## 4 Results and Discussion
### Relationship between EC GPP, PAW, VPD, and API  
Cross-site relationships of PAW and VPD with EC GPP were fairly moderate (R\({}^{2}\) = 0.36 and 0.43, respectively). The relationships at the individual sites were varied with R\({}^{2}\) values of VPD ranging between 0.59 and 0.75 and those of PAW between 0.59 and 0.76 (Table 3). API was the worst-performing variable, averaging a cross-site R\({}^{2}\) of 0.33. This is probably due to the fact that rain events during the growing season were sporadic and API could not capture the lag effect each event had on EC GPP. The combination of PAW and VPD explained the largest variance across all three sites (R\({}^{2}\) = 0.47, RMSE = 2.06 g C m\({}^{-2}\) d\({}^{-1}\)), with the highest R\({}^{2}\) and lowest RMSE at Wankama Fallow (R\({}^{2}\) = 0.83, RMSE = 0.77 g C m\({}^{-2}\) d\({}^{-1}\)) (Table 3). An overview of the relationship between EC GPP, VPD and PAW across all sites is shown in Figure 4. The dry season is characterized by high VPD (>2 kPa), which is linked to the reduction in evapotranspiration due to low PAW. With the reduction/closure of stomatal conductance, plants deactivate metabolic activity in the shoots to reduce consumption of water (and subsequent transpiration) while simultaneously enhancing uptake of water and nutrients by the roots to mitigate the effect of the dry season (75). In the wetter savanna systems of the humid region (>750 mm of mean annual precipitation, MAP), it has been shown that a considerable amount of C is stored in the roots at the start of the dry season and is subsequently used by the herbaceous vegetation to re-sprout at the start of the next growing season (76). If the dry season is intense, prolonged, or both, this memory effect allows vegetation to tap into these C reserves and maintain root respiration (77). A recent global study by [PERSON] et al. (2017) lends support to this, finding that the intensity and duration of the dry season has a larger impact on annual primary productivity than increased rainfall in the rainy season. Thus, a similar mechanism may take place in the poorer soils of the Sahel, where the herbaceous species are specialized to grow quickly at the beginning of the rainy season (13). These subsurface mechanisms can in turn be influenced by anthropogenic activity as soil moisture amount and availability are influenced by land management practices (36; 79). When the rainy season commences, not only is soil water replenished and root water uptake increased, but VPD is reduced due to humid air masses from the northward movement of the ITCZ. In Wankama Fallow, maximum EC GPP is reached when PAW is greater than 125 mm and VPD is below 1.5 kPa, while in Demokeya maximum EC GPP occurs when PAW is above 90 mm and VPD is below 2 kPa. At Dahra, maximum EC GPP was reached at VPD levels similar to Demokeya and Wankama Fallow, however, PAW was relatively low compared to the other two sites at \(\sim\)60 mm. The partial correlation analysis revealed that the increase in C uptake during the greening phase, shortly after the onset of the first rains, is governed by the availability of PAW and a reduction in VPD (Table 4). The increase in soil moisture also decreases LST (Spearman's Rho, \(r_{s}=-0.28\)) due to evaporative cooling. These are further illustrated by scatterplot in Figure 5. The browning phase is characterized by a steep decrease in GPP and a reduction in PAW due to a cessation of the rains. The relationship of EC GPP, VPD and PAW were generally strong during both the greening and browning phases (Figure 5). The mechanism by which VPD and PAW interact is further illustrated in Figure 6, which uses the 2009 growing season in Demokeya as an example. The first rains occurred on July 4, however, the relative humidity began to increase from a mean of 15% in April and May, which is typical of the dry season, to 32% in June. Between 4 July and 1 September (the larger shaded area in Figure 6), the mean relative humidity was 64%.