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Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Evaluating rainwater harvesting systems in arid and semi-arid regions
    Ammar, Adham Ali - \ 2017
    Wageningen University. Promotor(en): C.J. Ritsema, co-promotor(en): M.J.P.M. Riksen; M. Quessar. - Wageningen : Wageningen University - ISBN 9789463431460 - 207
    water harvesting - rain - water - arid zones - semiarid zones - geographical information systems - water management - climatic change - tunisia - iraq - regenwateropvang - regen - water - aride klimaatzones - semi-aride klimaatzones - geografische informatiesystemen - waterbeheer - klimaatverandering - tunesië - irak

    Rainwater harvesting (RWH) is an ancient traditional technology practised in many parts of the world, especially in arid and semi-arid regions (ASARs). ASARs represent 40% of the earth’s land surface and are characterised by low average annual rainfall and uneven temporal and spatial distributions of that rainfall. In these regions an efficient use of the limited amount of rainfall available is important, e.g. by collecting and using surface runoff (water harvesting). Lately, access to water for agriculture and domestic use has become worse because of increasing population, higher levels of human activity and the impacts of climate change. The inhabitants of ASARs have developed several RWH techniques to increase the water availability, thus coping with water shortages. RWH is an important mitigation strategy to the impact of climate change on water availability in ASARs.

    Four main methodologies of site selection were categorised, ranging from those based only on biophysical criteria to more integrated approaches that include socioeconomic criteria. Our analysis suggests that the integration of multi-criteria analysis (MCA) with a geographic information system (GIS) is the most advanced approach. It offers high potential in data-poor regions; GIS-based hydrological modelling is always recommended for data-rich regions.

    The potential for RWH in wadi Horan (western desert of Iraq) was identified using a GIS-based suitability model. The method for selecting suitable sites for RWH was then further developed into an evaluation and decision support tool for assessing the overall performance of existing RWH systems by integrating engineering, biophysical and socioeconomic criteria using MCA supported by GIS. It was tested in the wadi Oum Zessar in southeastern Tunisia.

    A simple but generally applicable water harvesting model (WHCatch) was developed to investigate and optimise the performance of the RWH systems under various scenarios of design and management, It was tested in wadi Oum Zessar. The advantages of simulating long-term water balances at the sub-catchment level for improving our understanding of hydrological processes in an RWH system are emphasised. Several solutions for optimising RWH performance in various scenarios are provided.

    Finally, the impact of climate change on existing RWH systems in the Oum Zessar watershed under current and future scenarios of climate was investigated. The downscaled maximum and minimum temperatures clearly indicated an increasing trend in the mean monthly temperature and the generated precipitation tended to decrease in the future. It was shown that the combination of changing the flow direction and the spillway height had a large impact on the performance of the RWH systems under current and future conditions. Water management and structural design at the sub-catchment level plays a more important role than climate change in the performance of RWH.

    Effect of water harvesting techniques on hydrological processes and sediment yield in Northern Ethiopia
    Woldegiorgis, Berhane Grum - \ 2017
    Wageningen University. Promotor(en): V. Geissen; C.J. Ritsema, co-promotor(en): R. Hessel; C.A. Kessler. - Wageningen : Wageningen University - ISBN 9789463431682 - 156
    hydrology - water harvesting - arid zones - semiarid zones - water availability - ethiopia - hydrologie - regenwateropvang - aride klimaatzones - semi-aride klimaatzones - waterbeschikbaarheid - ethiopië

    The study was conducted in the semi-arid northern Ethiopia aimed at selecting appropriate water harvesting techniques (WHTs) for implementation. A plot-scale experiment was set up, in the Gule catchment, on a farmland to monitor the effect of in-situ WHTs such as tied ridges and straw mulch mainly on event-based runoff, soil-moisture, and soil and nutrient losses. The off-site effect of WHTs such as check dams and percolation ponds on catchment-scale event-based runoff and sediment yield was also monitored in the Gule catchment (~12 km2) and Misbar sub-catchment (~2.4 km2), northern Ethiopia. First, a decision support approach was developed to aid the selection of WHTs in arid and semi-arid areas. The decision support approach was validated with a case study for WHTs in the upper Geba watershed in northern Ethiopia. Using the decision support methodology, eight potential WHTs were pre-selected for implementation in the watershed. Next, using suitability indicators for WHTs and a GIS-based multi-criteria analysis, suitable areas were identified for three of these WHTs, namely check dams, percolation ponds and bench terraces and suitability maps were generated. The multi-criteria analysis was validated by comparing the predicted suitable areas with the already existing locations of WHTs in the watershed. The result was that 90% of the existing check dams and 93% of the percolation ponds in the upper Geba watershed were correctly identified by the approach. The field study showed runoff reduction by WHTs from farmland between 40 to 88% and soil loss between 60 to 90%. Nutrient loss reduction from farmland by WHTs also ranged between 52 and 86%. Soil-moisture also improved due to the use of the in-situ WHTs. Model-based simulation at the Gule and Misbar outlets using LISEM showed that the current WHTs applied in the catchment are able to decrease event-based runoff by 41 and 45%, respectively. Similarly, sediment yield was reduced at both the Gule outlet and Misbar sub-outlet, by 67 and 55%, respectively. This study has verified that in semi-arid areas, such as the northern Ethiopian highlands, in-situ and catchment-scale WHTs can be used to improve the efficiency of rainwater harvesting and water availability for agricultural uses. Furthermore, these WHTs help to mitigate land degradation by decreasing soil and nutrient losses from farmland and sediment yield from catchments.

