Fourth Study Conference on BALTEX Scala Cinema Gudhjem

The evapotranspiration is affected by the available soil water
content W according to the formula (3) if its value drops
below the lower optimum limit W opt,1. The last depends on
soil type and corresponds in the model to 60 % of field
water capacity.
Using of formula (3) have given good results in yield
calculations. Also, calculated soil water content gave
satisfactory coincidence with measured values in Estonian
agro-meteorological service (at 12-14 areas during 10-20
years) for most Estonian soils, except the soils under the
influence of ground water. However, quite extreme weather
conditions in 2001-2002 field experiments suggested to
upgrade the equation (3) and runoff/infiltration calculation.
5. Field measurements 2001-2002
The field experiments to determine the model parameters for
some new potato varieties were carried out in the
experimental field of the Estonian Research Institute of
Agriculture at Üksnurme (59°17' N; 24°37' E) in 2001-2002.
The soil type was sod-calcareous (haplic luvisols in FAO-
UNESCO classification), in 2001 sandy silt loam and in
2002 sandy loam.
Soil moisture measurements were carried out three times per
month for every 10-cm layer up to the soil depth of 0.5 m.
The samples in four repetitions were taken by soil auger.
Soil moisture was determined by the method of weighing
and drying the samples in thermostat. The soil bulk density
was measured by layers for conversion to soil water storage.
The dynamics of LAI was determined based on leaves
biomass and specific leaf weight measurements.
6. Upgrades of the water block
The soil water conditions were quite anomalous in the
observed years. In 2001 the periods with rather frequent
intensive precipitation occurred. These periods caused
conceivable differences between the calculated and actual
water content, the calculated values being higher. Removing
of part of cumulative precipitation from the water budget
during a prolonged rain period was inevitable. The results of
calculations suggested additional inclusion into the runoff
precipitation exceeding 60 mm during ten succeeding days.
The summer of 2002 was extremely dry. In late August the
soil water content decreased in some places below the
wilting point. As model did not describe it, the evaporation
part of the evapotranspiration formula (3) was modified:
4
E = 0.
0872 Q { min [ 1,
( W + Wd
) /( W opt,1 + Wd
) ] } + (4)
+ 0.
0406 L min( 1,
W / W )
where Wd is the difference between the wilting point and the
maximum hygroscopicity. These changes included, the
computed available soil water content is compared with the
measured data in Fig. 1. for 0.5-m top layer. The mean
absolute errors were 10 mm and 7 mm in 2001 and 2002,
respectively.
7. Acknowledgements
The development of the model was funded by the Estonian
Science Foundation (Grant No 5020).
References
Gojsa, N.I., Bibic, V.V., About the study of impact of
hydrometeorological factors and canopy conditions on
the evapotranspiration dynamics of maize crops, in:
Trudy Ukr. NIGMI, 151, pp. 17-34, 1976 (in Russian)
opt,1
- 185 -
Available soil water
content, mm
Available soil water
content, mm
120
2001
100
80
60
40
20
0
01.06 16.06 01.07 16.07 31.07 15.08 30.08 14.09 29.09
120
100
80
60
40
20
0
2002
Model
Measured
01.06 16.06 01.07 16.07 31.07 15.08 30.08 14.09 29.09
Date
Figure 1. Computed, using model and measured
available soil water content in 2001 and 2002
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