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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.

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.

The role of water-repellency in forming preferred flow paths and in accelerating solute transport is explained on the basis of extensive field measurements in a grass-covered water-repellent sandy soil in the Netherlands. When the 9 cm thick humous topsoil was wetted to 20-25 vol.%, preferred flow paths, or fingers, were formed in the sandy subosil. The fingers started at the layer interface at `weak' spots with a relatively low degree of potential water-repellency, and protruded vertically into the sandy subsoil. The fingers disappeared during dry periods and recurred at the same locations during new rain events.

Meststoffen en andere chemicaliën komen vaak sneller in het grondwater terecht dan modellen voorspellen. De meeste modellen zijn namelijk gebaseerd op de veronderstelling dat de bodem homogeen is, het water in de onverzadigde zone verticaal infiltreert en het vochtfront evenwijdig is aan het bodemoppervlak. In werkelijkheid stroomt het water vaak via preferente banen door de bodem. Preferente stroming heeft diverse oorzaken en kan in nagenoeg alle gronden optreden. In dit artikel wordt naar voren gebracht dat regenwater zich in zavel-, klei- en veengronden niet alleen snel naar de ondergrond verplaatst door scheuren en gangen, maar dat tevens de bovengrond ongelijkmatig bevochtigd wordt, waardoor vochtpatronen in de matrix van deze gronden ontstaan

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

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.

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.

Water and solute input rates on the soil surface may vary considerably from place to place. Distribution flow, i.e., the process of water and solute flow in a lateral direction over and through the very first millimeter or centimeter of the soil profile, is an extremely important process in distributing the rainfall toward places where vertical transport occurs. This study was carried out to quantify the process the distribution flow and its underlying process mechanism. A KBr tracer was applied on two water repellent sandy soils to follow the actual flow paths of water and solutes in the upper part of the profile. On both experimental fields, distribution flow actually displaced the applied bromide laterally through a very thin layer on the top of the soil profile, referred to as the {open_quotes}distribution layer.{close_quotes} Distribution flow was directed to locations within the 0- to 2.5- cm layer, where the soil was the least water repellent. On these relatively wet areas, the highest concentrations of bromide could be found. There was an acceleration in the vertical transport of water and bromide on these sites. Effects of distribution flow on the local concentration of water and solutes can be expected to be evenmore » more pronounced in uneven terrains, where lateral displacements may increase from the millimeter-centimeter scale to tens of meters. Implications for modeling field-scale water and solute flow are outlined. 71 refs., 9 figs., 2 tabs.« less

A simple method is presented to assess, on a regional scale, the contamination-risk of shallow aquifers covered by cracking clay soils, with special emphasis on preferential flow of contaminants through shrinkage cracks. A water extraction area of several hundred hectares is divided into units with homogeneous soil types and hydrological conditions. For each unit, a one-dimensional computer simulation model is used to compute residence times in the unsaturated zone of the cracking clay soils. Analytical equations are used to estimate residence times in the saturated zone. Finally, a classification system is proposed to summarize and visualize the model computations in a simple way. The developed method is illustrated with a case study from the Netherlands. It is known how contamination-risk maps of water extraction areas can be obtained. Policies to prevent contamination of the aquifer can be focused on the most vulnerable areas as indicated on such a map. The contamination-risk maps, obtained using the present method, deviate sharply from the traditional, more or less concentric protection zones, computed with saturated groundwater flow models.

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

Pesticiden en andere chemicaliën blijken vaak sneller in het grondwater terecht te komen dan modellen voorspellen. De meeste van deze modellen zijn namelijk gebaseerd op de veronderstelling dat de bodem homogeen is, het water in de onverzadigde zone verticaal infiltreert en het vochtfront evenwijdig is aan het bodemoppervlak. In werkelijkheid stroomt het water vaak via preferente stroombanen door de bodem. Deze stroombanen kunnen in nagenoeg alle gronden optreden. Water en daarin opgeloste stoffen bereiken hierdoor veel sneller het grondwater dan op grond van de gemiddelde verblijftijd verwacht zou worden. Vooral ondiep grondwater en oppervlaktewater hebben door dit proces een vergrote kans op verontreiniging.

