Infiltration, runoff and sediment production in blanket peat catchments: implications of field rainfall simulation experiments

Blanket peat covers the headwaters of many major European rivers. Runoff production in upland blanket peat catchments is flashy with large flood peaks and short lag times; there is minimal baseflow. Little is known about the exact processes of infiltration and runoff generation within these upland headwaters. This paper presents results from a set of rainfall simulation experiments performed on the blanket peat moorland of the North Pennines, UK. Rainfall was simulated at low intensities (3-12 mm hr(-1)), typical of natural rainfall, on bare and vegetated peat surfaces. Runoff response shows that infiltration rate increases with rainfall intensity; the use of low-intensity rainfall therefore allows a more realistic evaluation of infiltration rates and flow processes than previous studies. Overland flow is shown to be common on both vegetated and bare peat surfaces although surface cover does exert some control. Most runoff is produced within the top few centimetres of the peat and runoff response decreases rapidly with depth. Little vertical percolation takes place to depths greater than 10 cm due to the saturation of the peat mass. This study provides evidence that the quickflow response of upland blanket peat catchments is a result of saturation-excess overland flow generation. Rainfall-runoff response from small plots varies with season. Following warm, dry weather, rainfall tends to infiltrate more readily into blanket-peat, not just initially but to the extent that steady-state surface runoff rates are reduced and more flow takes place within the peat, albeit at shallow depth. Sediment erosion from bare peat plots tends to be supply limited. Seasonal weather conditions may affect this in that after a warm, dry spell, surface desiccation allows sediment erosion to become transport limited.