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The sorption of nine pesticides to the aquatic macrophytes Chara globularis, Elodea nuttallii, and Lemna gibba was studied. A batch equilibrium method was used to study the sorption at five concentration levels to fresh shoots of the macrophytes. The results for the herbicides atrazine and linuron were described by nonlinear Freundlich equations, with Freundlich exponents ranging from 0.53 to 0.60. The results for the other compounds showed almost linear sorption isotherms, with Freundlich exponents ranging from 0.9 to 1.1. The highest sorption was measured for chlorpyrifos, with sorption coefficients ranging from 1,660 to 2,150 L/kg. Sorption coefficients for C. globularis tended to be lower than those for the other two macrophytes. Correlation (R(2) = 0.80) was found for the relation between the sorption coefficient (K(d)) of six pesticides and their solubility in water (S). The equation log K(d) = 3.20 - 0.65 log S can be used for a first estimate of the sorption coefficient of a pesticide to aquatic macrophytes.http://link.springer-ny.com/link/service/journals/00244/bibs/37n3p310.html

This article proposes a new approach for estimation of the transpiration rate in sparse canopies. The method relies on a combination of techniques; some of which having a successful background of solid experimental and theoretical justification, while others having only recently been introduced as promising tools for the extraction of environmental information from satellite data. The transpiration rate (λ E v ) is calculated by applying an energy balance approach to the vegetation component of the land surface: λ E v =R n v −H v , where R n v is the net radiation absorbed by the vegetation, and H v is the sensible heat flux between the leaves and the air within the canopy. R n v is calculated through the use of remote sensing and standard meteorological data by combining a conventional method for estimation of the land surface net radiation with a ground-calibrated function of NDVI (normalized differential vegetation index). H v is assessed as a linear function of the temperature difference between vegetation ( T v ) and the mean canopy air stream ( T 0 ). Because the surface temperature ( T s ) recorded by satellite contains combined information of both soil and vegetation, T v is evaluated on the basis of the linear NDVI– T s relationship for individual surface types. T 0 is assessed utilizing recent evidence that ( T s −T 0 ) is linearly related to the difference in surface temperature and air temperature above the canopy ( T s −T a ), with the slope coefficient depending only on canopy structure. The method is tested using remote sensing data ranging from ground-based, airborne, and satellite recordings. The modeled transpiration rates compared well to measurements of sapflow data and latent heat fluxes recorded for a wide range of surface types (agricultural crops, natural vegetation, forest vegetation).

We demonstrate with the help of a numerical simulation model for hysteretic water movement that for horizontal redistribution three types of flow can occur. Raats and Van Duijn [1995] previously identified these types as a reaction to a specific analysis by Philip [1991]. Depending on the initial conditions in two parts of an infinitely long soil column, there can be no flow, conventional flow from the wet part to the dry part, or unconventional flow from the dry part to the wet part. When flow occurs, the cumulative wetting and cumulative drying both show the expected linear relationship with the square root of time. Hysteresis is described according to the modified dependent domain model of Mualem [1984]. We also indicate that Kool and Parker [1987], who used this model as well, presented a erroneous expression for the differential moisture capacity of the scanning drying curve.

Many environmental and agricultural problems are not restricted to national boundaries and therefore require international cooperation if solutions are to be found. Often, these solutions require the ability to use soil data as input in simulation models, however, despite a number of recognised international standards, soil data are rarely compatible across national frontiers. This problem was encountered when creating the HY draulic PR operties of E uropean S oils (HYPRES) database. The data, which includes particle-size distributions, were collected from 20 institutions in 12 countries. Only a few of these institutions adhered strictly to a recognised international system. Therefore, interpolation of the cumulative particle-size distribution was required to achieve compatibility of particle-size distributions within the HYPRES database. In this study, four different interpolation procedures were evaluated. The accuracy of the different procedures was found to vary with size intervals between measured points of the particle-size distribution. The loglinear interpolation of the cumulative particle-size distribution has previously been used in various studies but was found to give the least accurate estimation of the four procedures. Fitting the Gompertz curve, which is a special asymmetric type of curve described by a closed-form equation, showed less sensitivity to size intervals between measured points. However, interpolation within some of the particle-size distributions was not sufficiently accurate and this procedure could not be applied to particle-size distributions where the number of measured size fractions was less than the number of model parameters. Fitting a nonparametric spline function to the particle-size distributions showed a considerable increase in accuracy of the interpolation with decreasing size intervals between measured points. As a novel approach, the similarity procedure was introduced which does not use any mathematical interpolation functions. It uses an external source of soil information from which soils are selected with particle-size distributions that match the distribution of the soil under investigation. This similarity procedure was capable of giving the most accurate interpolations. Once an extensive external reference data set with well-quantified particle-size distributions is available, the similarity procedure becomes a very powerful tool for interpolations. Based on the number and distribution of measured points on the particle-size distributions, a general rule was formulated to decide whether to fit a spline function or use the novel similarity procedure to estimate missing values. Results of this study were used to classify all soils in the HYPRES database into the same soil texture classes used in the 1:1.000.000 scale Soil Geographical Database of Europe.

The Global Soil Wetness Project (GSWP) is an ongoing land surface modeling activity of the International Satellite Land-Surface Climatology Project (ISLSCP), a part of the Global Energy and Water Cycle Experiment. The pilot phase of GSWP deals with the production of a two-year global dataset of soil moisture, temperature, runoff, and surface fluxes by integrating uncoupled land surface schemes (LSSs) using externally specified surface forcings from observations and standardized soil and vegetation distributions. Approximately one dozen participating LSS groups in five nations have taken the common ISLSCP forcing data to drive their state-of-the-art models over the 1987–88 period to generate global datasets. Many of the LSS groups have performed specific sensitivity studies, which are intended to evaluate the impact of uncertainties in model parameters and forcing fields on simulation of the surface water and energy balances. A validation effort exists to compare the global products to other forms...

