A Protocol for
Surveillance of Soil Degradation
We have developed a protocol for
surveillance of soil degradation in the Lake Victoria
Basin.
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We first delineated the
river basins of the Lake Victoria Basin using spatial databases of
digital elevation (1 km resolution) and stream networks
(1:1,000,000) for Kenya, Uganda, Tanzania, Rwanda and Burundi. |
Data derived from these spatial databases
on the physical characteristics of the river basins indicated that Kenya
river basins have larger average annual rainfall, average slope and
sediment transport capacity than the other basins. The largest river
basin, the Kagera, also showed large erosion potential. We conducted our
first studies in the River Nyando Basin in Kenya as this basin had the
largest erosion potential.
We confirmed the large
erosion potential of the River Nyando when we uncovered a massive
sediment plume flowing into the Winam Gulf of Lake Victoria from the
river mouth . In this image the red, green and blue colours indicate
different types of soil suspended in the lake water (yellow) [Follow
this link for details and original
image].
Press coverage:
Portait of an Invader. Science
Volume 286, Number 5445, Issue of 26 Nov 1999,
p. 1675.
Scientists
Move to Save Lake Victoria from Dying. Panafrica News Agency,
December 11, 2000.
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Ground survey
Satellite imagery (Landsat)
was used
to stratify the River Nyando Basin for ground sampling (river basin
is delineated by yellow line). [Description and photos of
soil degradation in the Nyando
River Basin]. Sampling locations
were chosen to span the range of dominant parent materials and
elevation range in the study area using 1:125,000 geological and
1:50,000 topographical maps. |
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During mid 1999
through early 2001, 522, 30 x 30
meter sized plots were surveyed in the Nyando River Basin. Total
sampling effort by the involved five-person team was 29 days
distributed over three separate dry seasons i.e., periods of minimal
vegetation cover. All plots were georeferenced at their center-points
using post-processed, differentially corrected GPS. Each plot was
visually classified as apparently intact, sheet, rill, or gully
eroded, or hardset using field criteria described by Schoeneberger
et al. (1998) for soil erosion and criteria after Mullins et al.
(1990) for soil hardsetting. |
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Detailed field
observations and measurements were recorded for vegetation cover and
composition, FAO land cover classification system classes, terrain,
slope and soil surface conditions. |
Three topsoil (0-20
cm) and 3 subsoil (20-50 cm) were recovered at the 5, 15, and 25
metre positions of the centre line of the plot and in direction of
the dominant slope gradient. In some instances subsurface
restrictions prevented recovery of subsoil samples. In these cases,
the restriction type and depth were recorded.
The
soil samples were air-dried and crushed to pass through a 2-mm
sieve. A library of soil diffuse
reflectance spectra was built from these samples. |
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Three sediment cores
were collected from 3 meters water depth, circa 750 meters offshore
of the outlet of the River Nyando in Lake Victoria using an 8.5 cm
diameter gravity piston corer. Approximately 1-cm-thick sub-samples
were extruded from each core (50 to 150 cm length). All samples were
dried at 60ºC and crushed through a 2 mm sieve after
removing macro-fossils and particulate organic matter. Preliminary
Lead-210 and Cesium-137 dating of the master core, conducted at the
University of Colorado at Boulder, indicated the bottom-most section
of the core (at cm 145) could be dated to 1852 (95% CI = ± 3.3 years; D. Rowan, personal communication)
using a constant sedimentation rate assumption. |
The
data generated by the protocol is being used to map prevalence of soil physical
degradation and soil fertility constraints in the River Nyando basin at a
spatial resolution of 30 m, at acceptable odds ratios for diagnostic
surveillance purposes. We have developed empirical models that identify
tentative risk factors for soil degradation and fertility constraints. The
models are being used to provide practical recommendations for management
of soil degradation that are quantitative and spatially explicit. For
example, areas where preventative strategies as opposed to rehabilitation
strategies can be located and critical levels of herbaceous ground cover
for minimizing soil physical degradation can be specified in relation to
slope and soil depth. Sequential diagnostic tests using imagery, ground
observations, and soil spectral tests provide increasing levels of
precision that are appropriate for different uses and audiences (e.g.
broad mapping for policy makers, site specific diagnosis for field
advisers and land users). The protocol is also
providing a sampling frame for more detailed studies, such as prospective
studies and spatial analogue surveys.
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