Staff Publications

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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    Effect of vegetation cover and transitions on regional wind erosion in drylands
    Youssef, I.F. - \ 2012
    Wageningen University. Promotor(en): Leo Stroosnijder, co-promotor(en): Saskia Visser; D. Karssenberg. - S.l. : s.n. - ISBN 9789461733429 - 132
    vegetatie - winderosie - landdegradatie - modelleren - modellen - meting - droge gebieden - vegetation - wind erosion - land degradation - modeling - models - measurement - arid lands

    Wind erosion is a critical environmental problem that threatens mainly the arid and semi-arid regions of our planet. Usually this problem is associated with desertification, poverty and other environmental and socioeconomic problems. Wind erosion causes the loss of fertile topsoil, and has a negative effect on agricultural production and on human health. When conditions favorable for wind erosion are present, the process can cause large scale environmental disasters like the Dust Bowl in the USA in the 1930s. This event is considered one of the worst environmental disasters of the 20th century, and was caused by a reduction in vegetation cover due to a change in land use combined with an increased dryness in the region. Wind erosion involves the detachment, transport and deposition of soil particles. Depending on their size, particles can move in three different types of transport: creep, saltation and suspension. Vegetation is one of the key factors in the protection of the soil against erosive winds. Although research on wind erosion has started a few decades ago there is still a big gap between the available knowledge provided by current measurement and modeling tools and the knowledge which is required by policy makers and land managers. This thesis focuses on improving the knowledge of the effects of vegetation cover and land use on regional scale wind erosion. The thesis covers improvement of wind erosion measurement techniques (Chapters 2, 3 and 5) and wind erosion modeling on the regional scale

    (Chapter 4 and 6).

    In Chapter 2, the efficiencies of the Vaseline Slide (VS) and Modified Wilson and Cooke (MWAC) catchers were determined with different sand particle sizes (<50, <75, 50–75, 200–400, and 400– 500 μm) at a fixed wind speed (13.3 m s–1) and with different soil textures at different wind velocities (10.3, 12.3, and 14.3 m s–1). The study showed that whereas the VS trap is better for catching fine particles, the MWAC trap is better for trapping coarse particles. In the experiments with different soil textures, the efficiency of each catcher considerably changed with the with wind speed. This also varies importantly between catchers: for instance, for sand the MWAC efficiency was relatively high, whereas the efficiency of VS catcher was relatively low. Results concluded that the efficiency of each catcher varies critically with particle size, soil texture and wind speed. Equipment or measurement techniques for the observation of saltation at the regional scale does not exist although these are essential for improving the understanding of wind erosion problem at that scale. In Chapter 3, the portable plot method for measuring regional scale wind erosion with a specific focus on the saltation process was developed. With this strategy the number of measurement locations is increased with limited budget and time. The portable plot method was applied at agricultural stability zones 4 and 5 in the Khanasser Valley in Syria in 2009 and 2010. During the measurement period, a meteorological station was installed at each plot together with MWAC sediment catchers. Results showed that, with this method, information on the effect of wind regime on the aeolian mass transport for different land uses in the region can be obtained. Also, insights into the interrelation between neighboring land units can be gained and the data for scaling-up a field scale model to the regional scale are obtainable. We concluded that this method provides insight into the wind erosion at regional scale and data collected through it are important for progressing the modeling of wind erosion at a regional scale. The new measurement method enabled the calibration and validation of the field scale model of RWEQ (Revised Wind Erosion Equation) at several land use areas in the Khanasser valley in Syria (Chapter 4). In this chapter, the RWEQ model was modified to estimate mass flux and soil loss at a field scale for different types of land use. We implemented this modified version of RWEQ that represents wind erosion as a transient process, using time steps of 6 