Effect of Urban Growth 

on Streamflow Regimen 

of Permanente Creek 

Santa Clara County 

California

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1591-B

Prepared in cooperation with the 

California Department of W^ater 

Resources
Effect of Urban Growth 

on Streamflow Regimen 

of Permanente Creek 

Santa Clara County 

California

By E. E. HARRIS and S. E. RANTZ

HYDROLOGIC EFFECTS OF URBAN GROWTH

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1591-B

Prepared in cooperation with the 

California Department of Heater 

Resources

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402
CONTENTS

Page

Abstract__ ______________________________________________________ Bl

Introduction._____________________________________________________ 1

Purpose and scope.____________________________________________ 1

Acknowledgments _____________________________________________ 2

Description of basin_______________________________________________ 3

Hydrology of basin________________________________________________ 5

Precipitation._________________________________________________ 5

Ground water.________________________________________________ 6

Surface water _________________________________________________ 6

Urban growth in the project area____________________________________ 9

Effect of urban growth..___________________________________________ 12

Summary____ _____________________________________________________ 16

References cited___________________________________________________ 18

ILLUSTRATIONS

FIGURE 1. Index map of southern San Francisco Bay area_ ___________ B3

2. Map of the Permanente Creek basin showing location of gaging stations-___-_______________-__-_-_-------_-------_ 4

3. Double mass curve of annual runoff for index area and Saratoga

Creek,1940-58______________________-__-_-_--__-____ 8

4. Aerial photograph of project area in 1948 _ _________________ 10

5. Aerial photograph of project area in 1960__ ________________ 11

6. Double mass curve of storm runoff from the index area versus 

total storm outflow from the project area and local storm 

runoff from the project area, 1945-58-___________________ 15

7. Double mass curve of precipitation and local storm runoff from

the project area, 1945-58_____________________-_--_-_--- 17

TABLES

TABLE 1. Annual runoff in Permanente and Saratoga Creek basins.____ B9

2. Percent of index and project areas impervious, as computed

from aerial photographs.._-___-_-______________________ 12

3. Storm runoff in Permanente Creek basin and storm precipitation at Palo Alto, 1945-58___________________________ 14

HYDROLOGIC EFFECTS OF URBAN GROWTH

EFFECT OF URBAN GROWTH ON STREAMFLOW REGIMEN

OF PERMANENTE CREEK, SANTA CLARA COUNTY

CALIFORNIA

By E. E. HARRIS and S. E. EANTZ

ABSTRACT

This report presents the results of an investigation of the effect of urban growth 

on the streamflow regimen of Permanente Creek in Mountain View, Santa 

Clara County, Calif. The data available did not permit a complete study of 

all hydrologic aspects, but there is conclusive evidence that the volume of storm 

runoff produced by rainfall on the valley floor has increased substantially as a 

result of urbanization. In 1945, storm runoff from the 5.12-square mile project 

area was insufficient to balance channel losses, and the streamflow entering the 

project area in the Permanente Creek channel was greater than that leaving the 

area. If, however, total outflow from the project area is considered to be the 

sum of the streamflow leaving the area plus channel seepage in the area, the ratio 

of total outflow to inflow was 1.18. By 1958, storm runoff from the project area 

was far in excess of channel losses and the ratio of total outflow to inflow was 

1.70. This increase in outflow is attributed to the fact that urban development 

during the period 1945 to 1958 increased the extent of impervious surface in the 

project area from about 4 percent to 19 percent.

The effect of urban growth in other basins in Santa Clara County should be 

investigated before any attempt is made to project the quantitative results of 

this study to other areas in the county.

INTRODUCTION

PURPOSE AND SCOPE

Santa Clara Valley has experienced a tremendous increase in 

population in the last twenty years, and particularly in the last 

decade. As with all rapidly growing communities, its water problems 

have grown accordingly. In recognition of this growth, county and 

independent water agencies were organized to deal with the increasing 

problems of pollution, flood control, water conservation, and the 

development of distribution systems for the increased domestic and 

commercial demands. Because much of the land that was predominantly agricultrual has been converted to residential and commercial

Bl
B2 HYDROLOGIC EFFECTS OF URBAN GROWTH

use, there has been an increase in impervious surface area; one of 

the prime concerns of the water agencies has been the impact of this 

urban growth on the natural recharge of ground water and on the 

adequacy of storm-drainage facilities. There is reason for concern. 

