Section 4
Limnd
01988,
Trophic dynamics
33(6, part 1), 1988, 1308-1320
by the American Society of Limnology and Oceanography, Inc.
Oceanogr.,
The pond rises and falls . . . six or seven feet; and yet the water shed by the surrounding hills
is insignljicant in amount, and this overflow must be referred to causes [not corresponding to
the general wet and dryness] which a~ect the deep springs. . . . It is remarkable that this
fluctuation, whelher periodical or not, appears thus to require many years for its accomplishment. [In thirty years] I have observed one rise and a part of two falls. . . . Flint’s Pond, a
mile eastward, . . . and the smaller intermediate ponds also, sympathise with Walden. . . .
The same is true, . . . . of White Pond.
Thoreau
Walden
Estimation of downward leakage from Florida lakesl
Edward S. Deevey, Jr. 2
Florida Museum of Natural History, University of Florida, Gainesville 32611
Abstract
The karst lake district of peninsular Florida shares hydrologic properties with certain glaciated
districts, such as outwash plains, but differs from most well-watered regions in losing much more
water to subsurface effluents than to surficial runof~. In nutrient-loading models that begin as
hydrologic throughflow models, groundwater inflow and outflow are commonly ignored or estimated from errors of closure of water budgets. Such models are inapplicable to seepage lakes,
unless deep-seepage outflow can be measured. Estimates of downward leakage from Florida lakes
with and without outlets suggest that conventional nutrient-loading models are inapplicable to any
lake not in a watertight-rock basin.
Downward leakage is measurable during months when Iak.e level falls by an amount exceeding
the net precipitation deficiency. To measure it, USGS stage data for 20 lakes from north, central,
and south sectors of the peninsula were compared with three 32- yr sets of mean monthly meteorologic measurements. Monthly pan-evaporation data were converted to lake evaporation by factors
estimated at Lake Okeechobee. Leakage estimates (geometric means) range from 28.4 to 50.9 cm
yr-’ in 18 of 20 lakes two exceptionally astatic lakes gave 90.1 and 141.2 cm yr-’. Presence of
outlets in 15 of 20 lakes makes no detectable difference in leakage. Adding evaporative losses (and
ignoring surface outflow if an y), residence times for the 20 lakes averaged 2.67t1.33 yr.
Florida lake levels fluctuate in sympathy with each other and respond with little or no lag to
monthly net precipitation. Longer term swings are also evident and are more highly correlated
with artesian pressure in the deep limestone aquifer than with meteorologic variables. Few if any
lakes receive direct injections from this aquifer, and artesian influence is exerted on lakes and local
water tables through many meters of surficial (Mio-Pliocene) deposits.
In limnological usage, drainage lakes are
also open lakes, whereas closed lakes lose
water only by evaporation (Wetzel 1975, p.
‘ Supported by the National Science Foundation
(DAR 79-24812, DEB 82- 11380) and by a grant from
the Whitehall Foundation.
2Died 29 November 1988.
40). If closed lakes are thus restricted to rock
basins in semi-arid regions, “all seepage
lakes are in this sense almost certainly open”
(Hutchinson 1957, p. 231); in other words,
seepage lakes are the subset of drainage lakes
that lack surface outlets. That seepage lakes
have effluents to groundwater has been surmised by many observers, beginning at least
with Thoreau. Considering drainage lakes
1308
b
Leakage from Florida lakes
like Thoreau’s Sandy (Flint’s) Pond, however, later limnologists have been misled by
the fact that “most small lakes not in rock
basins are separated from the ground water
by a clay seal, formed as an early lake sediment” (Hutchinson 1957, p. 248). Linsley
Pond is such a drainage lake. Although its
clay seal was found to admit some inflowing
14C-deficient
groundwater
(Deevey and
Stuiver 1964), the crude water budget gave
no hint of outflow via a second effluent.
Mirror Lake, downstream from the Hubbard Brook Experimental Forest, is also insulated from that system’s “watertight bedrock” (Likens et al. 1977) by a clay lens
overlying sandy till and outwash. Collectively known as drift, these glacial and early
postglacial deposits provide the only subsurface aquifers of the region. Their output,
the groundwater that enters streams and
lakes and nourishes forest trees, is deep
seepage. Although Mirror Lake itself is a
source of this output, the magnitude of
downward seepage through its floor has not
yet been measured directly (Winter 1985).
When estimated by the lake’s water budget,
measured inputs (rainfall plus deep and surface inflow) exceed surface outflow by 3.6
x 105 m3 yr– 1, or 430/0 of the total (Likens
1985: table IV. E-4). Proof that this huge loss
is in fact deep seepage was obtained during
1970-1972, when the lake level (measured
daily) was below the dam during 11 of 28
months. Leakage through the drift dam was
measured at V-notch weirs downstream and
appears as outflow in the budget. Accurately
measured and corrected nonevaporative
losses from the lake were then 3.1 x 105 m3
yr- 1; the remaining discrepancy of budget
closure, averaging 49.7 *22.3 x 103m3 during 10 yr or &5.90/0 yr– 1, was also assigned
to deep seepage.