hours. Beside this, a number of adaptations including the estimation of mass flux over the field boundaries and the routing of sediment have been added. The results showed that this modified version of RWEQ provided acceptable predictions for the average mass flux from our measurement plots compared with the results of previous tests of the model. While the portable plot method provided insights on the effect of land use and climate on the quantity and intensity of wind erosion in a region, more knowledge was required on the border effect between different land uses as the portable plot method provided only limited knowledge on the effect of vegetation pattern and border effect. To get sufficient knowledge on this subject a simulation of sediment transport in a regional scale environment was designed and tested in wind-tunnel experiments (Chapter 5). This simulation showed the effect of vegetation pattern on sediment transport within a land unit and at the border between land units. Wind tunnel experiments were conducted with artificial shrubs representing Atriplex halimus, a native shrub in Khanassar valley. In the experiments, a wind speed of 11 m s-1 was applied and after each 200-230 second wind run the sediment redistribution was measured using a graph paper. Results showed that: 1) the transport within a land unit is affected by the vegetation density and pattern for the land unit itself and for the neighboring units; 2) plans for re-vegetation of degraded land need to take into account the ‘streets’ effect; 3) the effect of neighboring land units includes a sheltering effect and the regulation of sediment passing from one land unit to the neighboring land units and 4) revegetation projects in regions vulnerable to wind erosion not only need to investigate the effect of vegetation pattern on erosion and deposition within the region in general, but also should consider the redistribution of sediment at smaller scales. In Chapter 6, The Regional Scale Wind Erosion Equation (RS-WEQ) was developed. This model takes the different land uses in a region into account, considers the interrelation between neighboring land units and considers saltation as the main transport mode. Although the RWEQ was the starting point for the development of RS-WEQ, the new model is not restricted in its application to a single field. RS-WEQ predicts mass flux, soil loss and deposition for all land uses in a region. The model considers the erodibility parameters for each land use independently and takes the effect of the borders into account. Its output provides a clear insight on wind erosion processes at the regional scale. RS-WEQ was run using scenarios that represent main land uses in dry regions in general and in Khanasser valley specifically. The model outputs showed that the wind speed, field length and land use patterns affect the quantity and severity of mass flux, soil loss and deposition in a region. Specifically, the results showed that the mean mass flux and its related abrasion risk increased with the increase in wind speed and field length. The model provides further details on the effect of field length and land use patterns on the severity of soil loss and deposition at the regional scale. Therefore the developed model can be considered as a useful tool for land managers and policy makers in regions that are vulnerable to wind erosion. This thesis showed that for comprehensive understanding of the aeolian sediment transport at regional scale a combination of measuring and modeling of wind-blown sediment transport is required. And the intensively calibrated and validated wind erosion models can be used in the framework of wind erosion mitigation.

    Desire for greener land : options for sustainable land management in drylands
    Schwilch, G. ; Hessel, R. ; Verzandvoort, S.J.E. - \ 2012
    Bern [etc.] : University of Bern [etc.] - ISBN 9789461733290 - 282
    droge gebieden - grondbeheer - duurzaamheid (sustainability) - duurzaam bodemgebruik - woestijnvorming - bodembescherming - waterbeheer - teeltsystemen - begrazingsbeheer - bosbedrijfsvoering - arid lands - land management - sustainability - sustainable land use - desertification - soil conservation - water management - cropping systems - grazing management - forest management
