Repellent Soils Research Articles

Effect of soil water repellency on soil hydraulic properties estimated under dynamic conditions

Summary Soil research done over the past decades has proven that water repellent soils are widespread in all climates. Water repellency enhances the leaching of contaminants in the unsaturated zone by introducing preferential flow. In order to predict soil water fluxes in the unsaturated zone the accurate knowledge of the soil hydraulic properties (SHP) is mandatory. In this study the effect of water repellency on both imbibition and drainage SHP was studied. Inflow/outflow experiments were conducted in the laboratory for two soils and two artificially created hydrophobic mixtures. In the inflow/outflow experiments the pressure head at the bottom of the soil column was increased/decreased and the estimated SHP functions were obtained by means of inverse modeling. Inflow/outflow experiments were also conducted using ethanol instead of water in order to estimate the effect of liquid wetting properties on the estimated characteristic curves of the materials under study. The results showed that the water retention functions and the unsaturated hydraulic conductivity functions estimated from the dynamic experiments are strongly dependent on the degree of water repellency and the wetting/drying process.

Specific processes during in situ infiltration into a sandy soil with low-level water repellency

Despite growing knowledge about water repellent soils it is not well investigated how soil water repellency (SWR) of a lower level influences the in situ water infiltration into soils. Hence, we investigated a sandy soil where we found subcritical water repellency and unexpected stable (persistent) contact angles (CAs) in the subsoil characterized by CA measurements in the laboratory. To characterize the influence of this persistence on in situ water infiltration, a ponded tracer infiltration experiment was carried out. During the infiltration, hydraulic sensors revealed a slight saturation at the wetting front which is a sign for unstable flow. The excavated dye-stained infiltration zone was conical and its lateral extent decreased with increasing depth, showing characteristics of a macro-finger. We suggest the subsoil to exhibit a strong hysteretic water retention characteristic governing the infiltration process, but we could not verify this by standard laboratory measurements, as strongly persistent SWR prevented the fast wetting of the soil samples. Close to saturation the persistence of SWR led to a wetting period of soil samples of at least 56 days to overcome SWR and reaching an equilibrium state. There are three major conclusions of this study: 1. In contrast to many other studies we found persistent SWR also in the subsoil and not only in the humic topsoil, 2. The shape of the wetting front is contrary to common expectations for homogeneous sandy soils and 3. The wettability characteristics found might also be relevant for assessing infiltration dynamics at other sites regarding the fact that those observations are not obvious.

Effects of hydrophobicity on splash erosion of model soil particles by a single water drop impact

ABSTRACTRaindrop impact can be a major contributor to particle mobilization for soils and other granular materials. In previous work, water repellent soils, comprised of hydrophobic particles, have been shown to exhibit greater splash erosion losses under multiple drop impact. However, the underlying principle differences in splash behavior between hydrophobic and hydrophilic granular surfaces have not been studied to date. In this study the effects of particle hydrophobicity on splash behaviour by a single water drop impact were examined using high‐speed videography. Water drops (4 mm in diameter) were dropped on beds of hydrophilic and hydrophobic glass beads (sieved range: 350–400 µm), serving as model soil particles. The drop velocity on impact was 2.67 m s‐1, which corresponds to ~30% of the terminal velocity of a raindrop of similar size. The resulting impact behaviour was measured in terms of the trajectories of particles ejected from the beds and their final resting positions. The response to the impacting water drop was significantly different between hydrophilic and hydrophobic particles in terms of the distance distribution, the median distance travelled by the particles and number of ejected particles. The greater ejection distances of hydrophobic particles were mainly the result of the higher initial velocities rather than differences in ejecting angles. The higher and longer ejection trajectories for hydrophobic particles, compared with hydrophilic particles, indicate that particle hydrophobicity affects splash erosion from the initial stage of rainfall erosion before a water layer may be formed by accumulating drops. The ~10% increase in average splash distance for hydrophobic particles compared with hydrophilic particles suggests that particle hydrophobicity can result in greater net erosion rate, which would be amplified on sloping surfaces, for example, by ridges in ploughed agricultural soils or hillslopes following vegetation loss by clearing or wildfire. Copyright © 2012 John Wiley & Sons, Ltd.

