Spatial variation of effective porosity and its implications for discharge in an upland headwater catchment in Scotland

Abstract

Spatial and temporal measurements of shallow sub-surface soil physical properties were made within a 1 km² upland catchment. The surface soil layer of the catchment was organic rich (>70% organic matter) with a corresponding total porosity of 81%. Monthly point observations of volumetric water content (Ɵ) were combined with point estimates of total porosity (Ɛ) and the porosity <50 μm (Ɛ'residual'), to define the ratio of water filled pore volume:pore volume in pores <50 μm =(Ɵ/Ɛ'residual'). Values of Ɵ/Ɛ'residual' were compared with discharge to test whether mass flow occurred when Ɵ/Ɛ'residual' >1. A correlation between water content and discharge was found, with discharge increasing rapidly when Ɵ/Ɛ'residual' approached unity. Similar relationships between water content and catchment discharge were identified for soil units adjacent to the stream when Ɵ/Ɛ'residual' approached unity. These data suggest that soil pores >50 μm are of crucial importance in determining catchment discharge. Spatial and temporal variations in soil properties related to moisture content of the soil were also observed. Under dry conditions, a clear division based on aspect was noted, the west-facing side of the catchment being wettest. In wetter months, total porosity and soil water content were significantly affected by soil type and the spatial pattern of soil water content was more variable than in the dryer months. The physical quantification of soil properties in the shallow sub-surface layer proved important in explaining different initial changes in discharge from the catchment in response to a rainfall event.