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

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.

Unexpected water content profiles under flux‐limited one‐dimensional downward infiltration in initially dry granular media

“…Hence, if the applied water flux is very low, this property would disappear and the wetting front would become stable [Yao and Hendrickx, 1996]. The water retention curve for the wetting process from air-dry (Figure 6), which is the equilibrium property, shows no such discontinuity, even if the curve is very steep, nor can the static property of hysteresis in the q-h relationship explain the property of a saturated wetting front.…”

“…Even under the condition of a continuous supply of water from the surface, finger flow can be observed by limiting the water flux with alternating layers of overlaying finer material having a lower saturated hydraulic conductivity than the medium and the ponding water [Tabuchi, 1961;Hill and Parlange, 1972;Glass et al, 1989;Baker and Hillel, 1988;Cho and de Rooij, 1999] or by applying a water flux that is lower than the saturated hydraulic conductivity into a uniform medium [Selker et al, 1992a;Yao and Hendrickx, 1996;Kawamoto and Miyazaki, 1999]. However, the occurrence of finger flow is limited to dry granular materials of relatively large [Diment and Watson, 1985] and relatively uniform particle sizes .…”

Morphology of Rain Water Channeling in Systematically Varied Model Sandy Soils

“…Later, field studies [9][10][11][12][13] demonstrated the existence of preferential water paths in sandy soils during rainfall or irrigation. At the same time, laboratory experiments [14][15][16][17][18] further confirmed that rain water channeling is a common feature that widely exists not only in sandy soils with structure heterogeneity, but also in uniform dry sands with almost no structure defects. The cause of the latter is due to instabilities that occur at the gravity-driven water wetting front [14,16].…”

“…At the same time, laboratory experiments [14][15][16][17][18] further confirmed that rain water channeling is a common feature that widely exists not only in sandy soils with structure heterogeneity, but also in uniform dry sands with almost no structure defects. The cause of the latter is due to instabilities that occur at the gravity-driven water wetting front [14,16]. Rainwater channeling largely reduces the water-reachable area in the plant root zone and results in a significant deviation of the predicted soil water capacity from measurements, which are usually started or performed at a fully saturated state.…”

“…To further clarify the influence of the rain rate, we fix the raindrop impinging speed to U T ¼ 1.0 m=s and vary the rain rate Q by changing the water level in the water container. Yao and Hendrickx [16] reported unusual water channel size changes at extremely low rain rates using real sands with different grain size ranges. Here, we focus on a relatively high range of rain rates, from 12 cm=h to 220 cm=h, and apply different types of rain water in a model sandy soil with well-known pore structure.…”