Front cover: Panoche Creek looking upstream from the Interstate-5 bridge. The photograph was 

taken at 10:30 a.m. on February 3, 1998 at a streamflow of 9,530 cubic feet per second. The peak 

streamflow of record for Panoche Creek (9,940 cubic feet per second) occurred two hours earlier. The 

suspended sediment sample collected at the time of this photograph had a concentration of 177,000 

milligrams per liter for an instantaneous sediment loading rate of over 4.5 million tons per day. The 

instantaneous total selenium loading rate at the time of this photograph was over seven tons per day. Note 

the trees and branches, standing waves, and the old gage house in the photograph. The current U.S. 

Geological Survey gage house is just downstream of the bridge. (Photograph by Willie B. Kinsey, U.S. 

Geological Survey)
Selenium and Sediment Loads in Storm Runoff

in Panoche Creek, California, February 1998

By Charles R. Kratzer1, Dina K. Saleh2, and Celia Zamora1

Prepared in cooperation with the

U.S. BUREAU OF RECLAMATION

Sacramento, California

2003

Water-Resources Investigations Report 02-4286

U.S. GEOLOGICAL SURVEY

1

U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, California 95819-6129

2

California State University Sacramento Foundation, 6000 J Street, Sacramento, California 95819-6129 6438-12
For additional information write to:

U.S. Geological Survey

Water Resources

Placer Hall, Suite 2012,

6000 J Street

Sacramento, California 95819-6129

http://ca.water.usgs.gov

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
THIS PAGE INTENTIONALLY LEFT BLANK
Contents iii

CONTENTS

Introduction .......................................................................................................................................................... 2

Methods................................................................................................................................................................ 2

Sampling Design ......................................................................................................................................... 2

Sample Collection ....................................................................................................................................... 2

Sample Processing and Laboratory Methods.............................................................................................. 4

Quality Control Samples ............................................................................................................................. 4

Regression Methods .................................................................................................................................... 4

Storm Hydrology.................................................................................................................................................. 6

Historical Hydrology................................................................................................................................... 6

Hydrology for Study Period (Water Years 1998 to 2000) .......................................................................... 7

Water Quality ....................................................................................................................................................... 8

Time Series Data During Storms ................................................................................................................ 8

Relations Between Constituents.................................................................................................................. 18

Loads .................................................................................................................................................................... 28

Suspended Sediment ................................................................................................................................... 28

Selenium...................................................................................................................................................... 31

Load Prediction Methods ............................................................................................................................ 36

Summary and Conclusions................................................................................................................................... 38

References Cited .................................................................................................................................................. 38
Figures iv

FIGURES

Figure 1. Map showing location of Panoche Creek Basin, California ............................................................. 3

Figure 2. Graph showing daily precipitation and streamflow in Panoche Creek, California, 

for water years 1959 to 1970 ............................................................................................................. 7

Figure 3. Graph showing daily precipitation and streamflow in Panoche Creek, California, 

for water years 1998 to 2000 ............................................................................................................ 8

Figure 4. Graph showing hourly precipitation, 15-minute streamflow, and sample collection times 

in Panoche Creek, California, February 1998.................................................................................... 9

Figure 5. Graphs showing streamflow and specific conductance in Panoche Creek, California, 

for February 1998 storms .................................................................................................................. 12

Figure 6. Graphs showing streamflow and suspended sediment in Panoche Creek, California, 

for February 1998 storms .................................................................................................................. 14

Figure 7. Graphs showing streamflow and dissolved and total selenium in Panoche Creek, 

California, for February 1998 storms ................................................................................................ 16

Figure 8. Graphs showing streamflow, suspended selenium (calculated), suspended sediment, 

and selenium attached to suspended sediment (calculated) in Panoche Creek, California, 

for February 1998 storms .................................................................................................................. 19

Figure 9. Graphs showing correlations between water quality constituents and streamflow 

in Panoche Creek, California, for February 1998 storm data ........................................................... 21

Figure 10. Graphs showing streamflow and suspended sediment loading rate in Panoche Creek, 

California, for February 1998 storms ................................................................................................ 29

Figure 11. Graph showing regressions for suspended sediment loading rate as a function of streamflow 

in Panoche Creek, California ............................................................................................................ 31

Figure 12. Graphs showing streamflow and selenium loading rate in Panoche Creek, California, 

for February 1998 storms .................................................................................................................. 34

Figure 13. Graph showing regressions for dissolved selenium loading rate as a function of 

streamflow in Panoche Creek, California. ......................................................................................... 36

Figure 14. Graph showing regressions for total selenium loading rate as a function of 

streamflow in Panoche Creek, California. ......................................................................................... 37
Tables v

TABLES

Table 1. Quality control samples for Panoche Creek at I-5, California, February 1998 ................................. 5

Table 2. Water quality data for storm sampling in Panoche Creek at I-5, California, February 1998 ............ 10

Table 3. Summary of regressions between water quality constituents ............................................................ 27

Table 4. Summary of regressions for calculating loading rates of suspended sediment, dissolved 

selenium, and total selenium.............................................................................................................. 32

Table 5. Loads of suspended sediment, dissolved selenium, and total selenium calculated by 

three different methods ...................................................................................................................... 33
Conversion Factors, Vertical Datum, and Symbols vi

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

CONVERSION FACTORS 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 

°C=(

 

°F-32)/1.8.

VERTICAL DATUM

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both the

United States and Canada, formerly called Sea Level Datum of 1929. 

Water Quality Units

Concentrations of constituents in water samples are given in either milligrams per liter (mg/L) or

micrograms per liter (μg/L). Milligrams per liter is equivalent to “parts per million” and micrograms

per liter is equivalent to “parts per billion.”

Water Year

In U.S. Geological Survey papers dealing with surface water supply, the 12-month period October 1 to

September 30. The water year is designated by the calendar year in which it ends and which includes

9 of the 12 months. For example, the year ending September 30, 1999, is called the “1999 water year.”

In this paper, unless otherwise defined, “years” refer to water years.

ABBREVIATIONS

I-5, Interstate 5

log, logarithm

MDL, method detection limit

NWQL, National Water Quality Laboratory

RPD, relative percent difference

Multiply By To obtain

foot (ft) 0.3048 meter

cubic foot per second (ft3/s) 0.02832 cubic meter per second

inch (in.) 2.54 centimeter

pound (lb) 0.4536 kilogram

pound per day (lb/d) 0.4536 kilogram per day

mile (mi) 1.609 kilometer

square mile (mi2) 2.590 square kilometer

ton, short (2,000 lb) 0.9072 megagram 
Abstract 1

Selenium and Sediment Loads in Storm Runoff in

Panoche Creek, California, February 1998

By Charles R. Kratzer, Dina K. Saleh, and Celia Zamora

ABSTRACT

Five to nine samples were collected per 

storm throughout the hydrograph of four storms in 

February 1998 from Panoche Creek at Interstate 5, 

California. The rainfall total of 10.40 inches for 

the month was greater than any other month 

during 1957 to 2000, and peak streamflows on 

February 3 and 7 exceeded the previous peak 

streamflow of record. Concentrations of suspended 

sediment, which were highly variable during the 

storms, ranged from 32,900 to 251,000 mg/L 

(milligram per liter) with a median of 

126,000 mg/L. Dissolved selenium concentrations 

in the storm samples ranged from 16 to 60 μg/L 

(microgram per liter), with a median of 30 μg/L. 

These concentrations were considerably higher 

during the first storm than during subsequent 

storms. Total selenium concentrations in the storm 

samples ranged from 57 to 320 μg/L, with a 

median of 115 μg/L. Total selenium in four 

replicate and four rerun samples was highly 

variable (relative percent differences ranged from 

0 to 57 percent), probably due to the extremely 

high concentrations of suspended sediment in the 

samples and possible interferences from other 

compounds. The calculated concentration of 

selenium attached to suspended sediment was less 

variable than suspended sediment or total 

selenium concentrations during storm runoff; 

concentrations ranged from 0.50 to 2.1 μg/g 

(microgram per gram) with a median of 0.89 μg/g. 

The logarithms of suspended sediment and 

total selenium concentrations were closely 

correlated to the logarithm of streamflow 

(R2 = 0.82, R2 = 0.71, respectively for all storm 

data). These relations for the first storm were 

significantly different from the later three storms. 

Because of these correlations, the logarithm of 

suspended sediment loading rate and the logarithm 

of total selenium loading rate were closely 

correlated to the logarithm of streamflow 

(R2 = 0.982 for both for all storm data). Loads of 

suspended sediment and total selenium were 

calculated for each of the four storms in three 

ways, including simple linear regression with 

streamflow for all storms, simple linear 

regressions for the first storm and the combined 

later three storms, and integration under the 

instantaneous load curves. The resulting 

suspended sediment loads for all four storms 

ranged from 1,793,000 to 2,555,000 tons; total 

selenium ranged from 4,909 to 5,830 lb (pound). 

Dissolved selenium concentrations 

correlated significantly with both the logarithm of 

streamflow and specific conductance. Simple 

linear regression with the logarithm of streamflow 

had an R2 of 0.45, and a multiple linear regression 

with the logarithm of streamflow and specific 

conductance had an adjusted R2 of 0.48 for all 

storm data. The relation between streamflow and 

specific conductance for the first storm and for the 

later three storms were significantly different. As 

for suspended sediment and total selenium, 

dissolved selenium loads were calculated three 

ways. The resulting loads for all four storms 

ranged from 773 to 1,007 lb.

No significant storm occurred during the 

remainder of the study period (water years 1998 to 

2000), and thus, no additional storm sampling took 

place. Assuming that future sediment and 

selenium transport is similar to that of 1998, a 
2 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

reasonable estimate of loading rates can be 

calculated from the streamflow record at the 

Interstate 5 gage. Additional storm sampling 

would improve the estimates and possibly allow 

for separate equations for the rising and falling 

limbs of storm hydrographs.

INTRODUCTION

The Panoche Creek Basin, which is a major 

source of selenium owing to extensive surficial 

exposures of selenium-rich shale formations (Presser 

and others, 1990), is located on the margin between the 

San Joaquin Valley and the Coast Ranges (fig. 1). Mean 

annual rainfall in the basin ranges from about 20 in. 

