Disentangling slope-scale spatial variability of saturated hydraulic conductivity in the black soil region of northeast China using noise-assisted multivariate empirical mode decomposition

Abstract

Reliable estimates of saturated hydraulic conductivity (K_s) are usually difficult to obtain, as K_s is regulated by a variety of soil processes acting at different spatial scales that may obscure each other's impacts. We hypothesized that these scale-specific relationships could be well characterized with the aid of the noise-assisted multivariate empirical mode decomposition (NA-MEMD), thereby serving as a solid foundation for an accurate K_s prediction. The objective was to evaluate whether the incorporation of NA-MEMD could improve the estimation of K_s based on the multiscale associations it unraveled. On a typical slope transect of 860 m in the black soil region of northeast China, K_s, mollic epipedon thickness, bulk density, soil organic carbon content, total (ϕ) and effective porosities (ϕ_e), and particle size distribution were investigated at every 20 m. Prior to NA-MEMD, K_s was most strongly correlated with ϕ_e, and the linear regression models based on ϕ_e solely were satisfactory for K_s estimation at the scale of investigation. Adding other predictors significantly improved K_s prediction in calibration, but impaired it in validation. Upon decomposition by NA-MEMD, K_s was found to be significantly associated with each attribute at two scales of oscillation at least. Summing up the estimates of each K_s component derived from the properties at the equivalent oscillation scales, the results outperformed the traditional multiple linear regressions made at the investigation scale, when the same sets of predictors were used. The application of NA-MEMD, moreover, could save the tedious measurements of ϕ_e and ϕ. Excluding these two porosity-related properties, K_s estimates obtained by incorporating NA-MEMD were statistically similar or even better than those involving them before NA-MEMD. These findings demonstrate the great potential of NA-MEMD in untangling scale-dependent relationships of K_s with various processes and hold important implications for future estimations of K_s and other hydraulic properties in the black soil region of northeast China and similar regions.