indian Valley reservoir operate on an annual cycle, regulating stream flows between the wet and dry seasons within a given year. They lack the capacity to provide sufficient carryover storage from year to year in case of droughts. The availability of water imports into the County from the Sacramento River also depends on water storage capacity in the Sacramento River system reservoirs, which is unlikely to be increased.

A.2. Access and distribution
The aquifer is a natural distribution system, and thus reduces the need to build above ground distribution systems. Wells can be installed in different and distant places to tap recharge placed in the aquifer at another time and place. Water stored in the groundwater basin can be made available when and where needed simply by controlling flow at the pump, as long as the aquifer has water and the well is deep enough. On the other hand, surface water requires that distribution systems be constructed and operated to transport water from withdrawal points to areas where it is needed in the County.

A.3. Quality and treatment
Expensive water treatment systems generally are not necessary for municipal/domestic groundwater supply because the bacterial quality of groundwater is good and the TDS is acceptable. In a few localized areas in the County, problems with TDS may preclude using groundwater for some M&I uses without treatment. However decentralized point-of-use treatment options, such as reverse osmosis home units, might easily correct most TDS-related problems and are becoming more affordable. For agriculture the quality of groundwater in Yolo County is generally good, except in those localized areas having high boron levels mentioned in Chapter 3.

Surface water quality is superior to groundwater quality for agriculture because of its lower TDS. But surface water's poor bacterial quality, high turbidity, trace carcinogenic organics (especially trihalomethane precursors), and various other quality parameters make it less suited for M&I use and necessitate large expensive treatment plants. Unlike M&I uses, agricultural uses of surface water require no water treatment and benefit from lower TDS levels, as compared to those of groundwater. For some industrial and other applications, however, the significantly lower TDS of surface water can outweigh its other quality problems.

A.4. Legal rights
Legally, entitlements to ground and surface waters are very different. While rights to surface waters are well developed and clearly defined in California, groundwater rights remain undefined, and poorly controlled. This causes some difficulty for effective management of groundwater because legal or institutional controls on the quantities and locations of groundwater pumping are difficult to achieve (Kletzing, 1988).

A.5. Costs
On the cost side, for M&I users, although initial investment in well/pump installation can be risky and somewhat expensive, it is generally much cheaper compared to building a surface water treatment plant, and installing the larger pipes required for distribution from a centralized point. In agriculture, there can be extra costs of using surface water both for the conveyance system needed to get supplies from storage/withdrawal points to farm sites and for any pressurization required for special irrigation technology. Costs for groundwater supplies also involve the extra operating costs of pumping which vary with the depth to groundwater. In general, groundwater is much cheaper than surface water for small and moderate-sized M&I applications, while in agriculture the comparative costs of surface and groundwater depend strongly on local conditions and water subsidies.

A.6. Reliability and local control
From the perspective of reliability and local control, groundwater is superior to surface water as a supply source for the County. Surface water from the Sacramento River, the second single most important water resource after groundwater in the County, is susceptible to hydrologic fluctuations, institutional controls, decision-making power located outside the County, and competition from many other more powerful entities in the state.

B. SOURCES OF WATER FOR ARTIFICIAL RECHARGE
Excess surface water, under conjunctive use management, can be put to use recharging Yolo County's groundwater basin to recover storage after a period of heavy groundwater use or to mitigate localized overdraft problems. The first step in developing artificial recharge schemes is to identify possible sources of excess surface water and their availability.

B.1. Unmanaged Cache Creek surface water
Winter runoff from the Cache Creek watershed amounts to an average of 307,000 acre-feet and is theoretically available for artificial recharge. This figure is the sum of the historical (1903-1989) average flow for the months of December through May at the Yolo gage on Cache Creek plotted in Figure 7. Streamflow at Yolo represents runoff in Cache Creek after irrigation withdrawals and natural aquifer recharge (discussed in Chapter 3) have been removed.

Runoff water leaves the County through the Yolo bypass as unmanaged flows into the Sacramento River and onward to the Delta . In reality these unmanaged flows are highly variable quantities in any given month and year. During the period 1980-1989 (excluding data missing for 1984), the average amount of unmanaged water in Cache Creek at Yolo from December through May was 480,000 acre-feet (see Figure 7).

