REPORT OF INVESTIGATIONS NO. 48


 RAINFALL, INCHES
Figure 13.-Relation between the average of yearly rainfall at Quincy, Monti-
cello and Tallahassee, and the net yearly change of the level of Lake Jackson.


rainfall and lake-level change given in figure 13, the rainfall fre-
quency curve can be converted into a frequency curve for lake-level
change as shown in figure 16.
 Figure 16 can be used to approximate the probability of occur-
rence of a yearly lake-level change of a specified magnitude. The
reality of the approximation rests on the validity of three assump-
tions: (1) that yearly rainfall is a random event; (2) that the
relative frequency of rainfall has been established by records of
past rainfall; (3) that the relation between rainfall and lake-level
changes shown in figure 13 adequately represents the true relation.
 If the factors controlling the lake level are in basic equilibrium,
the frequency curve in figure 16 should be symmetrical about the
line of zero change. As drawn, the frequency curve suggests a