Studies in other parts of the country have shown that an increase 

in intensity and volume of storm runoff generally accompanies an 

increase in urban growth (Savini and Kammerer, 1961). The 

availability of streamflow data for the Permanente Creek basin in 

the northwestern part of Santa Clara Valley affords an opportunity 

to study one facet of the effect of urbanization in the valley, namely, 

the resulting change in volume of storm runoff.

The Santa Clara Valley Water Conservation District, in connection 

with the development and management of water resources in the 

valley, installed three gaging stations in the Permanente Creek basin 

in 1939. Two of the stations, one on Permanente Creek and the 

other on Magdalena Creek, a tributary, are located at sites where 

the streams enter the valley. (See figs. 1 and 2.) That part of the 

basin upstream from these gaging stations (Nos. 32 and 33) is referred 

to in this report as the index area. The downstream gaging station 

(No. 34) on Permanente Creek at the California Street bridge in 

the city of Mountain View is 1 mile downstream from the mouth 

of Magdalena Creek. That part of the basin downstream from the 

index area and upstream from station 34 is referred to in this report 

as the project area. The Santa Clara Valley Water Conservation 

District established the three gaging stations to obtain a measure of 

the water recharging the underlying ground-water basin through the 

streambed in the project area. Stations 32 and 33 measure the 

streamflow entering the project area; station 34 measures the streamflow leaving the project area. No measurement of the sediment 

load of the streams has ever been made.

The index area, occupying 10.9 square miles, was predominantly 

rural in 1939, and has since experienced little change in land use. 

The project area, occupying 5.12 square miles, has become almost 

completely urbanized since 1939. This report describes the effect 

of this urban growth on the volume of storm runoff produced by rainfall in the project area. For reasons that are discussed later in the 

report, the study period covered the years 1945-58.

ACKNOWLEDGMENTS

The study described in this report was made under a cooperative 

agreement between the U.S. Geological Survey and the California 

Department of Water Resources. The report was prepared under 

the general supervision of Walter Hofmann, district engineer of the 

Surface Water Branch of the Geological Survey.
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. B3

REDWOOD CIT 

oHalfltfoon,. I |p

STREAM-GAGING STATIONS 

® 32 Permanente Creek at Holly Ranch 

® 33 Magdalena Creek at Magdalena Road 

® 34 Permanente Creek at California Street 

® 60 Saratoga Creek at Saratoga

10 0

PRECIPITATION STATIONS

 1 Palo Alto

 2 Santa Clara University

 3 Stevens Creek Dam

10 MILES

FIGURE 1. Index map of southern San Francisco Bay area.

The cooperation of the Santa Clara Valley Water Conservation 

District in furnishing the streamflow and ground-water data used in 

this study is gratefully acknowledged.

DESCRIPTION OF BASIN

Permanente Creek heads in the Santa Cruz Mountains of westcentral California and in its upper course flows eastward in accordance 

with the drainage pattern of streams in the San Andreas Rift zone. 

The creek then turns north and flows through the Santa Clara Valley 

into the south end of San Francisco Bay. Maximum altitude in the 

Eermanente Creek basin in on Black Mountain, which rises to 2,810
B4 HYDROLOGIC EFFECTS OF URBAN GROWTH

STREAM-GAGING STATIONS

32 Permanente Creek at Holly Ranch

33 Magdalena Creek at Magdalena Road

34 Permanente Creek at California Street

Mountain View

FIGURE 2. Map of the Permanente Creek basin showing location of gaging stations. Project area shaded 

Base from Palo Alto quandrangle (1948).
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. B5

feet. Magdalena Creek is a left-bank tributary that heads in the 

foothills of the Santa Cruz Mountains and flows into Permanente 

Creek on the valley floor in the city of Mountain View. Maximum 

altitude in the Magdalena Creek basin is 1,148 feet. The terrain 

in the upper basin is mountainous and the bed slope is steep. In the 

first 4.4 miles the stream falls 1,540 feet for an average gradient of 

355 feet per mile. The slope decreases on the valley floor to 55 feet 

per mile and becomes very flat near the mouth. The total drainage 

area of Permanente Creek is 17.5 square miles.

