Landsat ETM+, Terra MODIS and NOAA AVHRR: Issues of scale and inter-dependency regarding land parameters.
2. Data and Processing
A MODIS satellite image was acquired on the July 10, 2000, approximately 15 -25 minutes after the acquisition of the Landsat ETM+ image. There are numerous little clouds covering the Zhanghe Irrigation District, but not exceeding 8 percent of the area. The haze which is observed in the north east and south part of the ETM+ image is not so evident in the MODIS image, probably due to the later time of overpass, or due to spectral differences. The NOAA AVHRR image used in this study is captured on July 9, 2000, since there were no images available in the Satellite Active Archive for July 10. It is a cloud free image, and possibly existing small clouds are not visible due to low resolution.
Except from the original band data, vegetation index products can also be ordered from the EOSDIS center. These are 16 days maximum NDVI (normalized difference vegetation index) and EVI (enhanced vegetation index), aggregated in 500 m pixel size, with a product name MOD13A1. A compositing algorithm is implemented from the MODIS Science Team to account for cloud cover in the 16 days period that each product covers (van Leeuwen et al., 1999). Two sets of products have been used, one covering maximum NDVI from June 25 to July 10, 2000 (MOD16a), and the other from July 18 to July 26, 2000 (MOD16b). Alth ough there are no missing data, the effect of partial cloud cover is evident in the data set MOD16a.
The spectral bands of Landsat 7 ETM+ were checked upon atmospheric disturbance by creating the broadband surface albedo (Equation 1) after the method found in Ahern et al. (1977) and Koepke et al. (1985). It was validated against the physical features value of albedo (Table 2) taken from Tasumi et al. (2000).
where r
0 is the broadband surface albedo (-), r
p the planetary reflectance (-),r
a the in-band path solar irradiance (-) and
tsw the single-way transmittance of the atmospheric layer (-).
Table 2: Expected broadband surface albedo after Tasumi et al. (2000)
|
| Physical Feature | Expected Albedo | Broadband | Surface |
|
| Grass or Pasture fields | 0.15 to 0.25 | | |
| Maize Field | 0.14 to 0.22 | | |
| Paddy Field (Rice) | 0.17 to 0.22 | | |
| Water (30° < solar elevation < 50°) | 0.025 to 0.06 | | |
|
