Controls on storm runoff behavior in a gullied blanket peatland

Many upland headwaters of the UK drain areas of blanket peat, much of which has been degraded through atmospheric deposition of pollutants, vegetation change, peat extraction, artificial drainage and erosion. These areas are increasingly the focus of interventions to restore some of the multiple-benefits lost through degradation. Understanding their runoff generation processes underpins analysis of their wider benefits including their potential to mitigate downstream flooding.

Using a series of multivariate analysis techniques we examine controls on storm runoff in ten blanket peat catchments of 0.2-3.9 hectares all within 5 km of one another. We find that: 1) rainfall intensity is the dominant hydro-meteorological driver for both magnitude and timing of peak discharge for all ten catchments, with antecedent rainfall only relevant in small storms; 2) most of the inter-catchment variability in discharge predictability from rainfall can be explained by catchment characteristics, particularly catchment area; 3) runoff responses, particularly in small storms, are sensitive to scale even in an apparently homogenous and saturation-excess overland flow dominated peatland landscape; 4) peak discharge in large storms is strongly controlled by attenuation processes associated with the travel time distribution, and thus drainage network geometry; 5) peak discharge in smaller storms underlines the importance of hydrological connectivity at scales <1 hectare, perhaps due to depression storage driven (dis)connectivity.

Together these results suggest a switching in rainfall-runoff behavior within these catchments where peak discharge is controlled by: catchment storage, connectivity and antecedent conditions in small storms; but runoff attenuation, travel time and thus and network structure and scale in larger storms. In the context of Natural Flood Management, our findings suggest that enhancing depression storage by creating distributed shallow peatland pools in addition to existing restoration methods could raise the threshold storm size below which catchment storage, antecedent conditions and connectivity remain important. However, changes in surface roughness and other measures that target runoff velocities are likely to be more effective in the largest (and thus most flood relevant) storms.