Runoff Characteristics and Washoff Loads from 

Rainfall-Simulation Experiments on a 

Street Surface and a Native Pasture in the 

Denver Metropolitan Area> Colorado

U.Sv GEOLOGICAL SURVEY 1PROFESSIONAL PAPER 144*

Prepared in cooperation with the 

Denver Regional Council of Gpx)ernments

Runoff Characteristics and Washoff Loads from 

Rainfall-Simulation Experiments on a 

Street Surface and a Native Pasture in the 

Denver Metropolitan Area, Colorado

By MARTHA H. MUSTARD, SHERMAN R. ELLIS, and JOHNNIE W. GIBBS

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1441

Prepared in cooperation with the 

Denver Regional Council of Governments

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987
UNITED STATES DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging-in-Publication Data

Mustard, Martha H.

Runoff characteristics and washoff loads from rainfall-simulation experiments on a street surface and a native 

pasture in the Denver metropolitan area, Colorado.

(Geological Survey professional paper ; 1441) 

Bibliography: p. 

Supt. of Docs, no.: I 19.16:1441

1. Urban runoff Colorado Denver Metropolitan Area. I. Ellis, Sherman R. II. Gibbs, Johnnie W. 

III. Denver Regional Council of Governments. IV. Title: Runoff characteristics and washoff loads from rainfallsimulation experiments on a street surface and a native pasture in the Denver Metropolitan area, Colorado. 

VII. Series. 

GB991.C6M87 1986 551.48'8'0978883 86-600105

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425 

Denver, CO 80225
CONTENTS

Page

Glossary ........................................... v

Abstract ........................................... 1

Introduction ....................................... 1

Purpose ........................................ 1

Acknowledgments ............................... 2

Study area ..................................... 2

Denver Federal Center simulated-rainfall runoff study on a

street surface ..................................... 3

Study plots ..................................... 3

Data collection methods .......................... 3

Simulated rainfall ............................ 3

Runoff ...................................... 5

Constituent deposition ........................ 5

Related measurements ........................ 5

Runoff quantity ................................. 6

Runoff quality .................................. 9

Page

Denver Federal Center simulated-rainfall runoff study on a 

street surface Continued 

Constituent deposition and washoff loads ........... 19

Statistical analysis ............................... 23

Rooney Ranch simulated rainfall-runoff study on a native 

pasture .......................................... 24

Study plots ..................................... 24

Data collection methods .......................... 24

Simulated rainfall ............................ 24

Runoff ...................................... 24

Soil moisture ................................ 24

Runoff quantity ................................. 25

Infiltration rates ................................ 26

Water-quality relations ........................... 27

Conclusions ........................................ 29

References ......................................... 30

ILLUSTRATIONS

FIGURE 1. Map showing location of Denver Federal Center study site and Rooney Ranch study site ................... 2

2. Schematic of the Denver Federal Center plots showing sprinkler and flume locations ....................... 4

3-11. Graphs of runoff and washoff loads of selected water-quality constituents for Denver Federal Center:

3. Plot 1, June 2, 1980 ....................................................................... 6

4. Plot 2, June 2, 1980 ....................................................................... 7

5. Plot 3, June 3, 1980 ....................................................................... 8

6. Plot 4, June 3, 1980 ....................................................................... 9

7. Plot 5, June 4, 1980 ....................................................................... 10

8. Plot 6, June 4, 1980 ....................................................................... 11

9. Plot 7, June 5, 1980 ....................................................................... 12

10. Plot 8, June 5, 1980 ....................................................................... 13

11. Plot 9, June 6, 1980 ....................................................................... 14

12-18. Graphs showing:

12. Total suspended solids washoff loads and deposition measured by vacuum-strip sampling versus number

of days since street washing for the Denver Federal Center plots, June 2-6, 1980 ............... 16

13. Total lead washoff loads and deposition measured by vacuum-strip sampling versus number of days since

street washing for the Denver Federal Center plots, June 2-6, 1980 ........................... 17

14. Total zinc washoff loads and deposition measured by vacuum-strip sampling versus number of days since

street washing for the Denver Federal Center plots, June 2-6, 1980 ........................... 18

15. Total manganese washoff loads and deposition measured by vacuum-strip sampling versus number of days

since street washing for the Denver Federal Center plots, June 2-6, 1980 ...................... 19

16. Total nitrogen washoff loads and deposition measured by vacuum-strip sampling versus number of days

since street washing for the Denver Federal Center plots, June 2-6, 1980 ...................... 20

17. Total phosphorus washoff loads and deposition measured by vacuum-strip sampling versus number of days

since street washing for the Denver Federal Center plots, June 2-6, 1980 ...................... 21

18. Total organic carbon washoff loads versus number of days since street washing for the Denver Federal Center

plots, June 2-6, 1980 .................................................................. 22

19. Topographic maps of plots 1 and 2 at the Rooney Ranch rainfall simulation site ........................... 25

20. Schematic of the sprinkler layout at the Rooney Ranch plots ........................................... 26

21. Graphs of rainfall, runoff, and infiltration versus time for the simulated rainfall storms at the Rooney Ranch plots,

May 20, 1981 ................................................................................ 28

in
IV CONTENTS

TABLES

Page

TABLE 1. Characteristics of simulated rainfall on the Denver Federal Center plots, resulting runoff, and impervious retention 15

2. Washoff and mean washoff loads of selected water-quality constituents from plots 1-9 at the Denver Federal Center 15

3. Event mean concentrations of selected water-quality constituents in the runoff from plots 1-9 at the Denver Federal

Center and concentrations in the applied water ....................................................... 15

4. Maximum, mininrnim, and mean event mean concentrations for all storm runoff sampled at station 06711635 North Avenue

Storm Drain at the Denver Federal Center, at Lakewood in 1980 and 1981 ............................... 20

5. Deposition of selected water-quality constituents measured by vacuum-strip sampling, open-space bucket accumulation,

and street-bucket accumulation on the Denver Federal Center plots ...................................... 23

6. Correlation coefficients between selected water-quality constituent washoff loads and number of days since street washing, 

intensity of rainfall, total rainfall, open-space constituent deposition, street constituent deposition, and traffic for 

the Denver Federal Center plots .................................................................... 23

7. Antecedent soil moisture, rainfall, runoff, and runoff-to-rainfall ratios for the simulated rainstorms at Rooney Ranch and

natural rainstorms at station 06710610 Rooney Gulch at Rooney Ranch, 1980 and 1981 .................... 27

8. Soil-moisture and bulk-density data for the Rooney Ranch rainfall simulation plots ............................ 29

9. Loads of selected water-quality constituents and percentage of loads carried by the first 25 percent of the runoff from

the Rooney Ranch rainfall simulation plots ........................................................... 30

10. Event mean concentrations of selected water-quality constituents in the runoff from simulated rainfall at Rooney Ranch 

and the range of event mean concentrations in the runoff from natural rainstorms at station 06710610 Rooney Gulch 

at Rooney Ranch ................................................................................. 30
GLOSSARY

METRIC CONVERSION TABLE

Multiply inch-pound unit By To obtain metric unit

acre 4047 square meter

bar 100 kilopascal

cubic foot 0.02832 cubic meter

cubic foot per second 0.02832 cubic meter per second

(ft3/s)

cubic inch (in. 3 ) 16.39 cubic centimeter

foot (ft) 0.3048 meter

inch (in.) 25.40 millimeter

inch per hour (in./hr) 25.40 millimeter per hour

mile 1.609 kilometer

part per billion (ppb) 1 microgram per liter

part per million (ppm) 1 milligram per liter

pound (Ib) 0.4536 kilogram

pound per acre (Ib/acre) 1.121 kilogram per hectare

pound per acre per inch 0.004413 kilogram per cubic meter

(Ib/acre-in.)

pound per second (Ib/s) 0.4536 kilogram per second

pound per square inch 6.895 kilopascal

square foot (ft2 ) 0.09290 square meter

GLOSSARY

constituent deposition. Material deposited from the atmosphere on the land surface by gravity, 

precipitation, or wind.

correlation coefficient (r). A number that expresses the degree of correlation between two mathematical variables. r= 1 indicates perfect positive correlation, r=-l indicates perfect negative correlation, and r=0 indicates no correlation.

event mean concentration. The average concentration of a selected constituent in storm runoff 

determined as the storm load divided by the storm runoff.

first flush of constituent load. The occurrence of a disproportionately high portion of a constituent storm load being carried by the first part of the storm runoff. In this paper, a first flush of 

a constituent load is considered to have occurred if the first 25 percent of storm runoff carries 50 

percent or more of the constituent storm load.

impervious retention. The amount of water falling on impenetrable surfaces that does not runoff, 

toad The material that is moved or carried by a stream or the quantity of that material in a given 

time.

moisture retention capability. The weight of water per unit weight of soil that a soil will hold 

at 0.22 bars of soil moisture retention force (Miller and McQueen, 1978). It is roughly synonymous 

with field capacity.

rational method of runoff calculation. Method of determining runoff based on the rational formula. 

The rational formula states that the peak rate of runoff is equal to the intensity of rainfall times 

the drainage basin area times a runoff coefficient.

steady-state condition. The condition wherein thstant in magnitude and direction. 

