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Many sandy soils in the Netherlands have a water-repellent surface layer covering a wettable soil with a shallow groundwater table. Fingers form in the water-repellent surface layer and rapidly transport water and solutes to the wettable soil in which the streamlines diverge. Although several field observations are available, this system has not yet been studied systematically. In this paper, we present a model with a steady-state water flow to which solutes are added as a pulse. The model predicts the flow through the distribution zone and through the finger in the water-repellent surface layer with a closed form solution and transport in the wettable subsoil numerically. Model calculations show that the travel time through the water-repellent surface layer and the thickness and hydraulic conductivity of the wettable soil have the strongest effect on the arrival time of the solute pulse at groundwater level. The calculations also show that, assuming transport in the wettable subsoil to take place in fingers, the travel time is considerably shorter than when the diverging flow in the wettable soil is included.

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Bij interacties van grond- en oppervlaktewater is kennis van de fysische eigenschappen van de weerstandsmedia op het overgangsgebied van beide regimes noodzakelijk. Infiltratieproeven, d.m.v. kunstmatige verlaging van grondwater, vormen een geschikte methode voor de bepaling van weerstandswaarden. Afhankelijk van de heersende hydrologische condities in het overgangsgebied van grond- en oppervlaktewater bedragen de bodemweerstanden in lagere veen- en zandgebieden ca. 1 a 2 dagen, oplopend tot ca. 4 a 7 dagen op de middelhoge zandgronden. Visuele waarneming van slootbodemprofielen, gekoppeld aan lokale hydrologische condities, vormen een onmisbaar hulpmiddel bij deze studies.

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In this report a perspective is outlined for the development of area- oriented environmental assessments (AEAs) for integrated regional policy. These documents should hold scientific information for the development and foundation of an integrated policy for town and country planning, water management and environment. The present planning structure for environmental policy and the policy for town and country planning and watermanagement is described. The position and function of AEAs within the present framework of national, regional and local planning is explained. The advisable contents of AEAs is surveyed. Possible items for the contents of AEAs are: the physical environment and the relation with other regions, regional (economic) activities and developments, impacts on the environment, use of space, groundwater and surface watersystems and sustainability on a regional level. The interrelations between these items are emphasized. Furthermore the user prospects of AEAs for regional and local authorities is clarified.

In this report a perspective is outlined for the development of area- oriented environmental assessments (AEAs) for integrated regional policy. These documents should hold scientific information for the development and foundation of an integrated policy for town and country planning, water management and environment. The present planning structure for environmental policy and the policy for town and country planning and watermanagement is described. The position and function of AEAs within the present framework of national, regional and local planning is explained. The advisable contents of AEAs is surveyed. Possible items for the contents of AEAs are: the physical environment and the relation with other regions, regional (economic) activities and developments, impacts on the environment, use of space, groundwater and surface watersystems and sustainability on a regional level. The interrelations between these items are emphasized. Furthermore the user prospects of AEAs for regional and local authorities is clarified.

Het stroomgebied van de Hupselse beek is gelegen in het zandgebied van Oostelijk Gelderland. De hydrologische grenzen van het gebied zijn goed bekend. Water-, stikstof- en fosfaatbalansen voor het gebied zijn berekend van april 1985 tot april 1994. De gemiddelde neerslag in die periode bedroeg 845 mm j-1, de afvoer van water door de beek 330 mm j-1 en de actuele verdamping 515 mm.j-1. Afvoercomponenten zijn afvoer over de oppervlakte (3% van het totaal), grondwaterafvoer via drainbuizen (55-75%) en afvoer van grondwater naar sloten en beken (20-40%). Ca. 40% van het water heeft een verblijftijd van maximaal 1 jaar in de bodem en ruim 90% komt binnen 5 jaar tot afvoer. De gemiddelde concentratie aan nitraat in de beekafvoer is 32 mg.l-1 en die aan totaal-fosfaat is 0.21 mg.l-1. Ongeveer 20% van de bodembelasting met stikstof komt tot afvoer, dit is gelijk aan 112 kg ha-1 j-1 N. In de onderzoeksperiode is geen trend aanwezig in de stikstofvracht van de beek. De jaarlijkse gemiddelde fosfaatvracht van de beek is 1 kg ha-1 j-1 P, dat is 1% van de bodembelasting. De fosfaatvracht in de onderzoeksperiode is dalend in de tijd

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Het stroomgebied van de Hupselse beek is gelegen in het zandgebied van Oostelijk Gelderland. De hydrologische grenzen van het gebied zijn goed bekend. Water-, stikstof- en fosfaatbalansen voor het gebied zijn berekend van april 1985 tot april 1994. De gemiddelde neerslag in die periode bedroeg 845 mm j-1, de afvoer van water door de beek 330 mm j-1 en de actuele verdamping 515 mm.j-1. Afvoercomponenten zijn afvoer over de oppervlakte (3% van het totaal), grondwaterafvoer via drainbuizen (55-75%) en afvoer van grondwater naar sloten en beken (20-40%). Ca. 40% van het water heeft een verblijftijd van maximaal 1 jaar in de bodem en ruim 90% komt binnen 5 jaar tot afvoer. De gemiddelde concentratie aan nitraat in de beekafvoer is 32 mg.l-1 en die aan totaal-fosfaat is 0.21 mg.l-1. Ongeveer 20% van de bodembelasting met stikstof komt tot afvoer, dit is gelijk aan 112 kg ha-1 j-1 N. In de onderzoeksperiode is geen trend aanwezig in de stikstofvracht van de beek. De jaarlijkse gemiddelde fosfaatvracht van de beek is 1 kg ha-1 j-1 P, dat is 1% van de bodembelasting. De fosfaatvracht in de onderzoeksperiode is dalend in de tijd