The upper part of the basin is underlain by bedrock consisting 

principally of consolidated sedimentary rocks but which includes 

minor areas of metamorphic and igneous rock. Overlying this consolidated bedrock is the Santa Clara Formation, which consists of 

semiconsolidated conglomerate, sandstone, siltstone, and claystone. 

The conglomerate and sandstone are poorly sorted and have a finegrained matrix; thus, the formation has a low permeability and yields 

only small to moderate quantities of water to wells. Within the 

valley proper, unconsolidated alluvial and bay deposits of clay, sand, 

and gravel overlie the Santa Clara Formation and form the valley 

floor. As shown by well logs, the alluvial and bay deposits attain 

a thickness of 1,000 feet or more in the valley trough. However, the 

lower parts of these wells may be in the Santa Clara Formation, which 

to date has not been differentiated from the overlying unconsolidated 

alluvium in drill holes (Poland and Green, 1962, p. 2).

HYDROLOGY OF BASIN 

PRECIPITATION

The Santa Clara Valley lies within the path of storms which periodically sweep inland from the North Pacific during winter months. 

Rainfall resulting from these storms ranges from moderate to heavy, 

and direct precipitation provides a substantial part of the water supply 

of the area. More than 80 percent of the seasonal precipitation in 

the Santa Clara Valley occurs during the 5 months from November 

through March. The remaining months of the year are correspondingly dry, and only traces of rainfall occur in July and August (California State Water Resources Board, 1955, p. 23, 26).

There are no precipitation stations within the Permanente Creek 

basin, but stations are operated nearby at Palo Alto, Santa Clara 

University, and Stevens Creek Dam (fig. 1). The first two stations 

are part of the climatological network of the U.S. Weather Bureau; 

the precipitation station at Stevens Creek Dam is operated, by the 

Santa Clara Valley Water Conservation District. Because virtually 

all the precipitation in the basin occurs during general storms that 

cover large areas, records for any one of the precipitation stations

723-776 0 64 2
B6 HTDROLOGIC EFFECTS OF URBAN GROWTH

were considered adequate for use as an index of runoff. The Palo 

Alto station was chosen for this purpose, primarily because it is 

equipped with a recording rain gage and therefore provides a convenient record of the calendar dates on which rainfall occurred.

The Palo Alto station is now 5 miles northwest of Mountain View 

at an altitude of 23 feet, but prior to 1953 the station was 6 miles 

northwest of Mountain View at an altitude of 54 feet. A double masscurve comparison of annual precipitation recorded at Palo Alto and at 

Santa Clara University indicated that there is virtually no inconsistency in the precipitation catch at the two Palo Alto sites. Double 

mass-curve analysis is discussed in some detail on p. 17.

GROUND WATER

The effect of urban growth on ground-water recharge in the project 

area was not studied. The numerous, roughly parallel streams of the 

Santa Clara Valley form one large coalescing alluvial fan, and it is therefore not practicable to attempt to isolate the effect of a change in land 

use in a single small stream basin-. The ground-water aquifer is 

artificially recharged throughout the valley, but heavy pumping has 

kept the water table low, and Permanente Creek loses water through 

channel seepage as it flows through the valley. Records of the 

elevation of the ground water table obtained at four wells in the basin 

indicate the water table to be 150 to 300 feet below ground elevation 

in the project area. Inspection of the streamflow records gives no 

indication of any general alteration of the influent characteristics of 

the stream channel during the period studied.

SURFACE WATER

The upstream reaches of Permanente and Magdalena Creeks 

traverse a fairly impermeable mountainous area but have some flow 

all year except during dry years. After leaving the mountains, the 

streams become influent in the loosely consolidated alluvial material 

of the valley and lose water through the permeable streambed. The 

creeks flow in the valley only during and shortly after winter storm 

periods.

