APPENDIX A: Water Use And Supplies, Data and Calculations Municipal, Industrial and Domestic (M&I) Water Use   Population statistics and per capita water use rates were used to estimate Yolo County 1990 M&I water demands. Table A.1 gives the 1990 population data and future M&I projections for Yolo County. Two different annual water use rates for the County were selected: a value of 0.273 acre-feet/person for the city of Davis and a value of 0.316 acre-feet/person for the rest of the County (adjusted projection from Scott, et al., 1975). The lower level for Davis results from Davis's greater orientation towards water conservation. No attempt has been made here to forecast future changes in these use rates. In this study, West Sacramento has been excluded from the County's M&I demand because its water supplies are met separately with water from the Sacramento River.   Yolo County M&I annual water use levels for 1990 and 2010, as reported in Table A.2, are 37,500 and 53,700 acre-feet, respectively. The 1990 monthly M&I water use is listed in Table A.3. These numbers were estimated using the monthly average percentages of annual water use for Davis applied to the County-wide annual M&I total estimate.   Agricultural Water Use Estimating agricultural water use in Yolo is hampered by the lack of accurate and current land use data on the spatial distribution of irrigated crops. The most comprehensive summaries are provided by the Yolo County Agriculture Department in their biennial crop reports. However, these surveys are intended mainly for economic purposes and do not give precise details of irrigation practices. Uncertainties in determining irrigation water use are also inherent in the fact that large amounts of irrigation water come from unmetered supplies such as private groundwater wells, or private surface water withdrawals. Consequently many assumptions about irrigation practices had to be made for this study. Agricultural crop water requirements were estimated using the most recently available crop report (Yolo County Agriculture Department, 1989). Average year crop evapotranspiration (ETc) requirements, on a monthly basis, were determined for the local climate and cropping schedules. Irrigation efficiencies typical for farming practices in Yolo County were chosen. The different crop acreages under irrigation in Yolo County in 1989 are given in Table A.4. Crops listed in the first column come directly from the 1989 Yolo County Agricultural Crop Report. Those crops which were irrigated have been grouped into categories based on similar crop evapotranspiration requirements and irrigation practices. ETc values listed in Table A.4 and their monthly percentages listed in Table A.5 have been calculated from more detailed local CIMIS (California Irrigation Management Information System) data (Miyao, 1991) or estimated from regional data (State of California, Department of Water Resources, 1975) or from local experts.   By combining crop acres, annual ETc and overall irrigation efficiencies, the total annual on-farm water use given in Table A.6 was calculated. The last column in Table A.6 is the final 1989 crop water requirement adjusted for the County-wide reuse rate. Under average climatic conditions, rainfall is assumed to supply all water requirements for the months of November, December, January and February, and a portion of those for October and March as indicated in Table A.7. Effectively, this rainfall contribution to irrigated crop production, on average, amounts to about 0.27 feet of water per acre of irrigated land. In drought years, the precipitation contribution is reduced, resulting in a larger net irrigation water demand compared to normal years. The net on-farm irrigation water demand for Yolo county, based on maintaining 1989 crop production levels, has been estimated for three climatic conditions in Table A.7 -- normal year, 'moderate' drought, and 'severe' drought conditions. A 'moderate' drought represents a 25 percent reduction in the precipitation contribution, while a 'severe' drought reflects precipitation conditions such as those experienced in 1990 when a total of 12.51 inches fell. Monthly rainfall for 1990 is reported in Table A.7a. Using the estimates of the effective rainfall contribution to crop production for 1990 given in column 8 of Table A.7, a 'severe' drought results in a 53 percent reduction in the normal year precipitation contribution.   Estimation of Average Groundwater Pumping Table A.8 shows the total water use for Yolo County, combining M&I and agricultural demands. All M&I uses are supplied only by groundwater. Irrigation uses both surface and groundwater, but most of the surface water sources and all the groundwater supply for agriculture are not measured. In Table A.8 groundwater pumping for irrigated agriculture was estimated at 43 percent of the net irrigated water use total. If 100 percent of all contracted/permitted surface water (583,000 acre-feet/year, see Table 2) were used, groundwater would have to provide a theoretical minimum of 37 percent of total on-farm agricultural water use. A level of 43 percent reflects a surface water use rate, at the farm point, of 90 percent of all permitted/contracted quantities.   The estimates by (Scott, et al. ,1975) of actual and projected groundwater use in Yolo County are the most detailed and currently available estimates. In Scott's 1990 projections, groundwater accounts for 40 percent of agricultural water supplies. Adjusting his 1990 projections for new surface water supplies (19,000 acre-feet/year of Tehama-Colusa Canal water) and for higher actual irrigated crop acreages (302,600 acres over Scott's projected 253,200 acres) indicates that groundwater would have to supply 48 percent of agricultural water demands. This discussion points out the uncertainty involved in determining groundwater pumping. An accurate assessment would involve obtaining the electric consumption data and well characteristics for all irrigation wells in the County. If the range from 37 percent to 48 percent can be considered as reasonable for the percentage of agricultural demands supplied by groundwater, then the average current level of groundwater pumping in Yolo County is between 380,500 and 482,400 acre-feet/year. This study uses the mid-point value of this range.   Estimation of Drought Year Groundwater Pumping Two scenarios were developed to evaluate the possible impact of drought on groundwater pumping levels. Under a 'moderate' drought, a 25 percent reduction in surface water supplies from permitted/contracted amounts was assumed. Under the 'severe' drought scenario, reductions in surface water supplies at levels that were occurring or were being programmed for in 1991 have been assumed, i.e. no YCFCWCD supplies, 50 percent reduction in USBR contracted supplies, and 25 percent reductions in Sacramento River riparian and appropriative rights. While the USBR and Sacramento River projected reductions for 1991 did not fully materialize because of the unusually heavy March rains, out-of-County transfers of well over 100,000 acre-feet of mostly surface water from Yolo County effectively resulted in substantial surface water use reductions for the County.   Table A.9 shows the amount of groundwater and surface water that would be required to meet 1990 water demands under the two drought scenarios. The annual groundwater contribution to normal year water use increases from 45 to 56 percent in 'moderate' drought years and from 45 to 75 percent in 'severe' drought years. These increases translate into incremental volumes of 113,500 acre-ft and 324,000 acre-feet of groundwater pumping respectively. previous section | top of page | next section
		Preface     Title Page     Table of Contents 1. Introduction   2. Water Use    3.Groundwater Resources 4. Conjunctive Use     5. Planning     6. Conclusions/Recommendations List of Figures    List of Tables   References Appendix B     Appendix D