    Desire for Greener Land compiles options for Sustainable Land Management (SLM) in drylands. It is a result of the integrated research project DESIRE (Desertification Mitigation and Remediation of Land - A Global Approach for Local Solutions). Lasting five years (2007–2012) and funded within the EU’s Sixth Framework Programme, DESIRE brought together the expertise of 26 international research institutes and non-governmental organisations. The DESIRE project aimed to establish promising alternative land use and management strategies in 17 degradation and desertification sites around the world, relying on close collaboration between scientists and local stakeholder groups. The study sites provided a global laboratory in which researchers could apply, test, and identify new and innovative approaches to combatting desertification. The resulting SLM strategies are local- to regional-scale interventions designed to increase productivity, preserve natural resource bases, and improve people’s livelihoods. These were documented and mapped using the internationally recognised WOCAT (World Overview of Conservation Approaches and Technologies) methodological framework, which formed an integral part of the DESIRE project. The DESIRE approach offers an integrated multidisciplinary way of working together from the beginning to the end of a project; it enables scientists, local stakeholders and policy makers to jointly find solutions to desertification. This book describes the DESIRE approach and WOCAT methodology for a range of audiences, from local agricultural advisors to scientists and policymakers. Links are provided to manuals and online materials, enabling application of the various tools and methods in similar projects. The book also includes an analysis of the current context of degradation and SLM in the study sites, in addition to analysis of the SLM technologies and approaches trialled in the DESIRE project. Thirty SLM technologies, eight SLM approaches, and several degradation and SLM maps from all the DESIRE study sites are compiled in a concise and well-illustrated format, following the style of this volume’s forerunner where the land is greener (WOCAT 2007). Finally, conclusions and policy points are presented on behalf of decision makers, the private sector, civil society, donors, and the research community. These are intended to support people’s efforts to invest wisely in the sustainable management of land – enabling greener drylands to become a reality, not just a desire.
    Arid landscape dynamics along a precipitation gradient: addressing vegetation - landscape structure - resource interactions at different time scales
    Buis, E. - \ 2008
    Wageningen University. Promotor(en): Tom Veldkamp; N. van Breemen, co-promotor(en): B. Boeken. - S.l. : S.n. - ISBN 9789085049463 - 190
    droge gebieden - landschap - bodemmorfologie - neerslag - gradiënten - dynamica - watervoorraden - israël - landschapsanalyse - bodem-landschap relaties - arid lands - landscape - soil morphology - precipitation - gradients - dynamics - water resources - israel - landscape analysis - soil-landscape relationships
    This research is entitled ‘Arid landscape dynamics along a precipitation gradient: addressing
    vegetation – landscape structure – resource interactions at different time scales’ with as subtitle
    ‘A case study for the Northern Negev Desert of Israel’. Landscape dynamics describes the
    interactions and feedbacks among landscape structure, resource flows and organisms. This study
    focuses on the Northern Negev Desert of Israel, a semi-arid to arid rock desert with local loess
    and sand cover. Climate and humans are important driving factors of landscape dynamics here.
    Semi-arid and arid regions worldwide, are vulnerable to land degradation and desertification. A
    profound knowledge of the processes in these regions can help to avert land degradation and
    desertification. The objective of this thesis is to increase the knowledge on landscape dynamics
    and its drivers in semi-arid and arid regions by field and model studies in the Northern Negev
    Desert. This study can contribute to a sustainable future for the inhabitants of these areas.
    • Chapter 1 is the introduction of this thesis and discusses among others the four studied
    catchments along a precipitation gradient: Lehavim receives at average 280 mm precipitation per
    year, Sayeret Shaked 200 mm yr-1, Halluqim 93 mm yr-1 and Avdat 87 mm yr-1. Of the surface of
    Lehavim 53% is covered by vegetation and 15% by bedrock outcrops. The catchment is
    intensively grazed by livestock. Sayeret Shaked is covered by a thick layer of homogeneous
    loess. Vegetation cover is dense (62%). The catchment is taken out of grazing since 1987. In
    Halluqim only 20% of the surface is covered by vegetation. The catchment is very rocky, as
    bedrock crops out at 42% of the surface. Avdat, located close by, is much less rocky (7%). Here
    22% of the surface is covered by vegetation. Both catchments are extensively grazed.