In soils with fingered flow, surface-applied solutes can reach the groundwater more rapidly than in the case of homogeneous wetting. This study was undertaken to demonstrate the occurrence of finger-like wetting patterns in a silt loam soil and a silty clay loam soil, and to investigate the influence of dry bulk density and water repellency on the observed soil moisture patterns. In both soils, dry bulk density was not a main factor in inducing wet finger-like patterns and high water content variability. Spatial variability of degree and persistence of potential water repellency and, accordingly, of soil water contents were high.

Unstable water flow in water-repellent unsaturated soils can significantly affect the processes of infiltration and soil water redistribution. A field experiment was carried out to study the effect of water-repellency on water and bromide movement in a coarse-textured soil in the southwestern part of The Netherlands. The field data were analyzed using a relatively simple numerical model based on the standard Richards' equation for unsaturated water flow and the Fickian-based convection-dispersion equation for solute transport. Water-repellency was accounted for by multiplying the water content and the unsaturated hydraulic conductivity of the soil with F, a factor equal to the volumetric fraction of soil occupied by preferential flow paths resulting from the unstable flow process. The good comparison of simulated and measured bromide concentrations suggests that the model provides a viable method for simulating unstable water flow in water-repellent soils.

Water repellency in soils is currently receiving increasing attention from scientists and policy makers, due to the adverse and sometimes devastating effects of soil water repellency on environmental quality and agricultural crop production. Soil water repellency often leads to severe erosion and runoff, rapid leaching of surface-applied agrichemicals, and loss of water and nutrient availability for crops.In general, soils become water repellent through the coating of soil particles or structural elements with water repellent organic substances originating from decaying plant material. Soil water repellency manifests itself when the water content of the soil drops below a critical level. Water flow and solute transport patterns are complex under such conditions. The present study deals with flow and transport processes in an untilled, grass-covered water repellent sandy soil consisting of three layers.Extensive tracer experiments indicate that distribution flow dominates in the humous top layer, preferential flow in the water repellent sand layer, and diverging flow in the underlying wettable zone. Preferential flow paths or fingers occur almost throughout the year. Fingers develop rapidly during severe rain storms, causing significant portions of the infiltrating water to be preferentially transported to the deep subsoil. Fingers form at sites with relatively low degrees of water repellency, and finger diameters range from 10 to 25 cm.Model simulations show that fingered flow results from hysteresis in the water retention function, and the nature of the formation depends on the shape of the main wetting and drainage branches of that function. Once fingers are established, hysteresis causes them to recur along the same pathways during subsequent rain events. In the long term, recurrence of fingers may lead to changes in physical and/or chemical properties of the soil within the fingered flow pathways. It is only under initially dry conditions, with soil water contents below the critical level, that fingers will be formed during infiltration. Under wetter conditions, with soil water contents above the critical level, wetting fronts will remain stable and no fingers will develop.Future research should focus on improving our understanding of the origins, occurrence, hydrological responses and agricultural functioning of water repellent soils. Volgens vele onderzoekers is waterafstotendheid van gronden een van de slechtst begrepen verschijnselen in de bodemkunde en de hydrologie van de onverzadigde zone. Er is dan ook grote behoefte aan intensief onderzoek op dit gebied. Dit is des te belangrijker omdat in de komende tientallen jaren bodems in allerlei nieuwe gebieden binnen en buiten Nederland waterafstotend kunnen worden als gevolg van veranderingen in het landgebruik, bodem- en waterbeheer en weersomstandigheden.

