Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 157

Increase K sat ,
proportional increase
K sat
Increase λ ,
steeper slope
water content (cm 3 cm -3 )
Figure 4.22 Analytical equation for hydraulic conductivity function (Eq.( 4.24 )),
showing the effect of the two extra shape parameters.
soil layers. Vertical iniltration affects directly the runoff amount during high rainfall
intensity and therefore receives due attention by hydrologists.
Three main approaches exist to make simple, fast and accurate measurements
of iniltration behaviour (Smith, 2002 ): sprinkler methods, ring iniltrometer meth-
ods and permeameter methods. The challenge at sprinklers is to mimic natural rain
showers with suficient accuracy. Dificulties have been experienced in achieving a
wide range of application rates while maintaining a drop size distribution and kinetic
energy similar to that of natural rainfall. Most sprinkler devices are set up to measure
iniltration as the difference between applied rainfall and runoff from an experimental
plot. Typically, the plot is bounded and the runoff is routed through a small weir at
the downslope side of the plot. In case of ring iniltrometers, single or double rings
are inserted into the soil surface, and shallow ponded conditions are created inside
the rings. To create one-dimensional vertical low, either the conining ring must be
pushed very deep into the soil or an outer ring with ponded water should be used. A
permeameter is distinguished from an iniltrometer by its ability to control the pres-
sure head at the soil surface during iniltration. The major advantage of permeameters
is that they are portable and use relatively small volumes of water. This makes them
particularly useful for studies on soil spatial variability. Permeameters can also be
used to measure iniltration at different suctions to evaluate the effect of macropo-
res. Unlike sprinklers and iniltrometers, the analysis of permeameter experiments is
based on three-dimensional iniltration patterns (Smith, 2002 ).
In general, runoff at a particular location may occur from two types of soil hydrau-
lic limits. In either case the soil surface will be saturated during periods of runoff
generation. The irst type occurs when the rainfall rate exceeds the rate at which the
soil is able to transport water from the soil surface to the subsoil. This type is affected
by topography, soil depth, soil hydraulic functions and drainage. The second type