Wetting Front Instability in the Vadose Zone of New Mexico’s Soils

Field experiments were conducted in the Sevilleta dunes approximately 20 miles north of Socorro, New Mexico. After water application, the depth and stability of the wetting front was observed by digging an observation pit. Contrary to unstable wetting front theories in 1992, no instabilities were encountered and all wetting fronts appeared quite stable. These initial results indicated that wetting front theories were not yet adequate for the prediction of unstable wetting in New Mexico’s soils. Therefore, we pursued a laboratory study under well-controlled conditions.

Lysimeter experiments were conducted in the laboratory to validate current wetting front instability theories with four different grades of sieved and air-dried perlite and quartz sand. Water was applied by a sprinkler system at rates within the range of natural precipitation rates in New Mexico. The experimental results show that wetting from instability will cause fingering phenomena in a homogeneous soil system. This observation confirms experimental and theoretical results of other researchers. The diameter of fingers was observed to be a function of the sand’s grain size. 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 find 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 nearly 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 semi-stable; that is, there is incomplete wetting without distinct development of fingers. A similar trend is observed in the experimental results of sands with grain sizes of, respectively, 0.841-0.594 and 0.594-0.42 mm.

We used our laboratory results to develop a simple approach for evaluating wetting front instability in dry soils. The stability of wetting fronts in the top layer of soil depends on the soil type and the intensity of the precipitation. Our approach distinguishes stability criteria for wetting events that cause a high, intermediate, and low infiltration rate. At high infiltration rates, wetting fronts are stable if the infiltration rate exceeds or equals the soil’s saturated hydraulic conductivity. The stability criterion for low infiltration rates (less than approximately 0.2 cm/h for sand soils) is based on two characteristic times: a gravitational time and an infiltration time. The application of stability criteria is demonstrated with a case study from the Sevilleta dunes near Socorro, NM.