3d: Assessment of Sustained Yield

In this section, we investigate one of the key water resource issues for Yolo County's water supply system: whether and how current groundwater use levels in Yolo County are exceeding the sustained yield of the aquifer. Once again deviations from average groundwater use caused by spatial variations in pumping, drought response, and potential water transfers are particularly important phenomena in assessing problems arising from overdrafting, such as damage to the storage capacity of the aquifer, deteriorating groundwater quality, subsidence, and habitat changes.

Sustained yield and overdrafting are explained and damaging effects described in general terms in the first sub-section below. The magnitudes of the sources of groundwater recharge to, and discharge from, the aquifer are estimated in the next sub-sections. And finally a groundwater balance calculation is presented for both average and drought situations in the County.

D.1. Definition of Sustained Yield and Effects of Overdraft

In a natural equilibrium state, the recharge (R_o) to the groundwater aquifer is balanced by the discharge (D_o) such that R_o equals D_o. Groundwater pumping is a new disturbance on the natural system. In the words of Theis (1940) pumping is a "new discharge superimposed upon a previously stable system, and it must be balanced by an increase in the recharge of the aquifer, or by a decrease in the old natural discharge, or by a loss of storage in the aquifer, or by a combination of both" (quoted in Bredehoeft, 1982). The appropriate relationship can be expressed mathematically as

where Q is the rate of withdrawal due to pumping, ΔR_o and ΔD_o are the changes in mean recharge and natural discharge rates to the aquifer that result from pumping, and dV/dt is the rate of change of storage in the aquifer system (Bredehoeft, 1982). Whenever dV/dt is negative, an overdraft condition exists.

To prevent loss of storage (i.e. dV/dt = 0), Q must come from either an increase in recharge to, or a decrease in discharge from the aquifer, referred to together as 'capture'. Under these new equilibrium conditions, Q can be called the sustained yield. The balance equation for the sustained yield will have different values over different time intervals and at different spatial scales in the system owing to the non-uniform time patterns and spatial distributions of pumping in Yolo County as previously discussed in section 1.B and 2.A. Time delays in the response of the aquifer system in re-distributing pumping effects will also occur. While storage may be reduced during a drought year, the next year may involve sufficiently greater recharges to recover this storage. Over the long term, if the average Q exceeds average 'capture', then groundwater in storage will be steadily depleted.

One important physical impact of overdrafting is the compaction of clay layers and consolidation of unstructured sand and gravel formations in the aquifer with the resulting subsidence of the land surface (Freeze, et al., 1979; see Appendix D for a more detailed explanation). Compaction, consolidation and associated land subsidence are largely irreversible consequences of repeatedly overdrafting the groundwater aquifer. Subsidence of the land surface can result in structural damage, drainage problems, and increased flooding. Consolidation of aquifer layers reduces the aquifer's capacity to store water in the next and future rounds of recharge, and reduced transmissivities from compaction can negatively affect well yields and change the quantities and direction of groundwater movement. These are all major concerns in Yolo County.

Another important problem with overdrafting is potential water quality deterioration. This can come about when water of lesser quality is drawn into an overdrafted area to recharge it (e.g. salt water intrusion), or when changes in the paths and velocities of groundwater flow cause the groundwater to become contaminated by new constituents. Also, the concentrations of constituents or pollutants entering an overdrafted zone from the surface, or from other sources, can increase locally when recharge and dilution of the overdrafed area become reduced. (Freeze, et al., 1979.) Groundwater quality and its maintenance is of concern in Yolo County for both agriculture and M&I activities, but it has not been systematically investigated

D.2. Groundwater Recharge : Sources and Magnitudes

The sources of recharge to the groundwater aquifer under Yolo County are discussed and their annual average contributions estimated. A comprehensive review of many of the investigations, reports and studies conducted over the years on water issues in Yolo County was made by the author, and local experts were consulted, to assemble the information and quantities presented here. Because no primary data collection and analysis was conducted specifically for this project, where disagreement exists in the literature over recharge mechanisms and volumes, the author has tried to include the ranges of opinion and the general consensus.

Recharge from natural channels

Seepage from Cache Creek, Putah Creek, and the Sacramento River contribute recharges to Yolo County's aquifer. Each contribution is discussed below.