The streamflow records for the three gaging stations in the Permanente Creek basin (fig. 2) were reviewed and daily discharge 

hydrographs were compared. Examination of the rating curves for 

these alluvial channels showed that the ratings are not precisely 

defined at extreme high-water stages, and therefore no detailed study 

of instantaneous peak flows was made. The rating curves are sufficiently well defined, however, to permit a study of the volume of flood 

runoff, the bulk of which occurs at medium high stages.

All three stream-gaging stations were established in 1939 when the
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. B7

Santa Clara Valley Water Conservation District installed staff gages 

that were read twice daily. In March 1944, station 34, where runoff 

from the project area is measured, was equipped with a recording 

gage, and in April 1945 recording gages were installed at stations 32 

and 33, where runoff from the index area is measured. Because the 

runoff records became more reliable after installation of the recording 

gages, a test was made to determine if the Permanente Creek records 

prior to April 1945 were reasonably consistent with those obtained 

after that date. Double mass-curve analysis was used in the test.

A double mass curve is constructed by plotting cumulative totals 

of the variable being tested against cumulative totals of a second, or 

reference, variable, a common period of time being used for both. 

A change in slope of a line fitted to the plotted points indicates and 

dates a change in trend of the relation between variables. As a 

prerequisite to the use of double mass-curve analysis, there must be 

a simple ratio between the variables that are plotted. Thus, if y 

is the variable being investigated and x is the reference variable, 

the relation of y to x must take one of the three following forms:

y y y b -=ra: =ra; - =m. 

x x a x

These equations are obtained by fitting a straight line to a simple plot 

of the two variables, and in each equation m is the slope of the line. 

If the line passes through the point of origin of the graph, we have

7/ the first equation, - = ra. If the line does not pass through the

3/

origin, there will be an x intercept at a and a y intercept at b, and 

either of the two remaining equations will apply. It is particularly 

important that the intercept, if any, be used in the double mass plot 

if a series of dry years precedes or follows a series of wet years. Failure 

to use the intercept under these circumstances will result in a break 

in slope of the double mass curve at the year marking the end of the 

wet or dry period, even when there has been no actual change in 

trend of the relation between variables. It is also true that in double 

mass-curve analysis results obtained are usually better when both 

x and y are either streamflow records or precipitation records than 

when one variable is streamflow and the other precipitation a 

logical result of the fact that correlations of streamflow versus streamflow or precipitation versus precipitation are usually closer and more 

nearly linear than are correlations of streamflow versus precipitation. 

Because streamflow comparisons are invariably more satisfactory 

than rainfall-runoff comparisons, a nearby unregulated stream having 

a sufficiently long discharge record was sought for double mass-curve 

comparison with Permanente Creek. Only Saratoga Creek met these
B8 HYDROLOGIC EFFECTS OF URBAN GROWTH

20 40 60 80 100 

TOTAL SARATOGA CREEK RUNOFF, IN THOUSANDS OF ACRE-FEET

120

FIGURE 3. Double mass curve of cumulative total annual runoff for index area (stations 32 and 33) 

and Saratoga Creek (minus 1,800 acre-ft), 1940-58.

qualifications concerning proximity, nonregulation, and length of 

record. Actually there is a diversion for municipal supply upstream 

from the Geological Survey gaging station on Saratoga Creek near 

Saratoga (station 60, fig. 1), but a record of monthly diversions is 

available. Table 1 lists annual runoff for the gaging stations in the 

Permanente and Saratoga Creek basins. Column 4 of the table is the 

combined runoff at stations 32 and 33, or the runoff from the index 

area; column 6 is the combined gaged runoff at station 60 and the 

upstream diversion for municipal supply, or the natural runoff of Saratoga 

Creek. Figure 3 is a double mass plot of cumulative annual runoff 

from the index area (column 7) and cumulative annual natural runoff of 

Saratoga Creek minus 1,800 acre-feet (column 8). The 1,800 acre-feet 

subtracted from each annual value of Saratoga Creek runoff is the constant needed to transform the relation between variables to a simple ratio. 

