journal of hydrology, 122 (1991) 93 117 93 elsevier ... · journal of hydrology, 122 (1991) 93 117...

25
Journal of Hydrology, 122 (1991) 93 117 93 Elsevier Science Publishers B.V., Amsterdam [2] LONG-TERM ANNUAL AND SEASONAL TRENDS IN SURFACE SALINITY OF SAN FRANCISCO BAY J.P. FOX 1, T.R. MONGAN 2 and WILLIAM J. MILLER 3 '2530 Etna St., Berkeley, CA 94704 (U.S.A.) 284 Marin Ave., Sausalito, CA 94965 (U.S.A.) 3P.O. Box 5995, Berkeley, CA 94705 (U.S.A.) (Received February 26, 1990; accepted after revision April 26, 1990) ABSTRACT Fox, J.P., Mongan, T.R. and Miller, W.J., 1991. Long-term annual and seasonal trends in surface salinity of San Francisco Bay. J. Hydrol., 122: 93-117. Trends in surface salinity at seven stations in San Francisco Bay were studied. We found that annual surface salinity over the period 1920-1986 at the ocean boundary had slightly increased and at the river boundary, had slightly decreased. None of these trends were statistically different from zero even though upstream water use has nearly doubled. However, statistically significant seasonal trends were found. In most areas, salinity had increased from February through June and decreased at other times. Salinity in the bay is affected primarily by freshwater inflow and oceanic conditions. Seasonal salinity trends at stations near the freshwater boundary are probably largely owing to the operation of upstream water projects, which have redistributed freshwater between the months. At stations near the ocean, seasonal salinity trends appear to be strongly influenced by offshore conditions, including a local rise in sea level and increased upwelling. INTRODUCTION San Francisco Bay, CA. (Fig. 1) is a 1240 km 2 shallow estuary on the Pacific coast of the United States (Conomos et al., 1985). Many of the bay's biological resources, including fish, shellfish and tidal marshes, are sensitive to changes in salinity at some stage in their life cycle (Radtke and Turner, 1967; Alderdice and Forrester, 1968; Mall, 1969; Alderdice and Velsen, 1971; Herrgesell et al., 1983; Pearcy and Ustin, 1984). The bay's major source of freshwater is the Sacramento-San Joaquin Delta and its tributary river system, draining the area enclosed within the dashed line on Fig. 1. Water consumptively used within or exported out of the Sa- cramento-San Joaquin drainage would otherwise flow into San Francisco Bay. Since some of the freshwater had been diverted for agricultural and urban use (California Department of Water Resources (CDWR), 1983, 1988), it was generally believed that the salinity of San Francisco Bay had increased. Freshwater from this river system (delta outflow) is mixed seaward and ocean water is mixed landward (Smith, 1987). Most of the variability in bay salinity is caused by changes in delta outflow (Peterson et al., 1989). However, 0022-1694/91/$03.50 © 1991 Elsevier Science Publishers B.V.

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Page 1: Journal of Hydrology, 122 (1991) 93 117 93 Elsevier ... · Journal of Hydrology, 122 (1991) 93 117 93 Elsevier Science Publishers B.V., Amsterdam [2] LONG-TERM ANNUAL AND SEASONAL

Journal of Hydrology, 122 (1991) 93 117 93 Elsevier Science Publishers B.V., Amsterdam

[2]

LONG-TERM ANNUAL AND SEASONAL TRENDS IN SURFACE SALINITY OF SAN FRANCISCO BAY

J.P. FOX 1, T.R. MONGAN 2 and WILLIAM J. MILLER 3

'2530 Etna St., Berkeley, CA 94704 (U.S.A.) 284 Marin Ave., Sausalito, CA 94965 (U.S.A.) 3P.O. Box 5995, Berkeley, CA 94705 (U.S.A.)

(Received February 26, 1990; accepted after revision April 26, 1990)

ABSTRACT

Fox, J.P., Mongan, T.R. and Miller, W.J., 1991. Long-term annual and seasonal trends in surface salinity of San Francisco Bay. J. Hydrol., 122: 93-117.

Trends in surface salinity at seven stations in San Francisco Bay were studied. We found that annual surface salinity over the period 1920-1986 at the ocean boundary had slightly increased and at the river boundary, had slightly decreased. None of these trends were statistically different from zero even though upstream water use has nearly doubled. However, statistically significant seasonal trends were found. In most areas, salinity had increased from February through June and decreased at other times. Salinity in the bay is affected primarily by freshwater inflow and oceanic conditions. Seasonal salinity trends at stations near the freshwater boundary are probably largely owing to the operation of upstream water projects, which have redistributed freshwater between the months. At stations near the ocean, seasonal salinity trends appear to be strongly influenced by offshore conditions, including a local rise in sea level and increased upwelling.

INTRODUCTION

San Francisco Bay, CA. (Fig. 1) is a 1240 km 2 shallow estuary on the Pacific coast of the United States (Conomos et al., 1985). Many of the bay's biological resources, including fish, shellfish and tidal marshes, are sensitive to changes in salinity at some stage in their life cycle (Radtke and Turner, 1967; Alderdice and Forrester, 1968; Mall, 1969; Alderdice and Velsen, 1971; Herrgesell et al., 1983; Pearcy and Ustin, 1984).

The bay's major source of freshwater is the Sacramento-San Joaquin Delta and its t r ibutary river system, draining the area enclosed within the dashed line on Fig. 1. Water consumptively used within or exported out of the Sa- cramento-San Joaquin drainage would otherwise flow into San Francisco Bay. Since some of the freshwater had been diverted for agricultural and urban use (California Department of Water Resources (CDWR), 1983, 1988), it was generally believed that the salinity of San Francisco Bay had increased.

Freshwater from this river system (delta outflow) is mixed seaward and ocean water is mixed landward (Smith, 1987). Most of the variability in bay salinity is caused by changes in delta outflow (Peterson et al., 1989). However,

0022-1694/91/$03.50 © 1991 Elsevier Science Publishers B.V.

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94

34

33

J.P. FOX ET AL.

124 123 122 121 120 119 118 117 116 115 " - - " T - - - - - - - I f I I i i i i i

42

41

40

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MAJOR CANALS AND AQUEDUCTS

- - - - DRAINAGE DIVIDE 33 O COASTAL SALINITY STATION "~. ,~ ~ ~ / O MAJOR COASTAL POPULATION CENTERS

I l I I I I 1 2 4 1 2 3 1 2 2 1 2 1 1 2 0 1 1 9 l l u 1 1 7 1 1 6 1 1 5

Fig. 1. Major features of California's water delivery system that influence freshwater flow to and salinity of San Francisco Bay.

most of the salt and water is carried into the bay from the ocean on two flood tides every 24.8h. Consequently, changes in either delta outflow or oceanic conditions (e.g. upwelling, longshore currents, sea level, etc.) can alter the bay's salinity.

