59 research outputs found
Climate Change and Cereal Production Evolution Trend in the Sahel: Case Study in Mali from 1951 to 2010
No full textMali is a Sahelian country with a large climatic contrast from North to South. The current climatic and production evolutionary study is focused on the six major agro-climatic cereal production zones ranging from Kayes (400 mm) to Sikasso (>1000 mm) of rainfalls. Climatic data are rainfall records, daily maximum and minimum temperatures from 60 years of the six major synoptic weather observation stations. Data were analyzed on comparing average decades of the two normal periods of 30 years (1951-1980) and (1981-2010). Annual agronomic production data for millet, sorghum, maize and rice are derived from Mali's agricultural statistics base from 1984 to 2013. Main climatic results analyses indicate that climate change resulted in a decrease of 100 mm isohyets between the 2 periods of 30 years. The structure of the rainy season was little changed between these two periods since the average start of the season was delayed by 6 days and the average end date of the season became earlier by 4 days. Maximum temperatures increased significantly from + 0.44°C to + 1.53°C and minimum temperatures significantly increased from + 1.05°C to + 1.93°C in varying way depending on the sites. Statistics of major agronomic food crop production in Mali from 1984 to 2013 indicate an average increase of 985 to 4492 thousand tones, or 22% increase per year. There is a positive upward in saw tooth trend in Malian production from 1984 to 2013. This positive trend is the result of a combination of agricultural extension, agronomic research application and the management of small farmer holder in the Sahel. This evolution needs better study for drawing necessary right conclusions
Modelling landslide dynamics in forested landscapes
No full textThe research resulting in this thesis covers the geological, geomorphological and landscape ecology related themes of the project 'Podzolisation under Kauri (Agathis australis): for better or worse?' supported by theNetherlands Organisation for Scientific Research (NWO). The general objective of this thesis is to investigate landscape, soil and vegetation dynamics in theWaitakereRangesRegionalParkon the North Island of New Zealand, where also all the fieldwork was carried out. The main core of the thesis consists of the development of a dynamic landscape process model to simulate soil redistribution by shallow landsliding. Resulting spatial patterns of erosion and deposition, changes in landslide susceptibility over time and the relation of spatially explicit landscape attributes with vegetation patterns are further explored.·Chapter 1 is a general introduction elaborating on the geology, climate and socio-economic setting of the study area and explains the main objectives and research questions. The contents and overall structure of the thesis are also illustrated. Following this introductory chapter, the thesis is composed of 5 chapters based on scientific papers published in or submitted to peer reviewed journals. ·Chapter 2 deals with the general tectonic setting of the study area. Quaternary coastal and fluvial terrace morphology and chronology are explored to reconstruct the tectonic history of the south-west coast of the Northland region inNew Zealand. This chapter is situated on the geological timescale (1.8 Ma BP till present) and places the subsequent chapters dealing with the landscape process model and its applications, acting on a timescale of years to decades, in a broader spatio-temporal perspective. Field surveys and the analysis of aerial photography yield an inventory of 13 fluvial and 12 marine terrace levels. Due to poor exposure of clear field evidence in the form of e.g. wave-cut platforms or distinct river sediments, planar landscape morphology forms the main criterion for terrace remnant identification. Based on the record of terrace height spacings, sparse tephra age control and correlation with global paleoclimatic records, an attempt is made to reconstruct the regional Quaternary uplift rates. Because no hard chronostratigraphic marker is present within the fluvial terrace sequence, fluvial terrace levels are linked to the marine sequence by using the mean uplift rates calculated from the marine terraces (0.35 mm yr -1 from 0- 0.1 Ma and 0.26 mm yr -1 from 0.1-0.3 Ma). Both sets of terraces are then correlated with oxygen isotope fluctuations and the astronomically tuned timescale from ODP Site 677 and the Vostok ice core paleoclimatic records. Oldest marine and fluvial terrace levels are estimated 1.21 Ma and 0.242 Ma respectively. Although there seems to be some form of controversy about the uplift history and especially the preservation of terraces in the study area, a general regional uplift, superimposed on glacio-eustatic sea-level changes, is substantiated as the only possible mechanism leading to the maintenance of a considerable relief and active denudation processes inland.