    The thesis can be separated in three parts. In the first part the relationships between landscape
    structure and vegetation in the four catchments is studied by statistical analyses. This part gives
    insight in the landscape dynamics along a precipitation gradient and provides a system
    framework for the remainder of the thesis. The second part focuses on simulating water and
    sediment dynamics in the catchments using the landscape evolution model LAPSUS. The model
    is adapted to a semi-arid and arid climate, and vegetation cover is incorporated. The interactions
    between resource flows and vegetation is studied by model simulations. In the third part the
    system knowledge and modelling framework are applied at a longer time scale. Firstly the history
    of a valley fill is reconstructed by field observations, after which this valley fill is simulated with
    LAPSUS. Additionally the effect of land use on the valley fill development is tested by model
    Part 1: System framework
    • In chapter 2 the controls on functional surface cover types are studied in the four catchments
    along the precipitation gradient. First, four functional surface cover types are selected, based on
    their unique functionality in terms of water use and redistribution: shrubs, Asphodelus ramosus,
    other herbaceous plants and surface crusts (biological and physical). Percentage of surface cover
    of these functional surface cover types is estimated, and of bedrock outcrops and loose surface
    stones. Additionally, data is collected on soil depth, relative elevation, insolation, slope, profile
    curvature and plan curvature. Relations between functional surface cover types and landscape
    structure variables are analyzed with descriptive statistics, factor analyses and linear regressions.
    The landscape structure variables bedrock outcrop, relative elevation, soil depth and surface
    stones explain most of the cover variance in the catchments. In catchments with many bedrock
    outcrops, the occurrence of functional surface cover types is best explained by the landscape
    structure variables. In catchments with homogeneous soils reaching beyond the root zone,
    biological interactions between functional surface cover types are more important. Along the
    precipitation gradient the explanatory power of the biological variables decreases with decreasing
    precipitation, while the explanatory power of landscape structure variables appears unrelated.
    Only in homogeneous semi-arid catchments can regular vegetation patterns develop, in arid and
    heterogeneous catchments irregular vegetation patterns dominate.
    Part 2: Model framework
    • In chapter 3 the process of water redistribution at catchment scale is studied with the landscape
    evolution and erosion model LAPSUS. LAPSUS, formerly applied in Mediterranean regions, is
    modified to deal with the arid climate of the Northern Negev Desert of Israel. Daily model runs
    are used instead of yearly model runs, and the infiltration module is adapted to better represent
    the spatial diversity in water availability in an arid catchment. The model is calibrated for
    Halluqim and Avdat. First, a sensitivity analysis of the modified LAPSUS was done. Especially
    pore volume of the soil appears to have a strong influence on the modelling results. Second, the
    capability of LAPSUS to deal with varying surface characteristics was assessed by comparing the
    simulated water redistribution patterns in the two catchments with field data. Simulation results
    demonstrate that the catchments respond very different to precipitation. Water redistribution is
    larger in the dominantly bedrock-covered Halluqim compared to the dominantly sedimentcovered
    catchment of Avdat. Consequently, Halluqim has more positions with water
    accumulation than Avdat, and can sustain a larger vegetation cover including Mediterranean
    species. Finally the modelled infiltration patterns are spatially compared with vegetation cover in
    the catchments. The results indicate that there is a broad agreement between infiltration and
    vegetation patterns, but locally there is a strong mismatch indicating that part of the involved
    processes are still missing in the model.
