Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 32

2.2.2 The Hydrologic Cycle

Ironically, Viessman et al. (1977) stated that transpiration,
T , can be composed of both soil moisture and groundwater
but did not display this in the figure. Also excluded are the
interactions between surface water and groundwater.
Water removal from a particular basin also is not depicted
in the hydrologic cycle shown in Fig. 2.3 . Although only two
representations of the hydrologic cycle are reproduced here,
even casual observation of the many textbooks available on
water resources indicate that the removal of groundwater by
plants is not uniformly depicted or often even considered.
The common lack of uniform depiction of plant uptake
and transpiration of groundwater in the hydrologic cycle may
be explained as follows. As shown in Eq. 2.5 , the components
of precipitation, P , and river and groundwater discharge, R ,
are more amenable to direct observation and, therefore, were
the earliest components to be measured, as initiated by the
field measurements made by Perrault and Mariotte. Even the
process of evaporation from surface-water bodies could be
measured after the work of Perrault and Halley, and others
after them. So, the hidden nature of groundwater and the
common misunderstanding that it can supply the water to
plants led to its exclusion in many early, and even current,
water-budget studies. Moreover, the effects of surface-water
evaporation can exceed the effects of transpiration in certain
areas; because evaporation of groundwater was considered to

The deceptively simple equality of Eq. 2.5 often is presented
in the form of a variety of conceptual models that depict the
flow of water between the various compartments of
the hydrologic cycle. When it comes to the depiction of the
direct removal of water by plants, or the indirect removal by
evapotranspiration, there is considerable disagreement. Most
graphs, flowcharts, or descriptions of the hydrologic cycle
depict the outflow of water by plants in the form of precipi-
tation, soil moisture, or surface water if the plants are aquatic
macrophytes. Indeed, any subsurface water shown to be
removed by plants in a particular schematic is most often
assumed to be derived only from soil moisture in the unsat-
urated zone, not groundwater.
Although the method of presentation can differ widely,
two representative models are presented in Figs. 2.2 and 2.3 .
In the first example, evapotranspiration is explicitly
presented, but groundwater as a potential source for evapo-
transpiration is not (Fig. 2.2 ); in Fig. 2.3 , plant use of
groundwater is directly linked to water from aquifers.

Fig. 2.2 A common depiction of the hydrologic cycle that does not
include groundwater as a potential source of transpiration (Modified
from Viessman et al. 1977).

Fig. 2.3 A less common depiction of the hydrologic cycle that shows
that vegetation can use groundwater for transpiration (Modified from
Detay 1997).