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A.M.J. Meijerink and C.M.M. Mannaerts
15.4 Watershed management with the aid of remote sensing
15.4.1 Introduction
Watershed management contains all activities concerning sustained use, protection
or rehabilitation of the water, soil and vegetation resource base in the upper parts or
headwater systems of larger river basins. Various operational levels can be distinguished in watershed management, ranging from large basins, watersheds, subwatershed to local scale (Sheng, 1990). As the detail of survey increases, one fmally
arrives at the farm or community level of survey and inventory. The range of levels
is reflected by the range of spatial resolutions of aerospace images used. Compared
to aerial photographs, satellite images such as Landsat MSS, TM and IRS LISS II,
have a relative low spatial resolution. They provide overviews at regional scale,
particularly of land cover. With other information they are used for zoning of areas
or watersheds within larger catchments in order to list priority for treatment. In some
countries, the concept of 'critical' areas is used and the priority depends on the
proportion of critical areas within the catchment. Critical areas are those where the
land cover which offers little protection to erosion, such as row crops or overgrazed
rangelands, occurring in combination with certain lithologies - i.e. those where soils
which are susceptible to erosion -, and with dissected, sloping lands. At the other
end of the range, large scale aerial photographs, say, 1:10.000 or 1:20.000, are used.
Apart from the details of land cover and infrastructure visible on the photographs,
they offer the possibility of stereo graphic interpretation of the geomorphology and
morphometry of the terrain. The topographic information at large scales thus derived
can save much time and costs for the preparation of so called "engineering designs"
for the planning of soil conservation measures which follows the priority assessment
using smaller scales.
15.4.2 Hydrologic photo-interpretation for watershed management
Watershed management, and especially the planning of sustainable soil & water use,
requires detailed hydrologic information pertaining to the terrain and vegetation, as
well as to their interactions. The larger scales of stereo aerial photography are
eminently suitable for studying the interactions by visual interpretation. These
interpretations must be embedded in basic background knowledge of geology,
geomorphology and soils and on the effects of such terrain factors on the hydrology
of the area. This is demonstrated by the following example from Tanzania, East
Africa. Figure 15.4 shows a gneiss plateau (sub-areas 1 and 2) bounded by faults in
contact with a lower area of soft sedimentary shale and sandstone rocks (sub-area 3),
draining into a alluvial flood plain with swamps (Handeni area, Tanzania). The
lithology and geomorphological history exerts controls over the hydrology, as is
shown by the three aerial photo insets, whose locations are shown on the map. Subarea 1 is underlain by impermeable gneiss and has a thick weathered zone, associated
with a planation level, which supports a forest savannah. Infiltrated water seeps down
to grassy valley bottoms, which become saturated at the end of the wet season. Runoff
A.M.J. Meijerink and C.M.M. Mannaerts
15.4 Watershed management with the aid of remote sensing
15.4.1 Introduction
Watershed management contains all activities concerning sustained use, protection
or rehabilitation of the water, soil and vegetation resource base in the upper parts or
headwater systems of larger river basins. Various operational levels can be distinguished in watershed management, ranging from large basins, watersheds, subwatershed to local scale (Sheng, 1990). As the detail of survey increases, one fmally
arrives at the farm or community level of survey and inventory. The range of levels
is reflected by the range of spatial resolutions of aerospace images used. Compared
to aerial photographs, satellite images such as Landsat MSS, TM and IRS LISS II,
have a relative low spatial resolution. They provide overviews at regional scale,
particularly of land cover. With other information they are used for zoning of areas
or watersheds within larger catchments in order to list priority for treatment. In some
countries, the concept of 'critical' areas is used and the priority depends on the
proportion of critical areas within the catchment. Critical areas are those where the
land cover which offers little protection to erosion, such as row crops or overgrazed
rangelands, occurring in combination with certain lithologies - i.e. those where soils
which are susceptible to erosion -, and with dissected, sloping lands. At the other
end of the range, large scale aerial photographs, say, 1:10.000 or 1:20.000, are used.
Apart from the details of land cover and infrastructure visible on the photographs,
they offer the possibility of stereo graphic interpretation of the geomorphology and
morphometry of the terrain. The topographic information at large scales thus derived
can save much time and costs for the preparation of so called "engineering designs"
for the planning of soil conservation measures which follows the priority assessment
using smaller scales.
15.4.2 Hydrologic photo-interpretation for watershed management
Watershed management, and especially the planning of sustainable soil & water use,
requires detailed hydrologic information pertaining to the terrain and vegetation, as
well as to their interactions. The larger scales of stereo aerial photography are
eminently suitable for studying the interactions by visual interpretation. These
interpretations must be embedded in basic background knowledge of geology,
geomorphology and soils and on the effects of such terrain factors on the hydrology
of the area. This is demonstrated by the following example from Tanzania, East
Africa. Figure 15.4 shows a gneiss plateau (sub-areas 1 and 2) bounded by faults in
contact with a lower area of soft sedimentary shale and sandstone rocks (sub-area 3),
draining into a alluvial flood plain with swamps (Handeni area, Tanzania). The
lithology and geomorphological history exerts controls over the hydrology, as is
shown by the three aerial photo insets, whose locations are shown on the map. Subarea 1 is underlain by impermeable gneiss and has a thick weathered zone, associated
with a planation level, which supports a forest savannah. Infiltrated water seeps down
to grassy valley bottoms, which become saturated at the end of the wet season. Runoff