By appropriating some of this unmanaged water, artificial recharge schemes using excess Cache Creek water could be developed. The operation of such schemes would be limited to the rainy season of wet years. In practice, the amount of water which could be recharged is limited by the size of feasible spreading areas, by the rate of surface infiltration and deep percolation, and by the actual availability of these unmanged flows in the Creek, on a day to day basis, during the wet season. It would be impossible to recharge all of the excess water into the aquifer. In Figure 7, the wet month with the lowest historical average flow is May, at 12, 241 acre-feet, followed by March, at 22,703 acre-feet.

Systems designed to operate with the average May flow would be quite reasonable in size (see section C.5) and could be operated continuously for six months to provide about 60,000 acre-feet of groundwater recharge. Increasing the areas for infiltration, for example, to take advantage of the full average March flow, could achieve recharge approaching 100,000 acre-feet in average wet years. However, larger systems will more frequently operate below capacity during wet periods than more conservatively sized designs. Phasing in capacity is the most appropriate solution to uncertainty about predicting operations.

B.2. Treated urban wastewater
Treated wastewater from the cities of Davis and Woodland represents about 55 percent of their water use, or about 7,000 acre-feet/year from each city. Currently this water is discharged into drainage canals which flow into the Yolo Bypass. Much of the wastewater, during the irrigation season, ends up being withdrawn downstream for irrigation by farmers in the County. At other times of the year, it mixes with surface runoff and flows into the Sacramento River system. This water should be considered as a potential water supply for artificial recharge.

B.3. Surface water from outside Yolo County
Other sources of surface water for recharge activities are the Sacramento River and Putah Creek. Above average flows from either watershed during the rainy season could be used. Because the control of both of these sources lies outside the County's boundaries, getting their use for recharge projects will require more involved negotiations and arrangements with the respective institutions, and having to compete with other state water players who are also looking at artificial groundwater recharge. Access to Sacramento River water could be made either at the river along the eastern edge of Yolo County, or via the Tehama-Colusa canal and its possible extension into the County as far as Cache Creek. Putah Creek excess flows are also a potential source of water for a) enhanced recharge through the channel sytem; b) diversion to recharge sites in adjacent areas like those near Davis; or c) diversion to more distant sites that could be accessed with the YCFCWCD's distribution system and a hypothetical connector between Putah Creek and YCFCWCD's Winter's canal, near Winters. Much less has been studied about instream recharge in Putah Creek than in Cache Creek and very little information is available about the present and potential role Putah Creek plays in groundwater recharge. However, consideration of Putah Creek water for recharge into the southern edge of the County's aquifer would be worthwhile, especially as it might affect the groundwater balance around Davis.

C. ARTIFICIAL RECHARGE OPPORTUNITIES
Surface water recharge schemes can be designed to address specific groundwater problems, to prepare for drought year water demands, and to augment average year supplies. Unmanaged and extra winter flood flows in Cache Creek, the Sacramento River, and Putah Creek could be appropriated or imported into the County for artificially recharging different parts of the groundwater aquifer. This section will examine some artificial recharge opportunities that could form parts of a conjunctive approach to the management of Yolo County's water system.

A variety of artificial recharge methods are feasible. Surface water spreading operations such as spreading basins, infiltration pits, ponding, and stream modification, are the most commonly used techniques (Asano, 1985; Pettyjohn, 1981). Injection or recharge wells, where surface water is pumped directly into the aquifer, are more expensive to build and more costly to operate. Induced recharge is another mechanism, different in principle from either surface or injection methods, that operates at the interface between surface channels and the groundwater aquifer. Greater infiltration from an adjacent stream channel into the groundwater basin can be induced by pumping from a series of wells installed along side the channel. When pumped, these wells cause the water table gradients between the stream and the wells to steepen towards the wells, literally drawing in more water from the stream. Seven different artificial recharge concepts for Yolo County, mainly using unmanaged Cache Creek water, are conceptually presented in the following sections.

C.1. Recharge using irrigation distribution system canals
The YCFCWCD distribution system contributes about 30,000 acre-feet/year of recharge to the aquifer during the irrigation season from April to September. Assuming that this recharge quantity is evenly distributed in time, the system can recharge about 5,000 acre-feet/month. From information on the canal lengths and sizes in the distribution system (see Appendix C), approximately 440 acres of infiltration surface area exists when the system operates at design depth. Based on the system loss rate, and this area, an infiltration rate of 0.38 feet/day can be assumed. This is a reasonable rate for the soil conditions in Yolo County and in the canals (Fredericksen, Kamine and Associates, 1978, p.IV-19).

