Wind erosion is a widespread process in arid and semi-arid regions, and contributes to loss of soil fertility, alteration of atmospheric radiation, and air pollution, with important impacts on global and regional climates, agriculture, and human health. Erosion occurs when wind speed exceeds a certain threshold, the value of which depends on a number of factors, including surface soil moisture. It is argued that in arid regions, under air-dry conditions, variations in surface soil moisture can be significantly affected by changes in atmospheric humidity, with an important effect on wind erosion potential, and that this effect is not currently well understood or quantified. It is argued that in fact the effect of soil moisture on susceptibility to wind erosion is opposite in the air-dry range of soil moistures to its effect in the capillary range, i.e., that the drier an air-dry soil is, the less susceptible it is to wind erosion. The proposed research will test this hypothesis and quantify the effect by analysis of several important agricultural soils of the United States, which are strongly subject to wind erosion, as well as well-sorted, clean sands, which will function as "reference" soils. In particular, the project will (1) investigate the dependence of surface soil moisture (i.e., in a few grain layer) on air humidity and temperature; (2) determine the relationship between threshold wind velocity, and near surface air humidity and temperature by means of a number of wind tunnel tests, and (3) interpret the results in terms of a theoretical framework that will account for the dependence of interparticle forces on absorbed layer bonding on interparticle forces and soil matric potential. This approach will provide a theoretical equation expressing threshold wind velocity as a function of air humidity for air-dry soils. The parameters of this equation will be determined through the results of the wind tunnel experiments. The assessment of the type of dependence existing between soil erosion potential and surface moisture content is crucially important to the understanding and modeling of wind erosion and dust emission from dryland landscapes. Intellectual Merit. Most of the existing studies on the effect of surface moisture on the threshold wind velocity have concentrated on relatively wet soils, in which capillarity dominated interparticle bonding forces. This proposal recognizes the importance air-dry soils as sources of atmospheric dust and wind erosion in dryland landscapes. In these soils, water content contributes to interparticle bonding mostly as adsorbed-layer bonding. The proposal recognizes the different dependence of liquid-bridge bonding (capillarity) and adsorbed-layer bonding on soil water content, and investigates how it affects the relationship between soil erodibility and surface moisture. Moreover, because surface soil moisture measurements are seldom available, the dependence of surface soil moisture on air humidity will be investigated and used to predict the state of soil erodibility. To our knowledge, this is one of the very few studies concentrating on the erodibility of air-dry soils and the first one showing an increased erodibility associated with increasing moisture contents. The use air humidity as a surrogate for surface soil moisture conditions is also a unique and novel approach to the study of the hydrologic controls on soil erosion potential. Broader Impacts This project will involve the training of a Ph.D. student, and two high school science teachers in the methods of research and of presentation of research results. It will support education by integration into a course being developed by the PIs, and provide a basis for collaboration between different groups working on similar questions. Members of underrepresented groups will be sought for inclusion in the research team. The results of the proposed research will be disseminated through publications in scholarly journals and conference presentations. Society will benefit from the improved understanding of the causes of and the ability to predict wind erosion and dust generation.
