Estimating Regional Evapotranspiration in the Amazon Basin from the Atmospheric Water Budget

Estimates of regional evapotranspiration (ET) over the Amazon basin have been found to be highly dependent on the parametrization of transpiration. The inadequately understood and difficult to model vegetation control over ET under different conditions of energy and water availability leaves us with divergent estimates of the annual ET cycle in the basin. To avoid this problem, we estimate regional ET over the Amazon basin from the atmospheric water budget. The control volume is defined to be the atmosphere overlying the basin and the variables involved are area-averaged ET, precipitation (P), vertically integrated moisture convergence (Q), and change in the water vapor content of the atmospheric column (dW/dt). Significant differences exist between measures of these water budget components obtained from different data sources, reflecting the uncertainty of available data on precipitation, wind speed and atmospheric moisture. To reduce the uncertainty in the resulting ET estimate, we incorporate multiple estimates of each budget term in a constrained least squares estimator, weighting each estimate according to its uncertainty and constraining the optimization by the water balance equation: dW/dt= Q+ET-P. Data on windspeed and atmospheric moisture critical to this water budget study are obtained from various global reanalyses: NCEP-NCAR Reanalysis, NCEP-DOE Reanalysis, and the ECMWF Reanalysis ERA-40. Data on atmospheric moisture from the NASA Water Vapor Project (NVAP) is also utilized. TRMM and GPCP data products are used to obtain precipitation estimates. ET estimates from different land-surface models are also incorporated into the estimator. We explicitly account for the uncertainties associated with the available measures of the water budget components, and study their effect on the derived ET value and its error bounds. This approach is tested over the 5-year period 1997-2001. We evaluate our ability to resolve patterns of moisture fluxes over the basin and associated ET variations at the sub-monthly, seasonal and interannual timescales.