TESTIMONY OF STEPHEN GRINNELL, P.E., YUNG-HSIN SUN, Ph.D., AND STUART ROBERTSON, P.E.

YUBA RIVER INDEX:

WATER YEAR CLASSIFICATIONS FOR

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Unpublished Work © November 2000 TABLE OF CONTENTS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 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 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 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 Index (SRI) used in the SWRCB Decision 1485. The SJRI is a new index used to represent the water conditions in the . 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 , total inflow to Oroville Reservoir; Yuba River at Smartville; and the , 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 , total flow to New Melones Reservoir; the , total inflow to Don Pedro Reservoir; the , 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.

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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 mean in a relative scale. However, Sacramento Valley unimpaired flows have a higher coefficient of skew, implying that in a relative scale, the extremely high flows of Sacramento Valley unimpaired flow deviate more from the mean. The higher coefficient of excess suggests the distribution of Sacramento Valley unimpaired flows has a sharper peak than that of the Yuba River unimpaired flows.

Table 1 Selective Statistics of Annual Unimpaired Flows of Sacramento Valley and Yuba River Basin Mean 1 Standard Coefficient Coefficient Coefficient Watershed (1,000 AF) Deviation2 of Variation3 of Skew4 of Excess5 Sacramento Valley6 17,099 7,568.5 0.442 0.591 -0.538 Yuba River Basin 2,245 1,083.7 0.483 0.378 -0.800 1 = ∑ x i x i n , where n is the sample size, and xi’s are unimpaired flows. 2 2 = ∑ − 2 − s, the square root of variance. s i (x i x) (n 1) . 3 = cv, a measurement of variation. c v s x . 4 = ⋅∑ − 3 − ⋅ − ⋅ 3 cs; a measurement of skewness. c s n i (x i x) (n 1) (n 2) s . A positive coefficient suggests a distribution skewed to the right with the mean greater than the median that is greater than the mode; a negative coefficient suggests a distribution skewed to the left with the mean less than the median that is less than the mode. 5 ε ε = [∑ − 4 ⋅ 4 ]− , a measurement of peakedness. i (x i x) n s 3 . A positive coefficient suggests a leptokurtic distribution with a relative greater concentration of probability near the mean than the normal distribution; a negative coefficient suggests a platykurtic distribution with a relative smaller concentration of probability near the mean than the normal. 6 Consists of the Sacramento River above Bend Bridge, Feather River above Oroville Reservoir, Yuba River above Smartville, and American River above Folsom Reservoir.

The aforementioned statistical inferences can be visualized in Figure 1, which illustrates the distributions of the Sacramento Valley annual unimpaired flows and Yuba River annual unimpaired flows in nine equal intervals of flow magnitude. Compared with the distribution of Sacramento Valley annual unimpaired flows, Yuba River annual unimpaired flows appear to be more evenly distributed over a wide range of flow intervals with less extremely high flow conditions. On the other hand, the occurrence of low flow

Bookman-Edmonston Engineering, Inc. 3 Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR THE YUBA RIVER conditions in the historic Yuba River unimpaired flows appear to be more frequent than for the Sacramento Valley unimpaired flows. The Sacramento Valley unimpaired flows are aggregated values whose components originate from a wide range of climatic and precipitation characteristics. Yuba River unimpaired flows contribute only 13 percent of the Sacramento Valley unimpaired flows. Thus, the characteristics of the Yuba Basin unimpaired flows are only a minor part of the aggregated flow used to compute the SVI and thus exert only a small influence on the aggregated Sacramento Valley flow and the SVI.

The diversity of characteristics in various Central Valley subbasins was discussed in the Workgroup when the SJRI was being developed. For example, it was recognized that the proposed SJRI does not appear to reasonably predict water availability in the Tuolumne River Basin; however, it is considered adequate if the Delta is the location of use. Similarly, while the SVI may be a reasonable index for the Sacramento Valley when determining water quality standards at the Delta, it is not adequate or appropriate for local purposes in the Yuba River Basin.

