EOLOGICAL SURVEY CIRCULAR 105 

April 1951 

F======================--

SOME ASPECTS OF THE WATER SUPPLY 

IN THE SOUTH COASTAL BASIN, CALIFORNIA 

By 

H. C. Troxell, J. F. Poland, and others 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary 

GEOLOGICAL SURVEY 

W. E. Wrather, Director 

·washington, D. C. ; 

Free on application to the GeqJogical Survey, Washington 25, D. C. 
SOME ASPECTS OF THE WATER SUPPLY 

IN THE SOUTH COASTAL BASIN, CAUFORNIA 

By H. C. Troxell, J. F. Poland, and others 

CONTENTS 

Page Page 

Introduction................................................... 1 Surface reservoir storage............................... 4 

Precipitation in mountain and foothill areas Ground-water storage..................................... 6 

tributary to valley floor ................................. . Ground-water recharge................................... 6 

Runoff from mountain and foothill areas Deep penetration......................................... 6 

tributary to valley floor.................................. 2 Natural absorption of surface runoff............... 7 

Precipitation on valley floor............................. 2 Artificial means of ground-water recharge...... 8 

Waste to ocean............................................... 4 Local supply.................................................. 8 

Length of available records.............................. 4 Demand........................................................ 10 

Variability of annual runoff. .......... ,................... 4 Selected references........................................ 10 

ILLUSTRATIONS 

Figure 1. 

2. 

3. 

4. 

5. 

South Coastal Basin (including San Jacinto River Basin'} ........................................................... . 

Page 

2 

3 

5 

7 

9 

Precipitation and runoff in the South Coastal Basin, Calif. (including the San Jacinto River Basin) .. . 

Seasonal runoff distribution ................................................................................................ .. 

Fluctuation of ground-water level at key wells in the South Coastal Basin ................................... .. 

Annual water spreading in the South Coastal Basin .................................................................. . 

Introduction 

The South Coastal Basin is drained by the San 

Gabriel, Santa Ana, and Los Angeles Rivers. The area 

encompasses the San Fernando Valley, the San Gabriel 

Valley, the San Bernardino (or upper Santa Ana) Valley, 

the San Jacinto Valley, and the Coastal Plain. Figure 

1 shows an outline map of this area, in which the mountain and foothill areas are segregated from the alluvial 

valley floor area. The metropolitan and urban communities, as well as the agricultural areas of this basin, 

are entirely dependent upon the water supply originating in this 4, 550 square mile area, except for the importations of water through the Owens Valley and 

Colorado River aqueducts. This report is preliminary 

and is subject to revision. 

Precipitation in Mountain and Foothill Areas 

Tributary to Valley Floor 

The foothill and mountain areas, as much as 10, 000 

feet in altitude in northern and eastern parts of the basin, represent barriers to the incoming moistureladen maritime air. The lifting C>f this moist air results in considerable precipitation over the higher 

mountain areas, amounting to as much as a mean annual precipitation of 40 to 60 inches locally. However, 

the average annual precipitation is reduced to about 24 

inches when the entire foothill and mountain area of 

2, 410 square miles is considered. During the 28-year 

period of 1920-48 this basin-wide mountain and foothill 

precipitation has ranged from about 38 inches during 

the 1940-41 climatic (July 1 to June 30) year to about 

13 inches during 1927-28. This variability of precipitation is of considerable significance in that it has a 

major influence on runoff and recharge of the groundwater bodies. 

The basin-wide annual precipitation and runoff for 

the South Coastal Basin is shown graphically in terms 

of acre-feet on figure 2 for the period of 1920-48. The 

upper part of this figure giving the total precipitation 

in the mountain and foothill areas tributary to the valley 

floor indicates that the mean annual precipitation 

amounts to 3, 100, 000 acre-feet or about 24 inches over 

the entire area, and the median value amounts to 

3, 000, 000 acre-feet. However, the precipitation for 

individual years ranged from 5, 100, 000 acre-feet to 

1, 800, 000 acre-feet. Certain cyclic tendencies are 
2 WATER SUPPLY IN SOUTH COASTAL BASIN 

t 

Seal~ 

0 8 16 24 :sz MILES 

Explana1ion 

Mountain and foothill areas 

Volley floor areas 

Figure 1. -South Coastal Basin (including San Jacinto River Basin}. 

indicated by extended wet, as well as dry; periods. 

