flood flows of the san pedro river...pedro river, even within the seasonal framework, is evident in...

KATHERINE K.

HIRSCHBOECK

SIX TEEN

Flood Flows of the San Pedro River

IntroductionThis chapter addresses surface water fl ows of the San Pedro River with an

emphasis on fl ood fl ows. In terms of total volume, the storm runoff or fl ood

fl ow component of streamfl ow is much larger than the basefl ow. Although

individual fl ood fl ows can be very large, they are sporadic, with up to several

months or more between such fl ows, and thus provide only an intermittent

source of water to the stream. I discuss temporal patterns of fl ood regimes,

from intra-annual to decadal, as well as spatial patterns over the length of

the river.

Seasonal Patterns and VariabilityThe surface water hydrology of the San Pedro River basin has a distinctly

seasonal regime. It is characterized by moderate and relatively consistent

monthly streamfl ow during the winter wet season (primarily December

through March), low streamfl ow in the late spring and early summer dry

season (April through June), and greater and more variable fl ow during the

summer wet season ( July through September) (fi g. 1). This seasonal pattern

results from the spatial and temporal interplay of latitudinally shifting win-

ter storm tracks (Aldridge and Hales 1984), and infl uxes of moist air dur-

AZ-E-Stromberg-part3.indd 300AZ-E-Stromberg-part3.indd 300 11/11/2008 12:55:46 PM11/11/2008 12:55:46 PM

Katie

Text Box

Hirschboeck, Katherine K. (2009) Flood flows of the San Pedro River. Pages 300-312 in Stromberg, Juliet C. and Tellman, Barbara (eds.) Ecology and Conservation of the San Pedro River. University of Arizona Press: Tucson, 656 pp.

Flood Flows 301

ing the summer monsoon thunderstorm season (M. W. Douglas et al. 1993,

Adams and Comrie 1997). In addition, the occasional penetration of tropical

storm – related moisture during summer or fall (especially in September and

October) can trigger copious runoff-producing rains (W. Smith 1986, Roeske

et al. 1989).

Desert rivers throughout the world have highly variable fl ow regimes

(Tooth and Nanson 2000, Bunn et al. 2006). The high variability of the San

Pedro River, even within the seasonal framework, is evident in fl ow duration

curves (fi g. 2). While discharges are much larger during the summer months,

they also tend to be more variable. The steep fl ow duration curve calculated

for the warm season months implies high fl ow variability, while the fl atter

slope for the cooler months suggests that streamfl ow is being sustained more

consistently by either surface runoff or groundwater. Flow duration curves

calculated by decade show that while the low-fl ow portion of the curves has

steepened dramatically over the past several decades, the high-fl ow portion

has varied more idiosyncratically (fi g. 3). For example, the curve for 1976 –

1985 is distinctly fl atter than those for other decades. This decade experi-

enced an exceptionally large tropical storm – related fl ood (October 1977)

in addition to multiple occurrences of extreme winter fl ooding (December

1978, January 1979, December 1984).

FloodsFlood events contribute signifi cantly to the overall surface water regime of

the San Pedro, and thus the seasonality of annual fl ood peak occurrence

mirrors the seasonal pattern of mean monthly fl ows (fi g. 4a). The large dif-

ferences between mean and median discharge for nearly all months (fi g. 1)

NovOct

MeanMedian

12

10

8

6

4

2

0JanDec MarFeb MayApr JulJun SepAug

Mon

thy

disc

harg

e (m

3s-1)

Fig. 16.1. Mean monthly streamfl ow variations for the San Pedro River at Charleston (#09471000), for 1943 – 2005. Values shown are the median and the mean plus one standard deviation.


302 Katherine K. Hirschboeck

suggest that the fl ow from a relatively small number of extremely large events

(fl oods) is an important component of the mean monthly discharge. Runoff-

producing storms from three types of atmospheric patterns vary seasonally

and interannually to control the frequency, magnitude, and timing of fl oods.

Typically, the annual fl ow regime consists of a maximum peak in July or

August, caused by intense summer convective rainfall, and one or more sec-

ondary winter fl ows in November through March, caused by synoptic storm

precipitation (Hirschboeck 1991, House and Hirschboeck 1997).

