Elucidation of Controls of Soil Water Storage in the Landscape Using Hilbert-Huang Transform: II.

Soil varies considerably from place to place and its characterization is fundamental to the understanding of soil-landscape processes. Different aspects of soil spatial variability including spatial similarity, periodicity, scale dependency and nonstationarity have been characterized using geostatistics, spectral analysis, and wavelet analysis. However, these methods assumed soil processes to be linear. Hilbert Huang transform (HHT), a combination of empirical mode decomposition (EMD) and Hilbert spectral analysis (HSA), can deal with both nonstationarity and nonlinearity. There are no instances of use of HHT in delineating soil spatial variability. It is an intuitive, direct, a posteriori and adaptive method that differentiates dominant processes directly from data through the unique EMD. This method was used in analysing spatial variability of soil water storage, which influences a range of environmental processes in a nonlinear manner. Field soil water storage (140 cm depth) was measured along a 128 point transect from St Denis National Wildlife Area, Saskatchewan, Canada, using Neutron Moisture Meter. There were different factors controlling soil water storage at different scales, which can operate together at a particular location. EMD separated the data into 6 different mode functions (intrinsic mode function, IMF) based on their characteristic scales from small to large. The first IMF separated very small scale variations might have been caused from measurement errors, localized activities of roots, micro or macro organisms. While, the second and third IMF represented the control from elevation and organic carbon, the fifth and sixth IMF represented the control from soil textures. HSA was used to calculate the instantaneous spatial scales (from instantaneous frequency) of soil variation explaining the scale and location dependent variability of soil water storage. This adaptive method is useful in delineating nonstationary and nonlinear controls of different factors of soil water storage.