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

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積雪底部におけるCO<SUB>2</SUB>濃度をシミュレートする数値モデルを作成した.本モデルは, Fickの拡散第2法則に基づくもので,同時に,融雪水によるCO<SUB>2</SUB>の溶解・移流過程も含んでいる.モデルの妥当性を検討するため,農水省北陸農業試験場(新潟県上越市)のムギ圃場において,1991,1994年寒候期に積雪層底部におけるCO<SUB>2</SUB>濃度の連続測定をおこなった.その結果,積雪下のCO<SUB>2</SUB>濃度は大気中の値から約800ppm程度までの変動を示し,これらの変動には積雪深の増減に同期した長期的なものと,融雪水へのCO<SUB>2</SUB>の溶解によると思われる短期的なものがあることがわかった.モデルによるシミュレーションは定性的には実測値の挙動を再現したが,最大で数100ppm程度の誤差も見られた.誤差の原因のうち長期的な変動に関するものは土壌のCO2発生強度および積雪のガス拡散係数の見積もり誤差によるもので,短期的な変動に関するものは溶解係数の変動と融雪水流下の不均一性が原因であることが示唆された.

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.

収録刊行物

  • 雪氷  

    雪氷 60(5), 357-366, 1998-09-15 

    The Japanese Society of Snow and Ice

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各種コード

  • NII論文ID(NAID)
    10002384140
  • NII書誌ID(NCID)
    AN00131221
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    03731006
  • NDL 記事登録ID
    4558336
  • NDL 雑誌分類
    ZM43(科学技術--地球科学--気象)
  • NDL 請求記号
    Z15-23
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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