Water Repellent Sandy Soils Research Articles

Runoff response of a hydrophobic soil under high intensity rains

AbstractWater repellent soils represent some of the most responsive natural surfaces from the point of view of overland flow generation. Measurement of the runoff response of such soils can therefore provide characteristics that can help constrain hydrological models or improve the conceptual models of hillslope runoff generation. We have analysed the overland flow characteristics of water repellent sandy soils at three sites with different dominant vegetation (hereafter denoted as the lichen, moss and herb sites) and following vegetation removal (scalping) during extreme rainfall events created by a rainfall simulator (three consecutive sprinklings at 2.1 mm min−1 for 15 min each with a 5 min hiatus between the sprinklings). Despite the extreme water repellency (WR) and rainfall intensity, on average just up to a half of the rainfall left the plots as an overland flow. Whilst the overland flow mostly generated quickly (on average within 50–120 s), the vegetation substantially delayed its appearance (by approximately 100% compared to scalped plots). The median values of the runoff coefficients were 6% and 49% on the soil with and without vegetation, respectively. Good relationships between the overland flow characteristics and soil moisture were found only for the scalped plots. Although the time to runoff and runoff coefficients after each of the three consecutive sprinklings were not substantially different, the differences between the soil with and without vegetation remained substantial. The results indicated: (a) high variability within each of the sites, including the lichen site that appeared homogenous; (b) all vegetated plots retained a higher amount of water than the scalped plots; (c) although all sites exhibited high WR, the combined influence of preferential flow, higher hydraulic conductivity and specific vegetation (soapwort, black locust) mitigated its effects at the herb site; (d) the relatively lower overland flow at that site may point to subsurface flow.

Establishing selection criteria of water repellent sandy soils for use in impervious layer of engineered slope

In this study, the selection criteria of water-repellent sandy soils for use in the impervious layer of engineered slopes were examined. First, the hydrophobization of six kinds of sandy soils was performed via the silane treatment. Laboratory experiments to assess hydrophobicity and water-shielding performance—namely the water drop penetration time (WDPT) test, the contact-angle measurement test, and the water infiltration head (WIH) test—were conducted. Based on the required thickness and the required particle number of water-repellent soils, the mean particle sizes of sandy soils that achieved acceptable water-shielding performance were determined. It was found that a Cc value of less than 1.0 or a D40 value of less than 2.0 mm allowed good water-shielding performance in sandy soils. Lastly, some of the measured WIH values agreed with the results estimated by the capillary rise model, and the application of the particle size to this model was discussed.

Vacuum drying water-repellent sandy soil: Anoxic conditions retain original soil water repellency under variable soil drying temperature and air pressure

Monitoring land degradation due to soil water repellency (SWR) through space and time is often complicated by different soil drying temperatures and drying times that generate artefacts causing inconsistent SWR results. This study aimed to develop a reproducible soil drying method at room temperature for the severity of potential SWR determination. We compared conventional 105 °C oven drying under oxic conditions (101.3 kPa) and vacuum drying under anoxic conditions (20 °C and low air pressure 1.3 kPa)—both drying methods removed the same amount of water from soil. We then investigated the effect of aeration condition (anoxic or oxic), temperature (20 °C or 105 °C) and air pressure (1.3 kPa or 101.3 kPa) on SWR and residual soil water content after drying using a complete factorial design. We hypothesised that 1) SWR will decrease with low air pressure under both oxic and anoxic conditions, 2) SWR will remain unchanged with temperature under anoxic conditions, and 3) conventional air drying treatments will have the highest residual soil water content. We used sieved, air-dried soil (<2 mm) that had been collected from a cropping field. Soil water repellency, determined by the molarity of ethanol droplet (MED) test, did not change following drying under anoxic conditions, relative to soil that was air-dried at 20 °C and 101.3 kPa under an oxic condition but decreased significantly under oxic conditions at high temperature (MED from ~1.58 to 1.34 mol L−1) or low air pressure (from ~1.58 to 1.42 mol L−1). We recommend vacuum drying water-repellent sandy soil under anoxic conditions at 20 °C and 1.3 kPa when assessing SWR.