    African elephant in a cleft stick : choosing between starving or dying from thirst in arid savanna
    Wato, Yussuf - \ 2016
    Wageningen University. Promotor(en): Herbert Prins, co-promotor(en): Ignas Heitkonig; Frank van Langevelde. - Wageningen : Wageningen University - ISBN 9789463430005 - 126
    loxodonta africana - starvation - thirst - savannas - arid zones - animal ecology - mortality - drought - loxodonta africana - uithongering - dorst - savannen - aride klimaatzones - dierecologie - mortaliteit - droogte

    Elephant population studies have become important especially because of the long standing perception that high elephant densities have negative impact on vegetation and other wildlife species. Thus, in areas of high elephant density, managers attempt to re-distribute them or keep their numbers low through provision of water, translocation or culling. These approaches are thought to keep the population within the limits that can be sustained by the ecosystem, termed “the ecological carrying capacity”, a management option hinged on equilibrium theory. Equilibrium systems are considered stable, with resources and the animals that depend on them being at balance with each other. This stability is rarely the case in tropical savannas where the rule appears to be “a flux of nature” rather than “a balance of nature”.

    Tropical savannas, where over half of the African elephant live, are prone to constant environmental fluctuations, especially prolonged droughts, and hence there is a growing understanding that populations of wildlife species and their communities are rarely at equilibrium. Therefore, it is critical to understand how the constant environmental flux in this system affects wildlife populations and the implication for their management. In this thesis, the central focus is to investigate the role of drought occurrences on elephant population dynamics in tropical savannas. To address this question, it is important to have a good understanding of the historical changes of elephant population in relation to drought events and the ecology of elephant in semi-arid savannas - their distribution and density, their movements and behaviour. For the historical data, I analysed the best existing long-term data in Africa of wild elephant population that has been consistently monitored for over 40 years where life histories of over 3000 wild individual elephant are known, at Amboseli National Park in Kenya. In addition, I also analysed geo-referenced elephant mortality data collected daily for 10 years from Tsavo Conservation Area. Further, I analysed 2 years data from 8 GPS collared African elephant to investigate their movement response to seasonal water and forage distribution in Tsavo Ecosystem.

    First, I investigated the temporal effects of drought duration (number of consecutive dry months) and intensity (amount of rainfall) on elephant population structure in Amboseli National Park, Kenya. The result corroborates findings from past studies that calves (<2years) are more susceptible to drought caused mortality and the risk of dying decreased with age. A new finding in this study reveals that the effect of drought induced mortality for the adult elephant is sex and age dependent, with males older than 25 years being less likely to die as compared to females of the same age. This new result is because of the resolution of analysis in this study which focused on the length and severity of drought as opposed to past studies that restricted their analysis to seasonal and inter-annual differences in rainfall pattern. As they grow older and sexually mature, the foraging range of male elephant increase and they begin to take more risks and disperse to unfamiliar habitats to seek for quality forage and mates. Generally, foraging strategies between sexes in many species are more pronounced during periods of food scarcity, and the driving force in the differences appears to be driven by energy need requirements, reproductive status of an individual, body sizes and the social context, all of which differ between sexes.

    In the next study, I investigated the spatial pattern of elephant mortality in relation to drought occurrences in Tsavo National Park using MaxEnt. The results shows that elephant carcasses were aggregated and elephant mortality was negatively correlated with four months cumulative precipitation prior to death, forage availability and distance to water, while local elephant density showed a positive correlation. This finding rules out dehydration as the cause of elephant mortality in Tsavo as the river where the carcasses were aggregated is perennial. Furthermore, forage availability was low close to water sources and did not show a significant difference close to or further away from the river despite high elephant density around the river. Hence, these elephant mortalities may have occurred as a result of starvation.

    I went further to focus on two main limiting resources for elephants, namely forage and water, and their effect on elephant-habitat utilization in semi-arid savannas. I first investigated how water source distribution affect elephants’ seasonal movement patterns. Results indicate that male elephant moved maximally 20 km away from the nearest water source in the dry season while the female elephant foraged to a maximum of about 10 km and only moved further than this distances in the wet season. The strong directionality of elephant movement from a distance of 15km towards water sources (rho > 0.5) as they re-visited their watering source in the dry season suggest that elephant have information on location of the water sources.

    Next, I investigated the factors that determine selection of a foraging site for elephant with a focus on forage nutrients or biomass. Because of their large body size, it is thought that elephant can survive on a less nutritious but high biomass of forage. The results from this study shows that elephant selected foraging site based on forage biomass in dry seasons, whereas they selected areas with higher nutrients in the wet season. Moreover, females selected sites with a higher forage biomass as compared to males. This result may be explained by the difference in social organisation and foraging strategies between the sexes. In the previous studies on human-elephant conflict, for instance, male elephant raided crops more than the mixed herd, perhaps to seek for high quality forage.