In this paper, we analyze long-term surface salinity records from San Francisco Bay to quantify annual and seasonal trends. Our study was designed to answer three questions. (1) Have decreases in delta outflow caused the salinity of San Francisco Bay to increase? (2) Have changes in delta outflow altered the seasonal patterns of salinity? (3) Have changes in oceanic

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 95

conditions contributed to changes in bay salinity? The changes in delta outflow are reported elsewhere (Fox et al., 1990).

BAY SALINITY DATA

The seven stations selected to examine historic salinity trends in San Francisco Bay are located on Fig. 2. These stations were selected because their salinity records are longer and more complete than any other station. Descrip- tive statistics for each station are presented on Table 1.

Surface salinity, chlorinity, or electrical conductivity was measured ap- proximately daily at all stations in samples collected within the top 1 m of the water column. Sampling and analytical procedures varied from station to

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Fig. 2. Map of San Francisco Bay and the Sacramento-San Joaquin Delta showing the location of the seven Bay salinity s tat ions analyzed in this work.

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98 J.P. FOX ET AL.

station and also changed over the period of record at some stations (Table 1). Thus, our first step was to adjust the data to eliminate differences in units, sampling time, and averaging convention. Mean monthly salinity was then computed to eliminate tidal variability and random errors. The following sections describe the procedures used to generate the seven salinity data sets that were analyzed in this paper.

CD WR/USBR stations

Until about 1971, the California Department of Water Resources and its predecessor agencies measured chlorinity in grab samples from Antioch, Col- linsville, Port Chicago, Martinez and Point Orient. The samples were collected every 4 days within 2 h of higher high tide, because this is when the highest salinities occur (California Department of Public Works (CDPW), 1931). Chlorinity samples not taken within 2 h of higher high tide were eliminated to give a tidally uniform data set. From 1967 onward, the U.S. Bureau of Reclama- tion (USBR) continuously recorded electrical conductivity at these five stations and reported it as daily averages. Consequently, both 4-day chlorinity and continuous electrical conductivity are available at these five stations for the period 1967 through to about 1971, allowing conversion between the two data sets.

Monthly averages were computed from 4-day chlorinity and daily electrical conductivity values to minimize biases due to missing data and nonuniform sampling times. The monthly data were then converted to consistent units over the entire record by combining the overlapping monthly record for chlorinity and conductivity at four of the stations (Table 1) and finding the best-fit linear equation. The resulting regression equation (r 2 = 0.95, p < 0.01, n = 228) is:

Monthly average tidal-maximum C1 = 0.4519 × monthly average EC

Chlorinity (C1) is expressed in parts per million and electrical conductivity (EC) in gmhos cm 1 at 25 ° C. We converted conductivity to chlorinity because the majority of the data were reported as chlorinity. Statistical analyses at the CDWR/USBR stations were conducted using chlorinity data and converted to salinity for presentation using the relationships developed by the CDWR (Guivetchi, 1986).

NOS data

The National Ocean Service (NOS) has collected daily, almost random, grab samples at Alameda since 1939 and at Presidio since 1920. Samples were not collected at any particular tidal phase or time of the day. NOS computed salinity from water temperature and density using standard hydrographic tables (American Public Health Association (APHA) et al., 1985). We did not use the salinity computed by NOS because it contained errors. Instead, we obtained the raw data sheets and recomputed salinity from water temperature

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100 J.P. FOX ET AL.

and density using the Millero equations (Millero et al., 1976). These equations are comparable with the hydrographic tables and were used to facilitate computer analyses. We used notes made on the original data sheets to eliminate observations that were in error from factors such as poor calibration of hydrometers and thermometers. The daily salinity data were then used to compute monthly averages, which were used in all statistical analyses.

C O A S T A L D A T A

Since changes in costal conditions can significantly affect bay salinity, we also analyzed routinely measured coastal variables - - salinity, the upwelling index, and sea level - - to determine if they may have influenced bay salinity. Descriptive statistics for each coastal variable are reported in Table 2.

Salinity

Because bay salinity responds to changes in the salinity of coastal waters, we analyzed salinity at Pacific Grove about 160 km south of San Francisco Bay (Fig. 1). This is the only coastal salinity station close to the bay that is not influenced by freshwater from nearby rivers and that has a long, nearly continuous daily salinity record covering about the same period as the bay stations.

The Pacific Grove station was operated by the Hopkins Marine Station of Stanford University from 1919 until it was discontinued in 1975. Surface salinity samples were collected approximately daily from a beach on the north side of Point Cabrillo just north of Hopkins' main laboratory buildings. This location is exposed to the northwest swell as it sweeps past Point Pinos, so is representative of coastal conditions.

Salinity was determined by the standard hydrometric method until May 1955, by a Knudsen ti tration through July 1962, and from August 1962-1975 by induction salinometry. The smoothed data suggest that method changes have not affected long-term salinity trends at Pacific Grove. The daily data have been compiled by Scripps, and they also prepared the monthly summaries that were analyzed. The published monthly averages were modified for this work by Mantyla (A.W. Mantyla, personal communication, 1988) to eliminate daily samples inadvertently collected from local tide pools.

Coastal upwelling

Upwelling can increase bay salinity by transporting high salinity, deeper waters to the surface where they are carried into the bay on the tides. Intense upwelling occurs during the summer outside of San Francisco Bay (Bakun, 1973). Upwelling results from offshore transport of ocean water caused by wind stress on the ocean surface. The upwelling index estimates the average amount of water upwelled through the top 50-100m of ocean in m 3 s 1 per 100m of

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 101

coastline. Negative values indicate onshore transport or downwelling and positive values indicate offshore transport or upwelling.

We analyzed Bakun's monthly upwelling indices at the two points closest to San Francisco Bay (39°N 125°W and 36°N 125°W) for the entire record, January 1946-December 1987 (Bakun, 1973; Mason and Bakun, 1986 plus updates obtained from D. Mallicoate, NOAA Pacific Fisheries Environmental Group). We also analyzed weekly indices from 1967, when the weekly index was first published, through 1985 (Mason and Bakun, 1986). The data sources used to compute the monthly upwelling index over the period 1946-1987 changed, with increasingly detailed data used for later indices. The problem of data sources is particularly troublesome for the 1946-1963 period (Bakun, 1973).

Sea level

Sea level influences bay salinity because it in part determines the volume of ocean water entering the bay between the levels of high and low tide (i.e. tidal prism). If relative local sea level were to rise, for example, the volume of ocean water entering the bay would increase, causing average bay salinity to increase. Sea level has been measured daily at two of the bay salinity stations, Alameda and Presidio (Table 2). We analyzed the average monthly data reported by NOAA (Hicks et al., 1983 plus updates obtained from S. Lyles, NOS Tidal Datum Section) for the same period of record as the bay salinity data.

