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