Managing extremes in groundwater-dominated catchments: the Chalk of SE England (Invited)

The Chalk aquifers of Southern and Eastern England are a dominant local and regional water resource, and rivers located on the Chalk outcrop have a characteristic behaviour and support valuable and protected aquatic ecosystems. In these catchments, typically less than 2% of rainfall is translated into stormflow response. Infiltration occurs into an unsaturated zone that varies in depth from zero at the stream to 100m at the interfluve. Seasonal groundwater recharge is translated into a seasonal river hydrograph, and the length of flowing river expands and contracts seasonally - so-called ‘bourne’ behaviour. Groundwater catchment areas also vary temporally, and stream-aquifer interactions can be complex. Extensive and long duration flooding in 2000/2001 highlighted the vulnerability of these systems to extremes of long duration rainfall (weeks and months). Source areas for runoff expand into dry valleys, springs break out in ‘new’ locations and given the normally low %runoff, highly non-linear flow response occurs. Conversely, droughts are also of major concern, particularly given scenarios of climate change for SE England. Water quality issues are also important, particularly nutrient pollution. These management concerns have focussed attention on the need for improved understanding, and for appropriate modelling tools for flood risk assessment and drought and water quality management. The paper addresses recent research into the historic behaviour of these systems under extremes, the relative roles of fracture and porous matrix flow in the unsaturated zone under extremes, and the nature of stream-aquifer interactions, including detailed experimental studies. Challenges for modelling are identified; for groundwater flooding conventional flood design approaches are inadequate, but groundwater models lack appropriate topographic constraints and the required spatial and temporal resolution; for drought, recharge estimation and diffuse pollution, better representation of the unsaturated zone is required.