The relationship between evapotranspiration of potted Ficus benjamina plants on a flooded bench fertigation system and the distribution of water in the root zone was studied in detail for a range of fertigation schedules. The physical characteristics of the peat-based potting medium were described by models commonly used in soil physics. The functioning of the substrate in practical situations was greatly affected by hysteresis in the water retention characteristic. Daily evapotranspiration of Ficus benjamina plants ranged from 0.4 to 4.1 kg/msuperscript 2 per day, of which 19-41% was lost by evaporation. Water uptake by plant roots resulted in near hydrostatic equilibrium conditions as long as the volumetric water content was above 0.22. Lower average water contents resulted in locally drier conditions in the root environment. The absorption of water during irrigation depended on the soil physical conditions and increased with decreasing volumetric water content before flooding. The majority of the water absorption occurred within the first minutes, making frequent fertigation more effective than increasing the duration. Actual buffer capacity of the potting medium was defined and by combining measured absorption during flooding with data on evapotranspiration, the derivation of minimum requirements of the fertigation schedule is demonstrated.

In the present investigation the influence of exposure duration and lipophilicity on the lethal body burdens (LBBs) of thirteen organophosphorus insecticides are investigated. The LBBs are found to differ widely between compounds, varying from 0.011 to 29 μmol·g−1 wet weight. Compounds of high hydrophobicity (log Kow > 4) tend to have high LBBs (2–29 μmol·g−1 wet weight) which are not dependent on exposure concentrations. Some, but not all, of the less hydrophobic compounds had lower LBBs which were dependent on the exposure concentration. The use of LBBs for the interpretation of aquatic toxicity data of specifically acting chemicals is discussed. The wide variation in the observed LBBs makes the solely use of LBBs for toxicity assessment far from straightforward for more specificically acting compounds.

The variability of soil water content over short distances was studied at seven experimental fields, located in southern Sweden, eastern Germany, southern France, and in the Netherlands. The soils all in use as arable land, are a sandy soil, a stony sandy soil, a loamy sand, a sandy loam, and three clay soils. Samples with a volume of 100 cm 3 were taken at close intervals in trenches at several depths to determine potential water repellency and soil water content. When dry, the topsoils of the Mellby site in Sweden (loamy sand) and the Vredepeel site in the Netherlands (sandy soil) are water repellent, whereas the soils at the other five sites are wettable, according to the water drop penetration time (WDPT) test. The variation in water content within short distances was high at most depths at all sites, with differences of 7 to 18 vol% within a horizontal distance of only centimeters to decimeters. Drier as well as wetter soil areas were visualized by contour plots of the soil water content distributions in the transects. Large differences in wetting capacity between samples taken in the topsoil and in the subsoil at several sites were assessed by measurements of the wetting rate. In many cases, subsoil samples wetted faster than topsoil samples. The severity of water repellency of sandy samples from the Vredepeel site increased remarkably at oven temperatures above 65°C. The wetting rates of dried samples from this soil were also evidently influenced by the oven temperature; samples dried at 25°C wetted immediately and reached soil water contents of 23 to 32 vol% within one hour, whereas samples dried above 85°C hardly wetted during 72 hours, as a result of the increased water repellency at higher drying temperatures.

Water repellent soils are known to inhibit water infiltration, ultimately forcing water to flow via preferential paths through the vadose zone. To study water flow and transport in a water repellent sandy soil, a bromide tracer experiment was carried out, which started in the fall, after winter wheat had been sown. Field average soil water content and bromide profiles were determined seven times during a 474 d period. A one-dimensional convection–dispersion model was used to simulate observed flow and transport. It was found that water content and bromide profiles could be successfully simulated. The main reason was that despite water repellency no distinct preferential flow occurred within this particular experimental field. Only wavy wetting fronts were observed. This was due to a uniform vegetation cover and a relatively thick A-horizon (30 cm), which contrasts with known water repellent sandy soils with thin (10 cm) top layers in which typical fingered flow patterns had previously been found.

A method is presented to calibrate transfer function-noise (TFN) models, operating at the same frequency as the input (auxiliary) variables, to sparsely or irregularly observed time series of the output (target) variable. Once calibrated, the TFN models can be used to predict or simulate the output variable at the same frequency as the input variable. Consequently, the method provides a useful tool for filling in gaps of irregularly or sparsely observed hydrological time series. Although generic and suitable for any type of time series, the method is described through the modeling of a time series of groundwater head data with precipitation surplus (precipitation minus potential evapotranspiration) as input variable. First, the TFN model is written in vector notation, yielding the state equation of a linear discrete stochastic system. Subsequently, the state equation is embedded in a Kalman filter algorithm. The Kalman filter is then combined with a maximum likelihood criterion to obtain estimates of the parameters of the TFN model for small time steps (e.g., 1 day) while using sparsely (e.g., two times a month) or even irregularly observed time series of groundwater head data. The method is illustrated using (subsets of) time series of groundwater head data with varying regular and irregular observation intervals.

The spatial variability of turbulence above a forest has been examined. Two measurement towers were erected 800 m apart within a heterogeneous mixed forest located in the north east of the Netherlands. The measurements of u*/u were analysed and subsequently used to test a surface layer model. The model simulated the magnitude of the measurements reasonably well, but measured trends were not always reproduced by the model. The variable (du/dz)/u did not adapt as quickly to the new surface as u*/u. This is in agreement with Schmid (1994), and can be explained by a local decrease in mixing length. It is recommended to adapt the mixing length near a surface transition to improve the accuracy of surface layer models of heterogeneous landscapes.

Conversion of arable land (maize) to pasture will affect the soil organic matter (SOM) content. Changes in the SOM content were studied using a size- and density-fractionation method and 13C analysis. Twenty-six years of maize cropping had resulted in a depletion of carbon stored in the macro-organic fractions (>150 μm) and an increase in the 250 μm), light (b.d. 150 μm) and light (b.d. < 1.13 g cm-3) fractions were almost completely regenerated by input of root-derived SOM. The amount of medium-weight and heavy macro-organic fractions (>150 μm; b.d. >1.13 g cm-3) in the 0- to 20-cm layer was still 40-50% lower than in the continuous pasture plots. Average half-life times calculated from 13C analyses ranged from 7 years in the light fractions to 56 years in heavy fractions. Fractionation results and 13C data indicated that mechanical disturbance (plowing) during maize cropping had resulted in vertical displacement of dispersed soil carbon from the 0- to 20-cm layer down to 60-80 cm. Conversion of arable land to pasture, therefore, not only causes a regeneration of the soil carbon content, it also reduces the risk of contaminant transport by dispersed soil carbon.

