Estimation of annual forest evapotranspiration from a coniferous plantation watershed in Japan (2): Comparison of eddy covariance, water budget and sap-flow plus interception loss

Evapotranspiration (ET) was estimated from a planted coniferous forest in southwestern Japan by applying three methods: the eddy covariance method; the measurement of rainfall (P) and runoff (Q) in a small watershed; and a combination of rainfall interception loss (I_C), upper canopy transpiration based on a sap-flux density measurement in Japanese cedar (Cryptomeria Japonica D. Don) stands (E_UC), and modeled sub-canopy ET (E_SC). After inverse multiplication of the energy imbalance ratio, ET by the eddy covariance method (ET_EC) was 839.9 mm in 2007 and 811.8 mm in 2008. The yearly values of P–Q were partially affected by P in the previous autumn. After continuous data collection for more than 5 years, P–Q became stable. The 9-year (2000–2008) average P–Q, which was considered most reliable in this study, was 897.5 mm y⁻¹. The cumulative ET_EC during the daylight hours from the right stream bank, covered mainly with large Japanese cedars, was 894.1 mm from April 2007 to March 2008. The value was almost the same as that calculated as the components sum (ET_COMP = I_C + E_UC + E_SC: 911.4 mm), and the comparison suggested that the annual totals of ET_EC with an energy imbalance correction provide a reliable estimate of ET in a forest stand on a complex topography. Spatial variation in the watershed was likely caused by differences in soil water retention at each slope position. The slight difference in annual ET_EC in 2007 compared with 2008 was attributed to differences in the radiative energy input. In the monthly–weekly analysis, ET_COMP was frequently higher than ET_EC after heavy rainfall, while ET_EC was higher under dry conditions and during active ET. Even under dry canopy conditions, daily ET_EC was often higher than E_UC + E_SC. The results suggested a time-lag in evaporation from the ecosystem and/or under-estimated ET_EC after rainfall.