Characteristics of microscale evapotranspiration:: a comparative analysis

Evapotranspiration characteristics on the point-scale (several hundred square meters) and the local scale (several square kilometers) are analysed by comparing a deterministic and a statistical - deterministic surface energy balance model. The vegetation surface variability is represented by both the surface heterogeneity and inhomogeneity. Heterogeneity means the mosaic of wet (wif not equal 0) and dry (1-wif) fractions of vegetation surface, while inhomogeneity addresses small scale variations of soil moisture content. The microscale characteristics of evapotranspiration are considered in terms of analysing evapotranspiration E-nu versus soil moisture content theta, relative frequency distribution characteristics of E-nu(theta) and the aggregation algorithms for its estimation. The analyses are performed for loam soil type under different atmospheric forcing conditions. The main result is as follows: For dry vegetation surface (wif = 0), the relationship between the aggregated (theta(agg)) and the area-averaged (theta(m)) soil moisture content is nonlinear and depends on both the states of the surface and the atmospheric forcing conditions. In the study, we assumed that there are no advective effects and mesoscale circulation patterns induced by surface discontinuities. Based on this fact it seems unlikely to be able to construct an aggregation algorithm for calculating theta(agg) without inclusion of the atmospheric forcing conditions. This means that it will be difficult to construct a simple formula for calculating area-averaged transpiration, if it is possible at all.