FUNCTIONS AND PURPOSES OF EXISTING FACILITIES

All of the major storage facilities along the Sacramento, Feather, and American Rivers are on the main streams of the rivers, allowing more flexibility in flow regulation. The Yuba River Basin’s major storage facility, New Bullards Bar Reservoir, is located on the . Diversions from the and Oregon Creek provide additional inflow into New Bullards Bar Reservoir. However, because New Bullards Bar Reservoir is not located along the Lower Yuba River, a large portion of the runoff from the South and Middle Yuba Rivers, Dry Creek, and Deer Creek cannot be fully regulated, reducing the overall water availability of the basin.

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Figure 1 Distributions of Annual Unimpaired Flows

Distribution of Annual Sacramento Valley Unimpaired Flow (American, Feather, Sacramento, and Yuba Rivers) (1921-1994) 36,000-40,500 1

31,500-36,000 3

27,000-31,500 4

22,500-27,000 12

18,000-22,500 8

13,500-18,000 15

9,000-13,500 20 Unimpaired Flow (TAF) 4,500-9,000 11

0-4,500 0 Number of Years

Distribution of Annual Yuba River Unimpaired Flow (1921-1994)

4800-5400 1

4200-4800 1

3600-4200 6

3000-3600 12

2400-3000 10

1800-2400 15

1200-1800 14 Unimpaired Flow (TAF) Flow Unimpaired 600-1200 14

0-600 1

Number of Years

The ratio of the total storage to average annual unimpaired flow of the Yuba River Basin is about 0.56, compared to 0.82, 1.10, and 0.64 of the Sacramento River at , the Feather River at , and the American River at , respectively. In contrast to , Shasta, Oroville, and Folsom Dams are located on the mainstream of Sacramento River, Feather River, and American River, providing substantially better regulation of the unimpaired runoffs. The lower storage-runoff ratio makes the Yuba River Basin more vulnerable during droughts and, together with the lack of any major mainstream storage, requires the system to be operated more on a year-to- year basis.

The Yuba River system is operated mainly for local use, unlike Shasta, Oroville, and Folsom Reservoirs, which are operated to provide a water supply to other areas via the

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Central Valley Project and the State Water Project (the Projects). When these reservoirs are operated for the Projects, the SVI is an appropriate standard for water year classifications, since the location of use is south of the Delta. However, this principle does not apply to the operation of the Yuba River system, where local uses are the principal demands.

YUBA RIVER INDEX

INDEX DESIGN

The design of the Yuba River Index (YRI) follows the principles of the SVI and SJRI. The SVI and SJRI were designed through Workgroup discussions that incorporated opinions from all parties. No rigorous methodology for index design has been reported; nevertheless, we determined the approaches and objectives of the design by studying the reports of findings and meeting minutes and then used these approaches and objectives to design the YRI.

The YRI design is based on the unimpaired flow data for the period from 1921 to 1994, the current study period of the unimpaired flow analyses conducted by the Department of Water Resources. The index should be a proxy to classify the year according to water availability, should be able to forecast water availability reasonably well, and should be relatively simple. However, water availability is based on variables that change according to the level of development (e.g., demands and storage), as well as other variables that are not affected by the level of development (e.g., unimpaired flow). Therefore, the resulting index should be compared to the historical system operations (including deliveries, instream flow, and end-of-year deliveries) for verification.

The index should consist of three components: the current year’s April through July unimpaired flow, the current year’s October through May unimpaired flow, and the previous year’s index (with a cap that accounts for flood control operations and physical limitation of storage so that the influence of wet years will not be overstated). Similar to the determination of proper proportions for the SVI and SJRI in the Workgroup, the proportion of each component is obtained through a trial-and-error procedure. During an iterative procedure, several objectives are simultaneously targeted:

• The resulting index should reflect the appropriate importance of snowmelt in the system operation.

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• The resulting index (when fitted into a log-normal distribution1) should have a mean that is close to its medium. That is, the index mean will be more representative for a normal year and thus, above normal and below normal categories can be properly defined.

• The resulting index should compare favorably to historical operations in terms of water availability.

