Repellent Soils Research Articles

Water–soil interactions: Unravelling the processes and stages involved in the wetting of water repellent soils

The water repellent behaviour of soils is a widely studied phenomenon given its implications for infiltration, runoff, erosion and preferential flow. However, the principles underlying the eventual penetration of water into affected soils remain poorly understood. Theoretical considerations of the energetics and kinetics involved as a water drop makes contact with a water repellent soil surface and eventually penetrates into the soil suggest three distinct stages in the overall process. These stages are 1) adhesional wetting as soil and water first make contact, followed by 2) a kinetic barrier transitional stage in which molecular reorganisation of organics on soil reduces the water-soil contact angle to allow the water drop to sit deeper over soil particles of initial contact such that there is contact with particles in directly underlying soil layers, and finally 3) branching interstitial wetting as water penetrates into the bulk soil. Studies presented here of optical microscopy, mass of soil initially wetted, penetration time through layers of soil of different thicknesses, and time-dependent measurements of contact angle, volume of water penetrated, and mass of soil wetted, all give results consistent with this model. However, only for highly water repellent soils can distinct stages in wetting be clearly resolved experimentally, presumably because only these soils have a high enough kinetic barrier in the transitional stage for good separation between stages. For less water repellent soils, while the general time dependent behaviour remains consistent with the model, the distinction between the three stages is not so easy to resolve experimentally. The roles of contact angle, particle size distribution and drop size in determining the rates of these stages is considered, and the implications of the model for understanding soil water repellency are discussed.

How does calcium and sodium in saline water affect the potential of different rates and size of sugarcane biochar to soil water repellency remediation?

Soil water repellency (SWR) is a growing problem in arid and semiarid regions with non-saline water limitations. The objective of this study was to investigate the potential of different rates and sizes of sugarcane biochar on reducing soil water hydrophobicity when using saline and non-saline water. Eleven sugarcane biochar application rates from 0 to 10% with two different sizes including <0.25 and 0.25–1 mm were studied. The experiment was conducted in two strongly and extremely water-repellent soil. In addition, to investigate the effect of electrolyte concentration on biochar potential for SWR reduction, calcium chloride and sodium chloride electrolyte solutions with 5 concentrations including 0, 0.15, 0.3, 0.45 and 0.6 mol L−1 were considered. The results showed that both sizes of biochar reduced the soil water repellency. While in strongly repellent soil the 4% biochar was enough to change the soil from strongly repellent to hydrophilic, in extremely water-repellent soil, the use of 8% of fine biochar and 6% of coarse biochar changed the extremely water-repellent soil to slightly hydrophobic and strongly hydrophobic respectively. Increasing the electrolyte concentration caused the expansion of soil hydrophobicity and reduced the positive effect of biochar to water repellency management. Increasing the electrolyte concentration in sodium chloride solution has a greater effect on increasing hydrophobicity than in calcium chloride solution. In conclusion, biochar could suggest as a soil-wetting agent in these two hydrophobic soils. However, the salinity of water and the its dominant ion could increase the amount of biochar for soil repellency reduction.

Reforesting degraded hillslopes with exotic pines in Trinidad and Tobago: Infiltration, repellency and implications for runoff and recharge

In the Caribbean, planting resilient, fast growing exotic pine trees have commonly been used to reforest degraded landscapes. However, little is currently understood about how these practices alter soil water movement and subsequently runoff and recharge. We monitored the surface soil water repellency, hydraulic conductivity, and subsurface soil moisture response to individual storm events in an exotic pine plantation and native forest in the dry and wet seasons. During the wet season, neither forest had a repellent soil surface, and the hydraulic conductivity in the exotic pines (48 mm h−1) were not significantly higher than the native forest (36 mm h−1). The speed and magnitude of the soil moisture response did not differ substantially between both forests in the wet season. However, during the mid dry season, a threshold soil moisture level was reached in the pines which triggered the development of repellent/hydrophobic conditions across all locations. Repellency reduced the surface hydraulic conductivity to 1.6 mm h−1, which in turn severely reduced the soil moisture response to rainfall. Our findings illustrate that throughout most of the year, the exotic pine plantation does not have a substantial negative impact on runoff or recharge. Only under the driest conditions do they negatively affect these processes. Our work also highlights the importance of characterising soil water repellency, hydraulic conductivity, and subsurface soil water responses over longer time periods to account for temporal variability.

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.

Surfactant-coated Seed Emergence and Establishment Under Deficit Irrigation in Hydrophilic and Hydrophobic Soils

AbstractSoil water repellency inhibits seed germination and emergence, whereas soil surfactants improve the wettability of water-repellent or hydrophobic soils. An improvement in seed germination and emergence can occur when a soil surfactant is applied directly to the seed and/or to the water repellent soil at sowing. Therefore, a coating process was developed to utilize seed as a soil surfactant carrier. Greenhouse experiments were conducted to evaluate establishment of perennial ryegrass (Lolium perenne L.) seed coated with a soil surfactant (SCS = surfactant- coated seed), as compared to uncoated seed (CHK). Both SCS and CHK were sown in two hydrophilic substrates (100% sand [WSAND], 90:10% v/v sand:peat [WSP], and in two severely hydrophobic sands (100% hydrophobic stearic acid-treated sand [HSS], and hydrophobic 90:10% v/v sand:peat [HSP]). Due to the weight of the coating, SCS was sown at half the amount as compared to CHK; however, final turfgrass establishment in all rootzones with SCS was similar or better than CHK. In WSAND, WSP, and HSP, the volumetric water content was consistently higher in rootzones of SCS treatments versus CHK. SCS represents an opportunity to improve stand establishment and rootzone soil-water dynamics in challenging environmental conditions such as limited precipitation.Species used in this study: Perennial ryegrass, Lolium perenne L.