In this paper I follow the practice of Florida hydrologists in calling deep-seepage outflow downward leakage (Hammett 198 1). In
Mirror Lake it is the fraction that did not
seep outward through the dam. It amounted
to 420/o of the mean depth or 240 cm of
water per year. Implications of this extraordinarily large figure are discussed below.
In modern limnology, water budgets are
necessary first steps toward nutrient or solute budgets; hydrologists’ estimates of water
1309
throughflow are commonly
accepted by
professional courtesy. If this confidence is
misplaced
(Winter 198 1; LaBaugh and
Winter 1984), most limnologists would
probably prefer not to hear about it. Among
the hundreds of small lakes for which nutrient-loading models have been constructed (Chapra and Reckhow 1983), few if any
have water budgets as closely monitored as
that of Mirror Lake. Nearly all these lakes
lie in well-watered regions, where rainfall
exceeds evaporation by 70-850/0 (Deevey
and Brooks 1963; Kohler et al. 1959). It is
therefore not surprising that rapid hydrologic throughflow is a key component of the
nutrient-loading paradigm as used in eutrophication
studies. If deep seepage and
groundwater flow contribute some inconvenient variance to these models, it is easy
to overlook the variance in larger discrepancies in water or nutrient budgets.
Unfortunately for modelers, the nutrientloading paradigm is inapplicable to seepage
lakes., It is the thesis of this paper that it is
also highly questionable for drainage lakes
not in rock basins.
In this study of the rather special hydrology of the karst lake district of Florida,
I report some statistical measurements of
deep seepage. The study area is mapped in
Fig. 1. Its northern third is shown as seen
by satellite in Fig. 2. This northern sector
receives
1,359 .4~207.8
(*1
SE) mm
(53.52 *8. 18 inches) of annual rainfall; with
lake evaporation averaging 1,288.0* 73.7
mm (50.71&2.90 inches), net rainfall (the
difference, with a large SE) is 71.4 +220.5
mm (2.81 +8.68 inches). In the years of record (water-years 1954– 1986), net rainfall in
the central third of the lake district was
49.4 A228.6 mm (1.95*9.00 inches); in the
slightly drier southern third the net was negative, –38.4*260.9
mm (– 1.51* 10.27
inches).
As would be expected from the variance
of these figures most of the lakes are strongly
astatic. An extreme example (Pebble Lake)
has fallen 93 cm (3.05 ft) in a given month
and risen by 78.6 cm (2.58 ft) in another;
over 40 yr its mean depth has varied between 72 and 529 cm, i.e. by a factor of 7.3.
If a 2-m excursion of lake level is a Walden
unit, 11 of 20 lakes in our data set (Table
1310
Deevey
b- .,
L,
-,%7
{
‘%.
,4C
SCALES
1.0 ZO 3,04,0
LAKES
0“
1
L. Annie
2
L. PlaGld
3
4
L. Jackaon
6
L. Clinch
6
L, MarIon
L. June
I
T\
N
,*2
‘*+
&
1. Wlmer
~
1:
F
*J
)0
L. Lowery
8
L. Lou18a
9
10
L. Dora
L. Dorr
“i
08*
~E* \
.?.3
11
L, walr
12
L. Kerr
13
Orange
14
L. Qrandln
16
7“
!3
“
~0
M
2
)
L.
L. Qene.
a
76
L, Brooklyn
17
Mr,gnot{s
L.
18
Sandhill
10
20
L, Kingsley
Pc. bble L,
L.
METEOROLOGICAL
9TAT#NS
5[’
A
ala.
B
Galne8.llle
St.
C
Piilntka
D
Ocala
MWY, II
E
F
O,lnndo
Lisbon
Q
Clermont
H
Mountain
I
Avon
Park
J
Lake
Placld
K
A,chb
Old
Lake
Fig. 1. Peninsular Florida lake district location map
of 20 gauged lakes, 11 meteorological stations, and two
deep wells tapping the Floridan aquifer.
1) are 0.9-4.8 times as astatic as Walden. It
seems appropriate to begin analysis with
Thoreau’s questions: in this sievelike terrain do drainage lakes fluctuate in sympathy
with seepage lakes? Do the fluctuations correspond to “the general wet and dryness”?
Hydrologic data have been extracted from
the files oi7the-U.S. Geological Survey, with
help from G. H. Hughes and other members
of the Tallahassee, Tampa, Orlando, and
Jacksonville offices. Climatological
Data
(Florida section) are published monthly by
NOAA. Bathymetric maps, published for
some lakes by Kenner (1964), Hammett
(198 1), and Adams and Stoker (1985), were
drawn for others by R. A. Garren and T. J.
Whitmore. I am indebted to -K. Dorsey, M.
Brenner, and T. J. Whitmore for laboratory
and field assistance and especially to D.
Deevey for statistical advice; she also designed and printed the computer-generated
figures in this paper.