(inch) at the western boundary (ridge of the Coast 

Ranges) to about 7.5 in. at the Interstate 5 (I-5) bridge. 

The drainage area for the U.S. Geological Survey 

(USGS) gaging station at the I-5 bridge is 305 mi2 

(square mile). 

Several studies have focused on shallow ground 

water and surficial deposits in the Panoche Creek Basin 

owing to selenium-induced problems at Kesterson 

National Wildlife Refuge, which indicate the basin is a 

major source area (San Joaquin Valley Drainage 

Program, 1990). The work by Presser and others 

(1990) provided a preliminary look at surface water 

quality in the basin. Samples of storm runoff during 

water year 1988 had dissolved selenium concentrations 

of 44 and 57 μg/L in Panoche Creek at I-5 and of 

55 μg/L in Silver Creek at Panoche Road (Presser and 

others, 1990). Because the 57 and 55 μg/L (microgram 

per liter) samples were collected on January 17, 1988 

(during runoff from the same storm), this suggests that 

the selenium concentrations in the Silver Creek portion 

of the Panoche Creek Basin may be very similar to 

selenium concentrations in the rest of the basin.

The purpose of this study was to sample storm 

events during water years 1998 to 2000 at the I-5 gage 

and calculate the loads of dissolved and total selenium 

and suspended sediment. This work was funded by the 

U.S. Bureau of Reclamation primarily to help evaluate 

potential selenium loads from Panoche Creek to canals 

that can contribute to the San Luis Drain in the 

Mendota area during extremely high streamflow events 

(fig. 1).

METHODS

Sampling Design

The advance warning of possible storm runoff in 

Panoche Creek was based on information available 

from the California Data Exchange Center for two realtime rainfall gages in the basin (stations PCH and IDR 

on figure 1). The USGS streamflow gage at I-5, which 

is operated annually from December through June, was 

installed in December 1997. Gage height data are 

transmitted in 4-hour blocks to the USGS’s Automated 

Data Processing System (and subsequently to the 

USGS’s public website—http://ca.water.usgs.gov/). 

Sample Collection

All storm samples were collected from the I-5 

bridge using a rope-suspended grab sampler, which 

consists of a 3-L Teflon bottle strapped into a metal 

cage. Samples were collected at three equally spaced 

points across the channel and composited in a churn 

splitter. The sample-collection bottle and the churn 

splitter were cleaned between samples using the 

following protocol: after collecting a sample, the 

collection bottle and splitter were rinsed with deionized 

water, then with 5-percent hydrochloric acid solution, 

and then thoroughly rinsed again with deionized water. 

Before sample collection, the collection bottle was 

rinsed three times with native water (U.S. Geological 

Survey, 1997 to present). 

The grab sampler probably integrated the top 3 

to 4 ft (foot) of streamflow at the three points in the 

cross-section. Standard USGS width- and depthintegrated samples were not collected because of safety 

concerns related to bridge traffic, stream velocities, and 

debris; further, in many cases, stream velocities 

exceeded the operational range of such integrating 

samplers (U.S. Geological Survey, 1997 to present). 

Because of the high velocities, the streamflow appeared 

to be well mixed.
Methods 3

Figure 1. Location of Panoche Creek Basin, California.

Fresno

Bakersfield

Los Angeles

San Francisco

FRESNO CO.

SAN BENITO CO.

33

I-5

180

MENDOTA

180

PANOCHE

IDRIA

FIREBAUGH

COUNTY LINE

Panoche Creek

Panoche

Panoche

Creek

Silver Creek

Belmont Avenue

Fairfax Avenue

New

Idria

Road

CA. Aqueduct

San Joaquin

River

PCH

PNH

IDR

11255575

11255500

Delta-Mendota

Canal

San Luis

Drain

Fresno

Slough

San Luis Drain

Orestimba Creek

0 5 10 KILOMETERS

0 5 10 MILES

Precipitation Station:

PNH (Panoche 2W), National Weather Service

PCH (Panoche Road), CA Dept. of Forestry

IDR (Idria), CA Dept. of Water Resources

USGS gaging stations

EXPLANATION

Basin Boundary

Towns in the basin

Highway

Road

Sierra Nevada

Coast Ranges

Sacramento Valley

Sacramento

San Joaquin Valley Los Gatos Creek
4 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

Sample Processing and Laboratory Methods

Samples collected for dissolved selenium 

analyses were filtered through 0.45-μm (micrometer) 

glass-fiber filters. Both dissolved and total selenium 

samples were preserved using nitric acid. All selenium 

samples were sent to the USGS’s National Water 

Quality Laboratory (NWQL) in Denver, Colorado, for 

analysis using the atomic absorption spectrometric, 

hydride generation method (Fishman and Friedman, 

1989). The method detection level (MDL) is 1 μg/L. 

Suspended sediment samples were sent to the USGS’s 

California District Sediment Laboratory in Salinas. 

Two additional parameters were calculated for 

discussion in this report: suspended selenium 

concentration and selenium attached to suspended 

sediment. Suspended selenium concentration was 

calculated as the difference between total selenium and 

dissolved selenium concentrations. Selenium attached 

to suspended sediment was calculated as the suspended 

selenium concentration divided by the suspended 

sediment concentration.

Quality Control Samples

In addition to 32 environmental samples 

collected during February 1998, eight quality control 

samples were collected during the storms: four field 

blanks to evaluate possible contamination during 

sample collection and four replicates to evaluate the 

variability in selenium concentrations (table 1). The 

relative percent differences (RPD) for dissolved 

selenium replicates ranged from 0.0 to 18.2 percent. 

The RPDs for total selenium replicates ranged from 0.0 

to 50.0 percent. The dissolved and total selenium 

concentrations in the field blanks were all below the 

MDL of 1 μg/L. Thus, variability of dissolved selenium 

was acceptable, and contamination was not a problem. 

However, the variability in total selenium 

concentrations was a significant concern. This was also 

a problem in four samples that were rerun for total 

selenium (table 1). The RPDs for the reruns ranged 

from 8.0 to 57.1 percent.

The variability in total selenium concentrations 

increased with suspended sediment concentrations; 

suspended sediment may interfere by reacting with the 

reagents used to change the oxidation state of selenium 

prior to hydride formation (Glenda Brown, USGSNWQL, written commun., 1998). Also, the amount of 

suspended sediment is probably different each time the 

sample is run because the analyst shakes the bottle and 

pipettes a subsample; the amount of settling probably 

varies each time, especially for samples containing 

high suspended sediment. In addition, relatively high 

specific conductance and sulfate concentrations in the 

samples could interfere with selenium analyses and 

reduce the recovery of selenium (Jones and Garbarino, 

1999). Seven samples collected during the third storm 

had a median sulfate concentration of 1,350 mg/L as 

SO4 analyzed at a USGS laboratory in Menlo Park, 

California (Theresa Presser, U.S. Geological Survey, 

written commun., 1999). 

Regression Methods

Simple linear regressions were performed on 

several pairs of constituents. In some cases, one or both 

axes were transformed by taking the base 10 logarithm 

(log) to improve the linearity of the data. The goodness 

of fit is determined by the R2 value, and the 

significance of the slope of the fit is determined from 

the p-value. In some regressions one or more outliers 

were removed based on two criteria. An outlier in the 

x-direction was determined by a leverage point criteria 

(Helsel and Hirsch, 1992). The leverage value, hi, is 

calculated as:

where

An observation was discarded as a high leverage 

point if:

where

hi = leverage value

n = number of observations

xi = x value for i

th observation

= mean of all x values

SSx = sum of squares in x-direction

p = number of coefficients in regression model

(p = 2 for simple linear regression, b0 and 

b1).

hi 1 ⁄ n x

i ( ) – x

2 SS

x = + ⁄

x

hi > 3p n⁄
Methods 5

Table 1. Quality control samples for Panoche Creek at I-5, California, February 1998

[Numbers in parentheses are parameter codes from NWIS database. Relative percent difference is the absolute value of env. (environmental sample) minus 

repl. (replicate sample) divided by the average of env. and repl., multiplied by 100. —, not applicable; Se, selenium. ft3/s, cubic foot per second; mg/L, 

milligram per liter; mm, millimeter; μg/L, microgram per liter; μS/cm, microsiemen per centimeter; <, less than; %, percent]

Date, time

Streamflow,

instantaneous

(ft3/s)

(00061)

Specific

conductance

(μS/cm)

(00095)

Suspended

sediment

(mg/L)

(80154)

Suspended

sediment

(sieve

diameter % 

finer than

0.062 mm)

(70331)

Dissolved 

selenium

(μg/L as Se)

(01145)

Dissolved 

selenium;

relative

percent

difference

(%)

Total

selenium

(μg/L as Se)

(01147)

Total

selenium;

relative

percent 

difference

(%)

A. Blanks

2/3/98, 1405 3,920 2,610 229,251 48 <1 — <1 —

2/7/98, 0935 182 2,180 48,721 65 <1 — <1 —

2/21/98, 2345 924 2,890 135,448 56 <1 — <1 —

2/24/98, 0635 164 2,330 52,682 43 <1 — <1 —

B. Replicates

2/3/98, 1600 (env.) 2,420 2,480 176,800 53 45 — 230 —

2/3/98, 1601 (repl.) — — — — 54 18.2 170 30.0

2/6/98, 1530 1,620 2,280 135,800 52 32 — 120 —

2/6/98, 1531 — — — — 28 13.3 200 50.0

2/22/98, 0240 472 2,200 86,900 63 29 — 100 —

2/22/98, 0241 — — — — 26 10.9 100 0.0

2/24/98, 0630 164 2,330 52,700 43 21 — 73 —

2/24/98, 0631 — — — — 21 0.0 76 4.0

C. Reruns

2/3/98, 1400 3,920 2,610 229,300 48 60 — 450 —

Rerun — — — — — — 250 57.1

2/6/98, 1140 1,570 3,680 251,500 47 38 — 340 —

Rerun — — — — — — 260 26.7

2/6/98, 1240 2,420 2,850 156,700 58 37 — 120 —

Rerun — — — — — — 130 8.0

2/6/98, 1340 2,190 2,670 163,300 60 32 — 260 —

Rerun — — — — — — 200 26.1
6 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

An outlier in the y-direction was determined by 

using the standardized residual test (Helsel and Hirsch, 

1992). The standardized residual, esi, is calculated as:

where

An observation was discarded as an outlier in the 

y-direction if:

This is considered an extreme outlier and should occur 

only an average of three times out of 1,000 

observations if the residuals are normally distributed.

