Field measurements

Data used in this study are from two experiments on land surface processes carried out in China, namely the Hei He International Field Experiment (HEIFE) (Mitsuta, 1993) and the Inner-Mongolia Grassland-Atmosphere Surface Study (MGRASS) (Su et al 1999).

3.1.1 HEIFE

The large-scale field experiment HEIFE has been carried out in the arid zone of north-west China during several years (1989-1995). In the area, long-term measurements (Table 2) were made by means of towers, radiometers, automatic weather stations, and by means of additional eddy correlation and Bowen ratio devices during several short-term intensive observation periods. One of the basic sites, named Zhang-Ye, is selected for present study. Zhang-Ye site is located in the central part of a large oasis with crops such as bean, corn and a smaller fraction of orchard. Windbreaks are widely used to protect crops. At the 1 km (ATSR pixel size) scale the surface is relatively homogeneous in a statistical sense.

Surface radiometric temperatures on the ground were measured using a radiometer (EKO Thermo-Hunter) operating in the spectral range 7-20 jim with a radiometric resolution 0.1 °C and mounted at a 1.5 m height with zenith view angle 2° at HEIFE sites. Measurements of heat fluxes from the surface to the atmosphere were made at 2.9 m height by means of eddy correlation systems which consists of three-dimensional sonic anemometerthermometer (SAT: Kaijo, DAT-300 with TR-61A Probe), infrared hygrometer (Kaijo, AH-300), Clinometer (Kaijo, CM-100) and rotator (Kaijo,

502MSA). Profiles of wind speed, air temperature and humidity were measured on a 20 m high tower between the 0.5 and 20 m levels.

3.1.2 IMGRASS

IMGRASS's field experiment was carried out in Inner Mongolia grassland in north-east of China during May to August in 1998. Its main aim is to understand the effects of changing vegetation on the hydrologic and heat cycle of Inner Mongolia grassland and to provide surface measurements of water, heat and trace gases over various scales for development and validation of remote sensing algorithms. The land cover in the experiment area is grassland with various species of grass and fractional cover. The site in the present study, Baiyinsumu, has sparse grass cover and is a so-called degraded prairie.

Heat flux measurements near the surface were made by using the same method and instruments as those in HEIFE campaign but at 4.9m height. A 10m high tower was set up to measure wind speed, air temperature and humidity profiles with 5 levels at 0.5m, 1m, 2m, 4m and 8m height respectively. The baseline measurements, which include vertical profiles of wind speed, air temperature and humidity can be used to estimate sensible and latent fluxes. This method has been used successfully to estimate sensible and latent heat fluxes in early HEIFE studies (see Zuo et al, 1993). An Eppley pyrgeometer PIR with spectral range 4-50 |am was used to measure radiant flux density at IMGRASS site 4 from which surface temperature was obtained.

During the period of 26-31 July 1998, directional surface brightness temperature was measured using an IR-AH portable digital radiation thermometer operating in the spectral range 8-13 fim, footprint diameter = [distance/ 50] (m). Observations at nadir, 23° and 52° zenith view angle were done. The measurement height at nadir was 1.5 m and corresponds to a footprint diameter of about 3 cm. The field of view of the sensor is therefore small enough so that unobstructed bare soil can be seen at nadir view angle. Leaf area index of 0.5 was determined by counting grass leaf area in a meter square (Su et al, 1999).

At both sites, standard meteorological radiosounding data closest to the satellite overpass time were collected to perform atmospheric correction. At HEIFE site, lower level radiosounding and tethered-balloon measurements were also used for atmospheric correction of ATSR data.

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