    Together, the four studies in this thesis strongly suggest that elephant starve to death in prolonged drought contrary to the past studies that reported that adult elephant are less affected by drought. Even though prolonged droughts usually result in higher elephant mortalities, the resilience of semi-arid savannas may perhaps be as a result of these deaths that release the system from high browsing pressure and give it a window to regenerate. If that is the case, then drought induced elephant mortality may be a better way to regulate elephant numbers than culling. This finding strongly suggests that semi-arid savannas may in fact be a non-equilibrium system sustained by growth and crashes of herbivore populations. Maintaining the system as natural as possible may therefore keep elephant populations in savannas sustained for posterity. The modern day park managers have daunting challenges such as mass elephant deaths in drought, increased human-wildlife conflicts or changes in wildlife use of the landscape which may all be symptoms of wrong management interventions taken in the past or negative impacts of anthropogenic activities that have tipped the natural functioning of a non-equilibrium system. Therefore, park managers should undergo regular trainings on new conservation techniques and they should apply evidence-based science to make informed long term decision.

    Ecohydrology in water-limited environment using quantitative remote sensing - the Heihe River basin (China) case
    Jin, X. - \ 2009
    Wageningen University. Promotor(en): Michael Schaepman, co-promotor(en): Jan Clevers; Z. Su. - [S.l. : S.n. - ISBN 9789085852902 - 121
    hydrologie - ecologie - remote sensing - china - stroomgebieden - aride klimaatzones - ecohydrologie - hydrology - ecology - remote sensing - china - watersheds - arid zones - ecohydrology
    Water-limited environments exist on all continents of the globe and they cover more than
    30% of the Earth’s land surface. The eco-environments of these regions tend to be fragile and
    they are changing in a dramatic way through processes like land desertification, shrinking of
    oases, groundwater depletion, and soil erosion. These are either human induced or results of a
    changing climate. Implications of these changes for both the regional hydrologic cycle and the
    vegetation have been documented. Since these changes occur over a wide range of scales in
    space and time, remote sensing methods are needed to monitor the land surface characteristics,
    to observe changes in vegetation and hydrological states, and to compare these with
    predictions from hydrological models. It is widely accepted that remote sensing methods offer
    the ability to acquire spatially continuous measurements over large areas. Remote sensing can
    also help to visualize complex processes because the spatial data can be captured regularly
    over time.
    China is one of several countries with large arid and semi-arid areas. The Heihe River basin,
    situated in the arid inland of northwestern China, is one of the areas severely affected by ecoenvironmental
    degradation and recovery. The problem of the degraded environment is due to
    overexploitation of surface and ground water leading to shrinking of oases, including the
    decline and death of natural vegetation, and the lowering of the groundwater table. Exhaustive
    (over-)use of water resources is the main cause of land degradation in the lower reaches of the
    basin, called the Ejina oasis. The whole Heihe River basin is therefore selected as study area
    in this thesis to analyze the long-term eco-environmental changes. What happens in this river
    basin is likely to have a growing influence on regional hydrological cycles, even affecting
    human life. Effective management of eco-environmental problems in this critical zone of
    water-limited conditions will provide scientific evidence for protecting and improving the
    eco-environment in these Chinese northwestern arid regions, eventually resulting in land
    Studies on quantifying the relationship between the vegetation and the water resources are a
    critical step in developing an ecohydrological approach to resources management in order to
    minimize environmental degradation. Remote sensing measurements can help us to better
    understand the effects of changes in water management on hydrological processes and their
    subsequent feedback to the eco-environment at the regional scale. Remote sensing methods
    can also provide information to quantify heterogeneity and change at a large scale. Therefore,
    the main objective of this thesis is to develop a methodology for the quantitative assessment
    of eco-environmental changes at a large scale in arid regions by integrating remote sensing
    methods in ecohydrological approaches.
    Chapter 1 outlines the significance of quantitative assessment of eco-environmental
    changes using remote sensing methods and applying them for ecohydrology in northwestern
    China, resulting in the specific research objectives of this thesis.
    Chapter 2 quantifies both the vertical and horizontal distribution of vegetation in the Qilian
    Mountains area, representing the upper reaches of the Heihe River basin, based on MODIS
    NDVI images from the year 2000 - 2006. Our analysis reveals that elevation and aspect are
    two important impact factors for the vertical distribution of vegetation in a mountainous area.
    The NDVI increases with the elevation and reaches a maximum value at a certain elevation
    threshold, and then decreases as the elevation increases beyond this threshold. The optimal
    vegetation growth is on the shady side of the mountains because of less evapotranspiration.
    The best combination of temperature and precipitation is assessed providing good conditions
    for vegetation growth.
    Chapter 3 presents an efficient method to estimate the regional annual evapotranspiration
    (ET) based on the SEBS algorithm (Surface Energy Balance System) in the Zhangye basin,
    representing the middle reaches of the Heihe River basin. The method proposed is a
    combination of the daily SEBS results and data collected by meteorological stations. The
    result shows that the annual ET increased gradually during the period 1990-2004 and the main
    impact factor on the long-term increase of annual ET was the vegetation change. The
    accuracy of the ET result is validated using a water balance for the whole watershed and the
    validation reveals that the SEBS algorithm can be used to effectively estimate annual ET in
    the Zhangye basin.
    Chapter 4 establishes the quantitative relationship between the runoff of the Heihe River
    and the long-term vegetation change of the Ejina oasis, located in the lower reaches of the
    Heihe River. In this part, two time periods are distinguished corresponding to before and after
    the implementation of a new water allocation scheme in the Heihe River basin. The GIMMS
    NDVI and MODIS NDVI data sets are used to quantify the long-term change of the oasis
    vegetation in the first period 1989-2002 and the second period 2000-2006, respectively. The