The uncertainty in loads calculated from 

regression equations is expressed in this report as a 

95-percent confidence interval for predictions. This 

interval was calculated using the following equation 

(Helsel and Hirsch, 1992):

where

For the load calculation regressions using 29 data 

points, this interval can be simplified to:

For log-transformed regressions, this interval 

applies to the transformed x and y (e.g., x = log10 

streamflow, y = log10 suspended sediment load). The 

predicted load must be untransformed (10y) to calculate 

the confidence interval for the desired load.

The determination of significant differences 

between simple linear regressions was based on the 

Student’s t test between slopes. The method is 

analogous to the testing of differences between two 

population means (Zar, 1974). The t statistic is 

calculated from the following equation (Zar, 1974):

where

If this t value exceeds , then the

slopes are significantly different at the α significance 

level for sample sizes n1 and n2 for regressions 1 and 2.

Multiple linear regression was performed to 

predict dissolved selenium concentrations as a function 

of both log streamflow and specific conductance. As 

with simple linear regression, the goodness of fit is 

determined by the adjusted R2 value, and the 

significance of the slope of the fit is determined by the 

p-value. The adjusted R2 value accounts for the degrees 

of freedom in the regression (Helsel and Hirsch, 1992).

STORM HYDROLOGY

Historical Hydrology

Average annual precipitation (1911–60) in the 

Panoche Creek Basin ranges from about 20 in. in the 

upper part of the basin in the Coast Ranges to about 

7.5 in. in the lower part of the basin in the San Joaquin 

Valley (Kratzer and Shelton, 1998). The overall areaweighted average annual rainfall for the basin is about 

12 in. Historical averages (1911–60) for the three 

rainfall gages shown in figure 1 are about 14 in. at Idria 

(station IDR), 9 in. at Panoche 2W (station PNH), and 

7.5 in. at Panoche Road (station PCH). The average 

annual rainfall at the PNH rainfall gage for 1957–2000 

was about 8 in., with over 80 percent of the annual 

rainfall taking place during November through March 

(Gronberg and others, 1998).

esi = standardized residual

ei = yi – (= prediction residual in 

y-direction for ith observation)

yi = y value for ith observation

= predicted y value for i

th observation

s = standard error of regression

= predicted value of y for a given x from 

regression equation

tα/2,n–2 = t statistic for significance level α and 

n samples

s = standard error of regression

e

si ei ⁄ s 1 hi ( ) – 0.5 =

y

i

ˆ

yi

ˆ

e

si > 3

(yˆ t

α ⁄ 2, n – 2s yˆ tα ⁄ 2, n – 2 – ; + s)

yˆ

(yˆ – 2.05s y ; ˆ + 2.05s)

b1, b2 = slopes for regressions 1 and 2

= pooled sample standard deviation

t b1 b2 ( ) – Sb1 b2 – = ⁄ ( )

Sb1 b2 –

t

α 2 n1 n2 ⁄ , + –4
Storm Hydrology 7

The streamflow record for Panoche Creek is not 

as extensive as the rainfall record. The old USGS 

gaging station (station no. 11255500) recorded streamflows for 1950–53 and 1959–70. This site (fig. 1) is 4.8 

river mile upstream of the present gaging station at I-5 

(station no. 11255575) and has a drainage area of 293 

mi2 compared to 305 mi2 for the present site. The peak 

streamflow of record at the old site was 5,400 ft3/s 

(cubic foot per second) on February 24, 1969. 

Precipitation and streamflow records for Panoche 

Creek for 1959–70 are shown in figure 2. Overall, the 

Panoche Creek basin has a much lower frequency of 

high streamflows compared with other smaller Coast 

Range basins. For the 16 years of streamflow records, 

Panoche Creek (293 mi2) had daily mean streamflows 

of over 100 ft3/s for only 24 days; whereas, Los Gatos 

Creek (96 mi2) had 243 days in 54 years (water years 

1946 to 1999) and Orestimba Creek (134 mi2) had 851 

days in 64 years (water years 1936 to 1999) (see 

figure 1 and inset for locations).

Hydrology for Study Period (Water Years 

1998 to 2000)

Average daily precipitation (at PNH gage) and 

streamflow at the new USGS gaging station (at I-5; 

station no. 11255575) for the study period are shown in 

figure 3. Rainfall during water year 1998 at the PNH 

gage was 26.36 in., well above the long-term average 

of 8 to 9 in. Rainfall during water years 1999 and 2000 

were about average, 9.16 and 8.51 in., respectively. The 

rainfall total of 10.40 in. during February 1998 at the 

PNH gage was greater than any other month during 

Figure 2. Daily precipitation and streamflow in Panoche Creek, California, for water years 1959 to 1970.

PNH, Panoche 2W (National Weather Service) precipitation station.

Water Year

Daily mean streamflow, in cubic feet per second

Average daily precipitation at PNH, in inches per day

Daily mean streamflow at old USGS gage (site 11255500)

Daily precipitation at PNH

1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970

10

1

2

3

4

5

6

7

8

9

0

1

10,000

10

100

1,000
8 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

1957 to 2000. The 1998 streamflows also were 

unusually high compared with the 1950–53 and 

1959–70 periods. Peak streamflows on February 3, 

1998 (9,940 ft3/s) and on February 7, 1998 (8,550 ft3/s) 

both exceeded the peak streamflow of record at the old 

gage (fig. 4). The peak streamflow during water years 

1999 and 2000 was only 188 ft3/s on February 23, 

2000.

The relation between hourly precipitation in the 

basin and streamflow at the I-5 gage for February 1998 

is shown in figure 4. Since the PNH rainfall gage only 

records daily totals, we used the average of the PCH 

and IDR gages for the hourly rainfall. Although these 

two sites represent two extremes of rainfall in the 

basin, their average is close to the overall basin 

average. For February 1998, the average of PCH and 

IDR was 8.60 in. compared with 10.40 in. at PNH. 

Streamflow at the I-5 gage occurred about 6 to 12 hours 

after significant rainfall in the basin for the February 

1998 storms. The delay period is affected by 

antecedent moisture in the basin, rainfall intensity, and 

the initial water level in the pond at the gravel pit less 

than a mile upstream of the I-5 gage. 

WATER QUALITY

Time Series Data During Storms

A total of 32 environmental samples were 

collected during February 1998 (fig. 4). The first two 

samples were collected on February 2, before the first 

sampled storm. The other 30 environmental samples 

were collected throughout the hydrograph following 

four storms; five to nine samples were collected per 

storm runoff event (table 2). Seven of the 30 

environmental samples were collected on the rising

Figure 3. Daily precipitation and streamflow in Panoche Creek, California, for water years 1998 to 2000. 

PNH, Panoche 2W (National Weather Service) precipitation station. October December February April June August October December

February

April

June

August

October

December

February

April

June

August

October

0

1

2

3

4

5

6

7

8

9

10

Average daily precipitation at PNH, in inches per day

Daily mean streamflow at new USGS gage (site 11255575)

Daily precipitation at PNH

Daily mean streamflow, in cubic feet per second

10,000

1,000

100

10

1

1997 1998 1999 2000
Water Quality 9

limb of the storm hydrographs, four samples were 

collected within 4.1 percent of the peak streamflow, 

and 19 samples were collected on the falling limb. For 

five of the seven rising limb samples, falling limb 

samples were collected during the same storm at 

streamflows within 7.1 percent RPD (table 2 [see 

samples paired by color]).

Specific conductance peaked on the rising limb 

of the storm hydrographs for the last three storms and 

on the falling limb of the first storm (fig. 5). In all four 

storms, the specific conductance increased towards the 

end of the storm hydrograph. This could be a function 

of bank storage of high conductivity water being 

released or a second source of high conductivity water 

reaching the site from upstream in the watershed.

The suspended sediment concentrations 

generally followed the form of the storm hydrographs 

(fig. 6). As with specific conductance, suspended 

sediment concentrations peaked on the rising limb for 

the last three storms and on the falling limb for the first 

storm. The percentage of suspended material that is silt 

and clay (<0.062 mm [millimeter] diameter) varied 

from 43 to 71 percent during the storms, except for one 

sample on February 21 at the beginning of a storm that 

had only 31 percent fine material. The pre-storm 

samples on February 2 had 87 and 82 percent fine 

material. The percentage of fine material was usually 

lower near the peak streamflow than it was farther out 

on the falling limb of the storm hydrograph.

The concentration pattern of dissolved selenium 

was very similar to specific conductance (fig. 7). The 

concentration patterns of total and suspended selenium 

(calculated) were very similar to suspended sediment 

(figs. 7 and 8). One exception to this was near the peak 

streamflow of the first storm (figs. 7A and 8A). 

Suspended (calculated) and total selenium 

concentrations peaked on the rising limb while 

suspended sediment peaked on the falling limb of the 

storm hydrograph. The concentration of selenium 

attached to the suspended sediment (calculated in 

Figure 4. Hourly precipitation, 15-minute streamflow, and sample collection times in Panoche Creek, California, February 1998. 