Water vapour adsorption on water repellent sandy soils

AbstractSoil sorptivity is considered a key parameter describing early stages of water (rain) infiltration into a relatively dry soil and it is related to build-up complexity of the capillary system and soil wettability (contact angles of soil pore walls). During the last decade an increasing water repellency of sandy soils under pine forest and grassland vegetation has been frequently observed at Mlaky II location in SW Slovakia. The dry seasons result in uneven wetting of soil and up to hundredfold decrease in soil sorptivity in these vegetated soil as compared to reference sandy material, which was out of the reach of ambient vegetation and therefore readily wettable. As far as water binding to low moisture soils is governed by adsorption processes, we hypothesized that soil water repellency detected by water drop penetration test and by index of water repellency should also influence the water vapour adsorption parameters (monolayer water content, Wm, specific surface area, A, maximum adsorption water, Wa, maximum hygroscopic water MH, fractal dimension, DS and adsorption energies, Ea) derived from BET model of adsorption isotherms. We found however, that the connection of these parameters to water repellency level is difficult to interpret; nevertheless the centres with higher adsorption energy prevailed evidently in wettable materials. The water repellent forest and grassland soils reached less than 80% of the adsorption energy measured on wettable reference material. To get more conclusive results, which would not be influenced by small but still present variability of field materials, commercially available homogeneous siliceous sand was artificially hydrophobized and studied in the same way, as were the field materials. This extremely water repellent material had two-times lower surface area, very low fractal dimension (close to 2) and substantially lower adsorption energy as compared to the same siliceous sand when not hydrophobized.

Application of compost and clay under water-stressed conditions influences functional diversity of rhizosphere bacteria

Applications of compost and clay to ameliorate soil constraints such as water stress are potential management strategies for sandy agricultural soils. Water repellent sandy soils in rain-fed agricultural systems limit production and have negative environmental effects associated with leaching and soil erosion. The aim was to determine whether compost and clay amendments in a sandy agricultural soil influenced the rhizosphere microbiome of Trifolium subterraneum under differing water regimes. Soil was amended with compost (2% w/w), clay (5% w/w) and a combination of both, in a glasshouse experiment with well-watered and water-stressed (70 and 35% field capacity) treatments. Ion Torrent 16S rRNA sequencing and Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analysis of functional gene prediction were used to characterise the rhizosphere bacterial community and its functional component involved in nitrogen (N) cycling and soil carbon (C) degradation. Compost soil treatments increased the relative abundance of copiotrophic bacteria, decreased labile C and increased the abundance of recalcitrant C degrading genes. Predicted N cycling genes increased with the addition of clay (N2 fixation, nitrification, denitrification) and compost + clay (N2 fixation, denitrification) and decreased with compost (for denitrification) amendment. Water stress did not alter the relative abundance of phylum level taxa in the presence of compost, although copiotrophic Actinobacteria increased in relative abundance with addition of clay and with compost + clay. A significant role of compost and clay under water stress in influencing the composition of rhizosphere bacteria and their implications for N cycling and C degradation was demonstrated.

Scaling the Hydraulic Functions of A Water Repellent Sandy Soil

Abstract The heterogeneity of both unsaturated hydraulic conductivity and water retention was measured with a high spatial resolution on a transect using an evaporation method. Fifteen undisturbed 100 cm3 soil cores were taken on a transect every 10 cm from the topsoil of a water repellent sandy site. Five dynamic water retention curves and four unsaturated conductivity curves were determined for each core. We conducted measurements without further saturation in the laboratory in order to achieve field-like conditions. The initial water contents were heterogeneous, indicating different hysteretic conditions and water repellent areas. The scattering of the water retention curves was high, while the heterogeneity of unsaturated conductivity curves was unexpectedly low. Two scaling approaches were used to describe the heterogeneity: one with and one without considering hysteresis. The concept of scaling applies well to describing the heterogeneity of both hydraulic functions. Including hysteresis leads to similar results than excluding hysteresis. The distribution of the hydraulic conductivity and the water retention were independent from each other. The results give important information for numerical simulation of the water flow with heterogeneous hydraulic functions.

Effect of Soil Water Repellency on Energy Partitioning Between Soil and Atmosphere: A Conceptual Approach

Water repellency (WR) is a phenomenon known from many soils around the world and can occur in arid as well as in humid climates; few studies, however, have examined the effect of soil WR on the soil-plant-atmosphere energy balance. The aim of our study was to estimate the effects of soil WR on the calculated soil-atmosphere energy balance, using a solely model-based approach. We made out evapotranspiration to have the largest influence on the energy balance; therefore the effect of WR on actual evapotranspiration was assessed. To achieve this we used climate data and measured soil hydraulic properties of a potentially water-repellent sandy soil from a site near Berlin, Germany. A numerical 1D soil water balance model in which WR was incorporated in a straightforward way was applied, using the effective cross section concept. Simulations were carried out with vegetated soil and bare soil. The simulation results showed a reduction in evapotranspiration of 30–300 mm year−1 (9%–76%) at different degrees of WR compared to completely wettable soil, depending on the severity degree of soil WR. The energy that is not being transported away by water vapor (i.e., due to reduced evapotranspiration) had to be transformed into other parts of the energy balance and thus would influence the local climate.

