Temporal and spatial variations of meteorological elements and reference crop evapotranspiration in Alpine regions of Tibet, China.

Reference evapotranspiration (ET) is an important parameter for characterization of the hydrological cycle, and it is also important for agricultural, environmental, and other studies. In this study, by collecting the daily meteorological data of 31 base stations in high-altitude areas of Tibet for 35 consecutive years, the daily reference crop evapotranspiration (ET) of each base station is calculated by the ASCE Penman-Monteith formula. The Mann-Kendall method is applied primarily to test the trend of meteorological elements and ET. In addition, the software ArcGIS was used to generate spatial interpolation distribution maps of meteorological elements and ET so that the spatio-temporal variation trend of each base station is carried out. The ET in Shigatse shows an insignificant downward trend, while the other six regions show an upward trend, which is related to temperature and humidity directly with R of 0.23 and 0.67. The relative humidity (RH) and sunshine duration (SD) in Tibet show an overall three-step distribution in the east, middle, and west with RH decreasing from east to west and SD increasing from east to west. Annual cumulative ET of Ngari is the largest and decreases from west to east gradually, while ET increasing from east (lower elevation) to west (higher elevation) of Tibet with ranging from 730 to 1255 mm/year. This study not only is important for understanding of ET changes but also provides the preliminary and elementary reference for agricultural water management in Tibet with high elevation.