·Chapter 3 deals with the development and application of the LAPSUS-LS landscape process model. The model is constructed as a component of the LAPSUS modelling framework ( L andsc A pe P roces S modelling at m U lti dimensions and scale S ; -LS:L and S lide,refers to the process specific model component). LAPSUS-LS delineates the location of shallow landslide initiation sites and simulates the effects on spatial patterns of soil redistribution and resulting landslide hazard for a large watershed within the study area. Processes that need to be incorporated in the model are reviewed followed by the proposed modelling framework. The model predicts the spatial pattern of landslide susceptibility within the simulated catchment and subsequently applies a spatial algorithm for the redistribution of failed material on the basis of a scenario of triggering rainfall events, relative landslide hazard and trajectories with runout criteria for failed slope material. The model forms a spatially explicit method to address the effects of shallow landslide erosion and sedimentation because digital elevation data are adapted between timesteps and on- and off-site effects over the years can be simulated in this way. By visualisation of the modelling results in a GIS environment, the shifting pattern of upslope and downslope (in)stability, triggering of new landslides and the resulting slope retreat by soil material redistribution due to former mass movements is simulated and assessed.·Chapter 4 zooms in on a more theoretical aspect of the LAPSUS-LS model and evaluates digital elevation model (DEM) resolution effects on model results. The focus is on influences of grid size on landslide soil redistribution quantities and resulting spatial patterns and feedback mechanisms. Distributions of slope, specific catchment area and relative hazard for shallow landsliding are analysed for four different DEM resolutions (grid sizes of 10, 25, 50 and 100 m) for a 12 km 2 study catchment in theWaitakereRanges. The effect of DEM resolution proves to be especially pronounced for the boundary conditions determining a valid landslide hazard calculation. For coarse resolutions, the smoothing effect results in a larger area becoming classified as unconditionally stable or unstable. Simple empirical soil redistribution algorithms are applied for scenarios in which all sites with a certain landslide hazard fail and generate debris flow. The lower initial number of failing cells but also the inclusion of slope (limit) in those algorithms becomes apparent with coarser resolutions. For finer resolutions, much larger amounts of soil redistribution are found, which is attributed to the more detailed landscape representation. Looking at spatial patterns of landslide erosion and sedimentation, the size of the area affected by these processes also increases with finer resolutions. In general, landslide erosion occupies larger parts of the area than deposition, although the total amounts of soil material eroded and deposited are the same. Analysis of feedback mechanisms between soil failures over time shows that finer resolutions show higher percentages of the area with an increased or decreased landslide hazard. When the extent of sites with lower and higher hazards are compared, finer grid sizes and higher landslide hazard threshold scenarios tend to increase the total extent of areas becoming more stable relative to the less stable ones. It is concluded that extreme care should be taken when quantifying landslide basin sediment yield by applying simple soil redistribution formulas to DEMs with different resolutions. Rather, quantities should be interpreted as relative amounts. For studying shallow landsliding over a longer timeframe, the 'perfect' DEM resolution may not exist, because no resolution can possibly represent the dimensions of all different slope failures scattered in space and time. It is emphasised that the choice of DEM resolution, possibly restricted by data availability in the first place, should always be adapted to the context of a particular type of analysis.·Chapter 5 and 6 describe two distinct applications of the LAPSUS-LS model: in Chapter 5 , a sediment record is used, in combination with the LAPSUS-LS model, to reconstruct the incidence of high-magnitude/low-frequency landslide events in the upper part of theWaitakereRivercatchment and the history of the Te Henga wetland at the outlet. Sediment stratigraphy and chronology are interpreted by radiocarbon dating, foraminiferal analysis, andprovisionaltephrochronology. Gradual impoundment of the wetland began c. 6000 cal yr BP, coinciding with the start of a gentle sea-level fall, but complete damming and initial sedimentation did not begin until c. 1000 cal yr BP. After damming, four well-defined sediment pulses occurred and these are preserved in the form of distinct clay layers in most of the sediment cores. For interpreting the sediment pulses, the LAPSUS-LS modelisapplied to determine spatially distributed relative landslide hazard, applicable at the catchment scale. An empirical landslide soil redistribution componentisadded to determine sediment delivery ratio and the impact on total catchment sediment yield. Sediment volumesarecalculated from the wetland cores and corresponding landslide scenarios are defined through back-analysis of modelled sediment yield output. In general, at least four major high-magnitude landslide events, both natural and intensified by forest clearance activities, occurred in the catchment upstream of Te Henga wetland during the last c. 1000 years. Their magnitude can be expressed by a range of critical rainfall thresholds representing a LAPSUS-LS scenario.