A new approach for ecological restoration of floodplains of regulated rivers is presented. The emphasis is laid on restoring the most important abiotic factor, the water regime, which has undergone serious deteriorations due to anthropogeneous effects. The idea is to change the water regime on the floodplain for the benefit of ecology, by controlling the flow in the links connecting the floodplain depressions to the river and to each other. The viability of this approach is proven on a case study dealing with the restoration of a Danube floodplain. The flow control is proposed to be implemented on a passive way, by appropriately sizing the connecting channels and weirs of the floodplain. Accordingly, several restoration alternatives have been set up, evaluated and compared to each other. The evaluation is based on known relationships among ecology, hydrology and water quality. For the hydrological and water quality analysis, simulation models have been used. The comparison of alternatives is done by the ELECTRE II multi-criteria decision support technique.

Bestrijding van grondwateroverlast in stedelijk gebied vindt in veel gevallen plaats door middel van ontwatering en daarmee gepaard gaande verlaging van de grondwaterspiegel. Dit laatste is ongewenst met het oog op de bijdrage aan verdroging. Men zal daarom een ontwateringssysteem bij voorkeur zo ontwerpen, dat aan de ontwateringseisen die voortvloeien uit de bestemming van het gebied wordt voldaan, zonder dat meer water wordt afgevoerd dan strikt noodzakelijk is. Verhoging van de ontwateringsbasis is daarbij een voor de hand liggende mogelijkheid. Dit leidt behalve tot een hogere gemiddelde grondwaterstand echter ook tot verminderde (grondwaterlberging en daardoor tot een toename van de vereiste ontwateringscapaciteit. In veel gevallen zal zo de afstand tussen de ontwateringsmiddelen onaanvaardbaar klein worden. Een alternatief is een automatisch gereguleerd ontwateringssysteem met diepe (tijdelijke) ontwateringsbasis, dat alleen werkt als er grondwater moet worden afgevoerd en zichzelf uitschakelt als dat niet nodig is

Similarity models in the inner region of the unstable atmospheric boundary layer (ABL) are generally based on four dimensional parameters: buoyancy, friction velocity, surface heat flux, and the height above the land surface. In the free convection limit the friction velocity can be neglected, thus reducing the measurement needs in practical applications. Some field measurements of the second moment of temperature have indicated that free convection scaling of this statistic may be extended into more dynamic regimes, namely, the “dynamic-convective” and, perhaps, the “dynamic” regions of the ABL. An advantage of this approach is that the sensible heat flux can be estimated without shear stress measurements. This temperature variance similarity model is applied for a wide range of unstably stratified flows over the dry Owens Lake, in southeastern California. The simple free convection model of the temperature variance is accurate for sensible heat flux estimation across the full range of unstable atmospheric stability conditions.

The impact of hydrological manipulation of an unfertilised, Dutch peat grassland area on plant species composition on ditch banks and in ditchwater was studied. The hydrological manipulation involved raising the groundwater level by admitting nutrient-poor, Ca-rich groundwater in one compartment, and by retaining precipitation in another compartment. A third compartment served as control.

Een nieuwe, eenvoudige bepaling van de (botanische) kwaliteit van de natuur in Nederland. De resultaten worden weergegeven in de vorm van verspreidingskaarten van 28 ecologische groepen (ecotoopgroepkaarten)

When water enters a water-repellent topsoil, fingers develop and a large part of the soil is bypassed. Therefore fingering is expected to accelerate solute leaching to the groundwater. In soils with a groundwater-affected wettable sublayer, fingering yields a flow pattern consisting of three regions. In the top few centimeters, water flow converges toward the finger top. In the finger, water moves vertically downward. In the wettable subsoil the flow diverges due to matric forces. Analytical steady state solutions are presented for each of these regions. The resulting model provides the breakthrough curve of a pulse of an inert solute and reveals the influence of each region. Calculations showed that the enhancement of leaching caused by fingers could be largely undone by a thick (0.75 m) wettable subsoil. A very thin (0.05 m) wettable subsoil had a much smaller effect. This illustrates the need for accurate field estimates of finger depth.

Auteurs bestrijden de opvatting dat het model MOVE geschikt is voor het ontwikkelen van ecologische normen en voor risicobeoordeling

Data collected in an actual IMPASS experiment with a static gaseous mixture (methyl fluoride/ethylene/nitrogen) of preselected concentrations were analyzed with respect to uni(bi)modality, symmetry, skeweness and kurtosis. Bimodal probability distribution function is significantly different from Gaussian suggesting the presence of "mixed" data in the measurements ; the potential cause for this phenomenon is discussed.

The potentiality of combining large-scale inverse modelling of unsaturated flow with remote sensing determination of areal evaporation and areal surface moisture is assessed. Regional latent and sensible heat fluxes are estimated indirectly using remotely sensed measurements by parameterizing the surface energy balance equation. An example of evapotranspiration mapping from northern and central Egypt is presented. The inverse problem is formulated with respect to the type of information available. Two examples of estimation of soil hydraulic properties by the dynamic one-dimensional soil-water-vegetation model SWATRE are given: one refers to a classical lysimeter scale and another one to a catchment scale. It is concluded that small-scale soil physics may describe large-scale hydrological behaviour adequately, and that the effective hydraulic parameters concerned may be derived by an inverse modelling approach. Remotely sensed data on surface reflectance, surface temperature and soil moisture content derived from multifrequency microwave techniques provide a useful data set on the mesoscale. The inverse modelling approach presented combined with a meso-scale data set on evaporation and surface soil moisture, considerable potentialities arise to determine effective meso-scale hydraulic properties.