Soil evaporation: test of a practical approach under semi-arid conditions.

The role of soil evaporation in the field water balance is briefly reviewed. With an increasing demand for improved soil and water management, the common practice of combining evaporation with transpiration into a single term, i.e. evapotranspiration, is no longer justified. The approach described in this paper combines practical experience with computer modelling. Bare field soils evaporate at a potential rate only during one or a few days after rainfall. Thereafter, evaporation is reduced due to drying of the soil surface. Both deterministic and parametric modelling often show a roughly linear increase in cumulative evaporation with the square root of time. Theoretically, this holds only if the potential evaporation is constant. This is not the case in some climates: daily potential evaporation rates commonly fluctuate between 1 and 6 mm day-1 or more in temperate climates and in some (sub)tropical climates. In such cases it appears that cumulative actual evaporation relates better to the square root of cumulative potential evaporation than to the sqaure root of time. Evaporation can be described then with a simple equation containing only one soil parameter. The latter can be easily measured, even in tilled fields, with a fast and cheap microlysimeter technique. This practical approach is illustrated with measurements in the African Sahelian zone as well as in the Netherlands. (Abstract retrieved from CAB Abstracts by CABI’s permission)