## 1 Introduction  
The Amazon basin plays a crucial role in the regional and global climate system due to its large contribution to the global surface water and carbon cycle [1]. However, drier and warmer climatesover Amazonia have been predicted, due to climate change for the next 100 years [2], along with increasing drought frequency and severity. How tropical forests respond to these climate extremes is still an intense subject of debate [3; 4; 5; 6; 7]. Ecosystem evapotranspiration (ET) is an important component of the global water cycle and has a critical role in ecological and hydrological processes [8]. However, little is known about the mechanisms associated with the interannual variability of ET. While some ecosystem process models present ranges of interannual variability similar to those actually measured at flux towers (e.g., Soil-Plant-Atmosphere model [9]), others underestimate annual ET [9]. Deeper investigation into the factors determining the interannual variability of ET is therefore needed to address these discrepancies, particularly in tropical rainforests. Moreover, the seasonality of ET in the Amazon has been the subject of debate for over two decades. Field observations, flux measurements and modelling studies at multiple sites in Amazonian forests have generally shown higher ET rates during the dry season than during the wet season [10; 11; 12; 13; 14; 15], thus indicating that the Amazon forest transpires more in the dry season. In a changing environment, another major issue is how tropical rainforest ecosystems regulate and optimize the ratio between CO\({}_{2}\) assimilation (GPP, gross primary production) and evapotranspiration, as described by ecosystem water use efficiency (WUE). The equilibrium between assimilation and evapotranspiration reflects a set-point of foliar functioning and depends on how much the stomata of the tree open. Under limiting resource conditions (e.g., low radiation, lack of water), this equilibrium depends on the trade-off at the ecosystem level between limiting the rate of assimilation of CO\({}_{2}\) (benefit) and the transpiration speed (cost) [16]. Whether tropical rainforest ecosystems can adjust WUE over long time scales, particularly when soils are becoming drier, is a major question ecosystem models need to address. Several factors have been found to control ecosystem ET and WUE. Many studies have reported that solar radiation enhances annual [17] or seasonal [10; 11; 12; 18; 19; 20] ET rates. In contrast, hydraulic stress and/or stomatal control associated with depleted soil water restricts transpiration during drier conditions [21]. Yet, analyzing or simulating the drivers of WUE is more complex than meets the eye, since variations in WUE result from variations in ET and GPP, which can be decoupled. At the annual scale, ecosystem WUE was found to be positively correlated with radiation [22], precipitation [23] or temperature [24] in various tropical forest ecosystems. It should be noted that long-term trends for solar radiation and temperature do not vary much from year to year in the neo-tropical Amazon rainforest [25]. This means that fluctuations in soil water at the seasonal scale, brought about by changing rainfall patterns, may become predominant in determining the variations in the GPP/ET relationship. Studying the relations between climatic variables and water fluxes in the Amazon region may provide important new knowledge and reduce the uncertainty concerning the ecosystem's response to natural climate variations and possible extreme conditions in the future. In this study, we used a single eddy flux dataset collected continuously over an 11-year period at the Guyaflux site in the tropical rainforest of French Guiana to determine the interannual and seasonal variations in ET and WUE and their climatic drivers.