Average Cache Creek recharge into the Cache Creek sub-basin along the reach from Capay to Yolo was estimated at 22,400 acre-feet/year for the 1962-1971 period (Scott, et al., 1975), 25,600 acre-feet/year for 1953-59 period (Woodward-Clyde, 1976), and 27,620 acre-feet/year for the 1959-79 period (Wahler Associates, 1981). The low and high extremes were associated with the 1976-77 drought and the wet year immediately following it. In 1977, seepage from Cache Creek provided only 2,000 acre-feet of recharge. Then in 1978, the severe water table drawdown in the Cache Creek sub-basin (as much as 80 feet in some wells), combined with above average Cache Creek flows, caused recharge from the Creek to reach a record 113,000 acre-feet (Wahler Associates, 1981). Both Cache and Putah Creek recharge into Yolo County groundwater are affected by pumping patterns and groundwater gradients to the south and north along the Creeks. Scott, et al. (1975) estimated the contribution from Putah Creek at 19,000 acre-feet/year on average for 1958-1968. Since Scott's study, reduced pumping activity in Solano seems to have altered the water table gradients in the Davis area (Fredericksen, Kamine and Associates, 1978), maybe effecting an increase of recharge from Putah Creek to the Lower Cache-Putah sub-basin. In addition, pumping activity along some portions of the Creek may be inducing greater recharge into Yolo County.

Sacramento River seepage recharges the Colusa and East Yolo sub-basins along their eastern sides. Its contribution is not very significant because of the flat groundwater gradients in the area and the lower permeability of the basin materials (Yates, 1991). The flat gradients are the result of minimal groundwater pumping in the eastern parts of both of these sub-basins.

These gross recharge volumes are not evenly distributed along the reaches of the respective rivers and creeks, but depend strongly on local water table gradients, which depend on the impacts of pumping and other recharge/discharge mechanisms. Some reaches of Cache and Putah Creeks gain water from the groundwater rather than recharge it.

Recharge from irrigation and drainage canals

The YCFCWCD has an extensive network of unlined delivery canals and seepage from these canals is estimated at 20-22 percent of the average YCFCWCD supply (Frederiksen, Kamine, and Associates, 1978). This volume amounts to about 30,000 acre-feet during normal year deliveries and contributes recharge mainly to Cache Creek, Upper Cache-Putah, and Plainfield Ridge sub-basins. Seepage from the Colusa drain was estimated by Scott, et al. (1975) at 10,000 acre-feet/year and provides recharge to Colusa sub-basin.

Deep percolation of irrigation water

Assuming irrigation efficiencies of 70 percent and deep infiltration of 75 percent of these losses, this source provides approximately 208,600 acre-feet/year of recharge and can be treated as a relatively evenly distributed recharge source.

Deep percolation of rainfall

For average rainfall years, this contribution has been estimated at 157,200 acre-feet (Yates, 1991). Precipitation recharge is also relatively evenly distributed over the groundwater basin, with slightly higher volumes in the western areas of Cache Creek and Upper Cache Putah sub-basins near the foothill boundaries.

Subsurface inflows

Subsurface inflows across the boundaries of the basin vary considerably in time due to changes in water table levels, gradients, and associated transmissivities. They are difficult to accurately estimate without a good groundwater model. For the whole groundwater basin, inflows from outside the basin are considered small compared to the sources of recharge mentioned above (Scott, et al., 1975; Yates, 1991). However exchanges between sub-basins within the aquifer occur. The most important of these is the subsurface flow, driven by the dominant west to east groundwater flow gradient , which recharges Lower Cache-Putah sub-basin with groundwater from Upper Cache-Putah sub-basin as it moves across Plainfield Ridge. This flow pattern is a result of the groundwater elevations that are plotted in Figure 6.

Assessment of recharge situation

In summary, the major sources of groundwater recharge, in order of importance are: 1) on-farm irrigation seepage; 2) deep percolation of rainfall; and 3) seepage from natural channels and 4) from irrigation/drainage canals (see Table 5). Notice that human-induced, or what could be called 'artificial' sources of recharge are very significant. For example the 175 miles of YCFCWCD distribution canals contribute recharge comparable to that of Cache Creek, while irrigation inefficiencies exceed rainfall recharge.

D.3. Groundwater Discharge : Sources and Magnitudes

Natural discharges from the aquifer include subsurface outflow to Solano County, evaporation losses from areas with high groundwater tables, and phreatophyte transpiration losses along creeks. While the first one may be significant, estimates depend strongly on water table conditions in the area, which have varied considerably in response to the use of changing sources of irrigation water in Solano County. Reduced pumping activity there may have altered the groundwater elevations and gradients in the Davis area (Fredericksen, Kamine and Associates, 1978), possibly causing a reduction in subsurface groundwater outflows to Solano County. Yates (1991) estimated loss to Solano County at 102,000 acre-feet/year on average, while Scott, et al., (1975) estimated it at only 3,900 acre-feet/year for the period 1963-1972. The other two natural discharge sources are relatively small. Pumping, at 436,100 acre-feet/year, as seen in Table 5, clearly dominates the discharge picture and any natural quantities of discharge from the groundwater basin under Yolo County.

