Evaporation losses from bare soils as influenced by cultivation techniquesin 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 resour ces for rainfed cultivation of olives and almonds in semi-arid regions, wer e 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 system s since a large percentage of the soil is kept bare in order to maximise th e 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 (OM E) were tested. For the stony soil the effects of different rock fragment c ontents and distribution within the soil profile were tested. After thoroug hly wetting with simulated rainfall and allowing the soil moisture to redis tribute, 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 s oil with a high rock fragment content by volume (Rv = 0.35 m(3) m(-3)). The straw mulch and rock fragment mulch did not have an impact on the cumulati ve evaporation depth after 46 days. Furthermore, the time required to reach half of the total evaporation losses (d(0.5)) increased from 9 days for th e control soil (loamy sand) to 24 days for the soil impregnated with OME an d to 15 days for the straw mulch treatment. The same trend was observed for the stony soils: an increase in d(0.5) from 4 days for the control soil (R v = 0.19 m(3) m(-3)) to 7 days for the soil with Rv = 0.35 m(3) m(-3) and t o 8 days for the rock fragment mulch. These experiments show that the chang es in water retention capacity of the topsoil by treatment with a hydrophob ic substance (OME) or an increase in rock fragment content have a longer la sting effect on the reduction of evaporation losses, and result in a higher and more evenly distributed soil moisture content. (C) 2000 Elsevier Scien ce B.V. All rights reserved.