A.Pistocchi, F.Tomei

GECOsistema srl - Cesena

A 3D numerical model of surface and subsurface water flow has been developed using the technique of Integrated Finite Difference (IFD ) method. The need to develop such a system is to be linked to the need of an accurate description of soil water processes both at the field scale and at the small catchment scale to take into account physical features of the flow pattern which cannot be caught by traditional 2D models.

The governing equations involved in 3D characterization of flow patterns are 3D Richards equation for partially saturated porous medium flow, and depth averaged (2D) St Venant equations for surface runoff. In the latter, a parabolic approximation is introduced by ignoring convective and local inertia terms which are negligible in the problem of runoff formation.

The IFD method bases on describing the flow pattern in terms of an equivalent network of pipes each of which is represented by an appropriate equation (Manning’s equation for flow occurring on the surface, Darcy’s law for subsurface flow).

This leads to a strongly explicit physical description of the hydraulic processes in soil such as infiltration, redistribution, drainage, exfiltration, capillary uprise, runoff and flow accumulation. In this way, no empirical treatment of water balance terms is required.

Using the equations recalled above, for which a numerical integration scheme accordintg to the IFD method is implemented, it is possible to describe the behaviour of such hydrologic systems as a complex-terrain field or a small catchment, as shown in the figure below. This is accomplished through the specification of appropriate boundary conditions, such as: prescribed (no flow) bottom boundary or deep percolation bottom boundary condition or prescribed head bottom boundary condition; at the soil surface and at nodes falling within the root zone, either an inflow (precipitation) or an outflow (evapotranspiration) boundary condition is prescribed.

Spatially varying soil profiles are handled together with complex topography; soil behaviour is parametrized according to the Van Genuchten model for retention curves and relative hydraulic conductivity. In the case of surface flow, the only parameter to be assigned is Manning’s surface roughness coefficient.

The following figure gives an idea of the kind of problems the model can handle: a spatially and temporally varying solution is provided for both water depth at the soil surface, and soil moisture (expressed in normal (0-1) form) in the subsoil. A graphical user interface allows to visualize simultaneously soil surface conditions and soil moisture at three different levels in the soil profile considered.

The model has been successfully tested in some exploratory numerical case studies. Further developments of the research will concern full integration of the hydrodynamic model here presented within the CRITERIA system developed by SMR-ARPAER, and its application in field- to catchment- scale modelling.

Acknowledgements

The research has been funded by ARPA - Servizio meteorologico Regionale within the framework of the CLIMAGRI project, under the scientific supervision of F.Zinoni and V.Marletto. The authors wish toacknowledge the partners of the CLIMAGRI project for providing data, tips and helpful discussion on the topic of 3D soil water flow modelling.

For further information: Dr Alberto Pistocchi – GECOsistema srl: viale G.Carducci, 15, 47023 Cesena. www.gecosistema.it; e-mail: Alberto.pistocchi@gecosistema.it