Thiessen polygon method. A. method of calculating basin rainfall by weighting the rainfall measured 

at each rain gage by the percent of the basin area which is closer to that rain gage than to any 

other rain gage.

washoff load. The part of the constituent load carried by the runoff that has been washed off the 

basin, as opposed to the portion that is present in the runoff due to presence of the constituent 

in the applied water.

RUNOFF CHARACTERISTICS AND WASHOFF LOADS FROM

RAINFALL-SIMULATION EXPERIMENTS ON A

STREET SURFACE AND A NATIVE PASTURE IN THE

DENVER METROPOLITAN AREA, COLORADO

By MARTHA H. MUSTARD, SHERMAN R. ELLIS, and JOHNNIE W. GIBBS

ABSTRACT

Rainfall simulation studies were conducted in conjunction with the 

Denver Regional Urban Runoff Program to: (1) Compare runoff quantity and quality from two different intensities of rainfall on impervious plots having identical antecedent conditions, (2) document a first 

flush of constituent loads in runoff from 1,000-square-foot streetsurface plots, (3) compare runoff characteristics from a street surface 

subjected to simulated rainfall with those from a 69-acre urban basin 

of mixed land use subjected to natural rainfall, (4) perform statistical 

analysis of constituent loads in the runoff with several independent 

variables, and (5) compare the quantity and quality of runoff from 

400-square-foot plots of native grasses used for pasture and subjected 

to simulated rainfall with that from a 405-acre basin covered with 

native grasses used for pasture and sub-jected to natural rainfall.

The rainfall simulations conducted on the street surface showed that 

higher intensity simulated rainfall produced a higher percentage of 

runoff than lower intensity rainfall. A first flush of constituent loads 

occurred for most constituents in the runoff from most rainfall simulations on the street surface; however, a first flush did not occur in the 

runoff from simulated rainfall on the pasture. The event mean concentrations of constituents in the runoff from simulated storms on 

the street surface were generally much smaller than the event mean 

concentrations of constituents in the runoff from an adjacent urban 

basin.

Analysis of the data from the rainfall simulations on a street surface indicates that intensity of rainfall and total rainfall are important variables determining constituent loads. The design of the 

experiment was such that intensity of rainfall and total rainfall were 

highly correlated, thus precluding the development of useful regression equations to predict washoff loads.

The quality of runoff from the simulated rainfall on the pasture was 

influenced by the disturbed perimeters of the plots; however, the 

runoff-to-rainfall ratios for the simulated storms fell within the range 

of ratios measured for natural storms over the adjacent 405-acre basin.

INTRODUCTION

From 1979 through 1982, the U.S. Geological Survey, 

in cooperation with the Denver Regional Council of 

Governments (DRCOG), participated in the Denver

Regional Urban Runoff Program as part of the Nationwide Urban Runoff Program (NURP) sponsored by the 

U.S. Environmental Protection Agency and the U.S. 

Geological Survey. The purposes of the nationwide program were to:

1. Collect data to establish a data base for typical 

urban drainage basins;

2. Determine the water-quality constituent loadings from these typical basins;

3. Develop methods of estimating loadings from 

ungaged basins; and

4. Test the effectiveness of various Best Management Practices (U.S. Geological Survey/U.S. 

Environmental Protection Agency Technical 

Coordination Plan on urban runoff studies, 

written commun., 1979).

The U.S. Geological Survey collected storm runoff 

data from seven small basins in the metropolitan 

Denver area while DRCOG collected storm runoff data 

and dry-weather flow data at four stations on the South 

Platte River and at one tributary in metropolitan 

Denver (Gibbs, 1981; Gibbs and Doerfer, 1982). Regression equations for urban runoff based on precipitation 

variables and drainage basin characteristics have been 

developed from these data (Ellis and others, 1984).

PURPOSE

As a special study in the Denver Regional Urban 

Runoff Program, the U.S. Geological Survey conducted 

simulated-rainfall runoff studies at two locations in 

metropolitan Denver, Colorado. The first location was 

a moderately used, paved street surface on the Denver 

Federal Center and the second was a pasture of native 

grass used for horse grazing at Rooney Ranch.

i
RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

105°15' 105°00' 104"45'

envere "FedeVal Center! ^ADENVER

N. ^* rt*i i j-K ^nl.fc.n \ - A \ »^

15 MILES

R 71 W R 70 W R 69 W R 68 W R 67 W R 66 W

FIGURE 1. Location of Denver Federal Center study site and Rooney Ranch study site.

The objectives of the study on the street surface were 

to:

1. Compare the runoff quantity and quality 

resulting from simulated rainfall intensities of 

approximately one-half inch per hour and two 

inches per hour;

2. Determine if a first flush of constituent loads 

was observed;

3. Compare the runoff characteristics from the 

street surface with those of an adjacent basin 

monitored for natural storm runoff over a 

period of 2 years;

4. Compare the runoff constituent loads to the constituent deposition; and

5. Perform statistical analysis of washoff constituent loads with several independent 

variables.

The objective of the study on the pasture was to 

compare runoff volumes and water-quality loads, 

normalized to area, and concentrations from the 

400-square-foot plots with those from a 405-acre 

monitored basin. The runoff quantity and quality were 

compared to test the validity of data extrapolation from 

small basins.

ACKNOWLEDGMENTS

The Colorado Department of Highways supplied the 

traffic counter used in the Denver Federal Center study. 

The DRCOG conducted the vacuum sampling part of 

the Denver Federal Center study. William Jackson of 

the Special Studies Group, Colorado River Salinities 

Study, U.S. Bureau of Land Management, provided the 

equipment and assisted in the operation of the Rooney 

Ranch study. Gregg Lusby of the Public Lands Hydrology Program, Water Resources Division, U.S. Geological Survey, provided the equipment and personnel to 

conduct the Denver Federal Center rainfall simulations.

STUDY AREA

The study was conducted at two locations in the 

Denver metropolitan area, Colorado (fig. 1). The first 

location is the eastbound lane of North Avenue on the 

Denver Federal Center in Lakewood. The second location is Rooney Ranch, a pasture used for horse grazing 

about 4 miles southwest of the Denver Federal Center 

site. Both locations are on the edge of the plains next
SIMULATED RAINFALL ON A STREET SURFACE

to the foothills of the Rocky Mountains. The climate 

is semiarid, and about 14 inches of precipitation fall in 

an average year.

These locations were chosen because of their proximity to larger basins studied in the NURP. Data for the 

rainfall simulation study on a street surface were collected at the Denver Federal Center. In addition, data 

on natural rainfall runoff from an adjacent 69-acre urban basin were collected during a 2-year period at station 06711635 North Avenue Storm Drain at Denver 

Federal Center as another part of the NURP.

Data for the rainfall simulation study on a native 

pasture were collected at two 400-square-foot plots at 

Rooney Ranch. These rainfall simulation plots are approximately 1,000 feet downstream from a 405-acre 

basin monitored for rainfall runoff during a 2-year 

period at station 06710610 Rooney Gulch at Rooney 

Ranch as another part of the NURP.

DENVER FEDERAL CENTER

SIMULATED-RAINFALL RUNOFF STUDY

ON A STREET SURFACE

STUDY PLOTS

North Avenue, a four-lane major access to the Denver 

Federal Center, was selected as the street site. A 

grassed, elevated median divides the two westbound 

and the two eastbound lanes. Except during extremely 

intense rainfall, rain falling on the median does not run 

off to the street. There is a concrete curb on the median 

side of the street and an asphalt curb on the other side. 

The asphalt surface of the street was in good condition, 

having been repaired prior to the study. The study plots 

were in the eastbound lanes which are 25-feet wide and 

slope downward 4.9 percent from west to east and 1.7 

percent from north to south.

The street was divided into nine study plots of 25 feet 

by 40 feet with buffer zones of 50 to 60 feet between 

each plot to ensure that the simulated rainfall intended 

for one plot did not fall on another. The curbs provided 

boundaries on the north and south and other boundaries 

were created on the east and west ends using wooden 

boards backed with sand bags, covered with plastic 

sheets, and sealed with roofing sealant and duct tape. 

A 1-inch cut-throat flume was installed at the outfall 

of each plot. The slope of each plot is given below:

Plot

1

2

3

4

5

6

7

8

9

Slope (percent)

North to South

1.6

1.1

1.4

6.5

2.4

2.4

2.4

4.1

2.1

West to East

3.6

4.1

5.2

2.9

5.4

5.5

5.4

6.7

5.4

DATA COLLECTION METHODS

On June 1, 1980, the street used for this study was 

washed down by a fire hose and for the next 5 days 

simulated rainfall experiments were conducted on sections of the street using an assembly described by 

Lusby (1977). One storm of one-half inch per hour intensity and one storm of 2 inches per hour intensity 

were generated on each day for the 4 succeeding days 

and one rainfall simulation of one-half inch per hour intensity was conducted on the 5th day. Rainfall, runoff, 

and selected constituent deposition were measured. 

Samples of runoff were collected for water-quality 

analysis. Constituent deposition was measured by use 

of collection buckets and by vacuuming of the street 

by DRCOG personnel.

The 840-foot length of street was washed using highpressure fire hoses, 1 day prior to the first rainfall 

simulations, thus allowing approximately 1 day of constituent depositon. Starting at the lower end of the 

street, a rainfall storm was simulated on each plot and 

the resulting runoff was measured and sampled. The 

first simulation each day had an intensity of approximately one-half inch per hour and lasted about 1 hour. 