In the last decades, Dutch landscape conservation policy has developed from a sectoral into an integrated one, in which the values concerning the conservation of landscapes are strongly linked to the development of new local economic activities. This change is reflected in a new policy for the development of so-called valuable man-made landscapes (VMLs) that combines an integrated approach with a bottom-up strategy. Since 1994, 11 areas in the Netherlands have been designated as VMLs. The aim of this study is to identify and explain the differences in development between these areas. VML areas can be distinguished into three groups on the basis of their stages of development. Five main factors were found that explain the differences between these three groups: their history of regional co-operation (social), financial problems in agriculture (economic), awareness of regional identity (cultural), distance to the cities (physical, geographical) and size of the area (administrative). The problems in agriculture, regional identity and administrative size of the area explain why certain VMLs had a better start than other ones. For the future of VML-like-areas it is important that policy goals remain unchanged for decades and that VML policy is connected to urban developments.

Global satellite observations (NOAA/AVHRR) available for an extended time series (10 years) provide measurements of spectral reflectances in the red and near-infrared, which are useful to monitor the phenology of green vegetation. The methodology applied to study the phytophenological rhythms of vegetation involved the application of the Fourier analysis, used to summarize nine years of monthly NDVI-global/local area coverage (GAC/LAC) composites in a few parameters, ie, amplitude and phase values of the dominant periodic components. Continental-scale maps of vegetation units, each one characterized by homogeneous foliar phenology (maps of isogrowth zones), were calculated by using Fourier attributes such as the mean NDVI, the amplitude and the phase values at 12 and six months. Maps of isogrowth zones were obtained at continental scale for Southern Africa, South America and Europe by means of different temporal NOAA-AVHRR NDVI datasets. The main Fourier spectra of the isogrowth zones were compared with various ancillary data, ie, land cover maps, meteorological data, etc. The results highlight the importance of the Fourier analysis of the time series of NDVI observations as a reliable method of characterizing vegetation dynamics and correlating them with land cover characteristics at regional and continental scales.

The impact of the insecticide lindane (gamma-hexachlorocyclohexane) on the clearance rate (CR) of Daphnia magna was investigated using artificial beads. CR (24-h EC50: 65mgr;g L-1) was found to be a more sensitive endpoint than acute lethality for D. magna (48-h LC50: 516mgr;g L-1). The onset of the effect was rapid; after 2 h of exposure to approximately 241mgr;g L-1 of lindane a significant decrease in CR was observed. Daphnids recovered rapidly after transfer to clean water; after 24 h of exposure to approximately 250mgr;g L-1 lindane, transfer into clean water resulted in recovery to 80% of control levels within 2 h and complete recovery within 24 h.

In sandy regions of the Netherlands, current agricultural practices result in high nitrate concentrations in the upper groundwater, endangering water resources. A methodology to estimate costs of region specific solutions is applied to a groundwater protection area. It requires that for existing farms in the region variants of management are described that differ in nitrogen surplus. Costs of variants relate to investments and losses in income due to changes in farm management additional to those required by existing legislation. The expected nitrate concentration for each combination of variant and groundwater class is calculated from nitrogen and precipitation surplus and a leaching fraction. A linear programming model indicates the optimal spatial allocation of variants such that desired regional and sub-regional nitrate concentrations are reached at minimum regional costs. In the case study, current land use is grouped into non-dairy farms and various types of dairy farms and non-agricultural land use. Management variants are described for most of these groups. The LP model was used to analyze the effect of the following conditions: (a) the scale at which the tolerated maximum NO 3 concentration is effectuated; (b) the level of this maximum; (c) the hydrological situation; (d) the way farms are grouped; and (e) the biophysical situation of farms. Total regional costs show a negative, but not necessarily linear, relation with the resulting regional nitrate concentration. This relation is strongly influenced by the actual situation regarding management at the farms and the groundwater table depths in the region. For each region a specific cost–benefit relation might apply. Higher groundwater tables result in strongly reduced costs for required changes in farm management. Depending on costs and physical feasibility, measures that result in a higher groundwater table could therefore be used in addition to or replacement of measures at farm level. Calculated costs are influenced by the method of grouping of farms into farm types, the largest effect due to differentiating intensities of production. Differences between farms need therefore be considered when analyzing possibilities for and costs of measures to reduce nitrate leaching. Political decisions, specifically about the scale at which the maximum nitrate level should be effectuated (farm-type or region) also have a strong effect on the cost–benefit relation. Requiring all farm types to comply with a given maximum nitrate concentration, resulting in a specific regional average, is more costly than setting a similar regional maximum and allowing regional averaging. Methodology and preliminary results were discussed with policy-makers and representatives of farmers, environmentalists and groundwater extracting companies. They felt that these discussions helped focussing the interaction between the various groups of stakeholders. This is due to the fact that assumptions about effects of changes in land use and costs involved are made explicit and to the possibility of a more objective weighing of interests since wishes and conditions are translated into goals and constraints of the linear programming model.

A soil sampling scheme for estimating the mean extractable P concentration of fields is designed to be used as a tool for environmental regulation of the application rates of manure. The field to be sampled, is split up into geographically compact blocks of equal area that are used as strata. From each stratum one sampling point is selected by Simple Random Sampling. These samples are bulked into one composite for the field. The geographical stratification is performed by restricted least-squares clustering of raster cells using the coordinates of the midpoints as classification variables and the within-group sum of squares as the minimisation criterion. Using a variance model and a cost model, the numbers of sample points and laboratory analyses are optimised simultaneously, given a maximum allowed variance of the total error (sampling error plus measurement error). To predict the sampling variance, variograms have been estimated for 16 fields differing in land-use, soil parent material and phosphate level. A pooled relative variogram was used to predict the sampling variance for various sample sizes (5 to 50), field-areas (1 to 10 ha) and phosphate levels (for grassland 20 to 80 mg P 2 O 5 extracted in ammonimum lactate per 100 g soil, for arable land 20 to 80 mg P 2 O 5 extracted in water per 1 dm 3 soil). The cost model consists of three components: (i) fieldwork cost; (ii) field equipment cost and, (iii) laboratory cost. For the 16 fields, the predicted sampling variance of the Stratified Sampling design is 0.8 to 0.4 times the predicted variance of Simple Random Sampling if 40 points were sampled. To estimate the mean extractable P concentration with a total variance ≤9, replicate measurement of the composite only pays if the mean extractable P concentration of the field exceeds 40 to 50. This critical phosphate level increases with the maximum allowed variance of the total error.