    vegetation change shows a decreasing trend from 1989 to 2002 and an increasing trend
    between 2000 and 2006. Good relation between the runoff of the river and the vegetation
    growth are found at both stages and the time lag of the observed hysteresis effect of the runoff
    of the river on the oasis vegetation is one year. In addition, the yearly smallest water amount
    which sustains the demand of the eco-environment of the Ejina area is estimated to be 4×108
    m3 based on MODIS images.
    Chapter 5 explores a method to quantify the effect of the groundwater depth on the
    vegetation growth in the year 2000 in the oasis area by combining MODIS NDVI with
    groundwater observation data. The result demonstrates that the groundwater depth suitable for
    vegetation growth in this region ranges from 2.8 to 5 m, depending on species composition.
    Hardly any vegetation growth occurs when the groundwater depth is below 5 m because the
    rooting depth of the occurring species is limited and cannot maintain adequate water supplies
    to their canopies when the water depth is below 5 m. The situation changes after
    implementation of the new water allocation scheme since 2000. The mean NDVI increased
    and the annual conversion of bare land into vegetated land is about 38 km2 per year during the
    period 2000 – 2008. It reflects a potential recovery of the eco-environment of the Ejina area.
    Chapter 6 comprises the main conclusions and the outlook for possible improvements in
    future research. The main contribution of this study is the successful integration of remote
    sensing with ecohydrology in quantifying the relationship between water resources and
    vegetation occurrence at large scale. It provides a methodology to evaluate the long-term
    vegetation change and the water resources impact using remote sensing data in water-limited
    areas. The approach of vegetation dynamics, runoff and groundwater impacts presented in this
    thesis serves as a sound foundation for predicting the effects of future environmental changes.
    Science, technology and agency in the development of droughtprone areas: a cognitive history of drought and scarcity
    Vincent, L.F. - \ 2004
    The Open University. Promotor(en): D.V. Wield. - - 396
    droogte - aride klimaatzones - droog klimaat - woestijnvorming - waterbeheer - watervoorraden - sociaal milieu - sociale instellingen - politiek - schaarste - sociale factoren - agentschappen - bureaucratie - drought - arid zones - arid climate - desertification - water management - water resources - social environment - social institutions - politics - scarcity - social factors - agencies - bureaucracy
    The Impact of Climate Change on Drylands : With a Focus on West Africa
    Dietz, A.J. ; Ruben, R. ; Verhagen, A. - \ 2004
    Dordrecht/Boston : Kluwer Academic Publishers (Environment & policy vol. 39) - ISBN 9781402019524 - 469
    droge gronden - droge omstandigheden - klimaatverandering - milieueffect - west-afrika - aride klimaatzones - arid soils - dry conditions - climatic change - environmental impact - west africa - arid zones
    Sahelian West Africa has recovered from the disastrous droughts of the 1970s and 1980s. People have learned to adapt to risk and uncertainty in fragile dryland environments. They, as well as global change scientists, are worried about the impact of climate change on these West African drylands. What do the experiences of the last thirty years say about the preparedness for higher temperatures, lower rainfall, and even more variability? Detailed studies on Dryland West Africa as a whole, and on Burkina Faso, Mali and Northern Ghana in particular show an advanced coping behaviour and increased adaptation, but also major differences in vulnerability and coping potential. Climate change preparedness programmes have only just started and require more robust support, and more specific social targeting, for a population which is rapidly growing, even more rapidly urbanising, and further integrating in a globalised economy. This book is the first of its kind with a comprehensive analysis of climate change experiences in West African drylands, with attention for pathways of change and the diversity of adaptation options available.
    Using the depleted fraction to manage the groundwater table in irrigated areas
    Bos, M.G. ; Bastiaanssen, W.G.M. - \ 2003
    irrigatie - bodemwater - grondwaterspiegel - drainage - evapotranspiratie - modellen - aride klimaatzones - irrigation - soil water - water table - evapotranspiration - drainage - models - arid zones
    The depleted fraction, defined as the ratio of ETactual over (P + Vc ), relates parameters of the water balance of an irrigated area with each other in such a way that the (water) manager obtains information on the rate of change of water stored in the area (soil moisture and groundwater). If the depleted fraction equals about 0.6 water storage in the area is stable, while water is stored for lower values of the depleted fraction. If the value of the depleted fraction exceeds 0.6, the volume of water stored in the area decreases. Part of this decrease is due to natural drainage and part due to capillary rise into the root zone of the irrigated crop. Despite this capillary rise the actual evapo-transpiration drops below the potential ET-value. For most crops, a decrease of ET by about 15% would result to a higher productivity in terms of yield per cubic meter water. However, the yield per hectare (and thus farm income) would decrease. Management of an irrigation system is recommended in such a way that the monthly values of the depleted fraction range between 0.5 and 0.8. Such a management rule would provide sufficient water for leaching (at the 0.5 side of the range) and provide high crop yield per unit water consumed (at the 0.8 side).
    A tool for decision-making on water harvesting techniques in arid zones
    Graaff, J. de; Sghaier, M. ; Ouessar, M. ; Gabriels, D. - \ 2002
    In: Water harvesting in Mediterranean zones : an impact assessment and economic evaluation : proceedings from EU Wahia project final seminar in Lanzarote / de Graaff, J., Ouessar, M., - p. 125 - 138.
    regenwateropvang - economische evaluatie - watervoorziening - landgebruik - besluitvorming - aride klimaatzones - water harvesting - water supply - land use - decision making - economic evaluation - arid zones
    ILRI workshop: water and food security in (semi-)arid areas : proceedings of the [second] Wageningen water workshop 1998, [2-4 November]
    Schrevel, A. - \ 1999
    Wageningen : ILRI (Special report / ILRI ) - 182
    water management - irrigation - food supply - arid zones - semiarid zones - waterbeheer - irrigatie - voedselvoorziening - aride klimaatzones - semi-aride klimaatzones
    Catastrophic vegetation dynamics and soil degradation in semi-arid grazing systems
    Rietkerk, M. - \ 1998
    Agricultural University. Promotor(en): L. Stroosnijder; H.H.T. Prins. - S.l. : s.n. - ISBN 9789054859413 - 155
    beweidingssystemen - aride klimaatzones - bodemdegradatie - woestijnvorming - vegetatie - bodemwater - sahel - tanzania - burkina faso - grazing systems - arid zones - soil degradation - desertification - vegetation - soil water - sahel - tanzania - burkina faso - cum laude

