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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.

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

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

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.

Grass-covered heavy basin clay soils in the Netherlands appeared to be water repellent. Water repellency in the top layers of these soils occurred mainly as a coating on the aggregates. The variation in soil moisture content over short distances was studied by sampling the soil 10 times during the period August 31, 1993, to December 22, 1994. Each time, 35 samples (100 cm3) were taken in close order over a distance of 195 cm at depths of 0–5, 10–15, 20–25, and 30–35 cm. Differences between minimum and maximum soil moisture contents were high in all layers sampled, occasionally as much as 28 vol %. When the clay soil is dry, a major proportion of the water from precipitation or sprinkler irrigation may flow rapidly through shrinkage cracks to the subsoil, bypassing the matrix of the clay peds. However, preferential flow is not limited to macropore flow: irregular wetting patterns are also formed through the small pores of the matrix. The relationship between dry bulk density and volumetric water content was found to be positive when the clay soil was relatively dry and negative when it was relatively wet.

A knowledge of the area-averaged latent heat flux 〈λE〉 is necessary to validate large-scale model predictions of heat fluxes over heterogeneous land surfaces. This paper describes different procedures to obtain 〈λE〉 as a weighted average of ground-based observations. The weighting coefficients are obtained from remote sensing measurements. The remote sensing data used in this study consist of a Landsat thematic mapper image of the European Field Experiment in a Desertification-Threatened Area (EFEDA) grid box in central Spain, acquired on June 12, 1991. A newly developed remote sensing algorithm, the surface energy balance for land algorithm (SEBAL), solves the energy budget on a pixel-by-pixel basis. From the resulting frequency distribution of the latent heat flux, the area-averaged latent heat flux was calculated as 〈λE〉 = 164 W m−2. This method was validated with field measurements of latent heat flux, sensible heat flux, and soil moisture. In general, the SEBAL-derived output compared well with field measurements. Two other methods for retrieval of weighting coefficients were tested against SEBAL. The second method combines satellite images of surface temperature, surface albedo, and normalized difference vegetation index (NDVI) into an index on a pixel-by-pixel basis. After inclusion of ground-based measurements of the latent heat flux, a linear relationship between the index and the latent heat flux was established. This relationship was used to map the latent heat flux on a pixel-by-pixel basis, resulting in 〈λE〉 = 194 W m−2. The third method makes use of a supervised classification of the thematic mapper image into eight land use classes. An average latent heat flux was assigned to each class by using field measurements of the latent heat flux. According to the percentage of occurrence of each class in the image, 〈λE〉 was calculated as 110 W m−2. A weighting scheme was produced to make an estimation of 〈λE〉 possible from in situ observations. The weighting scheme contained a multiplication factor for each measurement site in order to compensate for the relative contribution of that site to 〈λE〉. It was shown that 〈λE〉 derived as the arithmetic mean of 13 individual in situ observations leads to a difference of 34% (〈λE〉 = 104 W m−2), which emphasizes the need for improved weighting procedures.

The development of accelerated degradation of nematicides was studied in incubation experiments with soils collected from trial plots at three long-term experimental fields. The plots had been treated annually for periods of 3–10 years with aldicarb, oxamyl, ethoprophos, fenamiphos or 1,3-dichloropropene (1,3-D). At one site (sandy soil; pH 7.3) highly accelerated degradation rates were measured for aldicarb, oxamyl and ethoprophos in soil from plots treated three times. After 5 years without further treatments, these high degradation rates were still present for aldicarb and oxamyl, but not for ethoprophos. At another site (sandy soil; pH 5.6) little or no increase in the degradation rates of aldicarb, oxamyl and ethoprophos was measured in soil from plots treated 10 times. Three annual applications of fenamiphos to a loamy soil (pH 7.7), doubled the rate of loss of the total toxic residue compared with the untreated soild. Fenamiphos and oxamyl did not show cross-enhancement when incubated in ethoprophos-adapted soils. After a lag-time of 4–9 days degradation of 1,3-D was very rapid in all soils including controls which had been treated only once or twice well before the sampling dates. Seven annual applications of 1,3-D resulted in only slightly shorter lag-times. An interval of 5 years without treatment resulted in only slightly longer lag-times. Simultaneous incubation of a commercial dose of the fumigant methyl isothiocyanate inhibited the fast degradation of 1,3-D.