A Summary of Soil Hydrophobicity Trials in U.S. Golf Course Greens

Recommendations for golf course green construction have been for rootzone mixes with at least 90% sand while the remainder is typically peat or other organic material. This type of construction mix initially facilitates water movement through the profile but provides an environment conducive to soil hydrophobicity. Several factors such as sand texture, frequent wet to dry cycles, and accumulation of organic matter and thatch contribute to the development of soil water repellency in golf course greens. In research studies conducted over the past fifteen years in several locations across the United States, water drop penetration tests (WDPT) on golf greens have shown slightly and strongly water repellent soil is prevalently found at 0 and 1 cm depths. In the majority of locations, as the depth of profile increased, soil hydrophobicity decreased and was mostly non-existent at the 6 cm depth. These results suggest that despite soil texture and management practices, soil hydrophobicity in a managed turfgrass area is most severe in the thatch and mat area of the profile. While the knowledge of what contributes to water repellency is critical for alleviating symptoms associated with hydrophobic coatings, management practices may be more successful if the focus was on the depth of the repellency.

Wetting properties of fungi mycelium alter soil infiltration and soil water repellency in a γ-sterilized wettable and repellent soil

Soil water repellency (SWR) has a drastic impact on soil quality resulting in reduced infiltration, increased runoff, increased leaching, reduced plant growth, and increased soil erosion. One of the causes of SWR is hydrophobic fungal structures and exudates that change the soil–water relationship. The objective of this study was to determine whether SWR and infiltration could be manipulated through inoculation with fungi. The effect of fungi on SWR was investigated through inoculation of three fungal strains (hydrophilic –Fusarium proliferatum, chrono-amphiphilic –Trichoderma harzianum, and hydrophobic –Alternaria sp.) on a water repellent soil (WR-soil) and a wettable soil (W-soil). The change in SWR and infiltration was assessed by the water repellency index and cumulative infiltration respectively. F. proliferatum decreased the SWR on WR-soil and slightly increased SWR in W-soil, while Alternaria sp. increased SWR in both the W-soil and the WR-soil. Conversely T. harzianum increased the SWR in the W-soil and decreased the SWR in the WR-soil. All strains showed a decrease in infiltration in W-soil, while only the F. proliferatum and T. harzianum strain showed improvement in infiltration in the WR-soil. The ability of fungi to alter the SWR and enmesh soil particles results in changes to the infiltration dynamics in soil.

Development of Unstable Flow and Reduced Hydraulic Conductivity due to Water Repellence and Restricted Drainage

The effect of water repellence and antecedent soil moisture on wetting front stability and infiltration rate are reported for a seasonally water repellent topsoil. The effect of water repellence on infiltration was determined by comparing the in situ infiltration of water to that of a 7M ethanol solution. Wetting front stability was measured during infiltration of water into repacked, wettable and water repellent soils, within a Hele‐Shaw chamber. Water repellence restricted in situ movement of water through large macropores (>500 μm), which decreased intrinsic permeability by 1 to 2 orders of magnitude. In repacked soils, water repellence caused the development of unstable wetting fronts and reduced infiltration from 240 mm h−1 to 101.7 mm h−1. Infiltration into wettable soils at moisture contents near field capacity was expected to result in rapid infiltration and stable wetting fronts. However in repacked soils, wetting front instability developed, and infiltration rates were 190% lower when air and/or water movement through the base of the chamber was restricted. Infiltration into in situ soil was also slower at high antecedent soil moisture. The hydraulic conductivity of the 7M ethanol solution decreased significantly from 112.3 mm h−1 in dry water repellent conditions, to 35.6 mm h−1 in wettable soils at high antecedent moisture contents. Consequently the previously reported development of wetting front instability and reduced infiltration into in situ wettable soils at high moisture contents were confirmed and attributed to difficulty displacing existing soil water during infiltration of new water.

An experimental setup to assess earthworm behaviour in compacted soil

A new experimental setup for studying the behaviour of earthworms in compacted soil is described. The main features are (1) the soil column containing the earthworms is surrounded by repellent soil, (2) a compacted soil block is placed on top of this column, (3) the soil block can be prepared with different attributes (e.g. amendments), and (4) the soil block can be removed for further investigation. The experimental setup was tested using adult individuals of the anecic Lumbricus terrestris and two different kinds of compaction (bulk density, 1.3 and 1.7 g cm−3). At the end of the experiment (15 days), earthworm activity at the compacted soil was recorded on transparency film. Soil blocks of 1.3 g cm−3 bulk density were penetrated within 1 day whilst blocks of 1.7 g cm−3 had not been penetrated by 15 days.

