SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

Heikinheimo M.J.& T.Koskela
Finnish Meteorological Institute
P.O.Box 503, 00101 Helsinki 10
Finland
Water budget of an agricultural field
Measuring techniques used in a Finnish experiment
1. Introduction
The concept of evaporation has for a long time been of great
interest in agricultural applications where information of the
available water for plants or moisture conditions in the canopy is
important. Various methods, such as gravimetric and
micrometeorological, have been developed to directly measure the
arnount of water evaporated into the atmosphere. Application of these
methods on a long term basis have proven tedious and costly since the
measuring instruments usually require great care and daily
maintenance. Therefore long term re cords of actual evapotranspiration
(AET) are rare to find.
Standard meteorological data or evaporation pans are, however,
cornrnonly used to deduce the so called potential evapotranspiration
(PET), which represents the rate of water loss from a surface of short
and extensive vegetation with an ample water supply from the soil.
Finnish Meteorological Institute, as an exarnple, provides daily data
about cumulative rainfall and PET to be used for water balance
calculations in agricultural applications, Fig. 1. The cumulative
evapotranspiration estimates are based on the Penrnan-Monteith method,
where the surface resistance for water vapour transfer is assurned to
have a fixed seasonal pattern.
The inconvenience in using the concept of PET is that it doesn't
realistically account for conditions where the surface resistance for
water vapour transfer has increased due to drought. Although AET will
be reduced due to drought the measured PET is subject to increase
during dry periods because the water vapour deficit of air is
increased. To obtain realistic estimates of the soil moisture it has
proven desirable to gain information also about AET for various types
of terrains and cultivated crops.
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