Quantification of Evapotranspiration Change in the Past 100 Years as Estimated by Landcover Data in the Headwaters Region of the Tama River

Abstract

Evapotranspiration from the Tama River basin (an aquatic resource of Tokyo) has been estimated by meteorological techniques or water balance method, based upon long term measurements. However, no calculation of evapotranspiration exists to differentiate the impact of forest, topography and weather in the basin. Water Cycle Act (established in 2015) demands the quantification of aquatic resources in the future. Therefore, this study estimated the evapotranspiration of the headwaters region of the Tama River using meteorological data observed for 99 years (i.e., 1914–2012) along with six landcover maps made from historical geographic maps and satellite images. The results show that forest area grew from 88.2％ in 1910 to 96.0％ by 2009, whereas the barren area decreased from 10.9％ to 0.6％ over the same time period. The quantity of evapotranspiration in the basin averaged 625 mm from 1914 to 2012; this average increased to 600–900 mm during 1986–1989. Evapotranspiration tended to increase with increases in forest area, reductions in relative humidity, and increases in air temperature, especially after 1957 (1957–1989). These trends did not account for the influence of forest trimming in the area. It’s thought that evapotranspiration will increase in the future when the density of evergreen conifers increases.