The results of this iterative procedure suggest that the percentage of the current year’s April-July runoff used in the index is the most sensitive in terms of achieving the above objectives; the second most sensitive is the percentage of the current year’s October-March runoff. The previous year’s index appears to be the least sensitive. This is consistent with the Workgroup’s findings for the SVI and SJRI.

For YRI, Water years would be classified into the five categories as in the SVI and SJRI: wet, above normal, below normal, dry, and critical. Consistent with the SVI and SJRI, YRI should fit with a log-normal distribution and the thresholds for classifications should be based on 15, 15, 20, 20, and 30 percent of the theoretical occurrence from the log-normal distribution for critical, dry, below normal, above normal, and wet years, respectively. The more detailed classifications for the low flow years are needed because the competing uses of the limited water resources require greater precision in system operation. The theoretical thresholds can be adjusted if needed to avoid excessive dry and critical years or to accommodate certain preferred year classifications suggested by historical operations.

INDEX DEFINITION

The YRI is defined by three components: the current year’s April-July Yuba River unimpaired runoff, the current year’s October-March Yuba River unimpaired runoff, and the previous year’s index in a 50-30-20 proportion. The previous year’s index is subject to a 1,400 TAF cap. The classification thresholds are 630, 790, 990, and 1,230 TAF. The detailed definition of the YRI is provided in Appendix A.

The resulting year classifications are generally acceptable, based on the historic operational records. The classifications were then compared with the results of a Yuba Basin HEC-5 operation model simulation, in which the system is stressed to provide the demand for full development and high fishery flow standards. The comparison was also favorable, based

1 A commonly used distribution to fit streamflow data, assuming the logarithm-transformed streamflow data conform to a normal distribution. The Workgroup provided no reason for using a log-normal distribution, but the major benefits of using a log-normal distribution are to avoid below- zero streamflow predictions and the log-normal distribution generally fits the right (positive) skewness of natural runoffs.

Bookman-Edmonston Engineering, Inc. 7 Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR THE YUBA RIVER on the end-of-year storage in New Bullards Bar Reservoir and imposed shortages on delivery and fishery flow.

The index reflects the fact that water availability in the Yuba River Basin relies on snowmelt more than water availability in the Sacramento Valley, but not as much as water availability in the San Joaquin Valley. The thresholds are rounded to the nearest 10 TAF without further modifications because all the thresholds appear to have a relative natural breakpoint (i.e., no change in year classifications with minor changes in the thresholds).

WATER YEAR CLASSIFICATIONS OF YUBA RIVER

Based on the above-defined YRI, the year classifications of the Yuba River from 1921 to 1994 are summarized in Table 2. Of the 74 years, 25 years are classified as wet years, 15 as above normal years, 15 as below normal years, 8 as dry years, and 11 as critical years, corresponding to historical occurrence frequencies of 34, 20, 20, 11, and 15 percent, respectively. The historical occurrence frequencies are close to the theoretical values from the assumed log-normal distribution. The actual index ranges from 2,087 TAF to 1,246 TAF for wet years, 1,227 TAF to 994 TAF for above normal years, 949 TAF to 792 TAF for below normal years, 741 TAF to 639 TAF for dry years, and 627 TAF to 252 TAF for critical years.

Table 2 Year List for Various Year Classifications of the Yuba River (1921-1994)

Total Number Year Type Years (in sequence of wetness) of Years 1983, 1982, 1952, 1938, 1974, 1958, 1942, 1969, 1956, Wet 1965, 1922, 1927, 1967, 1921, 1941, 1963, 1951, 1943, 25 1984, 1971, 1986, 1980, 1953, 1970, 1975 1993, 1936, 1928, 1978, 1940, 1973, 1923, 1946, 1935, Above Normal 15 1957, 1950, 1937, 1948, 1954, 1945 1932, 1979, 1989, 1972, 1925, 1962, 1964, 1968, 1944, Below Normal 15 1966, 1949, 1930, 1926, 1960, 1985 Dry 1959, 1955, 1947, 1981, 1990, 1933, 1929, 1939 8 1991, 1961, 1987, 1994, 1976, 1934, 1992, 1988, 1924, Critical 11 1931, 1977

COMPARISON OF WATER YEAR CLASSIFICATIONS

YUBA RIVER INDEX AND SACRAMENTO VALLEY INDEX

A detailed comparison of historical year-by-year classifications by SVI and YRI is provided in Appendix B. Table 3 shows the summary by year types. For the distribution of year types in the Yuba River Basin and Sacramento Valley, similar numbers of wet, below normal, and critical years are present. However, the Sacramento Valley has more dry

Bookman-Edmonston Engineering, Inc. 8 Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR THE YUBA RIVER years, but fewer above normal years. This is partly explained by the difference in the distributions of annual unimpaired flows in these two regions.

