A new perspective on the spatio-temporal variability of soil moisture: temporal dynamics versus time-invariant contributions

Knowledge about the spatio-temporal variability of soil moisture is essential to understand and predict processes in climate science and hydrology. A significant body of literature exists on the characterization of the spatial variability and the rank stability (also called temporal stability) of absolute soil moisture. Yet previous studies were generally based on short-term measurement campaigns and did not distinguish the respective contributions of time-varying and time-invariant components to these quantities. In this study, we investigate this issue using measurements from 14 grassland sites of the SwissSMEX soil moisture network (spatial extent of approx. 150× 210 km) over the time period May 2010 to July 2011. We thereby decompose the spatial variance of absolute soil moisture over time in contributions from the spatial variance of the mean soil moisture at all sites (which is time-invariant), and components that vary over time and are related to soil moisture dynamics. These include the spatial variance of the temporal soil moisture anomalies at all sites and the covariance between the site anomalies to the spatial mean at a given time step and those for the temporal mean values. The analysis demonstrates that the timeinvariant term contributes 50–160 % (on average 94 %) of the spatial soil moisture variance at any point in time, while the covariance term generally contributes negatively to the spatial variance. On the other hand, the spatial variance of the temporal anomalies, which is overall most relevant for climate and hydrological applications because it is related to soil moisture dynamics, is relatively limited and constitutes at most 2–30 % (on average 9 %) of the total variance. Nonetheless, this term is not negligible compared to the temporal anomalies of the spatial mean. These results suggest that a large fraction of the spatial variability of soil moisture assessed from short-term campaign may be time-invariant if other regions present a similar behavior. Moreover, we find that the rank (or temporal) stability concept, when applied to absolute soil moisture at the sites, mostly characterizes the time-invariant patterns. Indeed, sites that best represent the mean soil moisture dynamics of the network are not the same as those that best reflect mean soil moisture at any point in time. Overall, this study shows that conclusions derived from the analysis of the spatio-temporal variability of absolute soil moisture need not generally apply to temporal soil moisture anomalies, and hence to soil moisture dynamics.