The success of water management in large irrigation schemes with composites of soil, crop, wetness and micro-meteorological conditions is difficult to quantify. Performance assessment indicators, being among others a function of evaporation, are useful tools to evaluate the actual functioning of an irrigation system. The inevitable spatial variability of evaporation in large irrigation schemes makes its determination with conventional point measurements almost impossible. A new remote sensing evaporation parameterization algorithm has been tested with high resolution Landsat Thematic Mapper data for the Eastern Nile Delta, Egypt. Although the implementation of such an algorithm requires assumptions to be made, the current case study shows that these assumptions do not hamper the estimation of actual and potential evaporation at regional scale. The actual evaporation has been used to express the uniformity of crop water use which is related to the equity of irrigation water distribution. The coefficient of variance in actual evaporation between 53 differenent irrigation districts is 10% on average. The relative evaporation was considered to determine whether the crop was adequately irrigated. The relative evaporation was more than 75% for 48 out of 53 irrigation districts. It is concluded, that improved information on actual crop growth conditions through remote sensing provides an essential insight into the planning of real-time and seasonal irrigation water deliveries.

Soil physical characteristics are important input parameters for simulation modelling of unsaturated flow in soils and associated solute flow. The determination of soil water retention and hydraulic conductivity curves in the laboratory is laborious and expensive. For modelling studies that require characteristics for many soil horizons, such as regional studies or scenario studies, it may be impossible to measure all the necessary characteristics. An alternative would be to use characteristics inferred from readily available soil data by class-pedotransfer functions. In this study such a comparison was made for six sites on sandy soils in the Netherlands using the soil-water model SWACROP with soil physical characteristics from either laboratory measurements or from a standard series as input. For this the simulated pressure head values and moisture content values were compared with measured values at eight different depths using statistical criteria. Furthermore two functional criteria, i.e. the number of workable days and number of days with possible drought, were inferred from simulated pressure head values and again the different results were compared. It was found that simulation results were not significantly different, implying that standard series or class-pedotransfer functions could be used in studies like these for simulating the unsaturated water flow regime in sandy soils on field/farm level or regional level. Differences for specific criteria for individual sites were sometimes substantial and in such cases (at field level) it will make a difference which soil physical characteristics are used.

Wetting front instability resulting in fingered flow has been found in both wettable and nonwettable soils. Understanding how and when this phenomenon occurs under field conditions is greatly limited. Laboratory research has resulted in a number of expressions for finger diameter. In this paper we test the applicability of one of these equations for three different soils in the Netherlands where detailed soil sampling of moisture content was done earlier. In addition, information needed for finger prediction, such as the main wetting and drying loops of the soil moisture characteristic curves and the unsaturated and saturated soil conductivities, were measured in the laboratory. Results show that predicted finger diameters for the two sandy soils agreed well with the observed moisture patterns, while for the loess soil the wetting front was flat as predicted. The finger diameters in dry soil were based on the main wetting loop and in the wet soils were dependent on the main drying loop.

Simulated estimates of crop yield were made for rainfed rice in a 50 ha dryland rice area. The aim was to investigate how soil units and management units of different sizes with different yields could be used to obtain values for areas of land. Two procedures were applied. First, yields were simulated at six sites, that were representative of six soil mapping units. Second, yields were simulated using soil information from 133 auger sites, and were interpolated over management units using block kriging. Differences between the two procedures for the total area and for a test set of 22 additionally sampled locations were small. A 60% increase in precision was achieved when relatively large management units were defined. A Geographical Information System was used to identify areas with greatest yield potential for rainfed dryland rice. Statistical analysis showed that the six soil units could be grouped into three yield classes. The largest yields were obtained for a sub-area comprising 11% of the survey area which was associated with a slowly permeable Bg horizon in the soil profile. It was concluded that the best procedure for the spatial interpolation of simulated rice yield should be based on preliminary simulation of crop yields. A sensitivity analysis of the impact of weather variability and soil heterogeneity on the variation of yield was useful to detect the importance of their contributions. The procedures developed in this study are of value in obtaining a reliable estimate of average yield, and can consequently be used for associated cost-benefit calculations.

