Quaternary Catchments Research Articles

Hydrogeological Regions of Kolubarska Posavina (Central Serbia)

The groundwater catchments of the Kolubarska Posavina are of significant importance because if some contamination affects the groundwater system, it is necessary to find a quality groundwater source for the water supply of Obrenovac Municipality. Reconstruction of sedimentary evolution plays a significant role in the investigation of groundwater catchment. In this study, the variation in sedimentary characteristics and sedimentary evolution/paleogeography were studied for Quaternary catchment identification in Kolubarska Posavina. The groundwater catchment of the Kolubarska Posavina can distinguished into two main parts. The western part of Kolubarska Posavina is identified as the Sava groundwater catchment, where the Sava alluvial plain and the (paleo) meanders of the Sava are located. The east part belongs to the Kolubara groundwater catchment, characterized by the remnants of the (macro) alluvial fan of Kolubara and the Kolubara floodplain. The Kolubara catchment is located eastwards from the older meander of the Sava River.

The application of CHIRPS-based Pitman modelling in South Africa

Study regionThe Berg Water Management Area (WMA), drainage region G of South Africa is critical to supplying water for the City of Cape Town and the entire Western Cape region. Study focusDeveloping an approach for the application of CHIRPS-based Pitman modelling in South Africa. CHIRPS-based satellite rainfall data (with a spatial resolution of 0.05° × 0.05°) was used in the Pitman model to simulate observed flows. The model was set up for the Berg WMA which has 35 quaternary catchments and covers an area of approximately 12,965 km2 in assessing the suitability of CHIRPS-based flows. New hydrological insights for the regionThe simulated CHIRPS-based flows using the developed approach indicated 80% suitability of CHIRPS data as an alternative to the dwindling observed rainfall in simulating flows. CHIRPS rainfall data illustrated a strong correlation to observed catchment rainfall and calibration factors were developed for adjustments. Calibration (1981–2009) and validation (2010–2019) statistics indicated a good similarity between observed and CHIRPS-based flows for the period of analysis. The exceedance probability curves for the entire simulation period illustrated similar flow characteristics for the calibration and validation periods. In assessing for uncertainty, hydrological indices were used at a defined ±15% level and the results showed CHIRPS to be suitable. This research recommends the use of CHIRPS-based rainfall data as a suitable alternative in simulating flows for South Africa.

Calibration, validation and application of the SWAT model to determine the hydrological benefit of wetland rehabilitation in KwaZulu-Natal, South Africa

In South Africa, with highly variable and intense land-use practices, coupled with limited soil fertility and water resources, there has been a long history of encroachment of arable lands (sugarcane and timber plantations) into surrounding wetlands. Although wetland delineation within the timber and sugar sectors is well-defined in policy, and existing and proposed legislation, there are significant areas of non-compliance. The spatially-explicit Soil Water Assessment Tool (SWAT) was adopted to investigate the interactions of climate, land-use and soil on the water-use of natural and encroached wetlands. This paper documents the calibration, validation and application of the SWAT model on Quaternary Catchment (QC) U20G, which is a 498 km2 catchment that forms part of the uMngeni River basin. The SWAT-CUP parameter sensitivity and optimization model was tested with daily observed streamflow data for this catchment. Parameters were modified using the sequential uncertainty fitting (SUFI-2) analysis routine to calibrate the model. The simulated flow had a close fit to the observed flow with a regression coefficient (r2) of 0.87 and a Nash-Sutcliffe (NS) coefficient of 0.8. Through the buffer scenario analysis, the model showed that if the wetland and a 20-m buffer were to be returned to a natural state, there could be a 16% increase in the annual streamflow contribution, with an upper limit of a 60% increase in some hydrologic response units (HRUs). Thus there would be a hydrological gain if wetlands and sensitive buffer areas were to be cleared of commercial timber species and sugarcane.