The second simulation was on the nearest plot, had an 

intensity of approximately 2 inches per hour, and lasted 

about one-half hour. There were five storms of about 

one-half inch per hour intensity and four storms of 

about 2 inches per hour intensity. There was no natural 

rainfall during the time of the study.

SIMULATED RAINFALL

Rainfall was simulated using equipment described by 

Lusby (1977). Water was supplied by a tank truck and 

supplemental storage bags through a feeder hose and 

supply lines connected to sprinklers set on 10-foot 

risers. The sprinklers were arranged in a grid to allow 

rain from adjacent sprinklers to overlap. The pressure
RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

at each sprinkler was regulated to approximately 

28 pounds per square inch to provide uniform coverage at an intensity of about 2 inches per hour. The 

system was modified by deleting some sprinklers for 

the one-half inch per hour intensity storms. These 

intensities were target intensities the actual intensities were somewhat different, primarily because 

the relative elevations of the sprinklers to the tank

truck varied, and the feeder hoses leaked during the 

simulated rainfall on plots 5 and 7. The leaking 

feeder hoses did not introduce any water onto the plots. 

The rainfall was momentarily halted and the leaks 

repaired. Hose supports made of reinforcing bars 

elevated the supply lines above the street surface to prevent overland-flow restrictions. Figure 2 illustrates the 

layout of the sprinklers.

O Curb O O

Street

Flume

Curb

O O

O Sprinkler

0 10 20 FEET

Plots receiving about one-half inch per hour simulated rainfall

O

O O 

Curb

O O O O

O O O

s

O

Street

im

1

H

i ;i

iE

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O O

Flume

O

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O O O O

O O O O O

O Sprinkler

10

I

20 FEET

Plots receiving about 2 inches per hour simulated rainfall 

FIGURE 2. Denver Federal Center plots showing sprinkler and flume locations.
SIMULATED RAINFALL ON A STREET SURFACE

The simulated rainfall was measured using five 

storage rain gages in each plot. The average rainfall on 

each plot was calculated using the Theissen polygon 

method (Linsley and Franzini, 1972).

RUNOFF

The flow was measured at the outfall of each plot 

using a 1-inch cut-throat flume (Holtan and others, 

1962). The flume was rated at the study site using 

volumetric measurements. Discrete water-quality 

samples were collected by hand at the flume outflow and 

were analyzed for total suspended solids, total lead, 

total zinc, total manganese, total nitrogen, total phosphorus, and total organic carbon. The analyses were performed by the U.S. Geological Survey Denver Central 

Laboratory according to "Techniques of WaterResources Investigations of the United States Geological Survey" (U.S. Geological Survey, 1979). The total 

recoverable method was used for trace element 

analyses, and the total phase method was used for all 

other constituents.

CONSTITUENT DEPOSITION

Constituent deposition was determined daily by the 

amount of constituent accumulation in plastic buckets 

which had been scrubbed, rinsed with distilled water, 

and then placed either along the street or approximately 

300 feet from the street in an open space. Twenty clean 

buckets were placed during the day of street washing. 

On each day of rainfall simulation, two buckets from 

along the street were collected, their contents combined, and the contents analyzed by the U.S. Geological Survey Denver Central Laboratory. The constituents selected for analysis and the analytical methods 

used were the same as those for the runoff. The contents of the buckets in the open space were treated 

similarly.

Deposition of constituents was also determined by 

measuring the amounts of constituents vacuumed from 

the street surface each day by DRCOG personnel. Each 

day, for the 5-day period of rainfall simulations, a 

measured area of the street surface adjacent to the area 

selected for study was vacuumed and the contents of 

the vacuum cleaner sent to the Soils Testing Laboratory, Colorado State University at Fort Collins, Colo. 

for chemical analysis. The method of analysis for trace 

elements, unfortunately, did not correspond to the 

method used for the water samples analyzed in this 

study. The plant-available method (Lindsay and 

Norvell, 1978), a mild digestion, was used for the 

vacuumed samples. To compare the data from the

vacuumed samples with all other trace-element data in 

this study, replicates of five vacuumed samples of street 

deposition were analyzed by both methods and average 

adjustment factors determined. Adjustment factors of 

2.1 for lead, 5.9 for zinc, and 11 for manganese were 

developed from the replicate data. The plant-available 

method results were multiplied by these adjustment factors for comparability. Analytical methods for total 

nitrogen, total phosphorus, and total suspended solids 

were consistent with analysis methods for all other 

samples collected in the study. The Soils Testing 

Laboratory was unable to analyze for total organic 

carbon.

RELATED MEASUREMENTS

The temperatures of the street surface, runoff, and 

air were measured for each simulated storm. The street 

and air temperatures were measured before the simulated rainfall began. The runoff temperature was measured near the end of the runoff period. The data 

collected during this experiment are given below and 

may be used by other personnel for different 

applications.

Plot

Street

Temperature, in degrees Celsius 

Runoff Air

1

2

3

4

5

6

7

8

9

17.0

26.5

23.5

25.5

26.0

33.5

29.0

28.0

27.0

24.0

25.0

27.5

24.0

29.5

30.5

29.0

23.0

32.0

16.0

23.5

23.5

25.5

23.5

28.0

26.5

29.0

27.0

Vehicular traffic between the time of street washing 

and each rainfall simulation was measured by a traffic 

counter. The street was open to incoming rush-hour traffic each morning. The eastbound lanes were then closed 

to traffic only for the duration of the two experiments. 

The street was reopened by lunchtime. The traffic data 

are given below:

Plot Traffic since street washing, in number of vehicles

1, 2 

3,4 

5, 6

7, 8 

9

943

2,518

4,366

6,327

7,990
RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

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EXPLANATION

 Runoff

 Total suspended solids 

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EXPLANATION

 Total lead

 Total zinc

o Sample analyzed

EXPLANATION

 Total manganese

 Total nitrogen 

o Sample analyzed

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EXPLANATION

 Total phosphorus

 Total organic carbon 

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0.0 IFIGURE 3. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 1, June 2,1980.

RUNOFF QUANTITY

The characteristics of the simulated rainfall and the 

resulting runoff are presented in table 1. The runoff 

hydrograph for each plot is shown in figures 3-11. The 

average runoff-to-rainfall ratio was 0.82 for the lower 

rainfall intensity storms and 0.98 for the higher rainfall intensity storms. The difference between the ratios

may be partly due to: (1) More evaporation of the water 

from the street during the lower intensity rainfall than 

during the higher intensity rainfall, and (2) the impervious retention being a higher percentage of the rainfall during the lower intensity storms.

Another factor contributing to the difference between 

the two runoff-rainfall ratios may be the inaccuracy 

of flow measurements. The peak flows for each site
SIMULATED RAINFALL ON A STREET SURFACE

0.060

EXPLANATION

 Runoff

Total suspended solids 

o Sample analyzed

EXPLANATION

 Total lead 

Total zinc 

o Sample analyzed

EXPLANATION

 Total manganese

Total nitrogen 

o Sample analyzed

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EXPLANATION

 Total phosphorus

 Total organic carbon 

o Sample analyzed

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FIGURE 4. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 2, June 2,1980.

were predicted using the rational method (Linsley and 

Franzini, 1972) which states that the runoff rate from 

impervious surfaces will approach the rate of rainfall if the rainfall rate is constant. The measured 

peak flows were generally less than the predicted 

peak flows. Analysis of the peak-flow data shown 

in table 1 suggests that the rating of the cut-throat 

flume may have been low for the range of stage

measured during the one-half inch per hour intensity 

storm.

Two methods were used to determine the impervious 

retention at each site. The first method was to calculate 

the quantity of simulated rainfall normalized to area 

that fell before runoff began. The second method was 

to calculate: (a) The quantity of rainfall that fell before 

the runoff reached steady-state flow condition, (b) the
RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

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0^0

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0. - UJ

, Q- CO

0.000

2.0

1.0

0.0 

6.0

3.0

0.0 

4.0

3.0

2.0

1.0

0.0

900

EXPLANATION

 Runoff

 Total suspended solids 

o Sample analyzed

EXPLANATION

 Total lead

 Total zinc 

o Sample analyzed

EXPLANATION

 Total manganese

 Total nitrogen 

o Sample analyzed

EXPLANATION

 Total phosphorus

 Total organic carbon 

o Sample analyzed

1000 

TIME, IN HOURS

2.0

1.5

1.0

0.5

1.0

.

1100

CL QL O

W _ til

< ft0.0 g 

1.0

0.5

0.0 

2.0

to

z tf> <b

UJ Q r0.0

8.0 z

O 

00

cc. _

gO < Q »~ o.u ^ _,

4.0 <

5.2

OC - LU

o o w 

2.0 <<* _i in oc

0.0 H

FIGURE 5. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 3, June 3,1980.

volume of runoff before steady-state conditions were 

reached, and (c) the volume of runoff after rainfall 

ceased. The impervious retention calculated by this 

method is the difference between the quantity of rainfall and the sum of the volumes of runoff, all normalized to area. The results of these two methods and

their averages are presented in table 1. The average 

impervious retention for all nine plots was about 0.05 

inch. This value of impervious retention is fairly consistent with the values of impervious retention determined for a larger urban basin in the area (Ellis and 

others, 1984).
SIMULATED RAINFALL ON A STREET SURFACE

LU 

LU 

LL

OQ Z

38-7 LU

£= CO

u-'Pn LL LU o °-

O Q

 o

^T I °>S

^ -J *> 

CD O Q z °- z

< z o

t CO £

< Q.

