2026-04-24 東京大学
図1 日射量の年平均値と年水収支からもとめた年蒸発散量の1930年からの変化
1950年代から1980年代の年蒸発散量が少なかった期間は、日射量が減少していた期間とほぼ一致しています。毎年の観測値と5年移動平均値を示しています。
日射量の長期変化は白坂試験流域での観測値および名古屋地方気象台で観測された日照時間や日射量等を用いて推定しています。
＜関連情報＞
- https://www.a.u-tokyo.ac.jp/topics/topics_20260424-3.html
- https://onlinelibrary.wiley.com/doi/10.1002/hyp.70467
かつて荒廃した山岳流域における蒸発散量の長期的な変化：森林と土壌の回復および気候変動の影響に関する93年間のデータからの洞察 Long-Term Changes in Evapotranspiration in Once-Degraded Mountain Catchments: Insights From 93 Years of Data on Forest and Soil Recovery and Climate Change Effects
Yifan Bei, Yuko Asano
Hydrological Processes Published: 23 April 2026
DOI:https://doi.org/10.1002/hyp.70467
ABSTRACT
Understanding the evapotranspiration of mountainous catchments is vital for the effective management of water resources. Evapotranspiration is affected by changes in both field conditions, such as changes in land cover and soil, and external conditions, such as climate. However, the magnitude and interaction of these effects is not well understood. The Shirasaka Experimental Watershed of Ecohydrology Research Institute, established in 1929, has undergone precise rainfall-runoff and climate monitoring. During this period, the catchment experienced forest recovery following historical anthropogenic degradation and climate change. We first hypothesized that since the catchment has undergone substantial land cover changes, annual evapotranspiration has increased primarily due to changes in the land surface condition and climate change having a minor effect. To test this, we studied and measured changes in vegetation and soil, supplemented historical climate data such as precipitation, temperature, and solar radiation, calculated actual and potential evapotranspiration, and analysed long-term trends using the Mann-Kendall test. We then separated the effects of changes in land surface conditions and external conditions on annual evapotranspiration using catchment water and energy balances, as implemented in the Budyko model. The bare land is disappearing, and the stand volume and soil depth generally increased, yet evapotranspiration did not increase consistently, demonstrating that the hypothesis was rejected. Simultaneously assessing the impacts of land surface change and climate change demonstrated that relative impacts have changed with time. Between the mid-1950s and the 1980s, annual evapotranspiration was about 50 mm lower than the long-term mean and suggested that lower solar radiation and/or air pollution lowered evapotranspiration. While in the 2010s, changes in land surface conditions decreased evapotranspiration by more than 100 mm, although the climate conditions have changed in the direction of increasing evapotranspiration. Long-term data show significant changes in watershed evapotranspiration caused by previously unconsidered factors, necessitating further investigation.