Not all the years within an extended wet period are 

above normal, but they greatly predominate. Like·-

wise in an extended dry period not all the years are 

dry, but they tend to predominate. Within the 28-year 

period of 1920-48 the driest 10-year period was that of 

1923-33 when the average annual mountain and foothill 

precipitation anwunted to 2, 600, 000 acre-feet or 84 

percent o(the mean value for the 28-year period. · In 

contrast, the wettest 10-year period of 1936-46 produced an average precipitation of 3, 600, 000 acre-feet 

or 116 percent of the 28-year value. 

Runoff From Mountain and Foothill Areas Tributary to 

~all:!_!_!oor 

Of the precipitation in the mountain and foothill 

areas, that portion reaching the land surface at rates 

,of rainfall in excess of rates of infiltration tends to 

produce surface overland flow, or, basically, the flood 

runoff. However, in southern California major portions of the precipitation generally occur at lesser 

rates of rainfall, enter the soil mantle, and satisfy the 

soil moisture deficiencies resulting from the preceding 

season '.s evapotranspiration losses. After satisfying 

this soil moisture deficiency, the residual recharges 

the mountain ground-water bodies, from which water 

seeps into the stream channel to appear as sur.face 

flow. As a result the mean annual precipitation of 

3, 100, 000 acre-feet produces only about 500, 000 acrefeet of runoff, which includes both surface flood runoff 

and the more stabilized ground-water seepage. This 

runoff amounts to 3. 9 inches over the entire mountain 

and foothill areas, or 16 percent of the precipitation. 

The natural water loss therefore is indicated to be 20. 1 

inches annually. A part of this may be considered as 

the.cost in water of maintaining the native vegetation, 

which serves to retard erosion of the relatively steep 

slopes. Although the loss seems large, actual experience has shown that only a small amount of this water 

could be salvaged by removal of the vegetation, at the 

expense ·of accelerated erosion. 

The distribution of the annual runoff from the mountain and foothill areas is shown graphically as the second part Gf figure 2. Because of its method of origin, 

the annual runoff shows even greater variability than 

the precipitation. While the mean annual runoff 

amounted to 500, 000 acre-feet, the annual runoff has 

been less than this amount in 71 percent of the years. 

In fact, 47 percent of the entire runoff for the 38-year 

period occurred in the five wet or flood years of 1921-

22 , 1936-37, 1937-38, 1940-41, and 1942-43, a condition typical of mo!?t semiarid areas. Because of the 

unusually high discharges occurring during these years, 

and the lack of suitable surface reservoir storage facilities, a sizable portion of this runoff wasted into the 

ocean. Under these conditions of extreme variability, 

the median runoff of 370, 000 acre-feet (by definition --

half the years have a greater runoff and half the years 

have a lesser runoff) is by far' a better estimate of the 

normal runoff. During the driest 10-year period of this 

relatively short record, the mean annual runoff 

amounted to 280, 000 acre-feet, while the wettest 10-

year period had a mean annual runoff of 740, 000 acrefeet or 2.6 times greater. 

Precipitation on Valley Floor 

In addition to runoff from the mountain areas, the 

valley floor receives about 1, 800 , 000 acre-feet annually in the form of precipitation as shown in the third 

part of figure 2. This represents an annual valley-floor 

precipitation of about 16 inches. Because of the flatter 

ground surface and the generally high rates of infiltration of the soil mantle, very little of this water results 

in flood runoff. However, as the area becomes more 

urbanized a portion of this precipitation finds its way 
..... 

PRECIPITATION ON VALLEY FLOOR 

Total precipitation in mountain and foothill areas tributary to valley floor. 

5,000,000 -

4,000,000 -

3,000,000 

2,000,000 -

1,000,000 -

0 

10-YEAR MEAN= 2,600,000 ACRE-FEET 

10-YEAR MEAN = 3,600,000 ACRE-FEET 

t .,.; Runoff from mountain and foothill areas tributary to valley floor. 

1,000,000 

0 

~.ooo,ooo -

2,000,000 

1,000,000 -

0 

1,000,000 -

0 

10-YEAR MEAN= 

740,000 ACRE-FEET I 

I 10-YEAR MEAN= 280,000 ACRE-FEET 

-·?::~~~~Mean ( 500,000 acre-feet) 

JlJlJll~idltlti}i.llJlJltbliiJLJLJIILIIJUtJUUUI:I~I·;,-Jr-1_ Median (

3

TO,OOO acre-feet! 

(\j 

0 

N 

~ 

Total preciPitation on valley floor 

10-YEAR MEAN= 1,550,000 ACRE-FEET 

Waste 

co .., N 

.n 0 N .., ~ ~ 

to ocean 

10-YEAR MEAN= 2,180,000 ACRE-FEET 

I 

_rMeon (1,800,000 acre-fNf) 

---Median (1,600,000 acre-fHf) 

10.. YEAR MEAN = 315,000 ACRE-FEET 

co 

.., ; I 

II) 0 .., .,. ~ ~ 

Figure 2. --Precipitation and runoff in the South Coastal Basin, Calif. (including the San Jacinto River Basin). 
4 WATER SUPPLY IN SOUTH COASTAL BASIN 

to the stream channels as a result of street drainage. 

