Journal of Environmental Management (1998) 54, 189–203 Article No. ev980227 Spatial relationships between site hydrology and the occurrence of grassland of conservation importance: a risk assessment with GIS R. D. Swetnam†*, J. O. Mountford†, A. C. Armstrong‡, D. J. G. Gowing§, N. J. Brown†, S. J. Manchester† and J. R. Treweek† The UK’s Environmentally Sensitive Area (ESA) scheme provides financial incentives for farmers to undertake management which is compatible with the conservation of landscapes and wildlife species. Lowland wet grassland is an important component of a number of these ESAs. Management prescriptions relate to farming practices like grazing and weed management. For lowland wet grasslands, they may also include options to raise water levels for the benefit of species, many of which have declined following widespread drainage of agricultural land. This paper focuses on Southlake Moor in Somerset, south-west England, where raised water-level prescriptions have operated since 1992 and where an increased incidence of late spring flooding appears to be threatening important areas of nationally scarce Cynosurus cristatus–Caltha palustris grassland. A methodology is presented which makes use of a GIS to quantify the distribution of the nature conservation resource and link this to a hydrological model and a database of plant water-regime requirements. The model predicts water-tables on a field-by-field basis for each 10 day period throughout the year, allowing flood maps to be constructed. The database quantifies the water regime requirements for individual species on Southlake Moor. Using individual fields as the unit of study, these two are linked within the GIS to permit the extent of spring flooding to be identified and its potential impact assessed in terms of suitability for key species/communities. The paper describes how this approach could be used to determine whether deliberate management to raise water levels might be placing characteristic and scarce vegetation communities at risk.  1998 Academic Press Keywords: Environmentally Sensitive Areas, Geographic Information Systems, hydrological modelling, National Vegetation Classification, Cynosurus cristatus–Caltha palustris (MG8), spatial patterns. †Institute of Terrestrial Ecology, Monks Wood, Abbots Ripton, documented (Fuller, 1987; Baldock, 1990; Introduction Huntingdon, Adams, 1996). Escalating storage costs for Cambridgeshire surpluses of agricultural produce finally PE17 2LS, UK ‡ADAS, Gleadthorpe By the middle of the 1980s, certain un- prompted reform of the CAP. The 1986 Agri- Land Research Centre, desirable economic and environmental con- culture Act attempted to redress the balance Meden Vale, Mansfield, sequences of the European Union’s (EU) between increased productivity and as- Nottinghamshire NG20 9PF, UK Common Agricultural Policy (CAP) were be- sociated environmental degradation. For the §Silsoe College, coming apparent. The negative impacts of first time Agriculture Ministers had a stat- Cranfield University, agricultural intensification on the wildlife of utory obligation to balance conservation Silsoe, Bedfordshire MK45 4DT, UK farmed landscapes in the UK have been well against the support of a stable and efficient agricultural industry (Whitby, 1994). One of Received 13 August 1997; accepted 6 April 1998 Nomenclature follows Stace (1997) the most important provisions of Europe’s 0301–4797/98/030189+15 $30.00/0  1998 Academic Press