Abstract
Ph.D. (Geography)
Skillful management and planning of the earth's natural resources and of agricultural
production necessitates a great deal of Information regarding the resources and relevant
soclo-economlc factors, as well as better Information on crop yield expectations
throughout the year. These Intricate processes can often be simplified Into models. Most
of Nature's systems (for example climatic systems) are, however, too complex to allow
realistic models to be evaluated numerically and are therefore examined by means of
simulation models through which the Interaction with time between physical processes Is
established. The purpose Is to understand these processes In order to forecast the results
of these changes and Interactions.
The aim of this study Is to construct a composite climate model that (1) computes missing
climate values, and (2) extrapolates climate values until the expected date of harvesting,
by simulation using the random sampling of values from reference ("look-up") tables, In
order that (3) the climate files, with simulated climate values, could be used with the
parameter files as Input files for the CERES-Maize model. The CERES-Maize model uses
dally values to simulate the growth, development and yield of the maize plant. The
respective crop forecasting results obtained for actual and simulated climate values are
then to be evaluated.
Climate files, with four variables, were obtained on magnetic computer tape from the
South African Weather Bureau for the study area In the Eastern Transvaal. The
preliminary processing was done by the use of SA5-programmes and these files were then
exported from the mainframe computer to a personal computer and stored on floppy disks.
Climate reference flies were compiled from the original climate flies by sorting the
climate data according to the Julian date. The missing values In the climate reference
flies and the original climate files were restored from the files of neighbouring weather
stations, as calculated orestimated values by.means of a suitable method of computation.
Some of the methods used, were derived after comparing the graphs of the time-series
of a number of climate files. Aclimate simulation model was compiled In which climatic
elements were simulated by sampling values a set number of times randomly from the
climate reference files. The mean of these sampled values were adjusted by multiplying
It with a factor representing the climatic change over time. A climate file, also
containing simulated values, and a theoretical parameter Input file were then used as the
Input flies for a revised edition of the CERES-Maize model. A comparison of the results
obtained for the 1986/87 growing season when the climate files, with actual and simulated
values respectively, were used as Inputs for the CERES-Maize model, Indicated very
promising results. The values predicted for two climate flies (1962-1987) differed by
about 18%, whereas a difference of only about 8% between those predicted for two
smaller climate files (actual and simulated values respectively), representing only the
1986/87 season, was recorded. The difference between values predicted for the climate
file, mentioned last, and consisting only of simulated climate values, and those forecasted
for the original and complete climate data file, was only 5%. As Indicated by the
arithmetic mean, there is again a tendency towards the mean values.