South Africa needs a hydrological soil map: a case study from the upper uMngeni catchment

Accurate hydrological modelling to evaluate the impacts of climate and land use change on water resources is pivotal to sustainable management. Soil information is an important input in hydrological models but is often not available at adequate scale with appropriate attributes for direct parameterisation of the models. In this study, conducted in three quaternary catchments in the midlands of KwaZulu-Natal, three different soil information sets were used to configure SWAT+, a revised version of the Soil and Water Assessment Tool (SWAT). The datasets were: (i) the Land Type database (currently the only soil dataset covering the whole of South Africa), (ii) disaggregation of the Land Type database using digital soil mapping techniques (called DSMART), and (iii) a dataset where DSMART were complemented by field observations and interpretations of the hydropedological behaviour of the soils (DSMART+). Simulated streamflow was compared with measured streamflow at three weirs with long-term measurements, and the impact of the soil datasets on water balance simulations was evaluated. In general, the simulations were acceptable when compared to other studies, but could be improved through calibration and including small reservoirs in the model. The DSMART+ dataset yielded more accurate simulations of streamflow in all three catchments with Nash-Sutcliffe efficiencies increasing by between 9% and 67% when compared to the Land Type dataset. The value of the improved soil maps is, however, highlighted through the enhanced spatial detail of streamflow generation mechanisms and water balance components. The internal catchment processes are represented more accurately, and we argue that South Africa needs a detailed hydrological soil map for effective water resource management.

Assessment of groundwater vulnerability using the DRASTIC model: A case study of quaternary catchment A21C, Limpopo River Basin, South Africa

Groundwater is a vital resource for domestic, agricultural, industrial activities and ecosystem services. Despite its multi-ple purposes, the resource is under significant threat owing to increasing contamination from anthropogenic activities and climate change. Hence, in order to ensure the reliability and sustainable use of groundwater for the present and future gener-ations, effective management of groundwater (quality and quantity) is highly important. This can be achieved by identifying areas more vulnerable to contamination and implementing protective measures. The present study aims at assessing the vul-nerability of groundwater using GIS-based DRASTIC index in the Quaternary catchment (A21C) within Limpopo River Basin. The vulnerability index varied from 87 to 207. About 53.6% (408 km2) of the catchment area also exhibited high risk of groundwater contamination mostly in central, north-eastern and western part of the sub-catchment. The medium and low vulnerability classes cover only 18.1% (137.5 km2) and 21.7% (165.1 km2) of the study area, respectively. The shallow groundwater at the Doornfontein Campus belongs to very high vulnerability area. The sensitivity analysis indicates that depth to water level, recharge, aquifer media, soil and topography are the important contributors to vulnerability assessment. The correlation analysis performed to validate the final vulnerability map shows a moderate positive correlation, indicating the model’s applicability to the urbanised environment. The study indicates an area that is highly vulnerable to pollution, and hence protective measures are necessary for sustainable management of the groundwater resource in the study area. The result of this study can also be further improved and verified by using other vulnerability assessment models.

Tsallis Entropy for Assessing Spatial Uncertainty Associated with Mean Annual Runoff of Quaternary Catchments of the Middle Vaal Basin in South Africa.

This study assesses mainly the uncertainty of the mean annual runoff (MAR) for quaternary catchments (QCs) considered as metastable nonextensive systems (from Tsalllis entropy) in the Middle Vaal catchment. The study is applied to the surface water resources (WR) of the South Africa 1990 (WR90), 2005 (WR2005) and 2012 (WR2012) data sets. The q-information index (from the Tsalllis entropy) is used here as a deviation indicator for the spatial evolution of uncertainty for the different QCs, using the Shannon entropy as a baseline. It enables the determination of a (virtual) convergence point, zone of positive and negative uncertainty deviation, zone of null deviation and chaotic zone for each data set. Such a determination is not possible on the basis of the Shannon entropy alone as a measure for the MAR uncertainty of QCs, i.e., when they are viewed as extensive systems. Finally, the spatial distributions for the zones of the q-uncertainty deviation (gain or loss in information) of the MAR are derived and lead to iso q-uncertainty deviation maps.

Estimation of groundwater recharge response from rainfall events in a semi-arid fractured aquifer: Case study of quaternary catchment A91H, Limpopo Province, South Africa

This study estimated groundwater recharge response from rainfall events in a semi-arid fractured aquifer. This is crucial as recharge response has an influence on available groundwater in an aquife...

