Browsing by Author "Abiodun, Babatunde"

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    Open Access
    Characteristics of precipitation and extreme precipitation events over the Drakensberg Mountain range in past and future climates
    (2023) Takong, Ridick; Abiodun, Babatunde
    The Drakensberg Mountains is one of the most valuable natural resources in Southern Africa because precipitation over the mountains is the source of rivers that support socio-economic activities in Lesotho, South Africa, and Namibia. Meanwhile, extreme precipitation events over the Drakensberg are a threat to the communities around the mountains. While several studies have shown that mountains are among the most sensitive regions to climate variability and change, the potential impacts of global warming on precipitation and extreme precipitation occurrences over the Drakensberg are poorly understood. This thesis examines the characteristics of precipitation and extreme precipitation events over the Drakensberg in past climate and investigates how the characteristics might change in future climate at various global warming levels under RCP8.5 future climate scenario. Series of climate datasets were analysed for the study. These include observed precipitation datasets from eight satellite products, reanalysis datasets from National Centre for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR), and climate simulation datasets from the Model for Prediction Across Scales (MPAS), Weather Research and Forecasting Model (WRF), and the National Aeronautics and Space Administration (NASA) Earth Exchange Global Daily Downscaled Projections (NEX), and the Coordinated Regional Climate Downscaling Experiment (CORDEX). All the simulation datasets were evaluated against the observation datasets. Precipitation indices were used to characterize precipitation and extreme precipitation events over the Drakensberg Mountains, with emphasis on widespread extreme events (WEREs). Self-Organizing Map (SOM) technique was employed to group the synoptic patterns over southern Africa, WERE patterns over DMR, and the future climate change projections over the Drakensberg Mountains. Results of this dissertation reveal that the weak synoptic days, which are associated with highpressure systems or the ridging of highs, account for 16 − 20% of weather conditions in MarchAugust and 5% of annual rainfall over the Drakensberg. Wet weak synoptic days can induce widespread extreme rainfall (up to 20mm day−1 ) over the Drakensberg. CFSR underestimates the magnitude of the weak-synoptic-day rainfall but the WRF downscaling of the CFSR dataset enhances the quality of the simulated rainfall. All of the climate simulation datasets (WRF, MPAS, CORDEX, NEX) give realistic simulations of the precipitation indices over Southern Africa, especially over South Africa and DMR. In most cases, the biases in the simulations are within the observation uncertainties. SOM analysis reveals four major patterns of WERE patterns over the Drakensberg. The most prevalent WERE pattern usually occurs on the eastern side of the mountain, stretching from north-east to the south-west along the coastline, and it is usually induced by tropical temperate troughs, cold fronts, and the ridging highs. There is no agreement among simulations ensemble means on the annual precipitation projection over DMR. However, the ensemble means agreed on an increase in the intensity of normal precipitation and a decrease in the number of precipitation days and the number of continuous wet days. They also agreed on a future increase in frequency and intensity of extreme precipitation and widespread extreme events over DMR. SOM analysis, which elucidates the range of projection patterns that lie beneath the simulation ensemble means of the simulations, shows the most probable combination of projected changes in the annual precipitation and extreme precipitation events (intensity and frequency) over DMR: (i) an increase in both annual precipitation and extreme precipitation events; (ii) a decrease in both annual precipitation and extreme precipitation events; (iii) a decrease in annual precipitation but increase in extreme precipitation events. Results of this study can provide a basis for developing climate change adaptation and mitigating strategies over the Drakensberg.