D.4. Groundwater Balance Calculation

Yates (1991) has examined the current average year water balance for the groundwater aquifer under the County as a single unit, using various estimates for the recharge and discharge flows. His analysis suggested that the groundwater system may be very close to a balanced position again. The balance calculation in Table 5, using the estimated recharges and discharges given in the previous paragraphs, for average year conditions at 1990 water use levels, suggests a range for the County-wide overdraft from 0 to 35,700 acre-feet/year. The Scott, et al. (1975) study calculated an average overdraft of 12,000 acre-feet/year for the period 1963-1972, and projected an average 50,000 acre-foot/year groundwater overdraft for 1990, assuming that supplemental Cache Creek surface water from operation of the Indian Valley Reservoir would be available as anticipated when he conducted this study (and realized in 1978). The only additional surface water, which Scott omitted from his analysis, is the 19,000 acre-feet from the USBR via theTehama-Colusa canal, supplied to DWD since 1983. If included, this supply would reduce Scott's overdraft projection to an average of approximately 31,000 acre-feet per year, in line and consistent with this study's analysis reported in Table 5.

The recharges and discharges for a 'severe' drought year (see Figure 5) were also estimated (see Appendix B, Table B.3) to illustrate how droughts can affect the groundwater balance. The balance is reported in Table 6. Estimates of overdraft, in a 'severe' drought year, range from 490,900 to 533,900 acre-feet . Supporting these dramatic overdrafts during multi-year droughts, while also protecting the aquifer in the County from irreversible damage, would require careful planning and management of groundwater resources so that adequate storage could be maintained or recovered during wet years. To protect groundwater in storage for drought years would require changing the patterns of both surface and groundwater use in appropriate parts of the County during non-drought periods to compensate for heavy use of, and dependence on, groundwater in droughts.

Table 5. Annual average groundwater balance for Yolo County

Estimated recharge:
	Acre-feet
1) Deep percolation of irrigation water	208,600
2) Deep percolation of rainfall	157,200
3) Seepage from natural channels Cache Creek Putah Creek Sacramento River	27,600 19,000 0 ~ 55,000
4) Seepage from irrigation and drainage canals YCFCWCD Colusa Drain	30,000 10,000
5) Subsurface inflows	negligible
Subtotal	452,400 ~ 507,400
Estimated discharge:
1) Subsurface outflows	4,000 ~ 102,000
2) Evaporation from high water table	?
3) Phreatophyte transpiration	5,000 ?
4) Pumping withdrawals	436,100
Subtotal	445,100 ~ 543,100
Estimated overdraft:	(7,300) ~ 35,700

Source: See Appendix B

Table 6. Severe drought scenario groundwater balance for Yolo County

Estimated recharge:
	Acre-feet
1) Deep percolation of irrigation water	218,200
2) Deep percolation of rainfall	19,000
3) Seepage from natural channels Cache Creek Putah Creek Sacramento River	2,000 19,000 0 ~ 55,000
4) Seepage from irrigation and drainage canals YCFCWCD Colusa Drain	10,000 10,000
5) Subsurface inflows	negligible
Subtotal	278,200 ~ 333,200
Estimated discharge:
1) Subsurface outflows	4,000 ~ 102,000
2) Evaporation from high water table	?
3) Phreatophyte transpiration	5,000 ?
4) Pumping withdrawals	760,100
Subtotal	769,100 ~ 867,100
Estimated overdraft:	490,900 ~ 533,900

Quantifying the precise degree of overdraft would require far more data and a complete groundwater modeling analysis of how pumping in both average and drought years is affecting the long-term recharge and discharge balance on the aquifer. Thus far nobody in the County has undertaken such a water planning task. The most detailed studies (Woodward-Clyde, 1976; Wahler Associates, 1981) of groundwater balance have been done only for the Cache Creek sub-basin and these indicate a net decrease in storage in the area from Capay to Yolo of 166,000 acre-feet (spread over a 47,000 acre area) during the period 1959 through 1979 (Wahler Associates, 1981). Despite the inability to accurately pinpoint the exact state of overdraft, overdraft conditions in the County are evident from the progressive decline in groundwater elevations in many parts, from subsidence problems and water quality problems. A review of these groundwater problems follows.