Browsing by Subject "Rainfall"

Now showing 1 - 5 of 5
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    Open Access
    Assessing Intra-seasonal land surface change and long term trends in the Succulent Karoo biome using coarse resolution satellite data and interpolated rainfall surfaces
    (2005) Roberts, Jonathan Wesley; Hewitson, Bruce
    The Succulent Karoo is a biodiversity hot spot situated along the west coast of southern Africa. While it is predominantly recognized as a west coast vegetation type its borders stretch as far east as Steytlerville in the Little Karoo. The area contains the largest number of endemic succulent species in the world and harbors nearly 10 percent of the total number of succulent species worldwide. Furthermore, spring mass-flowering events draw thousands of tourists to the region, providing welcome input to the local economies. The floral diversity is however under threat from various environmental forces. These forces include inappropriate land use practices resulting in Land Degradation and the ever-present threat of Climate Change. With the pressures that are presently being placed on the biome and the threat of future changes to the climatic regime, it is necessary to develop methodologies to monitor vegetation changes within the biome. These methodologies should be able to discern between climate driven and land use driven changes that may denude the rich biological diversity of the biome. Monitoring regional changes in vegetation necessitates the use of coarse resolution satellite data in conjunction with statistically derived rainfall surfaces. For the purposes of this study intraseasonal land surface change is monitored using the Normalized Difference Vegetation Index derived from the National Oceanic and Atmospheric Administration (NOAA), Advanced Very High Resolution Radiometer (AVHRR). Rainfall surfaces are taken from Hewitson and Crane (2005). Ancillary temperature data taken from NCEP reanalysis is also used. Three analysis methodologies are used to answer questions relating to the relationship between rainfall and vegetation growth. Firstly, a Spatio Temporal Correlation filter (STCf) is used to investigate the spatial and temporal nature of seasonal vegetation changes (Where is seasonal vegetation change homogenous?). Secondly Principal Component Analysis is used to monitor seasonal vegetation change (What is the seasonal nature of NOVI in the Succulent Karoo?) and finally the relationship between precipitation and vegetation growth is classified using a Self Organising Map ( When and where is the relationship between rainfall and vegetation growth strongest?). Results from the three analysis methodologies are combined to determine zones of potential critical change. Within each of these zones drivers of vegetation change are identified and discussed. Most notably, regression analysis conducted on the output of the Self Organising Map indicates that temperature plays a more important role in vegetation growth than precipitation. This result indicates that the biome may be more vulnerable to the impacts of Global Warming than previously anticipated. Results also indicate that future studies of vegetation change using satellite imagery should be cognisant of the potential effects of soil background reflectance. In conclusion this thesis shows that much of the Succulent Karoo biome is under threat from both inappropriate land use and Climate Change, which may potentially lead to degradation of the biodiversity and rangelands that support the local population.
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    Open Access
    Exploring the environmental drivers of waterfowl movement in arid landscapes using first-passage time analysis
    (2016) Henry, Dominic A W; Ament, Judith M; Cumming, Graeme S
    BackgroundThe movement patterns of many southern African waterfowl are typified by nomadism, which is thought to be a response to unpredictable changes in resource distributions. Nomadism and the related movement choices that waterfowl make in arid environments are, however, poorly understood. Tracking multiple individuals across wide spatiotemporal gradients offers one approach to elucidating the cues and mechanisms underpinning movement decisions. We used first-passage time (FPT) to analyse high spatial and temporal resolution telemetry data for Red-billed Teal and Egyptian Geese across a 1500km geographical gradient between 2008 and 2014. We tested the importance of several environmental variables in structuring movement patterns, focusing on two competing hypotheses: (1) whether movements are driven by resource conditions during the current period of habitat occupation (reactive movement hypothesis), or (2) whether movements are structured by shifts in the magnitude and direction of environmental variables at locations prior to occupation (prescient movement hypothesis).ResultsAn increase in rainfall at a 32day lag (i.e., prior to wetland occupancy), along with tagging site, were significant predictors of FPT in both waterfowl species. There was a positive relationship between NDVI and FPT for Egyptian Geese during this 32day period; the relationship was negative for Red-billed Teal. Consistent with findings for migratory grazing geese, Egyptian Geese prioritised food quality over food biomass. Red-billed Teal showed few immediate responses to wetland filling, contrary to what one would predict for a dabbling duck, suggesting high dietary flexibility. Our results were consistent with the prescient movement hypothesis.ConclusionsUsing FPT analysis we showed that the proximate drivers of southern African waterfowl movement are the dynamics of rainfall and primary productivity. Waterfowl appeared to be able to perceive and respond to temporal shifts in resource conditions prior to habitat patch occupation. This in turn suggests that their movements in semi-arid landscapes may be underpinned by intimate knowledge of the local environment; waterfowl pursue a complex behavioural strategy, locating suitable habitat patches proactively, rather than acting as passive respondents.