Groundwater discharges to aquatic ecosystems associated with the Table Mountain Group (TMG) aquifer: A conceptual model

This paper reports on a conceptual model that was developed to describe the different groundwater discharge 'types' from the Table Mountain Group (TMG) aquifer, that contributes to the different components of the flow regime in each of the recognised river reaches for streams and rivers associated with the TMG. This model integrates hydrogeological, ecological and geomorphological understandings into an ecohydrological perspective linking ground- and surface water systems. Through geospatial intersections of existing GIS layers a GIS model was also developed to highlight the quaternary catchments containing sensitive aquatic ecosystems that could be vulnerable to groundwater use from the TMG. The conceptual model demonstrates the intimate link between groundwater from the TMG aquifer and aquatic ecosystems in the mountain and foothill reaches of streams and rivers in the Cape Folded Mountains in particular. It also identifies two primary zones of interaction between groundwater and surface water in the TMG, namely, the 'TMG aquifer daylight-domain', located in the recharge zone, and the 'TMG aquifer surface water interface-domain', located at the discharge end of the aquifer. The conceptual model clearly indicates the difference between real groundwater, and perceived groundwater contributions to streamflow in the TMG. It is the lower flows of the flow regime that will be most vulnerable to groundwater use from the TMG aquifer in the 'TMG aquifer daylight-domain', which are unfortunately also the most important flows from an ecological perspective. However, any groundwater use from the TMG aquifer will also affect the discharge end of the aquifer, located far from the higher elevation recharge areas, or the point of groundwater abstraction, in lowland settings in the 'TMG aquifer surface water interface-domain'. The GIS model integrated the conceptual understanding into a management tool by highlight all quaternary catchments associated with TMG containing sensitive aquatic ecosystems and gave the variable vulnerability for each.

Development of a framework for an integrated time-varying agrohydrological forecast system for Southern Africa: Initial results for seasonal forecasts

Uncertainty about hydro-climatic conditions in the immediate future (today), as well as the near (up to one week) and more distant futures (up to one season) remains a fundamental problem challenging decision makers in the fields such as water resources, agriculture, and many other water-sensitive sectors in Southern Africa. Currently many institutions, such as the SA Weather Service, provide weather and climate forecasts with lead times ranging from 1 d to one season. However, disconnects exist between the weather / climate forecasts and their links to agrohydrological models, and in the applications of forecast information for targeted agricultural and water-related decision-making. The skills level of the current weather and climate forecasts, and the mismatch in scales between the output from weather / climate models and the spatial scales at which hydrological models are applied, as well as the format of seasonal forecasts in that they cannot be used directly in agrohydrological models, are some of the problems identified in this study and are being addressed. This has necessitated the development of a GIS-based framework in which the 'translation' of weather and climate forecasts into more tangible agrohydrological forecasts such as streamflows, reservoir levels or crop yields is facilitated for all the inter-linked quaternary catchments for enhanced economic, environmental and societal decision making over South Africa in general, and in selected catchments in particular. For monthly and seasonal (i.e. 3-month lead time) forecasts, two methods, viz. the Historical Sequence Method and the Ensemble Re-Ordering Method have been developed to translate the triplet of forecast rainfall probabilities (i.e. above, near and below normal) into daily quantitative values of rainfall for use in hydrological models. The first method was applied together with the daily time step ACRU Model to simulate seasonal flow forecasts in the Mgeni catchment in KwaZulu-Natal, South Africa. In taking account of uncertainty in the seasonal rainfall forecasts through the process of translating these to daily streamflow simulations by the ACRU Model, some skilful initial forecasts of streamflows can be obtained which can assist decision makers to take protective action against the impacts of hydro-climatic variability.