    • In chapter 4 the interactions between resource flows and vegetation is studied and simulated in
    the loess-covered catchment of Sayeret Shaked. In semi-arid areas vegetation is scarce and occurs
    often as individual shrubs on raised mounds. The formation process of these mounds is still
    debated. In this chapter the hypothesis that shrub mounds are formed in part of the Northern
    Negev Desert by erosion and sedimentation is tested. Height and diameter of shrub canopy and
    shrub mounds are measured and micro-morphological techniques are used to reconstruct the
    formation process of the shrub mounds. The results suggests that shrub mounds are formed by
    accumulation of atmospheric dust and sedimentation of eroded material in the vicinity of the
    shrub, as well as by erosion of the surrounding crust. Model simulations are done for single
    events and longer time scale (100 years). In the simulations, mound formation appears most
    prominent at low shrub density and large shrub canopy diameter. Positive and negative feedbacks
    between shrubs and resource redistribution results in a meta-stable landscape. Long-term model
    simulations of the current climate indicates that initially formation rate of shrub mounds is high,
    but stabilized at lower rates. In dryer and wetter climates mound formation is unlikely to happen,
    as respectively too little or too many resources are redistributed, causing a stable or highly
    erosive landscape. Mound simulation with LAPSUS is successful and simulated shrub mounds
    resemble the actual shrub mounds in Sayeret Shaked. Consequently the model may prove to be
    valuable for the modelling of ecohydrological landscape processes in semi-arid areas.
    Part 3: Long-term application
    • In chapter 5 the interactions between climate change, human occupation and semi-arid
    landscape dynamics are studied to increase the insight in the effect of climate change and human
    land use. A Late Quaternary valley fill in the catchment of Sayeret Shaked is studied. The
    aggradation and incision history is reconstructed based on a transect study. The reconstructed
    valley fill is put in a temporal framework by correlation with local climate records and optically
    simulated luminescence and potsherd dates. Two Late Pleistocene and four Holocene aggradation
    and incision cycles are recognized, of which three in the last 2000 years. Contradictory to the
    expected positive relation between amplitude of climate fluctuations and cycles of aggradation
    and incision, the Late Holocene cycles are stronger than those in the Late Pleistocene and Early
    to Middle Holocene. The most significant cycle coincides with the rise and fall of the Byzantine
    Empire and appears related to the higher pressure on the landscape due to human occupation
    during that time. Human activity appears to have a strongly amplifying effect on aggradation and
    incision phases, which are initially triggered by climate fluctuations. This amplifying effect
    occurs only when human occupation crosses a threshold and triggers destabilization of the
    landscape. It causes collapse of the ecosystem and increases sediment redistribution.
    • Chapter 6 aims to quantify the effect of humans on semi-arid catchments, by reconstructing the
    infill history of Sayeret Shaked using LAPSUS. First, the infill history of Sayeret Shaked
    between about 800 BC and 800 AD is simulated. Second, three land use scenarios are tested to
    quantify the effect of extensive grazing, intensive grazing and intensive grazing combined with
    rainfed agriculture. Especially intensive grazing combined with rainfed agriculture leads to strong
    landscape dynamics. Extensive grazing causes almost no landscape dynamics, resulting in an
    almost stable landscape. The results seem to indicate that this catchment is formed by coevolution
    of human and natural induced processes. Rainfed agriculture leads to valley
    aggradation by tillage translocation, whereas intensive livestock grazing causes gully incision by
    increased slope runoff. Humans appear to be the main driven factor of landscape dynamics in this
    semi-arid catchment, much more than climate fluctuations. Only a short time period of strong
    human land use can irreversibly alter the development trajectory of a catchment. It is thus of high
    importance to manage the land sustainable, both in the present and future, to avoid further
    degradation of drylands.
    • In chapter 7 the results of the different chapters are combined and the most important
    conclusions discussed. The four catchments display very different landscape dynamics, caused by
    a high variation in climate, land use and landscape structure. In Lehavim and Halluqim the
    landscape dynamics is strongly influenced by the landscape structure, because bedrock outcrops
    regulate positions for vegetation grow and stimulate water redistribution. In Sayeret Shaked water
    redistribution depends mainly on biological surface cover. In Sayeret Shaked interactions
    between shrub and crust patches can, under a more intensive grazing regime, lead to regular
    vegetation patterns. When grazing pressure is released the herbaceous plant coverage recovers, as
    is happening today. Avdat is a divers catchment, with steep rock outcrop, a flat plateau and a
    loess covered wide gully. Though the whole catchment is characterized by a high aridity, each
    zone experiences different landscape dynamics.