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    Open Access
    Potential impacts of climate change and land-use change on hydrological drought in the Western Cape (South Africa)
    (2022) Naik, Myra; Abiodun, Babatunde
    The Western Cape (South Africa) recently witnessed the most severe drought on record. The meteorological drought, which was characterised by below-normal rainfall for three consecutive years (2015 – 2017), cascaded to agricultural and then hydrological drought, resulting in devastating socio-economic consequences. While some studies indicate that climate change may increase the severity and frequency of droughts in the Western Cape in the future, there is a lack of information on how to mitigate the effects of future climate change on hydrological drought. This dissertation therefore investigated the extent to which land-use changes could be applied to reduce climate change impacts on future hydrological drought in this region. For the study, the revised Soil Water Assessment Tool (SWAT+) was calibrated and evaluated over four river basins in the Western Cape, and the climate simulation dataset from the COordinated Regional Downscaling EXperiment (CORDEX) was bias-corrected. Using the bias-corrected climate data as a forcing, the SWAT+ was used to project the impacts of future climate change on water yield and hydrological drought in the four basins and to quantify the sensitivity of the projection to four feasible land-use change scenarios in these basins. The relevant land-use scenarios are the expansion of mixed forests (FrLand), the restoration of grassland (GrLand), the restoration of shrubland (SrLand), and the expansion of cropland (CrLand). The model evaluation shows good agreement between the simulated and observed monthly streamflow at hydrological stations, and the bias correction of the CORDEX datasets improved the quality of the SWAT+ hydrological simulations in the four basins. The climate change projection depicts an increase in temperature and potential evapotranspiration but a decrease in precipitation and all the hydrological variables. Drying is projected across the Western Cape, and the magnitude of such drying increases with higher global warming levels (GWLs). The land-use changes alter the impacts of climate change by influencing the hydrological balance. While FrLand mitigates the impacts of climate change on the frequency of hydrological drought by increasing streamflow, soil water and percolation, CrLand mitigates the impacts by increasing surface runoff. However, the magnitudes of these land-use change impacts are very small compared to the climate change impacts. Hence, the results suggest that land-use changes may not be an efficient strategy for mitigating the climate change impacts on hydrological drought over the region. The findings obtained from this 2 research provide relevant information towards mitigating the severity of future droughts and improving water security in Western Cape River Basins.
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    Potential impacts of climate change on hydrological droughts in the Limpopo river basin
    (2021) Makhanya, Nokwethaba Zamanguni; Wolski, Piotr; Abiodun, Babatunde
    Climate change possibly intensifies hydrological droughts and reduces water availability in river basins. Despite this, most research on climate change effects in southern Africa has focused exclusively on meteorological droughts. This thesis projects the potential effect of climate change on the future characteristics of hydrological droughts in the Limpopo River Basin (LRB). The study uses regional climate model (RCM) measurements (from the Coordinated Regional Climate Downscaling Experiment, CORDEX) and a combination of hydrological simulations (using the Soil and Water Assessment Tool Plus model, SWAT+) to predict the impacts at four global warming levels (GWLs: 1.5℃, 2.0℃, 2.5℃, and 3.0℃) under the RCP8.5 future climate scenario. The SWAT+ model was calibrated and validated with a streamflow dataset observed over the basin, and the sensitivity of model parameters is investigated. The performance of SWAT+LRB model was verified using the Nash-Sutcliffe efficiency (NSE), Percent Bias (PBIAS), Root Mean Square Error (RMSE), and coefficient of determination (R2 ). The study also examines the capability of the CORDEX SWAT+ system in reproducing the hydro-climatology and the influence of the quantile delta mapping (QDM) method on bias correction of CORDEX datasets. The Standardized Precipitation Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPI) have been used to detect meteorological droughts. The Soil Water Index (SSI) has been used to define agricultural drought, when the Water Yield Drought Index (WYLDI), the Surface Run-off Index (SRI), and the Streamflow Index (SFI) have been used to characterize hydrological drought. The performance of SWAT+ the model simulations over LRB is sensitive to the parameters CN2 (initial SCS runoff curve number for moisture condition II) and ESCO (soil evaporation compensation factor). The best simulation is generally performed better during the calibration period than in the validation period. In calibration and validation periods, NSE is ≤ 0.8, while PBIAS is ≥ ﹣80.3%, RMSE ≥ 11.2 m3 /s and R 2 ≤ 0.9. Although the CORDEX simulations capture the general spatial and temporal distribution of the hydroclimate variables over the LRB, they feature a cold and wet bias across the basin. However, the QDM bias correction reduces the bias and fosters better agreement among the simulations. The simulations project in all hydrological variables is projected over most parts of the basin, especially over the eastern part of the basin. The simulations predict that meteorological droughts (i.e., SPEI and SPI), agricultural droughts (i.e., SSI), and hydrological droughts (i.e., WYLDI, SRI) would become more intense and severe across the basin. SPEI-drought has a greater magnitude of increase than SPI drought, and agricultural and hydrological droughts have a magnitude of increase that is part-way between the two. As a result, this research suggests that future hydrological droughts over the LRB could be more severe than the SPI-drought projection predicts but less severe than the SPEI-drought projection. This research can be used to mitigate the effects of potential climate change on basin hydrological drought.