토양 소수성에 따른 미량원소의 흡착 및 이동

Water repellency which affects infiltration, evaporation, erosion and other water transfer mechanisms through soil has been observed under several natural conditions. Water repellency is thought to be caused by hydrophobic organic compounds, which are present as coatings on soil particles or as an interstitial matter between soil particles. This study was conducted to evaluate the characteristics of the water repellent soil and transport characteristics of trace elements within this soil. Capillary height of the water repellent soil was measured. Batch and column studies were accompanied to identify sorption and transport mechanism of trace elements such as Cu 2+ , Mn 2+ , Fe 2+ , Zn 2+ and Mo 5+ . Difference of sorption capacity between common and repellent soils was observed depended on the degree of repellency. In the column study, the desorption of trace elements and the spatial concentration distribution as a function of time were evaluated. The capillary height was in the repellency order of 0% > 15% > 40% > 70% > 100%. No water was absorbed in soil indicating >70% repellency. Using trace elements, Fe 2+ and Mo 5+ showed higher sorption capacity in the repellent soil than in non-repellent soil. The sorption performance of Fe 2+ was found to be in the repellency order of 40% > 15% > 0%. Our results found that transfer of Mo 5+ had similar sorption tendency in soils having 0%, 15% and 40% repellency at the beginning, however, the higher desorption capacity was observed as time passes in the repellent soil compared to in non-repellent soils.

Quantitative analysis of liquid penetration kinetics and slaking of aggregates as related to solid–liquid interfacial properties

Summary Aggregate stability is frequently shown to be enhanced by strong soil water repellency, however, there is limited systematic evidence on this effect for moderately (subcritically) water repellent soils. This study aimed to investigate the specific effects of interfacial properties on the liquid penetration kinetics in relation to the stability of subcritically water repellent aggregates (4–6.3 mm) from various arable and forest soils against breakdown by slaking. In contrast to many other studies, where aggregate stability was determined by wet sieving, we here assessed the stability by immersion of air-dry aggregates in water–ethanol solutions with surface tensions ranging from 30 to 70 mN m −1 . This approach allowed a highly sensitive discrimination of different stability levels and the determination of breakdown kinetics also for less stable aggregates. Interfacial properties were characterized in terms of contact angle measured on crushed aggregates, θ c , and calculated for intact aggregates, θ i , based on infiltration measurements with water and ethanol. Aggregate stability turned out to be higher in forest soils compared to arable soils with topsoil aggregates generally found to be more stable than subsoil aggregates. For water repellent aggregates, characterized by contact angles >40° and low water infiltration rates ( 3 s −0.5 ), the fraction of disrupted aggregates after 30 s of immersion was generally below 10%, whereas in case of the more wettable aggregates, characterized by contact angles 0.25 mm 3 s −0.5 ) more than 80% of the aggregates were disrupted. In accordance, we found a close relationship between aggregate stability and wettability with differences between θ c and θ i being generally small. In addition, aggregate stability turned out to be related to organic carbon content. However, correlation analysis revealed that both persistence of aggregate stability and kinetics of aggregate breakdown were more strongly affected by the contact angle, θ c ( r = 0.90 and r = −0.83, respectively) and θ i ( r = 0.89 and r = −0.76, respectively) than the organic carbon content ( r = 0.62 and −0.52, respectively), suggesting that stability was primarily controlled by aggregate interfacial properties. Calculation of liquid penetrativity as a function of surface tension and contact angle clearly demonstrated the importance of both solid and liquid interfacial properties in determining the stability of subcritically water repellent aggregates against slaking.

