| The practical application of simulation
models in the field is sometimes hindered by the difficulty of deriving the soil
hydraulic properties of the study area. The procedure so-called inverse
modelling has been investigated in many studies to address the problem where
most of the studies were limited to hypothetical soil profile and soil core
samples in the laboratory. Often, the numerical approach called forward-backward
simulation is employed to generate synthetic data then added with random errors
to mimic the real-world condition. Inverse modelling is used to backtrack the
expected values of the parameters. This study explored the potential of a
Genetic Algorithm (GA) to estimate inversely the soil hydraulic functions in the
unsaturated zone. Lysimeter data from a wheat experiment in India were used in
the analysis. Two cases were considered: (1) a numerical case where the
forward-backward approach was employed and (2) the experimental case where the
real data from the lysimeter experiment were used. Concurrently, the use of soil
water, evapotranspiration (ET) and the combination of both were investigated as
criteria in the inverse modelling. Results showed that using soil water as a
criterion provides more accurate parameter estimates than using ET. However,
from a practical point of view, ET is more attractive as it can be obtained with
reasonable accuracy on a regional scale from remote sensing observations. The
experimental study proved that the forward-backward approach does not take into
account the effects of model errors. The formulation of the problem is found to
be critical for a successful parameter estimation. The sensitivity of parameters
to the objective function and their zone of influence in the soil column are
major determinants in the solution. Generally, their effects sometimes lead to
non-uniqueness in the solution but to some extent are partly handled by GA.
Overall, it was concluded that the GA approach is promising to the inverse
problem in the unsaturated zone.
Keywords. Genetic Algorithm, inverse modelling, Mualem-Van Genuchten parameters,
unsaturated zone, evapotranspiration, soil water |