WATER LEVELS AND FLUXES DURING A DRY SUMMER PERIOD IN AN ESTONIAN RAISED BOG WITH A POOL-RIDGE ZONE

摘要

Water levels in different microtopes of Maennikjaerve Bog were recorded during August 9 through September 12 in 2002, when precipitation was negligible and discharge from the bog was also negligibly small or zero. Pool evaporation was measured using a GGI-3000 evaporimeter. A cross-section over the bog based on about 30 measuring points showed that the steepest horizontal hydraulic gradients occur in the pool-ridge zone, with the exception of the margin zone. Levels in seven pool systems were measured. The average fall of the water levels during the observation period exceeded the measured evaporation only slightly. The phreatic water levels on the bog outside the ridge-pool system mostly fell considerably more. This indicates that in spite of the relatively steep hydraulic gradients between the pool systems, the exchange of water between successive pool systems was small, if existing at all. Historic records of dry summers in 1969 and 1996 showed a similar behaviour. However, in view of the average annual excess precipitation, which is necessary for raised bog growth and of the position of the pool zone on the bog, the pools must have a throughflow and hence effectively redistribute water in the bog's flow system during the wet periods. During dry periods, the ridge-pool zone is merely a hydrologically isolated and hence isolating water body. The main flux is exchange with the atmosphere.