The soil hydrology of the Plynlimon catchments

This soil hydrology study was carried out in the Plynlimon Catchment Experiment to provide an assessment of the influence of the soils in regulating the relationship between rainfall and runoff. It was an observational study, and therefore the findings are interpretive and subjective. Indeed, it could not be otherwise, being the essential precursor to the design and interpretation of the field experiments and modelling studies that were to follow. One of the original criteria in the selection of this pair of catchments was that they should be watertight, with no possibility of ungauged inputs or outputs of groundwater between the catchments or sub-catchments that would invalidate their water balances. Of course this could never be proved completely, but the impermeable nature of the bedrock, coupled with several lines of indirect evidence was considered adequate to justify the assumption of water-tightness. Hence, the soil hydrology study was concerned only with the processes of water movement within and over the soils and in the weathered zone of the bedrock. The principal findings were that there are three main soil types which may be considered to be hydrologically distinct – Podzol, Creep Brown Earth and Peaty Gley/Peaty Gley Podzol. Importantly, each of these occurs in different and specific topographic positions. The determinants of these locations are the aspect of the slope (north- or south-facing) and position on the slope profile (i.e. hilltop, upper convex, lower convex, upper concave and lower concave/riparian slopes). Each soil type is associated with its own set of soil hydrological processes controlling the transfer of rainwater via the soil system to the stream channels. Water storage in the regolith and its transmission downslope within an underlying shallow fissure system remains a possibility that cannot be entirely precluded, and this needs further study. Field mapping of soils involves a great deal of subjective interpretation, relying heavily on intuition and experience in interpolating between exposures of the soil profile. A possible development would be to make use of modelling techniques, based on digitised topographic data in conjunction with parameters that define the different soil areas (“hydrological domains”), i.e. aspect, slope position and slope angle, thus producing a more accurate hydrologically-relevant soils map than is practicable by conventional field mapping techniques alone. If successful, the use of modelling techniques to define hydrological domains would be a potentially important aid to the improvement of physically-based models relating the hydrograph to rainfall.