Existence, origin and ecohydrological significance of soil water repellency: a review

Soil water repellency (SWR) is explained as a reduction of wetting rates and water entry into soils. Water-repellent soils does not wet spontaneously when water is placed on the surface. SWR is increasingly being recognized as a common phenomenon impacting the hydrological functions of soil systems. The main hydrological impacts of SWR are reduced infiltration rates, increased overland flow, spatially localized infiltration with fingered flow development, modifications of the three-dimensional distribution, and dynamics of soil moisture. SWR increases overland flow during rainstorms, and subsequently, topsoil erosion. It reduces the water entry into the root zone and retard plant growth, reducing the quantity and the quality of crop production. Water repellency is caused by the presence of hydrophobic organic matter in the soil as coatings on mineral particles intermixed materials. In addition to organic matter, soil moisture content is also an important factor that influences the SWR. Water-repellent nature can be theoretically explained based on the surface free energy and the contact angle of soil. The surface free energy of the soil and the contact angle measures the degree of water repellency, or how much the soil is water-repellent. In this review, the existence, origin, various impacts, theoretical concepts, and management of SWR are discussed using global and local research findings over more than ten decades.