3b: Impact of Drought on Goundwater Use

Under average wet year conditions (when the surface water supplies listed in Table 3 are fully available), groundwater use in Yolo County was estimated in this study at 436,100 acre-feet per year, for 1990 agricultural and M&I activity levels (see Appendix A). Agriculture is predominantly responsible for this extraction. With the exception of the University of California at Davis, the agricultural groundwater is pumped by individual private farmers and is presently neither officially metered nor monitored in the County. Only about 37,500 acre-feet of groundwater (about 8.6 percent) is currently pumped for municipal /domestic and industrial needs.

In drought years, agricultural groundwater use increases, as more groundwater is pumped by farmers to make up for shortfalls in rain and surface water supplies. Two possible drought scenarios and their potential impact on groundwater use are shown in Figure 5. Details of the analysis and calculations are reported in Appendix A.

The 'moderate' drought scenario assumes a 25 percent reduction in surface water supplies, and a 25 percent reduction in rainfall input to irrigation. To maintain 1990 levels of water use in the County under this scenario, 549,600 acre-feet of groundwater would have to be pumped -- a 26 percent increase over average conditions.

The 'severe' drought scenario assumes 1990 precipitation conditions, which result in an estimated 53 percent reduction in rainfall input to irrigation (see Table A.7 and A.7a), combined with a 57 percent overall reduction in surface supplies. This amount of surface water reduction was calculated from a set of conditions comparable to the 1991 drought had it not been for the unusual March rains; the County, at that time, was facing a situation with no YCFCWCD Cache Creek System water available, a 50 percent reduction in USBR contract supplies, and a 25 percent reduction in Sacramento River riparian and appropriative water rights. In such a severe drought case, a total of 760,100 acre-feet of groundwater would have to be pumped to sustain 1990 levels of agricultural activity -- a 74 percent increase in County-wide pumping over average supply conditions. The increased pumping load during these droughts has traditionally been driven by only the agricultural sector's need to make up for missing rain and surface water for cultivation. M&I pumping in drought years varies little from average year pumping, although M&I water use can often be reduced by 10-15 percent through special conservation programs with little harm to the urban economy.

The actual effects of droughts on groundwater pumping levels involve several competing and complicating agricultural trends, particularly for prolonged multi-year droughts. First, it has been common for irrigation requirements to get reduced from average year levels as farmers shift to more drought tolerant crops, or choose not to cultivate. Second, farmers experiencing a shortage of surface water at the beginning of a drought may not have the necessary pumping equipment installed to allow them to switch to groundwater supplies. But, as droughts recur, and one year events evolve into multi-year events, new wells are installed to maintain crop production. These two opposing trends make prediction of actual increases in agricultural groundwater usage during droughts more difficult. Agricultural and water use statistics for Yolo County are not systematically collected to allow such estimates to be made reliably. Nevertheless, historical practices in Yolo County have shown that agricultural groundwater usage intensifies significantly during droughts.

3c: Impact of Water Transfers on Goundwater Use

A significant new phenomenon has emerged in California with possibly for more serious and complicated consequences for what happens to the groundwater resources in Yolo County. Water marketing and transfers are the most recent California drought response innovation to come into practice (initiated in 1991). Water transfers are very likely, in the near future, to also become an established mechanism for meeting supplemental water needs in average years. In this section, the role and impact of water transfers on groundwater resources in Yolo County are explored.

During the 1991 state-wide drought, water transfers out of Yolo County under the State Drought Emergency Water Bank program reached 140,000 acre-feet and in the 1992 Water Bank they were 39,000 acre-feet. (Lund, et al., 1992). The full impacts of these transfers on Yolo County are being investigated in a study conducted by the Agricultural Issues Center at the University of California, Davis. Transfers took a number of forms, each with different impacts on groundwater. Transfers of Sacramento River surface water that involved land fallowing had an impact on groundwater by reducing recharge from deep percolation of applied surface water. In cases where groundwater was used to replace transferred surface water to cultivate, stresses on the aquifer were increased by an amount equivalent to the lost surface water. In other cases groundwater was directly transferred out of the County, in amounts associated with either fallowing land or with extra pumping beyond levels associated with the normal agricultural activities on the respective lands. At this point in time, the significance of the impacts of the 1991 transfers on the County's groundwater resources has yet to be formally evaluated. No transfers of water within the County occurred.

So far, all water transfers have involved temporary emergency arrangements. However, real interest and legislative support is growing in California to have an open water market where buyers and sellers could implement more permanent transfers of water around the state. Thus, the County faces the new possibility of permanently losing some surface water supplies through these transfers, at least during dry years. Negative consequences for the groundwater balance in both average and drought years, and overdraft problems are likely to arise from any net loss of surface water. More intensive use of groundwater to make up for losses in surface supplies will result in greater stresses on the aquifer, and diminish the resilience in the County's water system for surviving droughts. In addition, opportunities for the conjunctive management of Yolo's water system could be restricted by the reduced availability of surface water supplies in the County.

The 1991 and 1992 water transfers were unplanned and unmanaged events, and as such, probably exacerbated the negative impacts of the drought on the groundwater resources in the County. However, transfers and water marketing arrangements, when planned and managed, need not only play a negative role. Transfers of water, both within and outside the County, offer positive opportunities, in the context of conjunctive use management, to secure the County's water future. These kinds of water transfer and their potential roles in Yolo County's water system will be explored and discussed in the following chapter on conjunctive use management.