## 4 Discussion  
### GPP, Spectral Indices, and SM During the Study Period  
The results show that during dry seasons and years, there are no detectable increases in SM and growing seasons, with photosynthetic production being the most minimal during winter (Figure 3) [15]. In general, a quick response of SM at different depths is evident, even more during extremely wet periods, although the decay curve is less steep in deep horizons--typical behavior at depths where the influence of atmospheric water demand decreases (Figure 3a). It should be noted that less important wet pulses produce little changes in SM at 60 or 100 cm depths (e.g., February-March 2020). Ref. [33] reported that precipitations between 0 and 50 mm/week are associated with SM values around 0.10 m\({}^{3}\)/m\({}^{3}\) in deep horizons.  
The lags between GPP and SM observed in Figures 3 and A1 can be associated with antecedent SM. Refs. [14,33] suggested that subsurface SM acts as a buffer of the dry condition effect between rainfall events. Ref. [33] described how the reserve of water supported root function and small rates of photosynthesis without decreasing phenological and physiological responses to subsequent wet rainfall events. They suggested the existence of a hydraulic lift to shallow roots during dry periods; thus, these roots are maintained, and trees and grasses are synchronized. Then, vegetation has the capability to respond quickly to unpredictable rainfall events. Our results suggest that shorter delays in the response of vegetation to SM pulses can be associated with higher antecedent SM in the deepest horizons that support root systems and the capacity for faster vegetation response, as is the case of the 2017-2018 season.  
During dry periods such as in 2018-2019 (Figure 3), there is no clear vegetation growth associated with slight changes in SSM; this is related to the minimum water amount necessary to trigger a response [14]. Ref. [5], analyzing the EVI from MODIS, showed that acacia open woodlands and shrublands have high sensitivity to dry and wet periods. These authors and [35] showed that grass layers strongly respond to rainfall and significantly contribute to GPP during February-March in wet periods; thus, the rapid response of vegetation in late spring and summer should be due to the contribution of both vegetation layers. Some studies have analyzed the partitioning of three-grass functions and structure (e.g., [48,49]), with a detailed discussion of tree and grass contribution to GPP in [5].  
The weak response of vegetation to small increases in SSM is consistent with previous studies, like [12], who showed that vegetation in dry climatic zones generally shows great dependence on subsurface SM, especially in the grasslands and shrublands of interior Northern Australia. Some studies have analyzed SM estimates from microwave missions  
Figure 11: Relationship between daily in situ GPP (gC/m\({}^{2}\)), MOD LST (**left**), and MYD LST (**right**) in ASM (n = 232).  
in the study area [11]. These results suggest that those estimates of SSM could be useful to monitor vegetation response mostly in cases of vertical integration of surface and subsurface SM during significant wet pulses.