季節的積雪地帯における積雪下のCO_2濃度 Carbon dioxide concentration under a seasonal snow cover occurring on a coastal plain in the central Japan
A mathematical model to predict CO<SUB>2</SUB> concentration at the base of a seasonal snowpack was developed. The model was based on the one-dimensional diffusion equation and the wash-down process of CO<SUB>2</SUB> by melt water. Both the snowpack and the underlying soil layer were assumed to be horizontally uniform and infinite. These layers were characterized by their thickness, diffusivities for the CO<SUB>2</SUB>, and the flow rates of melt water. The CO<SUB>2</SUB> was assumed to evolve uniformly throughout the soil layer at a constant rate. An implicit finite-difference approximation was used to calculate the temporal change of CO<SUB>2</SUB> at the base of the snowpack. Measurements of CO<SUB>2</SUB> and snowhydrological elements were made for the snowpack over a wheat field in Joetsu, Niigata(37°06'31"N, 138°16'31"E;11m in elevation) in two winters 1990/1991 and 1993/1994. The CO<SUB>2</SUB> concentrations at the ground level were measured continuously with an electrode gas sensor. Two inputs to the model, the depth of snowpack and the flow rate of melt water through the snowpack, were also measured. These inputs, along with the other estimated inputs, were fed into the model to yield the predicted concentrations. Measured CO<SUB>2</SUB> concentration at the base of the snowpack varied between the atmospheric values and about 800 ppmv. The concentration generally increased as the snowpack developed. Superimposed on this slow variation were rapid fluctuations associated with runoff of melt water. The soil temperature at 5cm below the soil surface remained around 0.5°C, so that the runoff of melt water presumably took place almost all the time. Model generally reproduced the measured CO<SUB>2</SUB> concentrations, but occasional disagreement, as much as some hundreds ppmv, were noticed. Discrepancies in slow variations were probably due to the erroneous estimation of the rate of CO<SUB>2</SUB> generation from the soil and the CO<SUB>2</SUB> diffusivity through the snow, whereas spatially heterogeneous downward flow of melt water and the estimation of dissolution coefficient might have produced disagreement in the short-period fluctuations. Nevertheless, this study has provided evidence that melt water plays an important role to determine the CO<SUB>2</SUB> concentration beneath snowpacks occurring in regions where the soil temperature remains above freezing. The proposed model in this study is a useful means to analyze the carbon cycle in terrestrial ecosystems with a seasonal snow cover.
雪氷 60(5), 357-366, 1998-09-15
The Japanese Society of Snow and Ice