PCH, Panoche Road (California Department of Forestry) precipitation station; IDR, Idria (California Department of Water Resources) precipitation station.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

February 1998

1

10,000

10

100

1,000

0

0.32

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

Streamflow, in cubic feet per second

Average hourly precipitation at PCH and IDR, in inches per hour

Streamflow at site 11255575

Sample collected

Average hourly precipitation at PCH and IDR sites
10 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998 [Numbers in parentheses denote parameter codes in NWIS database. Each color indicates a paired sample (rising limb; falling limb) at similar streamflow during the sam

R, rising; F, falling; P, peak. Se, selenium; T/D, tons per day; ft3/s, cubic foot per second; lb/d, pound per day; mg/L, milligram per liter; mm, millimeter; μg/g, microgram 

liter; μS/cm, microsiemen per centimeter; —, not applicable; %, percent] Date, time Streamflow, instanta- neous (ft3/s) (00061) Storm hydro- graph Specific conduct- ance

(μS/cm)

(00095)

Suspended

sediment

(mg/L)

(80154)

Suspended

sediment

(sieve

diameter % 

finer than

0.062 mm)

(70331)

Dissolved 

selenium

(μg/L

as Se)

(01145)

Suspended 

selenium

(calculated)

(μg/L

as Se)4

Total 

selenium

(μg/L

as Se)

(01147)

Suspended 

sediment

loading

rate,

instantaneous

(T/D)5

Dissolved 

selenium

loading

rate,

instantaneous

(lb/d)6

Susp

sele

loa

ra

inst

ne

(lb

2/2/98, 09:30 122

— 11,420 124,000 187 17 13 120 7,888 4.6 8

2/2/98, 12:30 83

— 21,390 218,100 282 25 12 217 4,047 2.2 5

Storm 1:

2/3/98, 03:00 228

R 1,500 54,400 70 16 71 87 33,412 20 8

2/3/98, 07:00 3,860

R 1,670 140,000 71 26 294 320 1,455,747 541 6,11

2/3/98, 10:30 9,530

P 2,420 177,000 60 30 260 290 4,543,984 1,540 13,35

2/3/98, 12:00 6,840

F 2,540 230,000 48 56 204 260 4,237,937 2,064 7,51

2/3/98, 14:00 3,920

F 2,610 229,000 48 60 190 250 2,418,200 1,267 4,01

2/3/98, 16:00 2,420

F 2,480 177,000 53 350 150 3200 1,153,876 652 1,95

2/3/98, 19:00 1,250

F 2,130 131,000 58 46 114 160 441,115 310 76

2/4/98, 00:00 517

F 2,250 86,400 60 25 85 110 120,330 70 23

2/4/98, 07:00 263

F 2,270 63,900 70 24 86 110 45,272 34 12

Storm 2:

2/6/98, 11:40 1,570

R 3,680 251,000 47 38 222 260 1,061,559 321 1,87

2/6/98, 12:40 2,420

P 2,850 157,000 58 37 93 130 1,023,495 482 1,21

2/6/98, 13:40 2,190

F 2,670 163,000 60 32 168 200 961,617 378 1,98

2/6/98, 15:30 1,620

F 2,280 136,000 52 330 130 3160 593,505 262 1,13

2/6/98, 18:30 806

F 2,250 129,000 52 30 65 95 280,089 130 28

2/6/98, 22:30 722

F 2,390 122,000 47 30 80 110 237,284 117 31

2/7/98, 03:30 395

F 2,100 90,300 60 22 65 87 96,085 47 13

2/7/98, 09:30 182

F 2,180 48,700 65 25 48 73 23,877 25 4

Table 2. Water quality data for storm sampling in Panoche Creek at I-5, California, February 1998 
Water Quality 11

Date, time

Streamflow,

instantaneous

(ft3/s)

(00061)

Storm

hydrograph

Specific

conductance

(μS/cm)

(00095)

Suspended

sediment

(mg/L)

(80154)

Suspended

sediment

(sieve

diameter % 

finer than

0.062 mm)

(70331)

Dissolved 

selenium

(μg/L

as Se)

(01145)

Suspended 

selenium

(calculated)

(μg/L

as Se)4

Total 

selenium

(μg/L

as Se)

(01147)

Suspended 

sediment

loading

rate,

instantaneous

(T/D)5

Dissolved 

selenium

loading

rate,

instantaneous

(lb/d)6

Susp

sele

loa

ra

inst

ne

(lb

Storm 3:

2/21/98, 16:40 15 R 3,190 3,900 31 30 27 57 1,329 2.4 22/21/98, 22:40 735 R 3,140 150,000 56 44 146 190 296,995 174 572/21/98, 23:40 924 P 2,890 135,000 56 32 108 140 336,029 159 532/22/98, 00:40 824 F 2,430 116,000 57 25 75 100 257,487 111 332/22/98, 02:40 472 F 2,200 86,900 63 328 72 3100 110,492 71 182/22/98, 05:40 235 F 2,130 54,500 71 20 62 82 34,501 25 72/22/98, 08:40 176 F 2,250 51,500 61 20 62 82 24,417 19 52/22/98, 12:30 170 F 2,410 42,000 65 24 62 86 19,234 22 5

Storm 4:

2/23/98, 15:40 518 R 3,200 94,500 52 30 80 110 131,866 84 222/23/98, 16:30 1,480 R 2,750 150,000 52 31 89 120 598,031 247 712/23/98, 17:10 1,780 P 2,570 152,000 52 26 94 120 728,843 249 902/23/98, 21:00 458 F 2,060 106,000 59 21 99 120 130,780 52 242/24/98, 06:30 164 F 2,330 52,700 43 321 54 375 23,282 19 41Average of samples collected at 0920, 0930, and 1010.2Average of samples collected at 1230 and 1240.3Average of replicates for dissolved selenium (01145) and total selenium (01147).4Calculated by difference: total selenium (01147) minus dissolved selenium (01145).5Calculated: 0.002694  suspended sediment concentration (mg/L)  streamflow (ft3/s).6Calculated: 0.00538  dissolved selenium concentration (μg/L)  streamflow (ft3/s).7Calculated: 0.00538  suspended selenium concentration (μg/L)  streamflow (ft3/s).8Calculated: 0.00538  total selenium concentration (μg/L)  streamflow (ft3/s).9Calculated: suspended selenium concentration (μg/L)/(suspended sediment concentration [mg/L]  g/1000mg).

Table 2. Water quality data for storm sampling in Panoche Creek at I-5, California, February 1998—Continued
12 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

Figure 5. Streamflow and specific conductance in Panoche Creek, California, for February 1998 storms.

A. February 2–4.

B. February 6 and 7.

C. February 21 and 22.

D. February 23 and 24.

0800 1200 1600 2000 2400 0400 0800 1200

6 7

February 1998, time and date

0

4,000

500

1,000

1,500

2,000

2,500

3,000

3,500

1,500

4,000

2,000

2,500

3,000

3,500 Streamflow

Specific conductance

B

Specific conductance, in microsiemens per centimeter

Streamflow, in cubic feet per second

2400 0600 1200 1800 2400 0600 1200 1800 2400 0600 1200

2 3 4

February 1998, time and date

12,000

0

2,000

4,000

6,000

8,000

10,000

1,200

3,000

1,400

1,600

1,800

2,000

2,200

2,400

2,600

2,800

Streamflow

Specific conductance

A

Streamflow, in cubic feet per second

Specific conductance, in microsiemens per centimeter
Water Quality 13

Figure 5. Continued.

1200 1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200

23 24

February 1998, time and date

2,000

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

1,200

3,500

1,500

2,000

2,500

Streamflow 3,000

Specific conductance

D

Streamflow, in cubic feet per second

Specific conductance, in microsiemens per centimeter

1200 1600 2000 2400 0400 0800 1200 1600

21 22

February 1998, time and date

0

2,000

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

1,200

3,500

1,500

2,000

2,500

3,000 Streamflow

Specific conductance

C

Streamflow, in cubic feet per second

Specific conductance, in microsiemens per centimeter
14 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

0600 1200 1800 2400 0600 1200 1800 2400 0600 1200

2 3 4

February 1998, time and date

0

12,000

2,000

4,000

6,000

8,000

10,000

0

300

50

100

150

200

250

Streamflow

Suspended sediment concentration

Suspended sediment sieve diameter,

% finer than 0.062 mm (in percent)

A

Streamflow, in cubic feet per second

Suspended sediment, in milligrams per liter x 1,000

(concentration and % finer than 0.062 millimeter)

Figure 6. Streamflow and suspended sediment in Panoche Creek, California, for February 1998 storms. 

[%, percent; mm, millimeter]

A. February 2–4.

B. February 6 and 7.

C. February 21 and 22.

D. February 23 and 24.

0800 1200 1600 2000 2400 0400 0800 1200

6 7

February 1998, time and date

0

3,000

500

1,000

1,500

2,000

2,500

0

300

50

100

150

200

250 Streamflow

Suspended sediment concentration

Suspended sediment sieve diameter,

% finer than 0.062 mm (in percent)

B

Suspended sediment, in milligrams per liter x 1,000

(concentration and % finer than 0.062 millimeter

Streamflow, in cubic feet per second
Water Quality 15

1200 1600 2000 2400 0400 0800 1200 1600

21 22

February 1998, time and date

1,000

200

0

400

600

800

0

200

20

40

60

80

100

120

140

160

180

C

Streamflow

Suspended sediment concentration

Suspended sediment sieve diameter,

% finer than 0.062 mm (in percent)

Suspended sediment, in milligrams per liter x 1,000

(concentration and % finer than 0.062 millimeter

Streamflow, in cubic feet per second

Figure 6. Continued.

1200 1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200

23 24

February 1998, time and date

0

2,000

200

400

600

800

1,000

1,200

1,400

1,600

1,800

200

20

0

40

60

80

100

120

140

160

180

Streamflow

Suspended sediment concentration

Suspended sediment sieve diameter,

% finer than 0.062 mm (in percent)

Streamflow, in cubic feet per second

D

Suspended sediment, in milligrams per liter x 1,000

(concentration and % finer than 0.062 millimeter
16 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

0600 1200 1800 2400 0600 1200 1800 2400 0600 1200

2 3 4

February 1998, time and date

0

12,000

2,000

4,000

6,000

8,000

10,000

0

400

50

100

150

200

250

300

350

A Streamflow

Dissolved selenium

Total selenium

Selenium concentration, in micrograms per liter as Se

(total and dissolved)

Streamflow, in cubic feet per second

Figure 7. Streamflow and dissolved and total selenium in Panoche Creek, California, for February 1998 storms. 

