Environmental Engineering Reference
In-Depth Information
15.2.5 Soil Physical Properties
Compared to soils of croplands or of other land uses, forest soils have much higher
infiltration capacity Bruijnzeel,
2004
. Forest soils are known for their high organic
matter content derived from plant litter fall above ground and dead roots below
ground that are conducive to high activities of soil organisms, high soil porosity, and
higher hydraulic conductivity. For example, saturated hydraulic conductivity of the
top soil layer in a mature loblolly pine forest on the North Carolina coastal plain can
be as high as 700 cm/h (Diggs
2004
). Compaction caused by forest harvest operation
increased bulk density from 0.22 to 0.27 g/cm
3
, decreased saturated hydraulic
conductivity from 397 to 82 cm/h (Grace et al.
2006
). Soil disturbances in intensive
agriculture and forestry such as plowing, bedded, harvesting activities can dra-
matically degrade soil hydraulic properties. For example, deforestation and subse-
quently tillage practices in Iran resulted in almost a 20 % increase in bulk density,
50 % decrease in organic matter and total nitrogen, a 10-15 % decrease in soluble
ions comparing to the undisturbed forest soil (Hajabbasi et al.
1997
). The unique soil
physical properties of forests explain the high soil water retention, high soil infil-
tration rates, minor overland flow rate, low streamflow, and high groundwater
recharge commonly found in forests Zhou et al.,
2010
.
15.2.6 Interactions Between Forests, Climate,
and Streamflow
As illustrated in Fig.
15.1
and the following energy balance equation, forests affect
the redistribution of solar radiation into sensible and latent heat fluxes through
passive (i.e., light reflection) and active physiological processes (photosynthesis,
transpiration etc.). Latent heat is the energy source for evapotranspiration, a key
component of the hydrologic cycle. The changes in sensible heat flux and
evapotranspiration, which is accompanied with latent heat change, will modify air
temperature and humidity. The change in air temperature, together with the
changes in turbulence and wind, will modify atmospheric circulation. Precipitation
will be affected due to the changes in circulation, temperature, and humidity.
Rn
¼
1
a
ð
Þ
S
þ
L
;
and Rn
¼
LE
þ
H
þ
G
where Rn is net radiation, a is albedo, S is incoming shortwave radiation, L is net
long wave radiation. LE and H are latent and sensible heat flux, respectively.
According the principle of water balance below, water yield or streamflow (Q)
is largely controlled by ET, or LE, at a long term scale (e.g., a few years) when the
change in soil water storage is negligible. However, at short temporal scale (e.g.,
1 day, 1 month), soil water storage can be significant water source for Q and ET.
In this case, both soil water storage and ET are important.
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