·Chapter 6 is a more ecologically focused application of the model and links digital terrain analysis and landslide modelling with the spatial distribution of mature kauri trees. The use of topographical attributes for the analysis of the spatial distribution and ecological cycle of kauri ( Agathis australis ), a canopy emergent conifer tree from northernNew Zealand, is studied. Several primary and secondary topographic attributes are derived from a DEM for theWaitakereRivercatchment and the contribution of these variables in explaining presence or absence of mature kauri is assessed with logistic regression and Receiver Operating Characteristic (ROC) plots. The topographically based landslide hazard index calculated with the LAPSUS-LS model appears to be very useful in explaining the occurrence and ecological dynamics of kauri. It is shown that the combination of topographic -, soil physical - and hydrological parameters in the calculation of this single landslide hazard index, performs better in explaining presence of mature kauri than using topographic attributes calculated from the DEM properties alone. Moreover, this example demonstrates the possibilities of using terrain attributes for representing geomorphological processes and disturbance mechanisms, often indispensable in explaining a species' ecological cycle and forest stand dynamics. The results of this analysis support the 'temporal stand replacement model', involving disturbance as a dominant ecological process in forest regeneration, as an interpretation of the community dynamics of kauri. Furthermore, a certain threshold maturity stage, in which trees become able to stabilise landslide prone sites and postpone a possible disturbance by this process, together with great longevity are seen as major factors making kauri a 'landscape engineer'.·Synthesising, Chapter 7 reflects on the most important conclusions from the research resulting in this thesis and discusses the achievement of the main objectives and answers to the research questions postulated in Chapter 1. Three general themes are put forward covering the previous chapters. Finally some ideas for future research are suggested
Bridging the gap between the available and required soil data for regional land use analysis
No full textThe United Nations pledged to achieve the Sustainable Development Goals by 2030. Regional land use analyses (RLUA) have an essential contribution to achieving these goals. To better meet the needs for achieving sustainable development, RLUA became more quantitative and more interdisciplinary over recent decades. This change resulted in an increased use of quantitative simulation models, which changed the type and nature of input data as well. Soil data are one of the input data RLUA require. Available soil data often do not meet the soil data requirements anymore, due to the change in RLUA. Therefore, a gap exists between the available and required soil data. This thesis aims to find possible solutions to bridge this gap. In Chapter 2, different soil datasets are compared to identify the gap and to analyse the effect of using different soil datasets as input for a regional land use analysis (RLUA). Main challenges with soil data in RLUA are: i) understanding the assumptions in soil datasets, ii) creating soil datasets that meet the requirements for regional land use analysis, iii) not only rely on available soil data but also collect new soil data and iv) validate soil datasets. Chapter 2 demonstrated differences between soil datasets, which had significant effect on the results of RLUA. Three potential solutions on bridging the gap between the available and required soil data are given in Chapter 3, 4 and 5. A literature study showed that RLUA hardly combine available and newly collected soil data. Chapter 3 analyses what complementary data RLUA require by combining available soil data and newly collected soil data. Two case studies were carried out to illustrate how a combination can enrich the soil data for RLUA. Predicting soil properties, in particular soil organic matter, using newly collected soil data often result in soil maps of poor quality. The digital soil mapping techniques that are currently being used for predicting soil properties make dominantly use