Monitoring the transition from preferential to matrix flow in cracking clay soil through changes in electrical anisotropy

Transition from preferential flow to matrix flow in a vertisol was investigated by combined use of square array resistivity measurements and stable water isotopes. Two irrigation events with water of different water isotope composition were carried out on a 0.5 m deep initially dry and cracked soil profile in a weighing lysimeter. Throughout the irrigation events, depth profiles of resistivity square array measurements were collected to calculate profiles of the anisotropy index (AI) and mean resistivity. The stable isotope composition of the drainage water showed that water from the second irrigation bypassed the soil matrix with limited mixing to form the collected drainage. This bypass showed that even though the soil cracks at the surface were visually closed at the onset of the second irrigation, preferential flow paths must have remained open. Changes of the AI with applied irrigation volume were used to detect the dominant flow processes in the soil, with a stable AI indicating matrix flow and an unstable AI indicating preferential flow. The AI showed that over the course of the irrigation events the dominant flow process changed from preferential flow to matrix flow. The use of the AI allowed for the first time to detect the exact timing of this transition. Matrix dominated flow didnʹt occur until ponding water at the soil surface was observed. The onset of matrix flow started at the top of the profile and progressed downwards. Complete crack closure in terms of water flow occurred long after visual closure of soil cracks and could be detected by monitoring changes in the AI.