Geoscience Reference
In-Depth Information
Before land clearing, transpiration from
trees helps to keep the water table down
Evaporation of water from soil
2 m
C C C C C C C C C
C C
Capillary water movementll
C
C
C p p p p p p p
p
p p
p
w t
t t
t te r
r
r
r m m m m m
m m m m m m mo v v
v
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v v v
v
v m m m m m m m m m m
m n n n n n n n n
n n n
a a a a a a a a
a a r y
y y
y y y y y
y y y
y w w w w w w w w w
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S S Sa l l li n n nne g r r ro u u un d d dw a a t e e er
Saline groundwater
SSa inne g grrouunddwaateer
Saline groundwater
Unsaturated
sediment
Saturated
sediment
Water
table
Salt deposited
in the root zone
(a)
(a)
After land clearing, discharge through
transpiration decreases, causing the
water table to rise
Plants transpire water
Dead trees
Barren land
Saline water
flows to the
surface
Saline groundwater
(b)
Figure 20.12 Salt crust formation after native vegetation is
cleared. (a) Native vegetation keeps the water table down due
to transpiration. (b) Once the native plants are removed, more
groundwater recharge occurs because agricultural crops absorb
less water. The water table then rises, causing salinization in top-
ographic depressions.
Water
evaporates
Water plus salt
goes into soil
ater p
s s alt
plus s
lus sa
W a s
g o es into so if
W
ter p
goe
in
es into soil
nto s
o so
o
o
o
Salts remain behind
Another way that agriculture causes soil salinization is
through irrigation (Figure 20.13). In irrigated fields the water
applied to the soil can raise the water table. Evidence from
around the world indicates that irrigation can cause the local
water table to rise rapidly, with increases up to 3 m (~10 ft) re-
ported in some places. As with the clearing of native vegetation,
such a rise can lead to wicking of salt in the soil and formation
of salt crusts as the water evaporates. Especially in warm and
dry areas, if the applied water is already concentrated with salts,
the irrigated water will rapidly evaporate from the surface, leav-
ing higher levels of salt in the soil. Human-induced soil salini-
zation has been a problem since the dawn of civilization. One of
the best examples of the impact that increased soil salinity has
on people is ancient Sumeria in Mesopotamia.
(b)
Figure 20.13 Soil salinization due to irrigation. (a) Soil salini-
zation can occur in irrigated fields because application of water
causes the water table to rise close to the root zone. Once this
occurs, water and salts move to the surface by wicking. The wa-
ter dissolves, leaving salt behind. (b) Soil salinization in farm fields
can also occur if saline water is applied to fields. Once the water
evaporates, salt crusts form at the surface.
desert climate region ( BWh ), the Sumerians thrived for centu-
ries because they developed a complex irrigation network of
canals and aqueducts to divert water from the rivers to outlying
farm fields. It is believed that wheat was first grown as a domes-
tic crop during this time. As carrying capacity increased, the
first city-states (Figure 20.14b) evolved in this part of the world
and the first form of systematic writing began (Figure 20.14c).
At the same time that Sumerian society became increasingly
sophisticated, its foundation was apparently slowly collapsing
as soil salinity increased due to rising water tables and wicking.
Indirect evidence for increased salinity can be seen in the counts
of grain impressions in excavated pottery, which indicate that by
Ancient Sumeria
The Sumerians lived between about 4000 and 2300 b.c. in the
fertile alluvial plain of the lower Tigris and Euphrates river val-
leys in what is now southern Iraq. This region lies within the
southeastern part of the famed Fertile Crescent (Figure 20.14a),
which is one of the key places where early human civilization
developed. Although this region lies within the hot low-latitude
 
 
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