A. February 2–4.

B. February 6 and 7.

C. February 21 and 22.

D. February 23 and 24.

0800 1200 1600 2000 2400 0400 0800 1200

6 7

February 1998, time and date

0

3,000

500

1,000

1,500

2,000

2,500

0

300

50

100

150

200

250

Selenium concentration, in micrograms per liter as Se

(total and dissolved)

Streamflow, in cubic feet per second

Streamflow

Dissolved selenium

Total selenium

B
Water Quality 17

1200 1600 2000 2400 0400 0800 1200 1600

21 22

February 1998, time and date

0

1,000

200

400

600

800

0

200

20

40

60

80

100

120

140

160

180

Selenium concentration, in micrograms per liter as Se

(total and dissolved)

Streamflow, in cubic feet per second

C

Streamflow

Dissolved selenium

Total selenium

Figure 7. Continued.

1200 1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200

23 24

February 1998, time and date

0

2,000

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0

200

20

40

60

80

100

120

140

160

180

D

Selenium concentration, in micrograms per liter as Se

(total and dissolved)

Streamflow, in cubic feet per second

Streamflow

Dissolved selenium

Total selenium
18 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

microgram per gram) generally increased on the rising 

limb, decreased near the peak flow, and increased again 

on the falling limb of the storm hydrographs (fig. 8). 

Although concentrations at a given streamflow 

were usually greater on the rising limb of the storm 

hydrographs, there were many exceptions (table 2 [see 

samples paired by color], figures 5–8). The instances 

during which concentrations at a given streamflow 

were greater on the falling limb than the rising limb are 

as follows:

• Specific conductance in the first storm

• Suspended sediment in the first and fourth 

storms

• Total selenium in the first and fourth storms

• Dissolved selenium in the first storm

• Suspended selenium (calculated) in the first 

and fourth storms

• Some concentrations of selenium attached to 

the suspended sediment (calculated) in the 

second and fourth storms.

Relations Between Constituents

Several correlations between constituents were 

evaluated for the February 1998 storm data (figs. 9A–

9K). Most of the discussion focuses on regressions that 

had slopes significant at the 95-percent confidence 

level (p-value < 0.05). Because the water-quality timeseries data for the first storm are different from storms 

2–4 for several constituents (table 2, figs. 5–8), 

separate regressions were done for these two 

populations in addition to the combined regression. 

The linear regression slopes for storm 1 and storms 2–4 

are significantly different (at the 95-percent confidence 

level) for specific conductance versus log streamflow, 

dissolved selenium versus specific conductance, 

dissolved selenium versus log streamflow, total 

selenium versus log streamflow, selenium attached to 

suspended sediment (calculated) versus log streamflow, 

and log suspended sediment versus log streamflow 

(figs. 9A, B, D, and F–H, respectively).

For the regressions that are based on all storm 

data, no significant correlations (p-value of 

slope > 0.05) were found between the following 

constituents: specific conductance and log streamflow 

(fig. 9A); total selenium and specific conductance 

(fig. 9C); selenium attached to suspended sediment 

(calculated) and log streamflow (fig. 9G); and 

suspended sediment size and log streamflow (fig. 9K). 

Moderate correlations (p-value of slope < 0.05 and 

R2 < 0.5) occurred between dissolved selenium and 

specific conductance (fig. 9B) and between dissolved 

selenium and log streamflow (fig. 9D). Relatively 

strong correlations (p-value of slope < 0.05 and 

R2 > 0.5) occurred between log suspended selenium 

(calculated) and log streamflow (fig. 9E), total 

selenium and log streamflow (fig. 9F), log suspended 

sediment and log streamflow (fig. 9H), suspended 

selenium (calculated) and suspended sediment (fig. 9I), 

and total selenium and suspended sediment (fig. 9J). 

These relations suggest that an acceptable estimate of 

suspended sediment and total selenium could be made 

from the continuous streamflow record at the I-5 gage. 

A summary of regressions between water quality 

constituents is shown in table 3.

Separating data for the first storm from all storm 

data improved some regressions and made some worse. 

The regression for dissolved selenium versus specific 

conductance (fig. 9B) was substantially improved. The 

goodness of fit (R2) improved from 0.18 to 0.54 while 

the significance (p) of the correlation remained 

unchanged. No previously nonsignificant correlations 

became significant by separating storm 1 data from the 

other storm data. Significant correlations (p < 0.05) had 

substantially improved goodness of fit (R2) for log 

suspended selenium (calculated) versus log streamflow 

(fig. 9E) and total selenium versus log streamflow 

(fig. 9F). The regressions for dissolved selenium versus 

log streamflow (fig. 9D), suspended selenium 

(calculated) versus suspended sediment (fig. 9I), and 

total selenium versus suspended sediment (fig. 9J) 

were substantially worse. Significant correlations 

(p < 0.05) became nonsignificant (p > 0.05) for 

dissolved selenium versus log streamflow (fig. 9D) and 

suspended selenium (calculated) versus suspended 

sediment (fig. 9I). Although still significant, the 

goodness of fit (R2) was substantially worse than for 

the regression using all storm data for total selenium 

versus suspended sediment (fig. 9J). 

Similarly, separating data for storms 2–4 

improved some regressions and made some worse. The 

regressions for specific conductance versus log 

streamflow (fig. 9A), dissolved selenium versus specific 

conductance (fig. 9B), and selenium attached to 

suspended sediment (calculated) versus specific 
Water Quality 19

0600 1200 1800 2400 0600 1200 1800 2400 0600 1200

2 3 4

February 1998, time and date

0

12,000

2,000

4,000

6,000

8,000

10,000

0

400

50

100

150

200

250

300

350

A Streamflow

Suspended selenium (calculated)

Suspended sediment

Selenium attached to suspended

sediment (calculated)

Suspended selenium concentration, in micrograms per liter as Se

Suspended sediment concentration, in milligrams per liter x 1,000

Selenium attached to suspended sediment, in micrograms per gram x 0.01

Streamflow, in cubic feet per second

Figure 8. Streamflow, suspended selenium (calculated), suspended sediment, and selenium attached to suspended sediment (calculated) in Panoche 

Creek, California, for February 1998 storms. 

A. February 2–4.

B. February 6 and 7.

C. February 21 and 22.

D. February 23 and 24.

0800 1200 1600 2000 2400 0400 0800 1200

6 7

February 1998, time and date

0

3,000

500

1,000

1,500

2,000

2,500

0

300

50

100

150

200

250

B

Streamflow

Suspended selenium (calculated)

Suspended sediment

Selenium attached to suspended

sediment (calculated)

Suspended selenium concentration, in micrograms per liter as Se

Suspended sediment concentration, in milligrams per liter x 1,000

Selenium attached to suspended sediment, in micrograms per gram x 0.01

Streamflow, in cubic feet per second
20 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

1200 1600 2000 2400 0400 0800 1200 1600

21 22

February 1998, time and date

0

1,000

200

400

600

800

0

200

20

40

60

80

100

120

140

160

180

Suspended selenium concentration, in micrograms per liter as Se

Suspended sediment concentration, in milligrams per liter x 1,000

Selenium attached to suspended sediment, in micrograms per gram x 0.01

Streamflow, in cubic feet per second

Streamflow

Suspended selenium (calculated)

Suspended sediment

Selenium attached to suspended

sediment (calculated)

C

Figure 8. Continued.

1200 1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200

23 24

February 1998, time and date

0

2,000

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0

200

20

40

60

80

100

120

140

160

180

D

Suspended selenium concentration, in micrograms per liter as Se

Suspended sediment concentration, in milligrams per liter x 1,000

Selenium attached to suspended sediment, in micrograms per gram x 0.01

Streamflow, in cubic feet per second

Streamflow

Suspended selenium (calculated)

Suspended sediment

Selenium attached to suspended

sediment (calculated)
Water Quality 21

conductance (fig. 9G) were substantially improved. 

Nonsignificant regressions (p > 0.05) became 

significant (p < 0.05) for specific conductance versus 

log streamflow (fig. 9A) and selenium attached to 

suspended sediment (calculated) versus specific 

conductance (fig. 9G). The goodness of fit (R2) was 

substantially improved for dissolved selenium versus 

specific conductance (fig. 9B). No significant 

regressions became nonsignificant by separating data 

for storms 2–4 from all storm data. Although still 

significant, the goodness of fit was substantially worse 

for log suspended selenium (calculated) versus log 

streamflow (fig. 9E), total selenium versus log 

streamflow (fig. 9F), suspended selenium (calculated) 

versus suspended sediment (fig. 9I), and total selenium 

versus suspended sediment (fig. 9J).

The relations of log suspended sediment to log 

streamflow (fig. 9H), dissolved selenium to specific 

conductance (fig. 9B), dissolved selenium to log 

streamflow (fig. 9D), and total selenium to log 

streamflow (fig. 9F) are the basis for load predictions 

discussed in the next section. These relations are 

Figure 9. Correlations between water quality constituents and streamflow in Panoche Creek, California, for February 1998 storm data. 

A. Specific conductance versus log10 streamflow. 

B. Dissolved selenium versus specific conductance. 

C. Total selenium versus specific conductance. 

D. Dissolved selenium versus log10 streamflow. 

E. Log10 suspended selenium (calculated) versus log10 streamflow. 

F. Total selenium versus log10 streamflow. 

G. Selenium attached to suspended sediment (calculated) versus log10 streamflow. 

H. Log10 suspended sediment versus log10 streamflow. 

I. Suspended selenium (calculated) versus suspended sediment. 

J. Total selenium versus suspended sediment. 

K. Suspended sediment sieve diameter percent finer than 0.062 millimeter versus log10 streamflow.

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

012345

Specific conductance, in microsiemens per centimeter

Data points

Regression line

Student's t between storm 1

and storms 2–-4 regressions = 3.07

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

Data points

Regression line

ALL STORMS (n = 29):

Regression line

Log10 streamflow, in cubic feet per second

A

y = 1,200 + 313.3x

R2 = 0.24

p = 0.18

y = 1,044 + 523.4x

R2 = 0.23

p = 0.03

y = 1,717 + 238.6x

R2 = 0.08

p = 0.14
22 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

0

10

20

30

40

50

60

70

0 500 1,000 1,500 2,000 2,500 3,000 3,500

Specific conductance, in microsiemens per centimeter

Dissolved selenium, in micrograms per liter, as Se

B Data points

Regression line

Student's t between storm 1

and storms 2–-4 regressions = 8.79

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

Data points

Regression line

ALL STORMS (n = 29):

Regression line

y = – 30.3 + 0.031x

R2 = 0.54

p = 0.02

y = – 0.43 + 0.011x

R2 = 0.47

p < 0.01

y = 2.47 + 0.012x

R2 = 0.18

p = 0.02

Figure 9. Continued.

