10-year evapotranspiration estimates in a Bornean tropical rainforest

This study was undertaken to quantify 10-year evapotranspiration (ET) in a tropical rainforest, Sarawak, Malaysia. To this aim, a simplified big-leaf model was formulated, which can consider transpiration (Et) and rainfall interception (Ei). The model was independently validated using eddy covariance fluxes, rainfall interception based on throughfall and stemflow measurements, and sap flow measurements conducted for more than two years. Consequently, our big-leaf model could successfully reproduce Et and Ei. By using the model and a 10-year meteorological data set, Et, Ei, and ET was estimated in the period between 2000 and 2009. The annual Et , Ei, and ET averaged over 10 years were estimated as 1114, 209, and 1323 mm, respectively, with the small seasonal fluctuations. The derived estimations for 10 years showed conservative year-to-year variations in Et, Ei, and ET (CV = 5-7%) against considerable year-to-year variations in annual rainfall (CV = 11%). Specific rainfall characteristics in this site could be a reason for conservative year-to-year variations in Ei. Small interannual variations in meteorological conditions and no occurrence of unusually severe drought in this study period could be a reason for the small year-to-year variations in Et. As well, we compared ET, Ei at this site with those of other tropical forests. Our forest ET was smaller than global maximum value of ET estimated in other tropical forests because of the smaller Ei, relative to annual rainfall at this site. Based on the derived characteristics of ET, we also discussed possible changes in ET, Et, and Ei in response to changes in rainfall regime at this site.