The disposition of valley precipitation which does not 

run ·off is first to satisfy soil-moisture deficiencies 

and for the remainder to penetrate the ground-water 

zone. 

The diagrams on figure 2 indicate that the median 

annual precipitation amounts to 1, 600, 000 acre-feet or 

about 89 percent of the mean. During the driest 10-year 

period of 1923-33 the mean annual precipitation 

amounted to 1, 550, 000 acre-feet, while during the wettest 10-year period of 1936-46 the precipitation 

amounted to 2, 180, 000 acre-feet. 

Waste to Ocean 

Of the 1, 800, 000 acre-feet of precipitation falling 

annually on the valley-floor areas and the 5()0, 000 

acre-feet of the annual runoff reaching the valley floor 

from the mountain and foothill areas, only 190, 000 

acre-feet or 8 percent wastes to the ocean. In this instance the four wet years, 1921-22, 1937-38, 1940-41, 

and 1942-43 produced 52 percent of the total runoff 

wasted to the ocean during this entire 28-year period. 

The amount of wastage was greatest in 1922 although 

1941 was the year of highest precipitation. The influence of this flood runoff on the total waste to the ocean 

is clearly suggested by the lower diagram on figure 2. 

Effective conservation of this excessive runoff is extremely difficult because of its infrequent occurrence 

and the lack of natural storage sites in the steep and 

rugged mountain areas. 

Although the mean annual wastage to the ocean 

amounts to 190,000 acre-feet, the median value amounts 

to only 110, 000 acre-feet or about 58 percent of the 

mean. Because of the very irregular distribution of the 

annual precipitation, the drier 10-year period of 1923-

33 wasted annually only 72, 000 acre-feet to the ocean 

while the wet 10-year period of 1936-46 wasted annually 

315, 000 acre-feet or 4. 4 times that for the drier 

period. 

By way of summary: the 24 inches of annual precipitation on 2, 410 square miles of mountain and foothiil 

areas and the 16 inches of annual precipitation on the 

2, 140 square miles of valley lands results in a waste 

to the ocean of runoff equivalent to 0. 8 inch over the 

entire basin. Consequently the natural water losses of 

the mountain and foothill areas, together with the use 

on the valley floor, results in consumption of all but 4 

percent of the annual precipitation. 

Length of Available Records 

In an analysis such as this the investigator is always 

hampered by the shortness and incompleteness of the 

records. It was for this reason that it was necessary 

to confine this analysis to the 28-year period of 1920 

to 1948. The shortness of the record, coupled with the 

extreme variability of the precipitation, mountain runoff, ground-water recharge, and other factors affecting 

water supply, might have an eff~ct on this report. Consequently there is included figure 3 which gives the annu~l runoff distribution of some long-~erm records: 

namely, the San Gabriel River near Azusa, the Santa 

Ana River near Mentone, and the Santa Ysabel Creek 

tnear .. Mesa Gran,de_. 

During the entire period of record from 1895 to 1948, 

an interval of 53 years, the mean annual runoff of the 

San Gabriel River neaP Azusa ame>unts to 119, 000 acrefeet, which is substantially in agreement with the mean 

of 117, 000 acre-feet for the 28-year period of 1920-48. 

However, the median for the entire period of record of 

93, 000 acre-feet is considerably larger than the median 

of 76, 000 acre-feet for the period of 1920-48 and the 

normal annual runoff for the standardized base period 

of 1921-45 shown on figure 3. The wettest 10-year 

period of the entire record was in 1906-16 and produced 

7 percent more runoff than the 10-year period of 1936-

46, while the driest 10-year period of the entire record, 

which included the driest three years of record, was in 

1895-1905 and had a runoff of materially less than the 

driest 10-year period within the recent 28-year interval. 