0

50

100

150

200

250

300

350

0 500 1,000 1,500 2,000 2,500 3,000 3,500

C

Data points

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

Data points

Regression line

ALL STORMS (n = 29):

Regression line

y = 72.2 + 0.057x

R2 = 0.07

p = 0.5

y = 38.6 + 0.030x

R2 = 0.08

p = 0.22

y = 143 – 0.0017x

R2 < 0.01

p = 0.96

Specific conductance, in microsiemens per centimeter

Total selenium, in micrograms per liter, as Se

Student's t between storm 1

and storms 2–-4 regressions = 0.32
Water Quality 23

0

10

20

30

40

50

60

70

012345

Data points

Regression line

Student's t between storm 1

and storms 2–-4 regressions = 2.62

STORM 1 (n = 9):

Data points

Regression line

ALL STORMS (n = 29):

y = – 14.5 + 16.0x

R2 = 0.36

p = 0.09

y = – 2.7 + 11.1x

R2 = 0.45

p < 0.01

y = – 12.0 + 14.7x

R2 = 0.45

p < 0.01

D

Dissolved selenium, in micrograms per liter, as Se

Log10 streamflow, in cubic feet per second

Regression line

STORMS 2–4 (n = 20):

Figure 9. Continued.

0

0.5

1

1.5

2

2.5

3

012345

Data points

Regression line

Student's t between storm 1

and storms 2–-4 regressions = 1.20

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

ALL STORMS (n = 29):

Regression line

y = 1.03 + 0.35x

R2 = 0.88

p < 0.01

y = 1.02 + 0.33x

R2 = 0.56

p < 0.01

y = 0.92 + 0.37x

R2 = 0.74

p < 0.01

E

Log10 streamflow, in cubic feet per second

Log10 suspended selenium (calculated), in micrograms per liter, as Se

Data points

Regression line
24 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

Figure 9. Continued.

0

0.5

1.0

1.5

2.0

2.5

012345

Selenium attached to suspended sediment

(calculated, in micrograms per gram)

Data points

Regression line

Data points

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

ALL STORMS (n = 28):

y = 1.08 + 0.031x

R2 < 0.01

p = 0.91

y = 1.94 – 0.381x

R2 = 0.38

p < 0.01

y = 1.37 – 0.152x

R2 = 0.08

p = 0.35

G

Regression line

Student's t between storm 1

and storms 2–-4 regressions = 6.49

Log10 streamflow, in cubic feet per second

0

50

100

150

200

250

300

350

012345

Total selenium, in micrograms per liter, as Se

Log10 streamflow, in cubic feet per second

Data points

Regression line

Data points

Regression line

Student's t between storm 1

and storms 2–-4 regressions = 7.88

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

Regression line

y = – 230 + 133x

R2 = 0.87

p < 0.01

y = – 112 + 83.6x

R2 = 0.46

p < 0.01

y = – 207 + 120x

R2 = 0.71

p < 0.01

F

ALL STORMS (n = 29):
Water Quality 25

4

4.5

5

5.5

6

012345

H

Log10 streamflow, in cubic feet per second

Log10 suspended sediment, in milligrams per liter

Data points

Regression line

Data points

Regression line

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

ALL STORMS (n = 29):

y = 3.95 + 0.36x

R2 = 0.87

p < 0.01

y = 3.58 + 0.51x

R2 = 0.87

p < 0.01

y = 3.86 + 0.40x

R2 = 0.82

p < 0.01

Student's t between storm 1

and storms 2–-4 regressions = 10.05

Figure 9. Continued.

0

50

100

150

200

250

300

350

0 50,000 100,000 150,000 200,000 250,000 300,000

Suspended sediment, in milligrams per liter

Suspended selenium (calculated), in micrograms per liter as Se

I Data points

Regression line

Data points

Regression line

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

ALL STORMS (n = 28):

y = 47.8 + 0.0008x

R2 = 0.42

p = 0.06

y = 20.4 + 0.0006x

R2 = 0.62

p < 0.01

y = 11.8 + 0.0007x

R2 = 0.82

p < 0.01

Student's t between storm 1

and storms 2–-4 regressions = 0.99
26 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

Figure 9. Continued.

0

50

100

150

200

250

300

350

0 50,000 100,000 150,000 200,000 250,000 300,000

Suspended sediment, in milligrams per liter

Total selenium, in micrograms per liter as Se

J Data points

Regression line

Data points

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

y = 54.6 + 0.001x

R2 = 0.59

p = 0.02

y = 39.1 + 0.0007x

R2 = 0.66

p < 0.01

Regression line

ALL STORMS (n = 28):

y = 25.7 + 0.0009x

R2 = 0.84

p < 0.01

Student's t between storm 1

and storms 2–-4 regressions = 1.52

40

45

50

55

60

65

70

75

80

012345

log10 streamflow , in cubic feet per second

Suspended sediment sieve diameter, % finer than 0.062 millimeter, in percent

Data points K

Regression line

Data points

Regression line

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

ALL STORMS (n = 29):

y = 87.8 – 8.71x

R2 = 0.33

p = 0.10

y = 75.2 – 6.70x

R2 = 0.14

p = 0.10

y = 72.8 – 5.23x

R2 = 0.11

p = 0.07

Student's t between storm 1

and storms 2–-4 regressions = 1.51
Water Quality 27

Dependent variable Fig. 

9

All storm data;

storm 1 only; or

storms 2–4 only

Regression equation n R 2 p

Student's t

between

storm 1 and

storms 

2–4 

Specific conductance A All storm data 1,717 + 238.6 (log10 streamflow) 29 0.08 0.14 —

Storm 1 only 1,200 + 313.3 (log10 streamflow) 9 0.24 0.18 3.07

Storms 2–4 only 1,044 + 523.4 (log10 streamflow) 20 0.23 0.03 —

Dissolved selenium B All storm data 2.47 + 0.012 (specific conductance) 29 0.18 0.02 —

Storm 1 only –30.3 + 0.031 (specific conductance) 9 0.54 0.02 8.79

Storms 2–4 only –0.43 + 0.011 (specific conductance) 20 0.47 <0.01 —

Total selenium C All storm data 143 – 0.0017 (specific conductance) 29 <0.01 0.96 —

Storm 1 only 72.2 + 0.057 (specific conductance) 9 0.07 0.5 0.32

Storms 2–4 only 38.6 + 0.030 (specific conductance) 20 0.08 0.22 —

Dissolved selenium D All storm data –12.0 + 14.7 (log10 streamflow) 29 0.45 <0.01 —

Storm 1 only –14.5 + 16.0 (log10 streamflow) 9 0.36 0.09 2.62

Storms 2–4 only –2.7 + 11.1 (log10 streamflow) 20 0.45 <0.01 —

All storm data –21.3 + 10.1 (log10 streamflow) + 

0.0095 (specific conductance)

29 0.48 <0.01 —

Storm 1 only –43.3 + 8.46 (log10 streamflow) + 

0.024 (specific conductance)

9 0.61 0.06 NA

Storms 2–4 only –9.1 + 4.59 (log10 streamflow) + 

0.0098 (specific conductance)

20 0.67 <0.01 —

Log10 (suspended 

selenium)

E All storm data 0.92 + 0.37 (log10 streamflow) 29 0.74 <0.01 —

Storm 1 only 1.03 + 0.35 (log10 streamflow) 9 0.88 <0.01 1.20

Storms 2–4 only 1.02 + 0.33 (log10 streamflow) 20 0.56 <0.01 —

Total selenium F All storm data –207 + 120 (log10 streamflow) 29 0.71 <0.01 —

Storm 1 only –230 + 133 (log10 streamflow) 9 0.87 <0.01 7.88

Storms 2–4 only –112 + 83.6 (log10 streamflow) 20 0.46 <0.01 —

Suspended selenium/

suspended 

sediment

G All storm data 1.37 – 0.152 (log10 streamflow) 28 0.08 0.35 —

Storm 1 only 1.08 + 0.031 (log10 streamflow) 9 <0.01 0.91 6.49

Storms 2–4 only 1.94 – 0.381 (log10 streamflow) 20 0.38 <0.01 —

Table 3. Summary of regressions between water quality constituents

[Storm data (column 3): storm 1, data for 2/3–4, 1998; storm 2, data for 2/6–7, 1998; storm 3, data for 2/21–22, 1998; storm 4, data for 

2/23–24, 1998. Cells highlighted in yellow are significantly different at 95-percent confidence level (that is, >2.06 [for 

α = 0.05, n1 = 9, n2 = 20]). Cells highlighted in green are regression equations and are basis for load predictions. n, number of samples; 

—, not applicable; NA, cannot be calculated for multiple linear regression. p, number of coefficients in regression model; R2, correlation 

coefficient; Student’s t, test statistic. mm, millimeter; <, less than]

t

α 2 n1 n2 ⁄ , + – 4
28 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

highlighted in green in table 3. Log suspended 

sediment is well-correlated with log streamflow (fig. 

9H). Total selenium is well-correlated with log 

streamflow (fig. 9F) and is not correlated with specific 

conductance (fig. 9C). Dissolved selenium is about as 

well-correlated with specific conductance as with log 

streamflow; the correlation is better with specific 

conductance for storm 1 (figs. 9B and 9D). Thus, a 

simple linear regression with log streamflow is 

sufficient for explaining suspended sediment and total 

selenium concentrations, while a multiple linear 

regression with log streamflow and specific 

conductance was evaluated for explaining dissolved 

selenium concentrations. For all storm data, the 

multiple linear regression was not a substantial 

improvement over the simple linear regression 

(table 3). The multiple linear regression improved the 

goodness of fit for storm 1 and storms 2–4, but did not 

affect the significance of correlation. The multiple 

linear regression equation for storm 1 was still 

nonsignificant (table 3).