Effects of Isopropanol/Ammonia Extraction on Soil Water Repellency as Determined by Atomic Force Microscopy

Extraction of soil with solvents is commonly used to isolate fractions of soil organic matter (SOM) thought to cause soil water repellency. Previous studies have assessed the efficiency of extraction procedures by determining sample water repellency and SOM content, before and after extraction, and the ability of extracts to render wettable materials repellent. Here we use atomic force microscopy (AFM) for the direct, high‐resolution examination of the effects of the most commonly used solvent (isopropanol/ammonia) on physicochemical properties of soil particle surfaces. Particles, from three pairs of wettable and water repellent sandy soils, were examined before and after Soxhlet extraction and following extract application to extracted soil material. Complementary evidence was provided by comparison with the effects of exposing clean sand to organic compounds of known composition. The extraction rendered all soils wettable and resulted in a reduction in particle surface area covered by SOM. Subsequent exposure of these wettable particles to extracts (i) re‐introduced some level of water repellency despite achieving only a minimal increase in SOM; (ii) led to a more uniform scatter of soft regions, indicative of SOM; but (iii) did not consistently recreate original surface features. The outcomes suggest that (i) although isopropanol/ammonia extraction may be able to isolate key compounds involved in causing water repellency, these compounds appear to be obtained in a form with little affinity for clean mineral surfaces; (ii) the extent and distribution of SOM on soil particle surfaces appears to be a critical factor in the expression of water repellency.

Persistency of flow patterns in a water repellent sandy soil – Conclusions of TDR readings and a time-delayed double tracer experiment

Summary On a former waste water disposal field with water repellent sandy soil under grass vegetation we analyzed the persistency of flow patterns on a 150 m × 25 m plot by (i) continuous TDR-measurements on a 2 m × 1 m transect combined with seasonal soil moisture sampling campaigns, and (ii) a time-delayed double tracer experiment on a second 3 m × 1 m transect. Here, we applied bromide under wettable soil conditions in spring and chloride under water repellent soil conditions in autumn. At the end of the tracer experiment, after a travel time of 328 days for Br and 87 days for Cl, respectively, the transect was excavated and sampled in high spatial resolution. Tracer concentration, water content, water drop penetration times (WDPT), and soil organic matter content (SOM) of each sample were analyzed in order to characterize flow patterns. The TDR readings were used to predict the effective cross section (ECS) of subsurface flow and flow shifts over the season. During summer, when ECS is low and consecutive precipitation events occur, flow paths – once created – persist over time. However, over longer times (from autumn to autumn), the spatial arrangements of the flow paths can change completely. The Cl distribution showed typical fingering structures with high concentrations in the less water repellent flow paths. In contrast, Br was found mostly in the dry, hydrophobic areas indicating that it was transported before the soil became water repellent. Consequently, the flow patterns generated in spring and early summer differ completely from those in autumn and winter because of water repellent structures established during the vegetative period. These structures could be identified using a critical water content ( θ crit ) concept, considering both soil water content and SOM. As not all soil parts being active during to season, four flow categories could be identified: about 10% permanent (=stable flow paths), 45% periodic (i.e. water repellent in summer), 40% occasional (water repellent in summer and autumn), and 5% permanent water repellent.

Using spectral reflectance to document water stress in bermudagrass grown on water repellent sandy soils