Variability of Annual Runoff 

As tt.e San Gabriel River is an important source of 

water supply, the extreme variability of annual runoff, 

which ranged from 14 percent of normal for the water 

year of 1898-99 to 577 percent of normal for the water 

year of 1921-22, is of vital concern to the local water 

user. This variation in runoff is due to a combination 

of influences, paramount of which is the general magnitude of the mean annual precipitation and the absorptive and retentive qualities of the mantle rock. In the 

.aearby Santa Ana River, with somewhat less mean annual precipitation, the extremes of annual- runoff range 

from 32 percent of normal in 1898-99 to 462 percent of 

normal in 1915-16. This relatively greater annual runoff during extremely dry periods and smaller annual 

runoff during flood years suggest a much more absorptive and retentive mantle rock than that in the San 

Gabriel River drainage. Although the mean annual pre-

-cipitation is almost identica~ to that of the Santa Ana 

River the annual runoff distribution shown by the Santa 

Ysabel Creek record exhibits even greater extremes 

with 13 percent of normal runoff in 1947-48 and 1, 019 

percent of normal runoff in 1915-16. It is quite evident 

from the above that only with a sufficient length of rec-

;ord and with a complete understanding of the mountain 

mantle rock characteri~tics is it possible to predict or 

estimate the extreme conditions of flow which could occur in the many mountain and foothill areas tributary 

to the South Coastal Basin. 

Surface Reservoir Storage 

Efforts have been made since the first settlement of 

this area to develop mountain reservoir storage. The 

first of these structures is believed to be Bear Valley 

dam completed in 1884 followed by Lake Hemet dam in 

1893. Thes·e reservoirs were developed by the agricultural groups as conservation measures. It was not until 1920 when the urban areas began to encroach on natural stream courses that the first flood control dam at 

Devils Gate was built. The following table gives the capacity of the existing reservoirs in the South Coastal 

Basin. 
SURFACE RESERVOIR STORAGE 5 

Reservoir capacity, in acre-feet, in the South Coastal 

Basin 

Period in which Flood Control Conservation Total 

built 

Prior to 1930 7,000 120,000 127,000 

1931 to 1950 356,000 76,000 432.000 

Total 363,000 196,000 559,000 

In the case of multiple-purpose reservoirs it is often 

extremely difficult to separate the gross storage capacity of a reservoir into flood control and conservation. because most reservoirs are operated on a dayto-day schedule to give the maximum conservation with 

the minimum risk to possible flood damage. This generally means the assigning of greater and greater portions of the storage to conservation as the threat of 

floods diminishes with the advancement of the rainy 

season. 

As indicated in the above table. the existing storage 

capacity prior to 1930 was largely intended for water 

conservation. However. with the great increase in the 

100 -- Son Gabriel River near Azu•a, Calif . .,....----

~~------------~--------

_ Santa Yaabel Cr•k near Melo Grande, Calif. 

-------t----------··· 

w.t. • Tlhr,.,., 'f/IHIIf M MH tM Mo 

111«/itM ,_, lifo I$· IN' '.-ltHiol 1•11- 411 

population and industrial activities during the last 20 

years, flood control problems have become acute. As 

a result the major part of the reservoir capacity developed during these years was for flood control purposes. Because of unfavorable mountain sites, it has 

been necessary to create this storage capacity largely 

in the valley floor area. thereby practically eliminating 

the usefulness of the reservoir for conservation. 

At the present time the reservoir capacity allotted 

to flood control is almost equal to the median annual 

runoff from the mountain area. although during flood 

years such as those of 1937-38 this volume would 

amount to less than 25 percent of the annual runoff. 

The conservation facilities as furnished by reservoir 

capacity are of little value for long term carry-over 

because of the limited amount of capacity and the relatively protracted "dry" periods. 

Another problem encountered in management of storage reservoirs is that movement of heavy debris during 

floods reduces storage capacities. Some of this debris 

can be removed by sluicing. but some ca,n be removed 

only by hauling. 

--+-------+--------+---:-----··---

·-------+---------~---- - --·-

Figure 3. -Seasonal runoff distribution. 
6 WATER SUPPLY IN SOUTH COASTAL BASIN 

Ground-Water Storage 

Fortunately for the South Coastal Basin considerable 

ground-water storage capacity does exist in the deep 

alluvial-filled valley floor areas, where the depth of 

aquifers is as much as 3, 000 feet or more in certain 

places. The California Division of Water Resources in 

Bulletin 45, published by that agency, estimates the 

ground-water storage capacity for a zone 100 feet 

thick, 50 feet above and 50 feet below the water table 

of January 1933, to be 6, 860, 000 acre-feet. With the 

inclusion of the San_ Jacinto River Basin, this storage 

capacity increases to about 7, 500,000 acre-feet. However, the usable storage volume in this tremendous 

reservoir is limited to that portion which can be dewatered economically and then replenished effectively, 

either by natural or artificial means. Under present 

conditions these ground-water bodies are the principal 

source of the water requirements for the agricultural, 

industrial and a considerable portion of the municipal 

and domestic demands of the South Coastal Basin. 

