Specific-scale correlations between soil water content and relevant climate forcing factors across two climate zones

Soil water content (SWC) is determined through water exchanges between surface soil and the atmosphere, and changes in SWC will influence both energy conversions and water cycles. Being affected by different climate forcing factors (forcings), SWC is spatially and temporally variable, occurring at various intensities and scales. We compared scale-specific controls on SWC in a sub-humid and a semiarid climate zone based on a multivariate empirical mode decomposition (MEMD) method in conjunction with a multivariate dataset, including SWC (collected separately seven times within five soil layers to a depth of 500 cm) and eight measured climate forcing factors (i.e., precipitation, temperature, normalized differential vegetation index (NDVI), elevation, clay, silt, sand and soil organic carbon (SOC)). We also identified the percent variance of intrinsic mode functions (IMFs) and residuals relative to the original data. Two to three IMFs of SWC were further obtained in the two climate zones. IMFs explained the percent variance of SWC (79.3%), NDVI (77.3%), clay content (76.6%) and SOC (93.9%) to the original data in the sub-humid climate zone, while residuals from all environmental variables (except for SOC) yielded most of the total variance (55.5-93.2%) in the semiarid climate zone. Correlations between the two climate zones exhibited different IMF characteristics (scales) and different climatic conditions. Forcings of the two climate zones also played different roles in controlling SWC. Precipitation and temperature controlled large-scale SWC in both climate zones, namely, NDVI had an inverse effect on SWC between the two climate zones, SOC had a moderate and large-scale effect on SWC in the sub-humid climate zone and clay content had a comparatively critical effect on SWC in the semiarid climate zone. Scale-specific controls of soil properties on SWC were depth dependent in the sub-humid climate zone, while similar characteristics were obtained among the five soil layers in the semiarid climate zone. The specific scale dependence of SWC and relevant forcings were similar regardless of soil water condition (dry or wet). Our results offer insight into relationships between SWC and the main forcings of two different regional climate zones and their scale-dependent controls.