Geoscience Reference
In-Depth Information
23
Daily
Estimates of
Evaporation
Introduction
Broadly speaking, the understanding of processes and phenomena involved in
the interaction between the (often vegetation-covered) land surface and the
atmospheric boundary layer that were discussed in previous chapters is used in
two ways. The first way is to combine this understanding in the form of computer
sub-models, which then become important components of hydrological or
meteorological models. Such sub-models are often called soil-vegetation-
atmosphere schemes (SVATS) and they are described in the next chapter. This
chapter covers a second broad application of understanding of surface-atmosphere
interactions, which is to provide daily estimates of evaporation for use in other
hydrological and agricultural applications. This use of evaporation estimates
predates the creation of SVATS and, although the methods and formulae used
have the same basic origin, the applications are generally simpler and the methods
used involve making more assumptions.
Thus, the distinction between these two applications is in part related to
complexity. However, it is more fundamentally associated with the availability of
the meteorological data needed to calculate estimates of surface-atmosphere
exchange. In the case of SVATS, the meteorological data needed are generally
readily obtainable and they are usually available for sampling intervals of an hour
or less, perhaps from automatic weather stations in the case of hydrological
models, or as a byproduct of calculations made in other model components in the
case of meteorological models. The availability of relevant meteorological data is
usually a much more problematic issue when making the simple daily estimates
of evaporation discussed in this chapter. Moreover, data limitations can affect
the likely reliability of the method adopted to make an estimate. At best, the avail-
able data is that which might be provided by a standard agro-meteorological
climate station reporting daily. Hence, the meteorological variables used to