It is Dutch national policy to maintain the landscape’s identity. Spaciousness or landscape enclosure is the characteristic of identity investigated in this study. This study develops and validates a GIS model for evaluating and monitoring the spaciousness of Dutch landscapes. Six areas are selected to represent a range of landscape conditions. The degree of spaciousness from confined to exposed is evaluated in the field at approximately 12 views in each of the six study areas. The reliability and validity of these spaciousness ratings are demonstrated by a high correlation with ratings from 30 additional people of photographs of these 72 sites (r 2 a.70). These ratings are matched with attribute data from the 25 cells corresponding to the near and middle distance zones in each of these 72 views. A raster GIS database is created for each area. The attributes recorded for each cell are the length of tree rows, and the area of woods, isolated buildings and urban blocks. Three related GIS models predicting landscape spaciousness are developed. The principle used to structure the model algorithms is that initial introduction of a small amount of landscape objects into a cell creates a rapid increase in enclosure. Translation matrices are used to combine the landscape attributes into an overall enclosure rating. The Immediate (500) and Immediate (250) models describe nearly half of the spaciousness variation seen in the field (r 2 a.45 and .49, respectively). The Neighbor model is a revision of the Immediate (250) approach that incorporates the influence of the surrounding eight cells. The Neighbor ratings describe over half of the observed variation in spaciousness (r 2 a.54). This study demonstrates that perceived spaciousness is closely related to the degree that the landscape is filled with objects, such as trees and buildings. It is anticipated that results will improve with availability of more accurate digital data. The next steps include: (1) applying the approach to other aspects influencing landscape identity, such as urbanization, (2) developing a system that uses this information to monitor and manage landscape quality, (3) exploring the effect of possible land use change scenarios on landscape identity, and (4) investigating the possible existence of perceptions related to differences in landscape use or region. # 1998 Elsevier Science B.V. All rights reserved.

In a series of community food webs from native and agricultural soils, we modeled energetics and stability, and evaluated the role of the various groups of organisms and their interactions in energy flow and community stability. Species were aggregated into functional groups based on their trophic position in the food webs. Energy flow rates among the groups were calculated by a model using observations on population sizes, death rates, specific feeding preferences and energy conversion efficiencies. From the energetic organization of the communities we derived the strengths of the mutual effects among the populations. These interaction strengths were found to be patterned in a way that is important to community stability. The patterning consisted of the simultaneous occurrence of strong top down effects at lower trophic levels and strong bottom up effects at higher trophic levels. These patterns resulted directly from the empirical data we used to parameterize the model, as we found no stabilizing patterns with random but plausible parameter values. Also, the impact of each individual interaction on community stability was established. This analysis showed that some interactions had a relatively strong impact on stability, whereas other interactions had only a small impact. These impacts on stability were neither correlated with energy flow nor with interaction strength. Comparison of the seven food webs showed that these impacts were sometimes connected to particular groups of organisms involved in the interaction, but sometimes they were not, which might be due to different trophic positions in the food webs. We argue that future research should be directed to answer the question which energetic properties of the organisms form the basis of the patterning of the interaction strengths, as this would improve our understanding of the interrelationships between energetics, community stability, and hence the maintenance of biological diversity.

For the semi-detailed characterization of inland valleys systems in the Ivory Coast, transect sampling was used to describe valley morphology, soils, and land use characteristics. Initially, for each valley one transect with a width of 200 to 400 m was proposed. More recently, use of a larger number of transects has been suggested. However, there is no statistical basis for either the number of transects to be described or their optimal width. Two sets of aerial photographs (1979, scale 1:50 000, and 1996, scale 1:20 000) of the Boundiali key area were processed and the results digitized. Using GIS, sets of transects with different width were made, and used for the determination of the sample size for estimating the land use ratio and the valley bottom ratio at valley system level with given confidence intervals. These results were validated with data from the field and with sets of pre-selected transects. The calculated sample size is based on simple random sampling and depends on both transect width and variability in the study area. For the characterization of inland valleys with reasonable confidence intervals, the sample size for random sampling is too large for the semi-detailed characterization method. Smaller samples of transects selected on the basis of sub-recent aerial photographs gave comparable results for land cover characteristics estimation as the larger samples for random sampling. There is scope for reducing sample size by formalizing criteria for transect placement on the basis of the earlier aerial photographs.

In dune slacks in The Netherlands, a decline of rare mesotrophic basiphilous plant species and their plant communities has been observed in combination with an increase of more productive systems with common, taller acidophilous plant species. This has been attributed to both natural and anthropogenic changes. In a humid climate with a precipitation surplus, as in The Netherlands, the calcium carbonate content of a calcareous soil increases with depth. However, soils in coastal dune slacks, may have a higher carbonate content in the topsoil horizon than in the underlying layers. Carbonates which buffer the pH can prolong the presence of mesotrophic basiphilous plant communities which are of high conservation value. To explain the occurrence of calcareous surface horizons in dune slacks, hydrological and micromorphological analyses were carried out in three dune slacks. Two slacks are situated on the Wadden Sea islands in the northern part of The Netherlands; one on Schiermonnikoog and one on Texel. The third slack is situated in the dunes on the island of Goeree in the southwestern part of The Netherlands. In all three slacks, carbonate occurs as mollusc and gastropod fragments (silt- or sand-sized) and as micritic nodules in the topsoil layer, due to aeolian deposition and sedimentation by water.In situ carbonate accumulation (calcitans and calcareous crusts) due to CO2 release in inundated and/or capillary rise of calcareous groundwater near, or at the soil surface. Accumulation of carbonate also occurs as a result of biological activity by algae in the topsoil of the Goeree site. In general, hydrological processes maintaining high levels of calcareous groundwater are a prerequisite for the maintenance of high carbonate levels in topsoils. Such levels are necessary for the conservation and management of basiphilous pioneer vegetation.

Dry water repellent soils are known to inhibit water infiltration, ultimately forcing water to flow via preferential paths through the vadose zone. To study water flow and transport in a water repellent sandy soil, a bromide tracer experiment had been carried out, which started in the fall after winter wheat had been sown. Despite the uniform tracer application, soil core sampling indicated that bromide concentrations varied largely from place to place. Wetter sites in the experimental field received more bromide, due to lateral transport through a thin top layer. Wetting fronts infiltrated deeper here, leading to perturbed wetting fronts in the experimental field. In contrast to what was expected, the wetting front perturbations did not grow to fingers. Numerical results indicate that this was attributed to the relatively high soil water contents during the experiment, which caused the soil to be wettable instead of water repellent. The water-entry capillary pressure of the secondary wetting branch exceeds the air-entry capillary pressure of the primary drainage branch in this case. In the opposite situation, with the water-entry capillary pressure of the secondary wetting branch beneath the air-entry capillary pressure of the primary drainage branch, perturbations would have grown to fingers. Such a situation occurs during infiltration in initially dry, water repellent soil. The results presented illustrate the effect of antecedent moisture conditions on the formation of stable and unstable wetting fronts, and its relation to the moment of tracer application.

