Geoscience Reference
In-Depth Information
Fig. 9.10
The change of unsaturated hydraulic conductivity in the system of pores between the
aggregates shows a strong decrease with a slight decrease of soil water content below the full satu-
ration. In the system of fi ne pores inside of the aggregates, the decrease of unsaturated conductivity
is gentle with the decrease of soil water content or the pressure head
a clear indication that the gradients of both soil water content and pressure head are
zero. If no additional water fl ows through the surface of the topsoil (e.g., no rain and
no evaporation since the surface is covered), we shall fi nd after several hours or
much more distinctly after several days that the water content at the 10-cm depth is
smaller than that at depths of 30 or 40 cm. Indeed, water did fl ow down. From
sequential measurements of soil water content and pressure head, we notice what
Edgar Buckingham learned from his own experiments more than a century ago -
gradients of gravitational potential acting together with those of soil water pressure
potential provide the force responsible for vertical water movement through unsatu-
rated soils, and values of hydraulic conductivity of unsaturated soils depend upon
soil water content and as well as additional soil parameters and processes.
Buckingham (1907) was the fi rst person to signifi cantly modify and adapt the origi-
nal 1856 Darcy equation to quantitatively describe water movement through unsatu-
rated soils. His seminal equation, nowadays frequently called the Darcy-Buckingham
equation, constitutes a unique application of Fick's law of diffusion to compute the
rate of water movement within an unsaturated soil. Professionally active for 50 years,
his career was fi lled with an excellent mixture of diverse, unique achievements.
Although highly appreciated by theoretical physicists, soil scientists trying to under-
stand soil water behavior and its availability to agricultural plants essentially ignored
