Research Articles (Earth Sciences)

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Application of the standardised streamflow index for hydrological drought monitoring in the Western Cape Province, South Africa: a case study in the Berg River catchment.
(Multidisciplinary Digital Publishing Institute (MDPI), 2023) Mukhawana, Mxolisi Blessing; Kanyerere, Thokozani; Kahler, David
In many regions around the world, drought has been recurrent, more frequent, and more intense over time. Hence, scientific research on drought monitoring has become more urgent in recent years. The aim of this study was to test the applicability of the Standardised Streamflow Index (SSI) for hydrological drought monitoring in the Berg River catchment (BRC), Western Cape (WC) province, South Africa (SA). Using various methods described in this study, the sensitivity of the SSI to the commonly used Gamma, Log-normal, Log-logistic, Pearson Type III, and Weibull Probability Distribution Functions (PDFs) was tested. This study has found that all the tested PDFs produced comparable results for mild to severe drought conditions. The SSI calculated using the Gamma, Log-Normal, and Weibull PDFs is recommended for the BRC because it consistently identified extreme drought conditions during the 1990–2022 study period and identified the 2015–2018 droughts as the worst during the study period. Although more studies are required to test other PDFs not considered, this study has shown that the SSI can be applicable in the BRC. This study has provided a foundation for more research on the application of the SSI in the BRC and other catchments in SA.
Using multisource remotely sensed data and cloud computing approaches to map non-native species in the semi-arid savannah rangelands of Mpumalanga, South Africa
(Routledge, 2024) Maphanga, Thabang; Dube, Timothy; Sibanda, Mbulisi
Semi-arid savannah rangelands are diverse environments (in terms of species) that play an important role in sustaining biodiversity and providing ecosystem services. However, the emergence of non-native species, as well as bush encroachment, are currently threatening these (semi-arid rangeland and grassland) ecosystems. The purpose of this study was therefore to map and quantify the spatial extents of non-native woody vegetation in the Kruger National Park and surrounding communal areas in Mpumalanga, South Africa. To achieve the study’s objectives, Sentinel-1 and Sentinel-2 remotely sensed data were combined and analysed using the random forest (RF) machine-learning algorithm in the Google Earth Engine (GEE) cloud computing platform. Specifically, spectral bands and selected spectral derivatives, e.g. enhanced vegetation index (EVI2), normalized difference moisture index (NDMI) and normalized difference phenology index (NDPI) were computed and used to map non-native woody vegetation. After optimizing the model combination, the classification outputs had an overall accuracy of 70%, with class accuracies such as producer’s accuracy (PA) and user’s accuracy (UA) ranging from 67% to 95%. It was shown in this study that using Sentinel-2 and Sentinel-1 data together led to better overall accuracy than using single sensor models when mapping semi-arid savannah rangelands. It was also found in this study that the overall classification accuracy of non-native (invasive) species using optical sensors was higher than in previous studies. On a free platform like GEE, it was possible to utilize advanced classification processes to fully exploit the informative content of Sentinel-1 and Sentinel-2 data.
