Hp Hn Sec 0p Hn Hb Sec 0b Bp [ 1 fp bp

where, Hb is the base radiance of dark body pixels (water body for NIR and the cloud shadow region for visible band) due to atmospheric effects; Hn is the normalized atmospheric radiance for nadir, 0b is the scan angle for the base pixels; (f)p and (b)p refer to forward and backward scattering for the pixel at wavelength.

Saha and Pande (1995a) used Landsat TM optical bands data for computation of regional surface albedo following the approach suggested by Goita and Royer (1992) (Fig. 1). An albedo image was generated by Kant (2000) for the snow and forest covered Himalayan mountain of India by using NOAA - AVHRR Ch1 and Ch2 data following an empirical relationship relating broad band albedo and narrow band albedo (Fig. 2).

Figure 1: (a) FCC and ( b) Albedo image of part of Doon Valley generated by using Landsat-TM optical data
Figure 2: (A) FCC (NOAA-AVHRR-Ch2,Ch1,Ch1) and (B) Albedo image of Part of Himalayan mountain, India

Land Surface Temperature

It is the temperature of the land surface i.e. kinetic temperature of the soil plus the canopy surface (or in the absence of vegetation, the temperature of the soil surface).

Surface temperature can be used for various agro-meteorological applications -

• surface heat energy balance study

• characterization of local climate in relation with topography and land use

• mapping of low temperature for frost conditions (night-time) or winter cold episodes (day/night)

• derivation of thermal sums (using surface temperature instead of air temperature) for monitoring crop growth and development conditions.

The land surface temperature can be estimated from remote sensing measurement at thermal IR wavelength (8-14 ^m) of the emitted radiant flux (Li) and some estimate of the surface emissivity (e). Land surface temperature (Ts) can be expressed using inverse Plank's equation (Mansor and Cracknell, 1994) :

where, C and are the first and second radiation constants (C = 3.742 x 10-16 Wm2 & C2 = 0.01444 mK); X is wavelength in m; e is the emissivity and LX is the spectral radiance (mw/cm2/Sr/^m).

The Normalised Difference Vegetation Index (NDVI) is used as a parameter for evaluating emissivity. The surface emissivity of a surface can be calculated using the following relationship (van de Griend, 1993) :

e = a + b.1n (i) + Ae where, a = 1.0094 and b = 0.047, 'i' is the NDVI of mixed pixel, Ae is the error in emissivity values. 'i' can be estimated by using following expressions (Valor and Caselles, 1996) :

where, Pv is the vegetation proportion; 'i' is the NDVI value of mixed pixel; ig and iv are the NDVI values of pure soil and pure vegetation pixel, respectively, p2v and p1v are the reflectances in NIR and red region for pure vegetation pixels; p2g , p1g are the reflectances in NIR and red region for pure soil pixels.

where, Ae is mean weighed value taking into account the different vegetation in the area, their structure and their proportion in it.

The NOAA - AVHRR channels 4 & 5 (10.3 - 10.3 and 11.5 - 12.5 ^m) are widely used for deriving surface temperature for the day time passes. The temperatures derived from channels 4 and 5 are slightly different due to atmospheric water vapour absorption. Thus, in the land surface retrieval algorithm, the incorporation of the difference between channels 4 & 5 could be useful in correcting for the atmospheric water vapour effect as a first degree approximation. An approach based on the differential absorption in two adjacent infrared channels is called "split-window" technique and is used for determination of surface temperature.

In split-window algorithm, brightness temperatures in AVHRR channel

4 (T4) and channel 5 (T5), mean emissivity e, i.e. (e 4 + e$)/2; difference in emissivity Ae , i.e. (e 4 - e 5), have been used for the estimation of land surface temperature using the following relation (Becker and Li, 1990) :

where, A, B and C are co-efficients worked out by statistical analysis and given by :

A = 1.274; B = 1 + {0.15616 (1- e) / e} - 0.482 (Ae / e2) C = 6.26 + { 3.98 (1- e) / e} + 38.33 (Ae / e2)

Brightness temperature (TB) values have been calculated by using the inverse of Planck's radiation equation :

where, V is the wave number of (Cm-1) of channel filter;

C1 = 1.1910659 x 10-5 (mw/m2/Sr/cm4) C2 = 1.43883 cmok

Ei, is radiance (mw/m2/Sr/cm); C is digital number; Si scaled slope; Ii is the intercept value.

Flow diagram of the methodology for deriving land surface temperature following "split-window" approach using NOAA - AVHRR data is shown in Fig. 3. Landsat TM (Saha and Pande, 1995a) and NOAA - AVHRR derived (Kant, 2000) land surface temperature images generated following above approaches are presented in Fig. 4 and Fig. 5, respectively.

Figure 3: Flow diagram of methodology of retrieval of land surface temperature using NOAA-AVHRR data.

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Figure 4: Surface temperature image generated by processing of Landsat - TM in part of western Doon Valley, Dehra Dun

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