Agricultural and Forest Meteorology 106 2001 105–116
Evaluation of a water balance model using data for bare soil and crop surfaces in Middle Europe
J. Olejnik
a,∗
, F. Eulenstein
b
, A. Kedziora
c
, A. Werner
b
a
Agrometeorology Department, Agricultural University of Poznan, ul. Witosa 45, 60-667 Poznan, Poland
b
Center for Agricultural Landscapes and Land Use Research ZALF, Müncheberg, Eberswalder Str. 84, 15374 Muncheberg, Germany
c
Research Centre for Agricultural and Forest Environment, PAN, ul. Bukowska 19, Poznan, Poland Received 10 March 2000; received in revised form 7 August 2000; accepted 10 August 2000
Abstract
This paper compares the results of simulations and measurements of evapotranspiration from cultivated fields in Middle Europe. The modelling system THESEUS i.e. toolbox for hydro-ecological simulation and evaluation utilities from Wege-
henkel 1997a, 2000 was used in this study. Evapotranspiration was also calculated from the latent heat flux density LE measured using a modified Bowen ratio method. The model, as well as the modification of the Bowen ratio procedure are
described. Heat balance components for eight measurement periods 81 days were used in the comparison. There was good agreement between model simulations and measurements. The average relative error in evapotranspiration estimation by the
model was about 10 for time of integration of few days, and the error decreases with the increase of integration time. The error is smaller for higher daily rates of evapotranspiration. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Evapotranspiration; Modelling; Bowen ratio method
1. Introduction
In Western Poland and Eastern Germany large low- lands of the Elbe and Oder catchments are mainly
used for agriculture. The current and the future agricultural economic conditions will cause various
changes in land use, and thus, plant cover. Methods are needed that determine the impact of these changes
on the exchange of heat and water between the land and the atmosphere. In addition, land use strategies
providing sustainable agriculture and protection of
∗
Corresponding author. Tel.: +48-61-8487801 fax: +48-61-8487802.
E-mail addresses: olejnikjowl.au.poznan.pl J. Olejnik,
feulensteinzalf.de F. Eulenstein, kedanowl.au.poznan.pl A. Kedziora, awernerzalf.de A. Werner.
the water resources require information on the factors controlling the water and heat balances.
Water which reaches the earth surface partly re- turns to the atmosphere due to the evapotranspiration
process and partly goes to the sea as a runoff. The available energy and soil moisture are the most im-
portant physical factors which limit the amount of evapotranspiration. On the other hand, the species of
plants which cover the earth surface, as well as their development stage, are the most important biological
factors. So, models which simulate water balance, independently of simulation scale, should include
parameterisation of both, physical and biological processes which occur in the environment.
In recent years, many papers on modelling of heat and water balance components in different scales have
been published, i.e. Thom and Oliver 1977; Holtslag
0168-192301 – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 0 8 - 2
106 J. Olejnik et al. Agricultural and Forest Meteorology 106 2001 105–116
and Van Ulden 1983; Morton 1983; McNaughton and Spriggs 1985; Sellers and Dorman 1986; Ko-
vacs 1988, Holtslag and De Bruin 1988; Olejnik 1988a,b; Van De Griend and Van Boxel 1989;
Olejnik and Kedziora 1991; Beljaars and Holtslag 1991; Olejnik 1996; Wegehenkel 1997a. These
models can be applied at different scales: from a field to regional scale, but with the increase in scale some
important problems occur i.e. availability of input data such as soil moisture for large areas. One of
the most important problems during the procedure of modelling the water balance is coupling atmo-
spheric and land surface processes. The characteristic length of atmospheric processes is said to be 100 km
Kundzewicz, 1990, while the characteristic length of surface processes is smaller by many orders of
magnitude down to cellular scale for many biologi- cal processes. Land surface processes are too small,
too fast, too numerous and too heterogeneous to be represented explicitly in models, and therefore, the
parameterisation of these processes is necessary.
Recently, at
ZALF Institute
of Landscape
Modelling the THESEUS model was developed Wegehenkel, 1997a, 2000. Using this model, it
is possible to estimate the evapotranspiration of a chosen agricultural field or landscape with a one
day time step. To evaluate the results obtained using THESEUS i.e. toolbox for hydro-ecological simu-
lation and evaluation utilities model, measurements of evapotranspiration were made in some selected
fields in the eastern part of Germany and the western part of Poland. To estimate evapotranspiration, the
heat balance components of the selected fields were measured using the modified Bowen ratio method.
2. Investigation sites