AbstractTo conserve natural water resources, athletic fields may be mandated to follow phase water restrictions. In order to optimize irrigation reduction, early plant responses to water stress need to be identified to avoid loss in turfgrass performance and the requirement of additional irrigation to overcome water stress. Current irrigation scheduling based on using potential evapotranspiration (ET) and water stress indices are not suitable for all growing conditions, especially in the water repellent sand soils of South Florida. This experiment was conducted to use spectral reflectance as a means to document normal and progressive water stress in ‘Tifgreen’ bermudagrass (Cynodon dactylon (L.) Pers. X Cynodon transvaalensis Burtt Davy) grown on a native water repellent sand soil. Three consecutive trials were conducted, in which bermudagrass was subjected to either no irrigation until severe visual wilt (non‐irrigated) or irrigated daily (irrigated) over a dry down period. Visual assessment of turf quality and localized dry spot occurrence verified that bermudagrass receiving daily irrigation was not water‐limited and bermudagrass receiving no irrigation was experiencing increased water stress over time. An experimentally active near‐IR/red sensor was used to monitor changes in plant architecture as a means to determine early water stress. Monitoring near‐IR/red ratios determined mid‐day depression when water demand is at the highest as well as overnight rehydration in both bermudagrasses. At the end of the dry down period, pooled trial morning near‐IR/red ratios‐afternoon near‐IR/red ratios from non‐irrigated bermudagrass was significantly greater than irrigated bermudagrass (P = 0·0035), documenting severe water stress from the non‐irrigated bermudagrass. Spectral reflectance allows real‐time, site‐specific bermudagrass water stress evaluation before it is visually identifiable. Using spectral reflectance as an early warning detection tool can be used for better irrigation decisions including pre‐emptive irrigation to avoid water stress, compared to models based on predetermined ET and water stress indices. Copyright © 2007 John Wiley &amp; Sons, Ltd.

A new modelling approach to simulate preferential flow and transport in water repellent porous media: Model structure and validation

Water repellent soil and surface layers exhibit a complex flow and transport mechanism. Knowledge of the underlying principles is essential, for instance, to simulate water availability for crops and to estimate leaching potentials of agrichemicals. The present study aims to introduce and apply a new modelling approach to simulate preferential flow and transport in water repellent porous media, and to test the model on basis of an extensive field tracer experiment. The process of preferential flow and transport has been incorporated in the well-known SWAP model and applied to field data of tracer transport through a water repellent sandy soil in the Netherlands. Flow concept and model structure have been outlined, and simulation results presented. Results indicated early arrival times of bromide tracer in the subsoil in the case where preferential flow has been taken into account in the model. Comparison of measured and computed bromide concentration profiles and bromide recovery rates show close resemblances. Several strategies to alleviate soil water repellency and prevent the occurrence of preferential flow and transport processes are highlighted, such as irrigation scheduling, clay additions, and application of surfactants.

Role of heavy polar organic compounds for water repellency of sandy soils

Separation and chemical characterisation of specific compounds responsible for soil water repellency has not previously been achieved. Here we describe the extraction, separation and analysis by gas chromatography-mass spectrometry of organic compounds found in wettable and water repellent sandy soils from the Netherlands and United Kingdom. Fatty acids (C16–C24), amides (C14–C24), alkanes (C25–C33), aldehydes/ketones (C23–C31) and complex ring-containing structures were detected in all samples. We found a greater abundance of high molecular mass polar compounds in the water repellent samples.

The reduction of water repellency by added clay is influenced by clay and soil properties

The response of water repellency to added clays dominated by kaolin or smectite was investigated for 23 highly water repellent sandy soils from the West Midland Sandplains, Western Australia. The effects of soil properties, and wetting and drying cycles on this response were also examined. Two standard clays—Georgia kaolinite and Wyoming bentonite—were compared with two soil clays—Jarrahdale laterite pallid zone kaolin and Folly Flats soil smectite—in laboratory experiments. The clays were saturated with either sodium or calcium, and were applied at rates of 0%, 0.2%, 0.4%, 0.8% and 1.6% by weight. Water repellency was measured by recording water-drop penetration time (WDPT). Clay morphology and the distribution of clay on the surfaces of sand grains were examined using optical and electron microscopy. The response of WDPT to added clay could be described by the equation: WDPT= A (clay) − b , where A is the water-drop penetration time of the untreated soil and b is a coefficient that characterises a particular soil/clay combination. Clay type was the most important factor determining the slope of this response function. Kaolinitic clays were more effective than smectitic clays in reducing water repellency. The relative effectiveness of the four clays remained the same for all soil samples tested whether WDPT was measured on the dry mix of soil and clay or after a wetting and drying cycle had been imposed. Following a wetting and drying cycle, the effectiveness of kaolinitic clays increased, whereas that of the smectitic clays decreased. The nature of the exchangeable cation (sodium or calcium) and clay dispersibility had little effect on the ability of the clays to reduce water repellency. For smectitic clays, a negative relationship was found between the response of soil water repellency to clay application (i.e. the b coefficient) and either the clay or clay+silt contents of the soils. The b coefficient decreased as the content of these fine fractions increased. The type of vegetation overlying the soils varied depending on land use, but did not influence clay effectiveness. That is, clay applied to soil under bush or cultivation had the same effectiveness. A single wetting and drying cycle could lower the water repellency of the unclayed control samples, but subsequent wetting and drying cycles only caused a gradual decline in water repellency.

