3.3 Spectral reflectance characteristics of the SAS
The spectral reflectance of the calibration and validation dataset
exhibited variations which is evident from Figure 2. In order to
understand the effect of soil salinity on spectral reflectance, the
samples were classified in five different salinity classes (Nawaret al. , 2011, 2015) and their class-wise average spectral
reflectance was calculated. The classes were calculated as non-saline
(1.05 dS m-1, 0-2 dS m-1), slightly
saline (3.17 dS m-1, 2-4 dS m-1),
moderately saline (5.72 dS m-1, 4-8 dS
m-1), strongly saline (11.58 dS m-1,
8-16 dS m-1) and very strongly saline (22.15 dS
m-1, >16 dS m-1). The
spectral reflectance characteristics by salinity class are presented in
Figure 3. There were conspicuous absorption deeps in the region close to
wavelength 427, 487 and 1917 nm. Besides, weak absorption features were
also observed at regions near to 950, 1414, 2206, 2380 and 2460 nm. It
is noticeable that the absorption at these wavelengths varies with the
salinity class and generally increased with increasing salinity level.
The absorption features close to 1414 and 1917 nm represent the
stretching of oxygen (O)-hydrogen (H) and bending of H-O-H of the free
water and its overtones. As the salinity increased the features became
more asymmetrical and extended (Nawar et al. , 2014). The shape of
the spectral signature in the VIS-NIR region was more or less similar in
all five salinity classes. Overall, the reflectance spectra showed a
tendency to decrease with the increase in EC. Similar results were
reported by Nawar et al. (2014). . The SAS of the coastal region
primarily has chlorides and sulphates of Na, Mg, Ca and K (Mahajanet al. , 2016), of which few are hygroscopic in nature (e.g.
MgCl2). These can absorb water vapor and increase soil
moisture content. So, the soil moisture content increases with an
increase in soil salinity, resulting in reduced reflectance or higher
absorption. These results are concordant with those by Wang et
al. (2018), Nawar et al. (2014) and Sidike et al. (2014).