A multiscale investigation into the effects of agriculture on flood hydrology in southwest England
In the UK, perceived increases ih high impact flood events over the last decade and broad scale recognition of the enhanced flood risks associated with future climate change predictions, have reinforced the need for improved understanding and management of processes governing peak flow responses. This thesis investigates the effects of agricultural land uses on the hydrology of rural areas at a range of spatial and temporal scales. At the catchment scale, 48 catchments and subcatchments distributed across the south western counties of Cornwall, Devon, Somerset and Dorset were investigated. A suite of multivariate statistical techniques, including Direct (Redundancy Analysis) and Indirect (Principal Components Analysis) Ordination were used to explore catchment responses to four major storm events, selected from the wet autumn/winter of 2000-2001. A Geographic Information System. (GIS) incorporating the Hydrology of Soil Types (HOST) soil classification system and Land Cover Map 2000 satellite imagery data was developed to parameterise catchment physiographic variables and calculate the extent of 27 land use classes. Analysis of regional trends in erivironmental variables and two multivariate runoff datasets (R1 and R2) identified land use as the principal control of streamflow responses to extreme storm events. Land use, soil and geology parameters together explained 84% (R1) and 78% (R2) of the Variance in runoff for the same four storms. Grassland and improved grassland were consistent characteristics of catchments generating higher runoff volumes per unit area. Similarities in the hydrological behaviours of the Camel catchment and the De Lank subcatchment supported a dominant control on peak flows by runoff from grazed upland areas. A longer-term study of the River Camel catchment (1965-2000) revealed a 20% increase in the magnitude of the one in 25 year flow. Daily rainfall totals aggregated at monthly, seasonal and annual timescales and agricultural census data for the years 1969, 1979, 1988, 1997 and 2000 were examined to determine the influence of climate and land use changes on the enhanced streamflow response. Increases in the frequency and magnitude of peak flows were attributed to the cumulative impacts of a subtle, long-term rise in October rainfall totals, coupled with local urban development, the expansion of arable cultivation on highly connected slopes in the lower catchment and a rise in the intensity of grazing in the upper catchment At the field scale, characterisation of the textural, structural and ^hydraulic properties of soils subject to different land managements, including continuous cereal cultivation (CC), semi permanent pasture (SPP), permanent pasture (PP) and farm woodland (FW), identified a link between land use 'and the structural stability of the surface horizon. Marked differences in the percentage of water stable aggregates (WSA>2.8mm) between the topsoils of FW (66%) and.PP (71%), SPP (11%) and CC (6%) helped to explain differences in saturated hydraulic conductivity that were in the order FW>PP>SPP>CC, Laboratory rainfall simulations revealed slower wetting rates and higher average soil moisture percentages at near-saturation in FW and PP soil plots compared to SPP and CC soil plots that resulted from higher total porosities under FW and PP. Agricultural management systems are therefore capable of playing an important role in attenuating peak flow responses to storm events through considered land management which ameliorates or prevents soil structural deterioration and encourages the movement of water into storages within the hillslope. The adoption of specific measures, such as the introduction of buffer strips, widening of hedgerows or the introduction of forested areas to act as sinks, may serve to disconnect hydrofogical pathways from the main channel by providing a barrier to runoff, thereby reducing the upslope contributing area.