Table 3 indicates that the Yuba River Basin has slightly more wet, above normal, and below normal years, than the Sacramento Valley. However, the situation changes in the drier years. The Yuba River dry years correspond to Sacramento Valley year types ranging from below normal to critical years, and a Yuba River critical year can be a dry year in Sacramento Valley.

Table 3 Comparison of Water Year Types Defined by YRI and SVI (1921-1994) Count of Water Year Yuba River Index Year Type Sacramento Valley Index Above Below Year Type Wet Normal Normal Dry Critical Total Wet 23 23 Above Normal 2 7 9 Below Normal 851 14 Dry 10 4 2 16 Critical 3912 Total 25 15 15 8 11 74

Water availability in the Yuba River Basin is not consistently higher than that in the Sacramento Valley; however, when it is, the Yuba River Basin can be an important water source. During the period from 1921 to 1994, two extensive droughts have occurred: 1929 to 1934 and 1987 to 1992. Table 5 shows the YRI and SVI year types during these two drought periods. For the 1929-1934 drought, three years have a YRI year type wetter than its SVI year type; for the 1987-1992 drought, there are two such years.

Table 4 Year Type Comparison During Extensive Drought Periods (a) Drought Period: 1929 to 1934

Year 1929 1930 1931 1932 1933 1934 SVI Year Type Critical Dry Critical Dry Critical Critical Below YRI Year Type Dry Dry Critical Dry Critical Normal

(b) Drought Period: 1987 to 1992

Year 1987 1988 1989 1990 1991 1992 SVI Year Type Dry Critical Dry Critical Critical Critical Below YRI Year Type Critical Critical Dry Critical Critical Normal

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SWRCB DRAFT DECISION WATER YEAR CLASSIFICATIONS

The Draft Decision contains only two categories of water year classifications: normal and dry. A dry year, according to the Draft Decision, is a year in which the annual Yuba River unimpaired runoff near Smartville is less than 50 percent of the 50-year average (2,337 TAF, DWR Bulletin 120). Table 5 lists the historical dry years using the definition in the Draft Decision with the percentage of annual unimpaired flow. From 1921 to 1928, there are only 14 dry years, or 17.9 percent of the 78 years. The list of dry years includes many that are generally considered critical years (using the SVI or YRI) with very limited amounts of natural runoffs. Water year 1991 is a critical year using both the SVI and YRI; however, it would not even be considered to be a dry year under the Draft Decision because its annual unimpaired runoff is 50.4 percent of the 50-year average.

The annual unimpaired runoff, which is used in the Draft Decision, is not a good indicator for water availability for agricultural, municipal and industrial, environmental, and other beneficial uses. Water availability depends upon the amount of annual runoff and, importantly, the seasonal distribution of the annual unimpaired flow. Low water availability can be caused by one or more of several factors: (1) the runoffs in the winter cannot be stored because of the flood control requirements, (2) the snowmelt is insufficient to refill the reserved flood control space in the spring, (3) the reservoir is driven low because of the dry condition in previous year(s). The simple water year classifications based only on annual unimpaired runoffs used in the Draft Decision do not recognize any of these factors.