    When vegetation is drastically reduced as a result of drought or an increase in herbivore numbers, it does not simply recover if periods with normal rainfall follow or if herbivores are removed. These are commonly recognized catastrophic phenomena of semi-arid grazing systems in general and of the African Sahel in particular. The main aims of this thesis are to provide an effective explanation of the catastrophic properties of vegetation dynamics in these systems and to predict under which conditions they might be expected.

    We start with a description of Sahelian rangeland vegetation dynamics, to reveal its catastrophic properties. This exercise appeared a very useful first step in the growth of our ideas about catastrophic vegetation dynamics because: 1) it translated rather vague concepts into a verifiable format by deducing hypotheses about the conditions under which catastrophic vegetation dynamics might be expected, and 2) it generated the notion that soil degradation could somehow be an important factor attributing to catastrophic vegetation dynamics in semi-arid grazing systems. This is in contrast with models that emphasize herbivore feeding characteristics or plant competition as possible mechanisms underlying catastrophic vegetation dynamics. We tested the hypothesis that soil degradation, i.e. soil erosion by run-off and wind and the consequent loss of water and nutrients, is sufficient to explain catastrophic vegetation dynamics by mathematical modelling.

    Our model studies indeed show that soil degradation can effectively explain the catastrophic properties of semi-arid grazing systems. Soil degradation can cause a positive feedback between reduced resource (soil water and nutrients) availability and reduced vegetation biomass which may lead to collapse of the system. This positive feedback loop can be triggered by grazing. We argue on the basis of a large body of literature that this is an important mechanism causing catastrophic vegetation dynamics in semi-arid grazing systems. Furthermore, our model studies predict for which site-specific properties catastrophic vegetation dynamics may be expected, that is on loamy or clayey soils in case of water-limited vegetation biomass production, and on sandy soils in case of nutrient-limited biomass production. This is because sandy soils have higher water infiltration rates but are more vulnerable to nutrient loss through erosion than loamy or clayey soils.

    Based on our models, we hypothesized that the removal of aboveground herbaceous biomass would lead to a reduced soil water content and biomass production because of reduced water infiltration and increased run-off. We tested this hypothesis in a semi-arid savanna in Tanzania (East Africa). Indeed, as a consequence of biomass removal, a reduction in soil water content and biomass production occurred. But it appeared that increased loss of soil water through increased soil evaporation as a consequence of litter removal ultimately outbalanced all other effects on soil water content. Several factors might have contributed to the importance of increased soil evaporation, overriding that of reduced water infiltration and increased run-off. The soil in the research area was a sandy loam, with higher water infiltration rates than soils with a lower percentage sand and higher perentage clay, while rainfall primarily occurred in light showers. Thus, under these conditions, when the positive feedback between reduced water infiltration and reduced biomass does not operate, another positive feedback that is between increased soil evaporation and reduced biomass may become prominent.

