Summary of the workshop
20
discharges: absence of detailed base maps, limited relief, well defined seasons, and dominance of
mainly surface runoff over interflow. Several attempts have been made in this ares to link remote
sensing and hydrology since 1985. The method described during the workshop was based upon
the following elements:
• elementary hydrological knowledge based on the standard surface conditions of the Sahel,
obtained from a synthesis of measures with a rainfall simulators (1 m
2 test areas);
• surface feature mapping obtained from satellite imagery (LANDSAT TM) and ground data;
• hydrological validation obtained with field observations on a number of small Sahelian
watersheds.
Through the results it is possible to obtain a reasonable map of the potential runoff, that is
to say the basic runoff to be expected from each pixel. This map is an important tool for the
comparison of watersheds and the eventual hierarchization of basic runoffs in terms of total flow.
However, the map of potential runoff is not sufficient to quantify the runoff at the outlet of the
watershed. The downstream transfer of runoff and the modelling of flow within water courses still
pose a number of problems that are far from being solved. To use the map of runoff potential at
the watershed scale, it is currently necessary to use a calibration function. The parameters of this
function have been estimated and can be used for calculating the 10-year floods on small
watersheds of West Africa.
One presentation covers the use of the rainfall-runoff model elaborated by the Soils
Conservation Service of the United States Department of Agriculture (USDA/SCS) through the
runoff curve number (CN) to define the conditions of runoff (Colombo and Sarfatti). This is the
most traditional method and the simplest utilization of remote sensing for the estimation of runoff
conditions. The method, developed in the US, is tested in this case in Eritrea for the estimation of
peak discharges and of annual runoff. It includes two phases: a phase of image splitting based on
remote sensing and on the assignment of a global parameter to the hydrodynamic behaviour
(production coefficient) which is then applied prorata to land cover. The extrapolation of
calculation methods poses the problem of choosing the CN coefficient, considering that these
coefficients are used in a context that is different from the one for which they were initially
developed.
Another presentation also covers the issue of the hydrological response of landscape units
that are considered to be homogeneous from the hydrological point of view (Viné). The landscape
segmentation phase refers to “hydro-landscapes”, and insists on the hypotheses of models and of
the choice of segmentation. The space is divided into landscape units that are grouped according
to a limited number of land cover categories. These categories are defined from the image. The
choice of categories is based upon the hypothesis of each one having a specific hydrological
response function contribution. Putting in relation the global hydrological response of several
small watersheds (on a yearly, quarterly and monthly basis) with the different categories of land
use, the unit response for each category is obtained through a numeric deconvolution scheme,
based upon a disaggregation technique. This original analytical method appears well adapted to
the valorization of remote sensing to provide a global value of hydrological response per
landscape unit.
20
discharges: absence of detailed base maps, limited relief, well defined seasons, and dominance of
mainly surface runoff over interflow. Several attempts have been made in this ares to link remote
sensing and hydrology since 1985. The method described during the workshop was based upon
the following elements:
• elementary hydrological knowledge based on the standard surface conditions of the Sahel,
obtained from a synthesis of measures with a rainfall simulators (1 m
2 test areas);
• surface feature mapping obtained from satellite imagery (LANDSAT TM) and ground data;
• hydrological validation obtained with field observations on a number of small Sahelian
watersheds.
Through the results it is possible to obtain a reasonable map of the potential runoff, that is
to say the basic runoff to be expected from each pixel. This map is an important tool for the
comparison of watersheds and the eventual hierarchization of basic runoffs in terms of total flow.
However, the map of potential runoff is not sufficient to quantify the runoff at the outlet of the
watershed. The downstream transfer of runoff and the modelling of flow within water courses still
pose a number of problems that are far from being solved. To use the map of runoff potential at
the watershed scale, it is currently necessary to use a calibration function. The parameters of this
function have been estimated and can be used for calculating the 10-year floods on small
watersheds of West Africa.
One presentation covers the use of the rainfall-runoff model elaborated by the Soils
Conservation Service of the United States Department of Agriculture (USDA/SCS) through the
runoff curve number (CN) to define the conditions of runoff (Colombo and Sarfatti). This is the
most traditional method and the simplest utilization of remote sensing for the estimation of runoff
conditions. The method, developed in the US, is tested in this case in Eritrea for the estimation of
peak discharges and of annual runoff. It includes two phases: a phase of image splitting based on
remote sensing and on the assignment of a global parameter to the hydrodynamic behaviour
(production coefficient) which is then applied prorata to land cover. The extrapolation of
calculation methods poses the problem of choosing the CN coefficient, considering that these
coefficients are used in a context that is different from the one for which they were initially
developed.
Another presentation also covers the issue of the hydrological response of landscape units
that are considered to be homogeneous from the hydrological point of view (Viné). The landscape
segmentation phase refers to “hydro-landscapes”, and insists on the hypotheses of models and of
the choice of segmentation. The space is divided into landscape units that are grouped according
to a limited number of land cover categories. These categories are defined from the image. The
choice of categories is based upon the hypothesis of each one having a specific hydrological
response function contribution. Putting in relation the global hydrological response of several
small watersheds (on a yearly, quarterly and monthly basis) with the different categories of land
use, the unit response for each category is obtained through a numeric deconvolution scheme,
based upon a disaggregation technique. This original analytical method appears well adapted to
the valorization of remote sensing to provide a global value of hydrological response per
landscape unit.