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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
aspect, the sun exposition of the slopes, the slope limit variation
etc.
The intersection of the DEM with the vegetation cover layer
allows for example the determination of forest areas distributed
on altitudinal levels. By merging the map of the slope aspect
with the vegetation cover layer, a representation of the
vegetation distribution, function of the aspect of the slopes over
the basins may be obtained.
The dynamics of slope limit variations may be derived by
applying the gradient function (horizontal or vertical) on the
DEM.
The merging of LANDSAT-TM or SPOT satellite multispectral
imagery with the DEM is useful for updating the vegetal cover
and for the evaluation. of some parameters of the snow
accumulation or snow melting processes: retention coefficient,
infiltration coefficient, waterflow coefficient, etc.
4. SATELLITE DATA FOR HYDROLOGICAL MODEL
PARAMETER ESTIMATION
A more complex application of remote sensing data in
hydrology consists in the use of such data either for the
estimation of parameters of hydrological models or as input into
such models. For both purposes the application of GIS and
remote sensing improves the efficiency of such procedures
significantly.
The condition of the land surface (land cover, land use,
vegetation status etc.) influences hydrological process like
precipitation interception, infiltration and water runoff. In many
cases such information is derived from digitized maps which
are stored in GIS. This conventional technique has several
disadvantages because maps are often outdated and low
accuracy. These problems may be overcome by land cover/use
classification carried out with the aid of satellite imagery.
The snowmelt-runoff process is influenced by the vegetation
status. There are various indicators for this vegetation status,
like the “Normalized Difference Vegetation Index” (NDVI),
determined with the visible and near-IR satellite channels
(figure 3).
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Figure 3. NDVI map for Arges basin
obtained from LANDSAT 7 ETM- (04.07.2002)
265
Another type of information, which is frequently used in
hydrological modeling, is the subdivision of a drainage basin
into "Hydrological Similar Units" (HSU), which behave in the
same way. The determination of the HSU's requires data on soil
type, land use, on elevation, slope and aspect. So it is possible
to obtained the areas of equal hydrological behavior due to their
equal land use, slope and soil type. This is valuable information
for the water flow modeling in the snowmelt period.
5. REMOTE SENSING MONITORING OF SNOWPACK
DYNAMICS IN VIEW OF ESTIMATING THE SNOW
COVER WATER RESOURCES
The efficient and rational management of water resources in a
drainage basin involves the knowledge of snowpack conditions
evolutions and the snowcover phases of the snowmelt season.
In the National Institute of Meteorology and Hydrology in
Bucharest technical means and working methods were
developed for operational applications in the following field of
the snow hydrology: watershed snowcover areal extent and
snowline elevation determination, new snowfalls identification,
snowpack depth estimation, ablations monitoring and melting
zones discrimination.
The method for evaluation the water resources stored in the
snowpack (Stancalie, 1991) is based on the use of data from
three investigation levels: satellite, air and ground. This use is
required by:
- the complex physiographical peculiarities of the Carpathian
watershed and their relatively restricted area which calls for the
use of large scale imageries.
- the necessity of determining both the extent of the snow cover
and its condition which calls for the use of multispectral image-
data;
- the instability in time, during the winter-spring season, of the
maximum stored amount,
and the snowmelt process beginning which requires a
continuous surveillance of the snow cover , which imposes the
repetitive air and satellite imageries and data from the hydro-
meteorological network in rapid flux;
- the need to correlate remotely sensed data with the ground
truth.
Due to the variation in terms of altitude of the physiographical
and meteorological factors conditioning the snow cover, the
extension of watersheds over a relatively great altitude range
(600-2500 m) have led to:
- the division of each watershed under consideration into
altitude levels of 100-200 m;
- the estimation of the areas under the main types of vegetation
cover at each altitude step.
The need to obtain ground truth data has required the selection
and delimitation of some sampling areas that should cover the
most important types of land cover, classified according to their
capacity for snow storing.
The determination of the snow cover extent and the monitoring
of snow over the basin being investigated, at various moments
in the winter-spring season used:
- a direct method for singling out the areal with snow from the
imageries by means of the facilities provided by the processing
systems;
- an indirect method of converting the average snow line
altitude in the area by means of the hypsometric curve of the
watershed.
