## 2 Materials and Methods
### Data  
This study used SMAP Level 4 carbon products (L4C) available at 9 km spatial and daily temporal resolutions [47; 65]. Soil Moisture Active and Passive (SMAP), an environmental research satellite, was launched on 31 January 2015, by the National Aerospace Space Administration (NASA) to monitor global carbon fluxes, soil moisture and freeze/thaw state at different spatial and temporal resolutions using radar and radiometric instruments. However, with the failure of the SMAP active radar, only passive data were available from 31 March 2015, onwards. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Location** & **Latitude (\(\uparrow\))** & **Longitude (\(\uparrow\))** & **Climate Zone** & **Ecoreion** & **Land-Use** & **Soil Texture** \\ \hline
**UC Cropland** & 29.03 & \(-\)96.19 & UC & WGCP & Cropland & Sandy Loam \\
**HP Cropland** & 33.15 & \(-\)102.27 & HP & HP & Cropland & Sand \\
**ET Forestland** & 31.18 & \(-\)94.75 & ET & SCP & Forestland & Loam \\
**TP Forestland** & 30.69 & \(-\)104.12 & TP & CD & Forestland & Silt Loam \\
**ET Pasteland** & 31.04 & \(-\)95.89 & ET & ECTP & Pasture Hay & Loam \\
**NC Pasteland** & 31.48 & \(-\)96.88 & NC & TBP & Pasture Hay & Clay \\
**TP Shrubland** & 30.71 & \(-\)102.79 & TP & CD & Shrubland & Loam \\
**EP Shrubland** & 30.86 & \(-\)100.21 & EP & EP & Shrubland & Clay \\
**ET Developed land** & 30.08 & \(-\)95.98 & ET & WGCP & Developed land & Sandy Loam \\
**SC Developed land** & 29.93 & \(-\)97.69 & SC & TBP & Developed land & Clay \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Research site locations (center of the SMAP grid cell) and their corresponding climate zone, land-use, and soil texture. Note: Full names of climate zone and ecoregion are presented in Figure 1.  
Figure 2: Cumulative distribution of precipitation (**a**), and average annual distribution of temperature across Texas (**b**) using precipitation and temperature data from 2001–2018. The cumulative precipitation plot for each climate zone was developed using National Climatic Data Center (NCDC) daily precipitation data, whereas the annual average temperature (calculated using NCDC daily temperature data) was used to develop the spatial average temperature map across Texas. The x-axis of the cumulative distribution of the precipitation plot includes a fraction of time, which means 0.5 represents 50% of the time (9 years for this plot), and 1.0 represents 100% of the time (18 years for this plot). This study relied on model-derived data products of surface and root zone soil moisture and carbon net ecosystem exchange compiled using SMAP L4C products, which provided daily estimates of NEE to determine the spatial and temporal patterns of CO\({}_{2}\) fluxes across Texas. This study also analyzed modeled SMAP GPP component to understanding the spatial and temporal distributions of GPP for five major land-use categories in seven different climate zones across Texas. The SMAP NEE is not a direct measurement or observation from the satellite, rather, it is a modeled estimate which includes daily root-zone soil moisture inputs to constrain GPP, surface soil moisture to constrain heterotrophic respiration (Rh), and other input parameters to estimate daily NEE [60]. Uncertainty is reported on an annual basis as NEE root mean square error (RMSE) considering all sources of errors such as input data (e.g., temperature, soil moisture), heterogeneity (e.g., land-use heterogeneity), and model parameterization. The total annual and daily uncertainty are estimated as 30 g C m\({}^{-2}\) year\({}^{-1}\) and 1.6 g C m\({}^{-2}\) d\({}^{-1}\), respectively [47]. [PERSON] et al. [60] validated SMAP NEE using in-situ EC flux tower observations from 26 validation sites located all over the world. They found NEE performance within the targeted accuracy threshold (RMSE \(\leq\) 1.6 g C m\({}^{-2}\) d\({}^{-1}\)) for NEE over 66% of the global domain.