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Though erosion and its control is not a major issue in The Netherlands, in some regions of the country wind and water erosion have caused severe local damage and nuisance and continue to do so. The earliest farmers settled on the higher parts of The Netherlands but, over time, overexploitation has caused extensive tracts to become drifting sand. From 1500 ¿ 1900 ¿Erosion Boards¿ were set up to control the sand drifts. Shifting sand areas were visited twice a year and conservation measures were applied. Despite this promising approach, the area of sand drifts increased because land users were poor and fines were high. Reforestation was important for the battle against drifting sand. Also contributing to the decline of drifting sands were the abolition of communal land use, the introduction of fertilizers and the collapse of the wool industry. In recent decades, ¿modern agriculture¿ has aggravated wind and water erosion problems. Nowadays wind erosion is causing problems in the Peat Reclamation District (PRD) in the north-east of the country. Here, land consolidation is regarded as a possible solution to decrease wind erosion. As a result of water erosion in the hilly part of South Limburg, where loess soils are dominant, towns and villages in the valleys are regularly flooded. It is being attempted to mitigate the problems of flooding by means of legislation and cooperation between stakeholders: Redistribution of land, implementation of measures ¿on hot spots¿ in the drainage system and farm management plans. There have been major successes already, but sustainable land use is still a long way off

Meerjarige granen, waaronder Thinopyrum intermedium, wortelen dieper en intensiever en houden daardoor bodem, voedingsstoffen en water goed vast. Voor Nederlandse omstandigheden zou de teelt van dergelijke gewassen dan ook kunnen zorgen voor minder water en/of winderosie, minder milieubelasting (minder uitspoeling) en zuiniger gebruik van inputs.

Meerjarige granen, waaronder Thinopyrum intermedium, wortelen dieper en intensiever en houden daardoor bodem, voedingsstoffen en water goed vast. Voor Nederlandse omstandigheden zou de teelt van dergelijke gewassen dan ook kunnen zorgen voor minder water en/of winderosie, minder milieubelasting (minder uitspoeling) en zuiniger gebruik van inputs.

PPO heeft de alternatieven voor runderdrijfmest tegen stuiven onderzocht. Runderdrijfmest ingeregend met water werkt goed bij geringe meerkosten.

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 (

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 (

Rundveedrijfmest is in het verleden gebruikt als antistuifdek op zand- en dalgronden. De mestwetgeving verbiedt deze toepassing echter in verband met de verplichte emissiearme toepassing van mest. De praktijk is daarom al jaren op zoek naar alternatieven voor een goede bescherming van landbouwgrond tegen verstuiven in het voorjaar. In 2012 is op PPO proefboerderij ’t Kompas een proef uitgevoerd. Daarbij zijn de volgende maatregelen toegepast: houtmulch, cellulose (beide in 50% dosering èn 150% dosering) rundveedrijfmest, gevolgd door beregening (4 ton water per ha)

Rundveedrijfmest is in het verleden gebruikt als antistuifdek op zand- en dalgronden. De mestwetgeving verbiedt deze toepassing echter in verband met de verplichte emissiearme toepassing van mest. De praktijk is daarom al jaren op zoek naar alternatieven voor een goede bescherming van landbouwgrond tegen verstuiven in het voorjaar. In 2012 is op PPO proefboerderij ’t Kompas een proef uitgevoerd. Daarbij zijn de volgende maatregelen toegepast: houtmulch, cellulose (beide in 50% dosering èn 150% dosering) rundveedrijfmest, gevolgd door beregening (4 ton water per ha)

In dit rapport wordt beschreven welke maatregelen er in Nederland op dit moment genomen worden tegen water- en winderosie. Het rapport bespreekt de voor Nederland relevante water- en winderosieprocessen, meet- en modelleertechnieken voor winderosie, de maatregelen die mogelijk zouden zijn, en de maatregelen en verordeningen die op dit moment toegepast worden dan wel van kracht zijn.

In dit rapport wordt beschreven welke maatregelen er in Nederland op dit moment genomen worden tegen water- en winderosie. Het rapport bespreekt de voor Nederland relevante water- en winderosieprocessen, meet- en modelleertechnieken voor winderosie, de maatregelen die mogelijk zouden zijn, en de maatregelen en verordeningen die op dit moment toegepast worden dan wel van kracht zijn.

