Concerning the Relationship between Evapotranspiration and Soil Moisture

Evapotranspiration observations have traditionally been scaled by potential evapotranspiration as a means of unifying the soil moisture-evapotranspiration relationship under a variety of meteorological conditions. However, this scaling alone does not unify the relationship during the drying. supply-limited phase. In this paper, a second scaling parameter is identified which applies to this phase of evapotranspiration. The parameter is a maximum sustainable, or threshold evapotranspiration, which occurs in vegetation-covered surfaces just before leaf stomata close, and occurs when surface tension begins to significantly restrict the moisture release from bare soil pores. Simple expressions for this parameter are presented for the cases of vegetation cover and bare soil. The number of input variables required in these expressions is rather small. We examine the effect of natural soil heterogeneities on evapotranspiration as computed from the proposed model. It is shown that the observed natural variability in soil moisture resulting from these heterogeneities is large enough to seriously alter the relationship between regional evapotranspiration and the area average soil moisture when compared to the point or homogeneous relationship. The implications for remote sensing and grid point numerical models are discussed. As a consequence of these results, we propose some key elements of a very simple parameterization for regional evapotranspiration for use in numerical models.