    We further hypothesized that at a certain range of herbivore impact small initial differences in plant cover and amount of soil resources can magnify to alternative states which persist in time due to positive plant-soil feedbacks. We tested this hypothesis in a semi-arid grazing system in Burkina Faso (West Africa), where we studied vegetation patchiness along a gradient of herbivore impact. Indeed, the occurrence and likely persistence of a spatial pattern of vegetated patches alternating with bare soil at a certain range of herbivore impact could be explained by the positive plant-soil feedback between vegetation biomass and water infiltration.

    We stress the general applicability of our models by comparing catastrophic vegetation dynamics of the semi-arid grasslands of the African Sahel with the arctic salt marshes along the Hudson Bay in Canada. We argue that in both systems, an increase of herbivory triggered a catastrophic vegetation shift, which was ultimately caused by a positive plant-soil feedback, leading to desertification.

    One of our model assumptions was that herbivore density is not regulated by vegetation biomass. In the general discussion, I investigated the influence of a positive feedback between vegetation biomass and water infiltration on the dynamics of a plant-herbivore system, where herbivore density depends on vegetation biomass. As a consequence of the positive feedback and if herbivore reproduction is efficient, I predict that the plant-herbivore system could destabilize and collapse. In this chapter I also stress the practical relevance of our studies as our approach may finally lead to objective ecological criteria on which pastoral managers can base their decision how to evade the hazard of degradation of their rangelands.

    I highlight three topics which deserve more priority on the reseach agenda concerning semi-arid grazing systems in the near future. Hereby, I want to stress that it is important to put experimental and empirical studies into a clear theoretical framework, whereby mathematical modelling should play an important role. The three topics are:

    1. spatial heterogeneity and vegetation pattern formation,

    2. facilitation and competition between functional plant groups within the herbaceous layer and

    3. the effects of positive plant-soil feedbacks on herbivore dynamics.
    Influence of grazing regimes on cattle nutrition and performance and vegetation dynamics in Sahelian rangelands
    Ayantunde, A.A. - \ 1998
    Agricultural University. Promotor(en): H. van Keulen; H.M.J. Udo; S. Fernandez-Rivera. - S.l. : Ayantunde - ISBN 9789054859895 - 179
    rundvee - beweidingssystemen - extensieve weiden - agropastorale systemen - diervoeding - voeropname - prestatieniveau - dierlijke productie - veldgewassen - mest - vegetatie - sahel - aride klimaatzones - cattle - grazing systems - rangelands - agropastoral systems - animal nutrition - feed intake - performance - animal production - field crops - manures - vegetation - sahel - arid zones

    In the West African Sahel, common herd management practices such as night grazing and corralling influence time available for grazing. When animals are used to deposit manure in the cropping fields, conflicts often arise between the need for animals to graze long enough for adequate feed intake, especially in the dry season, and the need to collect manure.

    Grazing trials were carried out in Sadoré (13 o14'N and 2 o16'E) and Toukounous (14 o30'N and 3 o17'E), Niger, to determine the effects of timing (day or day-and-night) and duration of grazing on cattle nutrition and performance, and to quantify the short-term effects of grazing by cattle on vegetation dynamics in Sahelian rangelands. In addition, a survey was conducted among livestock herders in two villages of Niger, Kodey and Toukounous, on their perceptions of night grazing with the aim of identifying constraints to the practice of night grazing and opportunities to apply relevant experimental results in the management of herds in the region.

    There were no differences in the quality of the diet selected during the day and at night, but the quality of the available and ingested forage declined as the season progressed from wet to dry. During the dry season, there was a trend for day-and-night grazing cattle to be more selective during the day, than animals that grazed only during the day. Animals that had additional grazing time in the night consistently had higher forage intake and consequently, higher average daily gain than those that grazed only during the day in all seasons.

    However, additional grazing at night reduced the amount of manure that could be collected for crop fields. When animals are supplemented, night grazing appears less relevant as the length of night grazing time did not significantly affect average daily gain in the critical late dry season. Annual herbage production of four paddocks used in Toukounous was 1893 kg DM ha -1. Of this amount, consumption by cattle accounted for 48% on a year-round basis. The quality of the diet selected by the animals was consistently higher than that of the herbage grazed in all seasons. These results indicate that grazing ruminants tend to make better use of Sahelian rangelands than often predicted on the basis of pasture evaluation alone.

    The response of herders interviewed on their perceptions of night grazing indicates that ethnic group and herd size are critical characteristics for the decision on the practice of night grazing. Herders' perceptions of night grazing with respect to animal production parameters such as weight development, water consumption, faecal output and feeding behaviour are consistent with available experimental results. Therefore, the herders' current knowledge and herd management strategies need to be considered in the development of any animal or ecological innovation.

