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).