Geoscience Reference
In-Depth Information
3.3.3 Direct Estimation of Actual Evapotranspiration
There are now methods available for the direct measurement of actual evapotranspiration rates over a
surface using the
eddy correlation
technique (see, for example, Shuttleworth
et al.
, 1988) but, although
a global network of eddy correlation measurement stations is expanding, the main use of such instru-
ments has been during short field campaigns studying land-atmosphere interactions (Figure 3.2). More
generally, indirect methods of estimating evapotranspiration are used.
Another promising technique is the use of the laser scintillometer. This uses measurements of the
disturbance of a laser beam by rapid variations in the density of the lower atmosphere to estimate
the transfers of sensible heat away from the surface. Coupled with a closure of the energy budget for
the surface, this can also give an estimate of the latent heat fluxes and evapotranspiration. One nice
feature of this technique is that it can integrate the fluxes over the length of the laser beam and therefore
provides a larger scale measurement of evapotranspiration than eddy correlation measurements at a site
Figure 3.2
Measurements of actual evapotranspiration by profile tower, eddy correlation and Bowen ratio
techniques for a ranchland site in Central Amazonia (after Wright et al., 1992, with kind permission of John
Wiley and Sons).