Hydrological effects of a layer of vegetation ash on underlying wettable and water repellent soil

Hydrological processes after a wildfire may take place under soil conditions altered by heat and by the presence of ash. Soil and ash interact as a two-layer system with poorly understood hydrological properties, especially when ash covers water repellent soil. Here we quantify the effect of an ash layer (0, 5, 15 and 30mm depth) covering wettable and water repellent soil on (i) the hydrological response and the mechanism of runoff generation and (ii) the water repellency dynamics, for a rainfall event followed by different drying periods and a second rainfall event. Laboratory rainfall simulation experiments (82.5mmh−1 during 40min) at small plot-scale (0.09m2) were performed and surface and subsurface flow, sediment yield, splash detachment and moisture content evolution determined. Wettable soil without ash cover generated no surface runoff, but as a two-layer system temporary surface runoff was produced when ash became saturated, until water drained through the soil. Wetting and drying changed the hydrological properties of ash, increasing surface runoff for all ash depths. Over water repellent soil, the ash layer delayed and reduced surface runoff proportionally to ash depth (r=0.99), reduced soil water repellency and promoted fingered subsurface flow. Ash protected the soil from splash and sheet erosion, particularly for water repellent soil. The results demonstrate that (i) the presence of an ash layer can have contrasting effects on surface runoff, depending on the wettability of the underlying soil, and (ii) a single wetting and drying event can substantially modify ash hydrological properties.

Moment analysis description of wetting and redistribution plumes in wettable and water-repellent soils

Summary Water repellency has a significant impact on water flow patterns in the soil profile. Transient 2D flow in wettable and natural water-repellent soils was monitored in a transparent flow chamber. The substantial differences in plume shape and spatial water content distribution during the wetting and subsequent redistribution stages were related to the variation of contact angle while in contact with water. The observed plumes shape, internal water content distribution in general and the saturation overshoot behind the wetting front in particular in the repellent soils were associated with unstable flow. Moment analysis was applied to characterize the measured plumes during the wetting and subsequent redistribution. The center of mass and spatial variances determined for the measured evolving plumes were fitted by a model that accounts for capillary and gravitational driving forces in a medium of temporally varying wettability. Ellipses defined around the stable and unstable plumes’ centers of mass and whose semi-axes represented a particular number of spatial variances were used to characterize plume shape and internal moisture distribution. A single probability curve was able to characterize the corresponding fractions of the total added water in the different ellipses for all measured plumes, which testify the competence and advantage of the moment analysis method.

Effect of Particle Size on Droplet Infiltration into Hydrophobic Porous Media As a Model of Water Repellent Soil

The wettability of soil is of great importance for plants and soil biota, and in determining the risk for preferential flow, surface runoff, flooding,and soil erosion. The molarity of ethanol droplet (MED) test is widely used for quantifying the severity of water repellency in soils that show reduced wettability and is assumed to be independent of soil particle size. The minimum ethanol concentration at which droplet penetration occurs within a short time (≤ 10 s) provides an estimate of the initial advancing contact angle at which spontaneous wetting is expected. In this study, we test the assumption of particle size independence using a simple model of soil, represented by layers of small (~0.2-2 mm) diameter beads that predict the effect of changing bead radius in the top layer on capillary driven imbibition. Experimental results using a three-layer bead system show broad agreement with the model and demonstrate a dependence of the MED test on particle size. The results show that the critical initial advancing contact angle for penetration can be considerably less than 90° and varies with particle size, demonstrating that a key assumption currently used in the MED testing of soil is not necessarily valid.

Novel Alkyl Polyglucoside Surfactant Derivatives for Improving the Infiltration of Irrigation Water into Repellent Soil

Abstract Soil water repellency causes several agronomic challenges for turfgrass managers, such as reduced color, quality, and playability of golf courses and sports fields. Surfactants are commonly applied as water and soil treatments in order to manage water conservation, irrigation efficiency, and overall plant health. There is an ongoing need within the current sustainability initiatives for advanced materials with superior environmental and performance profiles. This research is the first to demonstrate the efficacy of anionic alkyl polyglucoside esters (AGEs) and AGE-nonionic surfactant blends in increasing the wettability of naturally derived and artificially repellent mineral soils. Surfactant compositions were diluted in distilled water to concentrations ranging from 8000 mg l−1 to 500 mg l−1. A standardized water droplet penetration test was used to measure infiltration times into soils of varying degrees of repellency. AGEs were up to 10 times more effective than alternative surfactants at improving water movement into soil. Performance differences became more significant as the application rates decreased. Our results demonstrate that AGEs can be highly effective at delivering irrigation water and aqueous compositions into the root zone of turfgrass when applied alone and/or in concert with traditional wetting agent classes. Patent applications have been submitted for this discovery.

Effect of brackish water irrigation on water and salt movement in repellent soils

Effect of brackish water irrigation on water and salt movement in repellent soils

Applicability of four infiltration models to infiltration characteristics of water repellent soils

Applicability of four infiltration models to infiltration characteristics of water repellent soils

Soil water repellency and its implications for organic matter decomposition - is there a link to extreme climatic events?