The present study suggests that many soils in the Netherlands, in natural as well as in agricultural areas, may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency can lead to the development of unstable wetting and preferential flow paths. Preferential flow has wide-ranging significance for rapid transport of solutes, such as agrichemicals, towards the groundwater and surface water, making it essential to understand this phenomenon.The persistence and degree of water repellency was examined in topsoils of nature reserves and cultivated soils, using the water drop penetration time (WDPT) and alcohol percentage tests. The severity of water repellency measured on dried soil samples, the so-called "potential" water repellency, can be used as a parameter for comparing soils with respect to their sensitivity to water repellency. In some cases, however, the severity of potential water repellency was found to be sensitive to the initial moisture content of the soil and the temperature during drying. Measurement of the "actual" water repellency on field-moist samples determines the soil fraction excluded from direct solute and water flow. However, preferential flow is a dynamic process, which is why the ratio between water repellent and wettable soil is time dependent. The "critical soil water content", below which the soil in the field is water repellent and above which the soil is wettable, was found to be a useful parameter in water repellency studies.Spatial and temporal variability in volumetric soil water content was studied in vertical transects by intensive sampling with 100 cm 3steel cylinders. Spatial variability in soil water content under grass cover was high, due to fingered flow. On arable land, vegetation and microtopography appeared to play a dominant role. This thesis provides examples of uneven moisture patterns in water repellent sand, loam, clay and peat soils with grass cover, and in cropped, water repellent sandy soils.

Water repellency in soils is currently receiving increasing attention from scientists and policy makers, due to the adverse and sometimes devastating effects of soil water repellency on environmental quality and agricultural crop production. Soil water repellency often leads to severe erosion and runoff, rapid leaching of surface-applied agrichemicals, and loss of water and nutrient availability for crops.In general, soils become water repellent through the coating of soil particles or structural elements with water repellent organic substances originating from decaying plant material. Soil water repellency manifests itself when the water content of the soil drops below a critical level. Water flow and solute transport patterns are complex under such conditions. The present study deals with flow and transport processes in an untilled, grass-covered water repellent sandy soil consisting of three layers.Extensive tracer experiments indicate that distribution flow dominates in the humous top layer, preferential flow in the water repellent sand layer, and diverging flow in the underlying wettable zone. Preferential flow paths or fingers occur almost throughout the year. Fingers develop rapidly during severe rain storms, causing significant portions of the infiltrating water to be preferentially transported to the deep subsoil. Fingers form at sites with relatively low degrees of water repellency, and finger diameters range from 10 to 25 cm.Model simulations show that fingered flow results from hysteresis in the water retention function, and the nature of the formation depends on the shape of the main wetting and drainage branches of that function. Once fingers are established, hysteresis causes them to recur along the same pathways during subsequent rain events. In the long term, recurrence of fingers may lead to changes in physical and/or chemical properties of the soil within the fingered flow pathways. It is only under initially dry conditions, with soil water contents below the critical level, that fingers will be formed during infiltration. Under wetter conditions, with soil water contents above the critical level, wetting fronts will remain stable and no fingers will develop.Future research should focus on improving our understanding of the origins, occurrence, hydrological responses and agricultural functioning of water repellent soils.

The present study suggests that many soils in the Netherlands, in natural as well as in agricultural areas, may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency can lead to the development of unstable wetting and preferential flow paths. Preferential flow has wide-ranging significance for rapid transport of solutes, such as agrichemicals, towards the groundwater and surface water, making it essential to understand this phenomenon.The persistence and degree of water repellency was examined in topsoils of nature reserves and cultivated soils, using the water drop penetration time (WDPT) and alcohol percentage tests. The severity of water repellency measured on dried soil samples, the so-called "potential" water repellency, can be used as a parameter for comparing soils with respect to their sensitivity to water repellency. In some cases, however, the severity of potential water repellency was found to be sensitive to the initial moisture content of the soil and the temperature during drying. Measurement of the "actual" water repellency on field-moist samples determines the soil fraction excluded from direct solute and water flow. However, preferential flow is a dynamic process, which is why the ratio between water repellent and wettable soil is time dependent. The "critical soil water content", below which the soil in the field is water repellent and above which the soil is wettable, was found to be a useful parameter in water repellency studies.Spatial and temporal variability in volumetric soil water content was studied in vertical transects by intensive sampling with 100 cm 3steel cylinders. Spatial variability in soil water content under grass cover was high, due to fingered flow. On arable land, vegetation and microtopography appeared to play a dominant role. This thesis provides examples of uneven moisture patterns in water repellent sand, loam, clay and peat soils with grass cover, and in cropped, water repellent sandy soils.