CO

CO Q O<-

i x

o ^ 

e:o

_1 D. CO

< CO DC

H < LJJ

O ^ D_

0.060

0.040

0.020

0.000

8.0

6.0

4.0

2.0

0.0

8.0

6.0

4.0

2.0

0.0

1.2

0.9

0.6

0.3

0.0

1000

EXPLANATION

 Runoff

 Total suspended 

solids

o Sample analyzed

EXPLANATION

. Total lead

 Total zinc 

Sample analyzed

EXPLANATION

 Total manganese

 Total nitrogen 

O Sample analyzed

EXPLANATION

 Total phosphorus

 Total organic carbon 

o Sample analyzed

1100 

TIME, IN HOURS

6.0

3.0

4.0

3.0

2.0

1.0

4.0

3.0

.

1200

CO

Q

O 

CO

T

0

0 £

LU Q X

Q Z Q

CO

< 

O

O ^

¥, o

^ o

- LU

Z

N "'

CO £5

^ Q.

LiJ D

2.0 Z

1.0

0.0

2 z

Z O 

 O

7f - LU 

< Z CO

O CO DC 

h-<LU

CO 

Q

0.9 z ot

0.6 O u Q

< < LU

fe Z °°

0.3 £ o £T "" CQ LU

	DC CL

o.o o

FIGURE 6. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 4, June 3,1980.

RUNOFF QUALITY

Samples for water-quality analysis were collected during the rainfall simulation at varied frequencies to provide data to calculate the washoff loads of selected 

constituents in the runoff from the nine plots. Trace

elements were analyzed using the total recoverable 

method and are referred to hereafter as total trace elements. Runoff-sample collection times are indicated on 

the hydrographs in figures 3-11. In addition, composite 

samples of applied water collected from sprinklers during various stages of each rainfall simulation were
10 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

O Q 

5 2

CO

o

2 

DC

0.016

0.012

0.008

0.004

0.000

2.0

1.5

1.0

0.5

0.0 

2.0

<- 1.5

1.0

< Q.

?^ X 

°- o Q

$2 2 

? "a^ °

_l CL CO

< CO DC

fe < ^

0.5

0.0 

4.0

3.0

2.0

1.0

0.0

900

EXPLANATION

 Runoff

 Total suspended solids 

o Sample analyzed

EXPLANATION

 Total lead

 Total zinc 

o Sample analyzed

EXPLANATION

 Total manganese

 Total nitrogen 

o Sample analyzed

EXPLANATION

 Total phosphorus

 Total organic carbon 

o Sample analyzed

1000

TIME, IN HOURS

4.0 CO 

Q

3.0 CO ?_

Q 2 Q

2 -° 2go

SD^iu

D Z CO

1.0 CO ~ QJ.

 I LU 

< 0,

o.o o

2.0

1.5 0 2 ^ 

1.0 ^^O 

0.5 "~ [^ CC

0.0 

2.0

1.5

1.0 ^

0.5

0.0 

4.0

3.0

2.0

1.0

'o.o

1100

2 CO b

LU Q i-

(D 2 *

O D ^

DC o Q

I- Ol 2

_i °

^ - m

< Z CO

O CO DC

H < "J

O

DQ

CO

20-2

< 2 O

DC - LU 

O CJ CO

_l CO DC 

<<UJ

O

FIGURE 7. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 5, June 4, 1980.

analyzed for the selected constituents. The constituent 

concentrations in the applied water samples were 

subtracted from the concentrations in the runoff 

samples. Washoff loads were computed using this difference and the flow rate. The washoff loads for each

plot and the mean washoff loads are presented in table 

2. Graphs of the washoff loads are shown in figures 

3-11.

The graphs of washoff loads from plot 6, figure 8, 

show a double peak in washoff loads, whereas the
(jo 2 co co Cfo

P CO HJ M p

» S « » * 5f

8 B g*ij w

«^OQ ^ 0> O

O M. 2 P. '"*'

^** £3 C Pi <J

| P | 5' 1

a» p < o

11 i I i

§ 

CO ^ tj*

» 2,* tt S3 o

s

r rT tr

2 f

-

ii

g 

d- d-

?!^l

» .

Hh K H3 i

p <

rSU P y> O (0 &-

*§' ^ $ s,

O CD

TOTAL PHOSPHORUS 

AS P, IN POUNDS 

PER SECOND X 10"6

TOTAL MANGANESE 

AS Mn, IN POUNDS 

PER SECOND X 1(T7

TOTAL LEAD 

AS Pb, IN POUNDS 

PER SECOND X 10'7

R

a 

2T

o

CD

U 

05

TOTAL ORGANIC CARBON

AS C, IN POUNDS

PER SECOND X 10"4

TOTAL NITROGEN 

AS N, IN POUNDS 

PER SECOND X 10"6

TOTAL ZINC 

ASZnJN POUNDS 

PER SECOND X 10~7

TOT

oo r CD
12 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

EXPLANATION

 Runoff

EXPLANATION

 Total lead

Total zinc 

Sample analyzed

EXPLANATION

 Total manganese

Total nitrogen 

Sample analyzed

4.0

3.0

g 2 O 2.0

°- ~ LLJ

_l Q- C/D

< CO CC 1.0

0.0

800

EXPLANATION

 Total phosphorus

Total organic carbon 

Sample analyzed

900 

TIME, IN HOURS

0.0

4.0

<

* ULI 

2 C/D

O CO OC 

H<£

O

CO in

OC CO k

< Q r- 

 % X

 O O 

20-2

C3 O

CC > LLJ (~) C_J c/5

_l C/D OC 

< < LLJ

0.0

1000

FIGURE 9. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 7, June 5,1980.

the plots but resulted from deposition and agitation of 

the washoff loads.

The concentration of total nitrogen in the runoff 

samples was not sufficiently higher than the concentration in the applied water for the difference to offset

analytical errors. Therefore, the results presented for 

concentrations and loads of total nitrogen may be in 

error by an order of magnitude or less. The possible 

error in the concentrations and loads of total nitrogen 

must be considered when these data are used in any
SIMULATED RAINFALL ON A STREET SURFACE 13

0.060

CO CO?

LU Q o 

< D X

i2i

< z o

o

~* LU Q_ CO

CO DC

0.0

1.5

1.0

0.5

0.0

3.0

2.0

1.0

0.0

1000

EXPLANATION

 Runoff

Total suspended solids 

Sample analyzed

EXPLANATION

 Total phosphorus

 Total organic carbon 

o Sample analyzed

1100 

TIME, IN HOURS

3.0

2.0

1.0

O b

CO r=-

Q CO X

pi^go

Q. g O

CO °- LU

=> Z CO

CO DC

N

LU 

0-

0.0

,\

' 1^

I

EXPLANATION

 Total manganese

o Sample analyzed

CT -fa. 5COO 3c NITROGEN TOTALAS N,IN POUNDS 10' PER SECOND6X 

3.0 ^

O 

CD

2.0

1.0 §

^8. LU

O CO 

CO DC

1200

0.0

FIGURE 10. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 8, June 5,1980.

interpretation. In particular, the use of these data

for statistical analysis could result in erroneous

conclusions.

A first flush of most of the constituent loads studied

took place in the runoff from the simulated rainfall. A

first flush of constituent load occurs when the first 25

percent of the runoff volume contains 50 percent or 

more of the constituent load. Data from larger urban 

basins in the metropolitan Denver area indicate that a 

first flush of loads is not common in basins of 33 to 167 

acres. This suggests that relatively small areas of impervious area may have a first flush of load, but due
14 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

0.016

LU 

LU

U Q "'"""

§§

O uj 0.008 

~ DC

LL LU 

£ °- 0.004

-z. -3

CL 

0.000

3.0 

§ X 2.0

< CL 

0.0 

4.0

g° 3.0 

=> X

^ Z 2.0 

1 O 

. O

C LU

^ 2 1 -°

co gj

< CL 

0.0 

5.0

W b 4.0

1*

0 Q 3 -°

CL Z

^ °

-^ 2-°

CL CO

w cc

< LU 1.0 

CL

0.0 

9C

_ / \ 1 o Sample analyzed _

-\ \ ____ ^

EXPLANATION

 Total lead

 A \ 

r \Q o Sample analyzed

/ \ \ 

- / \\ /

\\ 

x3"^-

O" " \ ^^.^

u

A EXPLANATION

I \ Total manganese 1 /\ \

IP \\ " ~ Total nitrogen 

_ M IR o Sample analyzed _

I 19 VV>-.

/ " ^ c ~"TVU

EXPLANATION

~ r\ Total phosphorus ~~

I/ ^ o Sample analyzed

/ ^l^s_

0 1000 11

TIME, IN HOURS

2.0 

1.0

0.0 

3.0

2.0

0.0 

2.0

1.0

0.0 

5.0

4.0 

3.0 

2.0 

1.0

0.0 

DO

TOTAL ORGANIC CARBON NITROGEN ZINC SUSPENDED £

AS C, INPOUNDS N, Zn,

SECOND XTO' 5 PER KT6 17(T

FIGURE 11. Runoff and washoff loads of selected water-quality constituents for Denver Federal Center plot 9, June 6,1980.

acres. This suggests that relatively small areas of impervious area may have a first flush of load, but due 

to timing of runoff from various sections of the basin 

the resultant basin loads are attenuated. That is, each 

small runoff event may progressively move the 

available constituents toward the basin outlet. The

distribution of available constituents throughout a 

basin prior to a runoff event may also be a determining 

factor in the distribution of the constituent load 

throughout the runoff period.