of statistical models, while much knowledge on the mechanistic processes that influence a soil property are available. To improve the prediction of soil organic matter, a mechanistic model for digital soil mapping (DSM) is developed and the potential of mechanistic digital soil mapping is explored in Chapter 4. Mechanistic digital soil mapping predicts soil properties by values that typically stay within realistic boundaries. Complex soil mapping techniques are increasingly being used to better meet the data requirements, because the use of quantitative simulation models in RLUA increased over recent decades. Chapter 5 analyses whether the required soil data can be obtained more targeted to RLUA. Three case studies were carried out to illustrate that the complexity of quantitative simulation models can differ from the complexity required by the RLUA. Therefore, the spatial variation at which the soil properties are provided need to be in line with the spatial variation at which the RLUA operate. In the synthesis (Chapter 6), the research findings, the implementation of the research findings, the hypothesis and future perspectives are discussed and recommendations towards the soil science community and the people involved in RLUA are provided. In this chapter, the ARDAIG approach is introduced, which aims to be an approach that helps obtaining the required soil data for RLUA more targeted. If the soil science community and the people involved in RLUA will implement the presented recommendations, there are opportunities to make soil science contribute more efficiently in RLUA.</p
Multiple impinging jet arrays. An experimental study on flow and heat transfer
No full textBecause of their high efficiency and their ability to provide high heat transfer rates, impinging jets are applied for rapid cooling and heating in a wide variety of industrial processes. However, the physical phenomena controlling the heat transfer from impinging jets are to a large degree unknown. The goal of the present project was to gain a better understanding of the interaction between the flow and heat transfer in impinging jet arrays. Experiments were performed in two different configurations: a single impinging jet and multiple impinging jet arrays. LDA and PIV velocity measurements in the single jet were compared: these measurements were aimed at serving as a reference for comparison of the multiple jet features. In a hexagonal and an in-line array of jets PIV was used to provide instantaneous velocity fields over the flow area of interest, what proved to be essential for detecting some salient features of the multiple-jet dynamics. These dynamics were investigated on the basis of POD filtered snapshots of the flow. In both arrays, large scale eddies in the development zone cause the impinging jets to break up or be severely displaced in the out-of-plane direction. A horse-shoe vortex appears around the outer jets of the hexagonal array, whereas the in-line array does not show this feature. This is most likely caused by the higher pitch in the in-line configuration. On the other hand, the flow field in the in-line arrangement appears to be diagonally asymmetrical, even though the symmetry of the nozzle arrangement would imply a symmetrical flow field. This flow asymmetry causes an elliptical heat transfer pattern in the impingement zone of the center jet. The flow asymmetry was also found in numerical simulations of a similar configuration, conducted in a parallel project. Heat transfer measurements were performed in the impingement surface for seven multiple jet arrays using LCT, which provided insight in the influence of different geometrical parameters of the arrays on the heat transfer. Additionally, a non-dimensional correlation was derived linking heat transfer to geometrical and flow parameters. Heat transfer profiles were subsequently compared to velocity and turbulence quantities just above the impingement plate to investigate the effect of the mean flow features on the heat transfer. The apparent correlation between the Nusselt number and the normal contribution to the production of turbulent kinetic energy is coincidental: the cause of the increased Nusselt number in the impingement points of the jets is a combination of an oscillating impingement position due to large vortical structures and a very strong acceleration of the fluid from the impingement point outwards. In the impingement point the boundary layer is very thin and the temperature gradient is very steep, enabling impinging vortices to encapsulate and remove heat in an effective way. The results presented in this thesis should aid in better understanding the role of turbulence and vortical structures in the interaction between adjacent impinging jets, and in the heat transfer from impinging jets. With the advent of a measurement technique capable of capturing instantaneous heat transfer distributions, a potential extension of the present work is a more detailed analysis of the influence of individual vortical structures on impingement heat transfer.Applied Science