Filling the canals during the off-season with excess Cache Creek winter water from mid-November through mid-March might provide approximately 20,000 acre-feet of additional groundwater recharge. Modifications to the distribution system would be necessary to maintain sufficiently high depths in the canals. Other aspects of the scheme might necessitate modifications to the headworks for silt and flow control during flood season, and minor changes to reservoir and flood operations. This scheme would entail costs for these modifications and for extra operating costs by the YCFCWCD. Recharge would directly benefit areas under the canal system, mainly Cache Creek, Upper Cache-Putah, and Plainfield Ridge sub-basins, and would also serve to increase subsurface recharge from the Upper Cache-Putah to the Lower Cache-Putah sub-basin. Secondary effects from higher groundwater tables would need to be explored. However, this scheme would make a general contribution to increased storage in the whole system.

Recovery of the recharged water could occur either through drought year pumping, through irrigation season pumping at YCFCWCD centrally managed well fields optimally located (i.e., in very full areas) in the affected sub-basins, or through increased pumping by private users in these sub-basins. The additional recharge would allow: a) the more even distribution of groundwater withdrawals throughout the Flood Control's district, by allowing them to enter into the conjunctive use and management of their Cache Creek surface water with groundwater pumped from these centrally managed well fields and distributed to farmers through the district's canal system or even transfered to other uses elsewhere within the County (e.g. Woodland or Davis); or b) diversion of more of YCFCWCD's surface water away from the fuller western sub-basins to YZWD, while western farmers directly pumped the recharged groundwater instead. These two activities might be used to help reverse the heavy overdraft in the YZWD area, reduce subsidence there, improve recharge to the Yolo-Woodland-Zamora area of the aquifer, and more effectively use the groundwater in the western part of the County.

C.2. Winter season irrigation applications
Pre-season irrigation of bare fields can enhance and contribute to deep percolation of either rainfall or subsequent irrigation. Excess winter stream flow could be made available in wet years to farms in overdraft areas(around Yolo and Woodland for example) or east of the Plainfield Ridge sub-basin, for one or two additional off-season irrigations to their fields. Because winter evapotranspiration demands are low, most of this water should contribute to recharge.

Two additional applications of 3-4 inches could yield as much as 500-660 acre-feet of recharge per 1,000 acres of farmland. If 25 percent of the YCFCWCD service area where involved, this operation could potentially contribute 22,000 to 29,000 acre-feet/year of additional recharge.

Costs would depend on whether new distribution canals would have to be constructed to reach overdraft areas. This would be the case for parts of Lower Cache-Putah sub-basin. YCFCWCD operational changes and extra costs would be similar in nature to those described above for scheme C.1. The irrigation applications would also involve some extra farm costs. As a recharge source, this scheme could benefit overdraft areas and their problems directly.

C.3. Cache Creek induced recharge for Davis/Woodland supplies The reach along Cache Creek from Esparto Bridge to half a mile upstream of Stephens Bridge gains flow from groundwater under the influence of the existing water table gradients in this region. This section of the creek does not currently contribute much if any recharge to the groundwater during the winter recharge season. Locating a network of induced recharge wells or infiltration galleries ('Raney wells') parallel and close to the Creek at the lower end of this reach, and pumping groundwater during the wet season to be delivered by pipeline to Davis and Woodland, could capture some of the unmanaged Creek flows and increase recharge in this area. Given the high permeability of the shallow basin and creek materials, this induced recharge mechanism should be feasible.

The benefits of this scheme would result from supplying the water demands of the cities of Davis and Woodland, the only significant water demands in the County for the operating period. If the combined water demands of these two cities for December through May, could be met by this operation, the cities could reduce their groundwater pumping in Lower Cache-Putah sub-basin. At current demand levels, 9,600 acre-feet of induced pumping would be needed. This is equivalent to a continuous pumping rate of only 27 cfs during the six month period, and suggests that future demands projected at 14,000 acre-feet in 2010 (see Table A.2 and A.3) could be met as well. A pipeline from near Stephens Bridge to Davis (13.5 miles) and Woodland (7 miles) would be required, but major water treatment might be reduced and even avoided because of the natural filtration process of passing the water through the ground. Furthermore, the potentially lower hardness of induced Creek surface water would be an improvement over existing groundwater quality in these cities. Boron in Cache Creek runoff might be a concern. Costs could be substantial but are likely to be much less, by avoiding treatment, than the costs of developing surface water supplies now being proposed for Davis and Woodland. Other benefits may easily offset these costs. Reducing groundwater pumping in the vicinity of both cities would help to control the overdraft problems in the whole Lower Cache-Putah portion of the aquifer and improve the cities' local groundwater balance going into a drought period.