Assessing the effects of non-stationarity on reservoir yield estimations: A case study of the Southern Okavango integrated water development system in Botswana
(John Wiley and Sons Ltd, 2024) Mazvimavi, Dominic; Kapangaziwiri, Evison; Gumbo, Anesu Dion
Streamflow data used for water resources planning should ideally be stationary, and any non-stationary behaviour is taken into account. However, with limited time series data, the influence of non-stationarity is often hidden and can result in unreliable estimates. This paper examines the impact of non-stationarity on the Southern Okavango Integrated Water Development (SOIWD) project that was carried out with streamflow collected between 1969 and 1989 against an extended time series from 1990 to 2019. To achieve this, (a) the statistics of these inflows and (b) the reliability of satisfying water demands from the proposed reservoirs were tested based on the two periods (1969–1989 and 1990–2019). The results show that average monthly flows for July, August and September significantly change when peak outflows from the delta occur. Given the expected variability of the flow regime, an uncertainty approach utilising flow perturbation from ±5% to ±90% was also used to investigate the system's response to changes in the driving flow conditions. The increasing availability of flow data from 1990 to 2019 has shown that the SOIWD system would not have satisfied the water demand as it would not be operationally viable. This confirms the importance of accounting for non-stationarity in reservoir yield estimation and reemphasises its importance in hydrological studies
Hydrological drought assessment using the standardized groundwater index and the standardized precipitation index in the Berg River Catchment, South Africa
(Elsevier B.V., 2024) Mxolisi B. Mukhawana; Thokozani Kanyerere; David Kahler
Study region: Berg River Catchment (BRC), South Africa (SA). Study focus: Hydrological droughts threaten water security and climate Change and groundwater abstractions may exacerbate their impact. Thus, early detection and prediction of droughts is essential. The integration of the Standardised Groundwater Index (SGI) with the Standardised Precipitation Index (SPI), has been applied worldwide, it but not explored in South Africa (SA). The aim of this study was to evaluate the integration of the SGI and SPI for hydrological drought assessment within the BRC, SA. New hydrological insights for the region: The sensitivity of the SGI to selected Probability Distribution Functions (PDFs) was examined, resulting in the Gamma and Log-Normal PDFs being preferred in the BRC. SGI-Gamma and SPI-Gamma comparisons validated the SGI method and shed light on the key factors influencing groundwater-related droughts in the BRC. The following was revealed: Hydrological droughts in the BRC are influenced by climate factors, surface and groundwater interactions, and groundwater abstractions. When conducting hydrological drought assessments using the SGI and the SPI, the initial focus should be on catchment-scale assessments before expanding to national-scale evaluations. Groundwater in the BRC exhibit significant resilience to droughts induced by climate factors, and thus useful to mitigate the risk of future droughts. Finally, The SGI performed satisfactorily in the BRC with limited groundwater level data.
Unveiling subsurface heterogeneity in porous aquifers: Insights from hydrogeophysics and derivative analysis
(Elsevier Ltd, 2024) Ndubuisi Igwebuike; Innocent Muchingami; Brighton Chunga
Groundwater is a crucial resource, particularly in urban areas where the need for alternative water sources is rising. Securing groundwater resources is paramount, requiring a concentrated effort to assess these resources and mitigate the increasing water shortages in urban regions. Despite its significance, the application of hydrogeophysics and derivative analysis in understanding aquifer dynamics is often overlooked and underused. Consequently, this study argues that neglecting the integration of hydrogeophysics dataset and derivative analysis in aquifer characterization leads to developing models that lack solution-oriented approaches, hindering effective groundwater management. This study aimed to enhance understanding of aquifer dynamics for solution-based modeling while emphasizing the importance of integrating hydrogeophysics dataset and derivative analysis to highlight aquifer system heterogeneities. The electrical resistivity tomography and derivative analysis of pumping test were used in this study to image the subsurface and amplify aquifer flow regimes respectively. The results of the electrical resistivity survey revealed a distinct layer of fine to medium-grain sand at depths of approximately 60 m in certain areas intercalated with medium-to-coarse-grain sand and thin layers of peat. Derivative analysis plots indicated that the predominant flow regime is linear and bilinear, with evidence of fracture dewatering during pumping cycles suggesting an unconfined aquifer. Additionally, aquifer heterogeneity and a no-flow boundary were evident during the pumping cycle. This study underscores the efficacy of combining hydrogeophysics and derivative analysis of pumping tests as a robust approach for imaging and validating subsurface conditions. The implications of these findings extend beyond the specific case study, offering valuable insights for groundwater utilization, monitoring, and management on a broader scale. By elucidating effective methodologies, this research enhances groundwater security and meets the escalating demand for sustainable water sources in urban areas. In conclusion, electrical resistivity tomography is an effective tool for characterizing subsurface aquifer systems, while derivative analysis of pumping tests is crucial for highlighting aquifer system heterogeneities. These methods offer a more practical interpretation compared to traditional drawdown versus time curve approaches, making the results invaluable for groundwater usage monitoring and management.
