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 unde rtaken to modify the topsoil characteristics (mulching, tillage) in or der to create a thin dry topsoil that reduces evaporation. However, li ttle attention was paid to the role of natural rock fragments in topso ils with respect to evaporation. This paper presents the results of la boratory 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 a t one (high) evaporative demand (E(o) = 7.7-9.2 mm day(-1)). Time doma in reflectometry (TDR) was used to investigate the relation between th e water content of the topsoil and the actual evaporation rate. For so ils 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 r ain (10 and 20 mm), an opposite behaviour was observed. Initial fine e arth water content and evaporation rates increase with rock fragment c ontent. A strong positive sigmoidal relation between relative evaporat ion 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 mulc h, there were no systematic differences in this relation between the t reatments, A rock fragment mulch reduces evaporation rate at a given s oil water content significantly, These trends explain the often ambiva lent effects of rock fragments on evaporation rates in the field.