UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY


MAP SERIES NO. 64


FLORIDA DEPARTMENT OF NATURAL RESOURCES
published by BUREAU OF GEOLOGY


.89 8Be 87 86 8as 84 830 82 81 80


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LOW STREAMFLOW IN FLORIDA-
MAGNITUDE AND FREQUENCY


by
Roy B. Stone

Prepared by
UNITED STATES GEOLOGICAL SURVEY
in cooperation with
FLORIDA DEPARTMENT OF NATURAL RESOURCES
BUREAU OF GEOLOGY
Tallahassee
1974

INTRODUCTION

Streamflow in Florida fluctuates and, at times, is not sufficient to
supply the water required for municipal or industrial supplies,
supplemental irrigation, maintenance of suitable conditions for fish,
and disposal of liquid wastes. Low-flow characteristics of a stream
indicate ground-water flow to a stream and can be used as parameters in
regional water-resource evaluation, or as legal hydrologic parameters for
pollution controL It is, therefore, important to know the magnitude
and frequency of annual minimum streamflow.
This report is meant to acquaint the reader with the concept of
minimum streamflows and their frequency of occurrence. The range of
minimum flow to be expected in the state can readily be seen on the
map. By using the procedures and references presented, low-flow
frequency curves can be constructed for a stream-gaging station. It is
also possible to estimate minimum flows at nongaged sites where some
discharge measurements have been made. Only gaging stations with 10
years or more of record were used in this report.

FREQUENCY CURVES-
RECURRENCE INTERVAL VS. DISCHARGE

The average interval of time within which annual minimum flows are
expected to occur is generally referred to as recurrence interval and is
estimated from a frequency curve. Frequency curves for selected
streams were prepared from annual minimum flows compiled through
1964 by Heath and Wimberly (1971) for stream-gaging stations in
Florida. These annual minimum flows are the minimum average flows
for periods of 1, 3, 7, 14, 30, 60, 90, 120, 150, 183, and 274
consecutive days. Annual minimum flows since 1964 were compiled
from records of the U.S. Geological Survey.
Frequency curves are prepared from annual minimum flows for
various consecutive day periods as follows:
(1) Array the annual minimum flows for a selected time period in
order of magnitude beginning with the smallest value as number 1.
(2) Compute the recurrence interval (T) in years, by T = (n plus
1)/m, where n is the total number of values (years of record) and m is
the order of magnitude.
(3) Plot each flow against its computed recurrence interval and fit a
smooth curve to the plotted points.
The low-flow frequency curve selected for use will depend on the
type of water problem to be solved. Frequency curves for several flow
periods are shown in figure 1. The 7-day frequency curve is frequently
used as a basis for comparing low-flow characteristics of different
streams.