As a result of these great extractions the groundwater storage has become depleted localiy. In some 

basins, long-time perennial yields have not been exceeded; in others, however, alarming overdrafts have 

been developed. These two conditions are best illustrated by fluctuations of the ground-water level at key 

wells as shown in figure 4. The upper part of this figure shows the altitude of the ground water in the Bunker 

Hill and San Gabriel ground-water basins, which are 

two of a group of more than 60 identifiable ground-water 

basins within th~ South Coastal Basin. In the Bunker 

Hill ground-water basin near San Bernardino the water 

level of less than 1, 130 feet above sea level in January 

1900 dropped to 1, 110 feet in the early spring of 1905; 

thereafter it rose to. an altitude of greater than 1, 150 · 

.feet in 1916 and 1923. As a result of the dry 10-year 

period of 1923-33, the ground-water 'level dropped 

steadily to a minimum elevation of about 1, 095 feet in 

the fall of 1936. However, during the subsequent wet 

10-year period of 1936-46 the level rose to 5tn elevation 

slightly less than 1, 150 feet. Thus the ground-water 

storage, depleted as a result of an extended dry period, 

was completely recharged during a subsequent period 

of above-normal precipitation. The ground-water extractions in this basin are less than the natural recharge and the record of ground-water fluctuation is 

assumed to be typical of those areas considered not to 

be overdrawn. The records obtained in the San Gabriel 

Basin near Azusa are included in this figure as confirmation of ground-water behavior in areas where the 

extractions are well within the limits of the recharge. 

The remaining records of ground-water fluctuation 

are obtained from key observation wells in groundwater basins showing various degrees of overdraft. 

The record of ground-water fluctuations beginning in 

1904 for the well in the West Coastal Basin located in 

the vicinity of Gardena and about 5 miles from the 

ocean shows the progressive depletion of the grouQdwater storage. From an altitude of about 10 feet above 

sea level in the fall of 1923 it dropped to 10 feet below 

sea level in 1933 the end of the dry 10-year period. 

However, during the subsequent wet period, in which 

the annual recharge vA:>uld be at its greatest, the altitude of the ground water shows continual decline so 

that by the end of the wet 10-year period in 1946 the 

ground-water altitude was about 20 feet below sea level. 

It must be assumed that during this period the pumping 

demand not only used all the accelerated annual recharge, but continued to draw on hold-over storage. 

The records for the remaining wells show the same 

typical pattern found in areas where the pumping draft 

exceeds the natural recharge. The overdraft in the 

South Coastal Basin is believed to be on the order of· 

100,000 acre-feet annually. 

This depletion of ground-water storage becomes extremely serious in those grO'Ilnd-water basins adjacent 

to the ocean. In the West Coastal Basin, an area of 

175 square miles along the coast between Redondo 

Beach and Santa Monica and inland to Beverly Hills and 

Inglewood, the ground-water level in the main waterbearing deposits in November 1945 was below sea level 

in at least 75 percent of the area, was 20 feet below 

sea level in at least half the area, and 50 feet below in 

nearly 10 percent of the area. Since that time the average water levels have been drawn down several feet 

more. This condition fosters sea-water encroachment, 

and as has been pointed out by Poland and others 

( 1948) 1/ ·this condition has actually existed for more 

than 20 years in this area. Sea-water encroachment 

has also occurred or is occurring in parts of the coastal plain in Orange County. 

Ground-Water Recharge 

The ground-water reservoirs of the valley-floor 

areas are recharged mainly by deep penetration of the 

precipitation falling on the valley floor, by absorption 

of surface runoff in the natural stream channels and 

by artificial means, such as water ~preading. It is 

quite obvious from figure 4 that in many parts of the 

South Coastal Basin the present rate of extraction exceeds the combined rate of recharge from all three of 

these sources. 

Deep Penetration 

Deep penetration is first influenced by the land slope 

and permeability of the surface soil mantle. Highly 

permeable soils having moderate land slopes are generally conducive to minimum flood runoff and maximum 

absorption of the storm precipitation. However, soils 

of low permeability will produce the reverse effect with 

a greatly reduced opportunity for deep penetration. 

After moisture enters the soil mantle it must first satisfy the existing soil moisture defic.iencies. In the fall 

just prior to the winter precipitation period these deficiencies may be as great as 18 inches in the chaparral 

or native brush-covered areas and as little as 3 inches 

. in the frequently irrigated citrus areas. Because of the 

many phases or influences affecting deep penetration, 

its evaluation is always quite complex. However, 

Muckel (Muckel and Aronovic.i., 1948) did estimate the 

deep penetration centered around the towns of Chino and 

Ontario in the Chino Basin .and i,n an integral part of the 

South Coastal Basin. He found the average annual deep 

penetration over this 237-square-tnile valley-floor area 

to amount to about 4. 5 inches or about 240 acre-feet per 

square mile. However, in the wet year 1940-41, Muckel 

found that the deep penetration amounted to 3. 8 times 

the average value or over 900 acre-feet per square mile 

of area. In fact the recharge for this single season ac1 See selected references, p. 
GROUND-WATER RECHARGE 7 

1160 Ground water areas havino ~ apparent overdraft 

· SIO .. 

