Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado

tEvapotranspiration (E) in the Amazon connects forest function and regional climate via its role in pre-cipitation recycling However, the mechanisms regulating water supply to vegetation and its demand forwater remain poorly understood, especially during periods of seasonal water deficits In this study, we address two main questions: First, how do mechanisms of water supply (indicated by rooting depth andgroundwater) and vegetation water demand (indicated by stomatal conductance and intrinsic water useefficiency) control evapotranspiration (E) along broad gradients of climate and vegetation from equatorialAmazonia to Cerrado, and second, how do these inferred mechanisms of supply and demand compareto those employed by a suite of ecosystem models? We used a network of eddy covariance towers inBrazil coupled with ancillary measurements to address these questions With respect to the magnitudeand seasonality of E, models have much improved in equatorial tropical forests by eliminating most dryseason water limitation, diverge in performance in transitional forests where seasonal water deficits aregreater, and mostly capture the observed seasonal depressions in E at Cerrado However, many mod-els depended universally on either deep roots or groundwater to mitigate dry season water deficits, therelative importance of which we found does not vary as a simple function of climate or vegetation In addi-tion, canopy stomatal conductance (gs) regulates dry season vegetation demand for water at all exceptthe wettest sites even as the seasonal cycle of E follows that of net radiation In contrast, some models sim-ulated no seasonality in gs, even while matching the observed seasonal cycle of E. We suggest that canopydynamics mediated by leaf phenology may play a significant role in such seasonality, a process poorlyrepresented in models Model bias in gsand E, in turn, was related to biases arising from the simulatedlight response (gross primary productivity, GPP) or the intrinsic water use efficiency of photosynthesis(iWUE). We identified deficiencies in models which would not otherwise be apparent based on a simplecomparison of simulated and observed rates of E. While some deficiencies can be remedied by parame-ter tuning, in most models they highlight the need for continued process development of belowgroundhydrology and in particular, the biological processes of root dynamics and leaf phenology, which via theircontrols on E, mediate vegetation-climate feedbacks in the tropics.NASA (LBA investigation CD-32 and the LBA-DMIP project - # NNX09AL52GNational Science Foundation (Amazon-PIRE, NSF # OISE - 0730305Gordon and Betty Moore Foundation’s Andes-Amazon InitiativeFAPESP - 08-581203GREF - U.S. DOE Global Change Edu-cation Progra