Environmental Engineering Reference
In-Depth Information
opposed to groundwater; meteorological events, such as
photosynthetically active radiation (PAR) and temperature;
plant growth and density, such as vigorous as opposed to cut;
proximity to surface water; and depth to water table.
A variety of studies to examine groundwater fluctuations
either in the field or using controlled-experiment tank studies
was conducted by the USGS in the 1960s. These include
work by McDonald and Hughes (1968) in the flood plain of
the Colorado River between California and Arizona and
north of Yuma, Arizona, and downgradient of the Imperial
Dam. Diurnal groundwater fluctuations ranged from 0.1 to
0.4 ft (0.03-0.12 m). The monitored well was located about
1,500 ft (456 m) from the gaining Colorado River in the flood
plain that contained predominantly arrowweed (
Pluchea
sericea
) from 1962 to 1965. These researchers made the
observation that the initial decrease in groundwater level in
the monitored well, actually called a transpiration well, from
the rebounded higher levels during nighttime was seen in less
than 15 min after the sun's rays hit the study area. The depth
of groundwater fluctuation was directly related to air temper-
ature and inversely related to relative humidity.
The groundwater-level fluctuations observed in these
studies indicate the possible interactions that occur between
components such as plant-root uptake rates, groundwater
movement rates, and precipitation and recharge frequency,
among others. The explanation behind the diurnal fluctua-
tion observed is that the roots respond to a
VPD
by removing
water from the capillary fringe or water table during the mid-
morning to afternoon period when solar radiation is not at a
maximum and the stomata remain open. The water table
decreases, if and only if the rate of removal is greater than
the rate of replenishment. Replenishment can be derived
from storage, lateral movement of groundwater from
upgradient areas, or recent recharge (Fig.
5.5
). These factors
are controlled by the climate and the hydraulic conductivity
of the saturated- and unsaturated-zone sediments. Once the
stomata close and evapotranspiration ceases, as occurs dur-
ing the hottest part of the day, the rate of groundwater
removal by plants is less than the rate of replenishment,
and the water table rises. If this occurs over a period of
time characterized by no precipitation, especially in uncon-
fined aquifers of low hydraulic conductivity, the nighttime
rebound groundwater level over time becomes progressively
lower, and the maximum daily decline also becomes pro-
gressively deeper. The amount of change remains the
same each day, however, as this is controlled by the VPD
and plant-resistance characteristics, as long as the plant is
healthy.
Szil´gyi et al. (2008) report that groundwater-level
fluctuations in wells located near gaining streams occurred
1-1.5 h after a fluctuation in the surface-water level in a small
forested watershed. The explanation given for the lag is the
decrease in the hydraulic gradient,
i
, between the wells in
Fig. 5.4
Effect of groundwater uptake by (
) willows near surface
water. The dashed lines in all figures represent equal head. (
a
) The
dashed water table represents a higher water table following infiltration
of surface water. (
b
) The lines with arrowheads indicate the direction of
groundwater flow, and
Q
t
represents water removed by transpiration
(Modified from Meyboom 1966).
c
methods that determine total water used, such as sap flow,
regardless of the source of water. Mower et al. (1964) placed
three wells in stands of phreatophytes such as saltcedar and
grasses. Evapotranspiration from the water table at the three
sites ranged from 2.5 to 5 ft/acre/year (0.76-1.52 m/acre/
year), with an average depth to water table near 7 ft (2.1 m).
Kluitenberg et al. (2005) performed an investigation to
determine the effect of phreatophyte evapotranspiration
demand on groundwater levels in a shallow aquifer at two
locations in Kansas—the Larned Research Site and the
Ashland Research Site. Both sites are riparian groundwater
and surface-water systems; the first is dominated by
cottonwoods and willows and the second is dominated by
Tamarix
. At each site, groundwater fluctuations were
measured in wells, and changes in soil moisture above the
water table were measured by neutron probes. The
researchers reported that the diurnal change in groundwater
level was related to temporal changes in the following: the
source of water to the plants, such as precipitation as
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