Environmental Engineering Reference
In-Depth Information
Fig. 1.14
Daily fluctuation of
the water table beneath an alfalfa
field from August 25, 1926
(
dashed horizontal line
) during
plant-uptake of groundwater,
until August 31, 1926, when the
alfalfa was cut. When the alfalfa
began to regrow, the
groundwater-level fluctuation
resumed (Modified from White
1932).
A study conducted by the USGS during 1943-1944 by
Gatewood et al. (1950) to determine the potential ground-
water quantity that could be made available to the War
Department (the original name of the Department of
Defense) from the Safford Valley, Arizona, also provided
additional, fundamental information on the interactions
between plants and groundwater. As part of that investiga-
tion, the amount of groundwater being used consumptively
by saltcedar trees (
Tamarix spp
.) was investigated before
and after removing the trees in order to make available that
volume of groundwater for other uses. These researchers
used a variety of methods to determine the volume of
groundwater used by plants, including tank, transpiration
well, seepage-run, inflow-outflow, chloride increase, and
slope-seepage methods. A major conclusion of their study
was that of the 28,000 acre-ft (3.45
10
7
m
3
) of water
calculated to be used by vegetation along 9,303 acres
(3.76
10
7
m
2
) of a 46-mi (mile) (74 km [kilometers])
reach of the Gila River during the 1-year study, 23,000
acre-ft (2.83
10
7
m
3
) was derived solely from groundwa-
ter. Hence, 82% of the water demand came from groundwa-
ter, and most of that was from saltcedar. Suffice it to say, this
study was the first of its kind on a scale large enough to
indicate that plant and groundwater interactions had signifi-
cant hydrogeologic, water budget, and economic effects.
During this time period, the effect of plants on ground-
water was included in textbooks such as
Hydrology
(Wisler
and Brater 1956), although inclusion of these interactions
fell out of favor in later textbooks. It took until the 1970s for
the effect of phreatophytes on groundwater to once again be
examined, although from the perspective of the effect of
plants on flood control in open channels. Such work was
done using analytical methods (Bouwer 1978). Still,
questions as to the magnitude of the effect of these plants
on groundwater, not
Fig. 1.15
Diagram of an experimental tank filled with an upward-
fining profile of sediment, artificial groundwater (
▾
depicts water-table
surface) supplied by an external reservoir through a central pipe, and
plants. Unfortunately, the plants used so much water that when it was
depleted, the plants died (Modified from White 1932).
zone along this gradient to replenish water lost to the plants
from the capillary fringe. This is purely a physical process. If
the uptake of water by plants from the capillary fringe and
replenishment by the upward movement of groundwater is
faster than the rate of recharge of the aquifer by hydrostatic
pressure, artesian flow, or lateral flow from upgradient areas,
the water-table level declines. When the plants do not
remove water from the capillary fringe, for instance at
night when transpiration is lower or ceases altogether, the
water table rises to replenish water that was lost.
just as indicators of the resource,
Search WWH ::
Custom Search