The washoff loads versus number of days since street 

washing are presented in figures 12-18 for the two
SIMULATED RAINFALL ON A STREET SURFACE 

TABLE 1. Characteristics of simulated rainfall on the Denver Federal Center plots, resulting runoff, and impervious retention

15

Plot

1

2 ... . .

3 . ... .

4

5 . ... .

6 . ....

7 . . ...

8

9

Rainfall 

(inches)

0.53 

1.08 

.53 

1.17 

.42 

1.22 

.59 

1.25 

.55

Rainfall 

duration 

(hours)

0.95 

.60 

.85 

.59 

.66 

.53 

.93 

.57 

.98

Rainfall 

intensity 

(inches 

per hour)

0.56 

1.80 

.62 

1.98 

.64 

2.30 

.63 

2.19 

.56

Runoff 

(inches)

0.43 

1.05 

.43 

1.13 

.37 

1.19 

.46 

1.22 

.46

Runoff-torainfall 

ratio

0.81 

.97 

.81 

.97 

.88 

.98 

.78 

.98 

.84

Measured 

peak flow 

(cubic feet 

per second)

0.012 

.041 

.012 

.044 

.014 

.053 

.013 

.050 

.011

Peak flow 

predicted by 

rational method 

(cubic feet 

per second)

0.013 

.042 

.014 

.046 

.015 

.053 

.015 

.051 

.013

Impervious retention

Method 

1 

(inches)

0.046 

.073 

.043 

.058 

.040 

.086 

.037 

.061 

.044

Method 

2 

(inches)

0.048 

.064 

.028 

.011 

.020 

.058

.022 

.046

Average 

(inches)

0.047 

.068 

.036 

.034 

.030 

.072

.042 

.045

1See page 7 for explanation of methods used to determine impervious retention.

TABLE 2. Washoff and mean washoff loads of selected water-quality constituents from plots 1-9 at the Denver Federal Center

[Except for intensity of rainfall, all units are in pounds per acre]

Plot

1 ....

2 ....

3 ....

4 ....

5 ....

6 ....

7 ....

8 ....

9 ....

Mean

Intensity 

of

rainfall 

(inches per

hour)

0.56

1.80

.62

1.98

.64

2.30

.63

2.19

.56

Days

since 

street

washing

1

1

2

2

3

3

4

4

5

suspended

solids

2.6

11

6.4

13

8.6

25

8.9

42

14

15

lead

0.0026

.0060

.0051

.011

.0049

.0099

.0063

.031

.0091

.0095

zinc

0.0016

.0014

.0063

.0047

.0030

.012

.0044

.018

.0068

.0065

Total

manganese

0.0047

.012

.0022

.014

.0081

.024

.0091

.043

.014

.015

nitrogen

0.055

.14

.046

.041

.13

.052

.20

.064

.082

.090

phosphorus

0.022

.026

.021

.014

.012

.034

.014

.049

.017

.023

organic

carbon

1.0

2.3

2.0

2.7

1.4

2.6

1.7

3.3

1.7

2.1

TABLE 3. Event mean concentrations of selected water-quality constituents in the runoff from plots 1-9 at the Denver Federal Center and

concentrations in the applied water

[mg/L = milligrams per liter; fig/L = micrograms per liter; concentrations in the applied water are shown in parentheses and are not included in the event mean concentrations]

Plot

1 .....

2 .....

3 .....

4 .....

5 .....

6 .....

7 .....

8 .....

9 .....

Intensity

of 

rainfall 

(inches 

per hour)

0.56

1.80

.62

1.98

.64

2.30

.63

2.19

.56

Days 

since 

street 

washing

1

1

2

2

3

3

4

4

5

suspended 

solids 

(mg/L)

97/1 QV

45(12)

£*£*try\ ob(7)

49(9)

100(10)

Q9/941

QK/9QI

150(20)

lead

(/tg/L)

97(41

25(14)

53(9)

44(8)

60(1)

37(36)

110(4)

zinc

17 (50)

5.8(90)

66 (80)

19 (50)

36 (50)

46 (60)

42 (30)

66 (60)

fifi (40)

Total

manganese

49(20)

52(10)

99/701

54(10)

98(10)

91(10)

87(10)

160(10)

140(20)

nitrogen

(mg/L)

0.57(1.2)

.58(1.1)

.47(1.7)

.16(1.7)

1.5 ( .66)

1Q/1 9J

I Q (O1\

.23(2.9)

.80( .95)

phosphorus 

(mg/L)

0.23 (0.15)

.11 ( .10)

.22 ( .01)

.051( .09)

.15 ( .01)

.13 ( .02)

.13 ( .01)

.18 ( .02)

.16 ( .01)

organic 

carbon 

(mg/L)

10 (4.4)

9.8(3.3)

20 (3.4)

11 (2.9)

17 (3.5)

9.9(3.7)

16 (3.4)

12 (3.8)

17 (4.0)
16 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

1000

500

DC 

(J

DC 

LLJ 

Q_

V) 

Q

O

Q_

CO 

Q

_i 

O 

CO

Q

LLJ

Q

Q_ 

(/)

(/)

?

O

CO 

Q

100

50

10

El El

EXPLANATION

0 Loads generated by simulated rainfall of about 

one-half inch per hour

A Loads generated by simulated rainfall of about 

2 inches per hour

El Deposition measured by vacuum-strip sampling

El A

A

A

A

0

O

0

0

0

0123456 

DAYS SINCE STREET WASHING

FIGURE 12. Total suspended solids washoff loads and deposition measured by vacuum-strip 

sampling versus number of days since street washing for the Denver Federal Center plots, 

June 2-6, 1980.

rainfall intensities. Also indicated are the deposition 

loads of each constituent measured by vacuum-strip 

sampling. The loads produced from the higher intensity rainfall generally increase with the number of days 

since street washing. The loads produced by the lower

intensity rainfall show an inconsistent pattern. This suggests that antecedent dry days may be a more important factor in determining loads from higher intensity 

rainfall-runoff events than from lower intensity events. 

Another possible explanation is that the variability and
SIMULATED RAINFALL ON A STREET SURFACE 17

0.5

DC

a

DC 

LLJ 

Q_

CO 

a

D 

O 

a.

< 

O

CO

a

0.1

0.05

0.01

0.005

0.001

I EXPLANATION ' I

O Loads generated by simulated rainfall of about 

one-half inch per hour

A Loads generated by simulated rainfall of about 

2 inches per hour

El Deposition measured by vacuum-strip sampling

H 

0

0

A

0

A

O

A

O

O

O

O

1234

DAYS SINCE STREET WASHING

FIGURE 13. Total lead washoff loads and deposition measured by vacuum-strip sampling 

versus number of days since street washing for the Denver Federal Center plots, June 2-6, 

1980.

error of the measurement are a larger percent of the 

value for the loads from lower intensity storms.

The event mean concentrations of the selected constituents in the runoff are presented in table 3. The 

event mean concentrations of total suspended solids,

total lead, total zinc, and total manganese generally increased with the number of days since street washing. 

The event mean concentrations of total phosphorus and 

total organic carbon remained fairly constant with the 

increase in the number of days since street washing.
18 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

1

0.5

DC 

o

DC 

LU 

Q_

1) 

O 

Q_

O

z

N

O

0) 

Q 

< 

O

0.1

0.05

0.01

0.005

0.001

EXPLANATION

0 Loads generated by simulated rainfall of about 

one-half inch per hour

A Loads generated by simulated rainfall of about 

2 inches per hour

El Deposition measured by vacuum-strip sampling

El

A

A

El 

©

A

©

©

©

El

© 

A

DAYS SINCE STREET WASHING

FIGURE 14. Total zinc washoff loads and deposition measured by vacuum-strip sampling 

versus number of days since street washing for the Denver Federal Center plots, June 2-6, 

1980.

This suggests that antecedent dry days may be more important in determining washoff loads of trace elements 

than of nutrients, however, this tendency was not substantiated in the data collected from a larger basin during the Denver urban runoff study (Ellis and Alley, 1979).

The event mean concentrations of the selected constituents in runoff from the rainfall simulations are 

generally much smaller than the event mean concentrations in storm runoff from the adjacent larger 

basin studied. The event mean concentrations in
SIMULATED RAINFALL ON A STREET SURFACE 19

0.5

cc 

o

DC 

LU 

Q_

ID 

O 

0.

0.1

0.05

< 

O

o 0.01

Q

3

0.005

0.001

EXPLANATION

O Loads generated by simulated rainfall of about 

one-half inch per hour

A Loads generated by simulated rainfall of about 

2 inches per hour

ElDeposition measured by vacuum-strip sampling

E)

E)

A

A

A

El

O

o

o

o

El

o

DAYS SINCE STREET WASHING

FIGURE 15. Total manganese washoff loads and deposition measured by vacuum-strip 

sampling versus number of days since street washing for the Denver Federal Center plots, 

June 2-6, 1980.

runoff measured at station 06711635 North Avenue 

Storm Drain at the Denver Federal Center are summarized in table 4 by the maximum, minimum, and 

mean values for all storm runoff sampled in 1980 and 

1981.

