Modelling the impacts of land cover change on flood hydrographs in upland peat catchments

There is global concern about headwater management and associated impacts on river flow. In many wet temperate zones peatlands can be found in headwater catchments. In the UK there is major concern about how environmental change, driven by human interventions, has altered the surface cover of headwater blanket peatlands. However, the impact of such cover changes on river flow is poorly understood. In particular, there is poor understanding of the impacts of different spatial configurations of bare peat or well-vegetated restored peat on river flow peaks in upland catchments. This thesis employs a numerical modelling approach to explore such impacts. TOPMODEL, due to its process representation which is very suitable for blanket peat catchments, was utilized as a prototype acting as the basis for a new distributed catchment hydrological model. A new overland flow module with a set of detailed stochastic algorithms representing overland flow routing and re-infiltration mechanisms was created to simulate saturation-excess overland flow movement. The influence of land cover on surface roughness could be represented in the model. The new model was tested in three upland peat catchments in different parts of the UK: Trout Beck in the North Pennines, the Wye in mid-Wales and the East Dart in southwest England. The model was found to work well in all three cases. Land cover scenarios were designed for the three catchments to investigate land cover impact on river flow through simulation runs of the new version of TOPMODEL. As a result of hypothesis testing three land cover principles emerged from the work as follows: Principle (1): A wider bare peat strip nearer to the river channel gives a higher flow peak and reduces the delay to peak; conversely, a wider buffer strip with higher density vegetation (e.g. Sphagnum) leads to a lower peak and postpones the peak. In both cases, a narrower buffer strip surrounding upstream and downstream channels has a greater effect than a thicker buffer strip just based around the downstream river network. Principle (2): When the area of change is equal, the size of land cover change patches has no effect on river flow for patch sizes up to 10000m2. Principle (3): Bare peat on gentle slopes gives a faster flow response and higher peak value at the catchment outlet, while high density vegetation or re-vegetation on a gentle slope area has larger positive impact on peak river flow delay when compared with the same practices on steeper slopes. These simple principles should be useful to planners who wish to determine resource efficiency and optimisation for peatland protection and restoration works in headwater systems. If practitioners require further detail on impacts of specific spatial changes to land cover in a catchment then the new model can be readily applied to new catchments of concern. The model also has the potential to provide useful information on potential sediment or contaminant transfers because it has a fully distributed overland flow module