## 4 Discussion
### Seasonal Variations in ET and WUE  
There was a strong seasonal pattern for both ET and WUE (Figure 4). ET showed a minimum value at the beginning of the year and reached a maximum around August (Figures 2 and 4); these seasonal variations could be driven by climatic conditions either directly or indirectly, through their effect on biotic mechanisms, such as leaf phenology. What precise functional mechanisms explain why ET is elevated during seasonal dry periods? First, several studies conducted in the Amazon region have reported that ET is mainly driven by global radiation, both annually [1; 19; 10] and during \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Location/Ecosystem** & **Study Period** & **Interannual Variations** & **Daily Average ET (kg H\({}_{2}\)O m\({}^{-2}\) d\({}^{-1}\))** & **References** \\ \cline{3-4}  & & **in ET (\%)** & **Annual Period** & **Water Depletion Period** \\ \hline Southern Brazil, & 2000–2002 & 5–10 & 2.7 & [18] \\ Tropical forests mostly & & & 3.75 & [19] \\ Amazon & & & & \\ (modelling study) & & & & \\ Amazon forests & 1996–2006 & & & [13] \\ -Manus & & & 3.3–3.9 & \\ -Santaram & & & 3.3–3.9 & \\ -Jaru & & & 1.3–3.3 & \\ -Javes & & & 1.3–3.3 & \\ -Simop & & & 1.3–3.3 & \\ -Pe de Gigante & & & 1.3 & \\ Eastern Amazon & 2000–2001 & & 3.51 & 3.96 & [40] \\ Amazon & & & & [10] \\ -Manus & 1999–2000 & & 3.58 & 3.70 & \\ -Santaram & 2000–2003 & & 3.49 & 3.59 & \\ -Jaru & 2000–2003 & & 3.57 & 3.27 & \\ -Simop & 2004–2005 & & 3.11 & 3.18 & \\ Amazon forest, Brazil & 2002–2006 & 18 & 2.4–3.76 & 3.41 & [17] \\ Sub-tropical forest in & 2003–2005 & 15.5 & 2.62 & & [34] \\ Tropical forest in & 2000–2009 & 12 & 3.62 & & [33] \\ Malaysia & 2004–2014 & 10.4 & 2.95 & 3.23 & This study \\ French Guiana & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of evapotranspiration (ET) in various studies from the Amazon and other tropical forests. seasonal dry periods [10; 11; 12] and Rg was indeed the main driver of seasonal variations in ET at our site (Table 2). High monthly radiation can be reached either when cloud cover is low during the dry season or when the Earth's axial tilt maximizes incident solar radiation at this latitude (i.e., at the end of March and the beginning of September). During these periods of high Rg, the large amount of solar energy induces strong evaporative demand resulting in high ecosystem evapotranspiration rates [10; 11; 12; 13; 18; 19; 20]. Second, at the ecosystem level, the potential reduction in leaf transpiration by stomatal closure is compensated for in part by dry air entrainment from above the boundary layer and by changes in leaf temperature [41], meaning that canopy evaporation may not be reduced despite stomatal closure. Third, tropical rainforests can continue transpiring during periods of limited precipitation, because canopy trees are able to use available water from deep soil layers, thanks to a deep rooting system [42; 43]. Hydraulic redistribution and upward soil capillary water flux may also help sustain ET during the dry season [39; 44], though such mechanisms have not been documented at our study site [42]. Fourth, the seasonal dynamics of leaf emergence and leaf fall (i.e., leaf phenology) has also been suggested as a possible explanatory mechanism for the dry-season maxima of carbon and water fluxes in tropical forests [17; 45], with leaf fall together with new leaf emergence peaking at the onset of the dry season. All of these patterns may, in one way or another, have contributed to the increase in ET during the WD period at our site. The seasonal pattern for WUE was different from that of ET. We expected a lesser decrease in ET than in GPP during water deficit conditions (WD period), which would lead to higher WUE. Actually, the opposite pattern clearly occurred. Within any given year, the WD period had the lowest WUE values (on average, 7.6% lower than during the rest of the year; 1%-17%) (Figure 4). Subtropical forest ecosystems in China suffering from frequent drought also showed lower WUE during dry periods [24].