DATA ANALYSIS METHODS

Time series of the data described above were analyzed for trend. A time series is a set of ordered observations on a parameter such as salinity taken at different points in time. The trend in each time series was estimated using classical parametric techniques (Draper and Smith, 1981) and nonparametric methods that have become popular in the past decade (Hirsch et al., 1982; Hirsch and Slack, 1984; Gilbert, 1987). We emphasize the results from the nonparametric procedures because the data sets violate some of the assumptions required for validity of the parametric methods.

Smoothing

Each time series was smoothed to portray the essential features of the data. We used LOWESS, which stands for "LOcally WEighted regression Scatter plot Smoothing" (Cleveland, 1979; Chambers et al. 1983). The number of points in the shortest data set (Point Orient salinity, n = 332) was used in the weighting function to achieve uniform smoothing. We chose LOWESS because outliers do not distort the smoothing and it accommodates missing values. The version of LOWESS implemented in the Systat computer package was used (Wilkinson, 1986).

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102 J.P. FOX ET AL.

Nonparametric trend estimation

Trend was estimated and its statistical significance assessed using non- parametric procedures because the data sets contain outliers and are not normally distributed. In all cases, we tested the null hypothesis of no trend against the alternative of either an upward or downward trend using a two- tailed test. We selected a significance level (a) of 10% as commonly used by the U.S. Geological Survey (USGS) in similar work (Smith et al., 1987).

We estimated trend in both monthly and seasonal time series to determine, respectively, annual changes and changes in each individual month. Annual trends were estimated using the entire monthly time series described previously (Tables I and 2). Seasonal time series were formed by separating the monthly time series into twelve monthly data sets where successive values are one year apart (i.e. a January series, a February series etc.). The resulting seasonal series for each month were separately analyzed for trend. Since trend procedures were different for seasonal and monthly series, the analysis of each type of series is discussed separately. The seasonal series are discussed first because the analytical procedures are simpler.

Seasonal time series Successive values in these series are separated by one year, and seasonal

cycles are not present. Trend was quantified using Sen's slope estimator (Sen, 1968) as implemented in a Fortran program developed by Battelle Pacific Northwest Laboratory (Gilbert, 1987). In the Sen method, the data are ranked in the order in which they are collected over time, and the slopes of the straight line segments linking all possible pairs of time-ordered data points are computed. These slopes are ranked, and the Sen slope is the median value. This method is preferred to least-squares linear regression for our data because it is not biased by the major floods and droughts at the beginning and end of the data sets.

The statistical significance of trend in these series was tested using the Mann-Kendall test (Mann, 1945; Kendall, 1975) as implemented in a Fortran Program developed by Battelle (Gilbert, 1987). In this test, the data are time ranked as in the Sen method and assigned the value + 1, - 1, or 0 according to the sign of the calculated differences of all possible time-ranked pairs. The Mann Kendall S statistic is then computed as the number of positive differen- ces minus the number of negative differences. Since the distribution of S is nearly normal for the sample sizes evaluated (n > 40), we used the standard normal deviate Z (computed from S and the variance of S) referred to as the cumulative normal distribution to evaluate significance (Gilbert, 1987, p. 211).

Monthly time series These series comprise monthly data over the entire period of record (Table

1), and successive values are separated by one month. The monthly series have strong seasonal cycles and serial correlation. Consequently, we used modifica-

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 103

tions of the Sen slope estimator and Mann-Kendall test that account for the seasonal effects and serial correlation.

Trend was quantified using the seasonal Kendall slope estimator (Hirsch et al., 1982) as implemented in a Fortran program developed by the USGS. This is a generalization of Sen's slope estimator. The Sen slope is computed for all data pairs for the same month from the entire data set, the Sen slopes are ranked, and the median of the ranked values is the seasonal Kendall slope.

The statistical significance of trend in monthly time series was evaluated with the seasonal Kendall test (Hirsch et al., 1982) corrected for serial correla- tion (Hirsch and Slack, 1984) as implemented in a Fortran program developed by the USGS. The seasonal Kendall test is an adaptation of the Mann- Kendall test for trend, in which each month is treated as an independent variable. The Mann-Kendall S statistic and the variance of S are computed separately for each of the twelve months over all years and the grand test statistic S* is computed as the sum of individual monthly Mann Kendall S statistics. Serial correlation is accounted for by estimating the covariance between successive months within the same year and including it in the computation of the variance of S*. Since the distribution of S* is nearly normal for the sample sizes evaluated here, we used the standard normal deviate Z (computed from S* and the variance of S*) referred to as the cumulative normal distribution to evaluate significance (Hirsch et al., 1982).

Parametric trend estimation

Trend was also estimated in each monthly bay salinity data set using ordinary least-squares regression (Draper and Smith, 1981). This procedure was used to provide a familiar reference point because it is simple and widely understood. However, it is important to recognize that least-squares trends are not good estimates of the average long-term trend in the data sets because outliers (i.e. flood, droughts) can strongly bias the trend estimates.

Trend was estimated by fitting a straight line to the data with time as the independent variable. Month was included as a dummy variable to account for seasonal variability (Draper and Smith, 1981, p. 241; Bowerman and O'Connell, 1987). The statistical significance of trends calculated in this manner is not reported because the data violate the underlying assumptions of the parametric significance tests (Smith, 1986; Helsel and Hirsch, 1988).

Component analysis

The two main factors affecting bay salinity are delta outflow and oceanic conditions. In addition to evaluating the effects of delta outflow on bay salinity trends, we wished to study the influence of oceanic conditions. Because the influence of oceanic conditions is most pronounced at the Presidio at the ocean boundary (Fig. 2), we further analyzed Presidio salinity to separate salinity trends owing to changes in freshwater flow from those due to all other factors,

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104 J.P. FOX ET AL.

primarily changes in oceanic conditions. These analyses were performed for the period June 1946 through the end of the record. June 1946 was selected because the LOWESS smoothed curve of Presidio salinity indicated that salinity uniformly increased after that date.

The monthly salinity data were separated into two component time series, one the result of changes in delta outflow (Fox et al., 1990) and one the result of all other factors. The method of Meade and Emery (1971) was used to form the component series, except we used robust regression and nonparametric trend procedures instead of the least-squares regression techniques used by Meade and Emery (1971). The two component series were analyzed for annual and seasonal trends using the nonparametric trend procedures described previously.

The delta outflow component time series were computed from 12 linear regression equations between salinity and delta outflow, one for each month. First, we formed twelve monthly flow and salinity data sets (i.e. a January series, a February series etc.). Since the flow-salinity relationship at the Presidio is linear (Peterson et al., 1989), we fit twelve straight lines to the data, one for each month. The regression equations were computed using the simplest robust M-estimator, minimizing the mean absolute deviations (Press et al., 1988). This robust procedure is less sensitive to outliers and departures from ideal assumptions than is least squares.

