Experimental evidence of lateral flow in unsaturated homogeneous isotropic sloping soil due to rainfall

Abstract

This paper describes laboratory experimental evidence for lateral flow in the top layer of unsaturated sloping soil due to rainfall. Water was applied uniformly on horizontal and V‐shaped surfaces of fine sand, at rates about 100 times smaller than the saturated hydraulic conductivity. Flow regimes near the surface and in the soil bulk were studied by using dyes. Streamlines and streak lines and wetting fronts were visually studied and photographed through a vertical glass wall. Near wetting fronts the flow direction was always perpendicular to the fronts owing to dominant matrix potential gradients. Thus, during early wetting of dry sloping sand, the flow direction is directed upslope. Far above a wetting front the flow was vertical due to the dominance of gravity. Downslope flow was observed during decreasing rainfall and dry periods. The lateral movement was largest near the soil surface and decayed with soil depth. Unstable downslope lateral flow close to the soil surface was attributed to non‐Darcian flow due to variable temporal and spatial raindrop distributions. The experiments verify the theory that predicts unsaturated downslope lateral flow in sloping soil due to rainfall dynamics only, without apparent soil texture difference or anisotropy. This phenomenon could have significant implications for hillside hydrology, desert agriculture, irrigation management, etc., as well as for the basic mechanisms of surface runoff and erosion.

Lateral water flux in the unsaturated zone: A mechanism for the formation of spatial soil heterogeneity in a headwater catchment

“…Lateral unsaturated flow has been detected in soils in the field (Genereux, Hemond, & Mulholland, ; Jackson, ; Logsdon, ; McCord, Stephens, & Wilson, ; Ridolfi, D'odorico, Porporato, & Rodriguez‐Iturbe, ; Torres, Dietrich, Montgomery, Anderson, & Loague, ) and laboratory (Cabral, Garrote, Bras, & Entekhabi, ; Lv, Hao, Liu, & Yu, ; Sinai & Dirksen, ). The resulting unsaturated water fluxes offer a process description for unsaturated soils supplying baseflow to streams (Anderson & Burt, ; Hewlett, ; Hewlett & Hibbert, ) and the variation of soil moisture with topography (Perry & Niemann, ).…”

Infiltration and Unsaturated Flow under the Influence of Surface Microtopography - Model Simulations and Experimental Observations

“…Based on their experiments for large and small soil boxes, Sande and Chu (2012) concluded that the wetting front movement was "faster" under higher peaks and smooth surfaces compared with the adjacent lower depressions, and that the microtopographic effects on wetting front movement were still strong for deeper soil and longer-duration rainfall. Sinai and Dirksen (2006) observed that during the initial stage of percolation, the wetting front was parallel to the soil surface. As the front moved deeper, the effect of soil surface microtopography became weaker and the wetting front became horizontally homogeneous.…”

“…Surface microtopography induces lateral flow movement near the soil surface, and this movement degenerates along soil depth (Sinai and Dirksen 2006). However, gravity force gradually becomes the governing force over time (Hillel 1998;Sande and Chu 2012).…”

Impacts of a capillary barrier on infiltration and subsurface stormflow in layered slope deposits monitored with 3-D ERT and hydrometric measurements

“…6 -60 min). Sinai and Dirksen (2006) described, with laboratory experiments for an unsaturated homogeneous slope, that the flow direction during early wetting may be directed upslope, and during a flow regime of steady infiltration, the flow vector is changed toward the vertical direction. Already after 1 h 35 min, a clear change of the orientation into the vertical direction is recognizable (Fig.…”