Fig. 2. Northern third of the peninsular Florida
lake district, from photo 018-039, 17 March 1976, 7,
CN 30- 18/W082-27,
NN 30- 18/W082-23,
NASA
ERTS E-2420-15 180-701, T2 S D E 011-1.0, COOO044. Gainesville at lower left. Lake Kingsley (No. 19
in Fig. 1) is the large doline at top; Orange Lake (No.
13) is the polje at bottom. Lake Geneva (No. 15) above
and left of center, has two swallets at the SW margin
of its basin.
It is a particular pleasure to dedicate this
paper to W. T. Edmondson. As “ecologists
reflect the properties of the ecosystems in
which they have grown and matured” (Margalef 1968, p. 26), he knows well the hydrologic properties that are reflected from
the wet Quinnipiac basin to the wetter Puget
Lowland.
Buried karst: Hydrology and
h’nno[ogy
The karsted limestone formations that
constitute the Floridan aquifer are of various Paleogene ages. Through much of the
northern district shown in Fig. 2, the uppermost limestone— the Eocene Ocala formation —lies 30-120 m below the land surface and 0-60 m below sea level. The first
over] ying formation is the Miocene Hawthorne formation–impermeable
beds of
which act as a confining layer. The second,
O-35 m thick, is a mix of marine and fluviatile sand and gravel with kaolini te stringers thought to be of Citronella (?Pliocene)
age (Cooke 1945; Alt and Brooks 1965; Ar-
1311
Leakage from Florida lakes
Table 1. Size, depth, number of observations,
Region*
Lake
North
Kingsley
Sandhill
Pebble~$
Brooklyn
Geneva
Magnolia
Grandin~
Orange
Weir
Kerr$
Dora
Dorr
Louisa
Marion
Lowery$
Clinch$
Jackson
June in Winter
Placid
Annie
Central
South
*
t
$
$
LOcation
(Fig. 1)
19
18
20
16
15
17
14
13
11
12
9
10
8
6
7
5
4
3
2
1
leakage + 1 SE (o/o),and residence time of 20 Florida lakes.
z
Area
(ha)
640.63
514.20
2.52
264.47
750.25
80.79
146.99
3,648.70
2,488.90
1,005.41
1,882.66
683.86
1,470.83
1,312.15
372.71
480.67
1,279.84
1,481.47
1,303.80
36.76
(m)
“x
24.4
- “ -
9.1
4.3
12.5
9.1
14.3
3.7
3.6
10.4
4.9
4.6
6.5
4.9
3.6
3.7
11.6
7.9
11.6
16.5
20.0
(:)
7.33
4.13
1.53
5.61
4.13
7.70
2.40
1.67
5.78
3.55
2.86
4.38
2.19
1.96
2.40
6.36
4.53
5.55
6.76
8.30
Not
Leakage
(cm)
~
SE
(Y,)
Res. time
(yr)
“12/3”12
35.5U
13.5
4.b4
31/190
93/384
66/271
63/266
57/258
19/91
107/372
36.10
141.20
90.06
32.40
41.50
50.90
47.50
18.3
10.7
11.9
14.1
14.1
27.8
11.5
87/372
78/342
83/372
38/230
94/333
65/300
75/288
34.20
34.10
42.70
29.40
49.70
28.35
30.94
37.74
31.63
33.31
40.43
29.82
10.2
13.0
15.0
17.3
12.7
15.9
16.7
10.7
13.1
2.51
0.57
2.57
2.57
4.53
1.34
0.95
3.69
2.27
1.74
2.88
1.28
1.30
1.56
124/353
87/316
69/363
103/324
16/79
3.79
2.80
11.0
3.40
3.97
3?;:
5.20
Mean lake evaporation: northern district 128.47 cm; central district 122.42 cm; southern district 129.93 cm.
1st figure, No. of months of Ieakagq 2nd figure, total No. of months of observation.
Seepage lake.
Data for Dccembcr 1986, stage 28.9 m asl (94.8 ft). Pebble lake varies in area from 0.599 to 6.0235 hw in stage from 25.8 to 35.1 m asI (84.6 to
115 ft); in mean depth from 72 to 529 cm; in residence time from 3.25 months to 1.96 yr.
rington and Lindquist
1987). [Brooks(198
1),
apparently
rejecting
this interpretation,
maps
most
of the lake district
as HawIn this texturally and chemically
thorne.]
Florida limnology has been reviewed by
Brenner et al. (in press).
Our attention to the problem of deep
seepage was prompted by the need, in studinhomogeneous
material “local water ta- ies of recent rates of change of trophic state
bles” are bewilderingly variable; “ground(Deevey et al. 1986), for estimates of flushwater aquifers” are locally stacked in tiers. ing rate and its reciprocal, residence time.