Earth system models associate the ongoing global warming with increasing frequency and intensity of extreme events such as droughts and heat waves. The carbon balance of soils may be more sensitive to the impact of such extremes than to homogeneously distributed changes in soil temperature (Ts) or soil water content (θs). One parameter influenced by more pronounced drying/rewetting cycles or increases in Ts is the wettability of soils. Results from laboratory and field studies showed that low θs, particularly in combination with high Ts can increase soil water repellency (SWR). Recent studies have provided evidence that the stability of soil organic matter (SOM) against microbial decomposition is substantially enhanced in water repellent soils. This review hypothesizes that SWR is an important SOM stabilization mechanism that could become more important because of the increase in extreme events. We discuss wettability-induced changes in soil moisture distribution and in soil aggregate turnover as the main mechanisms explaining the reduced mineralization of SOM with increasing SWR. The creation of preferential flow paths and subsequent uneven penetration of rainwater may cause a long-term reduction of soil water availability, affecting both microorganisms and plants. We conclude that climate change-induced SWR may intensify the effects of climatic drought and thus affects ecosystem processes such as SOM decomposition and plant productivity, as well as changes in vegetation and microbial community structure. Future research on biosphere–climate interactions should consider the effects of increasing SWR on soil moisture and subsequently on both microbial activity and plant productivity, which ultimately determine the overall carbon balance.

The wettability of ash from burned vegetation and its relationship to Mediterranean plant species type, burn severity and total organic carbon content

Immediately following a wildfire, a layer of ash and charred material typically covers the ground. This layer will gradually be reduced, being redistributed by rainfall, wind or animals, by partial dissolution and by incorporation into the soil. Ash can increase or decrease the post-fire runoff and erosion response, depending upon the soil and ash properties and the ash thickness. One aspect of ash that has remained unknown and which may affect the variability in the hydrological response of the burned soil is its wettability. This study examines the wettability of ash using the Water Drop Penetration Time (WDPT) test, its relationship with total organic carbon (TOC) content and colour, and its effects when incorporated into the soil. Ash samples ( n = 48) were taken from five wildfires in the Mediterranean basin encompassing a range of burn severities. Additional ash material was generated from exposing plant leaves ( Rosmarinus officinalis, Pinus halepensis and Quercus coccifera) to specific temperatures in a muffle furnace (200–700 °C). Selected laboratory ash types were also mixed with wettable and water repellent soil material to assess their effect on soil wettability. Over 30% of ash samples from the wildfire sites exhibited water repellency (WR), with WDPT levels ranging from low to extreme. This variability appears to be related to differences in fire severity, combustion completeness of the biomass, vegetation type and subsequent rainfall events. The laboratory-generated ash exhibited a greater frequency and persistence of water repellency at lower furnace temperatures (200–300 °C), with ash from R. officinalis being generally less water repellent than that derived from the other two species. The water repellency levels of ash correlated well with TOC (r = 0.80), whereas neither of these parameters correlated very well with ash colour (r = 0.57 for TOC and r = 0.59 for WR). This suggests that ash colour, which is widely used as a parameter in classifying burn severity in the field, may not necessarily be a very accurate indicator. Adding water repellent ash to wettable soil induced WR, whereas the addition of wettable ash to water repellent soil had the opposite effect. A wetting–drying cycle can reduce the water repellency of a soil–ash mixture. There were substantial differences between wildfire- and laboratory-generated ash in terms of organic carbon content and colour, suggesting that the combustion conditions in a furnace may not adequately represent those in wildfires. In contrast to what is generally assumed, our findings demonstrate that ash from vegetation fires can be water repellent. This is likely to have implications for runoff responses and nutrient fluxes not only when ash is present on the ground surface, but also following its redistribution and incorporation into the soil.