CONSTITUENT DEPOSITION AND 

WASHOFF LOADS

Constituent deposition was measured by three 

methods once a day for the 5 days following street
20 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

cc 

o 

<

tr LU

Q_ 

CO

D 

O 

Q_

O 

tr

RLL 

O

CO 

Q

0.5

0.1

0.05

0.01

EXPLANATION

O Loads generated by simulated rainfall of about 

one-half inch per hour

A Loads generated by simulated rainfall of about 

2 inches per hour

0 Deposition measured by vacuum-strip sampling

O

A

O

O

0 

A

0 

A

O 

A

0

-a234 

DAYS SINCE STREET WASHING

0 

O

FIGURE 16. Total nitrogen washoff loads and deposition measured by vacuum-strip sampling versus number of 

days since street washing for the Denver Federal Center plots, June 2-6, 1980.

TABLE 4. Maximum, minimum, and mean event mean concentrations for att storm runoff sampled 

at station 06711635 North Avenue Storm Drain at the Denver Federal Center, at Lakewood in 

1980 and 1981

[mg/L = milligrams per liter, pg/L = micrograms per liter]

Constituent 

(totals)

Suspended solids ...............

Lead ..........................

Zinc ..........................

Manganese ....................

Nitrogen ......................

Phosphorus ....................

Organic carbon .................

Units

..... mg/L

..... Mg/L

..... /tg/L

..... /tg/L

..... mg/L

Maximum

1,360

550

790

950

15

2.2

120

Minimum

16

80

50

50

2.0

.27

17

Mean

449

310

440

410

5.4

.77

55

Number 

of storms

30

31

31

31

30

31

29
SIMULATED RAINFALL ON A STREET SURFACE 21

0.5

LLJ 

DC 

O 

< 

DC 

LU

a.

c/3 O

z

D 

O 

a.

D 

DC 

O

a. 

O 

a.

£ PLL 

O

O 

O

0.1

0.05

0.01

EXPLANATION I

® Loads generated by simulated rainfall of about 

one-half inch per hour

A Loads generated by simulated rainfall of about 

2 inches per hour

0 Deposition measured by vacuum-strip sampling

m

A

m

A 

0

A

0

A

0

©

0

234 

DAYS SINCE STREET WASHING

FIGURE 17. Total phosphorus washoff loads and deposition measured by vacuum-strip sampling versus number 

of days since street washing for the Denver Federal Center plots, June 2-6, 1980.

washing. The three methods were vacuum-strip sampling, open-space bucket accumulation, and streetbucket accumulation. The amount of deposition 

measured using all three methods is shown in table 5. 

The values determined from vacuum-strip sampling 

were used in figures 12-17. The values determined by 

open-space bucket accumulation and by street-bucket 

accumulation are not shown in figures 12-18 because 

they are orders of magnitude smaller than either the

washoff loads or the depositions measured by vacuumstrip sampling. Apparently the major source of constituents on the street surface is not conventional 

atmospheric deposition; this indicates that the traffic 

itself may be the major source. Traffic may contribute deposition to a street surface in several ways. One 

way is by tracking the transport of constituents into 

the study area from nearby areas by the action of 

vehicle tires. Another way is the direct depositing of
22 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

10

EXPLANATION

0 Loads generated by simulated rainfall of about 

one-half inch per hour

A Loads generated by simulated rainfall of about 

2 inches per hour

CC 

CJ

QC 

LLJ 

CL

oCL

O 

00 

QC 

< 

CJ

CJ

(D 

QC 

O

CO

Q

A

A

©

A

O ©

©

©

0.5

0.1 234 

DAYS SINCE STREET WASHING

FIGURE 18. Total organic carbon washoff loads versus number of days since street washing for the Denver Federal

Center plots, June 2-6, 1980.

constituents on the street by exhaust emission. A third 

way traffic adds to deposition is by the physical action 

of the tires on the street surface, which erodes the surface, resulting in the production of fine particles which 

are available to be washed off by runoff.

The washoff loads normalized to area were generally 

smaller than the constituent deposition measured by 

vacuuming. Nitrogen washoff loads did not follow the 

same pattern, probably due to error introduced in

calculating the washoff load. There was only a small difference between the concentrations in the applied water 

and the runoff (see page 12).

The vacuuming process has the ability to remove particles from the cracks and crevices that are not available 

to be washed off by runoff. Constituent deposition as 

measured by street vacuuming is considered to be the 

total load on a street surface, including that which is 

not available to be washed off by runoff.
SIMULATED RAINFALL ON A STREET SURFACE 23

TABLE 5. Deposition of selected water-quality constituents measured by vacuum-strip sampling, open-space bucket accumulation, and streetbucket accumulation on the Denver Federal Center plots 

[In pounds per acre X 10"6]

Measuring 

technique

Vacuuming .....................

Open-space bucket ...............

Street bucket ...................

Vacuuming .....................

Open-space bucket ...............

Street bucket ...................

Vacuuming ,.,,..,,.............

Open-space bucket ...............

Street bucket ...................

Vacuuming

Open-space bucket ...............

Street bucket ...................

Vacuuming .....................

Open-space bucket ...............

Street bucket ...................

Days

since 

street 

washing

....... 1

....... 1

....... 1

0

....... 2

....... 3

....... 3

....... 3

....... 4

....... 4

....... 4

....... 5

....... 5

....... 5

suspended 

solids

43,000,000

1,400

3,100

130,000,000

1,700

2,600

690,000,000

2,000

3,800

750,000,000

3,400

12,000

750,000,000

4,400

19,000

lead

14,000

1.3

1.0

48,000

1.6

2.3

260,000

1 Q

5.7

300,000

2.0

13

260,000

2.3

14

zinc

2,200

4.4

9 7

8,600

7.3

9.2

47,000

4.6

1.0

59,000

7.3

13

74,000

4.8

12

Total

manganese

3,000

1.1

.91

11,000

1.8

2.7

84,000

2.7

5.5

120,000

1.8

14

180,000

2.3

15

nitrogen

10,000

65

150

20,000

67,000

29

60

83,000

240

180

98,000

49

87

phosphorus

37,000

4.4

2.7

120,000

370,000

3.7

18

450,000

6.4

34

370,000

5.5

19

organic 

carbon

160

16

160

300

470

230

1,100

170

1,100

270

STATISTICAL ANALYSIS

Selected constituent washoff loads were correlated 

with the number of days since street washing, intensity of rainfall, total rainfall, constituent deposition 

measured by open-space bucket accumulation and by 

street-bucket accumulation, and traffic count. The correlation coefficients are presented in table 6. The 

variables were chosen because values of these variables 

can be obtained by simple techniques or from the 

literature. Gibbs and Doerfer (1982) present values of 

constituent deposition measured in the Denver 

metropolitan area. The number of days since street 

washing is analogous to antecedent dry days (the 

number of days since some predetermined rainfall, 

usually about 0.10 inch, fell) and is often used in 

theoretical load computations. Data from Denver urban

studies have indicated that antecedent dry days are not 

an important consideration in actual loads in storm 

runoff from urban areas in semiarid climates. The 

deposition may be offset by some removal procedure, 

such as traffic or wind, causing the resultant accumulation rate to approach zero. Also, the previous rainfall 

may not have been sufficient to wash the accumulated 

deposition from the impervious surfaces, therefore, 

creating an uneven base upon which further accumulation could take place. Open-space bucket accumulation 

and street-bucket accumulation are roughly analogous 

to atmospheric deposition, depending on the placement 

of the collector. Although deposition measured by 

vacuum-strip sampling may be related to washoff loads, 

it was not used in the regression equations because 

there are generally no data currently available, except 

that collected during this experiment.

TABLE 6. Correlation coefficients between selected water-quality constituent washoff loads and 

number of days since street washing, intensity of rainfall, total rainfall, open-space constituent 

deposition, street constituent deposition, and traffic for the Denver Federal Center plots

Constituent

(totals)

Suspended solids ........

Lead ..................

Zinc ..................

Nitrogen ...............

Organic carbon .........

Days

since

street

washing

....... 0.44

....... .43

....... .54

....... .43

....... .10

....... .16

Intensity

of rainfall

(inches per

hour)

0.73

.61

.59

.73

....... No

.65

.89

Total

rainfall

(inches)

0.72

.63

.58

74

Open-space

deposition

(pounds per

acre)

0.38

.38

.40

.19

Street

deposition

(pounds per

acre)

0.34

.51

.61

.49

.65 -22 .23

.90 .25 .12

Traffic

(number

of

vehicles)

0.45

.44

.54

.44

.11

.15
24 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

Correlation coefficients in columns 1 and 6 in table 

6 are very similar because the number of days since 

street washing and the number of vehicles are related. 

Likewise, values in columns 2 and 3 are very similar 

because in this study rainfall intensity and total rainfall are related. Rainfall intensity and amount are the 

variables most highly correlated with washoff loads but 

are also highly correlated with each other in this study.

Regression analysis was used in an attempt to 

develop models to predict constituent washoff load from 

street surfaces in the Denver metropolitan area. The 

independent variables used in the analysis were total 

rainfall, rainfall intensity, and dry days. The logtransformed method and simple regression techniques 

were applied to the data. The results of the regression 

analysis were inconsistent with observations of the load 

graphs. Figures 3-11 indicate that both total rainfall 

and intensity of rainfall are important variables which 

must be independent of each other. In this experiment 

total rainfall and intensity of rainfall were highly 

correlated.

