Ja. Vandenberg et P. Ullersma, PONDED INFILTRATION AS A CAUSE OF THE INSTABILITY OF CONTINUOUS MACROPORES, Journal of hydrology, 159(1-4), 1994, pp. 169-186
A model is presented which yields the range of the hydraulic parameter
s that are critical for the collapse of continuous macropores in a giv
en soil under certain hydrological circumstances. These continuous mac
ropores are important, as they control the hydraulic conductivity at s
aturation and the infiltration capacity of most soils. The conceptual
model is based on data from earlier infiltration experiments on soil c
olumns in situ; it consists basically of a soil matrix with discontinu
ous macropores which is traversed by a vertical macropore channel. The
experiments suggested that under the transient flow conditions that o
ccur in the soil when ponded infiltration ends, the wall of a macropor
e channel can become unstable. Collapse will occur if the shear stress
caused by the seepage to the channel exceeds the shear strength of th
e soil particles in the wall of the macropore channel. The model ascer
tains the magnitude of this shear stress from the transient gradient o
f the hydraulic head G over a length L which is the distance between t
he wall of the macropore channel and air encapsulated by the water in
the soil matrix. For given parameters L, k (hydraulic conductivity) an
d mu (specific storativity) the behaviour of G as a function of time i
s derived from the equation for non-stationary, horizontal flow of soi
l water. The solution consists of a series of exponential functions; o
ne term is characterized by the emptying time of the macropore channel
and the others by the relaxation time tau = 4L2mu/pi2k. It is shown h
ow, for a certain soil, a domain of k and mu values conducive to colla
pse can be found. Each point within this domain defines a length scale
for L. The moment of collapse is found to be proportional to square-r
oot pi. The domains of k, mu and L are determined for three different
soil textures (silt loam, sandy loam and loamy sand). The application
of the model shows that ponded infiltration can create conditions appr
opriate for the collapse of macropores and, therefore, for the decreas
e of the saturated conductivity.