Environmental Engineering Reference
In-Depth Information
1.2.8 W.N. White, Plants, and Groundwater
Fluctuations
W.N. White observed a similar predictable, daily decline
and rise in the water table that suggested removal of ground-
water by plants (White 1932). He observed that during the
growing season, automatic water-level recorders indicated a
daily fluctuation in the water table in areas characterized by
what he called groundwater plants, with only a slowly
declining water level was observed in areas without such
plants. Overall, he assigned the cause of the fluctuations to
the fact that the water in the capillary fringe was being
depleted by plants during the day at a faster rate than the
capillary fringe could be resupplied with groundwater from
hydrostatic or artesian pressures. Conversely, the water table
increased at night because the evaporative demand on the
water in the capillary fringe was eliminated by a decrease in
transpiration, and local groundwater replenished what had
been transpired.
Data from the automatic water-level recorders indicated
that a groundwater fluctuation occurred daily and was
initiated between 9 and 11 a.m., with the deepest groundwa-
ter levels measured between 6 and 7 p.m. later the same day
after the time of highest transpiration. After 7 p.m., the
groundwater levels began to return to levels similar to
those measured in the morning. The lag time of a few
hours between maximum daily groundwater demand and
lowered water table is a result of the night-time replenish-
ment of groundwater back into the cells of plants to meet
structural needs, because much of the water originally there
was removed during the day. A similar phenomenon can be
readily observed in the typically wilted condition of most
plants at night after a long hot day that is reversed in the
morning before sunrise when the plants return to a non-
wilted state.
Daily groundwater fluctuations caused by plants also
were observed to have a seasonal pattern. Groundwater
fluctuations were observed to begin in the spring after
foliage emerged but were not observed to occur after a
hard frost, or in other areas characterized by plowed fields
or where plants grew but the water table was deep and
beyond the reach of roots. White's observations of ground-
water fluctuation also were correlated directly with changes
in air temperature, wind movement, and sunlight intensity,
and indirectly with humidity. These relations are similar to
the factors that control evaporation. White was not surprised
to note that the greatest drawdown, or declines in the
groundwater table of up to 2.5 in. (6.3 cm), was observed
on days that were hot and windy, and that even the amplitude
of the daily groundwater fluctuation varied with the various
stages of plant health and growth. An example of the effect
of plant uptake on groundwater levels observed by White, in
Fig. 1.10
Map showing an early depiction of a phreatophyte facies in
relation to the depth to groundwater as measured in wells, shown as
lines of equal groundwater level of 25-, 50-, and 100-ft (7.6-, 15.2-, and
30.4-m) below land surface. As the depth to groundwater increased, the
plants changed from chamiso to mesquite to creosote bush as the root-
depth increased (Modified from Meinzer and Hare 1915). The arrows
depict the direction of groundwater flow.
of trees, such as mesquite or cottonwoods. The application of
automatic water-level recorders in wells was a novel
approach at the time. Previously, such recorders had been
used by the USGS to measure surface-water elevation. Smith
was able to demonstrate that a decline in groundwater level
occurred on a daily basis as a result of the uptake of ground-
water by trees. This process was confirmed over time as the
water table declined only during periods of tree growth and
no declines were observed at night or after the leaves fell.
G.E.P. Smith described his observations in an unpublished
paper given before the Geological Society of Washington in
November 1922. It is not known whether Meinzer was aware
of this early research.
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