The fate of the urea-herbicide linuron was studied in experimental ditches. Eight ditches were sprayed three times at monthly intervals, while two served as controls. As a simulation of spray drift, four doses (nominal: 0.5; 5; 15 and 50 pg.dni3) were applied in duplicate. After each herbicide application, the ditches were kept stagnant for a week, followed by a flow period of three weeks with fresh water. No clear stratifican n of linuron could be found in the water compartment. During the stagnant period, half-lives ‘a for the disappearance of linuron in the water compartment ranged from 7.2 to 11.8 days. The rate of disappearance was slower in the ditches treated with the highest dose and in colder treatment periods. A maximum of 6% of the linuron dose was found in the sediment and 1% in the macrophyte compartment. Approximately 20% of the fraction in the sediment compartment was present in the pore water. 01998 Else&r Science Ltd. All rights reserved

Relationships between nitrogen (N) output with seepage water and forest floor C/N ratios were analysed by use of three independent datasets: (i) a compilation of input-output studies in temperate forest ecosystems in Europe; (ii) a seven-year nationalDanish survey of nitrate concentrations in forest soils; and (iii) a similar one year Dutch survey. Nitrate leaching and nitrate concentrations were negatively correlated with forest floor C/N ratios in all three datasets, though the correlation was weakin the Dutch dataset. Sites with a C/N ratio below 25 leached nitrate or had elevated nitrate concentration in the three datasets. Nitrate was not present in the subsoil at sites with C/N ratios above 30 in the European and Danish data. In the less intensively monitored Dutch forest soils nitrate concentrations at C/N ratios above 30 were variable. Forest floor C/N ratios may be used to assess risk for nitrate leaching in conifer stands using>30, 25 to 30, and

Biological halogenation of aromatic compounds implies the generation of reducing equivalents in the form of e.g. NADH. Thermodynamic calculations show that coupling the halogenation step to a step in which the reducing equivalents are oxidized with a potent oxidant such as O2 or N2O makes the halogenation reaction thermodynamically feasible without the input of additional energy in the form of e.g. NADH. In a current model on the halogenation of tryptophan to 7-chloro-L-tryptophan NADH and O2 are proposed as co-substrates in a reaction in which the aromatic compound is oxidized via an epoxide as intermediate. The thermodynamic calculations thus indicate that such a route hinges on mechanistic insights but has no thermodynamic necessity. Furthermore the calculations suggest that halogenation of tryptophan and other aromatic compounds should be possible with N2O, and possibly even with nitrate replacing O2 as the oxidant.

The distribution of the Eurasian badger (Meles meles, L.) in the Netherlands is fragmented and adult mortality is high in many places because of traffic casualties. Both these facts affect the survival and dispersal of badgers in a negative way and are suggested to be the main causes of the decline of this species. For this reason the species receives special attention from the government in the national Nature Policy Plan and also from the lower administration in the provinces in their policy on physical planning and nature conservation. To evaluate changes in land use by means of spatial scenarios and conservation strategies in favour of the species, an individual-based simulation model was built that describes population dynamics in space and time. The model was used to evaluate three scenarios. The results indicate that the survival of groups benefits strongly from measures directed at lowering adult mortality. Also the (re)colonization of suitable but not inhabited areas increases the survival and is favoured by measures that encourage dispersal. The results indicate that simulation models as described are useful tools for establishing the comparative effectiveness of plans or measures aimed at increasing the viability of the species.

Soil heat flux is difficult to measure accurately and soil heat flux plates are difficult to calibrate. In this research the reference heat flux was calculated from the temperature gradient and independent thermal conductivity measurements. Reference conductivities, as measured by the non-steady state probe method, have an error of about 2%, while the temperature gradient was measured to an accuracy better than 1%. This results in very reliable reference measurements. Compared with this reference, commercially available heat flux plates have significant inaccuracies. The 1 mm thin TNO PU 43 T sensor was the most accurate with an average relative error of only 4%. A promising new technique is the in situ self calibration, as performed by the Huksefluks HFP-01-SC disc sensor. With self calibration this sensor achieves an accuracy of 5% and confers several advantages for field use. The MIDDLETON CN3 and TNO WS 31S sensors had relative errors of about 20%. The ring shaped sensor Huksefluks SH1 gave relatively poor results, because it measured, in fact, the temperature gradient instead of the heat flux. The results of this sensor remained poor after correcting for the thermal conductivity of the sand. For all sensors the same conclusions hold for non-steady state conditions with evaporation. The often used Philip (1961) correction factor is shown to be not very accurate: in only half of our experiments its use decreased the relative measuring error; and in some cases it made it worse. However, the correction remains useful as a tool for designing soil heat flux sensors; also a positive relation exists between the magnitude of the correction and the inaccuracy of the measurement.

In order to enable application of the critical load approach in the international heavy metal and POP abatement policies, critical loads of lead, cadmium, copper, zinc, lindane and benzo(a)pyrene have been calculated for Dutch forest soils. The environmental risk of atmospheric deposition of these substances on Dutch forest soils has been described in terms of the difference between the calculated present atmospheric loads and critical loads. Results indicate that the calculated critical loads strongly depend on the type of environmental quality criterium which serves as a basis for the calculation. More information on the quality criteria to be used is thus essential to gain insight in the risk of the loads of heavy metals and POPs on the terrestrial ecosystem.