(C} ANT OC!f

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Fig. 3. M o n t h l y su r face sa l in i ty da ta (~ ) . The jagged l ines plot the m o n t h l y ave rage sa l in i ty . The heavier , smoo thed l ine is the L OW E S S t rend curve. It shou ld be no ted t h a t d i f ferent sca les are used on the y-axes.

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 105

T h e e s t i m a t e d s a l i n i t y f r o m e a c h r e g r e s s i o n e q u a t i o n w a s u s e d as t h e d e l t a

o u t f l o w c o m p o n e n t o f s a l i n i t y . T h e t o t h e r ' c o m p o n e n t w a s c o m p u t e d by sub-

t r a c t i n g t h e f l o w c o m p o n e n t f r o m t h e o r i g i n a l m o n t h l y s a l i n i t y t i m e s e r i e s a n d

is t h e r e g r e s s i o n r e s i d u a l s f r o m t h e r o b u s t r e g r e s s i o n o f s a l i n i t y o n d e l t a

o u t f l o w .

BAY SALINITY TRENDS

M o n t h l y s u r f a c e s a l i n i t y d a t a a r e p l o t t e d in F ig . 3a g, s u p e r i m p o s e d o v e r

L O W E S S s m o o t h e d c u r v e s . T h e L O W E S S c u r v e s s u g g e s t t h a t s a l i n i t y h a s

d e c r e a s e d a t A n t i o c h , C o l l i n s v i l l e , a n d P o r t C h i c a g o n e a r t h e r i v e r m o u t h s a n d

a t P o i n t O r i e n t ( t h r o u g h 1957 on ly ) . T h e L O W E S S c u r v e s a l s o s u g g e s t t h a t

s a l i n i t y h a s i n c r e a s e d a t M a r t i n e z i n t h e N o r t h B a y a n d a t A l a m e d a a n d

P r e s i d i o n e a r t h e o c e a n .

T a b l e 3 c o m p a r e s t h e ~ t rend ' o r a n n u a l r a t e o f c h a n g e i n s a l i n i t y e s t i m a t e d

by t w o s e p a r a t e m e t h o d s : l e a s t - s q u a r e s a n d t h e s e a s o n a l K e n d a l l m e t h o d . I t

a l s o c o m p a r e s t h e p e r c e n t c h a n g e a t e a c h s t a t i o n s i n c e t h e b e g i n n i n g o f t h e

r e c o r d . T h e m a g n i t u d e o f t h e l e a s t - s q u a r e s a n d K e n d a l l e s t i m a t e s d i f f e r s d u e t o

TABLE 3

Trend analysis of San Francisco Bay monthly salinity data

Salinity station (Fig. 2)

Parametric analysis Nonparametric analysis

Least Total % Seasonal Total % Two- squares change ~ Kendall change ~ tailed trend 1 trend p-value :~ (ppm year 2) (ppm year 1)

Antioch - 32 83 + 0.l v 2 0.88 Collinsville - 35 74 0.5 - 6 0.65 Port Chicago - 21 11 43 - 23 0.25 Martinez + 15 + 5 + 10 + 4 0.84 Point Orient 4 1 30 - 3 0.58 Alameda + 23 + 4 + 11 + 2 0.70 Presidio + 12 ~ 3 + 12 + 3 0.19

~Computed as the slope of the straight line fit to the data using linear least-squares regression. A dummy month variable (Bowerman and O'Connell, 1987) was included to account for seasonal variability. The test statistic is not reported because the data violate underlying assumptions of the t-test (Helsel and Hirsch, 1988). -~The total percent change is computed from: 100 × (trend x N)/[mean _+ 1/2(trend x N)f where N = number of years in the record. In this equation, the plus ( + ) sign is used for a decreasing trend and the minus ( - ) sign for an increasing trend because the change is computed with respect to the beginning of the record. ~This is the attained significance level of a test of the null hypothesis of no trend against the alternate hypothesis of either an upward or downward trend. It is the probability (p) of exceeding the absolute value of the Z-statistic in a two-tailed test. When the p-value is less than the significance level of 0.10, the nonparametric seasonal Kendall trend is statistically significant. Thus. none of the salinity trends is statistically significant.

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106 J.P. FOX ET AL.

outl iers, which bias the least-squares estimates. These da ta sets conta in severe droughts (1931 1933, 1976-1977) and major floods (1982-1983). The Kendal l method is not biased by these out l iers because it is based on the median. Thus, it provides a more accura t e es t imate of the true, long-term average trend.

The t rends in Table 3 largely agree with those por t rayed by the LOWESS smoothed curves (Fig. 3) and fu r the r indicate tha t none of these t rends are s ta t is t ica l ly different from zero, pr imari ly because the changes are small and the var iance is large (Table 1). Pe te r son et al. (1989, fig. 22) repor ted tha t sal ini ty had increased at the Presidio and Alameda since 1940, but did not repor t s ta t is t ical significance. One of the reasons tha t annua l sal ini ty has not s ignif icant ly changed in the bay since 1920 is because annua l del ta outflow also has not s ignif icant ly changed since 1920 (Fox et al., 1990).

However , there have been s ta t is t ical ly significant seasonal changes in sal ini ty t h r oughou t the bay. Since the seasonal pa t te rns differ at the r iver and ocean boundar ies of the Bay, sal ini ty t rends in these two areas are discussed separate ly . The discussion of seasonal t rends focuses on Antioch, Collinsville, and Presidio because these s ta t ions have re la t ive ly complete sal ini ty records from 1920 th rough 1986.

River boundary

The ups t ream por t ion of the es tuary is s t rongly inf luenced by f reshwater flow from the S a c r a m e n t o - S a n Joaqu in dra inage owing to its proximi ty to the Sac ramen to San Joaqu in Del ta (Fig. 2). Ci rcula t ion and mixing processes in this area which influence sal ini ty have been reviewed by Conomos et al. (1985) and Smith (1987). F re shwa te r on the surface is mixed seaward and ocean water on the bot tom is mixed landward. Dur ing high win ter flows ( > 2000 m a s ~ ), bay water nea r the del ta is most ly f reshwater tha t is well mixed, and sal ini ty is near ly uniform t h r o u g h o u t the water column. As the r iver mouths are approached, all flows above a threshold value cor respond to f reshwater or a sal ini ty of about 0.1%o. Dur ing low summer flows ( < 500 m a s 1 ), bay wate r nea r the del ta is only par t ia l ly mixed, ver t ica l sal ini ty gradients are present , and the re la t ionship between flow and sal ini ty is approximate ly l inear (Fig. 4).

