SOIL WATER CONTENTS: VARIABILITY, VARIATION, AND VARIETY

Understanding spatio-temporal variations of soil water contents becomes an imperative as the needs of diagnostics, monitoring, forecast, and management of soil water resources are to be met. Knowing variability of soil water contents is essential, in particular, in inverse modeling and vadose zone flow model calibration, sensitivity analysis and uncertainty characterization, SWC sensor data assimilation, validation of remote sensing products, calibration/validation of near-surface sensors, sampling campaign design, precision farming applications, and other project types. Techniques are in use to perform both additive and multiplicative decomposition of spatio-temporal variations of soil water content into spatial (temporally stable), temporal (spatially independent), and random components. An example will be presented to illustrate the variability inflation due to ignoring the temporally stable component of variation. Results of simulation studies will be presented that (a) quantify effects of several local controls, such as soil hydraulic properties and root water uptake on the temporally stable components of soil water variation, (b) evaluate the combined effects of climate and soil hydraulic properties on the uncertainty of the temporally stable component, (c) in the range of low water contents, imply a positive linear relationship between mean soil moisture and its standard deviation which is controlled by the parameter defining the shape of soil water retention curves and the spatial variability of saturated hydraulic conductivity. Scale effects on soil water variability will be illustrated, and the need in variability studies for old and new water will be discussed. Understanding interactions of soil water content variability controls presents the new frontier of soil water research.