Percent of time that indicated discharge was equalled or exceeded

0 20 40 60 80 100

Dis

char

ge (m

3s-1)

0.01

0.1

1

10

100Mean of all monthsMean of cool season months (Nov.-Apr.)Mean of warm-season months (May-Oct.)

Fig. 16.2. Flow duration curves

(based on 1943 – 2005 water year mean

monthly streamfl ow) for cool-season months, warm-season months, and all months

combined, for the San Pedro River

at Charleston.

Percent of time that indicated discharge was equalled or exceeded0 20 40 60 80 100

Dis

char

ge (m

3s-1)

0.01

0.1

1

10

100WY 1936-1945WY 1946-1955WY 1956-1965WY 1966-1975WY 1976-1985WY 1986-1995WY 1996-2005

Fig. 16.3. Flow duration curves for

seven decades of recorded daily mean

streamfl ow of the San Pedro River at Charleston. Note: Prior to 1942, the

Charleston gage was located at various positions along a six-mile reach of

the river north of its present location,

and thus the 1936 – 1945 decade may have included

fl ow from the Babocomari River.


Flood Flows 303

SEASONAL FLOOD TYPES

Floods from convective storms. During 1950 – 2005, at the gaging stations of Palo-

minas, Charleston, and Redington, almost 90 percent of the annual maxi-

mum fl ood peaks occurred during the months of July through October (fi g.

4a). Two-thirds of the annual peaks occurred during July and August, the

most active months of the summer monsoon thunderstorm season. Figure 4b

illustrates the daily mean fl ow regime at the Charleston gage for three typical

years during which summer fl ood peaks dominated. Several large peak-fl ow

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Perc

ent o

f ann

ual p

eaks

0

10

20

30

40A C

B

PalominasCharlestonRedington

Dai

ly m

ean

disc

harg

e (m

3s-1)

0

20

40

60

80

100

197919781977195319521951J F MAM J J A S ON D J F MAM J J A S ON DJ F MAM J J A S ON D

Dai

ly m

ean

disc

harg

e (m

3s-1)

0

100

200

300

400

500

Summer peaks

Summer peaks

Summer peaks

Late summer/fall peak

Winter peaks

J F MAM J J A S ON D J F MAM J J A S ON DJ F MAM J J A S ON D

Fig. 16.4. Seasonal variations in San Pedro River fl ood peaks. (A) percent of annual fl oods occurring in each month at the Palominas, Charleston, and

Redington gages for water years 1950 through 2005 (stations #9472000 and 9472050 combined for Redington), (B) three years of daily mean discharges

at the Charleston gage during a period of dominance by summer fl ood peaks (maximum instantaneous peaks for the three years are 162 m3s-1 on 7-2-1951, 222 m3s-1 on 8-17-1952, and 243 m3s-1 on 7-7-1953), and (C) three years of daily mean discharges at the Charleston gage during a period of dominance by fall

and winter fl ood peaks (maximum instantaneous peaks for the three years are 672 m3s-1 on 10-8-1977, 329 m3s-1 on 12-18-1978, and 334 m3s-1 on 1-8-1979).



events are evident in the daily discharge data, and instantaneous fl ood peaks

on a given day can be two to fi ve times greater than the daily mean value. The

fl ashiness of the fl ow regime and the occurrence of multiple peaks are indica-

tive of frequent and variable convective storm activity.

Floods from tropical storms. September and October typically are much dryer than

July and August, although susceptibility to intense, tropical storm – related

precipitation increases during these late summer/early fall months. Figure 4c

illustrates one such event, the severe fl ood of October 1977 linked to rainfall

from Tropical Storm Heather in the eastern Pacifi c Ocean, which caused wide-

spread fl ooding throughout southern Arizona (Aldridge and Eychaner 1984).

Floods from synoptic winter storms. A fl ood season occurs in winter in response

to less-intense but more-sustained precipitation events, which usually yield

moderate discharges. Only 10 percent of the annual maximum fl ood peaks

in the San Pedro River were recorded in the months of December through

March during 1950 – 2005. When these winter fl oods do occur, however, they

tend to be of longer duration than the more fl ashy summer fl ood peaks; hence,

the total fl ow volume of a winter event may exceed that of a summer event,

even though the summer fl ood has a larger instantaneous discharge peak.