With prolonged rainfall, infiltrating wetting fronts in water repellent soils may become unstable, leading to the formation of high-velocity flow paths, the so-called fingers. Finger formation is generally regarded as a potential cause for the rapid transport of water and contaminants through the unsaturated zone of soils. For the first time, field evidence of the process of finger formation and finger recurrence is given for a water repellent sandy soil. Theoretical analysis and model simulations indicate that finger formation results from hysteresis in the water retention function, and the character of the formation depends on the shape of the main wetting and main drainage branches of that function. Once fingers are established, hysteresis causes fingers to recur along the same pathways during following rain events. Leaching of hydrophobic substances from these fingered pathways makes the soil within the pathways more wettable than the surrounding soil. Thus, in the long-term, instability-driven fingers might become heterogeneity-driven fingers.

International audience; Field evidence of finger formation and reformation during Successive rain events over an eight months' observation period from June 1994 until January 1995 is presented. Fingered flow pathways were monitored in a no-tilled, grass-covered water repellent sandy field soil using an automated, stand-alone TDR device. Within a 2 m long and 0.7 m deep transect, 98 three-wire probes were installed horizontally at depths of 4, 12, 20, 30, 40, 55, and 70 cm. The horizontal distance between two adjacent probes was IS cm. Finger formation occurred during distinct rainy periods and was most pronounced under heavy rainfall with initially wet topsoil conditions. The percentage of water infiltrated and transported preferentially through the fingers to the deep subsoil varied between 0 and 80%, depending on the wetting history of the soil and the rainfall characteristics.

Water flow and transport through the vadose zone of water-repellent field soils take place mainly through preferred flow paths. For modelling purposes, it is essential to know when fingers can be expected and what their average dimension is. Thereforeten soil blocks, each 1.2 m long, 0.6 m wide and 0.52 m deep, were sampled in a water-repellent sandy field soil. Fingered flow patterns were distinct in soil blocks sampled after rain events. Fingers were found where the degree of potential water repellency in the upper part of the soil was low.

When water enters a water-repellent topsoil, preferential flow paths develop and the flow bypasses a large part of the unsaturated zone. Therefore, preferential flow caused by water- repellency is expected to accelerate solute leaching to the groundwater. In soils with water-repellent toplayers and groundwater-affected wettable sublayers, a flow pattern develops that consists of three regions. In the top few centimeters, water flow converges toward the tops of the preferential flow paths. Within the preferential flow paths, water moves vertically downward. In the wettable subsoil, the flow diverges due to matric: forces. A new analytical three-region model is developed to calculate convective transport of an inert tracer in this steady-state flow system. Calculations with this model show that the wettable subsoil largely determines the residence time in the unsaturated zone and the shape of the breakthrough curve. For a sandy soil with a wettable soil of 1.20 m thickness on average, model results indicate that spreading of the solute front on the field scale caused by soil heterogeneity can be much larger than the front spreading caused by the diverging flow in the wettable subsoil.A new type of lysimeter was built to study the spatial distribution of drainage and solute leaching with a resolution of 5 cm from an undisturbed sandy soil column with a water-repellent toplayer (1.00 m 2area, 0.55 m height). The experiments provided strong support for the concepts underlying the preferential flow model. Additionally, during an eight-month period of uninterrupted experimentation, unique observations of the long-term dynamics of unsaturated flow were made. Areas of high drainage moved over lateral distances of up to 0.25 m, and the distribution of drainage over different areas with large drainage amounts varied slowly but strongly. The three-region analytical model can reproduce without calibration the breakthrough of a chloride pulse reasonably well. In combination with the support provided by the lysimeter data for the underlying concepts, this makes the model a promising starting point for a transient, numerical model for solute transport in fields with water-repellent toplayers.