Smit en Brouwer (2009) hebben bodems met een landbouwkundige handicap geselecteerd aan de hand van biofysische criteria. De relevante biofysische criteria waren gerelateerd aan bodemeigenschappen: drainage, textuur, aanwezigheid van stenen en chemische eigenschappen. Textuur en aanwezigheid van stenen zijn onderverdeeld in verschillende klassen: grof materiaal, grof en middelgrof zand, zware klei, veengronden, zwel en krimpverschijnselen en het opkomen van stenen uit de ondergrond. Verzilting is de enige chemische eigenschap die relevant is voor Nederland. Naast een selectie van gebieden op grond van biofysische eigenschappen, moet ook een verfijnde selectie worden uitgevoerd. Deze verfijnde selectie is gebaseerd op de verschillen in saldo per hectare tussen gebieden met en zonder de landbouwkundige gevolgen van de biofysische eigenschappen. De belangrijkste landbouwkundige gevolgen voor de verschillende categorieën waren de gevoeligheid voor winderosie bij grof en middelgrof zand; de beperkingen van de bewerkbaarheid van de grond en de beperkingen in het bouwplan op de zware kleigronden en lagere netto productie en de onmogelijkheid om maïs te verbouwen op veengronden

Smit en Brouwer (2009) hebben bodems met een landbouwkundige handicap geselecteerd aan de hand van biofysische criteria. De relevante biofysische criteria waren gerelateerd aan bodemeigenschappen: drainage, textuur, aanwezigheid van stenen en chemische eigenschappen. Textuur en aanwezigheid van stenen zijn onderverdeeld in verschillende klassen: grof materiaal, grof en middelgrof zand, zware klei, veengronden, zwel en krimpverschijnselen en het opkomen van stenen uit de ondergrond. Verzilting is de enige chemische eigenschap die relevant is voor Nederland. Naast een selectie van gebieden op grond van biofysische eigenschappen, moet ook een verfijnde selectie worden uitgevoerd. Deze verfijnde selectie is gebaseerd op de verschillen in saldo per hectare tussen gebieden met en zonder de landbouwkundige gevolgen van de biofysische eigenschappen. De belangrijkste landbouwkundige gevolgen voor de verschillende categorieën waren de gevoeligheid voor winderosie bij grof en middelgrof zand; de beperkingen van de bewerkbaarheid van de grond en de beperkingen in het bouwplan op de zware kleigronden en lagere netto productie en de onmogelijkheid om maïs te verbouwen op veengronden

Summary Dutch inland drift-sands are of great value to nature and house several Red-Listed species unique for Europe. The inland drift-sand landscape consists of three different intertwined wind erosion zones. Together they form the conditions where a mosaic of vegetation at different stages of development can thrive. Under present environmental conditions these drift-sands stabilise and tend to disappear. So far there is a lack of a management strategy to maintain the drift-sands after restoration. This paper presents a new management concept based on process management. The main objective of our new management strategy is to maintain optimum field conditions for wind erosion in a manner and scale that allow it to act as a landscape differentiating process. This process management strategy consists of: (1) an overall management plan for a period of 10–15 years; and (2) a short-term (1–3 years) maintenance plan to keep the erosion potential in the selected deflation zones sufficiently high. Details of our general management strategy such as the potential of drift-sand areas, the alternative measures to be undertaken, economic and social considerations and the monitoring and evaluation of our process management approach are elaborated in separate sections. Process management with scheduled maintenance will result in a more stable drift-sand habitat in which the typical drift-sand vegetation and fauna species are better able to survive. In other words, the answer to the question ‘How to use wind erosion to restore and maintain the inland drift-sand ecotype?’ is blowing in the wind if we maintain the optimum conditions for wind erosion.

The Sahelian zone ofAfricais the region that is globally most subjected to land degradation, with wind erosion being the most important soil degradation process. By using control measures, the negative effects of wind erosion can be reduced. At present, adoption of wind erosion control measures by Sahelian farmers is low, as most recommended measures do not fit into the farming systems. Therefore, the possibilities of using the local agro-forestry system, i.e. scattered vegetation of trees and shrubs as a wind erosion control strategy were explored in this study. The study area was located in the Sahelian zone of Burkina Faso. A survey among farmers revealed that they generally have a good knowledge of wind erosion processes and the possible wind erosion protection by natural vegetation. Detailed field measurements of wind speed and sediment transport revealed that the fluctuations in horizontal wind speed mainly cause wind erosion. Measurements around isolated vegetation elements revealed that these elements reduce wind speeds and sediment transport, and they are effective in trapping material already in transport. The effectiveness in reducing wind speed and sediment transport of scattered vegetation depends on the number of vegetation elements, the type of vegetation element and the height, width and porosity of the canopy of the element. A model was developed to simulate field-scale wind erosion with different types and arrangements of vegetation elements. This model can be used to develop optimal vegetation cover densities and spatial arrangements for wind erosion control. Overall it can be concluded that the use of the local agro-forestry system as a wind erosion control strategy is promising.