Occasionally, a sequence of winter storms can lead to an exceptionally active

winter fl ood season, such as the sequence of peaks in the winter of 1978 – 1979

(fi g. 4c). Since 1965 there has been an increase in the frequency of maximum

annual fl ood peaks occurring in winter in the San Pedro basin.

TEMPORAL VARIABILITY

Extreme events. All of the extreme discharges of record for each of the gaging

stations in the river basin have occurred during May through October. All

are associated with convective storms or with convective rainfall enhanced by

moisture from dissipating tropical storms (table 1). The extremes of record in

tributaries with small drainage areas are more likely to be caused by convec-

tive storms in July or August, in contrast to the record fl oods on the main

trunk stream where all but two are associated with late-season eastern Pacifi c

tropical storms.

No single storm event stands out as the cause of the largest fl ood of record

at all the gaging stations, in part because of different periods of record at each

station. Two notable tropical storms, however, produced the largest fl ood of

record at several stations. The fl ood associated with the unnamed tropical

storm of September 1926 produced exceptionally large fl ood stages from

Palominas in the upper basin to Redington in the lower basin. The gage at

Charleston recorded the greatest discharge in the basin for this event, and it

dominates the annual fl ood time series for this station (fi g. 5), as well as that


Flood Flows 305

of the Redington station downstream (not shown). Three gages in the lower

basin experienced their greatest fl ood of record in response to intense pre-

cipitation linked to Tropical Storm Octave in October 1983 (water year 1984).

This event also produced major fl ooding throughout central and southern

Arizona, especially in the San Francisco, Gila, and Santa Cruz Rivers.

Intra-annual patterns. Limiting the discussion to only the largest fl oods in the

record, or to the annual fl ood time series, neglects other peak fl ow events that

play an important role in the fl ood hydroclimatology and riverine ecology of

the basin. As suggested in daily mean discharge plots (fi gs. 4b, 4c), a given fl ow

season in the San Pedro can be characterized by multiple fl ow events — some of

substantial discharge — although only one will be recorded as the annual fl ood

peak. Information on these lesser fl ood peaks can be obtained from a station’s

peaks-above-base record, which includes all discharges above a designated

base discharge at a given recording station.

Figure 6 displays frequency histograms of the peaks-above-base record for

the Charleston gage, separated by type of fl ood-causing storm for the period

1950 – 1985. The peaks were classifi ed according to storm type based on a

detailed analysis of weather charts and other sources (Hirschboeck 1987, 1988).

(Histograms for the Palominas and Redington gages have patterns similar to

that of the Charleston record.) This type of classifi cation is more climatically

accurate than a simple seasonal division of fl oods into winter, summer, and

fall months (to represent synoptic, convective, and tropical storm – related

fl oods, respectively) because some synoptic-scale circulation features (upper-

level cutoff lows and short wave troughs) have contributed to enhanced fl ood-

ing during the warm-season months, and some tropical storm – derived fl oods

Water year

Inst

anta

neou

s pe

ak d

isch

arge

(m3s

-1)

0

500

1000

1500

2000

2500

3000

20052000

19951990

19851980

19751970

19651960

19551950

19451940

19351930

19251920

1915

Figure 16.5. Annual fl ood peak time series of the San Pedro River at Charleston (values are instantaneous peak fl ows). The maximum recorded peak (September 1926) was 2,775 m3s-1.


TA

BLE

1.

Max

imum

sta

ge a

nd d

isch

arge

at

San

Pedr

o B

asin

sta

tion

s fo

r pe

riod

of r

ecor

d.

Stat

ion

num

ber

0947

-U

SGS

stat

ion

nam

e

Dra

inag

e ar

ea

(km

2 )

Peri

od

of r

ecor

dW

ater

yea

rD

ate

Stor

m t

ype

linke

d to

fl ood

ing

Stag

e (m

)

Peak

dis

char

ge

(m3 s

-1)

R.I.

3

(yrs

)

Main Channel

0500

San

Pedr

o a

t Pa

lom

inas

1909

1926

–051

1926

1940

09/2

8/26

08/1

4/40

Trop

ical

sto

rm (

TS)

7.3

5.0

— 623

—

20

1000

San

Pedr

o a

t C

harl

esto

n31

9619

16–0

519

2609

/28/

26Tr

opic

al s

torm

6.