Contrary to a widespread impression, the Palmer (1984) estimates that 700/0 of Florclassic karst features of this district (dolines,
ida’s 7,800 lakes lack surface outlets and
uvalas, poljes; see Jennings 1985) are coninlets. In another 250/o, the putative drainfined to the Mio-Pliocene mantle. Despite
age lakes, inlets and/or outlets fail to functhe abundance and frequency of formation
tion for periods ranging from a few months
of sinkholes (Upchurch and Littlefield 1987) to many years. Apparently unaware of these
there is no evidence that the underlying
facts, the National Eutrophication Survey
limestones are cavernous. The only cavern(U.S. EPA 1978) assumed Florida lakes to
ous sinkhole in the district has its base at have “net surface runofl” equal to the counthe top of the Hawthorne (Pirkle and Brooks
tywide mean figures estimated by Heath and
1959).
Conover (198 1). As a result, the survey rePersistence of lakes in such porous terrain ported flushing rates for some 33 Florida
is remarkable, but all Florida lakes examlakes, at least two of which (Marion and
ined by coring have existed for several thouDora) are in error by an order of magnitude.
sand years (Watts 1980). In general, little is A table of “detention times” given by Huknown of the hydrology of the cored lakes, ber et al. (1982: table 4-11) contains many
and nothing (prior to the century-long 210Pb errors based on the same misconception.
time scale) is known of the sedimentary his- For a few lakes, such as Conway and Apoptory of the lakes studied by other limnoloka (Fellows and Brezonik 1980), less invengists (Huber et al. 1982; Canfield 1981).
tive water budgets have been constructed.
1312
Deevey
Seepage inflow to these drainage lakes was
measured directly with seepage meters (Lee
1977). No seepage was detected >30 m offshore. If the discrepancy of budget closure
is assigned to seepage, seepage outflow is
11.70/0of the total inflow in both lakes. Noting that this volume is smaller than observed seepage inflow to Lake Conway by
7.5 x 105 m3 yr- 1, or 5.90/0 of the total,
Fellows and Brezonik (1 980) considered inflow to be confined to the margins with “recharge to the Floridan aquifer occurring at
the center of the lake” (p. 640). From the
estimates of seepage outflow, downward
leakage is at least 20.6 cm yr- ] in Lake
Apopka and 20.2 cm yr- 1in Lake Conway.
These lakes are about as leaky as 18 lakes
of our data set.
In this study, as the standard methodology at its best (i.e. with seepage meters) is
inapplicable to lakes without outlets, we take
as our model for procedure Hughes’ (1974)
investigation of Lake Kerr. This procedure
is identical to the Mirror Lake procedure
for 1970-1972, but Florida lakes provide a
much richer body of data on nonevaporative water losses.
Methods
Leakage–The
water balance of a seepage
lake is expressed (Hughes 1974) as
.AH=P–
E–S+I,W+ZP
(1)
where AH is change of lake level, P is rainfall on the lake, E is evaporation horn the
lake, S is leakage (seepage) from the lake,
1,. is surface inflow, and lW is groundwater
inflow. That is, if a lake has no suriiace outlet, water not lost to evaporation must exit
downward. Of the sources of lake water–
direct precipitation, sufiace runoff, and deep
seepage inflow —only the first is easily measured. If lake evaporation can be measured,
however, the sum of sufiace and deep inflow
([,W + lW) will raise the lake level by some
amount that exceeds the net income (P –
E) from direct rainfall. Conversely, if the
level falls by an amount exceeding the (negative) net income, the lost water can reasonably be called leakage. In Florida, where
rainfall is strongly seasonal, leakage is most
likely to be observed
in near-rainless
months, when P, I.w, and .Z~. are all near
zero, and (P — E) is negative. (l_W is :near
zero because under these circumstances
groundwater flows away from the lake.)
In this interpretation
leakage occurs
throughout the year at near--constant rates.
It is assumed to be driven by a hydrostatic
head that includes the lake depth plus many
meters of saturated groundwater conduits.
Lake-level fluctuation can change this head
by a few percent at most.
[n his study of Lake Kerr (1962-1969),
Hughes (1974) restricted leakage measurement to dry winter months when surface
inflow is negligible and deep seepage at the
lake margins is usually outward rather than
centripetal. We find this restriction to be
unnecessary,
and record leakage in all
months when water loss exceeds net precipitation deficiency. If we consider the large
RMS variance of P, E, and especially of the
difference (P – E), however, leakage values
very near zero are surely statistical artifacts.
Similarly, maximal values, typically occurring in the second, third, or fourth of a. run
of dry months, are also likely to be artifacts.
The problems are compounded by the fact
that P and E are regional means, and AH
is a local observation. “We treat these problems (see Fig. 6) by computing geometric
rather than arithmetic means.
By Hughes’ (1974) estimate Lake Kerr
leaks at 36.58 cm yr-l (0.1 ft month-]); our
figure (Table 1), the geometric mean for 78
dry months, 1957-1986, is 34.10 cm yr-l
+ 13.OO/..The difference, - 70/0,is well within the (geometric) standard error, which. we
find to be 10–1 70/0in 18 of 20 lakes.