Wind has played an important role in the geological development of the north-western Europe. Various aeolian deposits such as inland dunes, river dunes, cover sands, drift sands and coastal dunes, form the base of large areas in our present landscape. The role of wind erosion in today's north-west European landscapes is, besides in coastal dunes, mainly related to arable fields (on light sandy soils), and to some active remnants of the inland drift-sand areas. In both latter cases there is an urgent need to manage the wind-erosion process. The aim of this PhD thesis is to develop management tools and strategies to control unwanted and wanted wind erosion.On arable land the main objective of managing the wind erosion process is to minimize the on- and off-site damage by reducing erosion risk. The extent, frequency, intensity of wind erosion events, and thereby their on-site and off-site effects, is controlled by soil characteristics, climate and human activities. Field surveys in areas with light sandy soils in four European countries showed that the erosion risk on agricultural land on light sandy soils is controlled by the cropping system, openness of the landscape and farmers practices. Scenario runs with the WEELS wind erosion model give insight into the general change in erosion risk per month. The simulations point to major changes in erosion risk for changes in a wind-break network, but relatively small changes in erosion risk for the tested cropping systems and climate-change scenario's. With this information, the possible consequences for on-site and off-site damage can be estimated, and from this estimation, additional (policy) measures for controlling wind erosion can be formulated for the region in question. There are enough effective land-management techniques available to minimise the erosion risk to an acceptable level. Voluntary measures based on good agricultural practice work well in a cropping system with high valuable crops and farmers financial benefit from control measures. However, additional policy measures such as mandatory measures and subsidies are needed in case of cropping systems with low or negative net profits and in case of high risk for off-site damage for instance in the period after harvesting when the soil is not longer protected by a crop.Inland drift-sand areas, on the contrary suffer from a lack of wind erosion activity. In the remnant active drift-sand areas with nature as main land-use type, which were saved from the large scale forestation schemes at the begin of the twentieth century, an ecologically high valuable vegetation and fauna developed. The role of wind erosion as main landscape differentiating process has changed to a local process with limited impact on the further development of the drift-sand areas. The general trend is an ongoing reduction of wind erosion activity and further colonisation of the drift sands by vegetation. This development addresses the reduced openness of the landscape, change in land-use, the limited size of the remainder open drift-sand areas, the relative high nitrogen-deposition and the invasion by exotic plant species. Without sufficient erosion activity the drift-sand pioneer vegetation turns into a grass dominated vegetation and eventually in forest. To maintain the positive interaction between erosion and the pioneer vegetation in the remaining active drift-sand areas, it is necessary to keep the active (bare) areas open (process management). In practice several tillage techniques are used by the terrain managers. Of these the beach-sand cleaner and the rotary cultivator proved to be most effective to reactivate drift sand areas with a pioneer vegetation cover. At the same time the complete top soil needs to be removed to set back the succession (pattern management) in parts of the areas covered with higher succession stages. This can go hand in hand with reactivation of wind erosion on locations in order to slow down the succession rate. In that case the treated area should be at least 5 to 10 ha to guarantee a positive impact of erosion on the surrounding area. Other criteria are: the presence of a sufficient amount of erodible sand and an optimum erosive wind force.