In this ups t ream por t ion of the estuary, small increases in surface sal ini ty (+ 0.2 + 0.3 ppm per mon th at Ant ioch and Collinsville) have occur red from Februa ry th rough J u n e and larger decreases ( - 3.0 to - 9.3 ppm per month at Ant ioch and Collinsville) have occur red from August to October over the period 1920 1986. All of these t rends are s ta t is t ica l ly s ignif icant (Fig. 5). The seasonal pa t t e rn at Por t Chicago is similar to Ant ioch and Collinsville, while Mar t inez is more similar to s ta t ions near the ocean boundary . However , the seasonal t rends at Por t Chicago and Mar t inez are not s ta t is t ical ly s ignif icant in most months (Fig. 5c,d).

These t rends are probably largely owing to the opera t ion of ups t ream water projects, which have redis t r ibuted del ta outflow between the months (Fox et al., 1990). Sal in i ty has decreased in the ups t ream por t ion of the es tuary dur ing

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 107

i

~( ', ~()E' :L:V :,: ' :

d ) M~ '~ i ' L [ :

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Fig. 4. The re lat ionship between surface sa l in i ty and freshwater flow through the S a c r a m e n t o San Joaquin Del ta (Delta outflow) at seven Bay stations. The Del ta outflow data are described and analyzed in Fox et al. (1990).

summer and fall because freshwater flow in these months has more than doubled over the period 1920-1986 (Fox et al., 1990). Figure 4a-4c show that sal ini ty is nearly l inearly related to flow during the low-flow, summer fall period (at flows < 500 m ~ s- 1 ). Thus, the increase in delta outf low from reservoir

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108 J.P. FOX ET AL

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3 ~ 3

o 3 o

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Fig. 5. M o n t h l y t rends in sur face s a l i n i t y computed us ing Sen 's slope es t imator . Shaded bars

ind ica te the t r end is s t a t i s t i c a l l y s ign i f i can t a t the 10% level in a two- ta i led Z test . U n s h a d e d bars ind ica te the t r end is no t s t a t i s t i c a l l y s igni f icant . I t should be no ted t h a t different sca les are used

for the y-axes on each plot.

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 109

releases has caused salinity to decline in Suisun Bay (Fig. 2) during this period approximately in proportion to flow.

Salinity has remained relatively unchanged in the upstream portion of the estuary in spring even though freshwater flow has decreased by 48 to 64% since 1920 (Fox et al., 1990). This occurs because spring delta outflow is almost always adequate (except during droughts) to keep at least the top meter or so of Suisun Bay (Fig. 2) fresh. Figure 4a,b show that at Antioch and Collinsville, salinity is constant at ~ 0.1-0.2%o during the high-flow, winter-spring period when delta outflow is greater than ~ 500m3s-~. Figure 4c shows that at Port Chicago, salinity is constant at about 1%o when delta outflow is greater than

2000 m :~ s 1 The exceptions (e.g. those points around 1000 m :~ s i with salinities of 2~%o)

occur following extended periods of high salinity, as at the ends of the 1931 and 1977 droughts. Others have reported that these anomalously high winter salinities are caused by a memory effect (CDPW, 1931, p. 95; Winkler, 1985; Peterson et al., 1989). Large winter and spring flows (the norm) delay the advance of saline water upstream the succeeding summer. However, unusually low winter flows, which have occurred during droughts, allow salinity to intrude further upstream earlier the following summer, requiring higher winter flows the next year to flush out the salinity.

Ocean boundary

In the seaward portion of the estuary, both delta outflow and coastal conditions can influence salinity trends. The variability in salinity, on the other hand, is largely controlled by fluctuations in outflow (Peterson et al., 1989). During high winter freshwater flows (> 2000 m:~s 1), water in this region is only partially mixed and salinity gradients are present. During low summer flows (< 500m3s 1), water in this region is mostly ocean water, and salinity is nearly uniform throughout the water column.

The seasonal salinity trends at these stations (Alameda, Presidio) differ from trends at the river boundary (Antioch, Collinsville) in both magnitude and timing (Fig. 5f, g). Salinity has increased from April~luly at a much faster rate at Presidio and Alameda (+ 1.3 + 6.3ppm per month) than at the riverine stations (+ 0.2 + 0.3ppm per month), possibly because of shifts in coastal conditions. These increases in salinity are statistically significant at the 10% level. Salinity has decreased more slowly at the ocean boundary at Presidio from September to January < 1 ppm per month) than at the river boundary ( - 3 to - 9 p p m per month). The September-January salinity decreases at the Presidio are not statistically significant while the decreases at the riverine stations are statistically significant (Fig. 5).

O C E A N F A C T O R S T H A T I N F L U E N C E B A Y S A L I N I T Y

Long-term trends in bay salinity are influenced by many factors. These include changes in freshwater flow, changes in bay bathymetry (e.g. filling and

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110 J.P. FOX ET AL.

dredging), shifts in offshore oceanic conditions, and wastewater discharges. Considerable attention has been focused on changes in freshwater flow (Conomos et al., 1985; Smith, 1987; Peterson et al., 1989) because upstream water resources have been extensively developed for in-basin consumptive use and export (CDWR, 1983, 1988). While the variability in salinity can be explained largely by variability in freshwater flow (Peterson et al., 1989), freshwater flow trends are not the only factors that influence salinity trends. Effects of changes in ocean conditions would be expected to be most evident in data collected at the Presidio at the ocean boundary of the bay. Consequently, we investigated the potential influence of oceanic factors on bay salinity trends using the Presidio salinity record for the period June 1946-December 1986. We focused on the post-1946 period because the first major upstream reservoir (Shasta; see CDWR, 1983) commenced to store water in 1944.

The LOWESS smoothed curve (Fig. 6a inset) indicates that salinity has steadily increased at the Presidio since 1920 except for a dip centered around 1946. The seasonal Kendall trend for the post-1946 period is not statistically significant (30ppm year ~; two-tailed p = 0.16). The component analysis indicates that the delta outflow component of salinity in the post-1946 period ( - 6 p p m year ~; two-tailed p = 0.55) decreased and annual delta outflow increased (0.8m3s ~ year :; two-tailedp = 0.76). Therefore, changes in delta outflow are probably not primarily responsible for the increasing trend in Presidio salinity, as suggested by Peterson et al. (1989).

The %ther' component of Presidio salinity has increased (33ppm year ~; two-tailed p < 0.01), suggesting factors other than delta outflow are primarily responsible for the post-1946 salinity rise at the Presidio. Our analyses,

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/ 2 4 ~ . . . . . . . . . . . . . . . . . .