The design of the rainfall simulation precluded distinguishing whether total rainfall or intensity is the 

more important parameter. Analysis of figures 3-11 

indicates that the intensity of rainfall is the more important parameter, but that after about 15-20 minutes 

most of the washoff had occurred. The total rainfall and 

intensity of the simulated rainfall are not representative 

of most natural rain storms monitored in the adjacent 

basin. Most storms monitored in the adjacent basin had 

total rainfalls of less than 0.20 inch (average storm rainfall was 0.22 inch) and rainfall intensities of less than 

0.05 inch per hour (average rainfall intensity was about 

0.06 inch per hour), and the average storm duration was 

about 3.5 hours. The rainfall simulation was not designed to simulate average rainfall storms in the Denver 

area.

ROONEY RANCH SIMULATED

RAINFALL-RUNOFF STUDY

ON A NATIVE PASTURE

STUDY PLOTS

Two plots at Rooney Ranch were artificially delineated by trenches and berms. An H-flume (Holtan and 

others, 1962) was installed at the outfall of each plot, 

which was approximately 400 square feet. Plot 1 had 

a slope of 0.20 and plot 2 a slope of 0.06. Topographic 

maps of the two plots are presented in figure 19.

The plots were about 200 feet apart and about 1,000 

feet downstream from monitoring station 06710610

Rooney Gulch at Rooney Ranch near Morrison. 

Although the two plots differed slightly in soil type and 

greatly in slope, both are representative of the clayey 

loam pastureland in the Rooney Gulch basin.

DATA COLLECTION METHODS

In May 1981, two rainfall-simulation experiments 

were conducted on each of the two runoff plots. The 

paired experiments were conducted using different 

levels of antecedent soil moisture. Rainfall and runoff 

were measured and samples of runoff were collected 

throughout the runoff period for water-quality analysis. 

In addition, soil samples were collected before and after 

each experiment to characterize antecedent and resultant moisture contents.

SIMULATED RAINFALL

The rainfall was simulated by a system similar to that 

used at the Denver Federal Center site (see page 3). The 

setup of risers was a subset of the setup used at the 

Denver Federal Center site because the Rooney Ranch 

plots were smaller than the Denver Federal Center 

plots. A schematic is shown in figure 20. Hose supports 

were used to keep the supply lines from touching ground 

and restricting overland flow.

The intensity of the simulated rainfall during each experiment was about 2 inches per hour. The simulated 

rainfall in each plot was measured using 17 storage rain 

gages. The average rainfall was calculated as the 

average of the 17 measurements. Rainfall intensity was 

assumed to be constant.

RUNOFF

Runoff was measured volumetrically and samples for 

water-quality analysis of selected constituents were collected at the H-flume (Holtan and others, 1962). The 

constituents selected for analysis were the same as 

those selected in rainfall-simulation experiments conducted at the Denver Federal Center.

SOIL MOISTURE

Soil moisture samples were collected before and after 

each rainfall-simulation experiment. A 2-inch-diameter 

hand auger was used to collect samples at 3.9-inch increments to a depth of 11.8 inches. The samples were 

taken from locations adjacent to the plots until after
SIMULATED RAINFALL ON A NATIVE PASTURE 25

EXPLANATION

 Plot boundary

 Ditch boundary

2.00 Elevation above flume throat 

in feet

5 FEET

H-flume

PLOT 1

Channel'

H-flume-Q |

PLOT 2

FIGURE 19. Topographic maps of plots 1 and 2 at the Rooney Ranch rainfall simulation site.

the second experiment at each plot to avoid disturbing 

the soil within the plot. Duplicate samples were collected after the first experiment on plot 1 to detect variations in soil moisture.

RUNOFF QUANTITY

Antecedent soil moisture, rainfall, and normalized 

runoff values, as well as ratios of normalized runoff to

rainfall for the rainfall simulations and for natural 

storms at station 06710610 Rooney Gulch at Rooney 

Ranch, are shown in table 7. The runoff ratios for the 

natural storms ranged from 0.00 to 0.16, whereas those 

for the simulated storms ranged from 0.01 to 0.08. 

Because the ratio for natural storms is so variable, it 

is not possible to determine how good a predictor of the 

ratio the rainfall simulation data are; however, the ratios 

for the simulated rainfall experiments fall within the 

measured range of ratios for natural storms. The runoff
26 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

o

EXPLANATION

; Study plot 

O Sprinkler

O

o

o

o

o

10 FEET

FIGURE 20. Sprinkler layout at the Rooney Ranch plots.

ratios for the native pastureland are extremely low compared to the ratios for impervious areas. This indicates 

that runoff from pervious areas is generally negligible 

compared to runoff from impervious areas even when 

rainfall is intense and sustained and antecedent soil 

moisture is high. This inference applies only to areas 

of similar climate and soil types.

Flow on each plot occurred only in the humus during 

the first rainfall simulation. Overland flow on both plots 

was observed during the second experiment. This process was not observed during natural storms on the 

larger Rooney Gulch basin.

INFILTRATION RATES

Infiltration was calculated as the difference between 

rainfall and runoff, ignoring evapotranspiration. The 

magnitude of evapotranspiration is probably within the 

error of measurement for this study. The soil moisture 

data were not used to calculate infiltration because they 

were too variable and indicated a decrease in soil 

moisture after the last storm, which resulted in a 

negative infiltration value. The values of the density 

and moisture retention capability from the soil samples 

were used to characterize the soils at the sites (table 8).
SIMULATED RAINFALL ON A NATIVE PASTURE 27

TABLE 7. Antecedent soil moisture, rainfall, runoff, and runoff-to-rainfall ratios for the simulated 

rainstorms atRooney Ranch and natural rainstorms at station 06710610 Rooney Gulch atRooney 

Ranch, 1980 and 1981

Plot Storm

Antecedent 

soil moisture 

(pound/pound)

Rainfall 

(inches)

Runoff-to

Runoff rainfall 

(inches) ratio

Simulated rainstorms at Rooney Ranch

1 1

1 2

2 1

2 2

0.22 

.26 

.34 

.44

1.19

1.14

.91

.98

0.03 

.09 

.01 

.07

0.03 

.08 

.01 

.08

Natural rainstorms at station 06710610 Rooney Gulch at Rooney Ranch

1980

April 23-24 ..............

April 30-May 2 ...........

May 8-9 .................

May 15-16 ...............

May 16-18 ...............

June 19-20 ...............

July 1-2 .................

August 14-15 ............

August 25 ...............

September 8-9 ............

September 20 .............

1981

AprO 19-20 ..............

May 3 ...................

May 9 ...................

May 12-13 ...............

May 17-18 ...............

May 28-29 ...............

May 31 ..................

June 2-3 .................

June 3 ...................

July 26 ..................

August 9 ................

September 6-7 ............

1.77

1.97

.25

.59

.48

.28

.44

.29

.31

.87

.26

.71 

.45 

.36 

.67 

.91 

.87 

.31 

.30 

.42 

1.07 

.59 

.33

0.02 

.31 

.02 

.06 

.08 

.00 

.00 

.00 

.00 

.00 

.00

.00 

.00 

.00 

.00 

.01 

.00 

.00

.00 

.00 

.00

0.01 

.16 

.10 

.10 

.16 

.00 

.00 

.00 

.00 

.00 

.00

.00 

.00 

.00 

.00 

.01 

.00 

.00

.00 

.00 

.00

The infiltration rates for individual storms are 

presented in figure 21 together with the normalized 

runoff hydrographs and precipitation graphs. The infiltration rate did not reach a steady-state value during any of the rainfall-simulation experiments, even 

after overland flow was observed, possibly because 

steady-state runoff was not sustained during the 

simulations.

WATER-QUALITY RELATIONS

Selected water-quality constituent loads from the 

Rooney Ranch rainfall-simulation experiments are 

shown in table 9. The percentage of the load of each constituent that was carried by the first 25 percent of the 

runoff is also shown. These loads ranged from 21 to 46 

percent indicating that there is not a first flush of loads
28 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

ROONEY RANCH RAINFALL 

SIMULATION PLOT 1, STORM 1

ROONEY RANCH RAINFALL 

SIMULATION PLOT 1, STORM 2

DC

D 

O

I

DC 

LLJ 

0.

in

LLJ

I

O

0

EXPLANATION

 Runoff

- Rainfall

Infiltration

EXPLANATION

 Runoff

 Rainfall

 - Infiltration

0 10 20 30 40 50 60

ROONEY RANCH RAINFALL 

SIMULATION PLOT 2, STORM 1

10 20 30 40 50 60

ROONEY RANCH RAINFALL 

SIMULATION PLOT 2, STORM 2

0

EXPLANATION

 Runoff

 Rainfall

 Infiltration

0

EXPLANATION

 Runoff

- Rainfall

 Infiltration

0 10 20 30 40 50 60 0 10 20 30 40 50 60 

TIME, IN MINUTES SINCE START OF SIMULATED RAINFALL

FIGURE 21. Rainfall, runoff, and infiltration versus time for the simulated rainfall storms at the Rooney

Ranch plots, May 20, 1981.