Rapid response tensiometers or TDR (time domain reflectometry) probes in soil profiles, or early arrival of solutes in groundwater or drainwater after a rain event, provide direct evidence of preferential flow in soils. However, little information about the amount and magnitude of preferential flow paths is obtained by such measurement methods. Here, two intensively sampled vertical trenches illustrate the effect of sample spacing and the effect of sample size on the detection of preferential flow paths. In a water-repellent sandy soil, a sample spacing of up to 22 cm over a distance of several metres is just sufficient to collect information about preferential flow paths. Using larger sample spacings, the water content distributions apparently became more horizontally stratified. Increasing the sample size by pooling pairs of adjacent 100 cm 3 soil samples over a distance of several metres, still allowed the detection of preferential flow paths. Preferential flow paths were no longer observed for larger sample sizes. Enlarging the sample size reduces the calculated standard deviation and coefficient of variation. As preferential flow paths may vary in space and time, so the optimal number of samples to detect these paths in vertical trenches may vary, indicating that sampling strategies need to be flexible in design.

The transport of a bromide tracer was studied in a heavy clay soil. After applying the tracer on day 0, water and bromide content profiles in the unsaturated zone were determined on days 6, 46, 209, 335, and 572. Furthermore, water samples were taken frequently from the groundwater and drain discharge and analyzed for bromide content. In the clay soil studied, solute transport occurred in three domains: large continuous macropores, smaller more tortuous mesopores, and the pores inside the soil aggregates. Bromide transport through the first domain, i.e., preferential flow through large macropores, amounted only to a few percent. This resulted, however, in maximum bromide concentrations in groundwater and drain discharge. Quantitatively, the most important solute transport occurred through the mesopores. This transport was characterized by high spatial variability, significant lateral transport, and low mobile water volumes (6%). At the field scale this resulted in the rapid leaching of solute in comparison with earlier experiments on sand and loam soils. After only 6 days and 34 mm net precipitation, the average bromide concentration peak had reached a depth of 55 cm. In spite of the heterogeneous transport and the presence of preferential flow, field-average solute concentration profiles exhibited regular and smooth shapes. Solute transport inside the soil aggregates played only an indirect role, i.e., via solute retardation as a result of convection and diffusion into the aggregates.

The DLO Winand Staring Centre studied the purification efficiency of an installation based on the Carbo-Flo process in nine agricultural and horticultural sectors in the Netherlands. Waste water, contaminated with agricultural pesticides, was treated using the installation. Purification is carried out by flocculation and carbon adsorption processes. Chemical and toxicological analyses show that a high degree of purification is possible.

Pesticide transport through the unsaturated zone was modelled with an analytical solution of the convection-dispersion equation assuming steady water flow, a linear sorption isotherm and first-order transformation kinetics. Pesticide behaviour in the saturated zone was described with an analytical solution of the mass balance equation for a cylindrical flow system assuming steady flow, no dispersion, linear sorption and first-order transformation. This simplified model for the unsaturated-saturated soil system was developed to identify the processes and parameters with the greatest impact on the fraction of applied pesticide reaching a drinking water well. Leaching from the unsaturated zone was highly sensitive to the parameters describing travel time and transformation rate. Leaching increased when heterogeneity of the soil was taken into account. Pesticide arrival in the well was only moderately sensitive to the characteristic travel time and transformation rate in the aquifer. However, this sensitivity increases if zones without pesticide application were introduced around the wells (protection zones). For representative sandy soils under average Dutch rainfall conditions, processes in the unsaturated zone had a much larger impact on pesticide arrival in the wells than processes in the saturated zone. Protection zones reduced pesticide transport to wells substantially if their half-life was much smaller than the characteristic travel time of the pesticide in the aquifer.

Simplified steady state mass balance model for critical load (CL) estimation was applied to a test area in Japan to evaluate applicability of the model. Three different criteria for acidification limits (1: [Al3+]

The use of satellite remote sensing is indispensable to improving the understanding of the processes of land surface-atmosphere interactions on a scale of 10 000 kmr in HEIFE. Because of their high spatial resolution, Landsat TM observations can be compared directly with surface measurements and be used for this purpose. The relevant data set and a state-of-the-art data processing methodology are discussed. A summer scene of TM was used to produce a set of maps of surface albedo, vegetation index, surface temperature, net radiation and other basic energy balance components of the whole area. Statistical analysis based on these maps revealed some quantitatively significant land surface parameters. Future developments are discussed.

