The hydrological function of organo-mineral soil grasslands in UK uplands

Despite the common global occurrence of grasslands on organo-mineral soils, very little is known about their associated hydrology. The UK uplands, as source areas for flood waters, are of considerable interest as locations that could be managed to reduce downstream river discharge peaks, and these areas are frequently covered with organo-mineral soil grasslands. This thesis examines how the management of UK upland organo-mineral grasslands influences soil properties, the production and control of overland flow, and river flow peak response to large storm events. Fieldwork was conducted in Swindale, Cumbria, assessing six different grassland types. Soils were highly permeable with significant differences in properties as the result of natural heterogeneity. Shallow soil depth was considered to be the dominant control over soil water storage, where soils frequently became waterlogged, producing overland flow up to 60% of the time. Land management, and seasonal growth and decay of vegetation, significantly influenced surface roughness, strongly affecting overland flow velocity. Winter overland flow velocity was significantly higher than in summer, and significant changes also occurred following vegetation cutting or grazing density alterations. Using empirical data, SD-TOPMODEL was used to predict river discharge peak size and timing in response to major storm events for different seasons and management scenarios in Swindale, and Calderdale, Yorkshire. Seasonality altered river discharge peaks by -5.5% to +2.2% and conservation management reduced peaks by up to 42% compared to the same storms occurring on recent land use. Overall, soil hydrological function was associated most with physical catchment characteristics, whereas grassland vegetation, and its influence on overland flow, was strongly associated with season and management. Where physical characteristics cannot be changed, grassland management was recommended as an effective means of ‘slowing the flow’ for flood mitigation, accounting for their role within a mosaiced upland landscape, management practicalities, climate change and other ecosystem services.