From benthic to floating: phytoplankton dynamics in African freshwater lakes and reservoirs
(Elsevier, 2023) Dube, Timothy; Madzivanzira, Takudzwa C; Mungenge, Chipo P; Dalu, Tatenda
This chapter reviewed phytoplankton communities in African freshwater lakes and reservoirs and further assessed the latitudinal diversity gradient (LDG) which has been used to explain species variations in other taxonomic groups. The chapter also identified freshwater reservoirs on the continent, heavily impacted by anthropogenic impacts, and assessed how these have led to changes in phytoplankton communities. From the systematic review, phytoplankton information was available for 51 reservoirs in Africa with 1633 freshwater phytoplankton species belonging to nine taxa recorded from the 51 reservoirs. There was strong evidence that supports LDG for phytoplankton species richness increased from the poles toward the Equator. Species that highly occurred in all reservoirs included Microcystis aeruginosa among other species. Despite the basal trophic importance of phytoplankton, undesirable phytoplankton blooms have been reported from several reservoirs on the continent.
Multispectral remote sensing of wetlands in semi-arid and arid areas: A review on applications, challenges and possible future research directions
(Remote Sensing, 2022) Gxokwe, Siyamthanda; Dube, Timothy; Mazvimavi, Dominic
Wetlands are ranked as very diverse ecosystems, covering about 4–6% of the global land surface. They occupy the transition zones between aquatic and terrestrial environments, and share characteristics of both zones. Wetlands play critical roles in the hydrological cycle, sustaining livelihoods and aquatic life, and biodiversity. Poor management of wetlands results in the loss of critical ecosystems goods and services. Globally, wetlands are degrading at a fast rate due to global environmental change and anthropogenic activities. This requires holistic monitoring, assessment, and management of wetlands to prevent further degradation and losses. Remote-sensing data offer an opportunity to assess changes in the status of wetlands including their spatial coverage. So far, a number of studies have been conducted using remotely sensed data to assess and monitor wetland status in semi-arid and arid regions.
Impacts of climate variability and drought on surface water resources in sub-saharan africa using remote sensing: A review
(Remote Sensing, 2022) Bhaga, Trisha Deevia; Dube, Timothy; Shekede, Munyaradzi Davis; Shoko, Cletah
Climate variability and recurrent droughts have caused remarkable strain on water resources in most regions across the globe, with the arid and semi-arid areas being the hardest hit. The impacts have been notable on surface water resources, which are already under threat from massive abstractions due to increased demand, as well as poor conservation and unsustainable land management practices. Drought and climate variability, as well as their associated impacts on water resources, have gained increased attention in recent decades as nations seek to enhance mitigation and adaptation mechanisms. Although the use of satellite technologies has, of late, gained prominence in generating timely and spatially explicit information on drought and climate variability impacts across different regions, they are somewhat hampered by difficulties in detecting drought evolution due to its complex nature, varying scales, the magnitude of its occurrence, and inherent data gaps.
Trends in elephant poaching in the Mid-Zambezi Valley, Zimbabwe: Lessons learnt and future outlook
(African Journal of Ecology, 2023) Mpakairi, Kudzai Shaun; Ngorima, Patmore; Blessing, Kavhu; Gara, Tawanda Winmore
Background: The conservation of African elephants (Loxodonta africana) has important ecological, economical, cultural and aesthetic values, at both local and global levels (Pittiglio et al., 2014). Despite the important role elephants play as keystone species, their populations have been dwindling due to human activities (Sibanda et al., 2016). The most serious threats to elephant's survival across most of its range include illegal wildlife trade which has been exacerbated by an increase in organized poaching (Ouko, 2013). Poaching for both meat and ivory is by far the most acute problem across Africa according to data derived from the Monitoring the Illegal Killing of Elephants (MIKE) and Elephant Trade Information System (ETIS; WWF, 2017). This is a complex global threat to the survival of the African elephant across most of its range (Dejene et al., 2021; Ouko, 2013; Wittemyer et al., 2014).
