UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY


FLORIDA DEPARTMENT OF NATURAL RESOURCES
published by BUREAU OF GEOLOGY


MAP SERIES NO. 32, August 1969


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AVERAGE ANNUAL LAKE EVAPORATION


Rainfall in Florida is generally abundant, but man
opportunity to control or direct the use of a large
rainfall. Evaporation from land, water and plant sui
on much of the rainfall. Evaluation of the amount an
rainfall in excess of all evaporative losses is a n
appraising the outflow of water from an area. Ou
surface and ground water flow from an area, an
difference between rainfall on an area and actual ev
area.
Present technology does not permit accurate
evaporation from large areas of diversified soils an
where the availability of water may differ and vary b
space. Thus, outflow is not determined directly, b
determining the incremental outflow that results
rainfall in the area. Such an approach usually is p
where the total outflow occurs in well-defined su
Florida, however, many areas are underlain by pe
aquifers which transcend surficial drainage divides a
large quantities of water underground. Even though
transmitted through aquifers may reappear in seeps
downstream point, the areas or points of rec
underground water cannot be determined, nor
underground flow be accounted for with certain
analytical tool must be used to evaluate the outflow
concept which provides a common basis for appraising
area is the difference between rainfall and potential e
Outflow from an area can equal the difference be
potential evaporation only if water is maintained co
over the entire surface of the area. In Florida, som
largely covered by lakes or swamps. Furthermore, the
Florida tends to maintain the overall availability of
accord with the evaporative demand. Rainfa
innumerable storm events throughout the year and
during the warm months of the year, when evap
greatest, and least during the cold months, when eva
least. Consequently, in Florida the areal distribution
be grossly similar to the distribution of the different
and potential evaporation- large where the different
and potential evaporation is large, and small where
small.
Outflow cannot be less than the difference bel
potential evaporation because, by definition, pot
represents the maximum amount of water that car
this process. If rainfall exceeds potential evaporat
occur in an amount at least equal to the difference
Although the difference between rainfall and potential
not define precisely the outflow from an area, it does
limit of outflow from an area.
Potential evaporation herein is considered equival
occurring naturally from an extensive water surface
thus, the yearly potential evaporation for an area
evaporation from natural lakes within the area. Th
seasonally because most lakes have some capacity
.- temporarily rather than dissipate it immediately b
other heat transfer processes, as would a wet
appreciable thickness. However, yearly lake evapi
useful estimate of potential evaporation because wit
storage, and releases are balanced.
The areal distribution of potential evaporation in
map) is from a map of lake evaporation in the United
Nordenson, and Baker (1959, plate 2). The lake
provides a rational estimate of average yearly lake
the period 1946-55. At two evaporation stations i
and Belle Glade Experiment station), average evapora
1946-55 was virtually the same as for a longer per
evaporation for the period 1946-55 probably provide
long-term average for Florida.
The general reliability of the evaporation map ha
tested in Florida because few determinations of lak
been made since the map became available. One si
based on mass transfer techniques similar to tl
Harbeck (1962), was made during 1962 at Lake
Florida (Pride and others, 1966). Measured lake ev
Helene was 53 inches compared to an average of 4
the evaporation map for the general area involved.
During 1962, rainfall in central Florida was abo
than normal. Such a large deficiency in rainfall
accompanied by a relative decrease in cloud cover an
as a relative increase in air temperature, all of wh
promote evaporation. Consequently, lake evaporation
probably greater than the long-term average for the a
between the two values is within the range of probal
at a particular lake.


30' --


id humidity as well
ich would tend to
Dn during 1962 was
rea. The difference
tble yearly variation


of Florida. Much of the area is covered by sandy material of variable
thickness which readily passes rainfall to the shallow water table, which
in turn maintains the high base flow of the streams. An aquiclude
underlying the shallow sand aquiler in this area insures that most of the
outflow of the area appears in streams.


37, U.S. Weather Bureau.
Pride, R. W., Meyer, F. W., and Cherry, R. N.
1966 Hydrology of Green Swamp Area in Central Florida: Report
of Investigation No. 42, Division of Geology, Florida Board
of Conservation.
Stewart, H. G., Jr.
1966 Ground-water resources of Polk County: Florida Geol.
Survey Rept. Inv. 44.


a range of 3 inches. The figure represents one of the extreme values of
the range of variation for adjacent areas serving as an upper limit for
one area and the lower limit for the other.



