Browsing by Author "Warburton, Michele Lynn."

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Challenges in modelling hydrological responses to impacts and interactions of land use and climate change.
(2012) Warburton, Michele Lynn.; Schulze, Roland Edgar.; Jewitt, Graham Paul Wyndham.
To meet society’s needs for water, food, fuel and fibre the natural land cover throughout the world has been extensively altered. These alterations have impacted on hydrological responses and thus on available water resources, as the hydrological responses of a catchment are dependent upon, and sensitive to, changes in the land use. Similarly, changes in the climate through enhanced carbon dioxide (CO2) levels in the atmosphere have resulted in increased temperature and altered precipitation patterns that alter hydrological responses. In combination, land use change and global climate change form a complex and interactive system, whereby both human influences and climate change manipulate land use patterns, and changes in land uses feed back to influence the climate system, with both impacting on hydrological responses. Relatively few studies have been undertaken examining the combined impacts of climate change and land use change on water resources, with no consensus emerging as yet as to combined influence of land use change and climate change on hydrological responses and the role of geographical characteristics in determining the overriding influence. There is, however, agreement that the effect on hydrological responses will be amplified. Given that South Africa is currently water stressed and considered to be highly exposed to climate change impacts, an understanding of hydrological responses to the complex interactions between land use and climate change is crucial to allow for improved integration of land use planning in conjunction with climate change adaptation into water resources management. To determine the sensitivity of land use to changing climate, a sensitivity study assessing the potential impacts of climate change on the areas climatically suitable for key plantation forestry species was undertaken. Under sensitivity scenarios of climate change the climatically optimum areas for specific forest species were shown to shift, with optimum areas changing in extent and location between and within South Africa’s provinces. With potential for shifts in land use due to climate change shown, the imperative to improve understanding of the dynamics between land use and climate change as well as the subsequent impacts on hydrological responses was further established. For the assessment of climate-land use-water interactions, a process-based hydrological model, sensitive to land use and climate, and changes thereof, viz. the daily time step ACRU model was selected. In order to increase the confidence in results from the model in a study such as this, its representation of reality was confirmed by comparing simulated streamflow output against observations across a range of climatic conditions and land uses. This comparison was undertaken in the three diverse South African catchments chosen for the study, viz. the semi-arid, sub-tropical Luvuvhu catchment in the north of the country, which has a large proportion of subsistence agriculture and informal residential areas, the Upper Breede catchment in the winter rainfall regions of the south, where the primary land uses are commercial orchards and vineyards, and the sub-humid Mgeni catchment along the eastern seaboard, where plantation forestry is dominant in the upper reaches, commercial plantation sugarcane and urban areas in the middle reaches, and urban areas dominate the lower reaches. Thus, in effect a space for time study was undertaken, thereby reducing the uncertainty of the model’s ability to cope with the projected future climate scenarios. Overall the ACRU model was able to represent the high, low and total flows, and thus it was concluded that the model could be used with confidence to simulate the streamflows of the three selected catchments and was able to represent the hydrological responses from the range of climates and diversity of land uses present within the catchments. With the suitability of the model established for the theme of this research, the understanding of the complex interactions between hydrological responses and land use could be improved. The hydrological responses of the three selected catchments to land use change were varied. Results showed that the location of specific land uses within a catchment plays an important role in the response of the streamflow of the catchment to that land use change. Furthermore, it was shown that the contributions of different land uses to the streamflow generated from a catchment are not proportional to the relative area of those land uses, and the relative contribution of the land use to the catchment streamflow varies with the annual rainfall of the catchment. With an improved understanding of the dynamics between land uses and hydrological responses, the impacts of climate change on hydrological responses were assessed prior to analysing the combined impacts on land use and climate change. Five plausible climate projections from three coupled atmosphere-ocean global climate models covering three SRES emissions scenarios which were downscaled with the RCA3 regional climate model and adjusted using the distribution-based scaling (DBS) approach for bias correction were used as climate input to the ACRU model, with future projections applied to a baseline land cover scenario compared to historical climate applied to the same baseline land cover scenario. No consistent direction of change in the streamflow responses was evident in the Mgeni and Luvuvhu catchments. However, decreases in streamflow responses were evident for all five scenarios for the Upper Breede. With an understanding of the separate impacts of land use and climate change on hydrological responses, an analysis of the combined impacts was undertaken to determine which changes were projected to be of greater importance in different geographical locations. Results indicated that the drier the climate becomes, the relatively more significant the role of land use becomes, as its impact becomes relatively greater. The impacts of combined land use and climate change on the catchments’ streamflow responses varied across both the temporal and spatial scales, with the nature of the land use and the magnitude of the projected climate change having significant impacts on the streamflow responses. From the research undertaken, the key results were • that the climatic variable to which plantation forestry species are most sensitive is rainfall; • that optimum growth areas for plantation forestry are projected to shift under changing climates, having a potentially significant impact on the landscape and thus on the hydrological responses from the landscape; • that the daily time-step, physical-conceptual and process-based ACRU model is appropriate for use in land use change and climatic change impact studies as shown through a space for time study; • that the contributions of different land uses to the streamflow generated from a catchment is not proportional to the relative area of that land use and that, as the mean annual precipitation of a subcatchment decreases, so the disparities between the relative areas a land use occupies and its contribution to catchment streamflow increases; • that specific land use changes have a greater impact on different components of the hydrological response of a catchment; • that land uses which currently have significant impacts on catchment water resources will place proportionally greater impacts on the catchment’s water resources if the climate were to become drier; thus the drier the climate becomes, the more relatively significant the role of land use becomes; • that when considering any hydrological impacts of land use change, climate change or combined land use and climate change, assessments need to consider the scale where the localized impacts may be evident, the progression of the impacts as the streamflow cascades through the catchment, as well as the impacts at the whole catchment scale where the accumulation of the effects through the catchment are evident; and lastly • that each catchment is unique with its own complexities, feed forwards and feedbacks, thus each catchment will have a unique threshold as to where land use change or climate change begins to have a significant influence of the hydrological response. Given these complex interactions between land use, climate and water, there is a growing imperative to improve the understanding of the movement of water within catchments, to be receptive and adaptive to new concepts and information, and to developing resilient and adaptive water management strategies for the future in a way that minimises the risks and maximises the benefits to potential impacts of climate change.