When pesticides leach through the soil to the upper groundwater zone, it is important to know whether further transformation occurs before the pumping wells for drinking water are reached. Atrazine and bentazone were incubated (at 10°C) in five water-saturated sandy subsoils (collected at depths between 1·5 and 3·5 m), simulating the conditions in the field. In three subsoils with comparatively low pH and intermediate to high redox potential, atrazine was transformed gradually, to leave 1·9%, 6·2% and 17·5% of the dose after about five years. In one of these subsoils, hydroxy-atrazine was detected; the amount corresponded to half of the dose of atrazine. In one anaerobic subsoil with high pH, the transformation of atrazine was comparatively fast (half-life about 0·15 year). Another anaerobic subsoil, with similar pH and a somewhat higher redox potential, however, showed hardly any transformation. Sterilization of the first anaerobic subsoil had no effect on the rate of transformation. In the course of about five years, bentazone in the first three subsoils was transformed gradually to leave

Evaporation from a cutover raised bog in The Netherlands was modelled using a detailed, physically based evaporation model for heterogeneous vegetation and unsaturated soil water how ''SWAPS''. The model enables a quantification of the role of heterogeneity on evaporation. Micro-meteorological measurements above a vegetation of Molinia caerulea (purple moor grass) in the cutover raised bog ''Engbertsdijksvenen'' are used for calibration and validation of the model. In the present study, it is investigated how small scale heterogeneity affects the prediction of evaporation. Therefore, two hypothetical variants, which represent two possible types of small scale heterogeneity in peatlands, are evaluated: (1) peatland in which drainage has resulted in a vegetation cover of 50% purple moor grass and 50% birch, (2) peatland in which after rewetting the vegetation consists of 50% purple moor grass and 50% Sphagnum mosses. The results show that characteristics of the evaporation from purple moor grass in the two variants are dissimilar. The evaporation from purple moor grass, forming the lower components of a vegetation stand (variant I), can be accurately predicted via radiation based models. In case purple moor grass is the exposed part of the vegetation (variant 2), such models are not likely to predict the evaporation from purple moor grass correctly. An exposed, aerodynamically rough vegetation requires a model that accounts for the vapour pressure deficit as well. (C) 1997 Elsevier Science B.V.

Surface fluxes and soil moisture were measured in situ during the Echival Field Experiment in Desertification-threatened Areas (EFEDA), executed in Castilla la Mancha, Central Spain. Although the observation network had a high density (20 flux towers and 46 soil moisture plots), the area-average values at a scale of 100 km for a Mediterranean agricultural landscape were difficult to deduce. An attempt with remote sensing data has been made to study the possibility to extrapolate in situ measurements to a coarser scale. A remote sensing flux algorithm was applied to estimate (1) actual evaporation, (2) evaporative fraction, (3) bulk surface resistance and (4) top soil moisture from spectral radiances at different spatial scales. The spatial variability and area-average values were computed for two different golden days (June 12 and 29). The evaporative fraction (latent heat flux/net available energy) estimated from remote sensing was within the error bounds of the values derived from instrumented flux towers in 85% of the cases compared. The bulk surface resistance derived from the remote sensing flux algorithm could be related successfully to in situ near-surface soil moisture measurements conducted with Time Domain Reflectometers (TDR). Unfortunately, the relationship between resistance and soil moisture is shown to be space and time dependent. Nevertheless, after calibration with field scale measurements, instantaneous relationships between resistance and top soil moisture could be applied to estimate soil moisture at unsampled locations. Thereafter, the area-average top soil moisture could be assessed. It was concluded that an arithmetic means of the distributed field measurements of evaporation ( n = 13) and soil moisture ( n = 46) gives a wrong indication of the area-average values at a scale of 100 km, and that weighting factors for the areal integration can be derived from remote sensing data. During a 17-day drying period in the Special Observation Period, top soil moisture in Castilla la Mancha at a scale of 100 km decreased from 0.16 to 0.10 cm 3 cm −3 resulting in an increase of the areal bulk surface resistance from 661 to 1166 s m −1 . The associated daily evaporation at this scale decreased from 2.0 to 1.3 mm d −1 .

Classical sampling theory has been repeatedly identified with classical statistics which assumes that data are identically and independently distributed. This explains the switch of many soil scientists from design-based sampling strategies, based on classical sampling theory, to the model-based approach, which is based on geostatistics. However, in design-based sampling, independence has a different meaning and is determined by the sampling design, whereas in the model-based approach it is determined by the postulated model for the process studied. Design-based strategies are therefore also valid in areas with autocorrelation. Design-based and model-based estimates of spatial means are compared in a simulation study on the basis of the design-based quality criteria. The simulated field consists of four homogeneous units that are realizations of models with different means, variances and variograms. Performance is compared for two sample sizes (140 and 1520) and two block sizes (8 × 6.4 km 2 , 1.6 × 1.6 km 2 ). The two strategies are Stratified Simple Random Sampling combined with the Horvitz-Thompson estimator ( STSI , t HT ), and Systematic Sampling combined with the block kriging predictor ( SY , t OK ). Point estimates of spatial means by ( SY , t OK ) were more accurate in all cases except the global mean (8 × 6.4 km 2 block) estimated from the small sample. In interval estimates on the other hand, p -coverages were in general better with the design-based strategy, except when the number of sample points in the block was small. Factors that determine the effectiveness and efficiency of the two approaches are the type of request, the interest in objective estimates, the need for separate unique estimates of the estimation variance for all points or subregions, the interest in valid and accurate estimates of the estimation or prediction variance, the quality of the model, the autocorrelation between observation and prediction points, and the sample size. These factors will be assembled in a decision-tree that can be helpful in choosing between the two approaches. Models can also be used in the design-based approach. They describe the population itself, whereas in the model-based approach they describe the data generating processes. Errors in such models result in less accurate estimates, but the estimated accuracy is still valid.

Sealing, crusting and cracking of crusts of the soil surface has been observed in many parts of the world in areas with sandy, silty and loamy soils. Sealing and crust formation occurs under the influence of rain storm and drying weather. With prolonged drying, surface crusts might crack, leading to complex situations with respect to infiltration and runoff generation. Cracking of crusted loamy soils appears to be a general process. This study aims to measure the hydraulic properties of fully crusted and cracked-crusted areas and to evaluate the effects of these measurements on catchment discharge and soil loss in a loess region of the Netherlands, using the LISEM soil erosion model. Samples with minimum infiltration rates (fully crusted) and with maximum infiltration rates (cracked-crusted surfaces) were taken from fields with bare soil or winter wheat and their soil hydraulic functions were measured. The results of these measurements were used as input in the LISEM soil erosion model. Simulations of discharge and soil loss were done for each of these two land-uses and for two rain events. Additionally, simulated discharge and soil loss under actual recorded land-use were calculated. In all cases, soils with no surface cracks produced higher figures for discharge and soil loss than those where 10% of the surface crust was cracked. For a good interpretation of the results for soil loss, the spatial distribution of cracked-crusted areas and fully crusted areas has to be investigated in detail. To deal with cracked-crusted and fully crusted areas in simulation modelling, care has to be taken to accurately measure the soil physical functions representing the maximum and minimum infiltration rates. An assignment of these functions to calculation grids has to be made. As the LISEM model is capable of assigning different soil physical functions to each calculation grid, an improved prediction of the soil physical behaviour of the catchment can be simulated.

