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

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 (σ θ ) versus 〈θ〉 was studied (σ θ –〈θ〉). The closer σ θ is located to the edge of the envelope, the σ θ at the permanent wilting point (σ θ –〈θ wp 〉), and the σ θ at saturation (σ θ –〈θ s 〉)—the anchor points—the easier it is to apprehend whether σ θ will increase or decrease on a change in 〈θ〉. The σ θ –〈θ〉 relationship forms combinations of concave and convex hyperbolas reflecting the variability of soil texture and depending on σ θ in relation to the anchor points. Most σ θ –〈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 σ θ phase space can be used to test whether hydrological or land surface models capture a realistic range of soil water heterogeneity.