Re: Measuring evapotranspiration on islands!

[Rick Allen]

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Dear Nicole:

Yes, one can use the Penman-Monteith method to predict ET from 
sand dune vegetation.  However, the challenge is in estimating the 
bulk surface resistance (or conductance) for the vegetation.  This 
depends on the density of the vegetation, its degree of stomatal 
control (governed perhaps by salinity of the soil/sand solution), and 
other environmental parameters.  Generally the resistance needs to 
be determined using field measurements that you have described.  

The recently released FAO Irrigation and Drainage Paper No. 56: 
Crop Evapotranspiration includes a few chapters on predicting a crop 
coefficient Kc  by describing the fraction of ground cover or LAI of the 
vegetation along with the height of the vegetation (see pages 159 to 
205).  The Kc is used to estimate ET using the Kc ETo approach 
where ETo is grass reference ET.  There are also reductions in ET 
recommended for varying levels of salinity .  The FAO-56 publication 
is available from the FAO in Rome (www.FAO.org).

There is no substitute for direct measurement of ET in order to 
calibrate some reproduceable numeric procedure.  However, in an 
undulating dune type of environment, it may be difficult to use 
micrometeorological boundary layer techniques like eddy covariance, 
so that lysimetry may be the best approach, as you indicated you 
will use.
I recommend that you read some of the publications on lysimetry 
that provide various procedural ideas and precautions so that your 
resulting measurements are valid and representative.  ASCE 
published the proceedings of an International Symposium on 
Lysimetry held in Hawaii in 1991.  These proceedings contain several 
papers on recommended procedures and do's and don't's of 
lysimetry.  (www.ASCE.org)

One other approach might be to use neutron depth meters and TDR 
systems to compute ET from a complete soil water balance.  
However, it is always difficult, if not impossible, to apply this method 
unless there is a deep, dry profile below the active root zone.  You 
may not have this with the dunes.

Good luck

Rick Allen
Univ. Idaho


> 
> SOWACS members,
> 
> My name is Nicole Armbrüster from the Department of Plantecology of the
> University of Oldenburg (Germany). This year we started a project to
> estimate the evapotranspiration on two Eastfrisian Islands in the North
> Sea.
> Now my question is, if somebody has any experiences in measuring
> evapotranspiration of dune vegetation on islands already?
> At the moment our plan is to solve this problem by using
> micrometeorological data and lysimeters (not weighable), with
> temperature-sensors and Theta-probes (FDR).
> Is it possible to use the Penman-Monteith-equation to estimate
> evapotranspiration of hilly areas or which other possibilities exist?
> Which experiences with FDR in the field do you have?
> 
> Thanks in advance,
> 
> ********************************
> 
> Nicole Armbrüster
> 
> Universität Oldenburg
> Fachbereich 7/ Pflanzenökologie
> Carl-von-Ossietzky Str. 9-11
> 26111 Oldenburg
> 
> Tel: 0441 / 771 79
> Fax: 0441 / 798-3331
> e-mail: nicole.armbruester@mail.uni-oldenburg.de
> 
> ********************************
> 
>