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 radiosonde measurement programme of the boundary layer during the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) is described. Typical examples of boundary layer evolution at two sites for wet and dry periods are shown. The Central Site was consistently dryer and warmer than the Southern Site and also reached higher boundary layer depths in the dry period in October. A brief analysis of surface flux variability is used to determine the errors associated with using a single average value for boundary layer input. Budget calculations using a mixed layer model show that advection plays an important role in the early morning budgets of heat and moisture. In the dry period subsidence also contributed to boundary layer heating.

The behaviour of evaporation, sensible heat and canopy conductance of fallow savannah and patterned woodland in the Sahel is studied for the HAPEX-Sahel Intensive Observation Period. Both fallow savannah and patterned woodland reach evaporation rates of 4–5 mm day−1 during the rainy part of the IOP and show a decline, after the rains have ceased, to 2 mm day−1. Sensible heat fluxes are different for the two vegetation types. This is also reflected in the behaviour of the evaporative fraction. Analysis of the vegetation surface conductance of the vegetation part shows that maximum values for fallow savannah are around 10 mm s−1, and for patterned woodland up to 40 mm s−1. The response of the conductance to vapour pressure deficit is different for the two vegetation types. This is attributed to differences in C3 and C4 species composition. The consequences of these differences for modelling vegetation-atmosphere interaction in the semi-arid tropics are discussed.

Risks and opportunities associated with maize production and fertilizer use under rainfed conditions in Kenya, mainly resulting from rainfall variability, and possible effects of variations in cost/price ratios (fertilizer/product) were identified for a cross-section of Kenya's climate-soil-maize growing environments. Production and financial risks, as well as opportunities, were quantified for maize cultivated in various agro-ecological zones (AEZ) of Kenya under alternative management practices, with special reference to nitrogen and phosphorus fertilizer application. Comparison of results of different management practices under given climate-soil-maize cultivar combinations and results from selective sensitivity analyses gave indications for the scope for adaptation to potential risks. The risk assessment approach based on crop growth modelling is suitable for taking different production goals and attitudes towards risk of farmers into account.

In this study, a simple method is described and tested for deriving initial soil moisture fields for numerical weather prediction purposes using satellite imagery. Recently, an algorithm was developed to determine surface evaporation maps from high- and low-resolution satellite data, which does not require information on land use and synoptic data. A correction to initial soil moisture was calculated from a comparison between the evaporation fields produced by a numerical weather prediction model and the satellite algorithm. As a case study, the method was applied to the Iberian Peninsula during a 7-day period in the summer of 1994. Two series of short-term forecasts, initialized from a similar initial soil moisture field, were run in parallel: a control run in which soil moisture evolved freely and an experimental run in which soil moisture was updated daily using the simple assimilation procedure. The simple assimilation resulted in a decrease of the bias of temperature and specific humidity at 2-m height during the daytime and a small decrease of the root-mean-square error of these quantities. The results show that the surface evaporation maps, derived from the satellite data, contain a signal that may be used to assimilate soil moisture in numerical weather prediction models.

In a large-scale field study, sediments and eel ( Anguilla anguilla ) samples were collected from six Amsterdam freshwater sites with varying degrees of pollution. All sediment and eel samples were analyzed for organic trace pollutants, such as polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs). In addition, the pollution-induced responses of a suite of 21 biochemical parameters in eel (notably phase I and phase II biotransformation enzymes, antioxidant enzymes, PAH metabolites, DNA adducts and serum transaminases) were measured. The resulting comprehensive database was subjected to statistical analyses in order to determine the biomarkers which were most suitable to assess inland water pollution and to classify the environmental quality of the sites. Bivariate correlation analysis, principal component analysis (PCA) and residual maximum likelihood analysis (REML) all revealed that the eel tissue levels of most PCB and OCP analyte groups were suitable to assess exposure to these contaminants, whereas PAH tissue levels were not. The phase I biotransformation enzymes in eel were found to be the most responsive to organic pollutants in the environment. Phase II enzymes and cofactors, as well as DNA adducts, were found to be less sensitive biomarkers, whereas the antioxidant enzymes and the serum transaminases did not show statistically significant correlations with pollutant levels. Similar results were obtained by means of the postulated bivariate correlation-significance index (CSI) and the multivariate PCA analysis. Discriminant analysis (DA) was used to classify the pollution status of the various sites. It appeared that the best discrimination between reference sites, moderately polluted sites and heavily polluted sites was obtained using DA on data of the nine most responsive biochemical markers. The importance of monitoring biota for the classification of the pollution status or environmental quality of freshwater sites was demonstrated in the present study, since no clear discrimination between moderately and heavily polluted sites could be made using sediment pollutant levels only. The results indicate that biological effect monitoring is the only appropriate method providing a reliable environmental risk assessment.

