Tracer and hydrometric study of preferential flow in large undisturbed soil cores from the Georgia Piedmont, USA

Abstract

Abstract:We studied the temporal patterns of tracer throughput in the out¯ow of large (30 cm diameter by 38 cm long) undisturbed cores from the Panola Mountain Research Watershed, Georgia. Tracer breakthrough was aected by soil structure and rainfall intensity. Two rainfall intensities (20 and 40 mm hr 71 ) for separate Cl 7 and Br 7 amended solutions were applied to two cores (one extracted from a hillslope soil and one extracted from a residual clay soil on the ridge). For both low and high rainfall intensity experiments, preferential¯ow occurred in the clay core, but not in the hillslope core. The preferential¯ow is attributed to well-developed interpedal macrochannels that are commonly found in structured clay soils, characteristic of the ridge site. However, each rainfall intensity exceeded the matrix in®ltration capacity at the top of the hillslope core, but did not exceed the matrix in®ltration capacity at the middle and bottom of the hillslope core and at all levels in the clay core. Localized zones of saturation created when rainfall intensity exceeds the matrix in®ltration capacity may cause water and tracer to over¯ow from the matrix into macrochannels, where preferential¯ow occurs to depth in otherwise unsaturated soil.

Gypsies in the palace: experimentalist's view on the use of 3‐D physics‐based simulation of hillslope hydrological response

“…These data indicate soil core porosities ranging from 0Ð64 at depths of 15 cm to 0Ð52 at 40 cm and 70 cm depths. Similar estimates of porosity (0Ð65 to 0Ð51) were obtained from time domain reflectometry (TDR) within the large undisturbed hillslope core of McIntosh et al (1999). Constant head method testing of the small hillslope Figure 3.…”

Microbial mediated carbon and nitrogen cycling in the spatially heterogeneous vadose zone: A modeling study

“…Solute transport in the subsurface was affected by spatial heterogeneity, displayed in the varying impact on breakthrough time and in varying saturation (Figure 1). Lower velocity and lower saturation (as observed in the slow flow regime) increased the contribution of matrix flow for solute transport in heterogeneous domains (McIntosh et al., 1999). In contrast, preferential flow contributed more to solute transport in domains exhibiting higher degree of saturation, or with higher water fluxes (Koestel et al., 2012), as also observed in the medium to fast flow regimes in our study.…”

The evolving perceptual model of streamflow generation at the Panola Mountain Research Watershed

“…Before installation of the trench, tracer studies using chloride conducted on this same hillslope (Figure 2 "A"; Peters & Ratcliffe, 1998) and using lithium bromide to characterize subsurface flowpaths on ridgetop soils in the SW catchment (Huntington et al, 1994) supported the view that rainwater bypassed the unsaturated zone via macropore flow. Also, preferential flow was observed in laboratory infiltration experiments on undisturbed soil cores from hillslope and ridgetop sites near the trenched hillslope (Figure 2 "B", McIntosh et al, 1999). Thus, the perceptual model before trench installation was that macropore transport was important to shallow water table development, that in-turn induces downslope water flow and ultimately contributes to streamflow.…”