LOADS

Suspended Sediment

The instantaneous loading rates of suspended 

sediment at the time of sampling closely follow the 

storm hydrographs (fig. 10). These instantaneous 

loading rates are calculated as:

Suspended-sediment loading rate (tons/day) = 0.0027 

(conversion factor) 

suspended sediment concentration 

(mg/L)  streamflow (ft3/s)

Regressions of the logarithm of suspendedsediment loading rate as a function of the logarithm of 

streamflow are very strong (fig. 11). The regression 

using all data points from the four storms (minus one 

outlier) had an R2 of 0.982, the regression for storm 1 

had an R2 of 0.990, and the regression for storms 2–4 

had an R2 of 0.983 (table 4).

Table 3. Summary of regressions between water quality constituents—Continued

Dependent variable Fig. 

9

All storm data;

storm 1 only; or

storms 2–4 only

Regression equation n R 2 p

Student's t

between

storm 1 and

storms 

2–4 

Log10 (suspended 

sediment)

H All storm data 3.86 + 0.40 (log10 streamflow) 29 0.82 <0.01 —

Storm 1 only 3.95 + 0.36 (log10 streamflow) 9 0.87 <0.01 10.05

Storms 2–4 only 3.58 + 0.51(log10 streamflow) 20 0.87 <0.01 —

Suspended selenium I All storm data 11.8 + 0.0007 (suspended sediment) 28 0.82 <0.01 —

Storm 1 only 47.8 + 0.0008 (suspended sediment) 9 0.42 0.06 0.99

Storms 2–4 only 20.4 + 0.0006 (suspended sediment) 20 0.62 <0.01 —

Total selenium J All storm data 25.7 + 0.0009 (suspended sediment) 28 0.84 <0.01 —

Storm 1 only 54.6 + 0.001 (suspended sediment) 9 0.59 0.02 1.52

Storms 2–4 only 39.1 + 0.0007 (suspended sediment) 20 0.66 <0.01 —

Suspended sediment 

sieve diameter

<0.062 mm

K All storm data 72.8 – 5.23 (log10 streamflow) 29 0.11 0.07 —

Storm 1 only 87.8 – 8.71(log10 streamflow) 9 0.33 0.1 1.51

Storms 2–4 only 75.2 – 6.70 (log10 streamflow) 20 0.14 0.1 —
Loads 29

0600 1200 1800 2400 0600 1200 1800 2400 0600 1200

2 3 4

February 1998, time and date

0

12,000

2,000

4,000

6,000

8,000

10,000

Streamflow, in cubic feet per second

0

5,000,000

1,000,000

2,000,000

3,000,000

4,000,000

Suspended sediment loading rate, instantaneous, in tons per day

Streamflow A

Suspended sediment loading rate,

instantaneous

Load curve (method 3)

Figure 10. Streamflow and suspended sediment loading rate in Panoche Creek, California, for February 1998 storms. 

A. February 2–4.

B. February 6 and 7.

C. February 21 and 22.

D. February 23 and 24. 

0800 1200 1600 2000 2400 0400 0800 1200

6 7

0

3,000

500

1,000

1,500

2,000

2,500

0

2,000,000

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

1,600,000

1,800,000

Streamflow, in cubic feet per second

B

February 1998, time and date

Suspended sediment loading rate, instantaneous, in tons per day

Streamflow

Suspended sediment loading rate,

instantaneous

Load curve (method 3)
30 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

1200 1600 2000 2400 0400 0800 1200 1600

21 22

0

1,000

200

400

600

800

0

500,000

100,000

200,000

300,000

400,000

C

February 1998, time and date

Streamflow, in cubic feet per second

Suspended sediment loading rate, instantaneous, in tons per day

Streamflow

Suspended sediment loading rate,

instantaneous

Load curve (method 3)

Figure 10. Continued.

1200 1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200

23 24

0

2,000

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0

1,000,000

200,000

400,000

600,000

800,000

D

February 1998, time and date

Streamflow, in cubic feet per second

Suspended sediment loading rate, instantaneous, in tons per day

Streamflow

Suspended sediment loading rate,

instantaneous

Load curve (method 3)
Loads 31

Suspended-sediment loads for each storm were 

calculated using three different methods: (1) the 

regression equation for all data (table 4) with the 15-

minute streamflows for the I-5 gage, (2) the regression 

equations for storm 1 and for storms 2–4 (table 4) with 

the 15-minute streamflows for the I-5 gage, and (3) 

connecting instantaneous loads in figures 10A through 

10D and integrating under the curves.

The average suspended-sediment loads 

calculated using the three methods for the four storms 

were 1.72, 0.31, 0.06, and 0.11 million tons for a total 

of 2.21 million tons (table 5). The 26 hours of storm 

runoff on February 3 and 4 accounted for 78 percent of 

the total load. The load calculated by method 3 for the 

first storm was much lower than loads calculated by 

methods 1 and 2. Because of this, method 3 provided 

the lowest estimate of total suspended-sediment load 

overall. The total load estimates of methods 1 and 2 

were 42 and 27 percent higher than method 3, 

respectively. Method 3 is susceptible to considerable 

errors because of the inadequate sampling frequency, 

while methods 1 and 2 incorporate a streamflow versus 

concentration relationship that is applied to 15-minute 

streamflow data to better describe the loads during the 

entire storm duration. Because of this and the very 

strong regressions in methods 1 and 2, these methods 

are preferable to method 3. Because method 2 

incorporates information on the variability between 

storms, it is probably the preferred method for 

calculating loads in this circumstance.

Selenium

The instantaneous loading rates of dissolved and 

total selenium at the time of sampling also closely 

follow the storm hydrographs (fig. 12). These 

instantaneous loading rates are calculated as:

Selenium loading rate (lb/d) = 0.0054

(conversion factor) 

selenium concentration (μg/L) 

streamflow (ft3/s)

Figure 11. Regressions for suspended sediment loading rate as a function of streamflow in Panoche Creek, California.

4

5

6

7

234

Data points

Regression line

Data points

Regression line

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 19):

ALL STORMS (n = 29):

y = 1.3852 + 1.3579x

R2 = 0.990

p < 0.001

y = 1.0078 + 1.5122x

R2 = 0.983

p < 0.001

y = 1.2935 + 1.4021x

R2 = 0.982

p < 0.001

Log10 suspended sediment loading rate, instantaneous, in tons per day

Log10 streamflow, in cubic feet per second

95 percent confidence interval (all storms)
32 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998 Table 4. Summary of regressions for calculating loading rates of suspended sediment, dissolved selenium, and total selenium

[Storm data (column 3): storm 1, data for 2/3–4, 1998; storm 2, data for 2/6–7, 1998; storm 3, data for 2/21–22, 1998; storm 4, data for 2/23–24, 1998p, number of coefficients in regression model; 

R2, correlation coefficient; student’s t, test statistic. <, less than]

Dependent variable Figure

All storm data;

storm 1 only; or

storms 2–4 only

Regression equation n 

Log10 (suspended sediment 

loading rate)

11 All storm data 1.2935

+ 1.4021 (log10 streamflow) 29 

Storm 1 only 1.3852

+ 1.3579 (log10 streamflow) 9 

Storms 2–4 only 1.0078

+ 1.5122(log10 streamflow) 19 

Log10 (dissolved selenium 

loading rate)

13 All storm data –1.3708

+ 1.1951 (log10 streamflow) 29 

Storm 1 only –1.4195

+ 1.2117 (log10 streamflow) 9 

Storms 2–4 only –1.3198

+ 1.1758 (log10 streamflow) 20 

All storm data –1.5768

+ 1.1699 (log10 streamflow)

+ 0.0001 (specific conductance) 29 

Storm 1 only –1.7978

+ 1.1180 (log10 streamflow)

+ 0.0003 (specific conductance) 9 

Storms 2–4 only –1.4405

+ 1.1078 (log10 streamflow)

+ 0.0001 (specific conductance) 20 

Log10 (total selenium loading rate) 14 All storm data –1.1366

+ 1.3374 (log10 streamflow) 29 

Storm 1 only –1.0797

+ 1.3317 (log10 streamflow) 9 

Storms 2–4 only –1.0301

+ 1.2932 (log10 streamflow) 20 
Loads 33

Regressions of the logarithm of selenium loading 

rate (dissolved and total) as a function of the logarithm 

of streamflow are very strong (figs. 13 and 14). The 

regression for dissolved selenium using all data points 

from the four storms had an R2 of 0.973, the regression 

for storm 1 had an R2 of 0.959, and the regression for 

storms 2–4 had an R2 of 0.978 (table 4). For total 

selenium, the regression using all data points had an R2

of 0.982, the regression for storm 1 had an R2 of 0.995, 

and the regression for storms 2–4 had an R2 value of 

0.965 (table 4).

Selenium loads for each storm were calculated 

using the three different methods discussed for 

suspended-sediment loads: the average dissolved 

selenium load for the four storms were 695, 134, 38, 

and 51 lb for a total of 918 lb (table 5). The first storm 

accounted for 76 percent of this dissolved selenium 

load. As for suspended-sediment loads, method 3 gave 

the lowest estimate overall because of a much lower 

load for the first storm. The total load estimates of 

methods 1 and 2 were 26 and 30 percent higher than 

method 3, respectively. Again, method 2, using the 

storm-specific regressions, is the preferred method.

Dissolved selenium concentrations were better 

predicted using storm-specific multiple linear 

regressions versus storm-specific simple linear 

regressions (table 3). Thus, a multiple linear regression 

also was evaluated for dissolved selenium loading rates 

as a function of log streamflow and specific 

conductance also. There was only a slight improvement 

in the fit over the simple linear regression (table 4). 

However, loading rates cannot presently be calculated 

with this equation because there is no continuous 

specific conductance recorder at the site. If a recorder 

were installed at the site in the future, a slight 

improvement in predictions of dissolved selenium 

loading rates might be realized.

