(using the atmospheric temperature and humidity profile, 
cloud fraction) can be observed/inferred directly or 
indirectly via remote sensing (also see Figure 1). We 
describe in detail some of these variables below. 
Vegetation is characterized using the Normalized 
Difference Vegetation Index (NDVI), a ratio of the 
difference and the sum of the visible (0.55-0.681m) and 
near-infrared (0.725-1.10um) radiances measured by the 
Advanced Very High Resolution Radiometer (AVHRR) 
(Tucker, 1979, Goward et al. 1994). Currently, the 
vegetation index is measured by the MODIS - Moderate 
Resolution Imaging Spectroradiometer on the TERRA 
(1030am/pm overpass) and AQUA  (130am/pm 
overpass) The NDVI can be converted into leaf area 
index (LAI) using a Beer's Law (for exponential decay) 
as in Baret and Guyot (1991). MODIS has many more 
channels and much higher spectral and spatial resolution 
than AVHRR (Justice et al, 2002). Vegetation plays a 
role in the land surface hydrology, indirectly by (a) 
Providing a shadow effect on the ground for direct solar 
radiation (b) Interception of precipitation — (c) 
Transpiration by canopy (d) Changes the roughness and 
aerodynamic resistance of the surface for latent and 
sensible heat fluxes * 
Vegetation data is available at spatial resolutions of 
100s of meters. This may be inadequate for small basin 
studies. Additional higher spatial resolution data sets may 
be available from commercial satellite sensors (such as 
SPOT, IKONOS and LANDSAT TM). 
In the case of soil moisture, the microwave 
frequencies respond best to the variations in moisture 
content due to the polar nature of the water molecule. 
With the launch of the AQUA satellite (June 2002) and 
ADEOS I (December 2002), the data from the 
Advanced Microwave Scanning Radiometer (AMSR) is 
becoming available at two equatorial overpass times (130 
am/pm - AQUA and 1030 am/pm — ADEOS II) (Njoku et 
al. 2003). The retrieval algorithms for AMSR soil 
moisture include Jackson (1993) and Njoku and Li 
(1999). The C-band of the AMSR (6.9 GHz) has a better 
sensitivity than the 19.4GHz channels of the SSM/I to 
retrieve soil moisture. In the past soil moisture has been 
measured from space using the Special Sensor 
Microwave Imager (SSM/I) with the 19, 37 and 85 GHz 
channels (Lakshmi et al. 1997a, b, c, Jackson, 1997). 
This sensitivity is the maximum at lower frequencies (L- 
band: 1.4 GHz and C-band: 6.9 GHz) and decreases as 
the frequency of observation increases due to increased 
contribution from the atmosphere and vegetation. Soil 
moisture reflects the net land surface reaction to 
precipitation, infiltration, runoff and evapotranspiration. 
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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004 
Spatial resolution of soil moisture (50km) is 
inadequate for basin scale studies. Future microwave soil 
moisture remote sensing missions improve the spatial 
resolution to around 10 km, even so, this is yet 
inadequate for basin scale studies. Therefore, soil 
moisture data derived from satellite remote sensing needs 
to be used as a qualitative reference in the region in and 
around the basin. Research for 
disaggregation/decomposition of large soil moisture pixel 
data to higher spatial resolution using other sensors (viz., 
vegetation data) is currently underway. 
Surface temperature has been derived using 
the AVHRR (Price, 1984, Becker and Li, 1990); TOVS 
(TIROS Operational Vertical Sounder — Susskind et al. 
1997); AIRS (Advanced Very High Resolution 
Radiometer - Susskind et al. 2003); MODIS (Moderate 
Resolution Imaging Spectroradiometer - Justice et al. 
2002) and ASTER (Advanced Spaceborne Thermal 
Emission and Reflection Radiometer - Gillepsie et al. 
1998, Schmugge et al 1998). The energy emitted in the 
thermal channel that is observed by these sensors is 
directly related to the surface temperature. Surface 
temperature is directly related to the energy balance and 
the water budget, viz., increased evapotranspiration 
lowers the surface temperature. Surface temperature is 
currently observed at high spatial resolutions (90m) and 
can be readily used at the catchment scale to understand 
the thermal response of the catchment to radiation and 
evapotranspiration via the energy budget. 
Surface air temperature can be derived from 
the TOVS (TIROS Operational Vertical Sounder — 
Susskind et al 1997) and AVHRR (Advanced Very High 
Resolution Radiometer - Prince et al 1998). More recent 
data are available from AIRS (Advanced Infra-Red 
Sounder - Susskind et al 2003) as a Level 2 core product. 
The surface air temperature plays a dominant role in the 
determination of sensible heat flux by the difference 
through the difference in the land surface and the surface 
air temperature. 
Even though the air temperature is observed at 
coarse (50km) scales, the low spatial variability of air 
temperature over larger scales of 10s of km makes these 
measurements valid at the catchment scale. 
Precipitation is measured in the microwave 
region by the SSM/I, AMSR and TMI (Tropical Rainfall 
Measuring Mission Microwave Imager). The microwave 
frequency responds to the falling hydrometeors and this 
response can be translated into a rainfall rate.