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 1 l l

presented below, suggest that offshore oceanic conditions (e.g. longshore current strength, upwelling, sea level) could have contributed to the increase in Presidio salinity since 1946. We did three things to demonstrate this: (1) we tested the strength of the relationship between Presidio salinity and oceanic variables; (2) we tested oceanic variables to determine if statistically signifi- cant increases had occurred; (3) we estimated monthly trends in flow and other components of Presidio salinity and compared them with monthly trends in oceanic variables.

Regression analysis for Presidio salinity

First, we regressed Presidio salinity against four oceanic variables (coastal salinity at Pacific Grove, sea level at the Presidio, and upwelling at 36°N, 122°W and at 39°N, 125°W) using linear least-squares techniques. Seasonal variability was accounted for by including month as a dummy variable (Draper and Smith, 1981, p. 241). The resulting relationships were all statistically significant (f2 = 0.88-0.99; two-tailedp < 0.01). Although a statistically signifi- cant relationship between two variables does not prove cause and effect, it does support the hypothesis that Presidio salinity is influenced by coastal conditions.

Annual trends in coastal variables

Second, LOWESS trend curves (Fig. 6) were checked to see if changes had occurred in the coastal variables that would tend to increase Presidio salinity. The seasonal Kendall test (Hirsch and Slack, 1984) was used to test the statisti- cal significance of trend in each coastal variable.

Salinity of coastal waters A statistically significant increase in Pacific Grove salinity occurred over

the period 1919-1975 (2 ppm year ~; two-tailed p = 0.01). The LOWESS trend curves for Presidio and Pacific Grove (Fig. 6a) are strikingly similar. These two LOWESS curves show a dip in salinity centered around June 1946. Prior to June 1946 salinity was decreasing and after June 1946, salinity increased at both Presidio and Pacific Grove. Although the rate of change at the two sites differs by about a factor of 10, the common transition point and shape of the smoothed curves suggest that common oceanographic or climatic factors (Peterson et al., 1989., p. 422) affect salinity in both places. The different salinity increase rates at the two stations may be affected by the longer record at the Presidio (1920-1986) than at Pacific Grove (1919-1975) and differences in the intensity of offshore upwelling.

Some have attributed the transition at the Presidio to upstream water development because the first major upstream reservoir (Shasta) came on line in 1944. However, Pacific Grove salinity is not affected by freshwater flow from nearby rivers nor upstream reservoirs, and it displays the same transition in

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112 J.P. FOX ET AL.

1946 as Presidio does. Furthermore, the component analysis indicates that Presidio salinity has increased even when increases in delta outflow are acting to decrease Presidio salinity. Thus, nonriverine factors appear to be primarily responsible for the rising salinity trends at both Pacific Grove and the Presidio. Presidio salinity trends are more strongly affected by these nonriverine factors than Pacific Grove salinity trends.

The cause of the 1946 salinity transit ion and post-1946 salinity increase at both Pacific Grove and the Presidio is unknown. One explanation is that more high salinity water has been transported into the offshore region by a shift in the longshore current system. The oceanographic l i terature describes a shift in conditions along the California coast in the 1940s. Roden (1961, p. 105) noted a sharp discontinuity in magnitude and general character of salinity anomalies (large deviations from the average) in 1944 at Pacific Grove and other stations along the California coast. Reid (1960) noted a shift in the average northerly wind component (and thus upwelling) and ocean surface temperature along the California coast in the period 1950-1956 compared with 1920-1938. Analyses presented below support the hypothesis that upwelling has increased in offshore waters. Another contributing factor, as discussed below, is an increase in sea level at the Presido.

Sea level Statistically significant increases in local relative sea level (Fig. 6c) have

occurred at the Presidio over the period 1920-1986 (2.3mm year 1; two-tailed p < 0.01) and at Alameda over the period 1939-1986 (7.6 mm year 1; two-tailed p - 0.04). The Alameda rise is larger because land in the vicinity of Alameda has subsided at a rate of ~ 4 mm year 1 since 1939 while no change in local land elevation has occurred at the Presidio (Moffatt and Nichol, 1987). Others have also reported similar increases in sea level at the Presidio, Alameda, and elsewhere along the California coast (Hicks, 1978; Smith, 1980). Although some believe that the reported rise is due to local subsidence of land masses from development (Roden, 1989), others claim a rise when the data are adjusted for subsidence (Moffatt and Nichol, 1987). However, an increase in local relative sea level could cause Presidio and bay-wide salinity to increase because the volume of ocean water entering the bay on each tidal cycle would increase.

Increases in sea level, if due to factors other than land subsidence, could indicate a change in the relative strength of longshore currents. Sea level and geostrophic flow are strongly correlated with high sea level corresponding to a stronger than normal northward flow of the saltier undercurrent system (Huang 1972; Namias and Huang, 1972; Reid and Mantyla, 1976; Chelton et al., 1982). Thus, the trend of increasing sea level at the Presidio (Fig. 6a) could indicate an increased advection of high salinity waters into the region off the Golden Gate because of a change in the strength of the longshore currents. Huang (1972) documented this phenomenon for the southern California coast. This explanation is certainly consistent with the increase in salinity observed at both Presidio and Pacific Grove.

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S U R F A C E S A L I N I T Y T R E N D S OF SAN F R A N C I S C O BAY

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- 0 . 1

113

Fig. 7. Seasona l trends in Presidio sa l in i ty from 1946 to 1986 attr ibutable to De l ta outf low and to other factors. Shaded bars indicate the trend is s tat i s t ica l ly s ignif icant at the 10% level in a two-tai led Z test. Unshaded bars indicate the trend is not s tat i s t ica l ly significant.

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114 J P . FOX ET AI,.

Upwelling index Since salinity increases with depth along the California coast (Churgin and

Halminski, 1974), upwelling brings saltier bottom waters to the surface where they are carried into the bay on the tides. The LOWESS trend curves for the monthly upwelling index (Fig. 6b) suggest that monthly upwelling increased from 1946 when the record begins through about 1975, and then declined slightly. The rise at 39 ° N 125°W in the monthly index was statistically signifi- cant (0.70m3s 1 per 100m year 1; two-tailed p < 0.01). The rise in the other monthly index at 36°N 122°W and the weekly index were not statistically significant at the 10% level.

Seasonal trends

Third, we compared the seasonal pattern in delta outflow, coastal salinity, upwelling index, and sea level with seasonal patterns in the components of Presidio salinity attributable to delta outflow and ~other' factors.