C.4. The YCFCWCD Cache Creek recharge concept
The YCFCWCD has proposed a scheme (Borcalli and Associates, 1990) to use and operate the upper Cache Creek sub-basin as an underground water storage reservoir and to capture some of the unmanaged flows from Cache Creek. Artificial recharge basins created along Cache Creek between Capay and Esparto, and downstream of the Interstate 505 crossing as far as Stephens Bridge, would be used to enhance recharge infiltration from the Creek into the aquifer. A number of operational problems exist with this scheme. Because most of the sub-basin is presently full, the YCFCWCD proposes to pump the aquifer during the May to November period down to the historic low levels reached in the fall of 1977 (50-70 feet below average annual groundwater elevations) to create space for recharging the excess unmanaged Creek flows. Drawdown of the basin poses several difficult problems. First the impact on private wells in the area must be mitigated, both in terms of increased pumping costs and dried up wells. More importantly however are the consequences of this drawdown on the storage in the basin going into a drought period. The scheme requires intentionally drawing down the aquifer in anticipation of winter flood flows. The inability to forecast future rainfall poses serious problems in this scheme for managing resources for drought conditions, and ignores the critical dependence during drought years on groundwater supplies. It is not clear whether the objectives of this scheme are for drought management or for increasing the quantities of YCFCWCD's marketable water in normal years. Finally such a big drawdown has far reaching consequences for changing the groundwater flow patterns throughout the basin.

Customers or users will also have to be found for the significant amounts of water pumped, estimated at 30,000 acre-feet, that will be necessary to create the underground storage space. Irrigation is the only demand of this magnitude, however YCFCWCD customers appear to be more than satisfied with the quantities the district supplies in wet years (indicated by YCFCWCD irrigation sales in 1989). Many farmers may prefer to pump there own groundwater at a comparable or lower cost than purchasing YCFCWCD system water from this scheme in the wet years when it is available.

In terms of the water resource problems of the County, this recharge concept only addresses a questionable need for increased future agricultural water supplies. County-wide groundwater problems may actually be exacerbated by this operation -- firstly by reducing the flow of groundwater from west to east in the Cache Creek sub-basin and from the Upper to the Lower Cache-Putah sub-basin; -- secondly by drawing down groundwater storage during wet years when it should be being protected or built up for drought periods; and thirdly by increasing the likelihood of compaction of the clay lenses, consolidation of the sand/gravel mixes, and associated subsidence and loss of storage capacity in the Cache Creek groundwater sub-basin under the repeated draw down of the water table to the extreme low set in the 1977 drought.

C.5. Enhanced recharge along lower Cache Creek
Recharge basins or pits, located in the gravel areas along the lower reaches of Cache Creek, could be used to recharge overdraft areas located in Lower Cache-Putah, Colusa and eastern Cache Creek (east of Plainfield Ridge and Stephens Bridge) sub-basins. Infiltration rates for the permeable gravel along Cache Creek are in the range of 5 to 6 feet/day (Richardson, 1961). To recharge 60,000 acre-feet in three months, approximately 100-150 acres of basin area are required. Assuming lower infiltration rates, of 2 feet/day (Moreland, 1972), 330 acres would be required. These are small areas compared to the size of areas designatied for gravel sites along Cache Creek below Stephens Bridge. Areas along the lower portion of Cache Creek east of Plainfield Ridge provide two conditions conducive to artificial recharge, both of which are lacking in much of the Cache Creek sub-basin to the west of Plainfield Ridge. First, the basin in the lower area is not full, and therefore has considerable room to store recharge. Second, Cache Creek from Stephens Bridge to Yolo is always a source of infiltration to the aquifer, which is not the case for sections of Cache Creek.