(defined by 50 percent or higher) but that the first part 

of the runoff generally carried a higher proportion of 

the constituent loads than the remainder of the runoff. 

This compares with the 10 to 63 percent in the first 

quartile of runoff from natural storms at Rooney Gulch 

at Rooney Ranch.

The event mean concentrations of selected waterquality constituents are shown in table 10. The range 

of event mean concentrations for natural storms producing runoff at station 06710610 Rooney Gulch at 

Rooney Ranch is shown for comparison. The event mean

concentrations from the simulated rainfall storms are 

approximately an order of magnitude greater than the 

event mean concentrations for the natural storms producing runoff. This is in part due to the disturbance of 

the plot perimeters in constructing the small artificial 

drainage basins. An estimated 10 to 15 percent of the 

area of the runoff plots was disturbed by artifical boundary construction. The runoff from the disturbed area 

contributed additional sediment increasing the concentration of total suspended solids. Since the other constituents are highly correlated with total suspended
CONCLUSIONS 

TABLE 8. Soil-moisture and bulk-density data for the Rooney Ranch rainfall simulation plots

29

Plot

1

1

1

1

1

1

1

1

1

1

1

1

2

2

2

2

2

2

2

2

2

Sample

hole

1

1

1

22

2

2

23

3

3

4

4

4

5

5

5

6

6

6

7

7

7

Tune

Before storm

do.

do.

Between storms

do.

do.

do.

do.

do.

After storm 2

do.

do.

Before storm 1

do.

do.

Between storms

do.

do.

After storm 2

do.

do.

Depth

(inches)

0.0- 3.9

3.9- 7.9

7.9-11.8

0.0- 3.9

3.9- 7.9

7.9-11.8

0.0- 3.9

3.9- 7.9

7.9-11.8

0.0- 3.9

3.9- 7.9

7.9-11.8

0.0- 3.9

3.9- 7.9

7.9-11.8

0.0- 3.9

3.9- 7.9

7.9-11.8

0.0- 3.9

3.9- 7.9

7.9-11.8

Volumetric

soil

water

content

0.20

.24

.23

.29

.25

.23

.27

.30

.21

.24

.27

.36

.23

.36

.44

.31

.53

.47

.24

.50

.43

Average 

volumetric 

soil

water content

from 0 to 11.8

inches

0.22

.26

.26

.29

.34

.44

.39

Moisture

retention

capability

0.34

.36

.29

.33

.22

.33

.28

.28

.31

.28

.25

.29

.27

.30

.29

.35

.36

.34

.35

.33

.35

Bulk

density

(pounds per

cubic foot)

37

49

51

50

62

72

47

59

44

43

59

72

43

70

95

46

84

84

34

78

76

Average 

bulk density 

for 0 to

11.8 inches

(pounds per

cubic foot)

46

61

50

58

69

71

63

See glossary.

Holes 2 and 3 are replicates.

solids, probably being attached to the sediment, the 

other concentrations were likewise increased. The waterquality concentrations during the rainfall simulation experiments are not considered representative of those 

from undisturbed pasture.

The constituent loads and event mean concentrations 

from plot 1 were significantly greater than those from 

plot 2. There are two possible explanations for this difference. First, plot 1 had a much greater slope than plot 

2; plot 1 slope was 0.20 whereas plot 2 slope was 0.06. 

The difference in slope would result in greater flow 

velocities on plot 1, especially in the artificial drainage 

channels, resulting in greater total suspended solids 

loads. Since there was a high correlation between total 

suspended solids and the other constituents, the other 

constituents would likewise increase. Second, about 20 

percent more water was applied to plot 1 than to plot 

2. The greater amount of water applied to plot 1 than 

to plot 2 would have increased the sediment transport 

in the drainage channels as well as in the overland flow. 

The greater sediment transport from plot 1 than from 

plot 2 would again increase the constituent loads and 

concentrations for the above-stated reasons.

CONCLUSIONS

Runoff from simulated rainfall of two different intensities on a street showed higher runoff-to-rainfall ratios 

for the higher intensity simulated rainfall. A first flush 

of constituent loads occurred in the runoff from rainfall simulations on the street. However, no first flush 

occurred in the runoff from similar experiments on 

native grass covered plots.

The event mean concentrations of constituents in the 

runoff from simulated rainfall on the street were generally smaller than the event mean concentrations of 

constituents in the runoff from an adjacent urban basin. 

Comparison of results for the pasture plots and the 

natural basin are meaningless because the disturbance 

of the plots influenced the water quality of the runoff.

Observations of load graphs for the rainfall simulations on a street surface indicate that intensity of 

rainfall and total rainfall are important variables determining constituent loads. The design of the experiment 

was such that intensity of rainfall and total rainfall were 

highly correlated, thus precluding the development of 

useful regression equations to predict washoff loads.
30 RUNOFF CHARACTERISTICS AND WASHOFF LOADS, DENVER, COLORADO

TABLE 9. Loads of selected water-quality constituents and percentage of loads carried by the first 25 percent of the runoff from theRooney

Ranch rainfall simulation plots

[All units are in pounds. Percent of load carried by the first 25 percent of runoff is in parenthesis]

Plot

1 

1

2 

2

Storm

1 

2

1

2

suspended 

solids

0.58 (29) 

1.3 (30) 

.097(46) 

.30 (42)

lead

14 (30) 

27 (41) 

2.1(27) 

11 (32)

zinc 

<xl<T6)

34 (31) 

75 (36) 

6.9(31) 

30 (38)

Total

manganese

(xi<r6)

170(31) 

360(36) 

31(35) 

130(39)

nitrogen 

<xl<T3)

1.3 (33) 

2.8 (36) 

.23(38) 

.86(45)

phosphorus

0.18 (32) 

.48 (31) 

.017(35) 

.097(45)

organic 

carbon

2.7(31) 

15 (28) 

1.3(31) 

6.3(21)

TABLE 10. Event mean concentrations of selected water-quality constituents in the runoff from simulated rainfall at Rooney Ranch and 

the range of event mean concentrations in the runoff from natural rainstorms at station 06710610 Rooney Gulch at Rooney Ranch

[mg/L = milligrams per liter, /ig/L = micrograms per liter]

Plot Storm

suspended

solids

(mg/L)

lead

(Mg/L)

zinc

(Mg/L)

Total

manganese

(mg/L)

nitrogen

(mg/L)

phosphorus

(mg/L)

organic

carbon

(mg/L)

Simulated rainfall

1

1

2

2

1

2

1

2

8,500

6,900

4,800

2,200

200

140

100

79

500

400

340

220

2,500

1,900

1,500

950

18

15

11

6.2

2.6

2.6

.85

.70

40

79

66

45

Natural rainfall

Range1 ....................................... 79-870

Mean1 260

9.8-150 

25

40-180 

87

180-1,200 

350

1.4-6.1 

2.1

0.14-.66 

0.27

13-28 

21

JThe range of values and mean are given only for storms producing runoff (See table 7).

Simulated rainfall on small pasture plots produced 

runoff-to-rainfall ratios similar to runoff-to-rainfall 

ratios from a larger native pasture subjected to natural 

rainfall.

REFERENCES

Ellis, S.R., and Alley, W.M., 1979, Quantity and quality of urban 

runoff from three localities in the Denver metropolitan area, Colorado: U.S. Geological Survey Water-Resources Investigations 

Report 79-64, 60 p.

Ellis, S.R., Doerfer, J.T., Mustard, M.H., Blakely, S.R., and Gibbs, 

J.W., 1984, Analysis of urban storm-runoff data and the effects 

on the South Platte River, Denver metropolitan area, Colorado: 

U.S. Geological Survey Water-Resources Investigations Report 

84-4159, 66 p.

Gibbs, J.W., 1981, Hydrologic data for urban storm runoff from nine 

sites in the Denver metropolitan area, Colorado: U.S. Geological 

Survey Open-File Report 81-682, 142 p.

Gibbs, J.W., and Doerfer, J.T., 1982, Hydrologic data for urban storm 

runoff in the Denver metropolitan area, Colorado: U.S. Geological 

Survey Open-File Report 82-872, 560 p.

Holtan, H.N., Minshall, N.E., and Harold, L.L., 1962, Field manual 

for research in agricultural hydrology: Agricultural Handbook No. 

224, U.S. Department of Agriculture, Agricultural Research Service, 215 p.

Lindsay, W.L., and Norvell, W.A., 1978, Development of a DTPA soil 

test for zinc, iron, manganese, and copper: Soil Science Society 

of America Journal, v. 42, no. 3, p. 421-428.

Linsley, R.K., and Franzini, J.B., 1972, Water-resources engineering 

(2d ed.): New York, McGraw-Hill Book Co., 690 p.

Lusby, G.C., 1977, Determination of runoff and sediment yield by rainfall simulation, in Erosion: Research Techniques, Erodibility, and 

Sediment Delivery, Geophysical Abstracts, Ltd., Norwich, 

England, p. 19-30.

Miller, R.F., and McQueen, I.S., 1978, Moisture relations in rangelands, 

Western United States, in Proceedings of the first International 

Rangeland Congress: p. 318-321.

U.S. Geological Survey, 1979, Techniques of water-resources investigations of the United States Geological Survey, Book 5, Chapter 

Al, 626 p.

& U.S. GOVERNMENT PRINTING OFFICE: 1987 773-047/66,007 REGION NO. 8