The results of the seasonal pattern analysis are presented in Fig. 7, which shows that trends in delta outflow cannot account for most of the monthly salinity trends at the Presidio. During April, May and June, decreasing trends in delta outflow (Fig. 7b) have contributed to increasing trends in Presidio salinity (Fig. 7a) because these are the months when runoff is stored in upstream reservoirs or exported. Only the May increase in Presidio salinity is statistically significant. During July-November, increasing trends in delta outflow (Fig. 7b) have contributed to decreasing trends in Presidio salinity (Fig. 7a) because flow is augmented in these months by reservoir releases for salinity control (Fox et al., 1990). Only the July September decreases in Presidio salinity are statistically significant at the 10% level. In general, decreasing trends in delta outflow result in increasing trends in salinity and vice versa because delta outflow and Presidio salinity are inversely related in all months. However, this is not necessarily true (e.g. December) due to random errors in the data sets and missing values in the Presidio salinity record.

Trends in delta outflow account for only a part of the total salinity trends at the Presidio. Our analyses indicate that other factors, possibly oceanic, have contributed to increasing trends in Presidio salinity during all months (Fig. 7c). The trend in the residuals from a robust regression of salinity on flow (i.e. the ~other' component) are all positive. These increasing trends in Presidio salinity attributable to ~other' factors are statistically significant at the 10% level in November, February, and July September. Large, statistically signifi- cant increases have also occurred in most months in coastal variables. These increases would tend to increase Presidio salinity. The largest salinity increases at Presidio occur in the summer when coastal upwelling has also experienced statistically significant increasing trends (Fig. 7f). Although ev- aporation (E) and precipitation (P) over coastal oceans are known to affect surface salinity of coastal waters (Jacobs, 1951), they are not included on Fig. 7 because long-term data for the key variable (E - P) are not available.

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SURFACE SALINITY TRENDS OF SAN FRANCISCO BAY 115

CONCLUSIONS

Our pr incipal conclus ions are as follows. (1) Annua l surface sal ini ty over the period 1920-1986 has s l ight ly increased

at the ocean boundary and s l ight ly decreased at the r iver boundary . None of these t rends are s ta t i s t ica l ly different from zero.

(2) There have been s ta t i s t ica l ly s ignif icant seasonal changes in sal ini ty t h roughou t the bay. In most areas, sal ini ty has increased from F e b r u a r y - J u n e and decreased at o the r times. The decreases are caused by ups t ream reservoi r releases of f reshwater to control sal inity.

(3) Presidio sal ini ty t rends since 1946 seem to be pr imar i ly affected by t rends in coastal condit ions, r a the r than t rends in f reshwater flow to the bay.

ACKNOWLEDGMENTS

We are gra teful for ass is tance obta ined from the many agencies who have collected, compiled, and generous ly provided the da ta we analyzed. Bay sal ini ty da ta were obta ined from Cheryl Baughman of the U.S. Bureau of Rec lamat ion and Randy Brown, Shei la Green, K a m y a r Guivetchi , and Ed Winkle r of the Cal i fornia Depa r tmen t of Wate r Resources. The or iginal da ta sheets for the Presidio and Alameda sal ini ty measurement s and sea level da ta were obta ined from Steve Lyles, Na t iona l Ocean Service, Tidal Datum Section. Upwell ing da ta were obta ined from Donna Mal l icoa te of NOAA Pacific Fisher ies Env i ronmen ta l Group. Coastal sa l in i ty da ta were obta ined from Sari lee Anderson and Arnold Manty la of Scripps Ins t i tu t ion of Oceanography and from Nelson Ross of NOAA Data and In format ion Service Section. These individuals also provided helpful comments tha t were incorpora ted into this work. The t rend analyses were performed using F o r t r an programs developed by the U.S. Geological Survey and the Bat te l le Pacific Nor thwes t Lab o ra to ry and modified for this work by Don Kendall , Univers i ty of California, Los Angeles, to run on an IBM-compat ible personal computer . Sa l in i ty data reduc t ion was performed by Pe te r Russell, Russell Resources, San Rafael, CA using dBASE III (Developer 's Release), graphs were prepared by Alison Br i t ton using Sigmaplot, and addi t ional s ta t is t ical analyses were performed using Sys ta t Vers ion 3.0.

REFERENCES

Alderdice, D.F. and Forrester, C.R., 1968. Some effects of salinity and temperature on early development and survival of the English sole (Parophrys vetulus). J. Fish. Res. B. Can., 25(3): 495 521.

Alderdice, D.F. and Velsen, F.P.J., 1971. Some effects of salinity and temperature on early develop- ment of Pacific herring (Clupea pallasi). J. Fish. Res. B. Can., 28(10): 1545 1562.

American Public Health Association (APHA), American Water Works Association and Water Pollution Control Federation, 1985. Standard Methods for the Examination of Water and Wastewater. 16th edn.

Bakun, A., 1973. Coastal upwelling indices, West Coast of North America, 1946--71. National

Page 24: Journal of Hydrology, 122 (1991) 93 117 93 Elsevier ... · Journal of Hydrology, 122 (1991) 93 117 93 Elsevier Science Publishers B.V., Amsterdam [2] LONG-TERM ANNUAL AND SEASONAL

116 ,I.P. FOX ET AI,

Oceanic and Atmospheric Administration, U.S. Department of Commerce, NOAA Tech. Rep. NMFS SSRF-671.

Bowerman, B.L. and O'Connell, R.T., 1987. Time Series Forecasting. Duxbury Press, Boston, MA. California Department of Public Works, 1931. Variat ion and Control of Salinity in Sacramento

San Joaquin Delta and Upper San Francisco Bay. CDPW Bull. No. 27. California Department of Water Resources, 1983. The California Water Plan. Projected Use and

Available Water Supplies to 2010. Bull. 16(~83, Sacramento, CA. 268 pp. California Department of Water Resources, 1988. California Water: Looking to the Future. Statisti-

cal Appendix. CDWR Bull. 16(~87, Sacramento, CA. Chambers, J.M., Cleveland, W.S. Kleiner, B. and Tukey, P.A., 1983. Graphical Methods for Data

Analysis. Wadsworth Internat ional Group, Belmont, CA and Duxbury Press, Boston, MA. Cheltom D.B.. Pernal, B.A. and McGowan, J.A., 1982. Large-scale In te rannual Physical and

Biological Interact ion in the California Current. J. Mar. Res., 40(4): 1095 1125. Churgin, J. and S.J. Halminski, 1974. Temperature, Salinity, Oxygen, and Phosphate in Waters off

United States. Vol. III. Eastern North Pacific. National Oceanographic Data Center. Washington, DC.

Cleveland, W.S., 1979. Robust Locally Weighted Regression and Smoothing Scatterplots. J. Am. Stat. Assoc., 74: 829836.

Conomos, T.J.. Smith, R.E., and Gartner, J.W., 1985. Environmental sett ing of San Francisco Bay. Hydrobiologia, 129:1 12.

