## 3 Results and Discussion
### Interannual, Interlocation and Interseasonal Patterns in Ecosystem Water Use Efficiency  
In this study, we obtained daily eWUE values between 0.2 and 5.0 g C kg\({}^{-1}\) H\({}_{2}\)O d\({}^{-1}\), with an interannual average of 1.54 g C kg\({}^{-1}\) H\({}_{2}\)O and statistical monthly and interannual differences (\(p<0.01\)). As occurred with GPP and NEE, eWUE responses depend on the vegetation type [99] and environmental factors [23; 83]. In the case of agricultural ecosystems, the eWUE is especially vulnerable to water inputs and management prac  
Figure 6: Almond plot average NDVI temporal evolution across the six years examined at both locations.  
tices [99; 100]. In our study, irrigation was applied based on the FAO-56 methodology [101] to maintain well-watered conditions until a few days prior to harvest due to management purposes. Our mean annual eWUE presented relatively similar results to those obtained by [PERSON] et al. [23] reporting 1.77 g C kg\({}^{-1}\) H\({}_{2}\)O in citrus trees, and to those obtained in croplands in a 7-year study (1.76 g C kg\({}^{-1}\) H\({}_{2}\)O) [99]. The latter study [99] also reported WUE results in different vegetation types, with mean almond eWUE estimates being higher than those obtained in grasslands and lower than forests and shrublands. In terms of daily eWUE, the results are within the range of other studies, for example, those obtained by [82] in a six-year study of subtropical coniferous plantation (1.5 to 4.5 g C kg\({}^{-1}\) H\({}_{2}\)O d\({}^{-1}\)), being slightly higher than those reported in citrus trees (0.22-3.39 g C kg\({}^{-1}\) H\({}_{2}\)O d\({}^{-1}\)) [23] and lower than those reported in olive trees (2 to 10 g C kg\({}^{-1}\) H\({}_{2}\)O d\({}^{-1}\)) [47].  
Although the present discussion on eWUE is limited, due to the lack of studies quantifying eWUE in almond, and given that comparisons with other crops are not strictly fair, this first assessment on eWUE in almond trees paves the way for a deeper analysis and assessment of these results, with future research for improved irrigation management practices in almond orchards.  
In this context, we address how seasonal (weather), interannual (tree age) and management (tree density) variability can affect almond eWUE (Figures 3, 4B and 5E,F). Higher values of eWUE were observed in Tarazona (2020-2022) compared to Las Tiesas (2017-2019). Again, this is likely related to the higher assimilation capacity due to higher tree density and canopy cover (note higher NDVI) under similar seasonal climatic conditions. Across the growing season, the largest values of eWUE were observed in spring (April-May) and autumn (October, see Figures 3 and 5E,F). In those months, both temperature and evaporative conditions were less demanding compared to summer months (June-August, Figure 2). These higher eWUE values in less demanding months compared to those in summer can be explained by evapotranspiration processes [41; 82; 102]. In this sense, it is worth noting that ET encompasses both water loss through plant transpiration and water evaporation from the soil and plant surfaces. Higher eWUE values mean that plants are achieving a greater ratio between biomass gain and evapotranspiration. However, this productivity based on WUE is relative but not absolute in terms of carbon fixation or productivity. As our results show, greater GPP and lower eWUE values were found in June and July compared with April or October (Figures 3 and 5). Thus, higher eWUE does not mean higher carbon capture, biomass production or yield, as previously reported [103].  
Therefore, eWUE emerges as a valuable metric for evaluating the relationship between GPP and ET. However, this relationship is not necessarily linear and is highly dependent on the ET range considered, as will be further discussed in the following sections.