The use of stable isotopes to identify surface water–groundwater interaction in the Kruger National Park, South Africa
(South African Water Research Commission, 2023) Petersen, Robin; Nel, Jaco; Strydom, T
The role of groundwater, in general, is often overlooked in freshwater ecosystem management policies and in the management of South Africa’s flagship conservation area, the Kruger National Park (KNP). To address this gap, a generalised conceptual model of surface water–ground water (sw–gw) interactions in the southern and central regions of the KNP was developed. To do this, stable isotope ratios (δ18O and δ2H) of groundwater, rainfall and surface water were used to determine the extent to which the base flow of perennial, seasonal and ephemeral streams on different geologies (granite vs. basalt) is driven by rainfall or groundwater.
Modelling the distribution of the invasive Ziziphus mauritiana along road corridors in Zimbabwe
(Wiley, 2019) Mungate, Pauline N. P.; Masocha, Mhosisi; Dube, Timothy
We investigate how human fruit consumption affects the spread of the alien invasive Ziziphus mauritiana along road corridors in northern Zimbabwe. A field survey was conducted to identify and map Z. Mauritiana stems at 25 randomly located 6‐ha plots placed along two main roads connecting the Zambezi Valley to the Highveld region. The L‐function was used to test for evidence of significant spatial clustering of the stems. The inhomogeneous point model fitted by maximum likelihood was also applied to check whether distance from the road explains variation in the number of stems per unit area. Finally, a t test was executed on log‐transformed abundance data to test for significant differences in the mean number of saplings and adults between the Zambezi Valley and the Highveld. Results of the L‐function indicated that Z. mauritiana had a clustered and linear distribution along roads both in the Zambezi Valley and Highveld regions. Results of the t test showed that the mean number of saplings per plot in the Zambezi Valley (μ1 = 275) was three times higher than in the Highveld (μ2 = 78), with p < 0.01. The mean number of adult trees was also significantly higher in the Zambezi Valley than in the Highveld.
Two pulses of Mesoproterozoic intra-plate magmatism in the heart of the Palaeoproterozoic Ubendian Domain of northern Malawi
(Elsevier, 2023) Thomas, Robert J.; Boger, Steven D.; Fullgraf, Thomas
Two Mesoproterozoic anorogenic igneous events are recorded in the Palaeoproterozoic Ubendian Domain in northern Malawi. The oldest is represented by the Mwakikome orthogneiss, a small peraluminous, sub-alkaline and ferroan syenogranite that has a U–Pb zircon emplacement age of 1411 ± 3 Ma, an initial εNd of 􀀀 4.5 and a Nd TDM2 model age of 2.27 Ga. These data are interpreted to suggest that the Mwakikome intrusion was derived mainly from the melting of the adjacent Palaeoproterozoic country rocks during Kibaran-aged extension. The second anorogenic igneous event is defined by the more widespread Mwenga Suite, a series of small (<100 km2) NW-SE oriented and elongated plutons that occur in a narrow belt in the north of the Ubendian Domain. Seven samples from different intrusions gave U–Pb zircon emplacement ages between ~1150 and 1110 Ma. The Mwenga Suite is comprised of rocks that are also ferroan, potassic and sub-alkaline in composition and shows a range of major element compositions from monzogranite to alkali-feldspar granite. Nd isotopic data from one sample yielded an initial εNd of 􀀀 2.2 and a Nd TDM2 model age of 1.88 Ga, suggesting that the suite was also largely derived from melting of the Ubendian host rocks, but with some additions from an enriched mantle source.
