Soil moisture, water tension, and water table relationships in a managed cutover bog

This study evaluates the hydrological conditions in a harvested bog where various water management schemes have been implemented to ameliorate conditions limiting Sphagnum regeneration. The study sites included a natural bog (natural), a recently drained and harvested bog (drained), which provided the hydrological extremes. Also included are a drained harvested bog with ditches blocked with (1) no other management (blocked), (2) peat bounded by open water at 5-m intervals (5-m), and (3) with straw mulch on the surface (mulch). The study period from May to September 1995 was drier than normal. The water table in the drained site descended to -107 cm by late August, compared with -72.5 cm at the blocked site. Both the 5-m and mulch sites (ditches also blocked) had water table recessions similar to the natural site (minimum approx. 62 cm). In the drained and blocked sites, little variation in water table depth occurred after early July, suggesting water exchanges with the atmosphere occurred to and from the unsaturated zone only. Soil moisture in the upper 3 cm layer on the drained and blocked site were similar, in spite of greatly different water table depths, dropping below 20% by volume, compared with a minimum of 30% when mulch was used. Soil water tension profiles suggest most of this storage change occurred in the upper 30 cm. The water table depth, therefore, was not a good indicator of water availability at the surface. Pressure head in the unsaturated zone (1 cm below the surface) in the drained and blocked sites was maintained between 0 and -100 cm (mb) 18 and 34% of the time, respectively, and with water management with open water (5-m) and mulch, increased to 55 and 97% of the time, respectively. The greatest tension, however, was observed on the blocked site (-355 cm), rather than the drained site (-247 cm), suggesting lower suitability for Sphagnum at the former. This was attributed to the higher bulk density (hence smaller pore structure) at the blocked site (rho(b) = 92.1 kg compared with 55.7 kg m(-3)). Higher bulk density at the blocked site was ascribed to its longer time since disturbance. This implies' restoration should begin as soon as possible after harvesting is finished. (C) 1997 Elsevier Science B.V.