Evaluation of Multivariate Regression Models to Predict Electrical
conductivity Using Vis-NIR and MIR Spectra
Abstract
Salts in the root zone have high spatial variability, changes rapidly
and adversely affects soil quality and crop productivity. Rapid
detection of electrical conductivity (EC) using visible-near infrared
(Vis-NIR) and midinfrared (MIR) spectroscopy can alleviate the adverse
effects on soil and plant, which through conventional method is time
consuming. Soils were collected from the Indo-Gangetic plains and
analyzed for EC using conventional, Vis-NIR, MIR spectroscopy and there
was wide variation in EC measured by the conventional method. The
spectral regions in 460-500 and 1890-1906 nm in the Vis-NIR region and
4200-4310, 5275-5280, 6660-6670, 7305-7310 and 8290-8300 nm in the MIR
region were sensitive to detection of EC. Partial least square
regression (PLSR) outperformed random forest regression (RF), support
vector regression (SVR), and multivariate adaptive regression splines
(MARS) both in Vis-NIR and MIR region during calibration. The ratio of
performance deviation (RPD), coefficient of determination (R2) and root
mean square error (RMSE) of the validation dataset were used to assess
the prediction accuracy and the predictive performance of PLSR (2.44,
0.84, 0.21), RF (1.95, 0.81, 0.20), SVR (2.09, 0.78, 0.22) and MARS
(1.81, 0.73, 0.27) models. PLSR model performed very well in the Vis-NIR
range; however, in the MIR range, RF (1.43, 0.52, 0.20), followed by
PLSR (1.40, 0.55, 0.35), performed better than SVR (1.39, 0.53, 0.35)
and MARS (1.29, 0.44, 0.37). Vis-NIR spectroscopy with PLSR algorithm
predicted EC better than MIR spectroscopy and would be the method of
choice for rapid estimation and prediction of EC in the study region.