REPORT OF INVESTIGATIONS No. 42


precipitation (P,) for the preceding year that furnished the runoff
for the current year, or

 Po = aPo + bP,
The coefficients a and b are determined by statistical correlation.
 Long-term annual records of rainfall and runoff for the
Withlacoochee River and Palatlakaha Creek basins have been used
to determine the variations in water loss in the Green Swamp area.
Areal variations in water loss are caused by: (1) climatic factors,
the most important of which are rainfall, temperature, humidity,
and wind; (2) drainage basin characteristics, which include size,
shape, surface slope, the amount of water area, seepage as related
to the surface and sub-surface geology, and the condition and type
of vegetative cover; and (3) storage underground and in natural
lakes, ponds, swamps, and artificial reservoirs. The effective annual
precipitation determined for the Withlacoochee River basin above
Trilby and for the Palatlakaha Creek basin above Mascotte is

 P, = 0.8P,, + 0.2P,
 The annual water-loss curve for Withlacoochee River at Trilby
is shown in- figure 46. The Po = L line (dashed line) in figure 46
represents the theoretical limit of water loss which would occur if
the loss equaled the precipitation and none ran off as streamflow.
The average water-loss curve is shown by the solid line which was
drawn to average the annual figures of effective precipitation and
loss (P, R) for the basin. The departures of the yearly data
from the average curve may be caused in part by changes in
storage and seepage and in part by differences in the distribution
of precipitation within the year. No adjustment is made for these
factors in the water-loss equation for Withlacoochee River at Trilby
and they thus affect the apparent evapotranspiration.
 An effective annual precipitation of about 32 inches, indicated
by the point where the downward extension of the curve coincides
with the Po = L relation, is the most probable yearly amount below
which little significant runoff would occur. Under some conditions
of intensity and distribution of precipitation, there could be runoff
with less than 32 inches of precipitation.
 As shown by the curve in figure 46, the average water loss
increases with the precipitation until it becomes nearly a constant
for higher values of precipitation. This is about the maximum loss
that would occur regardless of the amount of precipitation and is
called the potential natural water loss for the basin. The potential