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FLORIDA GEOLOGIC SURVEY MAP SERIES


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THE DIFFERENCE BETWEEN RAINFAI L AND POTENTIAL EVAPORATION U ACRIST! 1 HUP PU.NAM
IN FLORIDA
B y FLAE"
F. N. Visher and G. H. Hughes 8

presently has little The values shown by the evaporation map depend on meteorological In northeast Florida, the difference between rainfall and potential __.
part of the total factors such as solar radiation, wind movement, air temperature, and evaporation ranges from 6 to 9 inches or about half that for the "ASON 4
rfaces has first call humidity, all of which are importantly related to lake evaporation, panhandle area. In the part of the Suwannee River basin that lies 8 VOLUSIA
d areal variation of Thus, the values are applicable to individual lakes as long as the lakes entirely or almost entirely in northeast Florida, surface runoff averages 0
necessary factor in are openly exposed to wind movement, a requirement met by most about 14 inches. Large areas of northeast Florida are swampy andO
outflow is the total large lakes in Florida. The same requirement may not be met by some evaporation losses are almost equal to potential evaporation. On the
d is equal to the small lakes covering only a few ac -es that may lie at the bottom of deep other hand, larger areas are not swampy and have no defined network Q LAKE
aporation from an sinkhole depressions or that may be entirely rimmed by tall and dense of surface streams. Much of the rainfall on these areas apparently moves < *
vegetation. downward to the water table and reaches the main throughgoing stream --
determination of Yearly potential evaporation varies only slightly compared to yearly without large evaporation losses.
d vegetative cover rainfall, which in many areas of Florida ranges from about 50 percent Over a large part of central Florida, the difference between rainfall
both in time and in greater than normal to about 5( percent less than normal. Although and potential evaporation is from 3 to 6 inches. In the northernmost"
ut is evaluated by some of the excess rainfall of wet years may be carried over as soil part of this area, including primarily Lake County and the northern half H NANDO
from the excess moisture or stored in lakes or it a shallow water table, much of the of Polk County, there are numerous lakes and swamps. Thus, for most o 3NE
racticable in areas excess rainfall flows out of the a'ea and does not remain available for of this area, evaporation and transpiration should approach the
irface channels. In evaporation in subsequent years. Similarly, but in a somewhat opposite evaporation potential and runoff should approach more closely the
rmeable limestone sense, the energy represented by potential evaporation that cannot be difference between the rainfall and potential evaporation. Surface _-\
and which transmit used because of a lack of water curing a dry year is not stored for later runoff from the area, as measured by gaging stations on the upper PAS
part of the water use but rather is dissipated. Because the yearly and seasonal Withlacoochee River, the Little Withlacoochee River, and the Oklawaha
or springs at some irregularities between the supply and demand of water for evaporation River, averages about 7 inches. Outflow is somewhat greater than the
charge of all the are not smoothed by storage, the average outflow of water tends to be surface runoff because this is a known recharge area for the Floridan 4
can all of the greater than the difference between average rainfall and average Aquifer. Pride and others (1966) showed that for the Green Swamp 6l -6 OSCELA 0
inty. Thus, every potential evaporation-substantial outflow can occur during a year area, which occupies an appreciable part of central Florida, recharge to ILLSROUG 9 9
ow of an area. One when rainfall may be much less than the yearly potential evaporation, the Floridan Aquifer averages about 2 inches annually. Thus, outflowH 5
g outflow from an The topography and geology )f an area also cause the outflow to from this part of central Florida may be about 9 inches. POLK
evaporation. deviate from the difference between rainfall and potential evaporation. This same general area of central Florida is drained to the south by 4
between rainfall and Of prime importance are the sl)pe of the land surface, the type of the Peace River. The Peace River heads in an area of recharge for the
intinually available vegetative cover, and the permeability of the soil and underlying Floridan Aquifer, but over most of its length the river can receive A I
ie sizable areas are materials. Those factors whict cause the rainfall to be rapidly discharge from the Floridan Aquifer. Surface runoff from the basin D I N V
e rainfall pattern in concentrated into streams and drained from the area, such as steep land averages about 13 inches at the most downstream gaging station, which 5
water generally in slopes and poorly permeable soils, for example, tend to make outflow probably represents the outflow from the basin, because, in the MANATEE HARDEEHLANDS
ll occurs during greater than the difference between rainfall and potential evaporation, upstream 30 percent of the basin where most of the recharge occurs, .
is usually greatest Permeable soils which permit wa er to quickly reach a deep water table natural surface runoff still averages about 10 inches. Thus, from the OKECHOE
orative demand is that feeds streams or springs also :end to make outflow greater than the same general area where the difference between rainfall and potential
porative demand is difference between rainfall and potentiall evaporation simply because evaporation ranges from 3 to 6 inches, outflow ranges from 9 inches in RAsor -
of outflow should the water does not remain on o, near the land surface to evaporate, a northern section to about 13 inches in a southern section. Part of this
ce between rainfall Factors which promote ponding aid retention of water in place, such as difference is probably due to a higher actual evaporation from lakes and -