72,

775

>100

1550

San

Pedr

o n

ear

Tom

bsto

ne44

8119

67–0

519

7810

/09/

77T

S H

eath

er3.

568

550

–100

1800

San

Pedr

o n

ear

Ben

son

6449

1966

–76

1972

08/2

6/72

Con

vect

ive

3.2

278

<10

2000

San

Pedr

o n

ear

Red

ingt

on75

8119

26–9

719

2609

/28/

26Tr

opic

al s

torm

6.6

2,54

9 2

>100

2050

San

Pedr

o at

Red

ingt

on

Bri

dge

near

Red

ingt

on80

1919

99–0

520

0307

/22/

03C

onve

ctiv

e4.

616

7—

3100

San

Pedr

o be

low

A

rava

ipa

Cre

ek n

ear

Mam

mot

h

1124

819

80–8

419

8410

/01/

83T

S O

ctav

e5.

43,

823

>100

3500

San

Pedr

o a

t W

inke

lman

1153

319

19–8

3119

8410

/01/

83 2

TS

Oct

ave

—3,

823

>100


Tributaries

0900

San

Pedr

o Tr

ibut

ary

nea

r B

isbe

e13

.619

63–7

919

6509

/04/

65T

S Em

ily3.

741

.3—

1087

Wal

nut

Gul

ch 6

3.11

1 n

ear

Tom

bsto

ne0.

619

62–8

119

7607

/27/

76C

onve

ctiv

e—

15.3

>100

1090

Wal

nut

Gul

ch 6

3.00

9 n

ear

Tom

bsto

ne23

.619

67–8

119

7207

/24/

72C

onve

ctiv

e—

74.8

25–5

0

1110

Wal

nut

Gul

ch 6

3.01

5 n

ear

Tom

bsto

ne23

.919

55–8

119

7108

/10/

71C

onve

ctiv

e—

35.4

10–2

5

1185

Wal

nut

Gul

ch 6

3.10

3

nea

r To

mbs

tone

0.7

1963

–81

1975

07/1

7/75

Con

vect

ive

—0.

9<1

0

1195

Wal

nut

Gul

ch 6

3.00

7 n

ear

Tom

bsto

ne13

.519

66–8

119

7208

/12/

72C

onve

ctiv

e—

73.3

50–1

00

2100

Peck

Can

yon

Trib

utar

y

nea

r R

edin

gton

20.8

1968

–81

1972

08/1

2/72

Con

vect

ive

2.8

123

>50

3000

Ara

vaip

a C

reek

n

ear

Mam

mot

h13

9119

19–9

4119

8410

/02/

83T

S O

ctav

e5.

12,

005

>100

3200

Gre

en L

ante

rn W

ash

nea

r W

inke

lman

9.4

1964

–761

1981

05/0

1/81

Con

vect

ive

2.7

105

20–4

0

1 Obs

erva

tion

s fo

r so

me

year

s in

rec

ord

are

mis

sing

.2 D

isch

arge

or

date

is e

stim

ated

. 3 R

.I. =

Rec

urre

nce

Inte

rval

(R

.I. e

stim

ates

bas

ed o

n Pe

rry

et a

l. 20

01).



have occurred in July and August, even though the tropical storm infl uence is

most pronounced in September and October in southern Arizona.

The fl ood-type classifi cation shows the dominance of frequent sum-

mer peaks that are produced by intense convective rainfall events, but also

emphasizes that most of these fl oods have moderate peak discharges (mean

= 153 m3s-1), whereas the smaller populations of fl oods having synoptic or

tropical storm infl uences have larger mean discharges (168 m3s-1 for synoptic

storms, 224 m3s-1 for tropical storms). Hence, a shift to a climatic regime

favoring more frequent synoptic or tropical storms could alter the magni-

tude and frequency of fl ooding in the San Pedro basin.