Disaggregation of fixed time interval rainfall to continuous measured rainfall for the purpose of design rainfall estimation

Design rainfall estimates are primarily used in single-event deterministic design flood estimation methods where estimates of the peak discharge are based on the critical storm duration or time of concentration (TC) of a catchment. Therefore, daily design rainfall depths used in flood estimations must either be decreased or increased from durations less than or longer than 24 hours to the design rainfall depths for a rainfall duration of TC. This paper presents the comparison of two South African methods used to convert or scale 1-day fixed time interval observed rainfall (08:00 to 08:00) to continuous measures of n-hour rainfall for selected TC durations at a quaternary catchment level, in the C5 secondary drainage region in South Africa as pilot case study. In each quaternary catchment, the annual maximum series (AMS) of the 1-day fixed time interval point rainfall were extracted, infilled, converted and scaled to appropriate continuous measures of TC-hour point rainfall using conversion factors (Adamson, 1981) and scaling factors (Smithers and Schulze, 2003), respectively. Thereafter, all the TC-hour observed point rainfall values were averaged to observed catchment rainfall at a quaternary catchment level using the Thiessen polygon method. In using the two methods to estimate continuous short-duration n-hour (TC ≤ 24 hours) and long-duration n-hour (TC > 24 hours) catchment rainfall from 1-day fixed time interval point rainfall, an acceptable (0.71 < r2 ≤ 0.86) and high (r2 ≥ 0.93) degree of association were achieved, respectively, despite the different approaches used in each method. Overall, the results confirmed that fixed time interval rainfall should be scaled to continuous measures of rainfall using the Smithers-Schulze scale invariance approach for various TC durations in the case study area. In comparison to the Adamson conversion methodology, the Smithers-Schulze scaling methodology is also based on a more extensive and recent rainfall database as incorporated in software for design rainfall estimation in modern flood hydrology practice in South Africa.

Exploring dynamics of evapotranspiration in selected land cover classes in a sub-humid grassland: A case study in quaternary catchment S50E, South Africa

Land cover change is a pervasive force and it influences the relationship between precipitation (P) and actual evapotranspiration (AET). The study sought to determine variations in catchment scale AET attributable to land cover change over a grassland in the Eastern Cape, South Africa. Remotely sensed rainfall and AET data were used. Land cover maps for the study area were used to extract annual AET from the MOD16 ET product. The method of cumulative residuals was applied to link dynamics in AET with land cover change to enable the application of an appropriate map for retrieving AET. Rainfall and AET were subjected to the Mann- Kendall and Pettitt's tests. Grassland and built-up cover classes AET showed step changes (p < 0.05) indicating sudden changes. Forest cover revealed a marginal trend (p < 0.1), since forest cover change was a slow process. Changes in the evaporative index suggest that land cover influenced AET and this was confirmed by dynamics in the catchment parameter (w). Average w (1.88) indicated that the catchment had a low water retention capacity and this suggests that the hydrological response was sensitive to land cover change. Management interventions are required to improve water retention in the catchment.

Cross Mean Annual Runoff Pseudo-Elasticity of Entropy for Quaternary Catchments of the Upper Vaal Catchment in South Africa.

This study focuses preliminarily on the intra-tertiary catchment (TC) assessment of cross MAR pseudo-elasticity of entropy, which determines the impact of changes in MAR for a quaternary catchment (QC) on the entropy of another (other) QC(s). The TCs of the Upper Vaal catchment were used preliminarily for this assessment and surface water resources (WR) of South Africa of 1990 (WR90), of 2005 (WR2005) and of 2012 (WR2012) data sets were used. The TCs are grouped into three secondary catchments, i.e., downstream of Vaal Dam, upstrream of Vaal dam and Wilge. It is revealed that, there are linkages in terms of mean annual runoff (MAR) between QCs; which could be complements (negative cross elasticity) or substitutes (positive cross elasticity). It is shown that cross MAR pseudo-elasticity can be translated into correlation strength between QC pairs; i.e., high cross elasticity (low catchment resilience) and low cross elasticity (high catchment resilience). Implicitly, catchment resilience is shown to be associated with the risk of vulnerability (or sustainability level) of water resources, in terms of MAR, which is generally low (or high). Besides, for each TC, the dominance (of complements or substitutes) and the global highest cross MAR elasticity are determined. The overall average cross MAR elasticity of QCs for each TC was shown to be in the zone of tolerable entropy, hence the zone of functioning resilience. This could assure that water resources remained fairly sustainable in TCs that form the secondary catchments of the Upper Vaal. Cross MAR pseudo-elasticity concept could be further extended to an intra-secondary catchment assessment.

