Characteristic diurnal fluctuations in soil water content in peatland were detected by time domain reflectometry (TDR) technique in Sarobetsu Mire, Hokkaido, Japan. The water content measurements were conducted in clear day from 3 to 7 August 1995 at a sphagnum peatland (Site M) and at a transition peatland which has been invaded by Sasa Palmata (Site S). At both sites, the volumetric water content increased by 1 or 2% from about 12:00 to about 0:00, and then decreased until about 12:00 the next day. These characteristic diurnal fluctuations occurred not only in the unsaturated zone but also in the saturated zone. These fluctuations cannot be explained either by the time variation of groundwater level or that of ground-surface height or soil temperature.
In contrast, the weight of the lysimeter decreased from about 6:00 to 18:00, and remained constant until about 6:00 of the next day. The volumetric water content by the TDR in the lysimeter, however, showed same tendency of the characteristic diurnal fluctuations in the volumetric water content. This indicates that the characteristic diurnal fluctuations of the soil water content cannot be explained only by net water balance in the lysimeter.
