A Rational Parameterization of Evaporation from Dry, Bare Soil

Abstract

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 evaporation that take account of this stage switching should be considered as rational.A rational parameterization of the soil-surface evaporation based on a three-layer model of the vertical distribution of soil moisture is proposed. Each stage of evaporation is characterized by one of the three soil layers that comes to the surface. The switching from one stage to another can be determined from the difference between surface soil temperature and screen-level air temperature. The evaporation rate in the first stage is equal to the potential evaporation rate Ep, and that in the second stage can be expressed by the product of Ep and "surface moisture availability" M, which depends on the wetness of the superficial layer of soil and the soil type. Since these are practically the same as the existing parameterizations, only a general idea of formulating the two stages is given in this paper. The third-stage process, however, is formulated by a newly invented scheme that will be called the "DSL bulk method." An example of the application of this method to the Tottori-Dune sand is also shown. This parameterization makes it possible to evaluate the evaporation rate in the third stage by the surface soil temperature and the average water content for the top 5cm of soil, and both can be measured by remote sensing.

Satellite Detection of Water Stress Effects on Terrestrial Latent Heat Flux With MODIS Shortwave Infrared Reflectance Data

“…Two MODIS‐based SWIRs were used to drive the revised PT algorithm for terrestrial LE estimation. The first SWIR index employed was the SWIR SM index ( SMI ) used to characterize the variation in SM for bare soil (He & Kobayashi, ; Sadeghi et al, ; Zarco‐Tejada et al, ), which is defined as where ρ swir, d , ρ swir, s , and ρ swir, w are the reflectances of the dry soil, regular soil, and saturated soil in the SWIR bands (band 7 in the MODIS data), respectively. In this study, ρ swir, d and ρ swir, w were determined as 0.75 and 0.001 based on our ground measurements.…”

“…In the original PT‐JPL model, f (SM) uses an indicator of the atmospheric evaporative demand (RH VPD ) as a proxy for SM. In the present study, we considered the effects of the atmospheric evaporative demand and surface SM supply on LE s (He & Kobayashi, ), where the response of LE s to SM stress was defined as where SMI is the SWIR SMI described in section , RHD is the relative humidity deficit (1 – RH; K. Wang et al, ), and β is a fixed parameter (0.50).…”