Environmental Engineering Reference
In-Depth Information
advantageous for plants to use surface water when it is
available but also to have deep roots to tap groundwater
when surface water is less or even unavailable. These deep
roots serve to allow the plant to survive drought conditions
when surface water derived from runoff is scarce.
Some plants thrive only if groundwater is the water
source. Harvey et al. (2007) investigated plant and ground-
water interactions at peatland fens, or wetlands, in central
Nebraska. This area of the United States is hot and arid,
where potential evapotranspiration is 2-3 times annual pre-
cipitation, and is perhaps the last place that one would expect
to find peatlands, which are more common in the humid
southeast states such as Florida. The fens of Nebraska
exist, however, because they are sites of groundwater dis-
charge. Plants that can be found in these fens include rush
aster (
Aster junciformis
), mud sedge (
Carex limosa
), and
water sedge (
Carex aquatilis
).
At least two reasons why fens rely on groundwater were
suggested by the work of Harvey et al. (2007). First, a
microclimate of humid conditions in the wetlands,
surrounded by the arid area, is created from the transpiration
of groundwater. Second, plant growth is supported by
nutrients in the groundwater. This same phenomenon of
plant-transpiration-mediated nutrient transport and uptake
from groundwater was seen in the Florida Everglades by
Ross et al. (2006), who investigated the possible reasons
why the groundwater beneath the many Tree Islands in the
ridge and slough landscape typical of the Everglades is
characterized by high phosphorus concentrations, even
though the surrounding surface and groundwater have pri-
marily low phosphorus concentrations under ambient
conditions. Ross et al. (2006) measured groundwater levels
in shallow wells on Tree Islands and observed that the lowest
groundwater levels occurred during the summer months
when evapotranspiration was the highest and the plants
were using groundwater. Moreover, the deeper wells had
higher groundwater levels than the shallower wells, which
indicated a net upward flow of groundwater to roots.
The relation between
ET
and streamflow in the humid
southeastern United States was investigated by Palmroth
et al. (2010). They report for conditions in North Carolina
that long-term records of streamflow, or discharge, can be
used successfully to estimate regional values for
ET
. They
show that the sum of annual stream discharge and
ET
was
equal to annual, and independently, measured, precipitation;
this result confirms the relation expressed in Eq. (2.5).
the opportunity, would become surface water. Riparian,
from the Latin
ripari
meaning the bank of a stream, plants
have been shown to affect the level and volume of surface
waters that receive groundwater discharge. The effect of
the riparian removal of groundwater is apparent for most
rivers in the United States that are characterized by the
lowest water levels and discharge during the growing
months even though precipitation is often the highest dur-
ing these times. An excellent review of the effect of ripar-
ian plants on hydrological processes is provided by
Tabacchi et al. (2000).
The effect of phreatophyte uptake of groundwater on
surface-water flows was first investigated in the early
1900s by White (1932) of the USGS, and in the 1940s by
Robinson (1958) also of the USGS. In 1944, Robinson
(1965) instrumented wells near the Gila River in Arizona
in a large stand of saltcedar (
Tamarix
). During March
1944, before the growing season for saltcedar, no daily
fluctuation of groundwater level was recorded in the well.
During the growing season in June, however, Robinson
observed up to 0.19 ft (0.05 m) in daily groundwater-
level fluctuations. In October, after the saltcedar went
dormant, little to no daily fluctuation was observed. During
this same period, the level of the water in the Gila River
also was monitored with an automatic recorder. During
June, the daily fluctuations observed in the stage closely
followed the groundwater-level fluctuations in the well.
During the week-long period that daily stage fluctuations
were observed, the river stage was lower at the end of the
period, which indicted that not only was there less flow in
the river each day because of transpiration, but the transpi-
ration of groundwater also decreased the volume of surface
water.
An additional effect of phreatophytes is a reduction in
gradient from the originally higher groundwater levels to the
river (Robinson 1958). Thomas (1952) reported that the
Green River, which is characterized by valleys dominated
by up to 40,000 acres (1.61
10
8
m
2
) of phreatophytes,
loses an average of 552 acre-ft (680,616 m
3
) annually,
roughly 278 ft
3
/s, because of the uptake of groundwater by
phreatophytes.
Other examples of the interaction between plants,
groundwater, and surface water exist. Peak stream flows
observed in the Rio Grande River decreased following an
increase in non-native species in riparian areas (Roelle and
Hagenbuck 1995). Another example is the removal of
groundwater by trees prior to discharge to surface water in
the Rio Grande and Rio Bravo Rivers at the Gulf of Mexico.
A narrow border of trees and tall grasses flanks the shoreline
on both sides of the bottom of steep cliffs that lead down to
the river. A similar reduction in stream flows that resulted
from the uptake of groundwater by phreatophytes was
observed in the Northern Great Basin by Nichols (1993).
5.2.1 Reduction in Surface-Water Flow
Plants that grow along rivers that use groundwater affect
surface-water quantity and flow by default. In short, plant
transpiration of groundwater intercepts water that, given
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