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Chapter 1 Purpose and Need TESTIMONY OF STEPHEN GRINNELL, P.E., YUNG-HSIN SUN, Ph.D., AND STUART ROBERTSON, P.E. YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR YUBA RIVER PREPARED FOR YUBA COUNTY WATER AGENCY PREPARED BY BOOKMAN-EDMONSTON ENGINEERING, INC. Unpublished Work © November 2000 TABLE OF CONTENTS INTRODUCTION...............................................................................................................................................1 SACRAMENTO VALLEY INDEX AND SAN JOAQUIN RIVER INDEX .................................................1 NEED FOR YUBA RIVER INDEX ..................................................................................................................2 DISTRIBUTION OF YUBA RIVER ANNUAL UNIMPAIRED FLOWS...........................................................................3 FUNCTIONS AND PURPOSES OF EXISTING FACILITIES..........................................................................................4 YUBA RIVER INDEX........................................................................................................................................6 INDEX DESIGN ...................................................................................................................................................6 INDEX DEFINITION .............................................................................................................................................7 WATER YEAR CLASSIFICATIONS OF YUBA RIVER ..............................................................................................8 COMPARISON OF WATER YEAR CLASSIFICATIONS...........................................................................8 YUBA RIVER INDEX AND SACRAMENTO VALLEY INDEX....................................................................................8 SWRCB DRAFT DECISION WATER YEAR CLASSIFICATIONS............................................................................10 CONCLUSIONS ...............................................................................................................................................11 REFERENCES..................................................................................................................................................12 APPENDIX A: DEFINITION OF YUBA RIVER INDEX APPENDIX B: YEARLY COMPARISON OF YRI AND SVI YEAR TYPES LIST OF TABLES 1 Selective Statistics of Annual Unimpaired Flows of Sacramento Valley and Yuba River Basin.................................................................................................................................3 2 Year List for Various Year Classifications of the Yuba River (1921-1994) .......................8 3 Comparison of Water Year Types Defined by YRI and SVI (1921-1994) .........................9 4 Year Type Comparison During Extensive Drought Periods......................................................9 LIST OF FIGURES 1 Distributions of Annual Unimpaired Flows ........................................................................5 Bookman-Edmonston Engineering, Inc. i Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR YUBA RIVER INTRODUCTION Existing water quality and fishery flow standards in California vary with the natural availability of water, and water availability is generally characterized by water year classifications. The water year classification is used as a proxy to classify each year according to water availability. A water year classification should be able to forecast the water availability reasonably well and should be relatively simple. New water year classification systems for the Sacramento Valley and the San Joaquin River Basin were included in the State Water Resources Control Board’s (SWRCB) 1995 Water Quality Control Plan (WQCP) for the San Francisco Bay/Sacramento-San Joaquin Delta Estuary. The Draft Decision for the Lower Yuba River Hearing (Draft Decision), issued by the SWRCB in February 1999, specifies the instream flow requirements and temperature requirements in the Lower Yuba River under normal and dry year conditions. Under the Draft Decision, a dry year would be a year in which the annual Yuba River unimpaired runoff near Smartville is less than 50 percent of the 50-year average (2,337 thousand acre- feet [TAF], DWR Bulletin 120), and all other years would be “normal” water years. These two water year classifications are too simplified to recognize the substantial variation in yearly water availability in the Yuba River Basin, and are not consistent with the Sacramento Valley Index (SVI) and San Joaquin River Index (SJRI) in the WQCP. This report describes the year classification system that we developed, based on the principles established by the development of the SVI and SJRI, to more closely represent the unique hydrological conditions in the Yuba River Basin. SACRAMENTO VALLEY INDEX AND SAN JOAQUIN RIVER INDEX The SVI is the successor of the Sacramento River Index (SRI) used in the SWRCB Decision 1485. The SJRI is a new index used to represent the water conditions in the San Joaquin Valley. By establishing water year classifications for wet, above normal, below normal, dry, and critical years, water quality standards and fishery flow requirements can be established based on the water availability of the corresponding year types. Consequently, a greater ability to provide more water for water quality and fishery purposes can be facilitated in above normal years to compensate for reduced flow in drier years. This will allow a better balance of instream flow and quality requirements and other competitive uses of water, such as agricultural and M&I uses. Bookman-Edmonston Engineering, Inc. 1 Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR THE YUBA RIVER The definitions of the SVI and SJRI were developed by the Year Type Classification Sub- Workgroup (Workgroup) during the proceedings that led to the 1995 WQCP. Both SVI and SJRI are calculated from three components: (a) the unimpaired flow from April to July, (b) the unimpaired flow from October to March, and (c) the index of the previous year. The unimpaired flows from April to July are generally considered snowmelt water; the unimpaired flows from October to March are from seasonal and storm runoff. The use of the previous year’s index accounts for the carryover storage in the system that may be used during the following year. However, the rollover of the previous year’s index is subject to a cap because the carryover effect of wet years must be limited to account for reservoir flood control operations and physical limits on available storage. The unimpaired flow used in the SVI is the sum of the flows at the following locations: the Sacramento River above Bend Bridge, near Red Bluff; the Feather River, total inflow to Oroville Reservoir; Yuba River at Smartville; and the American River, total inflow to Folsom Reservoir. SVI uses a 40-30-30 weighting for the three categories of flows (the unimpaired flow from April to July, the unimpaired flow from October to March, and the index of the previous year), and a cap of 10 million acre-feet (MAF) for the previous year’s index to account for flood releases and to limit the sequential impact of wet years. For the SJRI, the unimpaired flow is the sum of the flows at the following locations: the Stanislaus River, total flow to New Melones Reservoir; the Tuolumne River, total inflow to Don Pedro Reservoir; the Merced River, total inflow to Exchequer Reservoir; and the San Joaquin River, total inflow to Millerton Lake. The weighting is 60-20-20, and the cap of the previous year’s index is 4.5 MAF. The different weighting proportions reflect the local hydrologic characteristics and the year-to-year operation of reservoirs in the upper San Joaquin River. By using index thresholds, the five water year types defined by SVI and SJRI, wet, above normal, below normal, dry, and critical, roughly correspond to 30, 20, 20, 15, and 15 percent of the theoretical occurrence of water year types. NEED FOR YUBA RIVER INDEX The Yuba River is a tributary of the Sacramento River. The unimpaired flow of the Yuba River at Smartville is one component in the SVI computation. Although the SVI is adequate for a proxy of water availability in the Sacramento Valley when the Delta is the location of concern, the SVI does not adequately represent the water availability of the Yuba River Basin. The uniqueness of water availability in the Yuba River Basin can be explained from two aspects: the distribution of annual unimpaired flows and the functions and purposes of existing facilities. Bookman-Edmonston Engineering, Inc. 2 Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR THE YUBA RIVER DISTRIBUTION OF YUBA RIVER ANNUAL UNIMPAIRED FLOWS Table 1 shows the selective statistics of annual unimpaired flows of the Yuba River above Smartville and the Sacramento Valley (sum of the unimpaired flows in the American River at Folsom Reservoir, the Feather River at Oroville Reservoir, the Sacramento River at Bend Bridge, and the Yuba River at Smartville). These statistics, based on the unimpaired flows in water years 1921 to 1994, can be used to quantitatively specify the shape of the distribution of annual unimpaired flows. The average Yuba River unimpaired flow is about 13 percent of the average Sacramento Valley unimpaired flow. The coefficients of variation for the Yuba River unimpaired flows and the Sacramento Valley unimpaired flows are similar in magnitude, indicating these flows vary similarly from their respective
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