The hydrological response of headwater and sideslope areas / La réponse hydrologique des zones de cours supérieurs et des zones de pente latérale

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Abstract
This paper shows that the hydrological response of convergent headwater and divergent sideslope areas, within a broadly homogenous basin, may be expected to be significantly different. A field study of three headwater and two sideslope areas of the order of 0.25 km2 in the Crimple Beck catchment, Yorkshire, England, is described. Spatial variations in gauged rainfall inputs to the sub-basin areas are shown to be small relative to differences in hydrological response. Results from winter and spring storms show that the headwater areas will generally provide significantly higher and earlier peak flows per unit area, and more total storm discharge, than the sideslope areas. Areas of surface saturation of the soil are most commonly found during and between storms in convergent hollow areas associated with ephemeral channels, expanding as variable contributing areas of surface flow during storm rainfall. In this basin, however, there was also a tendency for both headwater and sideslope sub-basins to generate surface flow on large areas of saturated soil during storms. Measured unchannelled surface flow velocities were very slow (~20 m/h) and suggest that, in this particular situation, the differences in the response of headwater and sideslope areas may be largely due to the role of the network of minor channels commonly found in convergent headwater areas. These, often ephemeral, channels transmit surface water to the permanent stream network efficiently and rapidly. In the sideslope areas, generally lacking such channels, surface water will be delayed, and may be delayed sufficiently to reinfiltrate before reaching a channel when rainfall ceases.