## 4 Discussion
### Variations in ET and WUE within the WD Period  
The impact of seasonal drought on ET in tropical forest ecosystems was found to be inconsistent among studies (Table 3). During water depleted periods, evapotranspiration of tropical rainforests was found to be reduced due to stomatal closure resulting from limited soil water content [10; 24]. However, other studies claim that ET is greater during seasonal water depleted periods [13; 14; 15; 19], pointing out that the positive effect on ET of increased solar radiation during seasonal dry periods was greater than the possible negative effect of soil water limitation. At our site, strong variations in ET (25%) occurredduring the WD period (Figure 4). In general, ET was highest at the beginning of the WD period, then linearly decreased during the period, clearly indicating that the changing environmental conditions during the seasonal dry period had a negative effect on ecosystem transpiration. A combination of different abiotic drivers may explain this pattern. During the WD period, Rg decreased over time, mainly because of the rotation of the earth, while relative water content strongly decreased because of reduced rainfall (Figure 3). This combined decrease in Rg and REW may have induced a decrease in ET during the WD periods. Yet, Rg was found to be the strongest predictor of ET during the WD periods over the 11 years (Table 2). Air humidity also varied during the dry season (Figure 3) and the decrease in vapor pressure deficit could also have contributed to the decrease in ET. Stronger stomatal control during the day when air humidity is low and vapor pressure deficit is high can indeed result in lower ET. However, this potential impact was impossible to disentangle from the impacts of Rg and REW (see PCA on climatic factors in [28]). When the effect of reducing solar radiation from September to November was removed to analyze soil water effect alone (Figure 5), Rg-normalized ET exhibited a strong sensitivity to water stress, which was proportional to WD intensity, with lower evapotranspiration occurring when soil water content worsened. This occurred because low REW, likely linked to water demand approaching or exceeding supply, induced stomatal regulation to restrict transpiration [21]. As drought conditions became harsher, the sensitivity of transpiration to water deficit was stronger, suggesting a predominant down-regulation of stomatal conductance, which lowered tree transpiration under extreme droughts. It is noteworthy that interannual variations in the intensity and the duration of the WD period at our site (Figure 1) caused in-between-year Rg-normalized ET slopes to differ significantly (Table 11), except in 2007, 2010 and 2011 (Table 1). This indicates that soil water stress levels induce different levels of evapotranspiration change when radiation is not limiting. This year-to-year variation in the slopes between Rg-normalized ET and SWSI may have occurred because, at the tree level, while some species may have suffered from the drought conditions, others some may not have, depending on drought intensity levels [47; 48]. Within the WD period, variations in WUE were low (Figure 4) and the variations observed were not consistent among years (Figure 4). We did not expect this pattern, owing to the large variations in ET (Figure 4) and GPP [25] observed during the WD period at this site. Yet, as WUE represents a trade-off between these two fluxes at the ecosystem level, the absence of major variations in WUE during the WD period confirms that, even though all the processes involved in the carbon and water fluxes may vary individually, all together they converge towards rather stable water use efficiency. This is a very important result for ecosystem modeling as it suggests that, despite strong variations in environmental factors and carbon and water fluxes during water depleted periods, ecosystem WUE can be parameterized with a constant value during water depleted periods in these models.