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    Open Access
    Relationship between Aspalathus linearis (Burm. F.) R. Dahlgren (rooibos) growth and soil moisture in a glasshouse and in the DSSAT-CSM crop model
    (2021) Adaramola, Rhoda Fiyinfoluwa; Chimphango, Samson; Abiodun, Babatunde
    Climate change and drought pose a major threat to agriculture and water resources globally and for rooibos (Aspalathus linearis (Burm. F.) R. Dahlgren) production in the Western Cape province of South Africa. Rooibos is adapted to the coarse, nutrient poor, acidic, well-drained, deep sandy soil of the Fynbos biome. The region has a Mediterranean climate, which is characterised by wet cold winters, with an average annual rainfall of about 375 mm, and dry summers. The growth of rooibos peaks in the summer months, implying a reliance on soil moisture. The current study aims to investigate the relationship between rooibos growth and soil moisture. The objectives of the study were: 1) to determine the effect of soil moisture on growth and evapotranspiration in rooibos under glasshouse conditions, 2) to adapt the CROPGRO model in DSSAT to simulate the shoot biomass yield of rooibos, using the rooibos CROPGRO model, 3) to investigate the effect of rooibos growth on soil moisture, and 4) to determine the effect of different levels of mulching and irrigation on rooibos yield and soil moisture. Some of the results obtained in the glasshouse study in Objective 1 and observational field data from the literature were used in the adaptation of the CROPGRO model. The glasshouse study was carried out at the University of Cape Town, using soils from Clanwilliam and Citrusdal sites to grow rooibos seedling for 16 weeks in pots before exposing them to drought treatments. The pots were arranged on trays in the glasshouse using a completely randomized design. Two drought treatments were used: moderate drought stress (MDS), set at 20% FC, and severe drought stress (SDS), during which watering was completely withdrawn, were applied to 10 pots per treatment per site. Data on plant growth, root morphology, evapotranspiration, soil moisture, chlorophyll fluorescence and leaves to determine chlorophyll and carotenoid concentration were collected from the plants in the glasshouse after 10 days of these drought treatments. The SDS plants were re-watered for 8 weeks for recovery, and together with the MDS and control plants were transferred into a growth chamber for measurement of gas exchange parameters and biomass. The CROPGRO model in DSSAT was adapted for rooibos by changing some parameters in a pigeon pea (Cajanus cajan L. Millspaugh) CROPGRO model. The adapted rooibos model was used to set up an experiment that compared the cumulative evapotranspiration and soil moisture from the rooibos field and bare soil under rainfed conditions. Also, in a simulation experiments, the model was used to determine the effect of three levels of mulching by means of wheat residue at 8000 kg/ha, 4000 kg/ ha and 2000 kg/ha and drip irrigation at 25.4mm and at 12.5mm once a week from December to March, both separately and in combination, on rooibos shoot biomass and soil moisture. The results from the glasshouse study showed a 40% decrease in biomass under MDS conditions for 12 weeks, while SDS plants could not survive beyond 10 days in the glasshouse. Root morphological features changed under severe drought stress, resulting in longer and thinner roots relative to the control plants. The reduced biomass accumulation under drought conditions was followed by reduced photosynthesis, stomata conductance, transpiration, and concentration of chlorophyll and carotenoids. Changes in both maximum quantum efficiency of photosystem II (Fv/Fm) and fluorescence quantum yield (Fq'/Fm') were observed in the later stages of the SDS plants (days 9 and 10) compared to the control plants but were unaltered in the MDS plants. The soil moisture correlated negatively with evapotranspiration and stomata conductance in control plants, while these relationships were absent in MDS plants. Changes in temperature in the glasshouse correlated positively with stomata conductance and transpiration in the control plants, but these correlations were also absent in MDS plants. However, changes in temperature correlated negatively with soil moisture in both the control plants and the MDS treated plants. The CROPGRO model in DSSAT was successfully adapted to simulate shoot biomass in rooibos under field conditions and the rooibos model had an agreement of 94% with observational shoot biomass under field conditions. Furthermore, the model simulated cumulative evapotranspiration in rooibos plants in the field, with an agreement of 56%. The simulated