Climatological Basin-Scale Amazonian Evapotranspiration Estimated through a Water Budget Analysis

Abstract

Spatially averaged evapotranspiration [ET] over the Amazon Basin is computed as the residual of the basin?s atmospheric water balance equation, at the monthly time scale and for the period 1988?2001. Basin-averaged rainfall [P] is obtained from the Global Precipitation Climatology Project (GPCP) dataset, and alternative estimates of the net convergence of atmospheric water vapor flux over the basin [C] are derived from three global reanalyses: the NCEP?NCAR and NCEP?Department of Energy (DOE) reanalyses and the 40-yr ECMWF Re-Analysis (ERA-40). Additionally, a best estimate of [C] is obtained by taking a weighted average of data from these three sources, in which the weight factors are based on the random error attributed to each reanalysis? [C] estimates by comparison to river discharge data. The resulting time series is dominated by ERA-40?s contribution, which was found to be the most accurate over the study period. Data products from the three reanalyses are also employed to compute the monthly tendencies of total precipitable water over the basin. While the seasonal signature of this ?accumulation term? provides important insight into the Amazon Basin?s hydrological cycle, its magnitude is found to be negligible relative to the other components of the water budget. The value of mean annual [ET] presented in this work is significantly lower than other published estimates that are based on simulations by various land surface models. Furthermore, when the best estimate of [C] is used, the resulting [ET] time series exhibits a seasonal cycle that is in phase with that of basin-averaged surface net radiation, suggesting that Amazonian evapotranspiration is prevalently limited by energy availability. In contrast, most land surface models, including that of the NCEP?NCAR reanalysis, simulate water-limited evapotranspiration in the Amazon Basin. The analysis presented here supports the hypothesis that most Amazonian trees sustain elevated evapotranspiration rates during the dry season through deep roots, which tap into large reservoirs of soil water that are replenished during the following wet season.