    At a larger spatial scale, in the whole Northern Negev Desert, the most relevant interactions and
    feedbacks between landscape structure, resource flows and organisms are related to water
    availability and redistribution as well. Since the Late Holocene, the main driving factor of
    landscape dynamics is human land use, especially tillage and intensive livestock grazing. Climate
    fluctuations seem to have much less influence on the region. The influence of humans, even
    confined in a small period in time, strongly affects landscape development in the whole Northern
    Negev Desert, causing co-evolution and formation of cultural landscapes.
    On dew and micrometeorology in an arid coastal ecosystem
    Heusinkveld, B.G. - \ 2008
    Wageningen University. Promotor(en): Bert Holtslag; A.F.G. Jacobs. - [S.l. : S.n. - ISBN 9789085049326 - 197
    droge gebieden - droge gronden - ecosystemen - dauw - micrometeorologie - arid lands - arid soils - ecosystems - dew - micrometeorology
    This study investigated intriguing aspects of dew within a sandy arid ecosystem situated in the NW Negev desert, Israel. The goal was to quantify dew formation and evaporation processes through sensor design, field measurements and modelling. To do this, two new sensors were developed. The first sensor is an automated microlysimeter for measurements of water content changes within the upper soil layer. This sensor uses a sensitive inexpensive Load Cell to provide daily records of dew input and evaporation. Dew amounts reached almost 0.4 mm day-1 but were found to depend strongly on the soil type. The second sensor developed was a remote optical surface wetness sensor that was designed and tested in the laboratory and in the Negev desert. The novelty of the sensor is that it measures surface wetness directly at the interface of the atmosphere, it is non-destructive and allows for repetitive measurements on the same sample, and it can operate outdoors without being affected by direct sunlight. The sensor was found to be capable of measuring leaf water content and near soil surface water content. Data obtained for an Anabasis articulata bush showed that leaves change their leaf water content during the evening.
    In general, errors in flux measurements can lead to difficulties in closing the energy balance at the surface, especially if turbulent flux or soil heat flux measurements are underestimated. Soil heat flux was assessed during the field experiments using a new approach. By placing the soil heat flux sensor near the surface, depth corrections are no longer needed. It is shown that the large soil heat fluxes in hot desert regions are very important in energy balance studies. The soil heat flux measurements led to a very good closure of the surface energy balance and highlighted the importance for careful soil heat flux measurements.An analysis of dew formation conditions revealed that the actual dew is greater than the calculated potential dew. The dew formation process seems to be enhanced by the extreme dryness of the soil. The dry soil pores reduce vapour pressure and increase the downward vapour flux. This is demonstrated by using a Penman model and incorporating the vapour pressure reduction of the soil surface.
    The dew amount that is available within a soil is limited by the soil physical properties. The term FAD “Free Available Dew” is introduced to define the amount of water available for biological activity. The FAD would be higher for a biological crust with cyanobacteria than for vascular plant roots. The FAD for cyanobacteria is around 20% of total dew. To improve our understanding of the distribution and flow of dew water within a sandy soil covered with a biological crust, a multilayer soil model was constructed. The model was tested with the microlysimeter data and gravimetric soil moisture profile measurements. Diurnal soil moisture variation was found to be confined to the top 15 mm. The FAD is available in the upper half of a 4 mm thick biological crust only and limited to < 25% of the day.
    The possibility of harvesting dew from an artificial flat surface within the interdune was investigated by constructing a dew collector and analyzing the measurements. This resulted in a detailed dew model to simulate dew yield in general. Dew yields of 0.30 mm day-1 are possible in the NW Negev desert at a location 40 km from the coast at the end of the dry season. The artificial flat surface tested here provided 0.10 mm day-1. Modelled yields were found to be larger near the coastline.