Meteorology— We divided the peninsular
lake district into three regions, centered on
three meteorological
stations (Fig. 1) for
which pan evaporation is reported from
1954 onward. Monthly rainfall was averaged for the three or four -stations selected
in each district. Pan-evaporation data always overestimate lake evaporation; our
monthly means were reduced to lake-evaporation estimates by application of monthly
factors computed for Lake Okeechobee
(1940-1946)
by Kohler (1954). These
monthly factors, ranging from 0.69 (February) to O.91 (July–August), were normalized for our central and northern districts
by the long-term pan-evaporation means.
1313
Leakage from Florida lakes
1
i
+25-
+25
o-
o
-25
545218–
Kingsley
Orange
16–
32-
Geneva
“\~
30 39 –
+I
37-1
Magnolia
I
54
I
62
~P”-~
I
70
-“
1
I
1
78
86
.-
Calendar
Year
Fig. 3. Northern part of the lake district: monthly precipitation,
lake; (NOAA and U-.S. Geol. Surv. data.)
Thus three sets of mean monthly net rainfall
figures (precipitation minus lake evaporation) were used to compute leakage for the
five to eight lakes selected in each district,
during a period of 32 water-years (October–
September 1954-1 986).
Artesian wells – When Thoreau wrote
Walden, he noted that lake-level fluctuation
needed many years for its accomplishment.
Figures 3 and 4 show by inspection that
variable net rainfall accounts for some but
not all of the fluctuation. Abnormally low
levels in the mid- 1950s and early 1980s suggest that some of the statewide fluctuation
is related to the “deep springs.” Accordingly, we examined hydrography for two tightly
cased observation wells which tap the Floridan aquifer near Orlando and at Ocala (Figs.
1 and 5). Unlike the first (OR 47), which
shows severe drawdown since about 1960,
the second (Sharpe’s Ferry, Marion 5) appears relatively free of disturbance; its main
purpose is to monitor and predict flow of
Silver Spring. The hydrography are shown
in Fig. 5. In the statistical treatment (see
Fig. 7) we (algebraically) summed monthly
gains and losses for water-years, thus de-
net precipitation,
and hydrography of four
trending the Orlando well data and condensing month-to-month
variation in all
data. A different, more penetrating analysis
of periodicity will be reported elsewhere.
Lake-stage data 1942-1986
The 20 lakes of our data set were selected
primarily because USGS stage data were
available for them. Daily measurements
were recorded for a few lakes for periods of
varying length; for most lakes observations
began after 1957 and were made between 3
and 12 times per month. Monthly means
and their differences (@
were computed
for all lakes; some single observations were
accepted but missing months were interpolated only when annual sums (see Fig. 7)
were tabulated. When the data set was complete, 20 geographically well-distributed
lakes of various sizes were seen to include
some complacent and two extraordinarily
astatic examples. True seepage lakes were
not as well represented as might be desired
because long runs of measurements are
available for few lakes without outlets.
Counting Lake Kerr, which is said to have
an inlet, 5 of 20 lakes are without outlet (see
1314
Deevey
17155452-
Kingsley
Weir
1816-
Dora
2018-
Clinch
33-
.J”~~\~~
31-
June
23–
2128263129~
%“’
in Winter
-’-v-f--”
Placid
-“
Jackson
./-.’-““’—--’—’””~
b,, ~.i~
42
50
v“
66
58
Calendar
Fig. 4.
74
82
Year
Hydrography for eight lakes with long records. (U.S. Geol. SurV. data.) Lake Clinch is a seepage lake.
tables and figures). As far as the data can
show, 14 “drainage” lakes are just as leaky
as 4 of the 5 “seepage” lakes, and the outlet
of the 15th (Lake Brooklyn) plays no part
in its extraordinary fluctuation. In one case
(Lake Weir: Brezonik and Messer 1977),
outflow removed 4.5 cm of water in calendar 1974, reducing its residence time by
5 weeks, 2.80/o below the 3.69 yr given in
Table 1.
By inspection, lake levels of the 20 lakes
are fairly well correlated with each other
(Fig. 4), but less well correlated with monthly direct precipitation (Fig. 3). For eight lakes
with long records (28 paired comparisons;
eight hydrography shown in Figs. 4 and 5)
only two between-lake correlations (Kerr
and Orange with June in Winter) were nonsignificant. Significant (P S 0.005) r values
ranged from 0.270 to 0.812. The highest
degree of sympathetic fluctuation (r values
between 0.711 and 0.812) was observed in
three adjacent lakes (Orange, Kerr, and
Weir). Correlations
between levels and
monthly direct precipitation were mostly
nonsignificant. When monthly gain or loss
in lake level (AH) was compared with
monthly net precipitation, however, 16 of
17 lakes tested gave similar and highly significant r or R values ranging from O.584 to
0.786. (R is the multiple correlation coefficient, in this case combining net precipitation for the same and the previous 1 or 2
months. Only in a few cases were r values
numerically higher for lagged than for samemonth data.) Lake Annie, with the poorest
record, is the nonsignificant outlier in this
set.