Wind has played an important role in the geological development of the north-western Europe. Various aeolian deposits such as inland dunes, river dunes, cover sands, drift sands and coastal dunes, form the base of large areas in our present landscape. The role of wind erosion in today's north-west European landscapes is, besides in coastal dunes, mainly related to arable fields (on light sandy soils), and to some active remnants of the inland drift-sand areas. In both latter cases there is an urgent need to manage the wind-erosion process. The aim of this PhD thesis is to develop management tools and strategies to control unwanted and wanted wind erosion.On arable land the main objective of managing the wind erosion process is to minimize the on- and off-site damage by reducing erosion risk. The extent, frequency, intensity of wind erosion events, and thereby their on-site and off-site effects, is controlled by soil characteristics, climate and human activities. Field surveys in areas with light sandy soils in four European countries showed that the erosion risk on agricultural land on light sandy soils is controlled by the cropping system, openness of the landscape and farmers practices. Scenario runs with the WEELS wind erosion model give insight into the general change in erosion risk per month. The simulations point to major changes in erosion risk for changes in a wind-break network, but relatively small changes in erosion risk for the tested cropping systems and climate-change scenario's. With this information, the possible consequences for on-site and off-site damage can be estimated, and from this estimation, additional (policy) measures for controlling wind erosion can be formulated for the region in question. There are enough effective land-management techniques available to minimise the erosion risk to an acceptable level. Voluntary measures based on good agricultural practice work well in a cropping system with high valuable crops and farmers financial benefit from control measures. However, additional policy measures such as mandatory measures and subsidies are needed in case of cropping systems with low or negative net profits and in case of high risk for off-site damage for instance in the period after harvesting when the soil is not longer protected by a crop.Inland drift-sand areas, on the contrary suffer from a lack of wind erosion activity. In the remnant active drift-sand areas with nature as main land-use type, which were saved from the large scale forestation schemes at the begin of the twentieth century, an ecologically high valuable vegetation and fauna developed. The role of wind erosion as main landscape differentiating process has changed to a local process with limited impact on the further development of the drift-sand areas. The general trend is an ongoing reduction of wind erosion activity and further colonisation of the drift sands by vegetation. This development addresses the reduced openness of the landscape, change in land-use, the limited size of the remainder open drift-sand areas, the relative high nitrogen-deposition and the invasion by exotic plant species. Without sufficient erosion activity the drift-sand pioneer vegetation turns into a grass dominated vegetation and eventually in forest. To maintain the positive interaction between erosion and the pioneer vegetation in the remaining active drift-sand areas, it is necessary to keep the active (bare) areas open (process management). In practice several tillage techniques are used by the terrain managers. Of these the beach-sand cleaner and the rotary cultivator proved to be most effective to reactivate drift sand areas with a pioneer vegetation cover. At the same time the complete top soil needs to be removed to set back the succession (pattern management) in parts of the areas covered with higher succession stages. This can go hand in hand with reactivation of wind erosion on locations in order to slow down the succession rate. In that case the treated area should be at least 5 to 10 ha to guarantee a positive impact of erosion on the surrounding area. Other criteria are: the presence of a sufficient amount of erodible sand and an optimum erosive wind force.

Though erosion and its control is not a major issue in The Netherlands, in some regions of the country wind and water erosion have caused severe local damage and nuisance and continue to do so. The earliest farmers settled on the higher parts of The Netherlands but, over time, overexploitation has caused extensive tracts to become drifting sand. From 1500 ¿ 1900 ¿Erosion Boards¿ were set up to control the sand drifts. Shifting sand areas were visited twice a year and conservation measures were applied. Despite this promising approach, the area of sand drifts increased because land users were poor and fines were high. Reforestation was important for the battle against drifting sand. Also contributing to the decline of drifting sands were the abolition of communal land use, the introduction of fertilizers and the collapse of the wool industry. In recent decades, ¿modern agriculture¿ has aggravated wind and water erosion problems. Nowadays wind erosion is causing problems in the Peat Reclamation District (PRD) in the north-east of the country. Here, land consolidation is regarded as a possible solution to decrease wind erosion. As a result of water erosion in the hilly part of South Limburg, where loess soils are dominant, towns and villages in the valleys are regularly flooded. It is being attempted to mitigate the problems of flooding by means of legislation and cooperation between stakeholders: Redistribution of land, implementation of measures ¿on hot spots¿ in the drainage system and farm management plans. There have been major successes already, but sustainable land use is still a long way off

A windbreak model was used to evaluate the effect of the various types of windbreak near Barnham, Suffolk, England. The model showed that the sheltering effect of the present windbreaks is limited and needs to be extented to provide full protection

A windbreak model was used to evaluate the effect of the various types of windbreak near Barnham, Suffolk, England. The model showed that the sheltering effect of the present windbreaks is limited and needs to be extented to provide full protection

A field study in four areas with a high wind erosion risk showed that soil and landscape characteristics determine the potential - and land use and farmers the actual - risk for off-site damage

Op verzoek van de ministeries van LNV en VROM is door Alterra en RIVM een quick scan uitgevoerd van de Nederlandse bodem, betreffende de thema's van de EU soil strategy. Per thema is de huidige bodemkwaliteit in beeld gebracht.Dok is per thema beknopt aangegeven wat de stand van zaken is met betrekking tot monitoring (kunnen we betrouwbaar aangeven In hoevere de gehalten aan organische stof, metalen, nutriënten, etc. veranderen?) en onderzoek (wat weten we van biodiversiteit, van te lage gehalten aan organsche stof in de bodem, van de effecten van winderosie?).

