Hydrological responses to moorland land-use change
Most documented investigations of the effects of land-use change on hydrological systems have considered the modification of forest areas. In this thesis, a headwater area in the North York Moors is used to examine the consequences of maintaining a land management regime which has received comparatively little observation in this context: controlled heather burning (muirburn). The effects of coniferous afforestation are also evaluated for selected variables. Particular attention is given to the responses of soil, moisture and evapotranspiration and the relationship between these two components.Simulated soil moisture deficits derived from empirical models are tested against measured values. Predictions based on Penman-Monteith evapotranspiration and 'layer' moisture deficits, along with an optimised soil-drying parameter, were found to simulate observed conditions most closely. A land-use change from open heather moorland to burnt ground promoted reductions both in evapotranspiration levels, especially at potential demando and in moisture deficits. In contrast, following afforestation, deficits were maintained or enhanced throughout the year, with higher moisture losses to interception than found under heather, due to the higher aerodynamic resistance of the latter. Predictions of actual evapotranspiration, determined from soil moisture models, were generally found to be reliable estimates of those 'observed' from the moorland water balance.Antecedent catchment conditions and storm characteristics were used in analysis of runoff distribution over time, quantified in terms of 'unit hydrographs' and linear regression models. Land-use effects were manifested most significantly in a doubling of hydrograph peak discharge following muirburn, the lower measured soil moisture deficits under a burnt catchment rendering more water available for storm runoff. A secondary, underlying control, that of a slower response from a wet catchment, lent 'support to evidence for the existence of variable source areas.