Shallow upland drains, grips, have been hypothesized as responsible for
increased downstream flow magnitudes. Observations provide
counterfactual evidence, often relating to the difficulty of inferring
conclusions from statistical correlation and paired catchment
comparisons, and the complexity of designing field experiments to test
grip impacts at the catchment scale. Drainage should provide drier
antecedent moisture conditions, providing more storage at the start of
an event; however, grips have higher flow velocities than overland flow,
thus potentially delivering flow more rapidly to the drainage network.
We develop and apply a model for assessing the impacts of grips on flow
hydrographs. The model was calibrated on the gripped case, and then the
gripped case was compared with the intact case by removing all grips.
This comparison showed that even given parameter uncertainty, the intact
case had significantly higher flood peaks and lower baseflows, mirroring
field observations of the hydrological response of intact peat. The
simulations suggest that this is because delivery effects may not
translate into catchment-scale impacts for three reasons. First, in our
case, the proportions of flow path lengths that were hillslope were not
changed significantly by gripping. Second, the structure of the grip
network as compared with the structure of the drainage basin mitigated
against grip-related increases in the concentration of runoff in the
drainage network, although it did marginally reduce the mean timing of
that concentration at the catchment outlet. Third, the effect of the
latter upon downstream flow magnitudes can only be assessed by reference
to the peak timing of other tributary basins, emphasizing that drain
effects are both relative and scale dependent. However, given the
importance of hillslope flow paths, we show that if upland drainage
causes significant changes in surface roughness on hillslopes, then
critical and important feedbacks may impact upon the speed of
hydrological response. Copyright (c) 2012 John Wiley & Sons, Ltd.