Detection of changes in temperature and streamflow parameters over Southern Africa.
(2005) Warburton, Michele Lynn.; Schulze, Roland Edgar.
It has become accepted that long-term global mean temperatures have increased over the twentieth century. However, whether or not climate change can be detected at a local or regional scale is still questionable. The numerous new record highs and lows of temperatures recorded over South Africa for 2003, 2004 and 2005 provide reason to examine whether changes can already be detected in southern Africa's temperature record and modelled hydrological responses. As a preface to a temperature detection study, a literature reVIew on temperature detection studies, methods used and data problems encountered, was undertaken. Simple statistics, linear regression and the Mann-Kendall non-parametric test were the methods reviewed for detecting change. Southern Africa's temperature record was thereafter examined for changes, and the Mann-Kendall non-parametric test was applied to time series of annual means of minimum and maximum temperature, summer means of maximum temperature and winter means of minimum temperature. Furthermore, changes in the upper and lower ends of the temperature distribution were examined. The Mann-Kendall test was applied to numbers of days and numbers of 3 consecutive days abovelbelow thresholds of 10th and 90th percentiles of minimum and maximum temperatures, as well as abovelbelow threshold values of minimum (i.e. 0°) and maximum (i.e. 40°C) temperatures. A second analysis, using the split sample technique for the periods 1950 - 1970 vs 1980 - 2000, was performed for annual means of daily maximum and minimum temperatures, summer means of daily maximum temperatures, winter means of daily minimum temperatures and coefficients of variability of daily maximum and minimum temperatures. Two clear clusters of warming emerged from almost every analysis, viz. a cluster of stations in the Western Cape and a cluster of stations around the midlands ofKwaZulu-Natal, along with a band of stations along the KwaZulu-Natal coast. Another fmding was a less severe frost season over the Free State and Northern Cape. While certain changes are, therefore, evident in temperature parameters, the changes are not uniform across southern Africa. Precipitation and evaporation are the primary drivers of the hydrological cycle, with temperature an important factor in the evaporation process. Thus, with changes in various temperature parameters having been identified over many parts of southern Africa, the question arose whether any changes were evident as yet in hydrological responses. The ACRU model was used to generate daily streamflow values and associated hydrological responses from a baseline land cover, thus eliminating all possible human influences on the catchment and channel. A split-sample analysis of the simulated hydrological responses for the 1950 - 1969 vs 1980 - 1999 periods was undertaken. Trends over time in simulated streamflows were examined for medians, dry and wet years, as well as the range between wet and dry years. The seasonality and concentration of streamflows between the periods 1950 - 1969 and 1980 - 1999 were examined to determine if changes could be identified. Some trends found were marked over large parts of Primary Catchments, and certainly require consideration in future water resources planning. With strong changes over time in simulated hydrological responses already evident in certain Primary Catchments of South Africa using daily rainfall input data from 1950 1999, it, therefore, became necessary to examine the rainfall regimes of the Quaternary Catchments' "driver" rainfall station data in order to determine if these hydrological response changes were supported by changes in rainfall patterns over time. A splitsample analysis was, therefore, performed on the rainfall input of each Quaternary Catchment. Not only were medians considered, but the higher and lower ends of the rainfall distributions were also analysed, as were the number of rainfall events above pre-defined daily thresholds. The changes evident over time in rainfall patterns over southern Africa were found to vary from relatively unsubstantial increases or decreases to significant increase and decreases. However, the changes in rainfall corresponded with the changes noted in simulated streamflow. From the analyses conducted in this study, it has become clear that South Africa's temperature and rainfall, as well as hydrological responses, have changed over the recent past, particularly in certain identifiable hotspots, viz. the Western Cape and KwaZulu-Natal where significant increases in temperature variables and changes in rainfall patterns were detected. These detected changes in climate need to be considered in future water resources planning.