## 3 Results
### Carbon Fluxes at Different Climate Zones  
Annual and monthly GPP and NEE values were compared in each climate zone for the selected months from 2015 to 2018 (Figures 5 and 6 and Table 3). While some climate zones showed higher GPP and NEE values in one year than another year, others showed lower. For example, each climate zone showed high GPP values from spring to fall and low in winter each year; however, in comparison, UC and ET climate zones showed the highest GPP values and TP and EP climate zones showed lowest GPP values during the same period. Each of the selected climate zones showed highest GPP (168-479 g C m\({}^{-2}\) mo\({}^{-1}\)) in August 2018 except EP, S, and LRP climate zones, which had highest GPP in June 2015 (154-190 g C m\({}^{-2}\) mo\({}^{-1}\)), and TP climate zone which had highest GPP in September 2016 (82 g C m\({}^{-2}\) mo\({}^{-1}\)). Each of the selected climate zones showed the lowest GPP in December, but not necessarily in the same year. For example, all of the climate zones had lowest GPP in December 2017 (22-51 g C m\({}^{-2}\) mo\({}^{-1}\)), except HP, S, and ET climate zones, which had lowest GPP in December 2016 (17-51 g C m\({}^{-2}\) mo\({}^{-1}\)) because hurricane Harvey (late summer in 2017) brought heavy precipitation, which may cause change in vegetation growth in some of the climate zones (Figure 5). In addition, each climate zone showed the highest carbon emission (+ve NEE) and carbon uptake (\(-\)ve NEE), respectively, in December (13.3-52.1 g C m\({}^{-2}\) mo\({}^{-1}\)) and in June (\(-\)74.3 to \(-\)47.3 g C m\({}^{-2}\) mo\({}^{-1}\)), except HP and TP climate zones, which had the highest carbon emissions and carbon uptakes, respectively, in May (6.8-12.2 g C m\({}^{-2}\) mo\({}^{-1}\)) and in June (\(-\)49 to \(-\)22.6 g C m\({}^{-2}\) mo\({}^{-1}\)). UC and ET climate zones had the highest NEE in December and April 2016 (43.8 and 52.1 g C m\({}^{-2}\) mo\({}^{-1}\)), whereas the lowest NEE (\(-\)63.2 and \(-\)58.5 g C m\({}^{-2}\) mo\({}^{-1}\)) in June 2016 and 2015. In 2015, NC and LRP climate zones had the highest and lowest NEE in December and June (NC/LRP; 21.3/13.3 and \(-\)66.2/\(-\)47.3 g C m\({}^{-2}\) mo\({}^{-1}\)), respectively. Five climate zones (SC, ET, S, LV, and EP) showed highest NEE (12.4-43.8 g C m\({}^{-2}\) mo\({}^{-1}\)) in December 2016 and lowest NEE (\(-\)74.3 to \(-\)47.7 g C m\({}^{-2}\) mo\({}^{-1}\)). However, two climate zones (NC and LRP) showed the highest and lowest NEE (NC/LRP; 21.3/13.3 and \(-\)66.2/\(-\)47.3 g C m\({}^{-2}\) mo\({}^{-1}\)) in 2015. Figure 4: Daily and monthly SMAP and in-situ GPP and NEE at PVAMU Research Farm (2016–2018). The unit of RMSE is the same as of GPP and NEE. Figure 5: Distributions of monthly GPP in each climate zone (2015–2018). Compared to August 2015, the largest spatial change in GPP was observed in August 2018 in UC, ET, SC, S, and some portions of EP and NC climate zones. However, the lowest change in GPP was observed in December in all climate zones except some portion of the UC climate zone (Figure 7). In addition, all climate zones except some portions of UC and ET climate zones showed a change in  
Figure 6: Distributions of monthly NEE in each climate zone (2015–2018). In comparison, carbon uptake changed into carbon emission in most of the climate zones in 2018 (Figure 9). One of the main reason for this change was the impact of wetness (rain), which increased from 2015 to 2017 and decreased in 2018. Each climate zone had different annual GPP, but UC and TP climate zones had the highest (1941-2070 g C m\({}^{-2}\) year\({}^{-1}\)) and the lowest (408-525 g C m\({}^{-2}\) year\({}^{-1}\)) annual GPP each year, respectively. In addition, TP climate zone had the lowest spatial variability (SD = 103-133 g C m\({}^{-2}\) year\({}^{-1}\)) each year, but UC climate zone had highest spatial variability (SD = 352 g C m\({}^{-2}\) year\({}^{-1}\)) in 2018, whereas LV climate zone had highest spatial variability (SD = 369-375 g C m\({}^{-2}\) year\({}^{-1}\)) in 2016 and 2017.