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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.

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.

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.

Met behulp van modelberekeningen (SIMFLOW en MWW) is nagegaan in welke mate het opzetten van het waterpeil als anti-verdrogingsmaatregel in een gebied de piekafvoeren doet toenemen en wat dit betekent voor de frequentie van het maaionderhoud. Voorbeeldberekeningen worden gegeven voor twee waterlopen in het stroomgebied van de Poelsbeek (Twente)

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.

De ruimte in Nederland is beperkt, waardoor de behoefte groeit om functies te combineren binnen een bepaalde ruimte. Er is dan ook sprake van een 'slag om de ruimte'

Resultaten van onderzoek naar de beperkende factoren bij natuurlijke verjonging van bossen op arme zandgronden. Naast factoren als kroonbedekking, samenstelling van de ondergroei en vraat, blijkt de dikte en aard van de humuslaag van belang te zijn. Dit heeft consequenties voor de verjongingspraktijk in oude bossen; windworp geeft de beste kansen

Langs verschillende wegen tracht men er achter te komen, welke natuur de belangstelling van mensen heeft. Nieuwe beleidsnota's en onderzoekprogramma "operatie boomhut", waarbij Intomart ingeschakeld is, getuigen daarvan

Microbial activity in soils depends on the status or the soil water, which is expressed by pressure head (h) or water content (θ). There is no unique relationship between θ and h because moisture relations exhibit hysteresis. For convenience microbial activity has usually been related to the main drying curve but in general this will lead to bias in computer models that aim to simulate microbial activity. This article aims to evaluate the magnitude of the bias resulting from the use of computer simulation models of mineralization coupled to a model that follows the hysteresis of water relations in soil. Simulations of mineralization were found to accumulate a bias of more than 15% of the mean annual mineralization by harvest. This bias was found to be most serious where soil is continually subject to severe wetting and drying cycles such as can be found in irrigated and tropical agriculture. | Microbial activity in soils depends on the status of the soil water, which is expressed by pressure head (h) or water content (θ). There is no unique relationship between θ and h because moisture relations exhibit hysteresis. For convenience microbial activity has usually been related to the main drying curve but in general this will lead to bias in computer models that aim to simulate microbial activity. This article aims to evaluate the magnitude of the bias resulting from the use of computer simulation models of mineralization coupled to a model that follows the hysteresis of water relations in soil. Simulations of mineralization were found to accumulate a bias of more than 15% of the mean annual mineralization by harvest. This bias was found to be most serious where soil is continually subject to severe wetting and drying cycles such as can be found in irrigated and tropical agriculture.

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.

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 effect of the rate and method of nitrogen application on nitrogen uptake and utilization by broccoli (Brassica oleracea var. italica) was studied in four field experiments. The methods of application were broadcast application vs band placement and split application. Maximum uptake of nitrogen by the crop was around 300 kg ha-1. In one experiment, band placement positively influenced nitrogen uptake. Split application did not influence nitrogen uptake. Nitrogen application resulted in a higher head dry matter production, but the efficiency of nitrogen utilization for the production of head dry matter decreased with higher amounts of nitrogen applied. Nitrogen application decreased the dry matter content of the heads. In half of the experiments, band placement of nitrogen fertilizer resulted in extra head dry matter production and lower head dry matter contents. At the optimum rates of band placed nitrogen application, the nitrogen harvest index in the experiments ranged from 27 to 30%. The amount of mineral nitrogen in the soil at harvest generally increased with increasing amounts of nitrogen applied. Band placement resulted only in one experiment in lower amounts of mineral nitrogen in the soil at harvest. The mineral nitrogen in the soil at harvest can be unevenly horizontally distributed, both with broadcast application and band placement of nitrogen fertilizer. The amount of nitrogen unaccounted for at harvest increased with increasing amounts of nitrogen applied, but was always less than the amount of nitrogen in crop residues. At the optimum rates of band placed nitrogen application, the amount of nitrogen in crop residues ranged from 120 to 155 kg ha-1. With broccoli cultivation, the nitrogen in the crop residues forms the single largest source of potential loss of nitrogen to the environment.

Traditioneel is door hydrologen en micrometeorologen aan verdamping in de zomer meer aandacht gegeven dan aan verdamping in de winter. Dit is te verklaren uit het grote belang dat de landbouw heeft bij het juist voorspellen van vochttekorten in de zomer in verband met mogelijke opbrengstdalingen. De winter is echter de belangrijkste periode voor aanvulling van grondwater. Bovendien bepaalt de hydrologische balans van de winter in hoge mate de voorjaarsgrondwaterstand, die weer grote invloed heeft op het mogelijke voorkomen van plantensoorten in de natuur. Daarnaast geven berekeningen met regionale hydrologische modellen in de winter vaak een te hoge afvoer te zien. Een van de mogelijke redenen hiervoor is dat de verdamping tijdens de winterperiode onderschat wordt

Gegevens uit het landsdekkend onderzoek door Intomart, in opdracht van het ministerie van Landbouw, Natuur en Visserij, zijn behandeld. De invloed van verstedelijking en leeftijd op de natuurwensen blijkt groot te zijn

Studies have been carried out in two fully stocked, fast growing Douglas-fir plantations of the Dutch ACIFORN project in three consecutive years, to obtain information on fine root densities (Olsthoorn 1991). For the present paper, data collected in early summer 1987 were used to study the relation of fine root density and proximity to the nearest tree or the dominant tree. A large number of samples (37 in one site and 55 in another) was collected in a small plot (10 x 1 m). Two distances were measured at each sampling point: the distance to the nearest tree and the distance to the tree with a dominant crown above that point. There was large variability in fine root density in the samples. Tests with different regression models showed a distinct rooting pattern for one of the two locations. It is concluded that systematic errors in the assessment of fine root density can arise when sampling points are chosen at a constant distance from trees. For Douglas-fir, this systematic error could have been an overestimation of the fine root density by up to 10%. These systematic errors can be avoided easily, using a stratified random design or a random sampling design. When trees are spaced irregularly, a grid sampling design is also appropriate.

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.

Besproken wordt "switch on", de winnende inzending van de ontwerp-prijsvraag levende energie

As part of the preparation for the Large-Scale Biosphere Atmosphere Experiment in Amazonia, a meso-scale modelling study was executed to highlight deficiencies in the current understanding of land surface atmosphere interaction at local to sub-continental scales in the dry season. Meso-scale models were run in 1-D and 3-D mode for the area of Rondonia State, Brazil. The important conclusions are that without calibration it is difficult to model the energy partitioning of pasture; modelling that of forest is easier due to the absence of a strong moisture deficit signal. The simulation of the boundary layer above forest is good, above deforested areas (pasture) poor. The models' underestimate of the temperature of the boundary layer is likely to be caused by the neglect of the radiative effects of aerosols caused by biomass burning, but other factors such as lack of sufficient entrainment in the model at the mixed layer top may also contribute. The Andes generate patterns of subsidence and gravity waves, the effects of which are felt far into the Rondonian area. The results show that the picture presented by GCM modelling studies may need to be balanced by an increased understanding of what happens at the meso-scale. The results are used to identifykey measurements for the LBA atmospheric meso-scale campaign needed to improve the model simulations. Similar modelling studies are proposed for the wet season in Rondonia, when convection plays a major role.

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.

Beschrijving van een methode waarbij m.b.v. groeigrafieken voor boomsoorten de bijgroeiverandering van bomen ten gevolge van grondwaterstandverandering (bijv. door grondwaterwinning) kan worden vastgesteld. Dit om de financiele schade te kunnen kwantificeren

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.