Investigating seawater intrusion in republic of South Africa’s heuningnes, Cape Agulhas using hydrogeochemistry and seawater fraction techniques
(MDPI, 2023) Xaza, Abongile; Mapoma, Harold Wilson Tumwitike; Abiye, Tamiru A
The Heuningnes Catchment in the Republic of South Africa was used as a case study in this research to describe the application of saltwater fraction/quantification and hydrogeochemistry methods to evaluate the extent of saline intrusion in the coastal aquifers. The argument of the research is that the presence of seawater incursion may be conclusively determined by combining the examination of the major ions, seawater fraction, stable isotopes of water, bromide, and geochemical modeling. Using stable isotopes of oxygen (18O) and deuterium (2H), major ions chemistry, seawater composition, and geochemical modeling, the genesis of salinity and mixing of different water masses were examined. Twenty-nine (29) samples of groundwater were examined. All samples showed water facies of the Na-Cl type, indicating a seawater-related origin. The significance of mixing in coastal aquifers under natural conditions was shown by the hydrogeochemical characteristics of key ions derived from ionic ratios, which demonstrated substantial adherence to mixing lines among endmembers for freshwater as well as saltwater (seawater).
Impacts of the spatial configuration of built-up areas and urban vegetation on land surface temperature using spectral and local spatial autocorrelation indices
(Remote Sensing Letters, 2022) Dube, Timothy; Kowe, Pedzisai; Mushore, Terence Darlington; Ncube, Amos
Understanding how the spatial configuration of land cover patterns of built-up areas and urban vegetation affect urban surface temperatures is crucial for improving the sustainability of cities as well as optimizing urban design and landscape planning. Because of their capability to detect distinct surface thermal features, satellite data have proved useful in exploring the impacts of spatial configuration of land cover on land surface temperature (LST). In this study, we examine how the spatial configuration of built-up and urban vegetation affects the LST in the Harare metropolitan city, Zimbabwe. In order to achieve this objective, we combined the LST, local spatial statistics of Getis-Ord Gi* and local Moran’s I statistic, Normalized Difference Vegetation Index (NDVI) and the Normalized Difference Built-Up Index (NDBI) derived from multi-date Landsat satellite data (1994, 2001 and 2017)
Correction: sibanda et al. application of drone technologies in surface water resources monitoring and assessment: a systematic review of progress, challenges, and opportunities in the global south. drones 2021, 5, 84
(Drones, 2022) Dube, Timothy; Mazvimavi, Dominic; Sibanda, Mbulisi; Mutanga, Onisimo
In the original publication [1], “Fahad Alawadi. Detection of surface algal blooms using the newly developed algorithm surface algal bloom index (SABI)”, “Proc. SPIE 7825, Remote Sensing of the Ocean, Sea Ice, and Large Water Regions 2010, 782506 (18 October 2010); https://doi.org/10.1117/12.862096” [2] was not cited. The citation has now been inserted in “3.5. The Role of Drone Data Derived Vegetation Indices and Machine Algorithms in Remote Sensing Water Quality and Quantity” as reference [60] and should read: “Numerous vegetation indices were derived from drone remotely sensed data for characterizing surface water quality and quantity. The most widely used sections of the electromagnetic spectrum in detecting water quality parameters were the visible section (blue and green) and the NIR wavebands. In this regard, vegetation indices such as the red and near-infrared (NIR), Surface Algal Bloom Index (SABI) [60], two-band algorithm (2BDA) [26], NDVI, and Green NDV [33], as well as band combinations and differencing such as (R+NIR/G) were used mostly in characterizing chlorophyll content as well as TSS.
