## 3 Results
### Spatial Distributions of Carbon Fluxes for the Selected Major Land Uses
#### 3.3.2 Spatial Distributions of NEE  
NEE is used to quantify carbon uptake (\(-\)ve NEE) and carbon release or emission (\(+\)ve NEE) of a given ecosystem. All climate zones showed carbon emission in December except some portion of the climate zones located along the western boundary of Texas. UC and ET climate zones had high carbon emission in December, whereas UC and LV climate zones had high carbon uptake in June. However, the HP climate zone had high carbon uptake in August (Figure 8).  
NEE (either carbon emission or carbon uptake) values from two different land uses at one climate zone, or the same land-use at two different climate zones were not consistent during the study period.  
For example, cropland of UC and HP climate zones had, respectively, carbon emission and carbon uptake in April 2015, which were reversed in April 2018 (Figure 8). The carbon emission rate decreased from April to June and increased from August to December each year in these two climate zones.  
Forestland of ET and TP zones had significantly different NEE each month except August and September. For example, ET and TP climate zones had carbon emissions and carbon uptakes, respectively, in April 2015, which were reversed in April 2018. In December, TP climate zone had low carbon emission, whereas ET climate zone had high carbon emission. Pastureland of ET and NC climate zones showed similar NEE distributions each month, except September and December. Pastureland of ET climate zone had high carbon emission, whereas the NC climate zone had low carbon emission in December each year. However, ET and NC climate zones had low carbon emission and low carbon uptake, respectively, in September 2018, whereas both climate zones had similar NEE distributions in September 2015.  
Shrubland of EP and TP climate zones had low carbon uptakes in April and May 2015 and low carbon emissions in April and May 2018, respectively. Both climate zones had average carbon uptakes in June 2015 and low carbon emissions in June 2018. However, both climate zones had similar NEE distributions from August to December. Developed land of both climate zones showed similar NEE distributions from April to June 2015 and April to May 2018 and different NEE distributions during the rest of the months. For example, developed land of the ET climate zone showed an average carbon emission, whereas the SC climate zone showed low carbon emission in December 2015 and 2018. Similarly, developed land of the ET climate zone had an average carbon uptake, and the SC climate zone had low carbon uptake from August to September in 2015 and 2018 (Figure 8).  
In May, the area with high carbon uptake (\(<\)\(-\)75 g C m\({}^{-2}\) mo\({}^{-1}\)) increased by 0.94% in 2018 from 2015; then in June and August, it decreased by 9.1% (Table 2). While there were no consistent changes in spatial coverage observed for particular NEE ranges in any month, some of the months had higher changes in spatial coverage for an average (\(-\)74 to \(-\)25 g C m\({}^{-2}\) mo\({}^{-1}\)) NEE range, whereas other months had higher spatial changes for low NEE range (\(-\)24 to 0 g C m\({}^{-2}\) mo\({}^{-1}\)). Areas with carbon emission range \(-\)74 to \(-\)25 g C m\({}^{-2}\) mo\({}^{-1}\) in June 2018 were 55.37% lower than in June 2015. However, each month in 2018, carbon uptake range 1 to 25 g C m\({}^{-2}\) mo\({}^{-1}\), had a higher area than in 2015 (Table 2). The changes in spatial coverage analysis showed 4.41% more area had low carbon uptake (\(-\)24 to 0 g C m\({}^{-2}\) mo\({}^{-1}\)) in December 2015 than in December 2018 and 15.92% more area had low carbon uptake in April 2015 than in April 2018. In May, 30.9% more area had low carbon uptake in 2015 than in 2018, whereas, in June, 32.3% more area had low carbon uptake in 2018 than in 2015 (Table 2).  
The analysis showed, in comparison, warmer and dryer climate zones and months had more carbon emission than the warmer and wetter zones. However, the dryer and cooler climate zones and months had more carbon uptakes across the state.