The assessment on band sensitivity to changes in crown, trunk and soil parameters at different surface roughness.

The previous section assesses the effect of surface roughness to the possible effectivity of X-, C-, and L-band in detecting changes within crown, trunk and ground layers of oil- palm canopies. This preliminary judgement is based on the observation on how varying surface roughness has caused the shift in dominance from one scattering mechanism to another, thus resulting in varying band sensitivity to different parts of tree canopies. This section will further assess this preliminary assessment by further investigating the band sensitivity to varying crown, trunk and soil parameters at different surface roughness.

7.3.1. Band sensitivity to changes in crown parameters at different surface roughness.

The crown parameters to be investigated are as follows: i) leaflet parameters and ii) frond parameters. Leaflet parameters to be investigated are leaflet radius and moisture content, whereas frond parameters are frond radius and moisture content. The model simulates radar backscatter using oil-palm tree parameters from 12 years old plantation.

7.3.1.1 Sensitivity to leaflet parameters.

SAR sensitivity to leaves plays an important role while assessing the suitability of employing a particular SAR configuration for monitoring different types of vegetation. This sensitivity depends mainly on the size of the leaves with respect to the incoming wavelength, with highest sensitivity achieved when the size of the leaves are comparable to the incoming wavelength. Different responses have therefore been observed using different SAR wavelengths for different vegetation. Baronti et al (1995) have shown that L-band is capable of identifying well developed " broad leaf" crop (e.g. sunflower), while C-band data allows discrimination among different kinds of herbaceous crops with smaller leaf sizes. In the investigation using Japanese Cypress trees comprising small leaves of around 2 mm in diameter, Hiroshawa et al (1989) have also shown similar trend. This work has demonstrated that the effect of removing leaves from such trees is more significant to short wavelength X-band than to C- and S-band.

Better SAR sensitivity to vegetation comprising leaves of comparable size to the wavelength also plays an important role while the attempt is to retrieve crop or forest parameters such as LAI. Using both a radar backscatter model and airborne/spaceborne SAR, Macelloni et al. (2001) have shown that while L-band is sensitive to changes in LAI of broad leaf crops such as sunflower, it is insensitive to LAI of narrow leaf crops such as wheat. Albeit less sensitive than L-band, C-band has also been shown to be reasonably sensitive to LAI of sunflower crops and insensitive to LAI of wheat crops. The diameter of sunflower leaves is in the order of 20 cm which is comparable to L-band wavelength, thus explaining the band sensitivity. While C-band is still sensitive to LAI of leaves of much larger dimension to the wavelength, it appears to be insensitive to leaves with much smaller dimension such as wheat (leaf width around 2 cm).

The main aim of this research is to assess SAR sensitivity to oil-palm age. Chapter 5 has shown that one important growth parameter for oil-palm is the increase in leaflet radius at the middle frond, from 1.85 cm at 4 years to 2.25 cm at 12 years old. This radius range is comparable to C-band wavelength. Results from chapter 6 therefore suggest that the higher sensitivity achieved by ERS Cvv may be due the strong interaction with oil-palm leaflets. This section therefore attempts to further assess the sensitivity of X-, C-

and L-band to detect the growth stage of oil-palm plantation via their sensitivities to oil- palm leaflet parameters. As the size of the leaves is the most important factor, the first part of this section therefore investigates the sensitivity of X-, C- and L-band to changes in leaflet radius at different surface conditions. The amount of radar interaction with leaves, to a large extent, also depends on the leaf moisture content. The second part therefore investigates the band sensitivity to changes in leaf moisture content. In the first investigation, the leaflet radius is increased from 0.5 to 15.0 cm while keeping other leaflet, frond, trunk and ground parameters constant. In the second investigation, the effect of increasing leaf gravimetric moisture content from 0.1 (very dry leaf) to 0.9 (very wet leaf) on both small and large leaves are investigated.

i) Sensitivity to changes in leaflet radius.

Results from the simulation show the strong dependence of SAR sensitivity to the dimension of leaves under observation. Figures 7.5a, 7.5b, and 7.5c show that all co- polarised bands are very sensitive to changes in leaflet radius. Their sensitivities appear to follow similar behaviour in which they show some "resonance effect" with peak sensitivities at leaf dimensions similar to the wavelengths. These peaks are at leaf radius 1.5, 2.5 and 10 cm for X-, C- and L- band respectively. After reaching these peaks, the sensitivities appear to decrease significantly, before starting to increase again in a slower manner.

Surface roughness appears to have more effect on the sensitivities when the radius of the leaves are smaller than 5 cm. At radii larger than this, the sensitivities appear to be independent of surface roughness. At smaller leaf radii, higher availability of gaps within the crown appears to have allowed more signal to reach the trunk and ground to produce surface related scattering (GS and TGS). This effect, however, is only observed at certain range of surface roughness: i) smooth surfaces for X-band, ii) smooth, and rough surfaces for C-band and iii) smooth to medium surfaces for L-band (see figures

7.5a, 7.5b, and 7.5c). The best sensitivity for each band is therefore achieved when

band, ii) medium surfaces at C-band and iii) rough surfaces at L-band.

However, when the leaf dimensions are comparable to the wavelengths, the surface roughness appears to have no effect on band sensitivities to leaflet radius. Under this circumstance, the interaction between radar signal and leaves becomes very high, and the contribution from CVS becomes consistently large to overcome any disturbances from GS and TGS. There is also a significant contribution from CGS at this range of radius, which add significantly to band sensitivities (see figure 7.6a and 7.6b). Section 7.2 has suggested that the sensitivities of X-, C-, and L-band to crown parameters may be hampered by the strong occurrence of TGS and GS at certain range of surface roughness. These results, however, show that when the leaf sizes are comparable to the incoming wavelengths, such sensitivities becomes more independent of the ground condition. This observation is also supported by previous works which have demonstrated the good sensitivity of C-band for discriminating vegetation comprising small leaves, and L-band for vegetation comprising broad leaves without having to consider the condition of the ground (Baronti et al, 1995; Ferrazoli et al, 1997).

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