Land surface models describe the exchange of heat, moisture and momentum between the land surface and the atmosphere. These models can be solved regionally using remote sensing measurements as input. Input variables which can be derived from remote sensing measurements are surface albedo, surface temperature and vegetation cover. A land surface model using those land surface characteristics is presented i.e. the Surface Energy Balance Index (SEBI) model. This model uses the observed temperature difference between the land surface and atmosphere as an indicator for evapotranspiration.Spatially distributed land surface model results can be used as a boundary condition for numerical weather predicton models. The results should therefore be aggregated from the remote sensing pixel scale to the atmospheric model scale. However aggregated values will differ when derived from remote sensing data with different resolutions. This difference, the error due to aggregation is caused by two different aspects: land surface heterogeneity and non-linearity of the land surface model. Two approaches are presented to quantify the error due to aggregation: the linearization approach, where the land surface model is approximated by a Taylor expansion and a geometrical approach where the range of valid results for the land surface model is derived using a convex hull.To measure the heterogeneity of land surfaces, the concept of length scale is introduced. The wavelet transform is being used to derive the length scale of the land surface characteristics. The wavelet variance derived from the Fast Wavelet Transform using the Haar wavelet is a good indicator for the variability of land surface characteristics at different spatial scales. For three different data sets the length scale of land surface characteristics have been derived: Barrax, Spain, the Jornada Experimental Range, USA and the Central Part of the Netherlands.The two approaches for quantifying the error due to aggregation have been verified using the three data sets. The results obtained by the linearization show that aggregation error can indeed be estimated. For the three test sites the large scale error did not exceed 10 %. However the results based on the convex hull analysis show that the large scale error due to aggregation can be much larger than observed for the three test cases. Therefore low resolution remote sensing data cannot be used a priori as input for land surface models.

In large parts of the Netherlands, agricultural potential is largely determined by water table depth. In turn, the water table depth is controlled by the surface water levels of a network of drainage ditches. Consequently, the gross return from agricultural production can be maximised by manipulating the surface water levels in these ditches. The traditional mode of surface water control consists of a weir with a fixed crest level at the outlet of a control area or catchment. A recent development is the use of groundwater dependent surface water control: the crest level of an adjustable weir is set according to measurements of the water table depth. For both modes of surface water control the crest levels of the controlling weir are adapted to the surface elevation at some reference location. The gross return from agricultural production thus depends on both the mode of surface water control and the choice of the reference location. A method is developed to estimate the performance of a mode of surface water control when adapted to a given reference location. Performance is measured in terms of the estimated average gross return of a control area (in Dutch guilders per ha per year). The method combines a soil water model with detailed surface elevation data from laser scanning and accounts for the unknown spatial variation of soil-physical and hydrological properties by means of a Monte Carlo approach. Although it is developed for the specific purpose of evaluating modes of surface water control, the method presented here is more generic. It is in fact a general procedure that uses densely sampled auxiliary information for mapping the results of expensive model calculations and the associated uncertainty. The method is applied in a control area in the north of the Netherlands. It is used to compare the performance of a traditional mode of surface water control with a groundwater dependent mode of surface water control and to estimate the best reference location for both modes. It is found that the traditional mode of surface water control is best adapted to a reference location from the higher parts of the control area, while the groundwater dependent surface water control is best adapted to a reference location from the lower parts. Maximum values of estimated average gross return that can be obtained for both modes of surface water control are approximately the same. However, contrary to the traditional mode of surface water control, the maximum value for groundwater dependent surface water control is obtained almost without adversely affecting the farmers in the lower parts of the control area.

A review is given of the state of knowledge in the field of assessing climate change impacts on agricultural crops and livestock. Starting from the basic processes controlling plant growth and development, the possible impacts and interactions of climatic and other biophysical variables in different agro-environments are highlighted. Qualitative and quantitative estimations of shifts in biomass production and water relations, inter-plant competition and crop species adaptability are discussed. Special attention is given to the problems encountered when scaling up physiological responses at the leaf and plant level to yield estimates at regional to global levels by using crop simulation models in combination with geo-referenced, agro-ecological databases. Some non-linear crop responses to environmental changes and their relations to adaptability and vulnerability of agro-ecosystems are discussed.

