Evaluation of a methodology to translate rainfall forecasts into runoff forecasts for South Africa.
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Date
2002
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Abstract
South Africa experiences some of the lowest water resource system yields in the
world as a result of the high regional variability of rainfall and runoff. Population
growth and economic development are placing increasing demands on the nation's
scarce water resources. These factors, combined with some of the objectives of
the new National Water Act (1998), are highlighting the need for efficient
management of South Africa's water resources.
In South Africa's National Water Act (1998) it is stated that its purpose is to ensure
that the nation's water resources are protected, used, conserved, managed and
controlled in a way, which takes into account, inter alia,
i. promoting the efficient, sustainable and beneficial use of water in the public
interest, and
ii. managing floods and droughts.
Efficient and sustainable water resource and risk management can be aided by the
application of runoff forecasting. Forecasting thus fits into the ambit of the National
Water Act and, therefore, there is a need for its operational application to be
investigated. In this document an attempt is made to test the following hypotheses:
Hypothesis 1: Reliable and skilful hydrological forecasts have the ability to prevent
loss of life, spare considerable hardship and save affected
industries and commerce millions of Rands annually if applied
operationally within the context of water resources and risk
management.
Hypothesis 2: Long to medium term rainfall forecasts can be made with a degree
of confidence, and these rainfall forecasts can be converted into
runoff forecasts which, when applied within the framework of water
resources and risk management, are more useful to water resource
managers and users than rainfall forecasts by themselves.
The validity of Hypothesis 1 is investigated by means of a literature review. South
Africa's high climate variability and associated high levels of uncertainty as well as
its current and future water resources situation are reviewed in order to highlight the importance of runoff forecasting in South Africa. Hypothesis 1 is further
examined by reviewing the concepts of hazards and risk with a focus on the role of
effective risk management in preventing human, financial and infrastructural
losses.
A runoff forecasting technique using an indirect methodology, whereby rainfall
forecasts are translated into runoff forecasts, was developed in order to test
Hypothesis 2. The techniques developed are applied using probabilistic regional
rainfall forecasts supplied by the South African Weather Service for 30 day periods
and categorical regional forecasts for one, three and four month periods for
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regions making up the study area of South Africa, Lesotho and Swaziland. These
forecasts where downscaled spatially for application to the 1946 Quaternary
Catchments making up the study area and temporally to give daily rainfall forecast
values.
Different runoff forecasting time spans produced varying levels of forecast
accuracy and skill, with the three month forecasts producing the worst results,
followed by the four month forecasts. The 30 day and one month forecasts for the
most part produced better results than the more extended forecast periods. In the
study it was found that hydrological forecast accuracy results seem to be inversely
correlated to the amount of rainfall received in a region, i.e. the wetter the region
the less accurate the runoff forecasts. This trend is reflected in both temporal and
spatial patterns where it would seem that variations in the antecedent moisture
conditions in wetter areas and wetter periods contribute to the overall variability,
rendering forecasts less accurate. In general, the runoff forecasts improve with
corresponding improvements in the rainfall forecast accuracy. There are, however,
runoff forecast periods and certain regions that produce poor runoff forecast
results even with improved rainfall forecasts. This would suggest that even perfect
rainfall forecasts still cannot capture all the local scale variability of persistence of
wet and dry days as well as magnitudes of rainfall on individual days and the effect
of catchment antecedent moisture conditions. More local scale rainfall forecasts
are thus still needed in the South African region.
In this particular study the methods used did not produce convincing results in
terms of runoff forecast accuracy and skill scores. The poor performance can
probably be attributed to the relatively unsophisticated nature of the downscaling
and interpolative techniques used to produce daily rainfall forecasts at a
Quaternary Catchment scale. It is the author's opinion that in the near future, with
newly focussed research efforts, and building on what has been learned in this
study, more reliable agrohydrological forecasts can be used within the framework
of water resources and risk management, preventing loss of life, saving
considerable hardship and saving affected industry and commerce millions of
rands annually.
Description
Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2002.
Keywords
Rain and rainfall--Forecasting., Flood forecasting., Water resources management., Water resources management--South Africa., Theses--Bioresources Engineering and Environmental Hydrology.