流域特性(黒ボク•非黒ボク•土地利用)が支配するP，Nの河川流出プロセス

摘要

Phosphorus (P) and nitrogen (N) concentrations were measured at 75 points in the upper reaches of rivers draining into the agricultural watersheds south of the Ibaraki prefecture during the summer and winter of year 2009. A geographic information system (GIS) was used to combine the environmental information regarding the watersheds such as land use, topography, and soil with water quality data for a statistical analysis. As andosols have a stronger tendency to adsorb P than other soil types, watersheds were classified into areas with andosol and non-andosol soils. No correlation was observed between land use or topography and the P levels in the rivers for andosol areas, whereas a positive correlation was observed between the proportion of the built areas or paddy fields in the watersheds and the P levels in the rivers for non-andosol soil areas. These results indicate that a difference in the adsorption property among soils is an extremely important environmental factor affecting the P discharge process. Although the differences related to the soil type were not observed for N， there was an extremely strong positive correlation between the N levels in the rivers and the proportion of fields in the watersheds: summer N concentration declined greatly for watersheds with a high proportion of dry fields. These results clarified that dry fields are the major sources of N， but they could also act as N sink during summer. In watersheds with soils other than andosols, the effects of land use and seasonal changes on the load sources were relatively large in terms of the P and N discharge processes. It was difficult to discern the effects of topography, and a correlation between the nutrient concentrations and TWI (Topographic Wetness Index) was observed only for a few cases.%農耕地河川の上流部において，2009年夏•冬に75地点でリン(P),窒素(N)の測定を行った。GISで整備した土地利用，地形，土壤等の流域の環境情報と水質情報を組合せた統計解析を行った。黒ボク土は他の土壌より高いリン酸吸着特性を持つ事から，対象流域を黒ボク地帯と非黒ボク地帯の二者に分類した。この結果，黒ボク地帯ではPと土地利用や地形との相関が全く見られなかった。一方，非黒ボク地帯ではPと流域の建物割合や田割合との間に正の相関があった。この事からPの流出プロセスでは，土壌による吸着特性の違いは非常に重要な環境因子であることが示唆された。Nでは土壌での違いは見られなかったが，流域の畑割合との間に非常に強い正の相関があった。更に流域の畑割合が大きい流域において夏のN濃度が大きく下がっていた。Nの流出プロセスでは畑は主な発生源であり，更には夏におけるNの消費場所としての寄与が明らかとなった。黒ボク地帯以外の一般的な土壌の流域におけるP,Nの流出プロセスでは，負荷発生源としての土地利用や季節変化の影響は相対的に大きく，地形の影響は見えにくく，いくつかのケースでTWI(湿潤度指標)との相関が見られたに過ぎなかった。