Runoff, flood peaks and proportional response in a combined nested and paired forest plantation/peat grassland catchment

Abstract In an unusual combined nested and paired catchment experiment in northern England, the 1.5-km 2 Coalburn catchment (90% mature plantation forest with a dense drainage ditch network) is paired with the 1.4-km 2 Flothers catchment (64% peat grassland), both being nested within the 6.9-km 2 Throssburn mixed forest and peat grassland catchment. In only the second such UK quantification, a 100% forest cover reduces annual runoff relative to a 100% grassland cover by 24%. The study provides rare field (as opposed to model) evidence showing that the impact of land use on flood peaks is moderated by catchment saturation and by extreme rainfall. Flood frequency curves differ according to land use at the smaller return periods but the data are not sufficient to confirm that the pattern extends to the highest floods. Against conventional expectation, flood peaks tend to be larger and flashier in the forest catchment, apparently the consequence of the forest ditching. Springtime monthly runoff/rainfall ratios in the peat grassland catchment may reach 100%, representing a seasonal sponge effect which is eliminated from the forest catchment through the effects of the ditches. Combination of the paired and nested catchments provides a highly unusual opportunity to show that, at the annual scale, the integrated catchment response can be obtained by combining the responses of the separate land covers according to the proportions of the catchment that they occupy, an assumption widely made but rarely, if ever, tested.