Simulation of hydrologic and geomorphic changes affecting a shrinking mire

Abstract

The Kushiro Mire in subarctic Japan has significantly reduced in size because of the complex interaction of many processes, including snowmelt runoff, sediment transport and vegetation dynamics. This study uses a coupled integrated catchment-based eco-hydrology model with a mass transport process (NICE-MASS) to investigate the influence of hydrologic and geomorphic changes on the Kushiro Mire. Coefficients of the sediment-rating curve in the snowmelt runoff are very different from those in the snow-free period. This empirical relation shall underestimate the observed suspended sediment (SS) concentration in the snowmelt period, indicating a difference in runoff mechanisms between the two periods and the necessity of a process-based model. The simulation model reproduced well the spatial distribution of elevation aggradations by sediment deposits from rivers flowing into the mire. NICE-MASS clarified that river channelization works carried out in the past resulted in groundwater degradation and drying phenomenon in the downstream area because of increased sedimentation and a decrease of seepage infiltration from the river to the aquifer. It is assumed that these hydrologic and geomorphic changes are closely related to the invasion of alder (Alnus japonica) into the mire. The occupation rate of alder is positively correlated with groundwater degradation relative to the ground surface. The model simulation derived by addition of this limiting factors related to submerged depth is important for prediction of vegetation succession when devising future policies for restoration of meandering river channels.

Shrinkage of shrub forest and recovery of mire ecosystem by river restoration in northern Japan

“…Up to now, the author has developed the distributed NICE model series (Nakayama, 2007(Nakayama, , 2008Nakayama and Watanabe, 2004, 2008aNakayama et al, , 2007, which includes surface-unsaturated-saturated water processes and assimilates land-surface processes describing variations of LAI (leaf area index) and FPAR (fraction of photosynthetically active radiation) derived from MODIS satellite data (Fig. 2).…”

“…The author expanded the process-based NICE (NIES Integrated Catchment-based Eco-hydrology) model series (Nakayama, 2007(Nakayama, , 2008Nakayama and Watanabe, 2004, 2008aNakayama et al, , 2007 to include mass transport and vegetation succession processes in the Kucyoro River catchment, Japan (NICE-VEG). In the first step of applying the model to this objective, it is important to distinguish two dominant vegetation types in the Kushiro Mire, socalled, between invasive alder (shrub forest) and species indigenous to the mire (reed-sedge vegetation).…”

Hydrological influences on the distribution of vegetation and tree height of Alnus japonica (Thunb.) Steud. in the Kushiro Mire, Japan

“…On the other hand, the sediment content (ash content) and elevation were highest at the upper river. Nakayama (2010) reported that these area increased elevation by sedimentation. From these results, the large effect on the alder tree height, such as from sediment inflow from rivers, a change in elevation, and the water table drawdown caused by a high sediment content may be restricted to the area of sediment deposition in the inflow area or to the river banks where previous research was carried out.…”

“…7). These differences were related to channelization and land-use change in the upper basin which caused aggradation and elevation change in the mire (Nakayama, 2010). This change would cause an immediate drying out of the alder sites, which was indicated by the time of water table drawdown.…”

“…Many researchers have reported that the decline in the water table affects the height or the distribution of alder trees as a result of sediment inflow and change in elevation, and then decided that alder trees are an indicator of the desiccation of mires (Okazaki, 1975;Shinsho and Tsujii, 1996;Kameyama et al, 2001;Nakayama, 2008aNakayama, , 2008bNakayama, , 2010Nakayama, , 2013. Fluctuations of the water table under a peat surface layer would be controlled by the sediment content, as represented by the ash content in another study on the Kushiro Mire (Negishi, 2008(Negishi, , 2009.…”