E 

:z•o r----- --~r-------~---------+--------~~-- 0 

.... 

" 

.. 

:! 

. 

> 

.! 

270 

Ground water areas 

~ or----------t----~--+-------~--------~~~ 

" 

"' 0 

.. 

! 

~ 

"' \ 

" 

'" .. 

e ':.: ...... -y 

: 

0 

.... 

0 

.!: 

0 

0 

,. 

----

on 

0 

,. 

----

550 -+---------+------c 

0 

N 

!! 

on 

... 

!! 

0 

~ "' on 

"' ,. 0 

• !! 

... 

• !!' 

Figure 4. -Fluctu ation of ground-water level at key wells in the South Coastal Basin. 

counted for 19 percent of the total recharge by deep 

penetration for the period of 1927-47 (the period of the 

Muckel Investigations), witJ:t the four wettest years of 

1936-37, 1937-38, 1940-4 1, a nd 1942-43 producing 

about 59 percent of the total recharge fr.om this source. 

This distribution in r egard to time is believed to b e 

typical of that occurring over the entire area. 

It cannot, however, be safely assumed that all parts 

of the South Coa stal Basin received a recharge of a 

magnitude similar to that for Chino gro und -water ba -

sin. As alr eady indicated this type of recharge is de -

pendent upon the precipitation, topography, soil type, 

type of cultur e, geo logy, and many other features of a 

less-controlling nature. 

Natural Absorption of Surface Runoff 

The natural absorption of the surface runoff a long the 

bottom of the str eam c hann e ls represents an important 

contribution to valley floor g round-water recharge . Of 

course, s ubstantia l absorption occurs only when he altitude of the water table is well below that of tli.e · 

stream-bed. , However, this condition exists in many of 

the valley areas of the South Central Basin and conditions for recharge are very favorable . 

The large recharge from this source during flood 

periods is often surprising to many observers. During 

the month of March 1938 when mos t of the streams 

were in extreme flood the runoff absorption in the val-
8 WATER SUPPLY IN SOUTH COASTAL BASIN 

ley floor areas amounted to 400, 000 acre-feet or 

slightly more than 40 percent of the flood waters dis-

·charging into the area from the many streams in the 

tributary mountains and foothills. An equally large 

portion of the extreme flood runoff for shorter intervals of time such as the maximum day is often absorbed over_ large sections of the valley floor. During 

such wet 10-year periods as 1936-46 at least 425,000 

acre-feet was absorbed annually into valley-floor 

ground-water storage or diverted for irrigation. 

Artificial Means of Ground-Water Recharge 

The means of recharge generally known as water 

spreading has been practiced for many years in parts 

of southern California. The oldest organized spreading operation for which factual data are available is 

that on the cone of the Santa Ana River east of San 

Bernardino. Records of spreading at this location are 

continuous since 1912; annual amounts of spread range 

from 68,000 acre-feet in 1921-22 to none in the dry 

years of 1924-25, 1928-29, 1930-31, and 1932-33. In 

more recent years this has been augmented by spreading in other parts of the Santa Ana River Basin as well 

as in other portions of the South Coastal Basin. The 

annual distribution of this spreading by organized 

groups is shown on figure 5 for the Santa Ana and San 

Gabriel River Basins. 

In the Santa Ana River Basin this annual spreading 

has a mean value of 15, 200 acre-feet and a median of 

10, 000 acre-feet. Of the total amount of the water 

spread since 1912, 17 percent was spread in 1921-22 

and together· with that spread in four of the other wettest years represents 41 percent of all the water spread. 

During the very dry 10-year period of 1923-33, 12, 400 

acre-feet was spread annually, increasing to 21, 300 

acre-feet during the wet 10-year period of 1936-46. 

However, because of certain legal requirements, these 

upstream spreading operations of the Santa Ana River 

are greatly restricted under all but extreme flood 

conditions. 

The second part of figure 5 gives the annual distribution of the organized spreading in the San Gabriel River 

Basin sponsored by the Los Angeles County Flood Control District and its associates. As in the preced:.ng 

case the major portion of this increment to groundwater recharge occurred during the wetter years. For 

example, 33 percent of the entire recharge from this 

source since 1920 occurred in the four years 1936-37, 

1937-38, 1940-41, and 1942-43. 

