Crop suitability mapping for underutilized crops in South Africa.
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Date
2022
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Abstract
Several neglected and underutilised species (NUS) provide solutions to climate change and
create a Zero Hunger world, the Sustainable Development Goal 2. However, limited
information describing their agronomy, water use, and evaluation of potential growing zones
to improve sustainable production has previously been cited as the bottlenecks to their
promotion in South Africa's (SA) marginal areas. Therefore, the thesis outlines a series of
assessments aimed at fitting NUS in the dryland farming systems of SA. The study successfully
mapped current and possible future suitable zones for NUS in South Africa. Initially, the study
conducted a scoping review of land suitability methods. After that, South African bioclimatic
zones with high rainfall variability and water scarcity were mapped. Using the analytic
hierarchy process (AHP), the suitability for selected NUS sorghum (Sorghum bicolor), cowpea
(Vigna unguiculata), amaranth and taro (Colocasia esculenta) was mapped. The future growing
zones were used using the MaxEnt model. This was only done for KwaZulu Natal. Lastly, the
study assessed management strategies such as optimum planting date, plant density, row
spacing, and fertiliser inputs for sorghum. The review classified LSA methods reported in
articles as traditional (26.6%) and modern (63.4%). Modern approaches, including multicriteria
decision-making (MCDM) methods such as AHP (14.9%) and fuzzy methods (12.9%),
crop simulation models (9.9%) and machine-learning-related methods (25.7%), are gaining
popularity over traditional methods. The review provided the basis and justification for land
suitability analysis (LSA) methods to map potential growing zones of NUS. The review
concluded that there is no consensus on the most robust method for assessing NUS's current
and future suitability. South Africa is a water-scarce country, and rainfall is undoubtedly the
dominating factor determining crop production, especially in marginal areas where irrigation
facilities are limited for smallholder farmers. Based on these challenges, there is a need to
characterise bioclimatic zones in SA that can be qualified under water stress and with high
rainfall variation. Mapping high-risk agricultural drought areas were achieved by using the
Vegetation Drought Response Index (VegDRI), a hybrid drought index that integrates the
Standardized Precipitation Index (SPI), Temperature Condition Index (TCI), and the
Vegetation Condition Index (VCI). In NUS production, land use and land classification address
questions such as “where”, “why”, and “when” a particular crop is grown within particular
agroecology. The study mapped the current and future suitable zones for NUS. The current
land suitability assessment was done using Analytic Hierarchy Process (AHP) using
multidisciplinary factors, and the future was done through a machine learning model Maxent.
The maps developed can contribute to evidence-based and site-specific recommendations for
NUS and their mainstreaming. Several NUS are hypothesised to be suitable in dry regions, but
the future suitability remains unknown. The future distribution of NUS was modelled based on
three representative concentration pathways (RCPs 2.6, 4.5 and 8.5) for the years between 2030
and 2070 using the maximum entropy (MaxEnt) model. The analysis showed a 4.2-25%
increase under S1-S3 for sorghum, cowpea, and amaranth growing areas from 2030 to 2070.
Across all RCPs, taro is predicted to decrease by 0.3-18 % under S3 from 2050 to 2070 for all
three RCPs. Finally, the crop model was used to integrate genotype, environment and
management to develop one of the NUS-sorghum production guidelines in KwaZulu-Natal,
South Africa. Best sorghum management practices were identified using the Sensitivity
Analysis and generalised likelihood uncertainty estimation (GLUE) tools in DSSAT. The best
sorghum management is identified by an optimisation procedure that selects the optimum
sowing time and planting density-targeting 51,100, 68,200, 102,500, 205,000 and 300 000
plants ha-1 and fertiliser application rate (75 and 100 kg ha-1) with maximum long-term mean
yield. The NUS are suitable for drought-prone areas, making them ideal for marginalised
farming systems to enhance food and nutrition security.
Description
Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.