Solar heating of the soil and evaporation from a soil surface

In 1994 a system for classifying landfills for municipal solid waste by local climate was introduced in South Africa. If it can be shown that potential evaporation from the landfill surface will exceed average annual rainfall, it is accepted that no significant leachate will be generated in the landfill, and a leachate collection system, impervious underliner and leachate treatment system is not legally required. This represents very large financial savings on the establishment of a new landfill or the extension of an existing one. Hence the measurement and prediction of evaporation from a soil surface has become a subject of considerable importance to the waste disposal industries in South Africa and neighbouring countries (e.g. Botswana and Namibia) that have adopted similar climatic approaches. Over the past 20 years it has become apparent that the long-established solar energy balance approach to assessing evaporation from a soil may be in error. This is because it regards solar energy expended on heating the soil as reducing the energy available for supplying energy of evaporation. This paper describes experiments aimed at exploring the role of heating of the soil in the evaporation process, and concludes that, far from subtracting energy from the process, it actually provides most of the energy for evaporation.