Environmental and anthropogenic determinants of the spread of alien plant species: insights from South Africa’s quaternary catchments

Alien plants invasion has negative impacts on the structure and functionality of ecosystems. Understanding the determinants of this process is fundamental for addressing environmental issues, such as the water availability in South Africa’s catchments. Both environmental and anthropogenic factors determine the invasion of alien species; however, their relative importance has to be quantified. The aim of this paper was to estimate the importance of 32 explanatory variables in predicting the distribution of the major invasive alien plant species (IAPS) of South Africa, through the use of Species Distribution Models. We used data from the National Invasive Alien Plants Survey, delineated at a quaternary catchment level, coupled with climatic, land cover, edaphic, and anthropogenic variables. Using two-part generalized linear models, we compared the accuracy of two different sets of variables in predicting the spatial distribution of IAPS; the first included environmental correlates alone, and the second included both environmental and anthropogenic variables. Using Random Forest, we explored the relative importance of the variables in producing a map of potential distribution of IAPS. Results showed that the inclusion of anthropogenic variables did not significantly improve model predictions. The most important variables influencing the distribution of IAPS appeared to be the climatic ones. The modeled potential distribution was analyzed in relation to provinces, biomes, and species’ minimum residence time.

The Gauteng Conservation Plan: Planning for biodiversity in a rapidly urbanising province

Background: Gauteng, the smallest of South Africa’s nine provinces, is rich in biodiversity; yet it is also the most densely populated province and thus faces significant development pressures.Objective: A project was therefore initiated in 2001 to identify areas of biodiversity importance in the province, using the systematic spatial biodiversity planning approach that has been adopted in South Africa. This article reports on the final version of the provincial conservation plan as completed in 2011.Method: Vegetation types and quaternary catchments constituted the coarse filter biodiversity features, while rare and threatened taxa constituted the fine filter features. Ecological processes were captured by a range of landscape features, while planning for climate change primarily involved the design of a corridor network. Planning was undertaken within the ArcView linked C-plan decision support system, where a cost surface preferentially directed the selection of available sites towards low-cost areas.Results: Forty-four per cent of the province is required to achieve targets. Only 8% of features are close to having their targets met or are adequately conserved in the current protected area network of 23 protected areas covering 2.4% of the province, while 73% of features are absent or poorly represented.Conclusion: The existing protected area network is inadequate for the conservation of biodiversity in Gauteng. The Gauteng Conservation Plan identifies a set of areas that are required to achieve conservation targets. It is important that identified areas currently not in the protected area network are protected either formally or through legislated land use management processes.

Mapping soil erosion in a quaternary catchment in Eastern Cape using geographic information system and remote sensing

In South Africa, soil erosion is considered as an environmental and social problem with serious financial implications particularly in some rural areas where this geomorphological phenomenon is widespread. An example is the Umzimvubu Local Municipality, where most households are strongly reliant on agriculture for their livelihood. Sustainable agriculture and proper land management in these rural areas require up-to-date and accurate information relevant to the spatial distribution of soil erosion. This study was therefore aimed at generating such information using Landsat8 Operational Land Imager (OLI)-derived vegetation indices (VIs) including the Normalised Difference Vegetation Index (NDVI), Soil Adjusted Vegetation Index (SAVI), as well as Soil and Atmospherically Resistance Vegetation Index (SARVI). Raster calculator in ArcMap10.2 was used to classify soil erosion features based on selected suitable thresholds in each VI. SPOT6/7 ( Systeme Pour Observation de la Terre ) multispectral data and Google Earth images were used for ground truth purposes. NDVI achieved the highest overall classification accuracy of 85% and kappa statistics of 69%, followed by SAVI with an overall accuracy and kappa statistic of 83% and 64%, respectively. SARVI produced very low overall accuracy (68%) and kappa statistic (25%) relative to other indices. Using these indices, the study successfully mapped the spatial distribution of soil erosion within the study area albeit there were some challenges due to coarser spatial resolution (30mx30m) of Landsat8 image. Due to this setback, image fusion and pan-sharpening of Landsat8 with high multispectral resolution images is strongly suggested as an alternative to improve the Landsat8 spatial resolution.