The role of water-repellency in forming preferred flow paths and in accelerating solute transport is explained on the basis of extensive field measurements in a grass-covered water-repellent sandy soil in the Netherlands. When the 9 cm thick humous topsoil was wetted to 20-25 vol.%, preferred flow paths, or fingers, were formed in the sandy subosil. The fingers started at the layer interface at `weak' spots with a relatively low degree of potential water-repellency, and protruded vertically into the sandy subsoil. The fingers disappeared during dry periods and recurred at the same locations during new rain events.

The research yielded the following conclusions and results: preferential flow can be expected in recently deposited, loosely packed, wettable dune sands; preferential flow is common in most water-repellent sandy soils; distribution flow in topsoils isa process of major importance, resulting in a spatially heterogeneous distribution of water and solutes in soils; with the potential water repellency parameter soils can be compared with respect to their sensitivity to water repellency; a two-dimensionalwater flow model, including hysteresis, has been developed, which is able to simulate finger growth and persistence.

In the Netherlands, water repellent soils are widespread and they often show irregular moisture patterns, which lead to accelerated transport of water and solutes to the groundwater and surface water. Under grasscover, spatial variability in soil moisture content is high due to fingered flow, in arable land vegetation and microtopography play a dominant role. Examples are given of uneven soil moisture patterns in water repellent sand, loam, clay and peat soils with grasscover, and in cropped water repellent sandy soils. In addition, the influence of fungi on inducing soil moisture patterns is illustrated as well.

When water enters a water-repellent topsoil, preferential flow paths develop and the flow bypasses a large part of the unsaturated zone. Therefore, preferential flow caused by water- repellency is expected to accelerate solute leaching to the groundwater. In soils with water-repellent toplayers and groundwater-affected wettable sublayers, a flow pattern develops that consists of three regions. In the top few centimeters, water flow converges toward the tops of the preferential flow paths. Within the preferential flow paths, water moves vertically downward. In the wettable subsoil, the flow diverges due to matric: forces. A new analytical three-region model is developed to calculate convective transport of an inert tracer in this steady-state flow system. Calculations with this model show that the wettable subsoil largely determines the residence time in the unsaturated zone and the shape of the breakthrough curve. For a sandy soil with a wettable soil of 1.20 m thickness on average, model results indicate that spreading of the solute front on the field scale caused by soil heterogeneity can be much larger than the front spreading caused by the diverging flow in the wettable subsoil.A new type of lysimeter was built to study the spatial distribution of drainage and solute leaching with a resolution of 5 cm from an undisturbed sandy soil column with a water-repellent toplayer (1.00 m 2area, 0.55 m height). The experiments provided strong support for the concepts underlying the preferential flow model. Additionally, during an eight-month period of uninterrupted experimentation, unique observations of the long-term dynamics of unsaturated flow were made. Areas of high drainage moved over lateral distances of up to 0.25 m, and the distribution of drainage over different areas with large drainage amounts varied slowly but strongly. The three-region analytical model can reproduce without calibration the breakthrough of a chloride pulse reasonably well. In combination with the support provided by the lysimeter data for the underlying concepts, this makes the model a promising starting point for a transient, numerical model for solute transport in fields with water-repellent toplayers.

Meststoffen en andere chemicaliën komen vaak sneller in het grondwater terecht dan modellen voorspellen. De meeste modellen zijn namelijk gebaseerd op de veronderstelling dat de bodem homogeen is, het water in de onverzadigde zone verticaal infiltreert en het vochtfront evenwijdig is aan het bodemoppervlak. In werkelijkheid stroomt het water vaak via preferente banen door de bodem. Preferente stroming heeft diverse oorzaken en kan in nagenoeg alle gronden optreden. In dit artikel wordt naar voren gebracht dat regenwater zich in zavel-, klei- en veengronden niet alleen snel naar de ondergrond verplaatst door scheuren en gangen, maar dat tevens de bovengrond ongelijkmatig bevochtigd wordt, waardoor vochtpatronen in de matrix van deze gronden ontstaan

The research yielded the following conclusions and results: preferential flow can be expected in recently deposited, loosely packed, wettable dune sands; preferential flow is common in most water-repellent sandy soils; distribution flow in topsoils isa process of major importance, resulting in a spatially heterogeneous distribution of water and solutes in soils; with the potential water repellency parameter soils can be compared with respect to their sensitivity to water repellency; a two-dimensionalwater flow model, including hysteresis, has been developed, which is able to simulate finger growth and persistence.