    Runoff and sediment transport in the arid regions of Argentina and India - a case study in comparative hydrology
    Sharma, K.D. ; Menenti, M. ; Huygen, J. ; Fernandez, P.C. ; Vich, A. - \ 1996
    Annals of Arid Zone 35 (1996)1. - ISSN 0570-1791 - p. 17 - 28.
    argentinië - woestijnen - economische impact - economie - india - regen - relaties - oppervlakkige afvoer - gebruikswaarde - aride klimaatzones - argentina - deserts - economic impact - economics - india - rain - relationships - runoff - use value - arid zones
    The arid zones of Argentina and India have been compared. In both regions run-off is often generated by the Hortonian infiltration surplus overland flow, and run-off response to precipitation input tends to be rapid. The sediment transport is governedby the transport capacity of run-off rather than by the availability of erodible material. The magnitude of hydrological processes is different in response to the different rainfall regimes.
    Modelling spatial sediment delivery in an arid region using Thematic Mapper data and GIS
    Sharma, K.D. ; Menenti, M. ; Huygen, J. ; Vich, A. - \ 1996
    Transactions of the ASAE 39 (1996)2. - ISSN 0001-2351 - p. 551 - 557.
    toepassingen - woestijnen - geografische informatiesystemen - modellen - remote sensing - onderzoek - watererosie - aride klimaatzones - applications - deserts - geographical information systems - models - remote sensing - research - water erosion - arid zones
    A distributed-parameter sediment delivery model is linked with a personal computer-based, low-cost geographical information system to facilitate preparation, examination, and analysis of spatially distributed input parameters and to link the sediment delivery from a micro-scale to the watershed scale. The model accurately predicts, even for the same values of flow shear stress, the higher sediment delivery from bare surfaces as compared to surfaces protected by vegetation cover. This study enables the identification of vulnerable regions within a watershed, thus facilitating improvements in the planning of soil conservation systems.
    An intercomparison of techniques to determine the area-averaged latent heat flux from individual in situ observations: a remote sensing approach using the European Field Experiment in a Desertification-Threatened Area data
    Pelgrum, H. ; Bastiaanssen, W.G.M. - \ 1996
    Water Resources Research 32 (1996)9. - ISSN 0043-1397 - p. 2775 - 2786.
    toepassingen - woestijnen - interacties - microklimaat - remote sensing - bodemtemperatuur - thermische bodemeigenschappen - thermische geleiding - vegetatie - aride klimaatzones - applications - deserts - interactions - microclimate - remote sensing - soil temperature - soil thermal properties - thermal conductivity - vegetation - arid zones
    Different procedures to obtain the area-averaged latent heat flux as a weighted average of ground-based observations of latent heat flux are described. Weighting coefficients are obtained from remote sensing data. A newly developed remote sensing algorithm, SEBAL, which solves the energy budget on a pixel-by-pixel basis, was successfully applied with EFEDA data. Two other methods for retrieving weighting coefficients were tested against SEBAL. The second method combines satellite images of surface temperature, surface albedo and Normalized Difference Vegetation Index (NDVI) into an index on a pixel-by-pixel basis. The third method uses a supervised classification.
    A methodology for the assessment of surface resistance and soil water storage variability a mesoscale based on remote sensing measurements. A case study with HAPEX-EFEDA data.
    Bastiaanssen, W.G.M. ; Hoekman, D.H. ; Roebeling, R.A. - \ 1994
    Wallingford : IAHS - ISBN 9780947571931 - 66
    toepassingen - woestijnen - evaporatie - land - remote sensing - bodem - bodemwatergehalte - oppervlakten - aride klimaatzones - applications - deserts - evaporation - land - remote sensing - soil - soil water content - surfaces - arid zones
    Groundwater monitoring in (semi-) arid regions. A general outline.
    Lanen, H.A.J. van - \ 1994
    In: International workshop on groundwater monitoring and recharge in semi - arid areas, Hyderabad, India, January 18-20, 1994 - p. SI1 - SI13.
    woestijnen - grondwater - hydrologie - instrumenten (meters) - meting - monitoring - pampa's - semi-aride klimaatzones - steppen - waterbeheer - waterkwaliteit - aride klimaatzones - netwerken - deserts - groundwater - hydrology - instruments - measurement - pampas - semiarid zones - steppes - water management - water quality - arid zones - networks
    Spectral reflectance in the Tunesian desert
    Epema, G.F. - \ 1992
    Agricultural University. Promotor(en): S.B. Kroonenberg; M. Molenaar. - S.l. : Epema - 150
    remote sensing - toepassingen - absorptie - reflectie - ruimtelijke ordening - landgebruik - zonering - woestijnen - tunesië - aride klimaatzones - remote sensing - applications - absorption - reflection - physical planning - land use - zoning - deserts - tunisia - arid zones


    Satellites provide the possibility to give a synoptical view of the earth surface at regular time intervals. Satellites operating in the optical wavelengths have however as disadvantage that monitoring of the surface characteristics becomes impossible as soon as clouds are present. Deserts and desert margins are for that reason much more appropriate for monitoring by optical satellites than temperate and wet tropical areas. Potential hazards, possibilities and often inaccessibility makes use of optical remote sensing very reasonable.

    Landsat Thematic Mapper (TM) satellites provide a much better spectral resolution than other satellites at reasonable spatial and temporal resolution. Ile presented research was hence focused on the spectral possibilities of Landsat Thematic Mapper for determining surface characteristics and their dynamics in desert areas. Field measurements of reflectance in different seasons were performed to evaluate the effect of different factors and their dynamics on reflectance. A field radiometer (MMR) with TM compatible bands was used. The study was performed in southern Tunisia in an area with large variation: footslopes, dunes and dynamical salt plains, all with few or absent vegetation cover. Dominant mineralogy is representative for many and areas and comprises gypsum, carbonate, quartz and halite.

