Revealing the Controls of Soil Water Storage at Different Scales in a Hummocky Landscape

Soil water storage is controlled by topography, soil texture, vegetation, water routing processes, and the depth to the water table. Interactions among these factors may give rise to scale-dependent nonstationary and nonlinear patterns in soil water storage. The objectives of this study were to identify the dominant scales of variation of nonstationary and nonlinear soil water storage and delineate the dominant controls at those scales in a hummocky landscape using the Hilbert-Huang transform (HHT). Soil water storage (up to 140 cm) was measured along a 128-point transect established at St. Denis National Wildlife Area, Saskatchewan, Canada, using time domain reflectometry and a neutron probe. Empirical mode decomposition was used to decompose the measured soil water storage series into six different intrinsic mode functions (IMFs) according on their characteristic scales. The first IMF represented the variations at small scales, the second IMF might characterize the variations associated with microtopography and the landform elements. The IMF 3 was highly correlated with elevation and had the largest variance contribution toward the total variance among all the IMFs. The fourth IMF was correlated to organic C (OC), showing the long-term history of water availability, which may be a reflection of topographic setting or the elevation. The fift h and sixth IMFs were associated with elevation, soil texture, and OC but they contributed a small fraction of the total variance. Therefore, decomposition made through HHT was physically meaningful and provided improved prediction of soil water storage from topography, soil texture, and OC.