Runoff production in blanket peat covered catchments

Abstract

Blanket peat catchments exhibit flashy regimes but little is known about the exact nature of runoff production processes within these catchments. Catchment, hillslope and plot-scale monitoring results are presented from the blanket peats of the northern Pennines, UK. Catchment efficiency for three study catchments with areas of 11.4 km(2), 0.83km(2) and 0.44 km(2) were 72 %, 77 % and 82 % respectively. Mean rainfall peak to discharge peak lag times were 2.7 hours, 2.1 hours and 3.2 hours respectively. Runoff from hillslopes in the catchments was gauged using runoff troughs. Mean hillslope lag times ranged from 0.6 to 2.2 hours and despite large differences in the scale of study there was little difference in response time. This indicates that slopes and channels are strongly hydrologically coupled in these peat systems. Saturation-excess overland flow dominates hillslope runoff, particularly on more gentle slopes, and on footslopes where overland flow occurs most frequently. Overall 81.5 % of the total overland flow and matrix throughflow collected by runoff troughs occurred at the peat surface, with 17.7 % between the surface and 5 cm depth, 0.7 % between 5 and 10 cm depth, and less than 0.1 % from below 10 cm depth, despite the thickness of the peat deposit. Most stormflow is produced as saturation-excess overland flow, whereas most low flow is produced by throughflow from 1 to 5 cm depth. Topography and preferential flow paths are important controls on the spatial pattern of runoff even on low-gradient peat. On steeper midslope regions, more flow occurs within the upper 10 cm of peat rather than at the surface. Preferential flow paths are identified at both the hillslope and plot-scale. Discharge deep within the peat is only produced from small macropore and pipe outlets that are well-connected to the peat surface. Response to rainfall from these macropore networks is rapid and they generate around 10 % of streamflow