Estimation of annual forest evapotranspiration from a coniferous plantation watershed in Japan (1): Water use components in Japanese cedar stands

To increase the ability to control forest ecosystem water and carbon cycles using forest management, we estimated watershed-scale evapotranspiration (ET) and its components, i.e., upper-canopy stand transpiration (EUC), sub-canopy vegetation transpiration (ESC), and canopy interception (IC), in a Japanese cedar (Cryptomeria japonica D. Don.) plantation over a whole year. For EUC, xylem sap flux density was measured in three plots: an upper (UP) and lower (LP) plot on a northeast-facing, and one on a south-facing slope (SP). Mean stand sap flux density (JS) in the UP, LP, and SP was similar despite differences among plots in tree density and size, implying that JS measured in a partial stand within the watershed is a reasonable estimator of the values of other stands, and that stand sapwood area is a strong determinant of the EUC. Prior information on annual variations in ET and its components was insufficient and urgently needed in Japan. Using a combination of observations and modeling, we obtained reliable estimations of ESC and IC, and thus, of annual variations in ET and its components (911.4, 359.3, 126.9, and 425.2 mm/year for ET, EUC, ESC and IC, respectively). We found a conservative ratio of IC to rainfall (P) (IC/P) throughout the year, a significant contribution of IC/P to the ratio of ET to P (ET/P) during heavy rainfall conditions, and an increase in IC and ESC when EUC was decreasing, resulting in a constant monthly ET/P in the growing season and winter. These support the idea of the conservative process of forest water use in that P mainly controls ET on a monthly and longer time scale.