## 4 Discussion
This could be due to a water deficit during winter, which caused a reduction in ecosystem respiration, and GPP tended to decline in an ecosystem as water supply reduced, changing the ecosystem from a carbon source to sink [73]. In addition, each land-use and climate zone showed an increase in carbon emission or decrease in carbon uptake after the rain events. These findings support the previous studies, which showed CO\({}_{2}\) dissolved in rainwater released when soil pore occupied by CO\({}_{2}\) was replaced by rainwater [74]. Each climate zone showed a decreasing trend in annual NEE and transition from carbon uptake to carbon release from 2016 to 2018. However, it showed higher annual variability in NEE between the climate zones. The humid climate zones (UC, S, and SC) located close to the coast and dominated by forest, pasture, and croplands, showed an annual balance of carbon uptake from 2016 to 2018 except LV climate zone, which had an annual balance of carbon uptake from 2016 to 2017 and annual balance of carbon release in 2018. All other climate zones (HP, LRP, NC, ET, TP, and EP) showed an annual balance of carbon uptake from 2016 to 2017 and annual balance of carbon release in 2018. In 2017, hurricane Harvey could have some impact in UC, SC, and S climate zones, which caused annual balance of carbon uptake until 2018. Change in spatial coverage of particular NEE and GPP ranges between selected months of 2018 and 2015 showed more GPP production in some parts of the state and less in others (Table 2). The reason for the low or high range of GPP and NEE values in any year in a climate zone of the state could be attributed to the types of vegetation and climate variability associated with the climate zone and ecoregion or the location. The study showed select land uses of two climate zones had significantly different temporal GPP and NEE distributions except for pasturelands of NC and ET climate zones (Figure 11). Quantitatively, the majority of land uses had a smaller range of GPP (119-213 g C m\({}^{-2}\) mo\({}^{-1}\)) and NEE (62-149 g C m\({}^{-2}\) mo\({}^{-1}\)) except developed land of ET, cropland of UC and shrubland of TP climate zones. Developed land of ET climate zone had the largest GPP range (362 g C m\({}^{-2}\) mo\({}^{-1}\)), and NEE (240 g C m\({}^{-2}\) mo\({}^{-1}\)) and shrubland of TP climate zone had the smallest GPP range (80 g C m\({}^{-2}\) mo\({}^{-1}\)) and NEE (45 g C m\({}^{-2}\) mo\({}^{-1}\)). The large variations in GPP and NEE for different land-use and climate zones indicated that the climate variability and land-use change, in combination, have a significant impact on GPP, carbon emissions, and carbon uptakes (Figure 11). The analysis showed that increased GPP and NEE during the summer and winter tended to be compensated by the reduced GPP and NEE during the winter and summer, respectively, resulting in little or no change in annual GPP and NEE. GPP peaked during summer and was lowest in winter, whereas NEE had the highest carbon uptake in summer and highest carbon release in winter. Overall, the study showed a significant impact of climate and land-use change on the spatial and temporal distributions of GPP and NEE across Texas. Moreover, cropland, forestland, and shrubland showed more carbon uptake than carbon emission, which can help in soil carbon sequestration.