A rational parameterization of evaporation from dry, bare soil

The evaporation of water from bare soil changes in mechanism as well as in magnitude as the surface dries. This evaporation proceeds in three stages, the third of which has a mechanism completely different from the other two stages. Therefore, only the parameterization schemes for soil-surface evapo ration that take account of this stage switching should be considered as ra tional. A rational parameterization of the soil-surface evaporation based on a thre e-layer model of the vertical distribution of soil moisture is proposed. Ea ch stage of evaporation is characterized by one of the three soil layers th at comes to the surface. The switching from one stage to another can be det ermined from the difference between surface soil temperature and screen-lev el air temperature. The evaporation rate in the first stage is equal to the potential evaporation rate E-p, and that in the second stage can be expres sed by the product of E-p and "surface moisture availability" M, which depe nds on the wetness of the superficial layer of soil and the soil type. Sinc e these are practically the same as the existing parameterizations, only a general idea of formulating the two stages is given in this paper. The thir d-stage process, however, is formulated by a newly invented scheme that wil l be called the "DSL bulk method." An example of the application of this me thod to the Tottori-Dune sand is also shown. This parameterization makes it possible to evaluate the evaporation rate in the third stage by the surfac e soil temperature and the average water content for the top 5 cm of soil, and both can be measured by remote sensing.