Water balance and water movement in unsaturated zones of Sphagnum hummocks in Fuhrengawa Mire, Hokkaido, Japan

This study examined the hydrological conditions and water balance of a tall hummock in Fuhrengawa mire for 2 years. We clarified the hydrological processes by which Sphagnum can survive on hummocks of a mixed mire. Surface-layer water content was almost constant irrespective of continuous sunny days and heavy-rain days. Water content of deeper layers and the overall water level fluctuated more widely. Estimates of water balance showed that an almost identical amount of water that was lost to evapotranspiration was re-supplied from deeper layers to the surface on sunny days. Conversely, during rainy periods, rainwater rapidly infiltrated through unsaturated layers to deeper layers. Thereby, some fraction of the rainwater was stored in deeper layers or as groundwater. These water movements are attributable to the physical properties of Sphagnum peat, which forms hummocks. During our 2-year observation, precipitation was 1.1–1.5 times higher than the evapotranspiration. The longest period without rainfall was 11 days. Such a frequent and abundant rainwater supply, which exceeds evapotranspiration loss, maintains wet conditions and quantitative predominance of rainwater in hummocks. Accordingly, Sphagnum hummocks in the Fuhrengawa mire maintain their wet and ombrotrophic conditions because of the physical properties of peat and humid climate. Sphagnum survives because of these suitable conditions for growth.