Decadal patterns. Figure 7, which shows the Charleston annual fl ood record

classified by f lood type, illustrates that abrupt shifts in f lood type have

occurred in the San Pedro basin over the twentieth century. This is seen most

Convective

0

5

10

15

20

25

30

35

40

45

Synoptic

Num

ber o

f occ

urre

nces

0

5

10

15

Tropical storm

Discharge classes (m3s-1)

0 50 100150200250300350400450500550600650700750800850900950M

ore

0

5

10

15

San Pedro at CharlestonPeaks-above-Base1950-1985

Base discharge = 85 m3s-1

_x = 153 m3s-1

_x = 168 m3s-1

_x = 224 m3s-1

Figure 16.6. Histograms of instantaneous peaks-above-base

discharge for convective, synoptic, and tropical storm fl oods recorded at the Charleston gage

(1950 – 1985).


0

-1

-2

-3

1

2

3

4

10 000

20 000

30 000

40 000

50 000

0

0

10 000

20 000

30 000

40 000

50 000

0

10 000

20 000

30 000

40 000

50 000

60 000

0

500

1000

1500

0

500

1000

1500

0

500

1000

1500

98 000 (f3s-1)

Convective Storm Floods

Synoptic Storm Floods

Tropical Storm Floods

Nino –3 SST Index

a

b

c

d 5Warm phase

Cold phase

-PDO + PDO -PDO + PDO - PDO

1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

Peak

Dis

char

ge (f

3s-1)

Peak

Dis

char

ge (m

3s-1)

Peak

Dis

char

ge (m

3s-1)

Peak

Dis

char

ge (m

3s-1)

Peak

Dis

char

ge (f

3s-1)

Peak

Dis

char

ge (f

3s-1)

Fig. 16.7. Annual fl ood time series of the San Pedro River at Charleston, classifi ed by fl ood-producing storm type: (a) convective, (b) synoptic,

and (c) tropical, plotted with (d) the Niño-3 SST Index. Also shown are El Niño years (gray vertical lines) and positive (+) (shaded) and negative (-) phases of the PDO. (Flood values are instantaneous peak fl ows. An

open bar is plotted for the October 1983 tropical storm fl ood which was the annual fl ood at downstream gages, but not at Charleston.)



dramatically after 1965 when there was a shift for annual fl oods to be pro-

duced more frequently by synoptic and tropical storms and less frequently by

convective storms. During the post-1965 period, the magnitudes of nearly all

synoptic storm fl oods exceeded that of the summer convective annual fl oods.

In addition, two very large tropical storm events (1977 and 1983) signaled the

return of tropical storm fl oods which had dominated the early record in the

1920s and 1930s, but then disappeared as an infl uence in the annual fl ood

record.

The causes for the post-1965 shift in fl ood type, which has been observed

on other regional rivers such as the Santa Cruz (Webb and Betancourt 1992),

have been much debated. The most probable cause is a circulation response,

through air-sea interactions, to changes in Pacifi c Ocean sea surface tem-

peratures. El Niño years tend to be characterized by wetter-than-normal

winter precipitation in the Southwest (Redmond and Koch 1991), and several

studies on fl ooding in Arizona have pointed to an association between El

Niño years and the occurrence of either large winter fl oods or late-season

tropical storm fl oods (Webb and Betancourt 1992, Ely et al. 1994). It should

be noted, however, that the Charleston record does not substantiate this

linkage in all cases. Figure 7 compares El Niño years and the Niño-3 sea

surface temperature index with the San Pedro annual fl ood series. There is

some agreement between synoptic storm fl ood occurrences and the increased

frequency of El Niño years since 1976 (especially in the 1990s), but the rela-

tionship is not consistent throughout the record. Several researchers have

speculated on the possible role of the Pacifi c Decadal Oscillation (PDO) as a

low-frequency infl uence on El Niño strength, such that positive phases of the

PDO (shaded in fi g. 7) enhance El Niño activity (see Gershunov and Barnett

1998, Gutzler et al. 2002). Seven of the nine tropical storm fl oods shown in

fi gure 7 occurred during positive phases of the PDO, suggesting a possible

low-frequency +PDO association, but winter synoptic fl ood occurrence does

not appear to favor either phase of the PDO.

