Determination of critical soil water content and matric potential for wind erosion

Purpose Soil strength and thus stability concerning wind erosion are controlled by the soil water content. The concept of soil critical water content (Θcrit.) for deflation was extended to include matric potential (Ψcrit.) as well. The focus of this paper is to quantify the Θcrit. and Ψcrit. as the upper boundary for wind erosion or as the lower boundary for soil strength, to model the Ψcrit. at the immediate soil surface (0-0.2 cm) and to evaluate the effect of soil moisture upon erosion as a function of time and sampling height. Materials and methods The influence of soil water content and matric potential upon wind erosion was tested under wind tunnel conditions. Both were investigated in N = 49 wind tunnel experiments for a sandy Podzol topsoil from north-western Germany under a typical, constant free-stream velocity of 6.8 m s⁻¹. The surface (0-0.2 cm) water content was measured using a microwave sensor; matric potential at 0.5 cm depth was measured by laboratory tensiometer and sediment discharge was measured by modified Wilson and Cooke and combined suspended sediment traps and saltiphone. The superficial (0-0.2 cm) matric potential cannot be measured directly and was thus modelled using a van Genuchten retention curve model. Results and discussion The range of Θcrit. was determined to be between 4.9 and 4.0 wt.% at the soil surface; the Ψcrit. ranged from −75 to −43 kPa at a depth of 0.5 cm. Modelled matric potential at the soil surface resulted in lower values between −1.3 and −2.3 MPa as a consequence of steep gradients within soil layers. The reduction of the Θcrit. and Ψcrit. led to a steeply significant, almost 20-fold increase in soil losses, projected at 0.56 up to 11.00 Mg ha⁻¹ h⁻¹. The ranges of Θcrit. and Ψcrit. were ensured by both statistical ascertainment and impulse detection method for validation. The latter yields the specific process-based Θcrit. and Ψcrit. at the initiation of erosion and is hence considered the more precise threshold parameter. The impracticability of measuring the soil surface matric potential directly requires its modelling or derivation from measurement of relative humidity--which is only applicable for soils in equilibrium with atmospheric humidity. The modelled matric potentials are much lower compared to those measured at 0.5 cm depth in consequence of steep gradients within soil layers. Erosion data derived by wind tunnel experiments are not directly comparable to field data, as the wind tunnel device facilitates neither the avalanching process nor the development of a flow saturated with sediment amongst others. Discrepancies of measured sediment discharge compared to those in the literature are most of all attributable to a different reference surface area of erosion in wind tunnel experiments. Conclusions Wind tunnel experiments showed that the initiation of wind erosion can be attributed to certain ranges of Θcrit. and Ψcrit.. Specifying the interparticle (capillary) forces Ψm provides a precise parameter for describing soil strength. The Ψm can be regulated by conservation tillage. Measurements of soil sediment discharge at typical Θ and wind velocity indicate that the sandur plains in north-western Germany are endangered by wind erosion. Besides the regulation of soil moisture and matric potential, conservation soil management or the installation of shelter belts and hedges increase the overall surface roughness across various scales and hence reduce the wind erosion risk.