Evaporation losses from bare soils as influenced by cultivation techniques in semi-arid regions

The impact of cultivation techniques on the evaporation from bare soils was investigated in the laboratory. Two soil-types, which are important resources for rainfed cultivation of olives and almonds in semi-arid regions, were selected: a loamy sand soil and a stony (loam) soil. Evaporation from the soil surface is an important loss of soil moisture in these farming systems since a large percentage of the soil is kept bare in order to maximise the water availability for the tree crop. For the loamy sand soil the impacts of a straw mulch and treatment of the topsoil with olive mill effluent (OME) were tested. For the stony soil the effects of different rock fragment contents and distribution within the soil profile were tested. After thoroughly wetting with simulated rainfall and allowing the soil moisture to redistribute, the columns were subjected to evaporation for 46 days. Cumulative evaporation depth of soils treated with OME was 28% lower than that of the control soil. A similar reduction, be it lower (16%) was observed for the soil with a high rock fragment content by volume (Rv = 0.35 m3 m−3). The straw mulch and rock fragment mulch did not have an impact on the cumulative evaporation depth after 46 days. Furthermore, the time required to reach half of the total evaporation losses (d0.5) increased from 9 days for the control soil (loamy sand) to 24 days for the soil impregnated with OME and to 15 days for the straw mulch treatment. The same trend was observed for the stony soils: an increase in d0.5 from 4 days for the control soil (Rv = 0.19 m3 m−3) to 7 days for the soil with Rv = 0.35 m3 m−3 and to 8 days for the rock fragment mulch. These experiments show that the changes in water retention capacity of the topsoil by treatment with a hydrophobic substance (OME) or an increase in rock fragment content have a longer lasting effect on the reduction of evaporation losses, and result in a higher and more evenly distributed soil moisture content.