In other words, the runoff relation has the form:

Animal index area runoff = m (annual Saratoga Creek runoff-1,800).
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. B9 

TABLE 1. Annual runoff, in acre-feet, in Permanente and Saratoga Creek basins

Water year

1

1940...... 

1941..............

1942 _ ...........

1943. 

1944 _ ........

1945 

1946 

1947 

1948 .

1949 .. 

1950.

1951 

1952. 

1953 ..

1954 

1955 

1956 

1957 ___ _ ..

1958 _____ . 

Index area runoff

Station 32

2

4,170 

8,802 

5,159 

3,117 

516 

1,714 

1,039 

282 

70 

600 

305 

2,600 

4,353 

2,028 

498 

236 

5,000 

371 

6,279

Station 33

3

1,122

2,600 

1,000 

431 

63 

119 

141 

1 

0 

123 

49 

297 

1,174 

403 

116 

153 

1,488 

147 

1,825

Total

4

5,290 

11,400 

6,160 

3,550 

579 

1,830 

1,180 

283 

70 

723 

354 

2,900 

5,530 

2,430 

614 

389 

6,490 

518 

8,100

Projectarea 

outflow

Station 34

5

3,950 

9,040 

4,880 

3,270 

589 

1,540 

486 

38 

0 

328 

55 

1,940 

4,740 

1,480 

120 

171 

8,320 

384 

10, 830

Saratoga 

creek 

natural 

runoff

Station 60

6

13, 240 

18,450 

12, 100 

7,040 

3,060 

4,930 

5,210 

1,810 

1,970 

4,190 

1,990 

7,910 

14, 870 

6,290 

4,000 

3,190 

15, 480 

2,850 

17,560

Cumulative totals

Stations 32 

and 33

7

5,290 

16, 690 

22, 850 

26, 400 

26, 980 

28,810 

29, 990 

30, 280 

30, 350 

31, 070 

31, 420 

34,320 

39, 850 

42,280 

42, 890 

43, 280 

49, 770 

50, 290 

58, 390

Station 60 

minus 

1,800

8

11,440 

28,090 

38, 390 

43, 630 

44,890 

48,020 

51, 430 

51,440 

51,610 

54,000 

54, 190 

60, 300 

73, 370 

77,860 

80, 060 

81,450 

95, 130 

96, 180 

111, 940

The break in slope of the curve on figure 3 at the year 1945 shows 

that the records prior to that year are not consistent with later records. 

The relative slopes of the two lines indicate that the annual discharges 

computed during the earlier period may be about 30 percent high. 

A similar conclusion was reached when the annual combined runoff 

for stations 32 and 33 was compared in a double mass-curve analysis 

with annual precipitation at Palo Alto. This type of analysis could 

not be repeated to test the early records for station 34 because, as is 

shown below, changes in land use caused changes in the runoff regimen 

of the project area.

The lack of consistency in the streamflow records for the index area 

before and after the installation of water-stage recorders is attributed 

to the quality of the early records of storm runoff. To' eliminate 

any distortion that might be introduced by the use of the earlier 

records, the period prior to April 1945 was not used in this study 

The latest runoff records for Permanente Creek available for analysis 

were those for 1958, and the study period accordingly terminated 

at that date.

URBAN GROWTH IN THE PROJECT AREA

Three sets of aerial photographs of the project area are available. 

Fortunately for this study, they were obtained at three different stages 

of urban development: 1948, 1953, and 1960. The 1948 photograph 

is shown on figure 4, the 1960 photograph on figure 5. These pictures
BIO HYDROLOGIC EFFECTS OF URBAN GROWTH

FIGURE 4. Aerial photograph of project area in 1948.

illustrate strikingly the change in land use, from orchard to urban, 

that occurred during those 12 years. The major element in the urban 

growth has been the construction of residential-tract homes. A 

computation of random samples indicated that each home represents 

an impervious area of 4,000 square feet. This area includes the house, 

driveway, patio, sidewalks, and the lot frontage to the center of the 

street. Paved areas in shopping centers, school playgrounds, and 

other thoroughfares were estimated also. These unit figures were
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. Bll

