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Het Geleenbeekdal wordt heringericht om de natuurwaarden te verbeteren, te anticiperen op het veranderende klimaat en de recreatieve waarden te verhogen. Hiervoor zijn reeds herinrichtingsprojecten uitgevoerd, maar de vraag bestaat welke gebieden verder in zijn te richten als natuurgebied of voedselbos. De onderzoeksvraag is: “Op welke wijze kan natuurontwikkeling in het Geleenbeekdal worden gerealiseerd met een potentiële rol voor voedselbossen?”

Het Geleenbeekdal wordt heringericht om de natuurwaarden te verbeteren, te anticiperen op het veranderende klimaat en de recreatieve waarden te verhogen. Hiervoor zijn reeds herinrichtingsprojecten uitgevoerd, maar de vraag bestaat welke gebieden verder in zijn te richten als natuurgebied of voedselbos. De onderzoeksvraag is: “Op welke wijze kan natuurontwikkeling in het Geleenbeekdal worden gerealiseerd met een potentiële rol voor voedselbossen?”

In het kader van natuurontwikkeling van het Geleenbeekdal in Zuid-Limburg wordt de Geleenbeek geanalyseerd en worden maatregelen aangedragen. Watersystemen worden teruggebracht in de natuurlijke staat. De Geleenbeek is vroeger voor de mijnbouw gekanaliseerd. Sinds 1990 ondervindt langs de beek herstelmaatregelen plaats. Het beekherstelproject van de Geleenbeek valt onder de doelstellingen van het Waterschap Limburg. Door de beek te herstellen en natuurontwikkeling binnen het gebied te bevorderen worden nieuwe kansen gecreëerd voor flora en fauna. Dit onderzoek dient als advies voor het Waterschap Limburg voor mogelijke natuurontwikkeling in en rondom de Geleenbeek. Door natuur te ontwikkelen kunnen problemen zoals zuurstofstress worden aangepakt.

Weirs, flumes and gates have been used for centuries to determine discharges in open channels. In this paper the distinct flow types, modular flow and non-modular flow, are described and applied to derive the head-discharge relations of weirs with a horizontal crest. The flow measurement structures are classified according to international rules. For a number of decades, the ability of flow measurement structures to allow fish migration has also been judged. On the other hand, some well-known fishways have been considered to act as an alternative flow measurement structure. The presentation of the Venturi gate, which combines the functions of flow measurement structure and fishway, concludes the paper

Uit een inventarisatie is gebleken dat in bijna een kwart van de zomereikenopstanden de laatste vijftien jaar veel sterfte is opgetreden. Deze sterfte vertoont duidelijke samenhang met ongunstige factoren in de (storende bodemlagen, grote grondwaterfluctuaties e.d.). De laatste paar jaar heeft de eikenprachtkever veel eiken aangetast en wellicht versneld doen afsterven. Vanwege de slechte vitaliteit van veel eiken is de komende jaren nog meer sterfte te verwachten.

There exists an impressive body of experimental evidence confirming the existence of power law relationships between various rainfall related variables. Many of these variables (such as rain rate, radar reflectivity factor and kinetic energy flux density) have a direct relevance for hydrology and related disciplines (hydrometeorology, soil erosion). There is one fundamental property of rainfall which ties all these variables together, namely the raindrop size distribution. It is the purpose of this article to explain (1) that there exist two fundamentally different forms of the raindrop size distribution, (2) how various hydrologically relevant rainfall variables are related to both these forms, and (3) how the coefficients of power law relationships between such rainfall variables are determined by the parameters of these two forms of the raindrop size distribution. The classical exponential raindrop size distribution is used as an example of a family of raindrop size distributions. Three groups of rainfall related variables are considered, namely properties of individual raindrops (size, speed, volume, mass, momentum and kinetic energy), rainfall integral variables (raindrop concentration, raindrop arrival rate, liquid rainwater content, rain rate, rainfall pressure, rainfall power and radar reflectivity factor) and characteristic sizes (median-volume diameter, volume-weighted mean diameter and mean-volume diameter). Six different consistent sets of power law relationships between these rainfall related variables and rain rate are presented, based on different assumptions regarding the rain rate dependence of the parameters of the raindrop size distribution.