Draper, N.R. and Smith, H., 1981. Applied Regression Analysis. John Wiley, New York, 2nd edn. Fox, J.P., Mongan, T.R., and Miller, W.J., 1990. Trends in freshwater inflow to San Francisco Bay

from the S a c r a m e n t ~ S a n Joaquin Delta. Water Resourc. Bull., 26(1): 101 116. Gilbert, R.O., 1987. Stat ist ical Methods for Environmental Pollution Monitoring. Van Nostrand

Reinhold Co., New York. Guivetchi, K., 1986. Salinity Conversion Equations. Calif. Dept. Water Resourc. Memo. Rep.,

Sacramento, CA. HelseL D.R. and Hirsch, R.M., 1988. Applicability of the t-test for detecting trends in water quality

variables. Water Resourc. Bull., 24(1): 201 204. Herrgesell, P.L., Schaffter, R.G., and Larsen, C.J., 1983. Effects of freshwater outflow on San

Francisco Bay biological resources. Calif. Dept. Fish & Game, Sacramento, CA, Rep. DO/SFB/ BIO-4ATR/83-7, 86 pp.

Hicks, S.D., 1978. An average geopotential sea level series for the United States. J. Geophys. Res., 83(C3): 1377 1379.

Hicks, S.D., Debaugh, Jr., H.A., and Hickman, Jr., L.E., 1983. Sea level var iat ions for the United States 1855 1980. U.S. Dept. Commerce, Nat. Ocean Serv., Rockville, MD.

Hirsch, R.M. and Slack, J.R., 1984. A nonparametr ic trend test for seasonal data with serial dependence. Water Resourc. Res., 20(6): 727 732.

Hirsch, R.M., Slack, J.R., and Smith, R.A., 1982. Techniques of trend analysis for monthly water quality data. Water Resourc. Res., 18(1): 107 121.

Huang, J.C.K., 1972. Recent decadal var ia t ion in the California current system. J. Phys. Oceanogr., 2:382 390.

Jacobs, W.C., 1951. The energy exchange between sea and atmosphere and some of its consequen- ces. Scripps Inst. Oceanogr. Bull., 6(2): 27- 122.

Kendall, M.G., 1975. Rank Correlation Methods. Charles Griffin, London, 4th edn. Mall, R.E., 1969. So i l -wa te~sa l t relationships of waterfowl food plants in the Suisun Marsh of

California. Calif. Dept. Fish & Game, Sacramento, CA, Wildl. Bull. No. 1. Mann, H.B., 1945. Non-Parametric test against trend. Econometrica, 13: 245~259. Mason, J.E. and Bakun, A., 1986. Upwelling index update, U.S. West Coast, 33N~8N latitude. Nat.

Oceanic Atmos. Admin., Nat. Mar. Fish. Serv., NOAA Memo. NMFS, NOAA Tech. Memo. NOAA-TM-NMFS-SWFC-67.

Meade, R.H. and Emery, K.O., 1971. Sea level as affected by river runoff, eastern United States. Science, 173: 425-428.

Miller, R.G., 1986. Beyond ANOVA. Wiley. New York.

Page 25: Journal of Hydrology, 122 (1991) 93 117 93 Elsevier ... · Journal of Hydrology, 122 (1991) 93 117 93 Elsevier Science Publishers B.V., Amsterdam [2] LONG-TERM ANNUAL AND SEASONAL

SURFACE SAI,INITY TRENDS OF SAN FRANCISCO BAY 117

Millero, F.J., Gonzalez, A. and Ward, G.K., 1976. The density of seawater solutions at one atmosphere as a function of temperature and salinity. J. Mar. Res., 34(1): 61 93.

Moffatt and Nichol, Engineers, 1987. Future sea level rise: predictions and implications for San Francisco Bay. Report Prepared for San Francisco Bay Conserv. Dev. Commission, San Francisco, CA.

Namias, J. and Huang, J.C.K., 1972. Sea level at southern California: A decadal fluctuation. Science, 177:351 353.

Pearcy, R.W. and Ustin, S.L. 1984. Effects of salinity on growth and photosynthesis of three California tidal marsh species. Oecologia, 62:68 73.

Peterson, D.H., Cayan, D.R., Festa, J.F., Nichols, F.H., Walters, R.A., Slack, J.V., Hager, S.E. and Schemel L.E., 1989. Climate variabil i ty in an estuary: effects of riverflow on San Francisco Bay. In: D. H. Peterson (Editor), Aspects of Climate Variabil i ty in the Pacific and the Western Americas. Geophys. Union Geophys. Monogr. 55, pp. 419-442,

Press, W.H, Flannery, B.P., Teukolsky, S.A. and Vetterling, W.T., 1988. Numerical Recipes. The Art of Scientific Computing. Cambridge University Press, Cambridge.

Radtke, L.D. and Turner, J.L., 1967. High concentrat ions of total dissolved solids block spawning migration of striped bass, Roccus saxatilis, in the San Joaquin River, California. Trans, Am. Fish. Soc., 96(4): 40~ 407.

Rei& Jr., J.L., 1960. Oceanography of the nor theas tern Pacific Ocean during the last ten years. Calif. Coop. Oceanic Fish. Invest. Rep. Vol. VII, pp. 77 90.

Reid, J.L. and Mantyla, A.W., 1976. The Effect of the geostrophic flow upon coastal sea elevations in the nor thern North Pacific Ocean. J. Geophys. Res., 81(18): 3100 3110.

Roden, G.I.. 1961. On nonseasonal temperature and salinity var iat ions along the west coast of the United States and Canada. Calif. Coop. Oceanic Fish. Invest , Rep. VIII: 95 119.

Roden, G.I.. 1989, Analysis and interpretat ion of long-term climatic variabil i ty along the west coast of North America. In: D.H. Peterson (Editor), Aspects of Climate Variabil i ty in the Pacific and the Western Americas. Am. Geophys. Union Geophys. Monogr. 55, pp. 93 111,

Sere P.K., 1968. Estimates of the regression coefficient based on Kendall 's Tau. J. Am. Stat. Assoc., 63(324):1379 1389.

Smith, L.H_ 1987. A review of circulation and mixing studies of San Francisco Bay, California. U.S. Geol. Surv. Circ. 1015.

Smith, R.A.. 1980. Golden Gate tidal measurements: 1854 1978. J. Waterway, Port, Coastal Ocean Div., Pro. Am. Soc. Cir. Eng. 106(WW3): 407 410.

Smith, R.A., Alexander, R.B., and Wolman, M.G., 1987. Water-quality trends in the nat ion 's rivers. Science, 235:1607 1615.

Wilkinsom L., 1986. SYSTAT: The System for Statistics. SYSTAT, Evanston, IL. Winkler, E.D., 1985. A statist ical approach to salinity modeling in the western Sacramento San

Joaquin Delta and Suisun Bay. Calif. Dept. Water Resourc. Tech. Inf. Rec. 1463 CD-04.