  The paper presents ASTERIX, an open source software for numerical integration
of an extended Saint-Venant system of equations used as a mathematical tool to
model the water flow from laboratory up to large-scale spatial domains applying
physically-based principles of fluid mechanics. Many in-situ observations have
shown that the plant cover plays a key roll in controlling the hydrological
flux at a catchment scale. The plant roots facilitate the infiltration
processes, the canopy intercept some proportion of rain, and plant stems slow
down the flow. In case of heavy rains, the infiltration and interception
processes cease in a short time, the remaining rainfall gives rise to the
Hortonian overland flow and the flash flood is thus initiated. In this context,
the following problem is also addressed in the article: how do the gradient of
soil surface and the density of the plant cover influence the water dynamics in
the Hortonian flow? The mathematical model and ASTERIX were kept as simple as
possible in order to be accessible to a wide range of stakeholders interested
in understanding the complex processes behind the water flow on hillslopes
covered by plants.
  The software is written in C programming language and it is free under GNU
license. It was tested on a series of benchmark problems, laboratory
experiments, and theoretical problems; and the results have shown a good
agreement with the theoretical or measured data.