The potential of airborne radar systems as tools for collecting information in support of the assessment of tropical primary forests and derived cover types was examined. SAR systems operating with high spatial resolutions and different wavelengths (i.e., X-, C-, L- and P-band) acquired data in Guyana and Colombia. Three fundamentally different information sources from the radar return signal were considered in the study: its strength or backscatter, polarization and phase, and spatial variability or texture. Radiometric, polarimetric, and textural attributes were computed from predefined image regions selected to represent five types of primary forest, selectively logged forest, secondary forest, and a mixture of nonforest cover types. Texture was found to be the most important source of information in high resolution X- and C-band images. Textural attributes computed per region made modest to good bases for automated classifications of the land cover types studied. Primary forests and logged-over forests were found to display particularly distinctive textural patterns. Backscatter values computed per region from L- and P-band radar images also made modest to good classification bases. Backscatter measurements in either a single L- or P-band channel enabled accurate classification of nonforest cover types. Reliable identification of secondary forests and logged-over forests generally asked for measurements in a minimum of two C-, L-, and/or P-band radar channels. Similarly, reliable assessment of primary forest types required observations in a minimum of three C-, L-, and/or P-band radar channels.

In plant ecology, site is a central concept. A site is the place where a plant species or plant community grows, and the site provides the set of conditions in which it lives. Within an initially homogeneous parent material, gravity-driven groundwater flow influences the site conditions through the spatial distribution of nutrients and other relevant chemical agents. Especially upward seepage may produce and maintain site conditions that are essential for various relatively rare plant species and communities. Increased attention to upward seepage among ecologists has resulted in cooperation with hydrologists and the emergence of a discipline of its own – eco-hydrology – on the boundary of two scientific fields, linked by the site concept.

In a series of three articles Calder, Hall and colleagues have recently presented an extension of Calder’s original stochastic model of rainfall interception. In their model, the dependence of rainfall interception on raindrop size is explicitly taken into account. All raindrops are assumed to have the same characteristic size, the so-called median-volume raindrop diameter D0, which is taken to be a power law function of the rain rate R. However, the D0–R relationship used in their model results from an erroneous interpretation of the general theory of the raindrop size distribution recently proposed by Sempere Torres and colleagues. As a result, this relationship is not consistent with the postulated underlying raindrop size distribution. Moreover, it seriously underestimates D0 for a given R as compared to the consistently derived D0–R relationships and to the D0–R relationship reported in a classical article on the subject of raindrop size distributions. This may seriously affect the model results presented by Calder, Hall and colleagues.

Soil moisture is considered to be the link between the water and energy balance at the land surface. The extraction of information on soil moisture from remotely sensed data, especially from microwave instruments, has always received a lot of attention. Most methods that are used to extract this information are empirical in nature. In this paper, a semiempirical model from Y. Oh et al., (1992) and a numerical inversion of the Integral Equation Method (IEM) model, introduced as "INVIEM," are applied to retrieve soil moisture over bare soil surfaces from active microwave data. The two models are compared using data from the Jet Propulsion Laboratory (JPL) air synthetic aperture radar (AirSAR) taken during the EFEDA-Spain'91 experiment and ERS-1 data from the HAPEX-Sahel'92 experiment. The comparison is performed within a general framework to estimate soil moisture from radar data, describing all necessary steps. The range of soil moisture values estimated by the INVIEM model, using the AirSAR and ERS-1 data, is in agreement with the soil moisture variation found in the field.

The structure of an aggregated soil or a dual-porosity medium is characterized by a function called the generalized surface-area-to-volume ratio, which can be measured as a distance distribution. For an isotropic structure the distance measurements may be done in the digitized image of a cross-section. A diffusion-reaction process in the soil matrix can be described by means of an equivalent cylinder system which preserves the generalized surface-area-to-volume ratio of the structure. The contribution of each cylinder radius is described by a weight function, which is obtained by transforming the measured cross-sectional distance distribution. In order to test the method, a diffusion-reaction model from the literature has been applied to spheres with a lognormal radius distribution and to an irregular, generated structure. Both geometries can be represented by just a few cylinder radii and cylinder weights. The method works best for dual-porosity structures with a convex shape. Deviations occur if parts of the matrix structure have a concave shape.

Because natural organic matter (NOM) can act as a carrier for contaminants, it is of great importance to understand its dynamic adsorption/desorption behavior. NOM is a mixture of organic molecules that vary both in chemical and physical properties. The adsorption/desorption behavior of a NOM mixture to the solid matrix cannot be adequately described using simple equilibrium sorption isotherms, like the Langmuir isotherm. Often adsorption/desorption hysteresis is found or “adsorption maxima” keep increasing slowly. In this paper a relatively simple model is developed to describe the kinetic adsorption/desorption of NOM. The model is calibrated using experimental data. Model simulations of experimental data show that adsorption/desorption hysteresis is an inherent property of a heterogeneous mixture of molecules. The composition and thus the properties of the mixture vary with the total amount of NOM added and with surface/volume ratios. Therefore the relationship between the overall adsorption and the ove...