The average total selenium load calculated by the 

three methods for the four storms were 4,385, 639, 159, 

and 237 lb for a total of 5,421 lb (table 5). The first 

Table 5. Loads of suspended sediment, dissolved selenium, and total selenium calculated by three different methods

[Method 1: simple linear regression of load as a function of streamflow using data from all storms. Method 2: simple linear regression of load as a function of 

streamflow using data from storm 1 and storms 2–4. Method 3: load determined from digitized area under curve for each specific storm]

Suspended sediment load (tons)

Storm 1 Storm 2 Storm 3 Storm 4 Total

Method 1 2,106,089 292,392 56,993 99,286 2,554,760

Method 2 1,765,694 334,937 66,127 108,620 2,275,379

Method 3 1,299,697 293,782 69,028 130,456 1,792,963

Average 1,723,827 307,037 64,049 112,787 2,207,701

Dissolved selenium load (pounds)

Storm 1 Storm 2 Storm 3 Storm 4 Total

Method 1 749 134 39 53 975

Method 2 774 141 39 53 1,007

Method 3 562 125 37 48 773

Average 695 134 38 51 918

Total selenium load (pounds)

Storm 1 Storm 2 Storm 3 Storm 4 Total

Method 1 4,438 683 159 243 5,523

Method 2 4,813 636 155 226 5,830

Method 3 3,904 600 163 243 4,909

Average 4,385 639 159 237 5,421
34 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

1200 1800 2400 0600 1200 1800 2400 0600 1200

2 3 4

February 1998, time and date

0

12,000

2,000

4,000

6,000

8,000

10,000

0

4,000

2,000

6,000

8,000

10,000

12,000

A 14,000

Streamflow

Total selenium loading rate,

instantaneous

Dissolved selenium loading rate,

instantaneous

Load curves (method 3)

Streamflow, in cubic feet per second

Selenium loading rate, instantaneous, in pounds per day

Figure 12. Streamflow and selenium loading rate in Panoche Creek, California, for February 1998 storms. 

A. February 2–4.

B. February 6 and 7.

C. February 21 and 22.

D. February 23 and 24.

0800 1200 1600 2000 2400 0400 0800 1200

6 7

3,000

500

0

1,000

1,500

2,000

2,500

0

3,000

500

1,000

1,500

2,000

2,500

B

February 1998, time and date

Streamflow, in cubic feet per second

Selenium loading rate, instantaneous, in pounds per day

Streamflow

Total selenium loading rate,

instantaneous

Dissolved selenium loading rate,

instantaneous

Load curves (method 3)
Loads 35

1200 1600 2000 2400 0400 0800 1200 1600

21 22

0

1,000

200

400

600

800

0

1,000

200

400

600

800

C

February 1998, time and date

Streamflow, in cubic feet per second

Streamflow

Total selenium loading rate,

instantaneous

Dissolved selenium loading rate,

instantaneous

Load curves (method 3)

Selenium loading rate, instantaneous, in pounds per day

Figure 12. Continued.

1200 1400 1600 1800 2000 2200 2400 0200 0400 0600 0800 1000 1200

23 24

0

2,000

200

400

600

800

1,000

1,200

1,400

1,600

1,800

0

1,400

200

400

600

800

1,000

1,200

D

February 1998, time and date

Streamflow, in cubic feet per second

Selenium loading rate, instantaneous, in pounds per day

Streamflow

Total selenium loading rate,

instantaneous

Dissolved selenium loading rate,

instantaneous

Load curves (method 3)
36 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

storm accounted for 81 percent of this total selenium 

load. As for suspended-sediment and dissolved 

selenium loads, method 3 gave the lowest estimate 

overall because of a much lower load for the first 

storm. The total load estimates of methods 1 and 2 

were 12 and 19 percent higher than method 3, 

respectively. Again, method 2, using the storm-specific 

regressions, is the preferred method. Because 

suspended selenium was calculated as the difference 

between total selenium and dissolved selenium, the 

average suspended selenium load would be 4,503 lb, or 

83 percent of the total selenium load.

Load Prediction Methods

Although method 2 is preferred for estimating 

storm loads after the fact, at this time, method 1 is 

probably a better prediction tool for estimating loads in 

future storms. The regression equation used in method 

1 incorporates the variability in data from four storms 

and is likely to be more representative of future storms. 

All of the regression equations would be greatly 

enhanced by the addition of more storm data from 

different years. However, this data is sparse because 

many years do not have significant storm runoff and the 

rising limbs of the storm hydrographs are very shortlived. More data showing differences in concentrations 

between the rising and falling limbs of the storm 

hydrograph would be especially useful. The data from 

the February 1998 storms was mixed with respect to 

this issue and, thus, separation of rising and falling 

limbs of hydrographs was not incorporated into the 

prediction equations presented here. In general, most 

concentrations were greater on the rising limb, but 

there were enough exceptions to make splitting the 

regressions problematic. With additional data, it 

probably would be possible to come up with separate 

regressions for the rising and falling limbs. Also, storm 

1 in February 1998 was extremely unusual, and having 

more storm data from other years and flow conditions, 

would help to refine the prediction regressions.

Figure 13. Regressions for dissolved selenium loading rate as a function of streamflow in Panoche Creek, California.

1.0

1.5

2.0

2.5

3.0

3.5

4.0

2.0 2.5 3.0 3.5 4.0

Log10 streamflow, in cubic feet per second

Log10 dissolved selenium loading rate, instantanous, in pounds per day

Data points

Regression line

Data points

Regression line

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

ALL STORMS (n = 29):

y = – 1.4195 + 1.2117x

R2 = 0.959

p < 0.001

y = – 1.3198 + 1.1758x

R2 = 0.978

p < 0.001

y = – 1.3708 + 1.1951x

R2 = 0.973

p < 0.001

95% confidence interval (all storms)
Loads 37

The suspended-sediment load calculated by 

method 1 was 12 percent higher than the load 

calculated by method 2. The loads calculated by 

method 1 were 3 and 5 percent lower than the loads 

calculated by method 2 for dissolved selenium and 

total selenium, respectively. The proposed loading rate 

prediction equations (from method 1) for suspended 

sediment, dissolved selenium, and total selenium in 

Panoche Creek at I-5 are:

log10 (suspended sediment loading rate,

in tons/day) = 1.4021 log10 (streamflow,

in ft3/s) + 1.2935 (1)

log10 (dissolved selenium loading

rate, in lb/day) = 1.1951 log10

(streamflow, in ft3/s) – 1.3708 (2)

log10 (total selenium loading rate,

in lb/day) = 1.3374 log10

(streamflow, in ft3/s) – 1.1366 (3)

The applicable range of streamflows for these 

prediction equations is about 150 to 9,500 ft3/s. At 

streamflows below 150 ft3/s, the predicted loading rates 

are less than about 22,000 tons/day suspended 

sediment, 17 lb/d dissolved selenium, and 59 lb/d total 

selenium. These loading rates are minor compared with 

the loading rates during relatively high streamflows.

The uncertainty in loading rates calculated using 

equations (1) through (3) was evaluated by looking at 

the 95-percent confidence intervals for the regressions. 

This interval is shown graphically for equations (1) 

through (3) in figures 11, 13, and 14, respectively. The 

95-percent confidence interval expressed in logtransformed units (i.e., where is the predicted log10

of the load) for equations (1) through (3) are 

0.194, 0.206, and 0.188, respectively. 

The 95-percent confidence interval expressed in the 

original untransformed units (i.e., where is the 

predicted untransformed load) for equations (1) 

through (3) are 0.64 to 1.56 , 0.62 to 1.61 , and 

0.65 to 1.54 , respectively.

yˆ

yˆ yˆ yˆ

yˆ

yˆ yˆ yˆ yˆ

yˆ yˆ

Figure 14. Regressions for total selenium loading rate as a function of streamflow in Panoche Creek, California.Figure 13.Regressions for dissolved 

selenium loading rate as a function of streamflow in Panoche Creek, California.

1

2

3

4

5

2 2.5 3 3.5 4

Log10 streamflow, in cubic feet per second

Log10 total selenium loading rate, instantaneous, in pounds per day

Data points

Regression line

Data points

Regression line

Regression line

STORM 1 (n = 9):

STORMS 2–4 (n = 20):

ALL STORMS (n = 29):

y = – 1.0797 + 1.3317x

R2 = 0.995

p < 0.001

y = – 1.0301 + 1.2932x

R2 = 0.965

p < 0.001

y = – 1.1366 + 1.3374x

R2 = 0.982

p < 0.001

95% confidence interval (all storms)
38 Selenium and Sediment Loads in Storm Runoff in Panoche Creek, California, February 1998

SUMMARY AND CONCLUSIONS

The rainfall and streamflow during February 

1998 were exceptionally high. The rainfall total of 

10.40 in. for the month was greater than any other 

month during 1957 to 2000. The peak streamflows on 

February 3 and 7 (9,940 and 8,550 ft3/s, respectively) 

both exceeded the previous peak streamflow of 

5,400 ft3/s at the upstream gage during 1950–53 and 

1959–70. The peak streamflow during water years 

1999 and 2000 was only 188 ft3/s.

In addition to 32 environmental samples 

collected during February 1998, eight quality control 

samples were collected (four field blanks and four 

replicates). No contamination was found in the field 

blanks, and the RPDs for dissolved selenium replicates 

ranged from 0.0 to 18.2 percent. However, the RPDs 

for four total selenium replicates and four reruns 

ranged from 0.0 to 57.1 percent. This variability in total 

selenium was attributed to the extremely high 

concentrations of suspended sediment in the samples 

and to possible interference from other compounds.

The suspended-sediment load transported in the 

four storms of February 1998 was about 2.21 million 

tons. About 78 percent of this load was transported 

during 26 hours of storm runoff on February 3 and 4. 

The dissolved selenium load transported in the four 

storms was about 918 lb, with about 76 percent of it 

transported during the first storm. The total selenium 

load in the four storms was about 5,421 lb, with about 

81 percent transported during the first storm. 

Suspended selenium accounted for about 83 percent of 

the overall total selenium load. 

Loading rate prediction equations were 

developed for suspended sediment, dissolved selenium, 

and total selenium as a function of streamflow at the I-5 

gaging station. The utility of these equations would be 

improved by additional storm sampling data. This 

improvement would not only provide better estimates, 

but also possibly would allow development of separate 

equations for the rising and falling limbs of the storm 

hydrographs.

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