Environmental Engineering Reference
In-Depth Information
under similar conditions. The depth to which
plant roots extend influences the efficiency
with which plants can extract water from the
subsurface. Trees, for example, are capable
of drawing moisture from depths of several
meters or more. In contrast, shallow-rooted
crops cannot access soil water that penetrates to
t h o s e d e p t h s . T h u s , e in h a in c e d r e c h a r g e r a t e s i in
areas with shallow-rooted vegetation are seen
in some semiarid regions when native peren-
nial vegetation is replaced by shallow-rooted
crops (Allison
et al
,
1990
; Scanlon
et al
.,
2005
;
Leblanc
et al
,
2008
). Nonirrigated agricultural
crops can have higher or lower evapotranspir-
ation rates than native plants; therefore, it is
dif icult to generalize as to whether the poten-
tial for recharge will increase or decrease due
to changes in vegetation alone. In most set-
tings, the influence of vegetation is seasonal;
in periods of senescence, the presence of
plants can actually promote recharge. Decay
or shrinkage of roots can expose cavities that
can act as preferential flow channels and
enhance infiltration. Plowing and tilling in
agricultural fields can have opposing effects -
breaking up surface crusts, thus increasing
the potential for infiltration - and destroying
preferential flow channels, thus decreasing
infiltration potential. Satellite remote sensing
(
Section 2.5
) can provide information on sur-
face characteristics, such as vegetation type
and percent coverage, leaf area index, and
land use that can be useful in formulating a
conceptual model (Brunner
et al
.,
2007
).
In the Murray Basin of Australia, native
eucalyptus trees were gradually replaced with
nonirrigated agricultural crops through the
1900s. Allison and Hughes (
1983
) estimated
natural recharge rates under native vegeta-
tion to be less than 0.1 mm/yr. After clearing
and subsequent cropping, estimated recharge
rates increased by up to two orders of magni-
tude (Allison
et al
.,
1990
). Unfortunately, the
increased recharge has led to increased leach-
ing of salts to groundwater and subsequently to
the Murray River and its tributaries.
Conversion from natural savannah to nonir-
rigated millet crops over large areas of south-
west Niger since the 1950s has produced soil
crusting on slopes, resulting in increased run-
off and focused recharge beneath ephemeral
ponds that collect runoff (Leblanc
et al
.,
2008
;
Favreau
et al
.,
2009
). Increased recharge rates
arising from the land-use change can explain
the paradoxical relationship between rising
groundwater levels (about 4 m between 1963
and 2007) and decadal droughts (23% aver-
age annual decline in precipitation from 1970
to 1998 relative to the previous two decades).
Areally averaged recharge rates are estimated
to have increased from 2 to 25 mm/yr (Favreau
et al
.,
2009
).
Irrigation can play an important role in
groundwater recharge.
Irrigation return flow.
is
any excess irrigation water that drains down
beneath the root zone or is captured in drain-
age ditches. It constitutes a significant amount
of recharge in many areas, especially in arid
or semiarid regions where natural recharge
rates are low. Fisher and Healy (
2008
) studied
recharge processes at two irrigated agricul-
tural fields in semiarid settings; virtually all
of the annual recharge occurred during the
irrigation season and was attributed to irriga-
tion return flow. Faunt (
2009
) used a complex
groundwater-flow model to show that, in add-
ition to the natural recharge that occurs dur-
ing winter to aquifers in California's Central
Valley, recharge also occurs during the grow-
ing season as a result of irrigation return flow.
Within the United States, flood irrigation has
gradually been replaced with more efficient
sprinkle or drip irrigation systems; conversion
to these new methods has reduced return flows
substantially in some areas (McMahon
et al
.,
2003
).
Urbanization brings about many land-
surface changes that can have significant rami-
fications for recharge processes. Roads, parking
lots, and buildings all provide impervious areas
that can inhibit recharge. Runoff diversions
are common features in urban landscapes.
Diversions may lead to surface-water bodies
or to infiltration galleries. In the former case,
overall recharge for the area is reduced. In the
latter case, recharge may not necessarily be
reduced, but at the very least it is redirected and
may change from a diffuse source to a focused
