Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 31

same time, with no storage, S , of water in either lakes
or groundwater. Mathematically, this can be expressed as
the change in storage,

water from a basin through plants, as transpiration, T , had to
be included in the water-budget equation. Since both evapo-
ration and transpiration are the conversion of water from a
liquid to a vapor, the two processes often are combined into
one term, called evapotranspiration, ET . As such, Eq. 2.3
becomes

D S , over time, D t , equals zero, or

D S / D t ¼

0.
This water-budget approximation can be considered
valid when looking at the inflows and outflows of water
for a particular basin over a period of many years, in which
any changes in storage that do occur can be considered
small relative to the larger quantities of inflow and outflow.
When shorter timeframes are studied, however,

P ð Q þ G þ ET Þ ¼ D S =D t ; (2.4)

t will
not be 0, and a transient form of Eq. 2.1 must be used,
such as

D S / D If we assume that the change in storage of water in a basin

can be considered negligible over a long period of study,
such as 0 in Eq. 2.4 , and that we can further

combine surface-water and groundwater return flow to the
oceans, R , then Eq. 2.4 can be simplified to

D S / D t ¼

W Inflow W Outflow ¼D S =D t : (2.2)

From Eq. 2.2 it can be seen that if water inflow is greater
than outflow, storage of water will occur (the sign of

P ET ¼ R : (2.5)

t
will be positive), and if water inflow is less than outflow,
then storage will be depleted (the sign of D

D S / D

To summarize, Eq. 2.5 states that within a particular
basin, the precipitation, P , not returned to the atmosphere
as water vapor from evapotranspiration will flow as surface
water in rivers or much more slowly as groundwater to the
ocean, unless springs supplement surface flows. As was
observed by Perrault (1674), because precipitation to the
Seine River valley was six times the volume of surface
water discharged from the basin, the balance could be
attributed to groundwater flow, storage, and evapotranspira-
tion. The removal of water by evapotranspiration occurs at a
much faster rate than by subsurface flow and at a fairly
constant annual rate, even though precipitation tends to
vary widely. Therefore, knowledge of the role of evapo-
transpiration, in particular plants, becomes important. For
example, in the continental United States the loss of water
by evapotranspiration accounts for almost 70% of precipita-
tion, or 2,871 bgal/d (billion gallons per day) (10,910 Mm 3 /
d [million cubic meters per day]). Of this total, a low
estimate for transpiration by plants not used for economic
purposes, called consumptive use, is about 106 bgal/d
(403 Mm 3 /d), or about 3% of total evapotranspiration,
although consumptive use can be much higher (Moran
et al. 2007). An excellent review of the impact of vegetation
on the hydrologic cycle at a global catchment scale is
presented by Peel et al. (2010).
In Alley et al. (2002), an example is provided that further
emphasizes that evapotranspiration is a large component of
the hydrologic cycle. In central Kansas, groundwater
recharge by precipitation accounted for about 10% of total
precipitation over a 6 year period. In addition, one explana-
tion for recharge being such a small fraction of precipitation
is that much of the precipitation that infiltrates into the upper
layers of soil is rapidly used by plants. For example, Healy
et al. (2007) stated that of the 76% of precipitation that
infiltrated, up to 85% of this volume either evaporated or
transpired.

t will be
negative). Perrault's work showed that precipitation, or
water inflow, provided six times more water than could be
accounted for in surface-water flow out of the valley.
Hence, there was either storage in the basin or water was
leaving the system through an unaccounted process. Was
this unaccounted water stored as groundwater or in lakes?
Or was this water leaving the basin in other ways than by
river discharge and evaporation? This imbalance of water
must have troubled Perrault, for as part of his study he
attempted to make what was probably the first measurement
of water outflow by evaporation from surface water, even
preceding the evaporation measurements made later by
Halley.
In addition to losses of water from a basin by stream
discharge and evaporation, there is the outflow of water
through porous soils and sediments (infiltration), which
recharges aquifers. Because of the interference of soil
particles to infiltration, the time it takes to return this subsur-
face water to surface-water bodies is considerably longer
than the time it takes water in rivers and streams to discharge
to the ocean. Considering these additional processes of water
outflow from a basin, Eq. ( 2.2 ) becomes

S / D

P ð Q þ G þ E Þ ¼ D S =D t ; (2.3)

where P represents precipitation, Q represents river dis-
charge, G represents groundwater flow or discharge, and E
represents evaporation.
Although the removal of surface water by evaporation,
E , was recognized by Perrault and Halley in the late 1600s,
it wasn't until the experiments by Stephen Hales in the
1720s, however, that the evaporation of water through
plant leaves was recognized and considered in water-balance
calculations. Therefore, this process of the evaporation of