SHAPE OF FREQUENCY CURVES
INDICATE GEOHYDROLOGIC CONDITIONS

Unless the streamflow is regulated, flat sloped low-flow frequency
curves suggest that the surficial materials in the stream basin are highly
permeable; much of the rainfall is absorbed where it falls and is stored
in aquifers which have sufficient capacity to sustain a high level of base
flow during dry weather (Searcy, 1959). The curves for Silver Springs
and Econfina Creek in figure 2 are typical of such streams. Steeper
sloped curves, such as shown for the St. Johns River, suggest less
permeable basin materials, and less water available to sustain
streamflow during dry weather (Hidaka, 1973, p. 14) although
evapotranspiration also affects the shape of low-flow frequency curves.
In the St. Johns River basin and in the upper Oklawaha River basin, for
example, streamflow is decreased by the evaporation of water stored in
the many lakes in the basin. Thus, the frequency curve for the St. Johns
and Oklawaha rivers (fig. 2) is steeper than the other curves. Although
evapotranspiration probably has considerable effect on the steepness of
the frequency curve, the ground-water contribution also has an effect,
and the respective amounts cannot be distinguished.
The similarity of the low-flow frequency curves for some selected
streams in northwest Florida such as the Choctawhatchee, Escambia,
and Shoal rivers (fig. 2) suggests that the low-flow hydrologic and
geologic characteristics of the stream basins are similar. This to some
extent might be expected because these streams originate in the same
general area in Alabama.
Diversion of water for irrigation, industry, or for municipal water
supplies affects low-flow frequency curves as illustrated by the curves
for two stations about 30 miles apart but on the same river (fig. 3). The
curve for Hillsborough River near Zephyrhills reflects substantial
ground-water contribution from Crystal Springs; whereas, the
Hillsborough River at Tampa has a very steep low-flow frequency curve
which is the result of regulation and the heavy withdrawals for the
Tampa municipal water supply.
Geologic and hydrologic characteristics often vary considerably over
a stream basin. Variation in the shapes of low-flow frequency curves for
four stations on the Suwannee River (fig. 4) illustrate the presumed
heterogeneity of hydrologic and geologic factors in the basin. The curve
for the most downstream station (near Wilcox) is fairly flat, suggesting
substantial ground-water discharge from the aquifer to the stream. The
slope of the frequency curves steepens progressively in moving to
upstream stations Branford and Ellaville until at White Springs very
little ground-water discharge is indicated. The White Springs frequency
curve reflects contrasting geologic differences in the upper reach of the
Suwannee basin. The steep part of the curve represents a streamflow
similar to the headwaters in the Okefenokee Swamp, where low flows
range from several hundred cubic feet per second to zero. The flatter,
lower part of the curve reflects an annual 5-10 cubic feet per second
ground-water inflow from a limestone aquifer just above White Springs.

LOW FLOW AT UNGAGED SITES

Minimum streamflow at ungaged sites cannot be accurately estimated
from known basin characteristics. However, low-flow characteristics for
an ungaged site may be derived from graphical correlations based on
low- flow discharge measurements obtained at the ungaged site and a
nearby gaging station (Riggs, 1972, p. 10, 11). Estimates should not be
made when the drainage area is less than 10 square miles.


RECURRENCE INTERVAL, YEARS
Figure 2. Magnitude and frequency of annual minimum 7-day
low flow for selected Florida streams.


DISCHARGE, CUBIC FEET PER SECOND
Figure 5. Relation of 7-day, 10-year low flow to drainage area for some major
rivers in peninsular Florida.


This public document was promulgated at a total cost of
$550.00 or a per copy cost of $.37 for the purpose of dis-
seminating hydrologic data.


k 21 0 a JAC SON





170 c u M son ADSON ILTON Ds VAL Note.-Flow in the lower reaches of the St.

____\-_ BAKER l reverses because of wind and tide effect.

4-- -- SUWANNEE *
WAKULLA M- WO L k
USE OF SPECIFIC RECURRENCE INTERVALS TL Il-T l Y O ,
FOR LOW-FLOW COMPARISONS 1 I

For specific comparisons of low-flow characteristics, a single point I LAFAYETTE NION
on the low-flow cuive is often used, such as the 2-year or 10-year 0 0 1670 i 9 r r s O
recurrence interval. The map shows the location of streamflow gaging I1,E A
stations and the value of the 7-day, 10-year low flow in cubic feet per .2---
second for each station. Under natural conditions this data indicates the 753 -
magnitude of ground-water contributions to the streams during 2810
low-flow conditions. In north Florida the major streams seldom cease c IS ALACHUA PUTAM
flowing while streams in south Florida often go dry or cease to flow. DIE 3680
Because of unequal or erratic distribution of rainfall, streamflow is
generally lowest in northwest Florida during late summer and fall and C8 54 00 FLAGLE R
in peninsular Florida during November, December, January, and May I -
(Rabon, 1971). I I I I I I S I LEV R
For a given recurrence interval, low flows usually increase with
drainage area. This relation is illustrated in figure 5 for the 7-day, -I
10-year low flows of selected streams in peninsular Florida. 5wiNac, R0.. r n5 r oi 65
1LY 10 2s -1. / MA on1.

