  The water uptake by roots of plants is examined for an ideal situation, with
an approximation that resembles plants growing in pots, meaning that the total
soil volume is fixed. We propose a coupled water uptake-root growth model. A
one-dimensional model for water flux and water uptake by a root system growing
uniformly distributed in the soil is presented, and the Van Genuchten model for
the transport of water in soil is used. The governing equations are represented
by a moving boundary model for which the root length, as a function of time, is
prescribed. The solution of the model is obtained by front-fixing and finite
element methods. Model predictions for water uptake by a same plant growing in
loam, silt and clay soils are obtained and compared. A sensitivity analysis to
determine relative effects on water uptake when system parameters are changed
is also presented and shows that the model and numerical method proposed are
more sensitive to the root growth rate than to the rest of the parameters. This
sensitivity decreases along time, reaching its maximum at thirty days. A
comparison of this model with a fixed boundary model with and without root
growth is also made. The results show qualitative differences from the
beginning of the simulations, and quantitative differences after ten days of
simulations.