A field study in four areas with a high wind erosion risk showed that soil and landscape characteristics determine the potential - and land use and farmers the actual - risk for off-site damage

In the semi-arid environment of northern Burkina Faso the processes of wind and water erosion occur almost simultaneously and may cause severe soil degradation. Especially in the early rainy season when soils are bare and unprotected, violent winds preceding intense rainfall events result in intense sediment transport by both wind and water. This Ph.D. project aimed at quantifying and modelling wind and water erosion processes, their interaction and related nutrient flows in a Sahelian environment. The source codes of EUROSEM and the stand-alone erosion sub-model of WEPS are translated to the dynamic modelling language PCRaster, further adapted to be applicable to the Sahelian situation and extended with nutrient components. From field measurement and modelling results it is concluded that for water erosion rain splash is the most important detaching agent at the scale of the field and that despite the large volumes of overland flow, erosion is transport capacity limited due to the general low slopes. The material detached by water is available for wind-blown transport. Due to the absence of non-eroding boundaries, intense mass transport under influence of violent winds does not always result in erosion. Depending on wind direction, cover and crust type, net deposition may occur. Further, due to the large spatial variation in wind erosion controlling parameters, areas with erosion and deposition can be identified within a field. Based on model results it is concluded that wind erosion is responsible that wind erosion is responsible for the loss of deposition of large amounts of fine sediment and the nutrients attached to these sediments. Though compared to wind erosion, the nutrient losses under influence of water erosion are small, these losses should not be underestimated; these nutrients flow to the nearest stream and are forever lost for the catchment. Due to the interaction between wind and water, nutrient and soil erosion at field scale may be large, but are limited at village scale. Provided a good management of natural resources as tree and shrub cover and a good insight in the distribution of cultivated and fallow fields around the village, long-term productivity is not at risk in the southern Sahelian zone.

In the semi-arid environment of northern Burkina Faso the processes of wind and water erosion occur almost simultaneously and may cause severe soil degradation. Especially in the early rainy season when soils are bare and unprotected, violent winds preceding intense rainfall events result in intense sediment transport by both wind and water. This Ph.D. project aimed at quantifying and modelling wind and water erosion processes, their interaction and related nutrient flows in a Sahelian environment. The source codes of EUROSEM and the stand-alone erosion sub-model of WEPS are translated to the dynamic modelling language PCRaster, further adapted to be applicable to the Sahelian situation and extended with nutrient components. From field measurement and modelling results it is concluded that for water erosion rain splash is the most important detaching agent at the scale of the field and that despite the large volumes of overland flow, erosion is transport capacity limited due to the general low slopes. The material detached by water is available for wind-blown transport. Due to the absence of non-eroding boundaries, intense mass transport under influence of violent winds does not always result in erosion. Depending on wind direction, cover and crust type, net deposition may occur. Further, due to the large spatial variation in wind erosion controlling parameters, areas with erosion and deposition can be identified within a field. Based on model results it is concluded that wind erosion is responsible that wind erosion is responsible for the loss of deposition of large amounts of fine sediment and the nutrients attached to these sediments. Though compared to wind erosion, the nutrient losses under influence of water erosion are small, these losses should not be underestimated; these nutrients flow to the nearest stream and are forever lost for the catchment. Due to the interaction between wind and water, nutrient and soil erosion at field scale may be large, but are limited at village scale. Provided a good management of natural resources as tree and shrub cover and a good insight in the distribution of cultivated and fallow fields around the village, long-term productivity is not at risk in the southern Sahelian zone.

PPO heeft de alternatieven voor runderdrijfmest tegen stuiven onderzocht. Runderdrijfmest ingeregend met water werkt goed bij geringe meerkosten.

De aanwezigheid van fijn stof (deeltjes kleiner dan 10 ~S^~m) in de buitenlucht kan gezondheidsproblemen veroorzaken. Om deze reden zijn in EU-verband normen voor fijn stof gesteld. Voor het bestrijden van de emissie van fijn stof is het van belang om deomvang van de verschillende bronnen te kennen. Over de emissie vanuit de landbouw bestond een aantal onduidelijkheden. De afdeling Monitoring en Informatiemanagement van het Ministerie van VROM heeft een opdracht gegeven aan Alterra om in samenwerking met RIVM de methodiek van emissieschatting te verbeteren. Het project heeft bestaan uit literatuuronderzoek en het bespreken van de problematiek met een aantal deskundigen tijdens een discussiedag. Uit het onderzoek blijkt dat stallen waarschijnlijk de grootste emissiebron vormen van fijn stof vanuit de landbouw. Daarnaast is er een aantal relatief kleine posten, met als belangrijkste de toediening van bestrijdingsmiddelen en kunstmest en de aanvoer van krachtvoer op het agrarisch bedrijf. Winderosie is vermoedelijk ook een grote bron, die variabel is in de tijd. De omvang ervan is echter moeilijk te schatten.