ce between rainfall flat land with surface depressions, tend to reduce outflow and make it swamps in the northern section. ESOTO e
e the difference is more nearly equal to the difference between rainfall and potential In the southernmost part of the state, which includes the ARTIN
evaporation. Caloosahatchee River basin, Lake Okeechobee, and the Everglades, the
tween rainfall and The accompanying map shows the approximate areal distribution of difference between rainfall and potential evaporation ranges from less
ential evaporation the difference between yearly rainfall and yearly potential evaporation than zero in the western part to 12 inches in the eastern part. The 0 ec obee
n leave the area by for Florida. Rainfall data used were for the most part U.S. Weather terrain in this part of the state is so flat that, for the few streams which Prepared by the 0 CHARLOTTE GLADES -
ion, outflow must Bureau normal rainfall values for the period 1931-60. For stations are identifiable, drainage basins are for the most part indeterminable. ep
between the two. where "normal" values were lacking, the average rainfall for the period Thus, the streamflow data available for this region cannot be evaluated U. S. GEOLOGICAL SURVEY 14 1
al evaporation may of record at the station was used; however, all stations so included have easily in terms of runoff per unit area. in cooperation with the O
Indicate the lower at least 25 years of record. Outflow from other regions of the state was found to be always BUREAU OF GEOLOGY /
Average yearly rainfall variesiappreciably in Florida-ranging from greater than the difference between rainfall and potential evaporation; I ENDRY BEACH
ent to evaporation about 46 inches at Miami Beacnm n south Florida to about 66 inches at consequently, this same gross relation should apply in the southern part FLORIDA DEPARTMENT 01 NATURAL RESOURCES LE L
of little thickness; DeFuniak Springs in north Florica-but the variations do not follow a of the state. The disparity between outflow and the difference was TALLAHASSEE, FLORIDA
essentially equals single statewide trend. In contrast to rainfall, average yearly potential greatest where surface drainage by streams was efficient and where 1969 -- 3
he two may differ evaporation decreases only slightly and somewhat uniformly from 54 lakes and swamps were generally absent, and least where drainage was -
y to store energy inches in south Florida to about 46 inches in north Florida, as shown poor and where lakes and swamps were prevalent. Outflow from the -
by evaporation, or on the small inset map. Consequently, on the accompanying large map, swamp and lake country in central Florida was estimated to be 9 inches- -
surface having no the somewhat cellular pattern of the isopleths is due more to variations for an area where the difference between rainfall and potential BROW D
oration provides a in rainfall than to variations in potential evaporation. The isopleths evaporation ranged from 3 to 6 inches. The disparity should be c
thin a year energy, accommodate the data for all but a few rainfall stations but the appreciably less for the southern part of the state where the availability OLLIER
configuration of the isopleths is ;iot expressly or adequately defined in of water to evaporate is much more complete for the entire area. Thus,
I Florida (see inset several areas of the state. As additional rainfall data become available, for southern Florida, outflow probably does not exceed the difference
d States by Kohler, the configuration of the isopleths may be more truly defined. Thus, the between rainfall and potential evaporation by more than 3 inches.
Evaporation map map should be interpreted only in terms of broad areas where the Actual outflow cannot be determined directly from the map. In
evaporation during average difference between rainfall and potential evaporation can be essence, the map portrays the minimum outflow from an area. Actual- -6
n Florida (Hialeah reasonably well established. outflow will almost always exceed the minimum, because basin
nation for the period As mapped, rainfall exceeds the potential evaporation over most of characteristics and variations in rainfall limit the availability of water ONROE
iod 1946-65; thus, Florida, but the difference vares greatly. The distribution of the for evaporation. EXPLANATION DADE
.es a representative difference is consistent in some ways with outflow, which also varies
appreciably across the state. However, no simple or direct relation REFERENCES 12
is not been widely between the two is apparent. The effect of other hydrologic and Harbeck, G. E., Jr. /
e evaporation have geologic variables is great enough in some areas to obscure the effect of 1962 A practical technique for measuring reservoir evaporation Rainfall station. Number represents the difference in inches between
uch determination, the difference between rainfall and potential evaporation on outflow, utilizing mass-transfer theory: U.S. Geological Survey Prof. rainfall at the station and potential evaporation in the general area of
hose described by The greatest difference between rainfall and potential evaporation Paper 272E. the station.
Helene in central occurs in the panhandle area of northwest Florida, where the greatest Healy, Henry G.
evaporation at Lake outflow in the state occurs and where the difference is from 15 to 18 1962 Piezometric surface and areas of artesian flow of the Floridan
9 inches shown by inches. Surface runoff averages asout 29 inches over stream basins that aquifer in Florida, July 6-17, 1961: Florida Geol. Survey 6
lie entirely or almost entirely within the panhandle area west of the Map Series No. 4. /
ut 25 percent less Choctawhatchee River. This area is well drained by a network of Kohler, M. A., Nordenson, F. J., and Baker, D. R. The line marks the approximate boundary of areas where the difference
unaiilv would hbe streams hnw hose basin are steep slnped relative to stream in other arts 1959 Evaporation Maps for the United States: Technical Paper No. between rainfall and potential evaporation is generally the same within 4


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