中国山地中部の源流域河川における融雪期の栄養塩濃度の特徴: 1年間を通じた週に1度の採水に基づく無雪期と融雪期の比較

摘要

中国山地中部の源流域河川における融雪期の栄養塩濃度の特徴を明らかにするために，水文観測と週に1度の河川水採取を通年で行った。調査流域では1月や2月であっても一時的に融雪が進み流量が上昇した。無雪期(5～8月と9～12月)と比較し，融雪期（1～4月）における溶存態の窒素（DN)とリン（DP)濃度は高く，懸濁態の窒素（PN)とリン（PP)濃度は同等かやや低かった。融雪期の河川水のDNとDP濃度には，土壌からの溶脱水よりも積雪からの融雪水の方が強く影響すると推察された。融雪期のPNとPP濃度には，河道内での輸送可能な土砂の蓄積量低下が反映されることが示唆された。結論として，中国山地中部の森林流域においてDN，DP，及びTNの年流出負荷量を推定する際に，融雪期の濃度を無雪期のそれで代用すると過小評価につながること，及び融雪期の実地調査では1～3日に1度の頻度で河川水採取が必要であることが示された。%To characterize stream water nutrient concentrations during snowmelt period (Jan-Apr) in a forested headwater catchment in the central Chugoku Mountains, hydrological observations and weekly stream water sampling were conducted for one year. Even in the coldest period (Jan-Feb), snow melting progressed temporally and generated a relatively large streamflow in the catchment. Compared with those during the no-snow period (May-Aug and Sep-Dec), the dissolved nitrogen (DN) and phosphorus (DP) concentrations during snowmelt period were high, whereas the particulate nitrogen (PN) and phosphorus (PP) concentrations were the same or slightly low. The DN and DP concentrations during the snowmelt period might have been strongly influenced by melting water from accumulated snow rather than by leaching water from the soil layer. On the other hand, the PN and PP concentrations during the snowmelt period seemed to reflect the decrease in the extent of accumulation of mobile sediments within the stream channel. In conclusion, in areas of the Chugoku Mountains around the catchment studied, 1) the use of DN, DP and TN concentrations measured during no-snow periods as substitutes for those measured during snowmelt period can underestimate the annual export load of the nutrients, and 2) stream water sampling at shorter intervals, i.e., every 1～3 days, is necessary for accurate nutrient load evaluation based on field investigation during snowmelt periods.