ANALYSIS OF WATER-LEVEL-LAKE JACKSON


surface in the underlying limestone aquifer. Active sinkholes have
existed in the lake bottom (Sellards, 1914 p. 128) and, with the lake
at a low level, the downward movement of lake water has been
visible. Thus, some water probably always moves from the lake to
the limestone aquifer system through sinkholes. A sink-hole depres-
sion, exposed when the lake was dry in 1932, is shown in figure 7.
 In addition to the limestone aquifer there is a surficial, sandy-
clay aquifer with a water table that probably approximates the
lake level in areas adjacent to the lake. During years of greater-
than-average rainfall the surficial aquifer probably contributes
significantly to the lake, but during dry periods its contribution
to the lake is probably small.
 Net ground-water movement to or from the lake may vary
depending on the relative amounts contributed by the surficial
aquifer and lost to the limestone aquifer. Evidence developed later
in this report indicates that net ground-water movement is into
the lake during most years having greater-than-average rainfall
and out of the lake during most years having less-than-average
rainfall.

 ANALYSIS OF DATA

 RAINFALL VERSUS RISE OF LAKE LEVEL

 Much of the rainfall at Lake Jackson is from thunderstorms
that are intense but of brief duration. The rise of the lake level
due to such storms is illustrated by graphs in figure 8. During
these brief periods of rise, loss of water by evaporation or ground-
water outflow is small. Thus, the rise of the lake level should be at
least as great as the rainfall; any inflow from the tributary area
during the storm adds to the lake-level rise.
 The over-all effect of inflow from the tributary area can be
 evaluated by comparing the lake-level rise to the rainfall. Rain-
 fall was not recorded at Lake Jackson and thunderstorm intensities
 vary greatly over wide areas, therefore, such a comparison cannot
 be made on a daily basis. However, the variability of rainfall at
 different points in the area tends to decrease as the measurement
 period is increased.
 Thus, to evaluate the effect of inflow to Lake Jackson from its
tributary area, the rise of the lake level was determined for all
detectable storm periods. All rises whether abrupt or gradual were
included, but only the months for which the recorder record was