In addition to a survey of the soils under 150 forest stands on non-calcareous sandy soils, the chemical composition of the soils under 40 stands on non-calcareous loess soil, 30 stands on non-calcareous clay soils and 30 stands on oligotrophous peat soils have been examined, to assess the current status with repect to acidification and eutrophication, and the provide data for further studies. Only the clay soils are not yet seriously affected by the atmospheric inputs. The loess soils are generally considerably acidified, except the alluvial loess soils. The peat soils show a considerable eutrophication, especially in the topsoil, whereas anthropogenic acidification can hardly be separated from their natural acidity. Most investigated heavy metals show elevated concentrations, both in the humus layer and in the top soil, but serious pollution is not found.

Spatio-temporal patterns of soil moisture status highly affect the heterogeneity of soil water and solute transport and leaching of chemicals to the groundwater. In order to quantify and describe spatial variability of ecologically highly relevant spatial and temporal processes linked to soil moisture at the land surface, the spatio-temporal covariance structure and the reasons for its change in time need to be identified. Therefore, soil water pressure head was monitored in two surface horizons between April and November 1995 at two field sites with a shallow ground water table, a sandy loam and a heavy clay soil in north-east Germany. For the 10- and the 30-cm depth of the sandy loam soil and for the 30-cm depth of the heavy clay soil the variance of soil water pressure head h (on log basis) was large under wet conditions. With decreasing soil water pressure head the variance of log 10 (− h ) decreased to a critical value, for which a spatial correlation structure disappeared. With further drying, the variance of log 10 (− h ) increased again, and a spatial range of correlation existed. During drying, temporally stable variation patterns developed at both field sites. The change of variance of log 10 (− h ) which is probably associated with changing degree of heterogeneity of flow conditions validates the findings of Roth (1995) obtained from stochastic flow model calculations. In general, the temporal correlation length was better defined than that of spatial correlation. At both depths, correlation lengths of the sandy loam were larger than those of the heavy clay soil. Random spatial variation of log 10 (− h ) occurred under conditions when the hydraulic gradient was close to zero. With our experimental design we could identify (i) temporal persistence of spatial patterns and correlation ranges of soil water status that can be used for representativity studies, (ii) the within-site variation of land surface moisture status, and (iii) a basis for description of spatial processes of effective soil properties that are linked to soil water status at the land surface.

A simple one-dimensional stochastic model was developed for quantifying groundwater dynamics and model accuracy. Due to its soil physical backbone, the model is somewhere between pure empirical models on the one hand, and complex physical-mechanistic models on the other. Hence, it is envisaged that it is more effective in scenario studies than pure empirical models, and more easily to operate than complex physical-mechanistic models. A case study showed that the model was capable of predicting various groundwater characteristics.

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.

Many environmental studies on the protection of European soil and water resources make use of soil water simulation models. A major obstacle to the wider application of these models is the lack of easily accessible and representative soil hydraulic properties. In order to overcome this apparent lack of data, a project was initiated to bring together the available hydraulic data which resided within different institutions in Europe into one central database. This information was then used to derive a set of pedotransfer functions applicable to studies at a European scale. These pedotransfer functions predict the hydraulic properties from parameters collected during soil surveys and can be a good alternative for costly and time-consuming direct measurement of these properties. A total of 20 institutions from 12 European countries collaborated in establishing the database of HYdraulic PRoperties of European Soils (HYPRES). This database has a flexible relational structure capable of holding a wide diversity of both soil pedological and hydraulic data. As these data were contributed by 20 different institutions it was necessary to standardise both the particle-size and the hydraulic data. A novel similarity interpolation procedure was successfully used to achieve standardization of particle-sizes according to the FAO clay, silt and sand particle-size ranges. Standardization of hydraulic data was achieved by fitting the Mualem-van Genuchten model parameters to the individual (h) and K(h) hydraulic properties stored in HYPRES. The HYPRES database contains information on a total of 5521 soil horizons (including replicates). Of these, 4030 horizons had sufficient data to be used in the derivation of pedotransfer functions. Information on both water retention and hydraulic conductivity was available for 1136 horizons whereas 2894 horizons had only information on water retention. Each soil horizon was allocated to one of 11 possible soil textural/pedological classes derived from the six FAO texture classes (five mineral and one organic) and the two pedological classes (topsoil and subsoil) recognised within the 1:1&unknown;000&unknown;000 scale Soil Geographical Data Base of Europe. Next, both class and continuous pedotransfer functions were developed. By using the class pedotransfer functions in combination with the 1:1&unknown;000&unknown;000 scale Soil Map of Europe, the spatial distribution of soil water availability within Europe was derived.