# A Typical Small Watershed in Southwestern China Is Demonstrated as a Significant Carbon Sink
## 2 Materials and Methods
### Eddy Covariance and Ancillary Data  
Three sets of integrated CO\({}_{2}\)/H\({}_{2}\)O open-path gas analyzers and three-dimensional sonic anemometers (IRGASON, Campbell Scientific Inc., Logan, UT, USA) were used to observe carbon flux between forest, cropland, and non-timber forest ecosystems and the atmosphere, as well as the three wind speed components (uy, uz, and ux). The instruments were installed on flux towers at three sites, with the instrument installation height at the forest site being 12 m (Figure 1d), and the instrument installation height at the cropland site (Figure 1e) and non-timber forest site (Figure 1c) being 6 m. Air temperature and relative humidity sensors (HMP155, Vaisala, Helsinki, Finland), photosynthetically active radiation sensors (LI190R, Campbell Scientific Inc., Logan, UT, USA), and tipping-bucket rain gauges (TE525 MM; Campbell Scientific Inc., Logan, UT, USA), which record precipitation, were  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sites** & **Geographical** & **Altitude** & **Vegetation** & **Observation** & **Observation Time** \\  & **Location** & **(m)** & **Types** & **Height (m)** & **Observation Time** \\ \hline Forest & 28\({}^{\circ}\)8\({}^{\prime}\)18.4\({}^{\prime\prime}\) N, & 2343 & _Pinus_ & & October \\  & 10\({}^{\circ}\)19\({}^{\prime}\)20.2\({}^{\prime\prime}\) E & & & dominates & \\ Cropland & 28\({}^{\circ}\)8\({}^{\prime}\)36.0\({}^{\prime\prime}\) N, & 2359 & _Potaetes_ and & & October \\  & 10\({}^{\circ}\)20\({}^{\prime}\)53.9\({}^{\prime\prime}\) E & & _ maize_ & 6 & 2021–September 2023 \\ Non-timber & 28\({}^{\circ}\)7\({}^{\prime}\)55.6\({}^{\prime\prime}\) N, & & _Zanthoxylum_ & & October \\ forest & 10\({}^{\circ}\)17\({}^{\prime}\)20.6\({}^{\prime\prime}\) E & & _Maxim_ & & 2021–September 2023 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Overview of eddy covariance sites for forest, cropland, and non-timber forest in the Reshuihe River watershed.  
Figure 1: (a) Location of the Reshuihe River watershed in Southwestern China; (b) watershed boundary and the distribution of forest, cropland, and non-timber forest within the watershed; eddy covariance systems installed in (c) non-timber forest, (d) forest, and (e) cropland.  
also installed on the flux towers at each site to record meteorological data. Soil temperature and moisture were obtained using time-domain reflectometer (TDR) moisture sensors (True TDR-315, Acclima, Inc., Meridian, ID, USA) installed in the soil at a depth of 10 cm near each flux tower. VPD was computed based on the air temperature and relative humidity measurements. A data collector (CR1000X, Campbell Scientific Inc., Logan, UT, USA) collected the carbon fluxes and 3D wind speed data at 10 Hz, as well as the averages or sums of meteorological data recorded at 30 min intervals. The instrument was maintained and inspected at an average of every 45 days.  
In this study, we collected carbon flux and environmental factor data from October 2021 to September 2023 at forest, cropland, and non-timber forest sites. Due to field instrument failures and power supply problems, some data were missing, including soil temperature data at forest site from 9 January 2022 to 25 June 2022, and carbon flux data and some meteorological data at the non-timber forest site from 1 March 2022 to 13 May 2022.