Stability of Wetting Fronts in Dry Homogeneous Soils under Low Infiltration Rates

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Abstract
Our understanding of unstable wetting phenomena in soils is limited. Therefore, lysimeter experiments were conducted in the laboratory to validate current wetting front instability theories. Four different grades of sieved and air‐dried perlite and quartz sand were used as the experimental material. Water was applied by a sprinkler system at rates within the range of natural precipitation rates in New Mexico. Experiments were conducted in small lysimeters (diameter 30 cm, height 50 cm) as well as a large one (diameter 100 cm, height 150 cm). The experimental results show that wetting front instability will cause fingering phenomena in a homogeneous soil system. This observation confirms experimental and theoretical results of other workers. The diameter of fingers was observed to be a function of the grain size of the sand. Small fingers (3–4 cm diameter) were found in coarse sand (grain size 1.41–0.84 mm); large diameter fingers (12 cm diameter) were observed in fine sand (grain size 0.42–0.25 mm). Our experimental results in the coarse sand show that, for infiltration rates varying between 0.3 and 12 cm/h, finger diameters remain more or less constant. This observation also agrees with existing theories. However, at infiltration rates lower than 0.12 cm/h, the coarse sand experiments show that the wetting fronts became stable. For rates between 0.3 and 0.12 cm/h, the wetting is semistable; that is, there is incomplete wetting without distinct development of fingers. A similar trend was observed in the experimental results of sands with grain sizes of 0.841 to 0.594 and 0.594 to 0.42 mm. This phenomenon has not been observed in previous experimental studies and is not predicted by current wetting front instability theories. Our experimental data under infiltration rates similar to natural precipitation intensities may explain why unstable wetting has rarely been observed in wettable field soils.

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