Effect of pore structure on redistribution of subsurface water in Sarobetsu Mire, northern Japan

We measured the diurnal change in groundwater level (GWL) and other hydrological factors in sphagnum peat in Sarobetsu Mire, Hokkaido, Japan. Field measurements confirmed that at nighttime, the GWL lowered just after a rainfall event and its rate of lowering slowed with time, until finally the GWL rose later in the night. At the study site, however, the groundwater inflow is minimal. The field experiment reproduced this tendency under the condition that the groundwater flow was completely interrupted. A new model was proposed to explain the GWL change during the night without groundwater flow, considering the peat-soil structure. Sphagnum peat has various scales of pores, including those found on the inside of plant cell walls. These pores were classified into two types based on their water infiltration capacity. One is the large pores that can easily be filled with water and also can be easily drained. The GWL is determined by the water in these large pores. The other types are the small pores that cannot be filled with water or drained of water so easily. Just after a rainfall event, the water potential in the large pores is larger than that in the small pores, and accordingly the discharge from the large pores to the small pores occurs. Since the GWL is determined by the large pores, the GWL decreases during the night. The GWL rate of decline slows, as the difference in the two pore potentials grows smaller. Further, when the water potential in the large pores in the night becomes lower than that in the small pores after intensive evapotranspiration, the water starts to move from the small pores to the large pores, and consequently the GWL increases later in the night. The diurnal change in the water loss from the large pores was compared with that caused by evapotranspiration. The comparison showed that all of the water loss from the large pores during daytime was evapotranspiration, whereas the water loss from the large pores during nighttime was mostly in the form of the storage change in the small pores. The storage change in the small pores in a day ranged from −0.19 to 0.21 cm day−1, and these values were a large proportion of the dayʹs total evapotranspiration. The characteristics of the subsurface water movement in sphagnum peat indicate that the sphagnum peat acts as a buffer zone to prevent drastic changes in the GWL.