The definition of dry years in the Draft Decision is identical to that in the 1965 YCWA/CDFG Stream Flow Release Agreement. (However, they were referred to as “critical dry years” in the Agreement.) The fact that these simple water year classifications fail to recognize important factors affecting water availability has not been as large of a concern under this agreement as it would with the 150 percent increase2 in instream flow requirements for the Lower Yuba River that is proposed in the Draft Decision. Even with the relative low flow requirements in the 1965 agreement, the dry year reductions vary in three levels (15, 20, and 30 percent), depending on the annual unimpaired runoff, to account for the variation of dry conditions. Thus, the agreement implicitly acknowledges three subcategories of dry years, providing different relief in instream flow requirements. On the other hand, under the Draft Decision, there would be only one level of dry year reductions in May, allowing only 56,886 acre-feet of reduction (13 percent). It should be

2 Under the Draft Decision, the instream flow requirements for a normal year has an annual water budget of 431,861 acre-feet below the Daguerre Point Diversion Dam in the Lower Yuba River. It was 173,722 acre-feet under the 1965 YCWA/CDFG Stream Flow Release Agreement.

Bookman-Edmonston Engineering, Inc. 10 Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR THE YUBA RIVER noted that the reduced instream flow requirement under the Draft Decision has an annual water budget higher than the annual unimpaired flow of Yuba River in 1977.

Table 5 Annual Unimpaired Runoff as Percentage of 50-Year Average of Historical Dry Years as Defined in SWRCB Draft Decision and Corresponding SVI and YRI Year Type Classification

Annual Yuba River Unimpaired Runoff near Smartville Sacramento Valley Index Yuba River Index Year (Percentage of 50-Year Average) Year Type Year Type 1924 25.8 Critical Critical 1929 43.3 Critical Dry 1931 27.4 Critical Critical 1933 46.2 Critical Dry 1934 42.3 Critical Critical 1939 38.8 Dry Dry 1961 48.2 Dry Critical 1976 34.4 Critical Critical 1977 15.8* Critical Critical 1981 47.0 Dry Dry 1987 37.4 Dry Critical 1988 39.3 Critical Critical 1992 39.0 Critical Critical 1994 37.4 Critical Critical *Historical Low

CONCLUSIONS

A water year classification system that follows the design principles of the SVI and SJRI has been developed for the Yuba River. The resulting YRI is defined by three components: the current year’s April-July Yuba River unimpaired runoff, the current year’s October- March Yuba River unimpaired runoff, and the previous year’s index in a 50-30-20 proportion. The previous year’s index is subject to a 1,400 TAF cap. The classification thresholds are 630, 790, 990, and 1,230 TAF. For the study period of 1921 to 1994, 25 years are classified as wet years, 15 as above normal years, 15 as below normal years, 8 as dry years, and 11 as critical years, corresponding to historic occurrence frequencies of 34, 20, 20, 11, and 15 percent, respectively.

The YRI developed in this study is needed to characterize the unique hydrological conditions in the Yuba River Basin index. The water year classifications in the Draft Decision are too simplified to recognize important factors affecting yearly water availability. The SVI may be a reasonable index for the Sacramento Valley when determining water quality standards at the Delta, but it is inadequate or inappropriate for local purposes in the Yuba River Basin. Considering historical operations and model

Bookman-Edmonston Engineering, Inc. 11 Yuba County Water Agency YUBA RIVER INDEX: WATER YEAR CLASSIFICATIONS FOR THE YUBA RIVER simulations, the year classifications based on the YRI are more representative than the SVI classifications of water availability in the Yuba River Basin.

REFERENCES

State Water Resources Control Board. Water Quality Control Plan for the San Francisco Bay/Sacramento-San Joaquin Delta Estuary (Draft). May 1995. State Water Resources Control Board. Decision in the Matter of Fishery Resources and Water Right Issues of the Lower Yuba River (Draft). April 1996. MacRostie, Wayne. Recommendation of a Water Year Classification System, Memorandum to R. R. Reynolds and R. B. Robie. Department of Water Resources. November 13, 1976. Water Year Classification Sub-Workgroup. Summary of Workshop Findings, in Summary of Workshop Activities, Bay-Delta Estuary Proceedings. November 1989. Water Year Classification Sub-Workgroup. Minutes of Meetings, in Summary of Workshop Activities, Bay-Delta Estuary Proceedings. November 1989. Water Year Classification Sub-Workgroup. San Joaquin River Index—Summary of Workshop Findings, Bay-Delta Estuary Proceedings. September 1991. Water Year Classification Sub-Workgroup. San Joaquin River Index—Minutes of Meetings, Bay-Delta Estuary Proceedings. September 1991.

