Geoscience Reference
In-Depth Information
activities, changes may occur even now against our desired goal. It happens when
we are confi dently persuaded that we are supporting sustainability without any
proof that our actions improve the function of soil in the hydrologic cycle on local
and global scales. Indeed, our actions may act in the opposite direction to worsen
the cycle and conditions for life and society. It should be apparent that knowledge
about soil hydrologic functions is unmatched and more vital than that about other
ecosystem functions.
The capacity of a soil to catch rainwater is the same as that of all natural lakes as
well as artifi cial lakes constructed by man for societal purposes. The amount of
water kept in soils equals one-third of the total amount of those residing in all lakes.
Comparing this volume to waters in all rivers, we fi nd that the amount of water in
soils is ten times more than all of the water in all rivers. If we add the volume of
groundwater to the volume of soil water, we obtain a volume that is 100 times larger
than the volume of all fresh surface waters in rivers and lakes. And when we study
the circulation of water on Earth and its atmosphere, we learn that the role of soil is
not negligible. Indeed, 74 % of global precipitation falls onto water and 26 % falls
on land surfaces, while evaporation from water surfaces makes 81 % and that
including transpiration from land surfaces is 19 %. The difference of 7 % between
distributions of precipitation and evaporation from land surfaces is balanced by
water running off soil surfaces.
Next, we are going to discuss details regarding the main hydrologic processes in
soils - infi ltration, evaporation, and transpiration - and also explore what happens
within a soil between two infi ltration events.
10.1
Infi ltration of Water into Soil
Rainwater as well as water from melting snow or ice is essentially absorbed by the
underlying soil at the Earth's surface. Accordingly, we speak about infi ltration of
water into soil. Water also infi ltrates into soil from fl ooded areas or slight depres-
sions where it accumulated by the surface runoff during the rain. Hydraulically, we
have to differentiate two types of infi ltration according to the source of water:
(a) Infi ltration from a suddenly fl ooded soil surface being permanently inundated
with a depth of water greater than that of the soil surface roughness. In a practi-
cal way, the free water level is more than 1 cm above the natural soil surface. It
is theoretically defi ned as infi ltration with a Dirichlet boundary condition
(DBC). It means from time zero at the beginning of infi ltration that a constant
water content is kept on the soil surface. Here, we mean the surface soil water
content remains water saturated. Furrow or check basin irrigation is solved
using this type of infi ltration boundary condition. The same condition is used to
calculate infi ltration into fl ooded plains along rivers.
(b) Infi ltration from rain or sprinkler irrigation knowing its prevailing intensity or
rate of water fl ow into the topsoil. Solutions of infi ltration equations using this
condition - known as the Neumann boundary condition (NBC) - provide
