Evaporation from cultivated soils containing rock fragments

Evaporation from bare soil surfaces represents an important water loss for agriculture in semi-arid regions, Numerous efforts have been undertaken to modify the topsoil characteristics (mulching, tillage) in order to create a thin dry topsoil that reduces evaporation. However, little attention was paid to the role of natural rock fragments in topsoils with respect to evaporation. This paper presents the results of laboratory experiments simulating evaporation from initially wet and air-dry soils containing a range of rock fragment contents, and compares them to field conditions, Evaporation was stimulated by blowing fans at one (high) evaporative demand (E(o) = 7.7-9.2 mm day(-1)). Time domain reflectometry (TDR) was used to investigate the relation between the water content of the topsoil and the actual evaporation rate. For soils at field capacity, initial fine earth water content decreases with rock fragment content, and consequently evaporation rates decrease in the same order. For air-dry soils that received a limited amount of rain (10 and 20 mm), an opposite behaviour was observed. Initial fine earth water content and evaporation rates increase with rock fragment content. A strong positive sigmoidal relation between relative evaporation rate (actual over open-pan evaporation rates) and fine earth water content in the centre of the wetted soil section was observed during the laboratory experiments. Except for the columns covered with a mulch, there were no systematic differences in this relation between the treatments, A rock fragment mulch reduces evaporation rate at a given soil water content significantly, These trends explain the often ambivalent effects of rock fragments on evaporation rates in the field.