Summary of the workshop
18
local topography provides a simple and operational method to calculate the volume of water
surfaces;
-
remote sensing can be used as a database, making it possible to reconstruct a series of
events. This is an original objective which will become increasingly important with the
growing availability of chronological images (for the past) and the possibility for the user to
select the viewing angle even at short notice (for the future).
The detection of fractures and their hydrological interest
Two presentations described the identification of linear features on the images as an indicator of
geological fractures, therefore potential areas for the utilization of groundwater. Two aspects
were covered:
-
a cartographic aspect: mapping of linear features with LANDSAT images for geological
(fractures) and hydrogeological purposes (for the selection of drilling sites). Aerial
photography was used routinely in fault areas, but the use of digital satellite imagery is
relatively new. Digital analysis of the images makes it possible to use powerful algorithms
for searching linear features and reduces, at least partially, the subjective nature of photointerpretation;
-
an analytical aspect: a study of frequencies and of the connection between drilling sites and
fractures through the analysis of the productivity of the sites in relation to their distance
from the fractures (through GIS) and the direction of the fractures themselves. The detection
of the most productive tectonic events facilitates the choice of areas for new drilling sites.
This point is particularly interesting, as it shows how these analyses with a GIS, can provide
a practical utilization of the results. Water resources can be supplied to people by greatly
increasing the chances of success in the search of high yield drilling sites.
The delimitation of watersheds
The use of digital elevation models (DEM) for the hydrological definition of watershed is
becoming increasingly important due to the relative ease of obtaining DEM at reasonable cost
and with a degree of accuracy that corresponds to that of the maps currently available. Most of
the hydrological applications of these DEM are connected to the use algorithms, particularly for
the definition of hydrological networks, the boundaries of watersheds, the measure of slopes, etc.
These algorithmic application are at the same time very valuable and rather easy to use, a fact
that has caused a multiplication of software available on the market. One example is provided in
the presentation of Bergaoui and Camus that describes the application of the DEMIURGE
programme created by ORSTOM for the definition of erosion potential and hydrological
modelling in Tunisia. The DEM is used to define slopes, the hydrological network and the Beven
indices. In the case of small and flat catchments, however, most presentations agree that it is
difficult to correctly derive the boundaries of the watershed and the hydrological network from
the DEM.
The importance of adapting the platform to the objectives
One presentation (Puech and Carette), shows the results obtained from a variety of platforms for
local studies in Africa, illustrated by three examples: amateur aerial photography, use of standard
aerial photography and SPOT satellite images. It is important to have a good correlation between
18
local topography provides a simple and operational method to calculate the volume of water
surfaces;
-
remote sensing can be used as a database, making it possible to reconstruct a series of
events. This is an original objective which will become increasingly important with the
growing availability of chronological images (for the past) and the possibility for the user to
select the viewing angle even at short notice (for the future).
The detection of fractures and their hydrological interest
Two presentations described the identification of linear features on the images as an indicator of
geological fractures, therefore potential areas for the utilization of groundwater. Two aspects
were covered:
-
a cartographic aspect: mapping of linear features with LANDSAT images for geological
(fractures) and hydrogeological purposes (for the selection of drilling sites). Aerial
photography was used routinely in fault areas, but the use of digital satellite imagery is
relatively new. Digital analysis of the images makes it possible to use powerful algorithms
for searching linear features and reduces, at least partially, the subjective nature of photointerpretation;
-
an analytical aspect: a study of frequencies and of the connection between drilling sites and
fractures through the analysis of the productivity of the sites in relation to their distance
from the fractures (through GIS) and the direction of the fractures themselves. The detection
of the most productive tectonic events facilitates the choice of areas for new drilling sites.
This point is particularly interesting, as it shows how these analyses with a GIS, can provide
a practical utilization of the results. Water resources can be supplied to people by greatly
increasing the chances of success in the search of high yield drilling sites.
The delimitation of watersheds
The use of digital elevation models (DEM) for the hydrological definition of watershed is
becoming increasingly important due to the relative ease of obtaining DEM at reasonable cost
and with a degree of accuracy that corresponds to that of the maps currently available. Most of
the hydrological applications of these DEM are connected to the use algorithms, particularly for
the definition of hydrological networks, the boundaries of watersheds, the measure of slopes, etc.
These algorithmic application are at the same time very valuable and rather easy to use, a fact
that has caused a multiplication of software available on the market. One example is provided in
the presentation of Bergaoui and Camus that describes the application of the DEMIURGE
programme created by ORSTOM for the definition of erosion potential and hydrological
modelling in Tunisia. The DEM is used to define slopes, the hydrological network and the Beven
indices. In the case of small and flat catchments, however, most presentations agree that it is
difficult to correctly derive the boundaries of the watershed and the hydrological network from
the DEM.
The importance of adapting the platform to the objectives
One presentation (Puech and Carette), shows the results obtained from a variety of platforms for
local studies in Africa, illustrated by three examples: amateur aerial photography, use of standard
aerial photography and SPOT satellite images. It is important to have a good correlation between