Surface soil moisture content is highly variable in both space and time. While remote sensing provides an effective methodology for mapping surface moisture content over large areas, it averages within-pixel variability thereby masking the underlying heterogeneity observed at the land surface. This variability must be better understood in order to rigorously evaluate sensor performance and to enhance the utility of the larger-scale remotely sensed averages by quantifying the underlying variability that remote sensing cannot record explicitly. In support of the Southern Great Plains 1997 (SGP97) Hydrology Experiment (a surface soil moisture mapping mission conducted between June 18 and July 17, 1997, in central Oklahoma) an investigation was conducted to characterize soil moisture variability within remote sensing footprints (approximately 0.64 km2) with more certainty than would be afforded with conventional gravimetric moisture content sampling. Nearly every day during the experiment period, portable impedance probes were used to intensively monitor volumetric moisture content in the 0- to 6-cm surface soil layer at six footprint-sized fields scattered over the SGP97 study area. A minimum of 49 daily moisture content measurements were made on most fields. Higher-resolution grid and transect data were also collected periodically. In total, more than 11,000 impedance probe measurements of volumetric moisture content were made at the six sites by over 35 SGP97 participants. The wide spatial distribution of the sites, combined with the intensive, near-daily monitoring, provided a unique opportunity (relative to previous smaller-scale and shorter-duration soil moisture studies) to characterize variations in surface moisture content over a range of wetness conditions. In this paper the range and temporal dynamics of the variability in moisture content within each of the six fields are described, as are general relationships between the variability and footprint-mean moisture content. Results indicate that distinct differences in mean moisture content between the six sites are consistent with variations in soil type, vegetation cover, and rainfall gradients. Within fields the standard deviation, coefficient of variation, skewness, and kurtosis increased with decreasing moisture content; the distribution of surface moisture content evolved from negatively skewed/nonnormal under very wet conditions, to normal in the midrange of mean moisture content, to positively skewed/nonnormal under dry conditions; and agricultural practices of row tilling and terracing were shown to exert a major control on observed moisture content variations. Results presented here can be utilized to better evaluate sensor performance, to extrapolate estimates of subgrid-scale variations in moisture content across the entire SGP97 region, and in the parameterization of soil moisture dynamics in hydrological and land surface models. | Surface soil moisture content is highly variable in both space and time. While remote sensing provides an effective methodology for mapping surface moisture content over large areas, it averages within-pixel variability thereby masking the underlying heterogeneity observed at the land surface. This variability must be better understood in order to rigorously evaluate sensor performance and to enhance the utility of the larger-scale remotely sensed averages by quantifying the underlying variability that remote sensing cannot record explicitly. In support of the Southern Great Plains 1997 (SGP97) Hydrology Experiment (a surface soil moisture mapping mission conducted between June 18 and July 17, 1997, in central Oklahoma) an investigation was conducted to characterize soil moisture variability within remote sensing footprints (approximately 0.64 km2) with more certainty than would be afforded with conventional gravimetric moisture content sampling. Nearly every day during the experiment period, portable impedance probes were used to intensively monitor volumetric moisture content in the 0- to 6-cm surface soil layer at six footprint-sized fields scattered over the SGP97 study area. A minimum of 49 daily moisture content measurements were made on most fields. Higher-resolution grid and transect data were also collected periodically. In total, more than 11,000 impedance probe measurements of volumetric moisture content were made at the six sites by over 35 SGP97 participants. The wide spatial distribution of the sites, combined with the intensive, near-daily monitoring, provided a unique opportunity (relative to previous smaller-scale and shorter-duration soil moisture studies) to characterize variations in surface moisture content over a range of wetness conditions. In this paper the range and temporal dynamics of the variability in moisture content within each of the six fields are described, as are general relationships between the variability and footprint-mean moisture content. Results indicate that distinct differences in mean moisture content between the six sites are consistent with variations in soil type, vegetation cover, and rainfall gradients. Within fields the standard deviation, coefficient of variation, skewness, and kurtosis increased with decreasing moisture content; the distribution of surface moisture content evolved from negatively skewed/nonnormal under very wet conditions, to normal in the midrange of mean moisture content, to positively skewed/nonnormal under dry conditions; and agricultural practices of row tilling and terracing were shown to exert a major control on observed moisture content variations. Results presented here can be utilized to better evaluate sensor performance, to extrapolate estimates of subgrid-scale variations in moisture content across the entire SGP97 region, and in the parameterization of soil moisture dynamics in hydrological and land surface models.