    Open path gas analyzers and ultrasonic anemometers have been used to measure water vapour fluxes. Dew can affect the performance of open path gas analyzers and thus affect data quality in an arid environment. A field experiment showed that additional sensor heating can avoid dew problems and yet greatly improve the quality of flux measurements. A detailed energy balance model of the gas analyzer window housings is presented so that existing datasets can be filtered for dew contamination events on the windows. Additional heating does influence the measured concentration but the effect is very minor.

    Land-surface and boundary layer processes in a semi-arid heterogeneous landscape
    Jochum, A.M. - \ 2003
    Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): H.A.R. de Bruin. - Wageningen : S.n. - ISBN 9789058088246 - 155
    atmosfeer - droge gebieden - droog klimaat - woestijnvorming - wiskundige modellen - spanje - aardoppervlak - grenslaag - atmosphere - arid lands - arid climate - desertification - mathematical models - spain - land surface - boundary layer

    The European Field Experiment in a Desertification-threatened Area (EFEDA) provides a comprehensive land-surface dataset for a semiarid Mediterranean environment. It is used here to study heat and moisture transport processes in the atmospheric boundary layer (ABL), to derive grid-scale surface fluxes for numerical weather prediction models, and to evaluate the performance of the High-Resolution Limited-Area Model (HIRLAM).

    Boundary layer budgets were derived for two sub-regions of the EFEDA area with different moisture characteristics. The budget analysis is based on the synergistic combination of the observational dataset and a simple coupled canopy-mixed layer model. The mixed layer (ML) warming is balanced by a combination of the heat flux divergence and the radiative divergence (directly measured by aircraft). The large radiative warming is consistent with the high aerosol load and low visibility observed in the area. The moisture budgets reflect the influence on non-ABL scales on the entrainment moisture flux, which changes sign over time as the ABL grows into the observed heterogeneous moisture structure of the residual layer (RL). Accurate high-resolution vertical humidity profiles are needed to properly estimate the moisture flux divergence, which varies in space and time. The coupled canopy-ML slab model proves to be a valuable tool in this complex environment, if it is regularly provided with updated RL gradients. The potential for moisture flux divergence and associated ML moistening is higher at Barrax, where irrigation enhances the surface evapotranspiration.

    The area-aggregated fluxes (in particular of moisture) depend strongly on the location of the area boundaries, whenever a significant fraction of irrigated land is present. This confirms the importance to adequately account for tiles of irrigated land in surface schemes and the corresponding physiographic databases of large scale models. A simple way to accommodate a minimum information on canopy water status is proposed in terms of the distinction of at least two seasonal classes of irrigated crops, one of spring and one of summer growing cycles.

    The HIRLAM performance evaluation reveals model shortcomings essentially in four areas. The moisture assimilation makes the model surface and ABL too moist. The ABL entrainment description cannot resolve the observed temporal and spatial variations of entrainment moisture fluxes. The landuse and soil classification with its associated physiographic database attribute too much green vegetation to the EFEDA grid cells, thus causing a wet bias in the surface energy balance. The aerosol parameterization in the radiation code does not account for the typically higher aerosol load of semiarid environments, which introduces a high bias in solar and net radiation. Practical steps for model improvement are proposed. They focus on the landuse classification and the aerosol parameters, both adapted to dry Mediterranean landscapes.

    Impact of climate change on drylands with a focus on West Africa
    Dietz, A.J. ; Verhagen, A. ; Ruben, R. - \ 2001
    [Bilthoven : National Institute op Public Health and the Environment] - ISBN 9789058510587 - 133
    klimaatverandering - geografie - landtypen - droge gebieden - west-afrika - climatic change - geography - land types - arid lands - west africa
    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. This book is of interest to scientists studying global and climate change, especially dealing with issues of adaptation. Social scientists, economists, geographers and policy makers concerned with West Africa should also read this book. Written for: Scientists studying global and climate change, especially dealing with issues of adaptation; social scientists, economists, geographers and policy makers concerned with West Af
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