Assessing Catchment Resilience Using Entropy Associated with Mean Annual Runoff for the Upper Vaal Catchment in South Africa

The importance of the mean annual runoff (MAR)-hydrological variable is paramount for catchment planning, development and management. MAR depicts the amount of uncertainty or chaos (implicitly information content) of the catchment. The uncertainty associated with MAR of quaternary catchments (QCs) in the Upper Vaal catchment of South Africa has been quantified through Shannon entropy. As a result of chaos over a period of time, the hydrological catchment behavior/response in terms of MAR could be characterized by its resilience. Uncertainty (chaos) in QCs was used as a surrogate measure of catchment resilience. MAR data on surface water resources (WR) of South Africa of 1990 (i.e., WR90), 2005 (WR2005) and 2012 (W2012) were used in this study. A linear zoning for catchment resilience in terms of water resources sustainability was defined. Regression models (with high correlation) between the relative changes/variations in MAR data sets and relative changes in entropy were established, for WR2005 and WR2012. These models were compared with similar relationships for WR90 and WR2005, previously reported. The MAR pseudo-elasticity of the uncertainty associated with MAR was derived from regression models to characterize the resilience state of QCs. The MAR pseudo-elasticity values were relatively small to have an acceptable level of catchment resilience in the Upper Vaal catchment. Within the resilience zone, it was also shown that the effect of mean annual evaporation (MAE) was negatively significant on MAR pseudo-elasticity, compared to the effect of mean annual precipitation (MAP), which was positively insignificant.

Determining fPAR and leaf area index of several land cover classes in the Pot River and Tsitsa River catchments of the Eastern Cape, South Africa

Determining the quantum (both annual maxima and minima) and the temporal variation in the leaf area index (LAI), and the fraction of photosynthetically active radiation (fPAR), are three fundamental biophysical characteristics of the plant canopy that should parameterise ecophysiological models of water use (evapotranspiration) and carbon sequestration. Although Earth observation provides values and time series for both these parameters, in-field validation of these values is necessary. Following a very wet summer season, we conducted field surveys of several land cover classes within two quaternary catchments in the Eastern Cape province, South Africa, to determine maximum values of LAI and fPAR that occur within each of these land cover classes. To assist in up-scaling these point measures to the landscape, we present a regression relationship between Landsat 8 NDVI and LAI measured using an Accupar Ceptometer (r2 = 0.92). Peak wet season LAI varied from extremely high (>7.0) under the canopy of invasive black wattle (Acacia mearnsii) trees to ∼2.0 under the canopy of a Eucalyptus plantation. Ungrazed native grassland displayed an intermediate LAI value of 3.84. The black wattle stand absorbed 97% of the available PAR, whereas the mature Eucalyptus plantation only absorbed 66% of PAR.

Estimating shallow groundwater availability in small catchments using streamflow recession and instream flow requirements of rivers in South Africa

Groundwater is an important resource for multiple uses in South Africa. Hence, setting limits to its sustainable abstraction while assuring basic human needs is required. Due to prevalent data scarcity related to groundwater replenishment, which is the traditional basis for estimating groundwater availability, the present article presents a novel method for determining allocatable groundwater in quaternary (fourth-order) catchments through information on streamflow. Using established methodologies for assessing baseflow, recession flow, and instream ecological flow requirement, the methodology develops a combined stepwise methodology to determine annual available groundwater storage volume using linear reservoir theory, essentially linking low flows proportionally to upstream groundwater storages. The approach was trialled for twenty-one perennial and relatively undisturbed catchments with long-term and reliable streamflow records. Using the Desktop Reserve Model, instream flow requirements necessary to meet the present ecological state of the streams were determined, and baseflows in excess of these flows were converted into a conservative estimates of allocatable groundwater storages on an annual basis. Results show that groundwater development potential exists in fourteen of the catchments, with upper limits to allocatable groundwater volumes (including present uses) ranging from 0.02 to 3.54×106m3a−1 (0.10–11.83mma−1) per catchment. With a secured availability of these volume 75% of the years, variability between years is assumed to be manageable. A significant (R2=0.88) correlation between baseflow index and the drainage time scale for the catchments underscores the physical basis of the methodology and also enables the reduction of the procedure by one step, omitting recession flow analysis. The method serves as an important complementary tool for the assessment of the groundwater part of the Reserve and the Groundwater Resource Directed Measures in South Africa and could be adapted and applied elsewhere.

