[A-HW23] 水循環・水環境

[AHW23-01] 水文条件の変化が中山間地域における面源からの栄養塩輸送に及ぼす影響

★招待講演

*井手 淳一郎¹、武田 育郎²、宗村 広昭³、森 也寸志³、作野 裕司⁴、米 康充²、高橋 絵里奈² (1.公立千歳科学技術大学、2.島根大学、3.岡山大学、4.広島大学)

キーワード：針葉樹人工林、面源汚濁、粗放化

This study aimed to examine the hypothesis that coniferous forests and agricultural lands including poorly managed ones contribute to increasing nutrient concentrations in downstream rivers during storms. For this, we collected river water samples and analyzed their total nitrogen, total phosphorus, nitrate, and phosphate concentrations during the 18 years in the eight subbasins of the Hii River basin located in the semi-mountainous area, western Japan. Then, a combination of time series and linear mixed-effects model analyses were used to elucidate how the effects of land cover on nutrient concentrations in downstream rivers change depending on hydrological conditions. Results revealed that the slope of regression between nutrient concentrations and agricultural area ratio was positive and increased with daily precipitation, indicating that agricultural lands made a large contribution to increases in nutrient concentrations during storms even though they accounted for a small percentage of the basin. On the other hand, the slopes of the multiple regression analysis between nutrient concentrations and coniferous and broad-leaved forest area ratios were negative and decreased with increasing daily precipitation. This suggests that forests more contributed to the dilution of river nutrient concentrations during periods of heavier precipitation regardless of whether they were coniferous or broad-leaved.

†This presentation is based on the published paper as follows: Ide, J., Takeda, I., Somura, H., Mori, Y., Sakuno, Y., Yone, Y., & Takahashi, E. (2019). Impacts of hydrological changes on nutrient transport from diffuse sources in a rural river basin, western Japan. Journal of Geophysical Research: Biogeosciences, 124(8), 2565-2581.
https://doi.org/10.1029/2018JG004513 (free access)