Spatial and Temporal Variability of Soil Water Content in Two Regions of Southwest Germany during a Three‐Year Observation Period

Abstract

Core Ideas A set of all possible states of spatial soil water content variability in relation to the mean spatial soil water content is a contingent phase space. The left and the right outermost points of the phase space and its lower bound are determined by the variability of soil physical properties. Generation or dissipation of soil water content variability depends on its momentary location in relation to these points and the phase space bounds. Soil water content temporal dynamics can be divided onto sequential wetting‐drying periods. At the event‐scale spatial soil water content variability moves along the hysteretic loop, triggered by rainstorms with spatially highly variable intensities. The topsoil water content (SWC) plays a key role in partitioning energy and water fluxes at the land surface. Knowledge about its spatial and temporal variability is crucial for improving climate and hydrology modeling. We investigated SWC variability, its relation to the mean spatial soil water content (〈θ〉), and the variability of rainfall on the regional spatial and event temporal scales. We used a 3‐yr data set, obtained at 15‐min resolution from two soil moisture sensor networks (spatial extent: 25 by 25 km), set up at croplands in the Kraichgau and Swabian Alb regions in southwest Germany. The relationship of SWC standard deviation (σθ) versus 〈θ〉 was studied (σθ–〈θ〉). The closer σθ is located to the edge of the envelope, the σθ at the permanent wilting point (σθ–〈θwp〉), and the σθ at saturation (σθ–〈θs〉)—the anchor points—the easier it is to apprehend whether σθ will increase or decrease on a change in 〈θ〉. The σθ–〈θ〉 relationship forms combinations of concave and convex hyperbolas reflecting the variability of soil texture and depending on σθ in relation to the anchor points. Most σθ–〈θ〉 clockwise hysteresis cases occurred in an intermediate and intermediate‐wet state of SWC. The initiation of a clockwise hysteretic loop is initiated by preferential flow. The σθ phase space can be used to test whether hydrological or land surface models capture a realistic range of soil water heterogeneity.