Shallow landslide susceptibility assessment in a semiarid environment — A Quaternary catchment of KwaZulu-Natal, South Africa

This paper presents an evaluation of landslide susceptibility maps based on the Frequency Ratio (FR) Landslide Susceptibility Index (LSI), logistic regression (LR), and artificial neural network (ANN) methods, applied within a Quaternary catchment in the KwaZulu-Natal region, South Africa. Landslide occurrence in this particular catchment is generally induced by heavy rainfall events where shallow landslips occur mostly on steep slopes thereby threatening informal housing at the base of slopes. The scope of this study is to compare the performance of landslide susceptibility models and rate the importance of landslide causal factors. Thirteen landslide causal factors, including altitude, slope angle, aspect, slope total curvature, slope plan curvature, slope profile curvature, proximity to stream network, Topographic Wetness Index (TWI), lithology, land cover, proximity to faults, proximity to dolerite sills and dykes, and proximity to the road network were considered in the study. Furthermore each causal factor was rated according to the above mentioned methods, and it was found that the most influential landslide causal factors are the, altitude, land cover, and lithology. The resulting susceptibility maps were classified into 5 classes ranging from Very low to Very high susceptibility. Model performance was tested using an independent validation set comprising 10% of all mapped landslides. For verification of the models' performance, receiver operating curves (ROCs) were calculated and the areas under curve (AUC) for the success rate curves were 0.736, 0.748, 0.827, and 0.809 for FR, LSI, LR and ANN respectively. The findings revealed that models showed promising results for shallow landslide susceptibility modelling since they all give accuracy values higher than 73%. The LR and ANN methods however proved to be superior in representing landslide susceptibility throughout the study area.

Spatiotemporal analysis of droughts using self-calibrating Palmer’s Drought Severity Index in the central region of South Africa

The loss of life and property from drought events has forced society to focus on the development of reliable early warning systems which may enable farmers and other stakeholders to correctly and timely adapt to the expected impacts of climatic hazard. However, a scientific approach to a reliable early warning system for a region requires, among others, characterisation of drought events in the region in terms of duration, magnitude, intensity and frequency using standard drought indices. Therefore, the objective of this study was to identify and characterise drought events in the Modder River basin, central region of South Africa, using a self-calibrated Palmer’s Drought Severity Index (sc-PDSI). Attempts were also made to establish a relationship between meteorological and hydrological drought events in the region. During the period of analysis, the total number of drought episodes identified in the study area ranged between eight and sixteen. It was found that the most severe drought episodes occurred during the period 1992–1995 followed by the period 1982–1987. Results of analysis of seasonal drought events in one of the quaternary catchments (C52A) revealed that peak drought events during the three summer months (November, December and January) occurred in the area in 1993. However, in terms of event magnitude and intensity, the worst drought events were recorded during the period December 1982–July 1987, followed by the event that ensued during December 1989–September 1995. Results of analysis of decadal variation of drought events showed that the number of extreme and moderate drought events recorded in the catchment showed statistically significant increasing trends during the five decades at 5 % significance level. Moreover, spectral analysis of sc-PDSI time series in the region identified periodicities in the time series ranging from 6 years (C52E) to 16 years (C52K). In terms of the spatial extent of extreme drought events, the maximum areal coverage (91 %) was recorded in November 1998, followed by December 1998 and December–January 1999 (43 %). Analysis of the relative frequency of droughts of varying categories revealed that extreme drought events were most prevalent in the C52E (2.72 %) quaternary catchment, followed by C52C (2.21 %). The study also found an average lag time of 10 months between the onsets of meteorological and hydrological drought events in the region.