Summary of the workshop
22
fact a number of serious problems of validity and interpretation, particularly as they manipulate
multiple plains, an operation that is often simple from a computing standpoint, but very uncertain
as far as the meaning and the coherence of results are concerned.
Modelling outside GIS
These are calculations carried out outside of a GIS framework, but whose base data are spatial.
Some of the results have been described above. The most effective applications are the simplest
and the most typical: hydrological balance or SCS (Soil Conservation Service) models in which a
runoff coefficient is applied to each land use.
Two presentations (Nonguierma and Dautrebande, Colombo and Scarfatti) presented the
applications of simple SCS models. Remote sensing is used for the segmentation into
homogeneous areas: an evaluation of CN by areas is proposed, based on the average soil
occupation, with the quantification of the runoff calculated at the rate of each homogenous area.
Another presentation (El Idriss and Persoons), uses a simple and effective conceptual
framework (linearity, permanence during a storm of production and transfer functions), to
simulate the hydrogrammes of the maximum flood events for the calculation of structures. As it
uses classes of pixels in a isochrone position in relation to the watershed outlet, the structure of
the grid hydrological model (GHM), enables a quick calculation in comparison to classical
distributed models. Its application consists in the creation of scenarios to test the influence of the
evolution of the physical characteristics of the watershed on the change in flood regime.
The reference to TOPMODEL modelling on the basis of spatial knowledge of relief
(Gineste) should also be noted. This is a simplified conceptual model of water tables that
proposes a precipitation-discharge transformation based on the notion of variable contribution
areas (during the storm), for calculating flood events. The areas can be defined using their
saturation index, that is a function of slope and of the surface drained upstream of the considered
point. Starting from a digital elevation model (DEM), it is possible to map this index, then
calculate its distribution that enters directly into the modelling. The increasingly easy access to
DEM encourages the use of this type of models and valorizes the inclusion of relief in
hydrological modelling. As a consequence of the hypotheses on the elementary streamflow
process, its application is a priori restricted to humid and temperate areas.
Modelling with GIS
Some examples of modelling based on spatial data managed through a GIS are shown both in
simple terms (regional hydrological balance constructed with a continental scale GIS, Bousquet et
al.) or more complex ones (modelling including production and transfer functions, Faurès). One
presentation (Perez et al.) illustrates the test of a event distributed modelling tool, the ANSWERS
model, that runs in a GRASS environment (a GIS software in image mode) to define runoff and
erosion in an agricultural catchment. The model requires the initial knowledge of several
parameters, several of which are obtained from field information or from remote sensing, while
others are obtained by calibration. The application is made simpler by the direct use of
information overlays (relief, land cover) in the modelling.
In the combination of overlays obtained from a GIS there is a problem in the interpretation
of map results obtained, particularly on the boundaries between homogenous areas. This problem
22
fact a number of serious problems of validity and interpretation, particularly as they manipulate
multiple plains, an operation that is often simple from a computing standpoint, but very uncertain
as far as the meaning and the coherence of results are concerned.
Modelling outside GIS
These are calculations carried out outside of a GIS framework, but whose base data are spatial.
Some of the results have been described above. The most effective applications are the simplest
and the most typical: hydrological balance or SCS (Soil Conservation Service) models in which a
runoff coefficient is applied to each land use.
Two presentations (Nonguierma and Dautrebande, Colombo and Scarfatti) presented the
applications of simple SCS models. Remote sensing is used for the segmentation into
homogeneous areas: an evaluation of CN by areas is proposed, based on the average soil
occupation, with the quantification of the runoff calculated at the rate of each homogenous area.
Another presentation (El Idriss and Persoons), uses a simple and effective conceptual
framework (linearity, permanence during a storm of production and transfer functions), to
simulate the hydrogrammes of the maximum flood events for the calculation of structures. As it
uses classes of pixels in a isochrone position in relation to the watershed outlet, the structure of
the grid hydrological model (GHM), enables a quick calculation in comparison to classical
distributed models. Its application consists in the creation of scenarios to test the influence of the
evolution of the physical characteristics of the watershed on the change in flood regime.
The reference to TOPMODEL modelling on the basis of spatial knowledge of relief
(Gineste) should also be noted. This is a simplified conceptual model of water tables that
proposes a precipitation-discharge transformation based on the notion of variable contribution
areas (during the storm), for calculating flood events. The areas can be defined using their
saturation index, that is a function of slope and of the surface drained upstream of the considered
point. Starting from a digital elevation model (DEM), it is possible to map this index, then
calculate its distribution that enters directly into the modelling. The increasingly easy access to
DEM encourages the use of this type of models and valorizes the inclusion of relief in
hydrological modelling. As a consequence of the hypotheses on the elementary streamflow
process, its application is a priori restricted to humid and temperate areas.
Modelling with GIS
Some examples of modelling based on spatial data managed through a GIS are shown both in
simple terms (regional hydrological balance constructed with a continental scale GIS, Bousquet et
al.) or more complex ones (modelling including production and transfer functions, Faurès). One
presentation (Perez et al.) illustrates the test of a event distributed modelling tool, the ANSWERS
model, that runs in a GRASS environment (a GIS software in image mode) to define runoff and
erosion in an agricultural catchment. The model requires the initial knowledge of several
parameters, several of which are obtained from field information or from remote sensing, while
others are obtained by calibration. The application is made simpler by the direct use of
information overlays (relief, land cover) in the modelling.
In the combination of overlays obtained from a GIS there is a problem in the interpretation
of map results obtained, particularly on the boundaries between homogenous areas. This problem