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    Open Access
    Mechanisms of observed spatial heterogeneity in rainfall variability over the winter rainfall zone of South Africa
    (2025) Conradie, Willem; Hewitson, Bruce; Piotr, Wolski
    Renewed interest in South Africa's Winter Rainfall Zone (WRZ) has emerged in the wake of the 2015–2017 “Day Zero” drought and water crisis. However, this has yielded little insight into the spatial heterogeneity in WRZ climatological seasonality and rainfall variability or their drivers, despite such understanding being crucial for interpreting past and robustly projecting future climate change patterns. Hence, the spatial heterogeneity of WRZ rainfall is investigated in relation to regional rainfall systems and hemispheric climate drivers, using both gauge-based and gridded data. A novel rainfall data quality control and gap-filling procedure involving Generalised Additive Models for Location, Scale and Shape (GAMLSS) is developed to obtain a continuous, high-density 41-year gauge-based rainfall dataset for the WRZ and its immediate surroundings. The effectiveness of this procedure in detecting common errors and accurately in-filling missing values is demonstrated. Hierarchical cluster analysis on these data is used to identify the WRZ and investigate its substructure. A uniform core WRZ is identified between the meridional Cape Fold Belt and west coast. Over the WRZ, strong spatial coherence between temporal variability patterns and seasonality is found. In places, the transition to the surrounding all-year and late summer rainfall zones is highly complex. Spatial heterogeneity in rainfall seasonality and variability is closely related to heterogeneity in the dominant rain-bearing winds and mid-tropospheric synoptic states. Throughout the year, the WRZ core receives more than 70% of its rainfall from westerly disturbances. In contrast, even during winter, the coastal all-year rainfall zone receives less than half its rainfall under dominant westerly or north-westerly synoptic flows. Correlations of rainfall timeseries between these zones is insubstantial. This demonstrates that the all-year rainfall zone represents a distinct climatic zone in its own right, rather than a superposition of summer and winter rainfall influences. Spatial heterogeneity in wind direction-rainfall relationships explain much of the heterogeneity in the evolution of Day Zero drought severity. The drought footprint expanded north-eastward into the transitional and surrounding areas even as drought relieving rains occurred around Cape Town in 2018 and 2019. This rainfall recovery was associated with the early winter frequency of mid-latitude mid-tropospheric troughs—which bring rain primarily to the south-western corner of the domain—returning to near pre-drought levels. The Day Zero Drought, although unprecedented (in more than a century of records) in 3-year intensity, coincides with a spatially localised but coherent quasi-cyclic multi-decadal pattern of repeated severe multi-year droughts over the WRZ. This low-frequency variability is not mirrored in the surrounding rainfall zones or any other winter rainfall region of the Southern Hemisphere extra-tropics. This finding may indicate a prominent role for zonally asymmetric climate modes. Multi-decadal rainfall variability is most closely associated with recently documented multi-decadal variability in the south-west Indian and Atlantic Oceans. We confirm that over certain periods a significant correlation with some established modes of hemispheric and global variability is apparent for sub-regions of the WRZ. However, none of these modes are consistently and strongly associated with annual or winter semester rainfall variability across the WRZ. Protracted early winter WRZ droughts are associated with amplitude changes and phase-shifts in the quasi-stationary Southern subtropical and mid-latitude zonal wave-4 through frequency changes in WRZ rain-bearing synoptic systems. Evidence suggests that these zonal wave-4 phase shifts are partially forced by regional tropical and subtropical convection. This thesis demonstrates an approach to linking sub-regional rainfall heterogeneity to occurrence fre-quency of regional-scale weather systems. These frequencies, in turn, are associated with global and hemi-spheric modes of variability. It is proposed that this can be extended to support more robust understanding of the drivers of rainfall variability at decision-relevant scales.
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    Open Access
    Navigating extreme weather events: An anthropological investigation
    (2025) Perin, Ridah; Morreira, Shannon
    The aim of this study was to investigate how do people respond to extreme weather events (EWEs), paying special attention to heavy rainfall. The data and literature gathered unearths strategies and tactics embedded in the social structure of the area of Crawford. These strategies and tactics are either manipulated or reconfigured in patterns of individuality, to gauge what necessary actions are required to mitigate the impacts of heavy rainfall. These approaches were used to answer the foundational question of this study, what roles do existing social relationships play in people's perceptions and responses to extreme weather events? By utilising qualitative research techniques, this study argues that residents of a middle-income neighbourhoods like Crawford respond, adapt and prepare for extreme rainfall in individualised manners. Contrary to areas of less socioeconomic standing who utilise communal strategies such as conviviality to address the challenges heavy rainfall brings.
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    Open Access
    Recent trends in the climate of Namaqualand, a megadiverse arid region of South Africa
    (2016) Davis, Claire L; Hoffman, M Timm; Roberts, Wesley
    Abstract Namaqualand is especially vulnerable to future climate change impacts. Using a high-resolution (0.5°x0.5°) gridded data set (CRU TS 3.1) and individual weather station data, we demonstrated that temperatures as well as frequency of hot extremes have increased across this region. Specifically, minimum temperatures have increased by 1.4 °C and maximum temperatures by 1.1 °C over the last century. Of the five weather stations analysed, two showed evidence of a significant increase in the duration of warm spells of up to 5 days per decade and a reduction in the number of cool days (TX10P) by up to 3 days per decade. In terms of rainfall, we found no clear evidence for a significant change in annual totals or the frequency or intensity of rainfall events. Seasonal trends in rainfall did, however, demonstrate some spatial variability across the region. Spatial trends in evapotranspiration obtained from the 8-day MOD16 ET product were characterised by a steepening inland-coastal gradient where areas along the coastline showed a significant increase in evapotranspiration of up to 30 mm per decade, most notably in spring and summer. The increase in temperature linked with the increases in evapotranspiration pose significant challenges for water availability in the region, but further research into changes in coastal fog is required in order for a more reliable assessment to be made. Overall, the results presented in this study provide evidence-based information for the management of climate change impacts as well as the development of appropriate adaptation responses at a local scale.