2.3 Meteorological data collection
All meteorological variables needed to calculate ET0 using the various models were obtained or estimated from the weather station at Haibei Station, and included relative humidity (RH), wind speed (WS), net radiation (Rn), total radiation (Rs), extraterrestrial solar radiation (Ra), soil heat flux (G), maximum air temperature (Tmax), vapor pressure deficit (VPD), minimum air temperature (Tmin) and mean air temperature (T). The radiation were measured by four radiometers (CNR4, Kipp&Zonen, Netherlands) at 1.5 m height; relative humidity, wind speed and mean air temperature were measured at 1.5 m height (HMP45C, Vaisala, Finland), and wind speed was converted to 2 m height for calculating ET0. The soil heat flux (G) was measured by three heat flux plates (HFT-3, Campbell, USA), which were separately buried 5 cm beneath the surface. Half-hourly means of meteorological data were stored by a data logger (9210 XLITE, Sutron, USA).