Environmental Engineering Reference
In-Depth Information
earth. An understanding of the hydrologic cycle took time.
The history of unraveling the hydrologic cycle depicts the
evolution of thinking in many aspects of science that began
with philosophical musings, led to direct observation of
physical processes, and finally to quantification of these
processes though direct experimentation and measurement.
In contrast to these ideas, in the first century BC , the
Roman architect Vitruvius (90-20 BC ) hypothesized,
among other things in De Architectura Libri Decem (Fetter
1988), that precipitation and snow falling in the mountains
reappeared as springs and streams in low-lying areas near the
ocean. Seen from today's perspective, this notion was very
insightful, in the sense that the source of water in the tops of
mountains did not invoke the presence of subterranean
reservoirs where water would have to flow uphill against
gravity. Aristotle thought that springs and surface water
were not connected and that springs were not caused by
precipitation of rainwater from the sky but by vapors from
underground cavities. In hindsight, Aristotle can be forgiven
for his incorrect notion, because his conclusions were based
on observations of the continual dripping of water from the
roofs of karst caves in Greece, thermal springs, and the fact
that some offshore springs flowed during high tide.
Most translations of the Bible contain a statement that
reveals a beginning of an understanding of the hydrologic
cycle:
For He draws up the drops of water, He distills His mist in rain,
which the skies pour down, and drop upon man abundantly.
Job 36:27-28 (Revised Standard Version (RSV) 1971)
2.1.1 Musings During the Early to Middle Ages
The first recorded musings about the movement and storage
of water were most likely derived from observations of water
that was readily available, such as precipitation and surface
water (Fetter 1988). In some places, an apparent dilemma
occurred between the observations of infrequent precipita-
tion but the seemingly constant presence of lakes, streams,
and springs. As early as 800 BC , the Greek philosopher
Homer stated in Book 21 of the Iliad that
The deep-flowing Oceanus, from whose deeps every river and
sea, every spring and well flows.
Anaxagoras of Clazomenae (500-428 BC ) wondered
about the source of water seen in rivers, and stated that
Rivers depend for their existence on the rains and on the water
within the earth, as the earth is hollow and has water in its
cavities.
Another, similar statement in the Bible is attributed to
King Solomon:
All streams run to the sea, but the sea is not full; to the place
where the streams flow, there they flow again.
Ecclesiastes 1:7 (RSV)
These writings (cited in Fetter 1988) hint at an initial
understanding of the hydrologic connection between oceans
and rivers. At the time, however, this connection was
believed to be made by the presence of large subsurface
reservoirs and not precipitation, because the amount of pre-
cipitation did not seem to be able to support observed
quantities of surface water. However, Plato (427-346 BC )
wrote about a potential source of subsurface water as the
connection, a cavern which he called Tartarus.
It has been shown, especially during droughts, that surface
water does consist of groundwater, called base flow, so Plato
and his contemporaries were not entirely incorrect in relating
surface water to an underground source of water. In this case,
however, they were referring to water being held in one large
cave, which supplied all the water seen in the rivers. Water
flowed from the oceans underground up to the tops of
mountains and then flowed back down in rivers to the
ocean. The fact that the landscape of Greece, which is dry
most of the year and consists of limestone, fostered these
observations is important in understanding the context in
which the observations were made. The Greeks used the
many sinkholes in the area as sources of drinking water and
as locations to convey excess surface water to the ocean
during flooding. Also, the Greeks noticed that there seemed
to be more water in rivers than was supplied by precipitation
and runoff, which supported the notion that the difference
was made up by water from underground caverns.
The role of the sun in the movement of water from ocean
to sky and back to land was noted in the writings of Aristotle.
He stated,
Now when the sun in its circular course approaches [sic], it
draws up by its heat the moist evaporation: when it recedes the
cold makes the vapour that had been raised condense back into
water which falls and is distributed through the earth. This
explains why there is more rain in winter and more by night
than by day: though the fact is not recognized because rain by
night is more apt to escape observation than by day.
Aristotle, Meteorologica , 359.27-360.25 (Ross 1927)
The Chinese and Arabian cultures represented the height
of civilization during the Middle Ages. Many advances were
made by these societies in the fields of chemistry and astron-
omy. Therefore, it is ironic that they left little record of
advanced understanding of the hydrologic cycle.
In Europe, Norse mythology reveals that the Viking uni-
verse consisted of nine worlds, and that the whole universe
was held up by a large tree (Helfman 1972), a common
theme in many ancient mythologies and often called the
World Tree. These nine worlds (nine being the most signifi-
cant number in Norse mythology) were located along the
tree, called Yggdrasil from Ygg meaning the terrible one and
drasil meaning horse, perhaps referring to the horse that the
wise elder Odin rode. The tree just happened to be an ash
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