The hydrological functioning of the Pevensey Levels wetland
The Pevensey Levels are a wet grassland of national importance in East Sussex, England. The site has been reclaimed from the sea since the Middle Ages, and has traditionally been used for grazing. A purpose-built hydro-ecological model that predicts water levels in ditches and relates them to the hydrological requirements of target species has shown that a traditional water level management regime for grazing is suitable for ditch flora and fauna, the flagship species of nature conservation importance on the site. During this century, the installation of pump-drainage has caused a decline in the range of these species. This causal link has also been confirmed using the hydro-ecological model developed. The Wildlife Enhancement Scheme (WES), a scheme that pays farmers to manage their land in an environmentally sensitive way, has been the main tool employed for the restoration of the site. The WES includes prescriptions to raise water levels. However, a catchment water balance model, quantifying all wetland inflows, outflows and sinks, indicates that the water demand associated with the WES in spring and summer coincides with a period of net water resource deficit. The dimensions of embanked channels, which are employed to feed lowland ditches in the summer are insufficient to provide the storage of winter runoff required to implement the scheme wetland-wide. Micro-meteorological studies also indicate that any attempts to capture winter rainfall in field ditches by raising sluice levels are offset by higher rates of evaporative loss in spring and summer. Consequently, higher water levels in winter do not necessarily lead to higher water levels in summer. In many summers therefore, higher water level targets cannot be attained. This suggests that areas targeted for restoration on the Pevensey Levels should be prioritised to account for water scarcity. Hydrological and ecological monitoring should be undertaken to identify the key areas to be targeted for restoration.