Spatial patterns. The magnitude and frequency of fl oods varies along longi-

tudinal (upstream-downstream) river gradients. Figure 8 illustrates how

selected San Pedro River fl oods linked to winter synoptic-scale precipitation,

summer convective rainfall, and fall tropical storm – related precipitation

each progressed downstream along the length of the river and were recorded

sequentially at the main trunk stream gages in operation at the time of the

fl ood. Overall, in the three types of fl oods, there is a tendency for peak dis-

charges to increase with increasing drainage area in the upper basin above

the Charleston gage, and then to decrease with increasing drainage area as the

fl ood wave progresses downstream and into the lower basin. This is not sur-

prising, because orographic effects cause more abundant precipitation in the


Flood Flows 311

mountains, and all of the large drainages from the high-elevation Huachuca

Mountain range (except for the Babocomari) enter the San Pedro between

the Palominas and Charleston gages. There have been exceptions to this

trend, including the July 1953 convective event which decreased in size rapidly

between Palominas and Charleston, suggesting a localized storm, and the

1983 fl ood associated with Tropical Storm Octave which increased dramati-

cally as it traveled downstream and produced the fl ood of record at the lower

basin stations near Mammoth and at Winkleman (table 1). Widespread heavy

precipitation in the lower basin contributed to the downstream increase in

1983. Additionally, the higher drainage density in the lower basin can result

in more rapid runoff and increased fl ood magnitude (see chap. 13).

Winter synoptic flood events

0

200

400

600

800 Dec 28-29, 1984Jan 18, 1979Jan 19, 1993

CharlestonPalominas Tombstone

Redington

Summer convective flood events

Peak

dis

char

ge (m

3s-1)

0

200

400

600

800 July 7, 1953Aug 16-17, 1952Aug 26-27, 1972

Tropical storm-related flood events

Increasing drainage area0

200

400

600

800

Oct 1-2, 1983 (Tropical storm Octave)Oct 7-10, 1977 (Tropical storm Heather)

Palominas Charleston Benson Redington

WinklemanTrib. nr.Bisbee

Charleston Tombstone Redington

WinklemanPalominas

Trib. nr.Bisbee

Fig. 16.8. Downstream progression of selected

fl ood events of different types. (Values are instan-

taneous peak fl ows.)



SummaryThe storm runoff or fl ood fl ow component of streamfl ow is much larger than

the basefl ow component, in terms of total volume, but is much more variable.

Seasonally varying storm types cause the San Pedro streamfl ow and fl ood

hydrographs to show a strong seasonal pattern. During the past century,

about 90 percent of the annual maximum fl ood peaks occurred during July

through October. Most of these were produced by summer monsoon convec-

tive thunderstorms which dominate in July and August. Floods associated

with eastern Pacifi c tropical storms occurred most often in late summer

and fall. Less than 10 percent of the annual fl oods occurred in December

through March. Though average annual rainfall has not changed over time,

the fl uctuations in the annual distribution of that rainfall have infl uenced

the fl ooding experienced by the San Pedro River. During the 1920s and 1930s,

the largest annual fl oods were dominated by tropical storm events, and this

gave way to dominance of the convective storm – driven annual fl oods until

1965, when the synoptic winter storms became dominant in the production

of annual fl oods. Twice in the last 30 years, annual fl oods caused by tropical

storms were among the largest fl oods on record in this basin.


Literature Cited in this Chapter

Adams, D. K. and A. C. Comrie. 1997. The North American monsoon. Bulletin of the American Meteorological Society 78:2197-2213.

Aldridge, B. N. and J. H. Eychaner. 1984. Floods of October 1977 in southern Arizona and March 1978 in central Arizona. U.S. Geological Survey Water-Supply Paper 2223.

Aldridge, B. N. and T. A. Hales. 1984. Floods of November 1978 to March 1979 in Arizona and west-central New Mexico. U.S. Geological Survey Water-Supply Paper 2241.

Bunn S.E., M.C. Thomas, S.K. Hamilton and S.J. Capon. 2006. Flow variability in dryland rivers: boom, bust and the bits in between. River Research and Applications 22: 179-186.

Douglas M. W., R. A. Maddox and K. Howard. 1993. The Mexican monsoon. Journal of Climate 6:1665-1677.

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flood flows of the san pedro river...pedro river, even within the seasonal framework, is evident in...

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