Ec150

The eddy covariance (EC) observation system was positioned 15 m above the ground. It comprises a three-dimensional ultrasonic anemometer (CSAT3, Campbell Scientific Ltd., Logan, UT, USA) and an infrared CO₂/H₂O analyzer (EC150, Campbell Scientific Ltd., Logan, UT, USA). Global radiation was monitored using a 4-component net radiometer (NR01, Campbell Scientific Ltd., Logan, UT, USA) installed at a height of 9 m. Additionally, the meteorological observation system includes air temperature and humidity sensors (HMP155A-L, Campbell Scientific Ltd., Logan, UT, USA), wind speed sensors (010C-1, Met One Instruments Inc., Grants Pass, OR, USA), wind direction sensors (020C-1, Met One Instruments Inc., Grants Pass, OR, USA), and an atmospheric pressure meter (CS100, Campbell Scientific Ltd., Logan, UT, USA). These instruments are programmed to automatically record routine meteorological data, such as average wind speed, temperature, and air pressure every 30 minutes.

In addition to daily evaporative losses measured by lysimeters, larger-scale alpine meadow ecosystem evapotranspiration rates (W·m⁻²) were measured by eddy covariance instrumentation. An eddy covariance tower was installed at a distance of 50 m from our experimental sites and at a measurement height of 5.8 m above ground in September 2011 (EC150, Campbell Scientific Inc., Logan, UT, USA) [49 (link)]. EC150 is an open-path analyzer specifically designed for measuring eddy covariance flux. Combined with the CSAT3A sonic anemometer, these two components of an open-path eddy-covariance system simultaneously measure carbon dioxide, water vapor, air temperature, barometric pressure, three-dimensional wind speed, and sonic air temperature. The half-hourly latent heat flux values measured by eddy covariance were converted to evapotranspiration rates and then were summed to daily evapotranspiration values. Net radiation was measured by four-component radiation (CNR1, Kipp & Zonen, Delft, The Netherlands). The measurements included short- and long-wave upwelling and downwelling radiation. Both variables are measured at the same height as the eddy covariance instrumentation.