De aanwezigheid van fijn stof (deeltjes kleiner dan 10 ~S^~m) in de buitenlucht kan gezondheidsproblemen veroorzaken. Om deze reden zijn in EU-verband normen voor fijn stof gesteld. Voor het bestrijden van de emissie van fijn stof is het van belang om deomvang van de verschillende bronnen te kennen. Over de emissie vanuit de landbouw bestond een aantal onduidelijkheden. De afdeling Monitoring en Informatiemanagement van het Ministerie van VROM heeft een opdracht gegeven aan Alterra om in samenwerking met RIVM de methodiek van emissieschatting te verbeteren. Het project heeft bestaan uit literatuuronderzoek en het bespreken van de problematiek met een aantal deskundigen tijdens een discussiedag. Uit het onderzoek blijkt dat stallen waarschijnlijk de grootste emissiebron vormen van fijn stof vanuit de landbouw. Daarnaast is er een aantal relatief kleine posten, met als belangrijkste de toediening van bestrijdingsmiddelen en kunstmest en de aanvoer van krachtvoer op het agrarisch bedrijf. Winderosie is vermoedelijk ook een grote bron, die variabel is in de tijd. De omvang ervan is echter moeilijk te schatten.

Bodemerosie is een wijdverbreid probleem in veel gebieden van de wereld. Op Europese schaal is bodemerosie ook aangemerkt als één van de belangrijkste bodembedreigingen in de Europese bodemstrategie, omdat het kan leiden tot verlies van vruchtbare grond, sedimentatie en vervuiling. Ook in Nederland komt erosie voor, en in het voorstel voor een Kaderrichtlijn Bodem wordt er dan ook aandacht aanbesteed. Dit artikel beschrijft waar erosie veroorzaakt door water en wind optreedt in Nederland, en welke maatregelen er tot nu toe zijn genomen om bodemerosie tegen te gaan. Deze informatie geeft een indicatie voor het eventueel aanwijzen van prioritaire gebieden in Nederland.

The dynamics of fine aeolian dust emitted from agricultural land was investigated over 15 months near Grönheim, Lower Saxony, Germany. The following aspects were studied: airborne dust concentration, the ratio of mineral versus organic dust, the vertical distribution of the particles in the atmosphere, horizontal and vertically integrated horizontal dust flux, vertical dust flux, dust deposition at ground level, grain-size distribution of the mineral dust component, and vertical distribution of organic matter in the dust. Standard meteorological parameters (wind speed and direction, precipitation) were measured as well. Dust activity in Grönheim is high in spring (March-May) and autumn (October-November) and low to very low during the rest of the year. There is a strong relationship between the periods of tillage and the intensity of dust activity. Also, there is high dust activity during wind erosion events. For the year 1999, dust emission due to tillage was 6?6 times higher than dust emission due to wind erosion. A dust transport of 15?8 ton km-1 a-1 was calculated for the first 10 m of the atmosphere in 1999. Total dust transport (in the entire mixing layer) was estimated between 16 and 20 ton km-1 a-1. About 25-30 per cent of this dust is mineral dust, emitted from the fields during tillage or during wind erosion events. In spring and autumn there is a strong vertical stratification in the airborne sediment, with much (coarse) dust in the lower air layers and significantly less (and finer) dust at higher altitudes. In summer and winter, when there is no local dust production, there is no stratification: equal amounts of dust are transported at all heights. The stratification in spring and autumn is exclusively caused by the mineral part of the dust. The organic particles are much better mixed in the atmosphere because of their lower density.

This paper analyzes the use of stratification in the modeling of dependence for regionalized variables in space and time. Stratification enables optimal use of available information, and it is used when deterministic, small-scale information is combined with large-scale random variation: on the one hand, the problem of having too few data to reliably estimate statistical parameters may be solved by stratification; on the other hand some variation may be so obvious that it serves as a prerequisite for obtaining any form of stationarity. The paper is illustrated with four case-studies. The first study analyzes optimization of an extant groundwater monitoring network within a catchment of the river Rhine. Stratification in space enabled 5 cm precision of predicted levels at critical locations, something that could not be reached without stratification. The second study analyzed variability in space and time of wind erosion in a field. Erosion by four storms was measured with 21 devices. Use of temporal stratification to overcome lack of spatial data resulted, in one instance, in a decrease of 17 percent in mean absolute error. The third study addressed changes in the soil solution influenced by environmental deposition over a period of six years. Stratification was made in space to enable analysis in the time domain and to obtain spatio-temporal maps. The fourth study used a general space-time model to evaluate the development of root-rot in soybean, caused by naturally occurring pathogens. Predictions were made at unvisited times to estimate the probability of occurrence of root rot. We conclude that use of stratification enhances the utility of space-time statistics for the solution of various environmental and agricultural problems.

