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. 
Allewijn, R., 1988, Regional Hydrological Systems 
Analysis using Satellite Remote Sensing Data and 
a Geographical Information System. International 
Journal of Remote Sensing : Special Issue - 1987 
EARSeL Symposium, 9(10 & 11):1775-1785. 
Beasley, D.B., 1960, ANSWERS: A Mathematical 
Model for Simulating the Effects of Land Use and 
Management on Water Quality. Ph.D. Thesis, 
Purdue University, West Lafayette, Indiana, USA. 
Fleming, G., 1969, Design Curves for Suspended 
Load Estimation, in Proceedings of the Institution 
of Civil Engineers, 43:1-9. 
Fleming, G., R.Walker, 1976, A Runoff Erosion 
Model for Land Use Assessment and Management. 
Technical Report, Department of Civil 
Engineering, University of Strathclyde, Glasgow, 
UK. 
Hardy, R.L., 1971, Multiquadric Equations of 
Topography and Other Irregular Surfaces. 
Journal of Geophysical Research, 76(8): 1905-1915. 
Jancaitis, J.R., J.L.Junkins, 1973, Modelling 
Irregular Surfaces. Photogrammetric Engineering, 
39(4):413-420. 
Ragan, R.M., J.D.Fellows, 1979, Computer-aided 
Watershed Analysis using Remote Sensing Based 
Regional Information Systems. The Contribution of 
Space Observations to Water Resources 
Management, Pergammon Press, pp. 181-193. 
Ritter, P., 1987, A Vector-based Slope and Aspect 
Generation Algorithm. Photogrammetric 
Engineering and Remote Sensing, 
42(12): 1539-1545. 
Stocking, M., 1987, A Methodology for Erosion 
Hazard Mapping of the SADCC Region. SADCC 
Coordination Unit, Report No. 9, Maseru, Lesotho. 
Wischmeier, W.H., D.D. Smith, 1978, Predicting 
Rainfall Erosion Losses: A Guide to Conservation 
Planning. USDA-SEA Agr. Handbook 537, 
Agricultural Research Service, USDA, Washington, 
USA.