Optimization of particle image velocimetry for turbulent stirred tank flow
No full textThis project was aimed at optimizing Particle Image Velocimetry (PIV) for the determination of velocity distributions in turbulent stirred tank flow. To resolve directional ambiguity, which is inherently linked to PIV, a rotating mirror was used. Two procedures to correct for the applied pre-shift were tested. At Re=5000, five different configurations of the PIV-system were tried. These configurations were different from each other in the choice of recording angle, particle concentration and pre-shift velocity. Two of these five configurations yielded pictures with sufficient quality. The pictures of these two configurations were analysed digitally by two correlation procedures, i.e. auto-correlation and cross-correlation with shifted interrogation areas (SIA). In the latter method, auto-correlation was used to determine the integer displacement of particle images. This integer displacement field was used as window shift field for the cross-correlation procedure with a view to minimizing the loss of pairs effect by in-plane movement. Additionally, a multiple pass method was proposed and its performance tested. The purpose of this method was to increase the resolution of the analysis by decreasing the size of the interrogation areas. Mean and RMS velocity profiles were constructed from the determined velocity fields. These profiles are compared to profiles resulting from LDA experiments in a similar tank.Kramers Laboratorium voor Fysische TechnologieApplied Science
Assessing climate change impacts and adaptation strategies for smallholder agricultural systems in Uganda
No full textThe debate on whether climate change will impact on peoples’ livelihoods and, hence, the need to act is essentially over and has instead shifted to the development of strategies needed by different regions and countries to adapt to climate change effects. However, there is still scanty information necessary to ably address climate change related issues. There is a considerable knowledge gap with respect to climate change impact, vulnerability and adaptation to increased climate variability and change. In this paper, using the trade off analysis model, the impact of climate change on peoples’ livelihoods and possible adaptation strategies to increase the resilience and sustainability of agricultural systems in three regions of Uganda (central, Masaka and southwest) are analysed. The results show that 70-97% of households will be adversely affected by climate change in Uganda. The southwest will be most affected due to smaller farm sizes and limited livelihood alternatives. There will be no positive gains from encroaching on swamps, which is one of the reported adaptation strategies to climate related stresses. Improving productivity of important crops (bananas for southwest, and sweet potatoes and bananas for central region), in addition to adoption of grade cattle will likely be a better adaptation strategy for climate change
Contribution of topographically based landslide hazard modelling to the analysis of the spatial distribution and ecology of kauri
No full textIn this paper the use of topographical attributes for the analysis of the spatial distribution and ecological cycle of kauri (Agathis australis), a canopy emergent conifer tree from northern New Zealand, is studied. Several primary and secondary topographical attributes are derived from a Digital Elevation Model (DEM) for a study area in the Waitakere Ranges. The contribution of these variables in explaining presence or absence of mature kauri is assessed with logistic regression and Receiver Operating Characteristic (ROC) plots. A topographically based landslide hazard index, calculated by combining a steady state hydrologic model with the infinite slope stability equation, appears to be very useful in explaining the occurrence and ecological dynamics of kauri. It is shown that the combination of topographical, soil physical and hydrological parameters in the calculation of this single landslide hazard index, performs better in explaining presence of mature kauri than using topographical attributes calculated from the DEM alone. Moreover, this study demonstrates the possibilities of using terrain attributes for representing geomorphological processes and disturbance mechanisms, often indispensable in explaining a species¿ ecological cycle. The results of this analysis support the `temporal stand replacement model¿, involving disturbance as a dominant ecological process in forest regeneration, as an interpretation of the community dynamics of kauri. Furthermore a threshold maturity stage, in which trees become able to stabilize landslide prone sites and postpone a possible disturbance, together with great longevity are seen as major factors making kauri a `landscape engineer¿
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