132
3.4. Rainfall
6. REFERENCES
Rainfall throughout the catchment was monitored
by a handfull of rain gauges, all of which were
in easily accessible lowland areas. These had
been used to produce mean annual rainfall
measures from which isohyets had been sketched.
This is not the best basis for estimating rainfall
on a per pixel basis, but is invariably the only
source of available data in developing countries.
The rainfall data was treated as contours and
spotheights and interpolated to produce a mean
rainfall surface.
Another possibility, investigated as a possible
future study, was the use of weather satellites
in the estimation of precipitation. For this a PC
based weather satellite receiving station would
be installed, in this case to receive the Japanese
GMS geostationary satellite and possibly the
polar orbiting satellites NOAA 10 and 11. This
would provide rainfall estimates on an hourly or
daily basis. Caibration of the satellite derived
estimates could be achieved using the existing
rain gauges. This would require the continual
monitoring of rainfall over large periods of time,
but would produce estimates of rainfall in the
not so easily accessible upland areas; and also
provide a source of data for use in surface
runoff and soil moisture models.
4. THE EROSION RISK MAP
The combined data set was then used in
production of an erosion risk map, based on the
SLEMSA model. This rated the relative risk of
erosion for each pixel in the database.
In this process empirical measures were used to
assess the effects of various plant canopy cover
on the interception of rain water and the
blanketing of erosive soil with plant litter. This
is therefore of particular significance in the
accuracy of the model, and only relative risk
could be produced. In the determination of the
potential benefits of re-forestation of certain
areas this is adequate.
Application of the Universal Soil Loss equation
(USLE) was also investigated, using parameters
derived for the Philippines. This gave a much
better quantification of erosion rates from the
catchment. However, it should be remembered
that both predictive techniques were being used
outside the parameter ranges for which they
have been designed and tested. Further work on
the quantitative modelling of sediment yield and
the routing of sediment through a series of
nested catchments is currently being undertaken.
5. CONCLUSIONS
This study has shown the application of remote
sensing and spatial information systems in the
support and monitoring of a typical catchment
management project. In this case the
identification of areas susceptible to soil erosion,
related to current land use, soil type and
topography. The same system can be used to
monitor the progress of such policies over a
number of years; and provides a means of
integrating data from satellites, cartography and
point sources in a form that facilitates use in
hydrological models.
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of Civil Engineers, 43:1-9.
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