Surfactant and irrigation effects on wettable soils: runoff, erosion, and water retention responses

AbstractSurfactants are chemical compounds that can change the contact angle of a water drop on solid surfaces and are commonly used to increase infiltration into water repellent soil. Since production fields with water repellent soil often contain areas of wettable soil, surfactants applied to such fields worldwide will likely be applied to wettable soil, with unknown consequences for irrigation‐induced erosion, runoff, or soil water relations. We evaluated surfactant and simulated sprinkler irrigation effects on these responses for three wettable, Pacific Northwest soils, Latahco and Rad silt loams, and Quincy sand. Along with an untreated control, we studied three surfactants: an alkyl polyglycoside (APG) in solution at a concentration of 18 g active ingredient (AI) kg−1, a block copolymer at 26 g kg−1, and a blend of the two at 43 g kg−1. From 2005 to 2009 in the laboratory, each surfactant was sprayed at a rate of 46·8 l ha−1 onto each soil packed by tamping into 1·2‐ by 1·5‐m steel boxes. Thereafter, each treated soil was irrigated twice at 88 mm h−1 with surfactant‐free well water. After each irrigation, runoff and sediment loss were measured and soil samples were collected. While measured properties differed among soils and irrigations, surfactants had no effect on runoff, sediment loss, splash loss, or tension infiltration, compared to the control. Across all soils, however, the APG increased volumetric water contents by about 3% (significant at p≤0·08) at matric potentials from 0 to − 20 kPa compared to the control. With a decrease in the liquid–solid contact angle on treated soil surfaces, surfactant‐free water appeared able to enter, and be retained in pores with diameters ≥ 15 µm. All told, surfactants applied at economic rates to these wettable Pacific Northwest soils posed little risk of increasing either runoff or erosion or harming soil water relations. Moreover, by increasing water retention at high potentials, surfactants applied to wettable soils may allow water containing pesticides or other agricultural chemicals to better penetrate soil pores, thereby increasing the efficacy of the co‐applied materials. Copyright © 2010 John Wiley & Sons, Ltd.

Synergistic effects of water repellency and macropore flow on the hydraulic conductivity of a burned forest soil, south‐east Australia

AbstractResearch shows that water repellency is a key hydraulic property that results in reduced infiltration rates in burned soils. However, more work is required in order to link the hydrological behaviour of water repellent soils to observed runoff responses at the plot and hillslope scale. This study used 5 M ethanol and water in disc infiltrometers to quantify the role of macropore flow and water repellency on spatial and temporal infiltration patterns in a burned soil at plot (<10 m2) scale in a wet eucalypt forest in south‐east Australia. In the first summer and winter after wildfire, an average of 70% and 60%, respectively, of the plot area was water repellent and did not contribute to infiltration. Macropores (r > 0·5 mm), comprising just 5·5% of the soil volume, contributed to 70% and 95%, respectively, of the field‐saturated and ponded hydraulic conductivity (Kp). Because flow occurred almost entirely via macropores in non‐repellent areas, this meant that less than 2·5% of the soil surface effectively contributed to infiltration. The hydraulic conductivity increased by a factor of up to 2·5 as the hydraulic head increased from 0 to 5 mm. Due to the synergistic effect of macropore flow and water repellency, the coefficient of variation (CV) in Kp was three times higher in the water‐repellent soil (CV = 175%) than under the simulated non‐repellent conditions (CV = 66%). The high spatial variability in Kp would act to reduce the effective infiltration rate during runoff generation at plot scale. Ponding, which tend to increase with increasing scale, activates flow through macropores and would raise the effective infiltration rates at larger scales. Field experiments designed to provide representative measurements of infiltration after fire in these systems must therefore consider both the inherent variability in hydraulic conductivity and the variability in infiltration caused by interactions between surface runoff and hydraulic conductivity. Copyright © 2010 John Wiley & Sons, Ltd.

Effect of soil water repellency on moisture distribution from a subsurface point source

Wetting and redistribution of water in air‐dried wettable, slightly repellent, and strongly repellent soils was investigated by tracking the spatial and temporal moisture‐content variation in a transparent flow chamber. Water was applied as a subsurface point source. The degree of water repellency had a substantial effect on the plume's shape, dimensions, and internal moisture‐content distribution. The high uniform moisture content in the repellent soil's plume surrounded by a narrow transition layer within which moisture content was sharply decreasing indicates that unstable flow shapes a finger‐like wetting front. Moisture redistribution in the repellent soils was limited and took place mainly in the vertical direction. The repellent soils were wetted immediately, very likely by local positive pressure buildup induced by the hydraulic resistance imposed by the initial >90° contact angle. The plume shape dimensions and internal moisture‐content distribution should be considered in the design and operation of subsurface drip irrigation in water‐repellent soils.