:

^* *- ^ *- - V. , v .£;« .rrvrr-.i-.i-, *J&f^ *.,«..« i£*w*s«3ej inr,. teansri

FIGUEE 5. Aerial photograph of project area in 1960.

used as a base, and the houses were counted from the aerial photographs and the total impervious area computed. The degree of urbanization can be expressed numerically as the percentage of the total 

project area that has been made impervious. The index area has 

had no appreciable change in impervious area, but the impervious 

part of the project area has increased from 5.7 percent in 1948 to 

19.1 percent in 1960. (See table 2.) Although no aerial photographs 

prior to 1948 are available, it is estimated that the impervious part
B12 HYDEOLOGIC EFFECTS OF URBAN GROWTH

of the project area was about 4 percent in 1945 when the study period 

started. The increase in urban development was greatest in 1955 and 

1956, and approximately 19 percent of the project area was impervious 

by the close of 1958, the end of the study period.

TABLE 2. Percent of index and project areas impervious, as 

computed from aerial photographs

Index area Project area 

Year (percent) (percent)

1948___________________ 0.7 5.7

1953_____________ _ 1.1 11.8

1960________ _.'______ 2.0 19.1

Accompanying the increase in urbanization was the usual construction of storm drainage facilities which undoubtedly influenced 

the magnitude of peak discharges from the project area. There was 

no basic change, however, in the drainage pattern during the study 

period, 1945-58. There is now (1964) a flood diversion channel from 

Permanente Creek to Stevens Creek, but it was constructed in 1960, 

after the close of the study period.

In the upper part of the basin, or index area, the only major development is the Permanente Cement Co. plant, which has been in 

operation since 1939, the year that streamflow records were first 

collected in the area.

EFFECT OF URBAN GROWTH

To study the time trend of the effect of urban growth on runoff 

characteristics, the precipitation and runoff records were inspected to 

obtain storm periods that were accompanied by appreciable runoff. 

An analysis of the change in annual volume of runoff was considered 

but was rejected because of the complicating effect of return flow 

from irrigation and because channel losses in Permanente Creek obscure the effect of light rainfall in producing runoff from the project 

area. To avoid these complications the analysis was therefore restricted to storm periods during which there was significant runoff. 

As mentioned earlier, the streams in the project area carry little flow, 

or may be dry, between storms. Storm runoff and rainfall was 

therefore considered to have started on the day on which an appreciable stream rise began. Because the response of runoff to storm 

rainfall is very rapid and because the project area is small, storm 

runoff was considered to have ended 3 days after the rains ceased. 

The use of runoff as an indication of storm periods minimizes, to a 

degree, the effect of antecedent soil-moisture conditions on the rainfall-runoff relation for the basin, because soil-moisture deficiencies 

would be at least partly satisfied before storm runoff began.
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. B13

Table 3 shows values of runoff for the Permanente Creek basin and 

precipitation at Palo Alto for storm periods during the years 1945-58. 

Column 1 of the table lists the days of storm rainfall. Where individual rain periods were separated by less than 3 rainless days, the 

individual rain periods were grouped together in column 1 as a single 

long storm period. Columns 2-4 are self-explanatory. Column 5, 

the duration of storm runoff, is equal to the number of rainy days 

indicated by column 1 plus 3 days, as explained in the preceding 

paragraph. Accurate determination of channel seepage rates from 

an inspection of the records is exceedingly difficult, but study of the 

records does indicate that a daily seepage rate of 10 acre-feet is of 

the proper order of magnitude for those days when the entire width 

of the streambed was submerged. Accordingly, the volume of channel 

seepage (column 6) is obtained by multiplying the storm duration 

(column 5) by 10. Column 8 is the gaged outflow from the project 

area. Column 7, representing local runoff from the project area, is 

obtained by subtracting column 4 from the sum of columns 6 and 8. 

For two storm periods this subtraction resulted in small negative 

values of local runoff. This result was attributed to inaccuracies 

in the data, and the local runoff was called zero for the two periods. 

Column 9 lists the precipitation at Palo Alto for the days shown in 

column 1.

