The description of field soil water content time series can be affected by
uncertainty due to measurement errors of the respective state variables, er
rors due to assumptions underlying the model, and errors in the determinati
on of boundary conditions. In this study, a simple state-equation was appli
ed for predicting field soil water contents at three different soil depths.
The simple state-model yielded large deviations of predictions from the me
asured soil water content, especially for the upper soil depth. Apparently,
the magnitude of the estimated evaporation rate was too high, The predicti
on result could significantly be improved when the calculated evaporation w
as reduced by a factor of 0.7. In order to account for uncertainty sources
associated with this simple approach, the state-equation was combined with
a stochastic technique, the so-called Kalman-Filter. Applying the Kalman-Fi
lter, the prediction quality significantly increased, even when the erroneo
usly high evaporation was assumed to be true. However, prediction uncertain
ty increased for the same time periods, for which it was shown earlier that
spatial correlation of soil water status was either random or very short.
When the Kalman-Filter was applied in a scenario to the surface layer only,
simulated soil water content in layers 2 and 3 agreed to measurements and
were highly improved compared to simulations when layer 1 was not filtered.
Hence, application of lab determined soil hydraulic property functions in
combination with state observations of upper soil horizon water content and
with the Kalman-Filter provides a promising opportunity to describe and pr
edict soil water contents for entire soil profiles even under the presence
of uncertainty sources. (C) 1999 Published by Elsevier Science B.V. All rig
hts reserved.