PONDED INFILTRATION AS A CAUSE OF THE INSTABILITY OF CONTINUOUS MACROPORES

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.