We conclude that response to wet and dry
months occurs with little or no lag and accounts for 34–620/o of the variance of level
in these 16 lakes. Readers who wish to inspect four sets of 136 correlation coefhcients
may request complete printouts.
Leakage estimates
Leakage is recorded in any month during
which lake level falls by an amount exceed-
1315
Leakage from Florida lakes
34
32
30
28
26
33
31
29
81
“v
Brooklyn
J
w
Kerr
‘m,?’j~’~v
18
Orlando
47
16
(well)
,
42
50
J
58
t
1
I
66
74
,
1
82
Calendar Year
Fig, 5. l+lydroma~hs for two very astatic lakes, a more no~al Me, andtwodew wells~amim@ ~Orida~
aqui~er. (U.S~ Ge61. %rv. data.) Pebble and Kerr are seepage lakes.
ing the negative net income or net precipitation deficiency. In terms of the water balance (Eq. 1), if Is. and Iw are negligible, (P
– l?) is – 13.72 cm (–5.4 in), and AH is
–15.24 cm (–6.0 in), leakage is 1.52 cm
(0.6 in) in that month. We include English
units, used by USGS and NOAA, to minimize errors of rounding. Four typical frequency distributions of the log of leakage
are shown in Fig. 6. We have mentioned
earlier the statistical artifacts that probably
affect both ends of these distributions. For
all lakes, with values of N ranging from 17
to 124 (modally 65 of 372) leaky months,
the distributions are approximately log-normal. This fact, suggestingmultiplicative interaction of two or more sets of controlling
variables, is consistent with the evidence
(Fig. 4) that abnormally low or high levels
in certain decades are not controlled by
rainfall or net rainfall alone. As arithmetic
means of such distributions are inappropriate, means in Table 1 are geometric
means, and standard errors express their
uncertainty as percentages.
The estimates (Table 1) have the large
errors associated with geometric means, but
if Pebble and Brooklyn are set aside the
means range between 28.35 cm yr- 1 &
15.90/0and 49.70 cm yr-~ ~ 12.70/o. Removal of two lakes with very poor records
(Grandin and Annie) makes no difference
to this range. Thus lake-to-lake variance is
much smaller than month-to-month variance, which Hughes (1974) considered to be
20-60%o. Presence
of outlets in 15 of the 20
lakes makes no detectable difference in leakage.
If mean leakage between -30 and -50
cm yr– 1 is typical of Florida lakes, some
22-390/o of direct rainfall is lost to groundwater. The rangeis probably typical, for Lake
Conway falls within it (Fellows and Brezonik 1980), as does the 1970–1973 estimate for Lake Jackson, Highlands County
(37.3 cm yr-l: Hammett 1981). Earlier
1316
Deevey
Frequency
Distribution
Of Log Leakage
40%
35%
30%
25%
20%
15%
10%
5%
o%
-1.1
-0.8
-().5
Interval
IZl
Orange
-0.2
0.1
Midpoint
U
Weir
0.4
0.7
1.0
1.3
1.6
(Log Centimeters/Month)
-
Kerr
EN
Dora
Fig. 6. Frequency distributions of log leakage for four lakes. Numbers of months are 107 for Orange Lake,
87 for Lake Weir, 78 for Lake Kerr, and 83 for Lake Dora. Kerr is a seepage lake. Local monthly leakage (cm)
is the difference of AH from (P – E) when the latter difference is negative. Frequencies are plotted as common
logarithms, for computation of geometric means. Many very low values and some maximal values are probably
statistical artifacts, resulting from large variance of the difference (P – E) between regional mean precipitation
and lake evaporation.
when Lake Jackson had some
surface outflow, leakage appeared to be lower, 12.4 cm yr– 1,but that estimate was made
by subtraction; our estimate for the same
33-month period was 18.7 cm yr- 1. On the
other hand, the notoriously leaky Lake
Jackson at Tallahassee leaked 91.76 cm yr-- 1
in 1973-1975 (Burton et al. 1978), just before one of its frequent disappearances. This
estimate is close to ours for Lake Brooklyn
and a little below that for Pebble Lake. Even
these high figures are not as extraordinary
as they may seem, as Mirror Lake leaked
240 cm yr ‘1 between 1970 and 1980.
(1954-1957),
Residence times
As leakage is a difference of lake level that
exceeds a deficiency in water income, we
treat it arbitrarily as negative and find it
distributed approximately log-normally below a zero bound. We have not inverted the
procedure, as did Brezonik and Messer
(1 977)–that is, we have not attempted to
estimate inflow from gains of lake level that
exceed net gains from rainfall. “Positive
leakage” in this sense would include surface
runoff and deep-seepage income but would
not separate them. Except for the five seepage lakes our water budgets are not closed
either -on the input or the output side. However, surface outflow is clearly negligible in
some, perhaps most, .of the 15 “drainage”
lakes.
