The agroclimatic potential of fuel crops in Zimbabwe

Abstract

The main objective of this thesis was to assess the agro-climatic potential of fuel crops (typically sorghum) in Zimbabwe using a revised map of solar radiation received at the surface. To achieve this objective climatic data, principally, maximum and minimum temperatures, and surface solar radiation were collected and analyzed.

Values of solar radiation data gathered from two terrestrial and one satellite source did not agree closely. Satellite estimates of solar radiation were found to be on average, at least 21 % lower than ground based measurements. Values from the Climatic Handbook, updated with more recent records from the Zimbabwe Meteorological Department were used to produce monthly radiation maps using the computer program Surfer.

Potential yields (biomass production) of sweet sorghum (Keller variety) under existing climate conditions were calculated using the Ceres model within the Decision Support System for Agrotechnology Transfer (DSSAT version 4.0) for four stations, representative of different Agroecological Zones of Zimbabwe. A few sensitivity tests were implemented to test the response of the potential yield to changes in environmental conditions, as well as a change in the sorghum variety. Potential yields assessment gave high productivity over Karoi (Natural Region IIa) and Masvingo (Natural Region IV) relative to Harare, and Buffalo Range. The lowest potential yield was found in Buffalo Range. However, while Karoi and Masvingo had high yields but, Masvingo displayed a shorter growing season, that means it has a relatively higher biomass production rate.

From the sensitivity tests, a 2 °C rise in maximum temperature (warmer climate) as well as a reduction in surface solar radiation of 5 MJ/m2/day resulted in a decrease in potential yields. The average potential yield decrease under a temperature modified environment was about 4 % and under reduced surface radiation about 9 %.

The potential yields of sweet sorghum (Keller variety) were compared with those of grain sorghum (Pioneer 8333) for the current climate of Karoi. A comparison of the potential yields of two sorghum varieties, Keller (sweet) and Pioneer 8333(grain) over the Karoi site did not give significant differences. Notable differences, however, were found in the length of the growing period (LGP); Pioneer displayed a short growing period relative to Keller.

The project illustrates the first part of the methodology required to evaluate the potential and economic benefits of different fuel crops in Zimbabwe. Further studies of alternative crops and varieties, and the costs of fuel production are required.