Télédétection et ressources en eau/Remote sensing and water resources
23
was raised by a number of participants: at the junction of zones, the variations may appear
artificially sudden, while in reality, they are usually gradual.
Even though a number of issues related to precision, validation and interpretation of
cartographic treatment of data with GIS remain to be solved, the results obtained are usually
more precise, more exhaustive and easier to use that the simple compilation of non systematic
point measurements. Regardless of the remaining difficulties, the use of GIS is an interesting
option for the extension, extrapolation and interpolation of observation for crossing information
overlays of differing nature.
Conclusions
Hydrological modelling
In addition to the results themselves, some presentations provided a basis for reflection on more
fundamental questions and they concentrate upon the connections between hydrological models
and spatial information. The analysis of discrepancies between the models and the observations in
Sahelian region emphasizes the limited precision of the latter. Spatial tools make it easier to take
into consideration local variations. In particular, remote sensing in its widest sense can be used to
explain the differences between watersheds through the representation of soil occupancy or of
drainage. It makes it possible to study watersheds in great detail. However, the majority of
hydrological models based on remote sensing assume the linearity and the invariability of the
process, and this is far from evident.
Remote sensing is presented as a tool for validating hypotheses. Radar images, for example,
can be useful for validating or invalidating the hypotheses on the dynamics of moisture in the
watershed. Internal behaviour of watershed can be explained by two spatial elements: morphology
(which explains the interest of DEM utilization in modelling) and land cover (from where the
interest in remote sensing). Several models using on or the other of these tools have been
developed at different scales.
The connection between spatial descriptors and hydrological indicators is also an important
subject. It shows the difficulties in associating the visual objects available on the image and the
objects required for hydrology, particularly because each connection is related to scale: the
description of hydrological processes varies with scale. Therefore, the choice of hydrological
objects (homogeneous areas) cannot be limited to one aspect. Their size depends on the objectives
of the study and should be based on a compromise between the consideration of the physical
phenomena underlying hydrological processes and the field data available for the elaboration of
models.
In the Sahelian context, each scale needs a specific approach. Mapping runoff potential is
possible at the pixel scale in areas where surface runoff prevails (this is the case for the Sahel)
and enables a good comparison between watersheds. However, due to the non linearity of
hydrological processes and to the relative difficulty of representing them at the different scales,
the discharge at the outlet of the watershed can only be obtained by the simple addition of
elementary runoff obtained on each pixel. It is necessary, therefore, to work on hydrological
objects of larger scale for which runoff measurements do not exist.
Data obtained from remote sensing and DEM now produce an almost continuous flow of
spatial descriptions, which makes it possible to analyse the internal variability of watersheds and
23
was raised by a number of participants: at the junction of zones, the variations may appear
artificially sudden, while in reality, they are usually gradual.
Even though a number of issues related to precision, validation and interpretation of
cartographic treatment of data with GIS remain to be solved, the results obtained are usually
more precise, more exhaustive and easier to use that the simple compilation of non systematic
point measurements. Regardless of the remaining difficulties, the use of GIS is an interesting
option for the extension, extrapolation and interpolation of observation for crossing information
overlays of differing nature.
Conclusions
Hydrological modelling
In addition to the results themselves, some presentations provided a basis for reflection on more
fundamental questions and they concentrate upon the connections between hydrological models
and spatial information. The analysis of discrepancies between the models and the observations in
Sahelian region emphasizes the limited precision of the latter. Spatial tools make it easier to take
into consideration local variations. In particular, remote sensing in its widest sense can be used to
explain the differences between watersheds through the representation of soil occupancy or of
drainage. It makes it possible to study watersheds in great detail. However, the majority of
hydrological models based on remote sensing assume the linearity and the invariability of the
process, and this is far from evident.
Remote sensing is presented as a tool for validating hypotheses. Radar images, for example,
can be useful for validating or invalidating the hypotheses on the dynamics of moisture in the
watershed. Internal behaviour of watershed can be explained by two spatial elements: morphology
(which explains the interest of DEM utilization in modelling) and land cover (from where the
interest in remote sensing). Several models using on or the other of these tools have been
developed at different scales.
The connection between spatial descriptors and hydrological indicators is also an important
subject. It shows the difficulties in associating the visual objects available on the image and the
objects required for hydrology, particularly because each connection is related to scale: the
description of hydrological processes varies with scale. Therefore, the choice of hydrological
objects (homogeneous areas) cannot be limited to one aspect. Their size depends on the objectives
of the study and should be based on a compromise between the consideration of the physical
phenomena underlying hydrological processes and the field data available for the elaboration of
models.
In the Sahelian context, each scale needs a specific approach. Mapping runoff potential is
possible at the pixel scale in areas where surface runoff prevails (this is the case for the Sahel)
and enables a good comparison between watersheds. However, due to the non linearity of
hydrological processes and to the relative difficulty of representing them at the different scales,
the discharge at the outlet of the watershed can only be obtained by the simple addition of
elementary runoff obtained on each pixel. It is necessary, therefore, to work on hydrological
objects of larger scale for which runoff measurements do not exist.
Data obtained from remote sensing and DEM now produce an almost continuous flow of
spatial descriptions, which makes it possible to analyse the internal variability of watersheds and