Interpreting soil and topographic properties to conceptualise hillslope hydrology

Abstract

The sustainability of agricultural practices is enhanced when vegetation makes optimal use of natural hydrological processes. For example, planting tree belts where slope gradient sharply decreases can enable harvesting of run-on water. This can be beneficial for example in reducing water logging and enhancing tree production. There is a need for rapid and low cost identification of water flow paths and conceptualisation of hillslope hydrology so that local landuse planning can reflect such opportunities. The collection of detailed hillslope hydrological data is prohibitively expensive for such applications and so the use of soil morphology and visual observation of topography and surface condition is evaluated as an alternative. At a study site in south eastern Australia the soil physical profile was described down a hillslope with additional measurements of the hydraulic conductivity, bulk density, cation exchange capacity, electrical conductivity, and particle size distribution of the key horizons. This data was used to identify the perceived significant hydrological flow paths down the hillslope. Measurement of the surface runoff, subsurface lateral flow, and the distribution of saturation measured in piezometers were subsequently used to test the conceptual hydrological model. Soil morphology, particularly the soil colour and presence of redox concretions were useful in identifying the locations and depths where saturation and lateral flow occur. The morphology provides an integrated reflection of the dominant hydrological conditions, but care must be taken to ensure that the observations reflect the current hydrological environment and not relic conditions. Other collaborating information such as the history of geomorphological events at the site, a validation of plausible water sources for the potentially transmitting layers, surface soil condition and landholder observations give improved confidence. Combining soil morphological understanding with visual observations of other site characteristics enabled rapid conceptualisation of hillslope hydrological behaviour as needed for local landuse planning.