It should be fully understood that the above values 

represent gross spreading, as a sizable portion of 

those quantities would have been absorbed naturally in 

the river channels further downstream. While the net 

contribution to ground-water recharge from this source 

may not be large, it does show that this means of water conservation is effective. 

Local Supply 

The gross average annual local supply of this basin, 

as shown on figure 2, is essentially the precipitation on 

the alluvial valley-floor areas of 1, 800, 000 acre-feet, 

plus the runoff from the mountain and foothill areas 

tributary to the valley-floor area of 500,000 acre-feet. 

However, not all of this water is available for the support of the agricultural crops or the demands of the 

domestic and industrial requirements of the South 

Coastal Basin. Immediately following and during a 

storm, there is a considerable loss by evaporation 

from the interception on the leaves of vegetative cover 

and from the surface of the soil. This evaporation represents an unrecoverable water loss to the atmosphere 

estimated by Blaney ( 1933) to be equivalent to about 

0. 5 inch per storm and ranging from 3 to 8 inches per 

year dependent on the wetness of the year. The average 

annual evaporation from precipitation at the land surface is roughly calculated to be on the order of 500, 000 

acre-feet. In addition to this unrecoverable loss, mean 

annual waste to the ocean is 190,000 acre-feet. The 

following table gives the estimated total recoverable 

water supply. 

Estimated total recoverable and available local annual water supply, in acre-feet, of the South Coastal Basin 

10-year 10-year 

Mean Median Mean Mean 

1920-48 1920-48 1923-33 1936-46 

Mountain and foothill runoff~/ 500,000 370,000 280,000 740,000 

Precipitation on the valley 

floor a/ 1,800,000 1, 600, 000 1,550,000 2,180,000 

Unrecoverable water losses of 4 

inches over valley floor area~/ -500,000 -500,000 -500,000 -500,000 

Waste to oceanb/ -190,000 -110, 000 -72,000 -315,000 

Change ir:t ground-water storage 0 0 0 0 

Total recoverable water 

supply~/ 1,600,000 1,400,000 1,300,000 2,100,000 

Soil mosture available only to 

vegetative cover~/ -900,000 -900,000 -900,000 -900,000 

Available surface and groundwater supply~/ 700,000 500,000 400,000 1,200,000 

a Reasonably accurate 

b Rough approximation only 
LOCAL SUPPLY 9 

100,000 

90,000 

10,000 

Santo Ana River Basin 

70,000 

60,000 

't 50,000 

~ . 

~ 40,000 

'<( 

30,000 

20,000 

10,000 

0 

100,000 

90,000 

80,000 

70,000 

60,000 

... 

~ 50,000 

t 40,000 

'<( 

30,000 

20,000 

10,000 

0 

~ ! . .. ... !! ! 

I(). YEAR MEAN • 

12,-400 ACRE-FEET 

·san Gabriel River Basin 

1(). YEAR MEAN~ I I 14,700 ACRE-FEET 

r----,. ---- :-------1--------------

.. ... ;;; . ':' 0 ., 0 01 .. .. • .. ~ 

"' ':' 

"' .. 

!! 

10-YEAR MEAN= 21,300 ACRE-FEET 

• 0 

• !! 

HI-YEAR MEAN m 

57,900 ACRE-FEET 

., 

• !! 

Figure 5. -Annual water spreading in the South Coastal Basin. 

However a portion of this recoverable water is in 

the form of soil moisture available only to vegetative 

cover. This soil moisture represents a first claim 

against the recoverable water, as it is first necessary 

for the precipitation to satisfy the existing soilmoisture deficiencies developed by transpiration from 

native or agricultural crops during the preceding growing season before recharge to the ground-water reservoir occurs by deep penetration~ These deficiencies, 

ranging from about 3 inches in the shallow-rooted citrus grove to 18 inches in the deep-rooted native chaparral, are estimated to be about 900 , 000 acre-feet or 

almost 8 inches.over the vall ey -floor area. Furthermore, the se deficiPnc ie s should remain fairly constant 

from year to year, as long as there is little change in 

type of land use. As shown by the table, the available 

mean annual ,supply of surface and ground water is estimated to be 700,000 acre-feet. This supply can be 

increased only to the extent that the average waste to 

the ocean (190, 000 acre-feet) can be reduced by additional conservation structures. 

