Behaviour and management of water repellent soils—Preface

Abstract

Recently, it has become clear that soil water repellency is much more widespread than formerly thought. Water repellency has been reported in most continents of the world for varying land uses and climatic conditions. Soil water repellency often leads to severe runoff and erosion, rapid leaching of surface-applied agrichemicals, and losses of water and nutrient availability for crops. At present, no optimum management strategies exist for water repellent soils, focussing on minimising environmental risks while maintaining crop production. One of the reasons is that knowledge on water repellent soils is scattered among researchers of different disciplinesworking at different places throughout the world. In order to obtain a more integral view on many aspects related to soil water repellency, a joint trans-national research project had been funded by several donor organisations, and executed by research institutes and universities in Europe, the USA, and Australia. The present issue contains a selection of papers prepared after conclusion of the project, including an extensive soil water repellency bibliography. The soil water repellency project, entitled ‘Development of amelioration strategies to reduce environmental deterioration and agricultural production losses in water repellent regions’, is the first executed to include and address these issues. The project has been jointly prepared and executed by a research consortium of 8 European, American andAustralian research institutes and universities, i.e. Alterra (Wageningen, The Netherlands, project coordinator), University of Wales (Swansea, United Kingdom), University of Aveiro (Aveiro, Portugal), Democritus University of Thrace (Xanthi, Greece), University of Minnesota (Minneapolis, USA), Cornell University (Ithaca, USA), Agriculture Western Australia (Geraldton, Australia), and Deakin University (Warrnambool, Australia). The project was funded by the European Commission (FAIR Program, contract CT98-4027), and the DutchMinistry of Agriculture, Nature Management and Fisheries, and took place between 1998 and 2002. In the first section dealing with the assessment and characterisation of water repellent substances, 4 papers are presented, the first by Doerr et al. dealing with extracting soil compounds associated with water repellency in sandy soils of different origin. Morley et al. focus on various organic compounds and their role in generating water repellency. The role of tree stem proximity in the spatial variability of soil water repellency in a eucalypt plantation in coastal Portugal is presented by Keizer et al. A second paper of Doerr et al. highlights the effects of heating and postheating equilibration times on assessing the degree of water repellency. The second section of the special issue focusses on the occurrence of soil water repellency. The first paper in this section by Leighton-Boyce et al. deals with the temporal dynamics of water repellency and soil moisture in eucalypt plantations. The paper by Ziogas et al. focusses on the occurrence of soil water repellency in the north-eastern part of Greece, and the effects of drying on the degree of water repellency. A study on soil water repellency under natural conditions in sandy soils of southern Spain is presented by Moral Garcia et al. A second paper of Keizer et al. deals with the occurrence of soil water repellency under dry and wet antecedent weather conditions for selected land-cover types in the coastal region of central Portugal. Coelho et al. discusses the impact of soil water repellency on hydrological and erosional processes under forest land use in the Mediterranean. A study on the spatial and temporal variability of soil water repellency of Amazonian pastures is presented by Johnson et al. The third section of the special issue is devoted to hydrology and modelling aspects. Ferreira et al. present a detailed paper on the influence of burning intensity on water repellence and hydrological processes at forest and shrub sites. The effect of soil water repellency on overland flow generation in forest stands is presented in a third paper by Keizer et al. Xiong et al. discuss the implications of using recycled water in relation to competitive sorption of metals in water repellent soils. Steenhuis et al. apply percolation theory to describe water flow in hydrophobic soils. Furthermore, Ritsema et al. present a new modelling approach to simulate preferential flow and transport in water repellent porous