Verdampings-, voelbarewarmte- en impulsflux, bodemvochtgehalte, weervariabelen en doorval zijn op vier boslocaties (populier, lariks, grove den, gemengd naald- en loofbos) in 1995 nagenoeg continu gemeten. In de zomer nam het interceptieverlies als percentage van de brutoneerslag af in de volgorde lariks (32,2%), populier (26,7%) en grove den (21,5%). De transpiratie nam af in de volgorde populier (422 mm), lariks (375 mm) en grove den (283 mm). Dit is een eerste schatting uit de metingen van de totale verdamping en interceptie voor het groeiseizoen van dag 120 tot en met dag 275. Het totale waterverbruik van populier (655 mm) en lariks (651 mm) is vrijwel gelijk, en hoger dan dat van grove den (539 mm).

The variation in water content of grass-covered peaty clay and clayey peat soils was studied at six sites in the Netherlands. The topsoils were water repellent during dry spells. When the topsoils were dry, they could only absorb water with difficulty, which is illustrated by wetting rate measurements. Precipitation could flow rapidly through shrinkage cracks towards the subsoil, bypassing the matrix of the peat. However, the measurements revealed that preferential flow was not limited to macropore flow: irregular, fingerlike wetting patterns were also formed in the soil matrix. Due to these typical wetting patterns, soil water content varied over short distances at all sites at all sampling dates.

Various factors contribute to the spatial variation of the soil water content and solute concentration in the unsaturated zone. Factors such as vegetation, microtopography, water repellency and soil layering can cause irregular wetting and fingered flow. Accelerated transport of water and solutes towards groundwater and surface water may be the result and, therefore, it is essential to understand this phenomenon. In the past years, sandy, loam, clay and peat soils in the Netherlands were sampled in vertical transects to study the spatial distribution of soil moisture content. Examples of soil moisture patterns caused by vegetation, microtopography, water repellency and soil layering are described.

Pesticiden en andere chemicaliën blijken vaak sneller in het grondwater terecht te komen dan modellen voorspellen. De meeste van deze modellen zijn namelijk gebaseerd op de veronderstelling dat de bodem homogeen is, het water in de onverzadigde zone verticaal infiltreert en het vochtfront evenwijdig is aan het bodemoppervlak. In werkelijkheid stroomt het water vaak via preferente stroombanen door de bodem. Deze stroombanen kunnen in nagenoeg alle gronden optreden. Water en daarin opgeloste stoffen bereiken hierdoor veel sneller het grondwater dan op grond van de gemiddelde verblijftijd verwacht zou worden. Vooral ondiep grondwater en oppervlaktewater hebben door dit proces een vergrote kans op verontreiniging.

In soils with fingered flow, surface-applied solutes can reach the groundwater more rapidly than in the case of homogeneous wetting. This study was undertaken to demonstrate the occurrence of finger-like wetting patterns in a silt loam soil and a silty clay loam soil, and to investigate the influence of dry bulk density and water repellency on the observed soil moisture patterns. In both soils, dry bulk density was not a main factor in inducing wet finger-like patterns and high water content variability. Spatial variability of degree and persistence of potential water repellency and, accordingly, of soil water contents were high.

Various factors contribute to the spatial variation of the soil water content and solute concentration in the unsaturated zone. Factors such as vegetation, microtopography, water repellency and soil layering can cause irregular wetting and fingered flow. Accelerated transport of water and solutes towards groundwater and surface water may be the result and, therefore, it is essential to understand this phenomenon. In the past years, sandy, loam, clay and peat soils in the Netherlands were sampled in vertical transects to study the spatial distribution of soil moisture content. Examples of soil moisture patterns caused by vegetation, microtopography, water repellency and soil layering are described.