The data from table 3 were used in a double mass-curve analysis 

to determine the effect of urban growth 011 the volume of storm 

runoff from the project area. The first relation tested for a time 

trend was that of storm runoff from the index area (combined flow 

of stations 32 and 33) and storm runoff leaving the project area. 

Inspection of columns 4, 6, 7, and 8 of table 3 shows that in the early 

years local storm runoff from the project area was insufficient to 

balance channel losses and that the streamflow entering the project 

area in the Permanente Creek channel was greater than that leaving 

the area. However, the total storm runoff leaving the project area 

is actually the sum of the gaged streamflow at station 34 and the 

estimated channel seepage in the area. Furthermore, it was found 

that when channel seepage was included in the outflow from the 

project area, the relation of total project-area outflow to index-area 

storm runoff was a simple ratio. Accordingly, estimates of channel 

seepage were considered in constructing the upper double mass 

curve of figure 6.

The upper double mass curve on figure 6 relates project-area storm 

outflow plus channel seepage (column 11, table 3) to index-area 

storm runoff (column 10). A sharp break in the curve appears in 

1955, the year that urban development greatly increased. Prior to 

1955, the ratio of storm outflow plus channel seepage to index area
TABLE 3. Storm runoff in Permanente Creek basin and storm prec

Storm period

1

Dec. 27, 1945---.. ...........

Mar. 10-11, 1949.... _ ......

Mar. 19-22, 1949 __ .........

Feb. 4-5, 1950...............

Nov. 18-20, 1950. __ . .......

Dec. 2-8, 1950--. _ .. ....

Dec. 1-5, 1951... ............

Dec. 28-30, 1951. ............

Jan. 6-7, 1952. - ..........

Mar. 14-19, 1952 ............

Dec. 5-7, 1952....... __ . ...

Dec. 26-30, 1952... ..........

Jan. 5-8, 1953__ ___ .. ...

Dec. 18, 1955-Jan. 28, 1956 

Feb. 19-29, 1956-..- . _ -

Feb. 21-28, 1957--- 

Jan. 23-26, 1958.- 

Feb. 2, 1958-.-...-....... _

Feb. 18-19, 1958-.- __ . _ ..

Feb. 24-25, 1958- 

Mar. 14-16, 1958. ........

Mar. 20-Apr. 7, 1958 ... ..

Runoff from index area 

(acre-feet)

Station 32

2

99 

196 

91 

44 

214 

464 

173 

153 

79 

131 

901 

676 

145 

280 

226 

2,825 

484 

123 

71 

194 

335 

371 

282 

2,632

Station 33

3

22 

77 

8 

38 

81 

69 

4 

52 

71 

60 

506 

188 

48 

91 

67 

1,234 

99 

65 

52 

177 

153 

135 

137 

899

Total

4

121 

273 

99 

82 

295 

533 

177 

205 

150 

191 

1,407 

864 

193 

371 

293 

4,059 

583 

188 

123 

371 

488 

506 

419 

3,531

Duration 

of storm 

runoff

(days)

5

4 

5 

7 

5 

6 

10 

5 

8 

6 

5 

9 

9 

6 

8 

7 

45 

14 

11 

7 

4 

5 

5 

6 

22

Channel 

seepage in 

jroject area 

(acre-feet)

6

40 

50 

70 

50 

60 

100 

50 

80 

60 

50 

90 

90 

60 

80 

70 

450 

140 

110 

70 

40 

50 

50 

60 

220

Local runoff from 

project area 

(acre-feet)

7

10 

31 

11 

18 

47 

69 

0 

59 

144 

22 

302 

125 

34 

157 

0 

2,683 

376 

93 

145 

207 

657 

389 

260 

2,013

Outflow 

from proj

ect area, 

station 34 

(acre-feet

8

9

25

4

5

28

50

10

18

23

16

1,61

89

16

44

21

6,29

81

17

19

53

1,09

84

6

5,32
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. B15 

24

'0 2 4 6 8 10 12 14 16 

TOTAL INDEX-AREA RUNOFF, IN THOUSANDS OF ACRE-FEET

FIGURE 6. Double mass curve of storm runoff from the index area (Sta. 32 and 33) versus 

cumulative totals of: (l; storm outflow plus channel seepage from the project area and 