On this latter assumption we calculate
water-residence times for all lakes by summing mean leakage and mean lake evapo-
9
Leakage from Florida lakes
ration (cm yr– 1, and dividing into the mean
depth (cm). If we omit Pebble Lake, the
figures (Table 1) range from 0.95 to 5.20 yr,
the mean for all 20 being 2.67 f 1.33 yr. The
small figure for Orange Lake may be an
overestimate, as the lake loses some water
to its outlet. However, of the four IBP lakes
whose carbon budgets were computed by
Richey et al. (1978) only Mirror Lake is
flushed as slowly as Orange Lake. The
streams that flush Marion Lake in 5 d and
Findley Lake in 7 weeks have no counterparts in Florida.
The importance of residence time has been
stressed by most students of eutrophication.
Richey et al. (1978) showed that flushing
rate, not trophic state, governs the proportion of throughflowing carbon that is cycled
through compartments
of the lacustrine
ecosystem. Residence times of Florida lakes
are longer by about an order of magnitude
than those expected for comparably small
lakes in regions with rapid hydrologic
throughflow. We presume that this fact goes
some way toward accounting for their vulnerability to “cottage eutrophication” and
acidification.
Groundwater control of leakage
As was shown above, monthly changes of
lake level respond with fair fidelity and little
lag to monthly net precipitation. Figures 3–
5 show that they also respond to longer term
variations of groundwater. Two kinds of
groundwater need to be considered: the deep
Floridan (“artesian”) aquifer and the surficial “water-table aquifer(s),” composed of
Mio-Pliocene
sediments of very variable
porosity. Most of the lakes are surrounded
by the latter but separated from the former
by many meters of “confining beds” whose
porosity can be guessed at but is rarely measured. Experimental studies of leakage and
porosity have been inaugurated by USGS
in the basin of Lake Lucerne (Lee and Winter unpubl.).
The surficial aquifer supplies lake water
during wet seasons and years but can be a
temporary sink when the shrinking “water
table” slopes away from the lake. At certain
places and times the Floridan aquifer can
appear to be feeding the lakes through their
floors (Pirkle and Brooks 1959), but such
1317
direct influence is ordinarily modulated by
interposition of one or more surficial aquifers. Water levels and flow rates in all aquifers are controlled in some part, somewhere,
by net rainfall (and local recharge), but while
the surficial aquifer may respond locally and
more or less annually, the weather governing recharge of the deep aquifer is likely to
be that of several previous years many kilometers away.
To get an idea of the relative influence of
weather and of long-term hydrologic fluctuations on lake levels, we summed (algebraically) monthly net precipitation and
monthly gains and losses of lake and of deepwell water by water years; downward leakage, an alternative dependent variable, was
also summed. The procedure condenses
monthly into annual variations of all variables and incidentally detrends the record
ofpumpage on well OR 47. Data for Orange
Lake (Fig. 7) are typical of all long (> 23 yr)
records examined-in this way. By inspection
it is clear that lake levels and leakage track
net precipitation and the hydrography of well
MAR 5, but are not so closely related to
annual rainfall. Other lakes, particularly in
the southern district, are better correlated
with well OR 47.
Inspection does not separate the possibly
independent and multiplicative influences
of weather and the deep aquifer on lake
levels. These influences are dissected statistically by partial correlation. Table 2, which
summarizes the results, shows the annual
sums of AH as the dependent variable, the
alternative (annual sums of leakage) having
been rejected by the statistical program. No=
tation of simple and partial values of r in
this table follows Snedecor and Cochran
(1967). Correlations between lake level and
net precipitation, though apparently high,
become nonsignificant in 4 of 10 lakes when
the effect of the deep aquifer is removed. In
8 of 10 lakes annual gains and losses (Z AH)
are more highly correlated with deep-well
fluctuations than with net precipitation. In
the best case, Lake Weir, the multiple correlation R, .23= O.878. The residual variance
1 – R2 (23V0 of the total) must include the
interaction with the surficial water table,
which cannot be evaluated by these methods.
1318
Deevey
ORANGE
c
.-o
2004
z“: :150-
:
LAKE
I
“: w 100-
ANNUAL
I1
FLUCTUATIONS
I
Precipitation-North
I
1
1
Area
b+50
&
-o
Net Precipitation-North
Area
-50
:
0-+75
;=
~
“- w
-75$
Orange
Well
Lake
Marion
Gain/Loss
Gain/Loss
o
a)
m
a_-25
XE
m 0-50
~w
-1
Orange
1~
1
Lake
Leakage
1
r
1955
19’60
I
1965
Water
1970
1
1975
1
1980
I
1985
Year
Fig. 7. Annual precipitation, net precipitation, gains and losses of lake level, and leakage for Orange Lake,
summed algebraically for water years 1954-1985. Lake data are compared with annual sums of gain and loss
of level in the Floridan aquifer, measured in well Marion 5. (NOAA and U.S. Geol. Surv. data.)
Table 2. Partial correlations between annual gain and loss of lake level, annual gain and loss of net precipitation, and annual gain and loss of level in the Floridan aquifer. Nonsignificant values in parentheses. All
variables are annual algebraic sums by water years (October+eptember).
