# Evapotranspiration in Dynamic of Highland Barley Using PML ET Product in Tibet
## 2 Materials and Methods
### Data Collection  
PML-V2 ET components product was used to investigate the characteristics and variations in ET components of highland barley in this study. This product was based on the PML-V2 model, which was initially developed by [PERSON] et al. [18] and [PERSON] et al. [19] and further revised by [PERSON] et al. [20] and [PERSON] et al. [1, 21] through the improvements in the surface conductance model. Coupled with MODIS data, including land use, albedo, leaf area index, surface-specific emissivity, and GLDAS meteorological data, including solar radiation, air temperature, precipitation, and water vapor pressure, a PML-V2 algorithm was employed to generate global ET components product with 500 m and 8-day resolution. These product data were evaluated using the measurements at 95 widely distributed flux towers globally, with the RMSE of 0.69 mm/day [1]. It was also validated across different regions in China and proved to be reliable data for investigating the variations in ET components across different ecosystems [36; 37; 38]. In this study, the PML-V2 ET product from 2001 to 2020 covering the study area was obtained from the Google Earth Engine ([[https://developers.google.com/earth-engine/datasets](https://developers.google.com/earth-engine/datasets)]([https://developers.google.com/earth-engine/datasets](https://developers.google.com/earth-engine/datasets)), accessed on 27 July 2021).  
The land use data of Tibet in 2015 were collected from the Resource and Environment Science and Data Center, Chinese Academy of Sciences ([[https://www.resdc.cn](https://www.resdc.cn)]([https://www.resdc.cn](https://www.resdc.cn)), accessed on 27 July 2021). These data were interpreted from the Landsat images, with 30 m resolution. Meteorological data, including sunshine duration (SD), air temperature (T), precipitation (P), and air pressure (AP), water vapor (WP), and relative humidity (RH) were obtained from the China Meteorological Data Service Center at 5 meteorological stations during the same period with ET product. The distribution of meteorological stations is shown in Figure 1, and detailed information on these stations is listed in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Site Name** & **Altitude** & **Longitude** & **Latitude** & **P** & **T** & **AP** & **WP** & **SD** & **RH** \\
**(m a.s.l)** & **Longitude** (\({}^{\circ}\)E)** & **Latitude** (\({}^{\circ}\)N)** & **(mm)** & **(\({}^{\circ}\)C)** & **(hpa)** & **(hpa)** & **(hpa)** & **(h)** & **(\%)** \\ \hline Lazi & 4000 & 87.60 & 29.08 & 356.28 & 7.70 & 623.76 & 3.98 & 2941 & 32.37 \\ Nannulin & 4000 & 89.10 & 29.68 & 500.88 & 6.39 & 625.85 & 4.56 & 2765 & 40.59 \\ Mongzhugongka & 3804 & 91.73 & 29.85 & 590.78 & 7.04 & 640.61 & 5.08 & 3020 & 44.57 \\ Zedang & 3551 & 91.77 & 29.25 & 402.80 & 9.40 & 660.53 & 5.46 & 2814 & 42.18 \\ Jiangzi & 4040 & 89.60 & 28.92 & 284.62 & 5.84 & 624.35 & 4.57 & 3104 & 44.64 \\ \hline \end{tabular}
\end{table}
Table 1: Detailed information on meteorological stations.  
Figure 1: Location of the study area and the distribution of meteorological stations and cropland of highland barley.