SELECTED REFERENCES oo -

Dalrymple, Tate i220 s o S mR Z -
1960 Flood-frequency analysis:U.S. GeoL Survey Water- Supply 40 -.0. fl 0 Z (fgj-\ LAKE

Heath, R.C... s
1971 (and Wimberly, E.T.) Selected flow characteristics of I L
Florida streams and canals: Florida Dept. Nat. Resources, 10
Bur. Geology, Inf. Circ. 69, 595p. 0 3SUTER .9 SE
1973 Low-flow characteristics of streams in the Puget Sound .,0 ,0orou R Ymr Tom E ANO
region, Washington: US. Geol. Survey open-file report, 0-s s m ORANGE

Pride, R.W. 0
1962 (and Crooks, J.W.) The drought of 1954-56. its effect on3 4e0
Florida's surface-water resources: Florida Geol. Survey, PASC
Rabon, J.W io os t 3 4 0 o10 to 2 S o 0So -10 6 0HA

1971 Streamflow data program for Flo Forida: mUS. GeoL Survey.6 I L
open-file report, 64p. RECUR.CE iIoNTERVAL EAgS 8 y Q caoscsEOLA
Rise, H.C. Figure 3. Magnitude and frequency of annual minimum 7-day low flow /

Water Resources Inv., Book 4, Chap. Bl, 18p. fIAR 5.8
Searcy, J.K. PKA
1959 Flow-duration curves. US. GeoL Survey Water-Supply 1 2 ? 40 FELLSiE
Paper 1542-A, 33 p. dn v _NrD Ju _, a May
Vernon, R.O. 10725 I I I -. .I I I I INIAN RIVER
1964 (and Puri, HS.) Geologic map of Florida: Florida Geol. SwAE RI 0u N R-
Survey, Map Ser. 18. 3
U.S. Geological Survey 4000oo MANATEE AREE HIGHLNDS
Water resources data for Florida, pt. 1, Surface water w, I 10
records: voL 1; Streams--northern and central Florida, vol. 2. 1 OKEECOBEE
2; Streams-southern Florida, Lake Okeechobee and the 0-1. 1 1 |,_\
Everglades: US. Geol. Survey, Tallahassee, Florida. 34 ST.RoT 0 LUCIE r

0,00o 0q o OoUo,

ot 0 !aI llieL -- -
1EXPLANATION ,

1000- ,o e-- ,*o d* \-T Okechobee
-- s 100d- GLADES I
I o0_.


SIENDRY BEACH
K10-0000 I I I I R


0ol l Is e 3 4 10o zo So 40 so 0

RECURRENCE INTERVAL YEARS PI 2340 s 6 2
Figure 1. Magnitude and frequency of annual ,- G .,,rp4.P I I
minimum flows for Withlacoochee River near Holder, isi 1s2 f 0 0 s ,o t o 40 so + \-A
Florida. \ a \ F D
RECURRENCE INTERVAL YEARS
Figure 4. Magnitude and frequency of annual minimum 7-day low flow IER
eo W i o:0US.iel. I- ,o for the Suwannee River. __ _

~EXPLANATION
5. 0.0...1a.. 3R 4 a... (Base Map)
44 ' Is '_T r to1': Average 7-day low flow, with a 10-year recurrence
Flria' surface t resRucs lr -delu veinterval ,
I000 1,0001,4- 0O S_,-.I~o 51 M R |OADE
|ow:i0o0 a Discharge in cubic feet per second
00 -Inv. 2oo6 ,-65p.* zero flow

a-- 0.01 to 5
4igs sf 6 e- --5 to 50 f "


'< 500 to 5000 ,.
-- More than 5000

0o S -0 Gaging station
40.No ta .R ..ca fF
o, 0o0 Number is cubic feet per second Note.-Flow in some qs the canals of
___________________________ south Florida occasionally reverses
0 a i I ,R el 11, I In ,1o k,4 l a B DEPARTMENT OF NATURAL RESOURCES because of regulabn.
0e 0. I L n IT AI (V


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