(2) local storm runoff from the project area, 1945-58.

runoff was 1.18. From 1955 to 1958, the period of maximum urban 

growth, this ratio was 1.70, an increase of 44 percent. This increase 

in outflow is attributed to the increase in the extent of impervious 

surface that resulted from urbanization of the project area. It is 

not to be inferred from the outflow-inflow ratio of 1.70 for storm 

periods that the 5.12-square-mile project area is now more productive 

of runoff, on a unit area basis, than is the 10.9-square-mile index 

area. Precipitation that infiltrates in the index area continues to 

feed the perennial streams there long after rainfall has ceased, and 

storm-period runoff from the index area therefore represents only a 

part of the streamflow attributable to the rain. On the other hand, 

in the project area there is little runoff after the end of a storm, and 

storm-period runoff from the project area represents almost all the
B16 HYDROLOGIC EFFECTS OF URBAN GROWTH

streamflow attributable to the rain. The data in table 1 (columns 

4 and 5) show that the index area is more productive of runoff than 

the project area.

The lower double mass curve on figure 6 relates local storm runoff 

from the project area (column 13, table 3) to index-area storm runoff 

(column 10). Again, a sharp break in the curve appears in 1955. 

It is seen, too, that a lesser change in trend of the relation occurred 

in 1951. After 1951, local storm runoff exceeded channel losses in 

most, though not all, years until the end of the study period.

As mentioned earlier, precipitation-runoff relations are generally 

not as satisfactory as simple runoff relations in studying trends in 

runoff, particularly when short time intervals such as storm periods 

are considered, because volume of precipitation is not the only factor 

that affects the runoff; rainfall intensity and antecedent soil-moisture 

conditions must also be considered. Despite these complicating factors, a double-mass curve was constructed (fig. 7) relating local storm 

runoff from the project area to storm precipitation totals at Palo Alto. 

In computing cumulative precipitation values for this graph (column 

12, table 3), 0.5 inch of rainfall was subtracted from each storm volume 

in order to transform the rainfall-runoff relation to a simple ratio. 

This subtraction is equivalent to saying that 0.5 inch of rain in each 

storm fails to contribute to storm runoff. The graph on figure 7 is 

less satisfactory than those on figure 6, but it too shows changes in 

trend in 1951 and 1955.

SUMMAKY

The streamflow records available for the Permanente Creek basin do 

not permit a complete study of the effect of urban growth on streamflow regimen, and this investigation was therefore limited to a study 

of trends in the volume of storm runoff produced by rainfall on the 

valley floor. It was found that a substantial increase in volume of 

storm runoff coincided with the period of major urban development. 

As a result of the increase in impervious surface in the project area 

from about 4 percent of the total area in 1945 to 19 percent in 1958, 

the ratio of outflow from the area (including channel seepage) to 

inflow increased from 1.18 to 1.70. The effect of urban growth in 

other basins in Santa Clara County should be investigated before any 

attempt is made to project the quantitative results of this study to 

other areas in the county.
STREAMFLOW REGIMEN, PERMANENTE CREEK, CALIF. BIT

10 20 30 40 

TOTAL PRECIPITATION, IN INCHES

50 60

FIGURE 7. Double mass curve of cumulative total storm precipitation (at Palo Alto, minus 0.5 inch) and 

cumulative total local storm runoff from the project area, 1945-58.
B18 HYDROLOGIC EFFECTS OF URBAN GROWTH

REFERENCES CITED

California State Water Resources Board, 1955, Santa Clara Valley Investigation:

California Water Res. Board Bull. 7, 154 p. 

Poland, J. F., and Green, J. H., 1962, Subsidence in the Santa Clara Valley,

California, a progress report: U.S. Geol. Survey Water-Supply Paper

1619-c, 16 p. 

Savini, John, and Kammerer, J. C., 1961, Urban growth and the water regimen:

U.S. Geol. Survey Water-Supply Paper 1591-A, 42 p.

O