L'humidite du sol, definie ici comme l'humidite volumetrique des 10 cm superieurs du sol, montre une tres grande variabilite. La variance observee dans un metre carre peut etre aussi grande que celle d'un champ entier. C'est pour cette raison que les mesurages d'un petit nombre de points ne peuvent pas etre representatifs de la moyenne de l'humidite du sol pour tout un champ. L'humidite du sol estimee par teledetection, en particulier l'utilisation des micro-ondes, peut fournir d'utiles informations sur la moyenne de l'humidite. Pour que ces techniques soient utiles en modelisation hydrologique, il est necessaire que l'on puisse relier les resultats obtenus ponctuellement et selon differentes resolutions spatiales. Malgre des differences de configuration des instruments il est possible dans certaines circonstances de degrader a une resolution plus grossiere les informations sur l'humidite des sols obtenues a haute resolution comme cela a ete demontre a l'aide des donnees de l'experience HAPEX-Sahel'92 et de celles de Washita'94.

Transport of reactive colloids in groundwater may enhance the transport of contaminants in groundwater. Often, the interpretation of results of transport experiments is not a simple task as both reactions of colloids with the solid matrix and reactions of contaminants with the solid matrix and mobile and immobile colloids may be time dependent and nonlinear. Further colloid transport properties may differ from solute transport properties. In this paper, a one-dimensional model for coupled colloid and contaminant transport in a porous medium (COLTRAP) is presented together with simulation results. Calculated breakthrough curves (BTC's) during contamination and decontamination show systematically the effect of nonlinear and kinetic interactions on contaminant transport in the presence of reactive colloids, and the effect of colloid transport properties that differ from solute transport properties. It is shown that in case of linear kinetic reactions, the rate of exchange of mobile and immobile colloids have a large impact on the shape of BTC's even if the solid matrix is saturated with respect to colloids. BTC's during the contamination and decontamination phase have identical shapes in this case. Moreover, the slow reactions of contaminants and colloids may lead to unretarded breakthrough of contaminants. Independent of reaction rates, nonlinear reactions lead to BTC's that are steeper during contamination than in the linear case. A characteristic aspect of nonlinear sorption is that shapes of BTC's differ during the contamination and decontamination phase. It has been observed that shapes of some of the simulated adsorption and desorption curves are similar as shapes found in experiments reported in literature. This stresses the importance of incorporating both kinetics and nonlinearity in models for coupled colloid and contaminant transport and the capability of COLTRAP to interpret experimental results. Finally, to figure out whether nonlinear processes play a role, it is very important to consider both contamination and decontamination in transport experiments.

The observations of a long-term experiment on the use of saline water were used to check the hypothesis whether models developed for drought conditions are valid for yield prediction in case of salinity. The models for the relation between yield and evapotranspiration, proposed by Stewart and Hagan (1973) and Stewart et al. (1977), were used for maize, sunflower, potatoes and soybean. The yield estimation for maize and sunflower was quite accurate, for potatoes somewhat less, but unsatisfactory for soybean. The estimated yield of soybean under saline conditions was higher than the measured one. This may be attributed to the differences in salt tolerance between soybean varieties or to an additional effect of salinity on the nitrogen supply of this legume.

Binding of radionuclides to natural colloids can significantly alter their transport behaviour in porous media. Dependent on the interaction between radionuclides, colloids and the solid matrix, radionuclide transport may be enhanced or retarded as a result of the presence of colloids. Often, equilibrium models are used to describe interactions between radionuclides, colloids and the solid matrix. However, experimental results indicate that kinetic processes may be important. In this paper, a model for coupled colloid and radionuclide transport in porous media is presented. Kinetic relationships are incorporated for the interaction between radionuclides, colloids and solid matrix. With this transport model, column experiments have been simulated, and modelling results are compared with experimental data reported in literature. It appears that kinetic interaction relationships are required to adequately model the experimental data.

