Geoscience Reference
In-Depth Information
agricultural development, river hydrology, and generally for healthy ecologic
parameters within the entire regions. Infi ltration declines as soil structure deterio-
rates as a result of soil humus decreasing together with diminishing contents of
humic acids, humins, and especially glomalin; see our earlier Sect.
6.2
.
Soil aggre-
gation is easily deteriorated by improper soil tillage and by repetitively planting the
same crops in monocultures. During the 1900s in the central plains of the USA,
continuous planting of wheat not just for several years but for decades resulted in
the wholesale destruction of soil structure followed by mediocre, unproductive
water management. With a few years of low precipitation, inadequate rainfall infi l-
tration resulted in an extreme drying of soils. Fine soil particles detached from the
structureless soil blown by winds and dust storms were transported as heavy clouds
depositing the hot silt and clay in leeward sites like huge snowdrifts, but instead of
snow, hot fi ne soil particles were burning the leaves of plants. When the long-
expected rain fi nally arrived, the infi ltration of rainwater was practically absent
because the soil surface was so dense and muddy. This impenetrability at the soil
surface magnifi ed the lack of water in the soil profi le and even in a short time after
the rain, the vegetation continued to permanently wilt and die from thirst. The
excess of non-infi ltrated water resulted in water ponding, surface runoff, and soil
erosion on even a slight slope. The long-lasting monoculture planting resulted in the
exhaustion of several important nutrients and the catastrophe was thus complete.
10.2
What Happens After Infi ltration
(About Soil Water Redistribution)
After infi ltration ceases, we detect a gradual decrease of the soil water content in the
top layer wetted by rain or fl ood even if the soil surface is protected by a cover
which does not allow evaporation. The decrease of the soil water content in the
originally piston-like wetted topsoil is caused by the downward fl ow of soil water.
As water drains from and through the wet topsoil and is conducted to soil below the
infi ltration front by the gradient of the water potential, a new quasi or secondary
wetting front is formed. The width of the piston-like wetted topsoil starts to be slim-
mer and slimmer, and the new secondary wetting front proceeds deeper and deeper;
see Fig.
10.6
. This transport of water inside the soil profi le after infi ltration is called
soil water redistribution. It contributes to an optimal source of water within the
entire root zone of plants since their roots may reach greater depths than the position
of any infi ltration front. Such soil water redistribution has other benefi ts - it opens
water-fi lled coarse pores for air to penetrate into the newly created nearly water-
saturated topsoil to reduce the danger of putrefaction of plant roots. It also opens
and prepares the soil profi le to benefi t from the next incoming rain.
Soil water redistribution after each infi ltration event eventually slows down in a
complicated hysteretic manner until a constant value of the soil water content exists
within the wetted portion of the soil profi le. The water held in the moist profi le
attracted early studies on soil physics and was called “fi eld capacity.” During the
