Comparative evaluation of BATS2, BATS, and SiB2 with Amazon data

Over the last decade, improved understanding of plant physiological process es has generated a significant change in the way stomatal functioning is de scribed in advanced land surface schemes. New versions of two advanced and widely used land surface schemes, the Biosphere-Atmosphere Transfer Scheme (BATS) and the Simple Biosphere Model (SIB), reflect this change in underst anding, although these two models make different assumptions regarding the response of stomata to atmospheric humidity deficit. The goal of this study was to evaluate the new, second version of BATS, here called BATS2, using: Amazon field data from the Anglo-Brazilian Amazonian Climate Observational Study (ABRACOS) project, with an emphasis on comparison with the original version of BATS and the new, second version of SIE (SiB2). Evaluation of Si B2 using a 3-yr time series of ABRACOS data revealed that there is an unrea listic simulation of the yearly cycle in soil moisture status, with a resul ting poor simulation of evaporation. Improved long-term simulation by SiB2 requires specification of a deeper rooting depth, and this requirement is g eneral for all three models. In general, the original version of BATS with a revised root distribution and rooting depth gave good agreement with obse rvations of the surface energy balance but occasionally showed excessive se nsitivity to large atmospheric vapor pressure deficit. Evaluation of BATS2 revealed that changes are required in the parameters that determine stomata l behavior in the model for realistic simulation of transpiration, time-ave raged respiration, and net carbon dioxide (CO2) uptake. When initiated with default values for carbon stores, BATS2 takes several hundred years to rea ch an equilibrium carbon balance. Aspects of the model's representation of instantaneous carbon allocation and respiration processes indicate that BAT S2 cannot be expected to provide a realistic simulation of hourly variation s in CO2 exchanges. In general, all three models have weaknesses when descr ibing the field data with default values of model parameters. If a few mode l parameters are modified in a plausible way, however, all three models can be made to give a good time-averaged simulation of measured exchanges. The re is little evidence of sensitivity to the different forms assumed for the stomatal response to atmospheric humidity deficit, although this study sug gests that assuming that leaf stress is related linearly to relative humidi ty is marginally preferred.