## 3 Results  
### Evaluation of Satellite \(Co_{2}\) Using In-Situ Measurements  
Open-developed land of the PVAMU research farm, located in East Texas climate zone, was used to evaluate SMAP NEE product. Net ecosystem exchange data were collected from the EC flux tower at 30-min intervals and converted into daily and monthly values. The footprint area of SMAP satellite is 9 km \(\times\) 9 km, while the flux tower has a 400 m \(\times\) 400 m footprint area. This study compared daily and monthly in-situ and SMAP GPP and NEE (Figure 4). Despite considerable scale difference between these two measurements, there were reasonable agreements between in-situ measurements and SMAP GPP and NEE estimates. In addition, the agreement was better during certain months than others. For example, Figure 4 shows reasonable agreement between in-situ measurements and SMAP NEE estimates during the June-August, and November-December periods. However, GPP showed better agreement during the winter. At most of the ecosystems, carbon productivity tended to increase during summer and reduce during winter while carbon exchange tended to be negative (carbon sinks) during summer and positive (carbon sources) during winter (Figure 4). For a quantitative assessment, the performance of SMAP NEE and GPP were evaluated using correlation coefficient (R), and root mean square error (RMSE). The correlation coefficient of 0.42 and 0.43 (daily) and 0.54 and 0.67 (monthly), respectively, for NEE and GPP, can be considered a reasonable agreement between in-situ measurements and SMAP estimates considering the scale difference between the two datasets. In addition, RMSE of 2.42 g C m\({}^{-2}\) d\({}^{-1}\) and 32.1 g C m\({}^{-2}\) mo\({}^{-1}\), respectively, for daily and monthly NEE values can generally be considered good, but are higher than the estimated uncertainty threshold which is mainly due to the land-use heterogeneity at this location. Additionally, RMSE of 2.96 g C m\({}^{-2}\) d\({}^{-1}\) and 62.5 g C m\({}^{-}\)2 mo\({}^{-1}\), respectively, for daily and monthly GPP values can also be considered reasonable at these scales.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Climate** & \multicolumn{3}{c}{**GPP (g C m\({}^{-2}\) year\({}^{-1}\))**} & \multicolumn{3}{c}{**NEE (g C m\({}^{-2}\) year\({}^{-1}\))**} \\ \cline{2-6}
**Zone** & **2016** & **2017** & **2018** & **2016** & **2017** & **2018** \\ \hline HP & 811 (207) & 811 (216) & 625 (199) & \(-\)90.2 (42) & \(-\)93.2 (47) & 23.2 (45) \\ LRP & 897 (139) & 819 (115) & 616 (117) & \(-\)102.4 (41) & \(-\)86.4 (32) & 57.1 (31) \\ NC & 1402 (198) & 1241 (194) & 1032 (191) & \(-\)160.7 (40) & \(-\)98.9 (37) & 41.3 (28) \\ ET & 1907 (161) & 1818 (187) & 1657 (217) & \(-\)102.4 (47) & \(-\)66.4 (47) & 42.1 (62) \\ TP & 525 (133) & 478 (121) & 408 (103) & \(-\)64.9 (45) & \(-\)47.7 (42) & 0.1 (34) \\ EP & 1093 (229) & 944 (204) & 820 (203) & \(-\)149.4 (51) & \(-\)87.1 (41) & 3.1 (42) \\ SC & 1623 (210) & 1490 (248) & 1366 (231) & \(-\)206.7 (56) & \(-\)150.0 (51) & \(-\)57.1 (44) \\ UC & 2070 (348) & 2027 (355) & 1941 (352) & \(-\)89.6 (62) & \(-\)90.6 (64) & \(-\)30.4 (66) \\ S & 1219 (211) & 1065 (188) & 1056 (198) & \(-\)149.7 (79) & \(-\)78.9 (67) & \(-\)44.9 (59) \\ LV & 1540 (369) & 1444 (375) & 1411 (325) & \(-\)65.7 (55) & \(-\)9.6 (56) & 48.8 (84) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Annual NEE and GPP estimates in each climate zone across Texas. The standard deviation (SD) values are presented inside parentheses.