The following table shows the present source of the 

surface- and ground-water supply available to the 

South Coastal Basin for the mean period of 1920-48, 

as well as for the dry 10-year period of 1923-33 and 

the wet 10-year period of 1936-46. 
10 WATER SUPPLY IN SOUTH COASTAL BASIN 

Estimated sources of the available surface- and ground-water supply,· in acre-feet 

10-year 10-year 

Source Mean Mean Mean 

1920-48 1923-33 1936-46 

Surface diversions of mountain and foothill runoff to valley floor!/ 200,00(1 200,000 200,000 

Recharge to ground-water storage {net) 

(a) Stream-bed absorption of surface runoff (including water 

~ spreading) 500,000 200,000 1,000,000 

(b) Deep penetration of precipitation below the root zone 

Total 700,000 400,000 1,200,000 

1 Probably about 25 percent of the diverted runoff subsequently moves downward to ground-water storage from the 

irrigated areas where it is applied. 

The first item in the above table represents the diversions of surface runoff from the mountain and foothill streams for use in the valley-floor areas. In most 

instances this runoff \s seepage from mountain groundwater bodies, which in turn have a very stabilizing effect on the magnitude of this diverted water. For this 

reason the amounts diverted are assumed to be constant 

for the mean periods used in this table, although these 

diverted quantities actually would vary somewhat from 

year to year, depending on the wetness of the year. 

The last item of this table represents the net recharge to the ground-water reservoir from stream bed 

absorption of surface runoff and deep penetration from 

the precipitation below the root zones of the vegetative 

cover in the valley-floor areas. Each phase of this recharge has been developed separately, but is here presented as a single item due to the many uncertainties 

and lack of reasonable accuracy in these determinations. It will be noted that this annual recharge to 

ground-water storage ranges from about 200, 000 acrefeet during the dry 10-year period of 1923-33 to about 

1, 000,000 acre-feet during the wet 10-year period of 

1936-46 w'ith a mean value of about 500, 000 acre-feet. 

Thus, it is possible to create a considerable apparent 

overdraft of the ground-water supply during extended 

dry periods and to replenish storage in the more favorable subsequent wet years. (See fig. 4, hydrographs 

for Bunker Hill and San Gabriel Basins.) 

Demand 

As of 1949, the estimated mean annual water demand 

of plants and man for the entire South Coastal Basin 

amounted to about 2, 100, 000 acre-feet. The sources 

from which this demand was taken are shown as 

follows: 

Mean annual total recoverable 

water supply 

Importations by Owens Valley 

and Colorado River aqueducts 

Overdraft of the valley floor 

ground-water storage as of 

1944-45 

Mean annual demand 

Acre-feet 

1,600,000 

400,000 

100,000 

2, 100,000 

The first item, mean annual total recoverable, was 

derived in the tabulation on page 8. The importations 

given as the second item are measured by the city of 

Los Angeles and the Metropolitan Water District of 

Southern California. The overdraft of the ground-water 

storage is largely based on the work of Gleason (1947) 

and Young, Ewing, and Blaney (1941) and relates chiefly to average conditions as of 1944-45. 

In a very general way the mean annual demand can 

be segregated into vegetal demands and those of the 

municipal, industrial, and domestic needs as indicated: 

Vegetal demands 

Soil moisture · 

Developed surface and ground-water 

Municipal, industrial, domestic 

Total use 

Selected References 

Acre-feet 

900,000 

500,000 

700,000 

2,100,000 

Blaney, H. F., 1933, Rainfall penetration, Ventura 

County investigation: California Water Resources 

Div., Bull 46, p. 89. 

Gleason, G. B., 1947, South Coastal Basin investigation, Overdraft on ground-water basins: California 

Water Resources Div., Bull. 53. 

Hamilton, E. L. , and Rowe, P. W., 1949, Rainfall 

interception by chaparral in California: California 

State Board of Forestry. 

Muckel, D. C., and Aronovici, V. S., 1948, Progress 

report on rainfall and irrigation water penetration 

and consumptive use in the Chino Basin, Santa Ana 

Valley: U. S. Dept. Agr., Soil Cons. Service (mimeographed rept. ) . 

Poland, J. F. , Garrett, A. A. , and Sinnott, A. , 1948, 

Geology, hydrology, and chemical character of the 

ground waters in the Torrance-Santa Monica Area, 

Los Angeles County, Calif.: U.S. Geol. Survey (unpublished rept. ). 

Veihmeyer, F. J., 1950, Soil moisture, runoff, erosion: California Agr. vol. 4, no. 10. 

Young, A. A., Ewing, P. A., and Blaney, H. F., 

1941, Utilization of the waters of Beaumont Plains 

and San Jacinto Basin, Calif.: U. S. Dept. Agr., 

Soil Cons. Service (mimeographed rept. ). 

lQterior-Oupll.catin& Sectioll, llasbin&ton, D· C. 9 ll71