Landuse and landcover change assessment in the Upper Runde sub-catchment, Zimbabwe and possible impacts on reservoir sedimentation
(Physics and Chemistry of the Earth, 2022) Dube, Timothy; Kusena, Winmore; Chemura, Abel; Nicolau, Melanie. D; Marambanyika, Thomas
This work assesses land cover changes on the Upper Runde sub-catchment, Zimbabwe, and associated effects on sedimentation rates and risks. The model was implemented using the common Geographic Information Systems tools. To achieve this objective, mean annual and monthly rainfall, as well as sediment data, were used (December 2016 and April 2017). Land use and land cover changes were assessed using time-series Landsat data acquired between the years 2000 and 2016. The Revised Universal Soil Loss (RUSLE) model was used to model sedimentation rates in the catchment
Spatio–temporal variation of vegetation heterogeneity in groundwater dependent ecosystems within arid environments
(Ecological Informatics, 2022) Mpakairi, Kudzai S; Dube, Timothy; Dondofema, Farai; Dalu, Tatenda
Climate change, land cover change and the over–abstraction of groundwater threaten the existence of Groundwater-Dependent Ecosystems (GDE), despite these environments being regarded as biodiversity hotspots. The vegetation heterogeneity in GDEs requires routine monitoring in order to conserve and preserve the ecosystem services in these environments. However, in–situ monitoring of vegetation heterogeneity in extensive, or transboundary, groundwater resources remain a challenge. Inherently, the Spectral Variation Hypothesis (SVH) and remotely-sensed data provide a unique way to monitor the response of GDEs to seasonal or intra–annual environmental stressors, which is the key for achieving the national and regional biodiversity targets. This study presents the first attempt at monitoring the intra–annual, spatio–temporal variations in vegetation heterogeneity in the Khakea–Bray Transboundary Aquifer, which is located between Botswana and South Africa, by using the coefficient of variation derived from the Landsat 8 OLI Operational Land Imager (OLI).
Mapping rangeland ecosystems vulnerability to Lantana camarainvasion in semi-arid savannahs in South Africa
(Journal of Ecology, 2022) Dube, Timothy; Maluleke, Xivutiso Glenn; Mutanga, Onisimo
We mapped and modelled the potential areas vulnerable to Lantana camara (L. camara) invasion in semi-arid savannah ecosystems in the communal lands of Bushbuckridge and Kruger National Park, South Africa. Specifically, we modelled potentially vulner-able areas based on remotely sensed data and environmental variables. The Maximal Entropy (Maxent) algorithm was used to model the vulnerable area. The reliability of the modelled results was assessed using Skills Statistic (TSS), Area Under Curve (AUC) and Kappa statistics. According to the results, Bushbuckridge communal lands are more susceptible to L. camara invasions than Kruger National Park.
Local community attitudes and perceptions towards benefits and challenges associated with biodiversity conservation in Blouberg Nature Reserve, South Africa
(Journal of Ecology, 2022) Dube, Timothy; Rampheri, Mangana B; Shoko, Cletah; Marambanyika, Thomas; Dhau, Inos
This study assessed the benefits and challenges associated with local community involvement in biodiversity conservation in the Blouberg Nature Reserve (BNR) of South Africa. To achieve this, a descriptive research design was used in the study. Three hundred and thirty-five households from four villages scattered around the nature reserve were selected using a stratified systematic sampling procedure to participate in a household questionnaire survey. The BNR Manager was purposefully selected for an in- depth structured interview so that an overview of socio-economic benefits and challenges to the community from the park's perspective could be known. Field data collection was conducted during the month of June 2019.
Impacts of eco-environmental quality, spatial configuration, and landscape connectivity of urban vegetation patterns on seasonal land surface temperature in Harare metropolitan city, Zimbabwe
(Taylor and Francis Group, 2022) Kowe, Pedzisai; Mutanga, Onisimo; Dube, Timothy
The study examined the impact of eco-environmental quality conditions, spatial configurations and landscape connectivity of urban vegetation on seasonal land surface temperature (LST) in Harare, Zimbabwe between May and October 2018. The results showed that densely built-up areas with sparse vegetation experienced extremely poor eco-environmental conditions. Clustered and highly connected were more beneficial in decreasing LST. These findings have important urban and landscape planning implications regarding how the spatial configuration and land-scape connectivity patterns of urban vegetation can be optimized to mitigate Urban Heat Island (UHI) effects and to improve the thermal comfort conditions in rapidly urbanizing cities.

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