Integration of Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 30

In 1687, the astronomer Edmond Halley (1656-1742)
partly answered these questions by making accurate
measurements of the amount of water being evaporated
from the surface of the Mediterranean Sea. He concluded
that the volume of evaporated water was sufficient to supply
the water discharging to the oceans from local rivers.
Halley's observation provided the foundation that water
returned to the land from the oceans not though subterranean
holes but as water vapor through the sky. Today, we know
that about once every 10 day the moisture in the air falls and
is exchanged with new water vapor.
Evaporation, therefore, became an important component
to consider when investigating water flow in a basin, but this
still only accounted for the removal of water from the rivers.
Similar evaporation investigations also were performed in
England and Wales. In 1802, for example, John Dalton
(1760-1844) calculated a water balance for many counties
in England by using the novel approach of a network of
raingages and was able to state

Upon the whole then I think that we can finally conclude that the
rain and dew of this country are equivalent to the quantity of
water carried off by evaporation and by the rivers.

Here is one of the initial records of the balance between
the input of water to a basin and the removal of water
from the basin, from which grew the concept of a water
budget. These early scientists, however, had not accounted
for all of the sources and sinks for water in a basin, espe-
cially from the standpoint of plants and groundwater.

Fig. 2.1 A conceptual depiction of the Seine River basin where it was
determined using a novel experimental method that the amount of water
delivered to the basin by precipitation, P , was much greater (Up to six
times) than the amount of water that left the basin by stream discharge,
Q . This observation provided empirical evidence that a source of extra
water existed to supply groundwater, evaporation, and ultimately
transpiration.

2.2.1 The Water Budget width, to determine the discharge of water. He concluded as

Perrault had stated that precipitation was sufficient to supply
the flow of water in the river and to springs. More than
100 years later, similar observations of the relation between
discharge and precipitation were made by J.F.D. Smyth in
the Colony of Virginia (Rosenshein et al. 1986).

These early investigations into quantifying the balance
between the inflow and outflow amounts of water in a
basin provided the foundation for early conceptual models
of the hydrologic cycle and the framework to quantify this
cycle in terms of a water budget. Because the total quantity
of water in the earth is finite, it can be handled mathemati-
cally using continuity equations. Thus, for any particular
valley or basin, such as the Seine River valley studied by
Perrault (1674), we can state the following equality shown in
Eq. 2.1 :

2.2 The Water Budget and Hydrologic Cycle

Although the measurements made by Perrault and Mariotte
indicated that precipitation was greater than six times the
river discharge, which refuted the need to invoke a source of
water from massive subterranean caverns, the question
remained: what happened to the balance of rainwater that
remained in the basin? Moreover, what was the source of
water to support the amount of precipitation being
measured? Perrault (1674) stated that

W Inflow ¼ W Outflow ; (2.1)

where W Inflow refers to water that enters a representative
basin, such as precipitation per unit time, and W Outflow
refers to water that leaves a representative basin, such as
by river discharge per unit time. Equation 2.1 represents
steady-state or equilibrium conditions, such that the
amount of water entering a basin during a certain period
of time is completely removed from the basin during the

to cause this river (the Seine) to flow for one year, from its
source to the place designated, and which must serve also to
supply all the losses, such as the feeding of trees, plants, grasses,
evaporation

...

...