Empirical relationships were derived to describe the climatic control of stomatal conductance for four of the most common species of the HAPEX-Sahel study area. These species included a C3 shrub (Guiera senegalensis), a C3 forb (Mitracarpus scaber), a C4 grass (Digitaria gayanus) and the most common crop of the area, millet (Pennisetum glaucum), which is also a C4 species. For all four species the controlling climate variables were photosynthetically active radiation (PAR), vapour pressure deficit (VPD) and soil water potential, and their responses to these variables differed in a manner that was consistent with their different photosynthetic physiologies. Air temperature was not found to be a significant variable. Canopy average stomatal conductances (ḡ(s)) were estimated from leaf-level measurements and then modelled in this paper. By relating the distribution of leaf-level stomatal conductances to light attenuation through the canopy it was shown that total PAR intercepted by the canopy, divided by leaf area index (LAI) (i.e. average PAR interception), can be used to estimate ḡ(s) to a good approximation of the non-linear canopy integral as long as LAI variation in the measurements is small (e.g. less than 1.0). This assumes that the other environmental variables are constant through the canopy, but is significant in that it simplifies the procedure for the estimation of ḡ(s) and canopy conductance (g(c)). The values of ḡ(s) were limited by PAR availability for brief periods at sunrise and, for the two C4 species, there was evidence that shading within the canopy reduced the average values. However, the strongest diurnal variation was correlated with the increase in VPD, associated with increasing air temperature, which resulted in decreasing conductances during most of the day, following the peak in the early morning. The long-term decline in stomatal conductances following the last rainfall was associated with increasingly negative soil water potential. G. senegalensis retained its leaves through December with low but measurable stomatal conductances. Canopy conductances (g(c)) were estimated form ḡ(s) and leaf area. Maximum values of g(c) occurred near the end of the rains, when environmental conditions were still near optimum and LAI was high. | Empirical relationships were derived to describe the climatic control of stomatal conductance for four common species of the Hydrological and Atmospheric Pilot Experiment (HAPEX) Sahel study area. These species included a C3 shrub, a C3 forb, a C4 grass and C4 millet. For all four species the controlling climate variables were photosynthetically active radiation, vapor pressure deficit and soil water potential, and their responses to these variables differed in a manner that was consistent with their different photosynthetic physiologies. Canopy conductances (gc) were estimated from ḡs and leaf area. Maximum values of gc occurred near the end of the rains, when environmental conditions were still near optimum and leaf area index (LAI) was high.

A high‐speed centrifugation method for obtaining several milliliters of soil pore water is described. The representativeness of the obtained pore water was assessed by carrying out toxicological tests in soil and isolated pore water using the springtail Folsomia candida as test organism. It was found that effects observed in both media were similar.

The variability between surface flux measurements at the fallow sites of the three HAPEX-Sahel supersites is examined over periods of three or four consecutive days. A roving eddy correlation instrument provided a common base for comparison at each supersite. The inhomogeneity of the surface and the instrumental layout did not provide the conditions to allow the separation of the effects of instrument error from those due to the spatial variability of vegetation cover and soil moisture. Surface fluxes of sensible and latent heat and energy balance terms were intercompared at each supersite over summation timescales of 1 hour and 3 days. It is shown that, generally, HAPEX-Sahel hourly sensible heat flux and latent heat values have confidence limits of 15% and 20% respectively. The three-day period energy balance shows the combined sensible and latent heat fluxes to have a confidence limit of 3%. It is concluded that, due to the averaging effect of longer time periods and larger flux footprints on spatial inhomogeneity, confidence in the surface flux measurements increases with longer summation periods and with neutral atmospheric surface layers which characterise the rainy period of the Intensive Observation Period.

An airborne laser profiling altimeter was used to measure surface features and properties of the landscape during the HAPEX-Sahel Experiment in Niger, Africa in September 1992. The laser altimeter makes 4000 measurements per second with a vertical resolution of 5 cm. Airborne laser and detailed field measurements of vegetation heights had similar average heights and frequency distribution. Laser transects were used to estimate land surface topography, gully and channel morphology, and vegetation properties ( height, cover and distribution). Land surface changes related to soil erosion and channel development were measured. For 1 km laser transects over tiger bush communities, the maximum vegetation height was between 4-5 and 6-5 m, with an average height of 21 m. Distances between the centre of rows of tiger bush vegetation averaged 100 m. For two laser transects, ground cover for tiger bush was estimated to be 225 and 301 per cent for vegetation greater than 0-5m tall and 190 and 25-8 per cent f...

The aquatic fauna in two freshwater biotopes, namely a Nymphoides peltata-dominated site and a macrophyte-free site, were studied quantitatively in a shallow oxbow lake of the river Waal. Water, sediment and macrophyte samples were analysed. Species richness, densities, and biomass of macro-invertebrates were considerably higher in the biotope with macrophytes. Sediment-dwelling fauna constituted a quantitatively important component in the Nymphoides-dominated site. It seems that macrophyte detritus plays an important role in determining the trophic structure of the macro-invertebrate community.

The performance of a newly developed column method for pesticide transformation rate measurements in the subsoil was tested using (Z)- and (E)-3-chloroallyl alcohol as model compounds. The subsoil columns were filled in situ. In the column experiment the half-life ranged from 0.5-5.2 d for (Z)-3-chloroallyl alcohol and from 1.0-5.5 d for (E)-3-chloroallyl alcohol. The transformation rates were compared with rates in laboratory incubation systems, in which the 3-chloroallyl alcohols were transformed gradually in the first three days, with a half-life of 1.9 d for both isomers. The two methods yielded similar results for these rapidly transforming compounds.

Surface run-off may be generated when rainfall intensity exceeds infiltration capacity, or when the soil profile is saturated. Both types of overland flow may occur in hilly agricultural loess regions. It was shown with pressure head and run-off measurements that Hortonian overland flow occurs during summer rain events. A two-dimensional water flow model could simulate pressure head changes and run-off. Simulated potential run-off for the transect studied was three times as high. This indicates effects of surface ponding and the probable location of this particular transect in a region with high run-off production.

Uncertainties in major input variables in water and solute models were quantified and their effects on simulated functional aspects of soil behaviour were studied using basic soil properties from an important soil map unit in the Netherlands. Two sources of uncertainty were studied: spatial variability of basic soil properties, and the use of pedotransfer functions to predict soil hydraulic functions. Depending on the type of process studied, variability of basic soil properties explained between 40 and 99.9% of the total variance. Uncertainty of pedotransfer functions is of importance mainly when physical soil behaviour (e.g. workable days and aeration status) is studied.

Pesticide transport from the soil surface to the ground-water table was modeled by means of an analytical solution of the convection-dispersion equation, assuming steady water flow, a linear sorption isotherm and first-order transformation kinetics. Pesticide transport in the saturated zone to a well was described by means of an analytical solution of the conservation equation for a cylindrical flow system, assuming steady flow, no dispersion, linear sorption, and first-order transformation. We derived an analytical solution for this coupled unsaturated-saturated system. We limited our analysis to systems that are difficult to protect with a protection zone. The strongest fluctuations in the pesticide concentration in the well were calculated for the lowest frequency of application, shortest characteristic travel time in the saturated zone and shortest half-life in the saturated zone. A simplified model, ignoring the variation of travel times in the unsaturated zone, resulted in concentration fluctuations in the well. The peak concentrations differed less than two times from those calculated with the analytical coupled unsaturated-saturated model, except for cases with the combination of a short characteristic travel time and a short half-life in the saturated zone.