    In order to compare results of field reflectance measurements with Landsat Thematic Mapper data, adequate processing of both data sets is necessary.
    General accepted assumptions that reference plates are ideal reflectors have to be rejected. Both wavelength and insolation angle dependant reflectance of the panel has to be determined. For large solar zenith angles also corrections have to be made for influence of diffuse irradiation (Chapter 2).

    Use of calibration coefficients given for Landsat Thematic Mapper data processed in Fucino will cause large errors in calculating reflectance data. It turned out that these coefficients were not updated for deterioration of the sensors during the flight and that calculations were made based on two different definitions of bandwidth. In order to achieve adequate values of reflectance these errors were evaluated and corrected (Chapter 8).

    The evaluation of factors affecting field reflectance can be separated in external and internal ones.
    The influence of external factors, solar zenith angle and atmosphere was evaluated (Chapter 3-4). Solar zenith angle dependant reflectance turned out to be limited for this area, if measurements are performed with solar zenith angles up to 65 degrees. In spring on the footslopes reflectance values at noon were about 10% higher than those at 65 degrees. Differences on the playa were even less. Both based on accuracy and applicability of the results (observations of this area at these latitudes with Landsat TM take place with angles ranging between 28 in June and 63 degrees in December), 65 degrees can be considered as a useful limit for performing adequate measurements.

    Influence of atmosphere on field reflectance is limited. The evaluation of the external factors leads to the conclusion that all measurements with a solar zenith angle less than 65 degrees and on clear days could be used for obtaining a field data reference set.

    Field measurements showed that Landsat TM-like bands are very useful in detecting the surface characteristics ( internal factors ) in this area (Chapters 5 - 7).
    Gypsum has absorption bands in both middle infrared bands (comparable with TM bands 6 and 7) and carbonate in TM band 7. Since on footslopes and in dunes quartz is the other important mineral, a high reflectance in these bands points to presence of quartz. Standard field reflectance measurements showed that on footslopes gypsum, quartz and carbonate dominated areas have a different spectral signature. Differences in eolian deposits are even more clear: relations between gypsum content and indices derived from spectral reflectance could be established under field conditions.

    Presence of halite on the playas could be detected by relatively high reflectance in the visible part of the spectrum, especially in the blue band. Field reflectance on plots showed that moisture content induces a relatively low reflectance in all bands with an extra low reflectance in the middle infrared bands. Linear relations between volumetric moisture content and reflectance in individual bands in near and middle infrared turned out to be feasible.

    Most dynamical parts of the area turned out to be the playas, where after storms moisture contents of the top layer were affected for a long time and halite efflorescences occurred shortly after the storms. Although dust slowly covering the surface, even in spring plots with higher halite content could be derived from the spectral signature. Field plots in dune parts showed a large variation in reflectance between November and May too due to changes by wind. Dynamics on footslopes were much less important than in other areas. Variation in vegetation appears to be relatively small, while also effect of storms was not visible for more than a few days after a storm.

    Results of field reflectance were extrapolated to Landsat TM satellite data (Chapter 9). It was possible to derive directly from Landsat TM data a number of useful classes for playas, footslopes and eolian material, having variation in surface mineralogy (gypsum, carbonate, quartz, halite) and variation in surface type. Also dynamics of factors like moisture and halite could be derived using multitemporal Landsat TM data.

    The presented methodology, implementing an extensive field reflectance measurement campaign, gives insight in possibilities of Landsat TM under a range of conditions. It corroborates that for operational application in and areas Landsat TM data will be a useful source of information in addition to other types of remote sensing as for instance aerial photography.

    Bio-economic capability of West-African drylands
    Wolf, J. ; Breman, H. ; Keulen, H. van - \ 1991
    Wageningen : CABO-DLO (Report / Centre for Agrobiological Research 147) - 83
    teelt - cultuurmethoden - agrarische bedrijfsvoering - landbouw - landbouwproductie - woestijnen - west-afrika - aride klimaatzones - cultivation - cultural methods - farm management - agriculture - agricultural production - deserts - west africa - arid zones
    A new simulation model of bare soil evaporation in arid regions (EVADES)
    Bastiaanssen, W.G.M. ; Kabat, P. ; Menenti, M. - \ 1989
    Wageningen : ICW (ICW note 1938) - 74
    woestijnen - evaporatie - land - modellen - onderzoek - oppervlakten - aride klimaatzones - deserts - evaporation - land - models - research - surfaces - arid zones
    Mapping evaporation from playa's in the Western Desert : approach and practical examples
    Abd El Karim, M.H. - \ 1989
    Wageningen : ICW (ICW nota 1972) - 39
    toepassingen - watervoerende lagen - woestijnen - egypte - evaporatie - hydrologie - remote sensing - aride klimaatzones - applications - aquifers - deserts - egypt - evaporation - hydrology - remote sensing - arid zones
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