——
(2;.
1)
13
20
16
19
11
9
12
5
3
2
* The
Lake
Orange
Pebble$
Brooklyn
Kingsley
Weir
Dora
Kerr$
Clinch$
June in
Winter
Placid
(1)
Dependent
variable
Level
Level
Level
Level
Level
Level
Level
Level
Level
Z
Z
Z
Z
Z
Z
Z
Z
Z
AH
AH
AH
AH
AH
AH
AH
N
AH
Level Z AH
(2)
Net prccip.
(cm)
(3)
Well*
Partial r*
Simple rt
(;)
r] 1
r13
r23
r123
r] 3.2
——
2.54*22.9
7.14*22.O
7. 14*22.O
2.54*22.9
2.54&22.9
2.54*22.9
2.54?22.9
–3.84*26.1
2.54&22.9
MAR 5
MAR 5
MAR 5
OR 47
MAR 5
OR 47
MAR 5
OR 47
OR 47
32
31
23
32
32
32
29
32
31
0.455
0.535
0.787
0.281
0.763
0.615
0.615
0.818
0.513
0.763
0.650
0.517
0.590
0.781
0.745
0.680
0.726
0.702
0.547
0.393
0.393
0.779
0.547
0.779
0.547
0.644
0.779
(0.069)
0.400
0.742
(–0.353)
0.642
(0.081)
0.395
0.665
(–0.078)
0.690
0.566
0,367
0.617
0.671
0.539
0.520
0.452
0.563
–3.84t26.l
OR 47
28
0.727
0.748
0.664
0.483
0.533
statistical program chooses the best-correlated net precipitation of three regional districts and the better correlated of two wells.
f’ r,, is a simple correlation coefficient between variable 1 and variable 2.
# r,,., is a partial comelation coefficient between variable 1 and variable 2 with variable 3 held constant.
$ Seepage lake.
Leakage from Florida lakes
Conclusions
Writing in water-year 1958, just after a
major drought that lowered lakes throughout the Peninsula (Fig. 4), Pirkle and Brooks
(1959) considered that Orange Lake had
barely escaped the swallow-hole or swallet
that converted nearby Alachua Lake into
Payne’s Prairie in 1891 (Hutchinson 1957,
p. 103-104). As the Ocala limestone crops
out both west and east of the region Pirkle
and Brooks assumed that Floridan aquifer
water regularly enters the lake. Using better
maps of the piezometric surface, Adams and
Stoker (1985) drew a similar conclusion for
Lake Placid and Lake Annie at the southern
end of the lake district. However, such direct influence seems to be ruled out by the
softness and low salinities of these and most
other Florida lakes (Canfield 1981).
At least in the northern (highest) part of
the district so intimate a connection is also
denied by abundant geologic and hydrologic
evidence (Arrington and Lindquist 1987;
Clark et al. 1964). Lakes Weir and Kerr and
Orange Lake, whose levels track each other
and the deep well MAR 5 between them,
all lie on the saddle between northern and
central domes of the piezometric surface
(Conover et al. 1984); but our data do not
suggest that any of the three are intersected
by the aquifer. Farther south, at least on the
sandy ridge system, the aquifer is even more
deeply buried.
We conclude that artesian pressure on
Florida lake levels is ordinarily exerted
through confining beds and surficial aquifers; that downward leakage, the sole nonevaporative output of seepage lakes, removes water from putative drainage lakes
at very similar rates; that both the modal
rates, 30-50 cm yr– 1, and the highest rates,
90-140 cm yr- 1, are to be expected in lakes
not in rock basins; and that water-residence
times of 1-5 yr are longer by an order of
magnitude than those found (or assumed)
where surficial runoff is rapid and groundwater exchanges are ignored.
As to periodic or aperiodic driving mechanisms, we find that whereas monthly fluctuations of lake level are fairly well correlated with same-monthly net precipitation,
1319
annual and decade-long variations are probably driven by slowly sloshing groundwater.
In 8 of 10 lakes with 23-40 yr of record,
the annual fluctuations are more highly correlated with those in the deep aquifer than
with net precipitation. One of the exceptions, Lake Clinch, is a seepage lake; the
exceptional “drainage” lake, Lake Brooklyn, has known swallets in its basin.
These conclusions have special relevance
for limnologists who have not yet considered the implications of the hydrology of
Mirror Lake for their own research. If water
budgets are in error by - 50Y0, owing to neglect of groundwater interchange, nutrient
and other solute budgets are likely to be in
error by several hundred percent. Neglect
of sediments in throughflow models may
then lead to omission of a storage reservoir
that is many times larger than the lake. For
hydrologists as well as for limnologists,
downward leakage of water through the sediment lens, even at rates as low as 30 cm
yr- 1, implies elution of solutes and irreversible contamination of deep aquifers. For
paleolimnologists and palynologists, we note
that no substance as soluble as 137CSC1will
remain for long in any particular stratigraphic position in a core and that downward migration of soluble carbon may invalidate 14C dates based on bulk lake
sediments.
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