experiments showed that cumulative evapotranspiration from the rooibos field was 33% higher than that of bare soil, and showed that rooibos plants extract moisture from deep soil layers to a depth of about 2 m. Furthermore, rooibos growth in deep soil, and in mulched or irrigated treatments, produced higher shoot biomass than control plants. In deep soil, the simulated irrigated rooibos plants, which received 25.4 mm water weekly from December to March, produced a higher biomass yield than only rainfed or mulched plants. However, the combined treatments of mulching at 8000 or 4000 kg/ha and irrigation at 12.5 mm was similar to irrigation at 25.4mm. The average extractable soil moisture was greater in deep soil for all the treatments and control plants compared to shallow soil. Overall, the rooibos crop model shows that an increased supply of soil moisture enhances the production of biomass yield in rooibos in the field. Also, rooibos extracts moisture from a deeper soil layer, which enables it to hydrate its leaves and to transpire during the summer period for better growth and biomass production. Water loss through evapotranspiration was high in rooibos fields, and thus mulching of the plants would be beneficial for increased biomass production. However, even better rooibos yields were obtained when mulching was combined with irrigation. The glasshouse experiments showed a yield decrease of rooibos biomass by about 40% when the moisture supply was reduced by about 50% of the adequate conditions. The thinner and longer roots of rooibos, among other drought tolerance traits, most likely enable it to cope with low rainfall and drought conditions, which are prevalent in the Cederberg region of the Western Cape. The production of rooibos in the farms is prone to water loss through evapotranspiration, and thus soil moisture conservation technologies such as mulching would greatly enhance its biomass yield.
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    Open Access
    Understanding the impacts of ENSO patterns on droughts over southern Africa using SPEEDY
    (2019) Gore, Michelle Jacqueline; Abiodun, Babatunde
    The El Niño Southern Oscillation (ENSO) is a major driver of southern Africa droughts, but the nonlinearity of ENSO variation inhibits accurate prediction of droughts. While studies have identified multiple patterns of ENSO, most drought predictions over southern Africa are still based on only two ENSO patterns. This study examines the relationship between southern African droughts and eight ENSO patterns: four El Niño SST conditions (EN1 - EN4) and four La Niña SST conditions (LN1 - LN4). In this study we analyzed multi-forcing ensemble simulations from SPEEDY (a general circulation model from the International Centre for Theoretical Physics) and used two drought indices (SPEI: Standardized Precipitation Evapotranspiration Index; SPI: Standardized Precipitation Index) to characterize drought. The capability of SPEEDY in reproducing southern Africa climate was evaluated by comparing the historical simulations (1979- 2008) with the Climate Research Unit (CRU) observation. To obtain the influence of ENSO patterns, we forced the SPEEDY simulations with SST of each ENSO pattern, analyzed the impacts on the simulated drought indices (SPEI and SPI), and studied the atmospheric dynamics that link each ENSO pattern to southern Africa droughts. The results show that SPEEDY generally captures the temporal and spatial distribution of climate variables over southern Africa well, although with warm and wet biases across the region. However, in most cases, these results are comparable with those from more complex atmospheric models. In agreement with previous studies, the results show that El Niño SST conditions weaken the Walker circulation and cause drier conditions over parts of southern Africa, whilst La Niña SST conditions strengthen the Walker Circulation and cause wetter conditions. However, the results show that the differences in the El Niño SST conditions (EN1 - EN4) alter the circulation, thereby influencing the spatial pattern and intensity of drought over the region. For instance, while EN2 induces the most severe drought in the tropical area, EN4 produces it in the southwestern region, because the two patterns feature different characteristics of anticyclonic moisture flux over southern Africa. The same is true of the La Niña SST conditions. Although, LN1 and LN4 show wet conditions across the southern part of the region, LN1 produces drought in the northern part, while LN4 induces it along the western coast. Hence, this study shows that accounting for the differences in El Niño (or La Niña) conditions may improve drought predictions in southern Africa.