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APPENDIX A: DEFINITION OF YUBA RIVER INDEX

The water year hydrologic classification for the Yuba River shall be determined by the following equation:

INDEX = 0.5⋅⋅⋅X + 0.3⋅⋅⋅Y + 0.2⋅⋅⋅Z

Where X = Current year’s April-July Yuba River unimpaired runoff

Y = Current year’s October-March Yuba River unimpaired runoff

Z = Previous year’s index.12

The Yuba River unimpaired runoff for the current water YEAR TYPE2 year (October 1 of the preceding calendar year through All Years for All Objectives September 30 of the current calendar year), as published in California Department of Water Resources Bulletin 120, is a forecast of the unimpaired flow of the Yuba River at Wet Smartville. Preliminary determinations of a year’s 1,230 classification shall be made in February, March, and April, with the final determination in May. These preliminary Above determinations shall be based on hydrologic conditions to Normal date plus forecasts of future runoff, assuming normal 990 precipitation for the remainder of the water year.

Below Classification Index Normal Thousand Acre-Feet (TAF) 790 Wet...... Equal to or greater than 1,230 Dry Above Normal ...... Greater than 990 and less than 1,230 Below Normal...... Equal to or less than 990 and greater than 790 630 Dry ...... Equal to or less than 790 and greater than 630 Critical Critical...... Equal to or less than 630 Index Thousand Acre-Feet

1 A cap of 1,400 TAF is imposed on the previous year’s index to account for required flood control reservoir releases during wet years. 2 The year type for the preceding water year will remain in effect until the initial forecast of unimpaired runoff for the current year is available.

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APPENDIX B: YEARLY COMPARISON OF YRI AND SVI YEAR TYPES

Water Year YRI Year Type SVI Year Type 1921 Wet Wet 1922 Wet Wet 1923 Above Normal Below Normal 1924 Critical Critical 1925 Below Normal Dry 1926 Below Normal Dry 1927 Wet Wet 1928 Above Normal Above Normal 1929 Dry Critical 1930 Below Normal Dry 1931 Critical Critical 1932 Below Normal Dry 1933 Dry Critical 1934 Critical Critical 1935 Above Normal Below Normal 1936 Above Normal Below Normal 1937 Above Normal Below Normal 1938 Wet Wet 1939 Dry Dry 1940 Above Normal Above Normal 1941 Wet Wet 1942 Wet Wet 1943 Wet Wet 1944 Below Normal Dry 1945 Above Normal Below Normal 1946 Above Normal Below Normal 1947 Dry Dry 1948 Above Normal Below Normal 1949 Below Normal Dry 1950 Above Normal Below Normal 1951 Wet Above Normal 1952 Wet Wet 1953 Wet Wet 1954 Above Normal Above Normal 1955 Dry Dry 1956 Wet Wet 1957 Above Normal Above Normal 1958 Wet Wet 1959 Dry Below Normal 1960 Below Normal Dry 1961 Critical Dry 1962 Below Normal Below Normal 1963 Wet Wet 1964 Below Normal Dry 1965 Wet Wet 1966 Below Normal Below Normal 1967 Wet Wet 1968 Below Normal Below Normal 1969 Wet Wet 1970 Wet Wet 1971 Wet Wet 1972 Below Normal Below Normal 1973 Above Normal Above Normal 1974 Wet Wet 1975 Wet Wet 1976 Critical Critical 1977 Critical Critical 1978 Above Normal Above Normal 1979 Below Normal Below Normal 1980 Wet Above Normal 1981 Dry Dry 1982 Wet Wet 1983 Wet Wet 1984 Wet Wet 1985 Below Normal Dry 1986 Wet Wet 1987 Critical Dry 1988 Critical Critical 1989 Below Normal Dry 1990 Dry Critical 1991 Critical Critical 1992 Critical Critical 1993 Above Normal Above Normal 1994 Critical Critical

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