In the Sahelian zone, future food supply is insecure due to increasing land degradation. Wind erosion contributes significantly to impoverishment of the sandy soils, which are often loose and sparsely covered by vegetation for most of the year. At the onset of the growing season (May - July), strong winds often precede rains and may cause damage to the young seedlings by abrasion and burial. A possible control measure is the implementation of windbreaks (WBs) that reduce the wind velocity near the soil surface, thus protecting soil and crops. Moreover, the change in air flow may lead to a more favourable crop microclimate. Higher crop yields and increases in the long-term sustainability of crop production may be the result of effective windbreaks integrated in crops. However, windbreaks also compete with crops for limited resources, which may outweigh their potential benefits.Windbreak experiments, reported in literature, illustrate various benefits as well as negative impacts of windbreaks on crop growth (Chapter 1). In addition to a more favourable microclimate and reduced wind erosion, increased soil fertility, and complementary use of resources by trees and crops are possible agroforestry benefits. On the other hand, competition and allelopathic interactions between species, as well as attraction of pests and diseases by trees may reduce crop production. Whether the overall windbreak effect results in yield increases or losses depends on many interacting factors, e.g. climate, soil properties, crop species and WB-design. The few studies performed in semi-arid regions give insufficient insights to formulate generally applicable rules that allow extrapolation of experimental results to other locations or from one WB-design to another. Understanding of the tree-crop interactions is required for the design of optimum windbreak-cropping systems that can provide an option for sustainable land use in the Sahel.This research aimed at enhancing the understanding of positive and negative influences of windbreaks on crop production by i) collecting field data (Part A) and ii) analyzing crucial tree-crop interactions in terms of dynamic and spatial occurrence by means of a model (Part B).Between 1991 - 1993, experiments were performed at the ICRISAT Sahelian Center (ISC), Niger, to study the effects of low, narrowly spaced windbreaks on microclimate, light, water and nutrient resources at the windbreak-crop interface, and the growth of pearl millet ( Pennisetum glaucum ). Millet, a C4 tropical cereal, is the principal food crop in Niger, since it is particularly adapted to conditions of high temperatures, nutrient-poor soils and low rainfall. The windbreaks were north-south oriented, i.e. perpendicular to storms and prevailing winds and, hence, protecting crops sown on their west side. The agroforestry system under study consisted of various shelter species and plots without windbreaks as control. Measurements were performed along transects across the tree-crop interface. Radiation, wind speed, relative humidity, and air temperatures were measured continuously and soil temperatures occasionally measured in Bauhiniarufescens plots. Bauhinia windbreaks were 2 and 3 m high and had a porosity of 0.9 and 0.2 at the onset and the end of the growing season, respectively.Crop yields adjacent to seven windbreak species and in control plots were determined. In two rather wet years of experiments it was found that the overall yield in windbreak plots was slightly (mostly not significant) higher than in the control plots. Up to a distance of 2.5 H (with H the windbreak height) from windbreaks lower yields were measured than in the middle of the plots.Growth reduction in the vicinity of the windbreak was more than compensated by increased production in the center of the plots when averaged over all plots. The magnitude of yield increases varied among the tree species, e.g. in Bauhina sheltered crops this positive effect was rather small. From the experiments the causes for yield increases remained unclear, since no amelioration in the microclimate was observed. Air temperature and relative humidity (at the top of the sheltered crop) were constant with distance from the Bauhinia windbreak, although the shelter reduced wind velocity up to a minimum of 5 H. Generally, the influence of shade and wind speed on temperature and relative humidity are opposite and, presumably, negated in the present system. Shade, however, lowered soil temperatures, but this had no measurable impact on crop emergence and yields. Protection against wind erosion, enhanced water availability and/or small microclimate modifications (too small to be recorded) may have caused higher yields.The zone of severe yield reduction (0.5 - 1.5 H) corresponded to that of the strongest reduction in radiation and soil moisture, indicating competition effects. To assess competition between crops and windbreaks, soil water and nutrient status were observed in the windbreak-crop interface and in control plots. Nutrient status did not vary significantly across the tree-crop interface or between windbreak and control plots, whereas soil moisture was strongly reduced in the vicinity of trees during dry periods. Then, trees were likely to compete with crops for water since they had no access to groundwater. Competition for water apparently occurred although total rainfall was rather high in the experimental years. From these results it was concluded that in dry years competition for water would be much stronger. That this is not necessarily the case, was shown by a simulation study covering a dry and two wet years (Part B).Competition for water and light, and wind speed reduction within the 0 - 5 H tree-crop zone were considered to be of major importance in the agroforestry experiment, as described in Part A of this