Preferential flow in water repellent sandy soils: principles and modeling implications

Leaching risks of surface-applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying processes, principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in the unsaturated zone of a water repellent sandy soil, and to indicate consequences for modeling. Soil blocks with a length, width and depth of 1.2, 0.6, and 0.52m, respectively, were sampled in the Ouddorp water repellent sandy soil to investigate three-dimensional soil water content distributions. Preferential flow patterns were clearly visible in soil blocks sampled after distinct rain events. Additional TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Further, preferred pathways recurred at the same sites during all rain events. Simulations with a two-dimensional flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the critical soil moisture content. Based upon the obtained results, indications are given on how to incorporate this preferential flow and transport process in current one-dimensional simulation models.

Wetting patterns and moisture variability in water repellent Dutch soils

The present study suggests that many soils in the Netherlands 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. The persistence of water repellency was examined in topsoils using the water drop penetration time (WDPT) test. 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. 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 100cm3 steel 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 study 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.

Hydrophobic compounds in sands in New Zealand—extraction, characterisation and proposed mechanisms for repellency expression

Organic compounds were extracted from samples of water repellent sandy soils from two development sequences found on the south-west coast of the North Island, New Zealand. A number of extraction procedures and solvent mixtures are effective at reducing the repellency of these soil samples as measured by the Molarity of Ethanol Droplet (MED) test. The extracted compounds were partitioned into lipid and water-soluble fractions. The lipid fraction includes neutral lipids (predominantly alkane hydrocarbons and triglycerides), acidic lipids (mainly long-chain fatty acids), and polar lipids. The water-soluble fraction exhibits amphipathic behaviour, and shares some similarities with the hymetamelonic fraction of the humic acid pool. When added to soil, this material binds very successfully and its effect on MED depends on the type of solvent in which it is added.There is no good correlation between repellency (MED value) and total carbon content, the quantity of water-soluble material, the quantities of total lipid extract or any of the lipid fractions. The repellency of soil samples can be modified in vitro by the addition of soil lipid extract (increases MED values), or by the addition of soil water-soluble extract (decreases MED values): this is true, regardless of whether the lipid and water-soluble extract have been sourced from repellent or wettable soil. When a sample of repellent Himatangi sand is serially extracted by fresh isopropanol:ammonia (7:3 v:v) mixtures interspersed with extraction by fresh solutions of ammonia, the MED value fluctuates markedly. The former solvent mixture removes lipid compounds and lowers the MED value; ammonia removes polar and amphipathic material and increases the MED value.The results of this study are best explained by the hypothesis that water repellency is determined by the composition or nature of the outermost layer of organic material, particularly amphipathic compounds, rather than the characteristics of the bulk of the organic matter. Four mechanisms, which are consistent with the results reported here, are proposed to explain the expression of repellency in both laboratory studies and in the field.

Simulation of water flow and bromide transport in a water repellent sandy soil using a one-dimensional convection-dispersion model

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

Quantifying the impact of preferential flow on solute transport to tile drains in a sandy field soil

The objective of this study was to quantify the impact of preferential finger flow on solute leaching to tile drains and shallow groundwater in a water repellent sandy soil. Measurements of site hydrology and bromide movement were made during a six-month period following application in autumn 1994 to a 900m2 plot. A dye tracing study was also conducted and this confirmed the presence of finger flow in the water repellent topsoil. Water flow and bromide transport was simulated using a mobile-immobile concept, in which a fraction of the soil volume is assumed not to participate in transport. With calibration of ‘difficult’ parameters (e.g. the fractional mobile volume), and accounting for significant water inflows to the plot from the surroundings, there was a good agreement between simulated and observed hydrology and solute transport patterns at the site. Mass balance calculations showed that ca. 46% of the bromide application was lost to the tile drains in the winter period following application, while 16% was lost in shallow lateral groundwater flow. Comparisons between one- and two-domain simulations showed that preferential flow reduced soil water contents and increased drainflow in the early autumn, leading to a more rapid bromide transport to depth. Nevertheless, the impact of preferential finger flow on leaching was minor for the non-reactive tracer bromide in this experiment, presumably because it was applied to relatively wet soil. However, scenario simulations for autumn application of a short half-life pesticide showed more significant effects, with leaching increased by ca. 80% because of preferential flow (from 1.2% to 2.2% of the applied amount).

Stable or unstable wetting fronts in water repellent soils – effect of antecedent soil moisture content

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.

Modeling and field evidence of finger formation and finger recurrence in a water repellent sandy soil

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.