The variation in soil texture, surface moisture or vertical soil moisture gradient in larger scale atmospheric models may lead to significant variations in simulated surface fluxes of water and heat. The parameterization of soil moisture fluxes at spatial scales compatible with the grid size of distributed hydrological models and mesoscale atmospheric models (∼ 100 km2) faces principal problems which relate to the underlying microscopic or field scale heterogeneity in soil characteristics. The most widely used parameterization in soil hydrology, the Darcy-Richards (DR) equation, is gaining increasing importance in mesoscale and climate modelling. This is mainly due to the need to introduce plant-interactive soil water depletion and stomatal conductance parameterizations and to improve the calculation of deep percolation and runoff. Covering a grid of several hundreds of square kilometres, the DR parameterization in soil-vegetation-atmosphere-transfer schemes (SVATs) is assumed to be scale-invariant. The parameters describing the non-linear, area-average soil hydraulic functions in this scale-invariant DR-equation should be treated as calibration-parameters, which do not necessarily have a physical meaning. The saturated hydraulic conductivity is one of the soil parameters to which the models show very high sensitivity. It is shown that saturated hydraulic conductivity can be scaled in both vertical and horizontal directions for large flow domains. In this paper, a distinction is made between effective and aggregated soil parameters. Effective parameters are defined as area-average values or distributions over a domain with a single, distinct textural soil type. They can be obtained by scaling or inverse modelling. Aggregated soil parameters represent grid-domains with several textural soil types. In soil science dimensional methods have been developed to scale up soil hydraulic characteristics. With some specific assumptions, these techniques can be extrapolated from classical field-scale problems in soil heterogeneity to larger domains, compatible with the grid-size of large scale models. Particularly promising is the estimation of effective soil hydraulic parameters from area averaging measurements through inverse modelling of the unsaturated flow. Techniques to scale and aggregate the soil characteristics presented in this paper qualify for direct or indirect use in large scale meteorological models. One of the interesting results is the effective behaviour of the reference curve, which can be obtained from similar media scaling. If the conclusions of this paper survive further studies, a relatively simple method will become available to parameterize soil variability at large scales. The inverse technique is found to provide effective soil parameters which perform well in predicting both the area-average evaporation and the area-average soil moisture fluxes, such as subsurface runoff. This is not the case for aggregated soil parameters. Obtained from regression relationships between soil textural composition and hydraulic characteristics, these aggregated parameters predict evaporation fluxes well, but fail to predict water balance terms such as percolation and runoff. This is a serious drawback which could eventually hamper the improvement of the representation of the hydrological cycle in mesoscale atmospheric models and in GCMs.

The concentration of soil phosphorus and pH correlated significantly with Landsat TM and Compact Airborne Spectrographic Imager (CASI) data, terrain position and aspect. Band ratios of the infrared and visible wavelengths are particularly significant for phosphorus. Exchangeable soil cations (that is calcium and sodium), as well as electrical conductivity, correlated less strongly with CASI and TM, but the correlations were statistically significant. Magnesium, potassium, and pH were significantly correlated with TM data, but not with the CASI data. pH and phosphorus were significantly correlated with terrain position. Three factors may individually, or in combination, be causing these results: illumination differences caused by terrain, direct reflectance of soil, or differences in vegetation (due to the influence of the soil on vegetation). A review of the literature appears to support the latter as the most likely causal factor. The results are important for establishing whether forest soils m...

The impact of soil heterogeneity on the spatially average water budget of the unsaturated zone is investigated using a framework of analytical solutions. This framework was previously shown to adequately represent the long-term average water budget as well as its transient evolution. Nine combinations of three climate and three soil types are considered, covering a wide range of different conditions. In highly conductive soils, where evapotranspiration is limited by percolation through the lower boundary, heterogeneity increases the spatially average evapotranspiration relative to the uniform (scaled average) soil. In contrast, for less conductive soils the decreasing infiltration rates due to soil heterogeneity cause evapotranspiration to become smaller. Formally derived “equivalent” parameters, which are intended to capture the effect of soil heterogeneity by means of one set of soil hydraulic parameters, are shown to depend not only on the spatially heterogeneous parameters, but on climate as well. Moreover, the “equivalent” parameters derived for the long-term average water budgets are not valid for transient behavior.

Maize and sunflower were grown in tanks filled with loam and clay, and were irrigated with water of three different levels of salinity. Predawn leaf-water potential and stomatal conductance were used as parameters for water stress. The predawn leaf-water potential of maize was higher than that of sunflower, but the effect of salinity and soil texture on the predawn leaf-water potential was the same for both crops. The stomatal conductance of sunflower was much higher and more severely affected by salinity and soil texture than the stomatal conductance of maize. Although salinity had a more serious effect on the development of leaf area and canopy dry matter of sunflower, its effect on evapotranspiration and grain yield was the same for both crops. Soil texture had a stronger effect on the development of leaf area and canopy dry matter of sunflower, which also appeared in the evapotranspiration and grain yield, indicating that sunflower is more sensitive to drought than maize.