thesis. Consequently, to assess the agronomic feasibility of windbreaks in the Sahel, those effects have to be quantified.Assessment of the competitive effects between trees and crop is complicated, due to difficulties in determining below-ground competition and interference with other tree-crop interactions. A model approach is helpful to quantify and integrate individual effects of tree-crop interactions. When starting this study, no windbreak-cropping system model was available. Existing crop models lacked spatial heterogeneity, and temporal detail as required for the description of agroforestry systems. Therefore, a model, WIMISA (WIndbreak MIllet SAhel), has been developed by combining methods from existing models with newly developed concepts, with special attention to resolution in time and space.WIMISA simulates crop growth as influenced by local soil moisture and radiation intensity simultaneously for a number of rows parallel to a windbreak in daily time steps. Soil moisture content and incoming radiation intensities are computed at process level in smaller time steps. Windbreak growth is not simulated, but windbreak characteristics are introduced either as fixed values or as time dependent forcing functions. As knowledge of the WB characteristics is limited, competition for light, tree water uptake, and wind speed reduction are described using a simple model of the windbreak component to restrict the number of assumptions. Reduction in wind speed is incorporated through an empirical factor for wind velocities in the first 5 H from the windbreak.Competition for light is incorporated as light interception by a two-dimensional WB-barrier, as a function of its height and density (both increasing during the cropping season). WIMISA calculates instantaneous fluxes of direct and diffuse radiation for each crop row separately at intervals of 6 minutes. The interval for the computation of CO2 assimilation was set in accordance with the radiation calculations. Soil water flow is simulated in two dimensions to account for horizontal gradients due to different water extraction by trees and crop, and horizontally varying evapotranspiration. Spatial detail in vertical and horizontal direction is introduced through an arrangement of compartments. The soil water module uses a time step that is adjusted automatically to water flow rate and thickness of the soil compartment. Competition for water is expressed by distributing available soil water between trees and crop in proportion to their uptake rates in a non-competitive situation. Water uptake is calculated on the basis of root length density distribution.WIMISA was parameterised for millet, the tree species Bauhinia rufescens and soil characteristics of ISC. Specific crop parameters of the model for a sole millet system were determined throughout the growing season in a separate plot outside the influence of a windbreak. Simulations were confined to the competition zone (i.e. 5 H).Simulation results of the windbreak-cropping system were evaluated with data from Bauhinia plots of the 1992 and 1993 growing seasons. WIMISA gave reasonable estimates of straw and grain yields for millet westward of a windbreak, considering the generally high spatial variability and the fact that nutrient limitations are not included. Grain yield beyond 3 m was underestimated, probably due to windbreak effects not included in the model. This suggests that in addition to light other microclimate factors might have been somewhat modified by the windbreak, although not observed. Since simulated global radiation intensities and soil water contents, as a function of time and distance from the windbreak, and percentage yield reductions normalized to 10 m corresponded well with field observations, it can be assumed, that the processes of competition for water and light are simulated in the correct proportion.In the present agroforestry system, species utilized the resources competitively rather than complementary, resulting in (ca. 20 - 30 %) yield reductions in the 0.5 - 2 H zone adjacent to the windbreak. This extent of competition zone and magnitude of yield losses correspond to field data and results reported for other windbreak-cropping systems in the Sahel.Three scenarios where competition effects for water and for light were separated by turning off one of the effects during simulations, were compared for the 1987, 1992 and 1993 growing seasons. Surprisingly, water and total competition were highest in the wet year 1992, intermediate in the dry year 1987 and lowest in the wet year 1993. In fields with no access to groundwater, competition for water between trees and crops is likely to occur at the beginning of the rainy season, when available soil water is restricted to the upper horizons and transpiration of the windbreak is higher than that of the crop. Neither horizontal water flow nor reduced soil evaporation as a result of shade could compensate for tree water use. Under severe drought, water remained the major factor (1992), otherwise light was of similar or major importance (1987 and 1993). Simulation results indicate that the degree of light and water competition also depends on rainfall distribution and the time laps between the seasonal onset of tree and crop growth.The model is appropriate to quantify the relative importance of competition for water and light in a windbreak-millet system, although some process descriptions need further refinement (i.e. LAI development) and evaluation for application in other environments. In particular, modelling of tree-root water uptake requires more data for input and validation. Once, nutrient, complete microclimate and erosion modules have been incorporated in WIMISA, the model allows quantification of the overall WB effects and screening of different windbreak-cropping systems.