Impact force of post-fire debris flows over erodible beds

After wildfire events, water repellent soil is often found in the subsurface layer of channel bed in the burnt area. Debris flows generated from burnt basins and ensuing entrainment of the channel bed pose imminent threat to infrastructure and human lives. However, the fundamental interaction mechanisms of debris flow overriding water repellent bed and resulting impact force on debris-resisting barriers have yet to be elucidated. In this study, physical flume experiments are conducted to simulate post-fire debris flows overriding and entraining a sand bed with varied wettability. Compared to a wettable bed, water repellent sediment exhibits a tremendous increase in the erosion depth and subsequent impact force on the barrier. The test results demonstrate that debris flows overriding water repellent sediment can be particularly hazardous and the effects of water repellency need to be captured by the design criteria of debris resisting barriers in burnt basins.

Soil water repellence increased early wheat growth and nutrient uptake

PurposeSoil water repellence causes uneven soil wetting which can constrain dryland crop and pasture establishment and yield. The same processes are likely to affect nutrient availability from soil and fertiliser, but the effects of repellence on crop growth and nutrition per se have seldom been reported. Here, we investigated early wheat (Triticum aestivum cv. Mace) growth and nutrient uptake responses to repellence.MethodsWheat was furrow-sown in severely repellent sandy loam soil (with a wettable furrow base to allow for germination) or completely wettable soil, under uniform plant density and variable topsoil thickness (20 or 100 mm) and fertiliser band placement (below or away from the seed). Tiller number, shoot dry matter, shoot N concentration, total nutrient uptake, and root length density (RLD) were determined.ResultsContrary to expectations, repellence significantly increased tiller number (by up to 2 tillers per plant), shoot dry matter (by 82%), shoot N concentration (by 0.3% N), and total nutrient uptake (by 87%) at 51 days after sowing, regardless of topsoil thickness and fertiliser placement. In the furrow, RLD of repellent treatments was also nearly double that in wettable treatments when fertiliser was banded below the seed. Results suggest that preferential soil wetting of the furrow in repellent treatments favoured plant nutrient uptake under regular but low water supply.ConclusionWe conclude that for water-repellent soils with limited water supply, water harvesting techniques such as furrow sowing and banding wetting agents could boost water and nutrient uptake and early crop growth.

Control Potentials of Three Entomopathogenic Bacterial Isolates for the Carob Moth, Ectomyelois ceratoniae (Lepidoptera: Pyralidae) in Pomegranates

Ectomyelois ceratoniae (Lepidoptera: Pyralidae) is the primary pest of pomegranates in Saudi Arabia and is mostly controlled using broad-spectrum pesticides. Providing environmentally sound choices to limit reliance on chemical management is a major challenge in the control of E. ceratoniae and, as a consequence, in the protection of pomegranate crops from its invasion. Entomopathogenic bacteria (EPB) symbiotically associated with entomopathogenic nematodes (EPNs) are well-known biocontrol agents of soil-dwelling or aerial pests. The bacterium symbiont (EPB) is the real insect-killing biocontrol agent, while the nematode (EPN) serves as a vector. We wondered whether the EPB vector, which is extremely vulnerable to adverse environmental conditions, like drought, high temperatures, and repellent soil microorganisms, could be omitted. We intended to evaluate the biocontrol potential of directly applied EPB cells and cell-free culture media (CFCM) on the larval instar E. ceratoniae. Xenorhabdus budapestensis DSM 16342 (EMA), X. szentirmaii DSM 16338 (EMC), and Photorhabdus luminescens ssp. laumondi (TT01) strains were used. After three days of exposure, the cells of EMA, EMC, and TT01 strains resulted in 100%, 88%, and 79.3% larval mortality rates, respectively. The applied EMA CFCM resulted in 53.7% larval mortality, indicating the presences of (at least) one extremely strong component produced by EMA. We concluded that the direct application of either the EPB cells or the CFCM must be a prospective alternative biocontrol of E. ceratoniae, especially to protect the important fruit (pomegranate, Punica granatum) cultivars. Especially, newly identified local EPB isolates could be applied as bio-pesticides for integrated management practices or organic pomegranate production.

Observation and simulation of water movement and runoff in a coarse texture water repellent soil

Water repellent or non-wetting soils show poor water infiltration, which leads to an increased runoff in both natural and agricultural setting. Some forest areas of northern Iran are characterized by extreme water repellency, which makes their soil hydrological response highly complex. A field experiment was carried out to study the effect of water repellency on water movement and runoff in coarse-textured soil covered in Pinus teada in northwestern Iran. An adapted version of the popular agrohydrological model SWAP (Soil, Water, Atmosphere and Plant) has been used to evaluate the modelling of soil water flow and runoff generation. Water repellency was accounted for by multiplying the soil water content and the unsaturated hydraulic conductivity of soil with the F factor. This factor is equal to the volumetric fraction of soil occupied by preferential flow paths. Twenty-seven rainfall experiments were carried out under artificial rainfall using soils with various wettability levels and various rainfall intensities (14 mm/h, 27 mm/h and 34 mm/h). The result show that during the summer and early autumn (May to October) the soil became extreme repellent and runoff was formed when the soils were dry (θ<2%) and the rain intensity was more than 27 mm/h. Also, we investigated the role of the repellent layer on evaporation losses. The results of laboratory and simulation data confirm that water repellency reduces evaporation from the soil surface. Comparing modelling results with field data shows that an assumption of uniform flow in water repellent soil leads to an overestimation of soil water content and an underestimation of runoff. After incorporating water repellency in SWAP with the mobile–immobile concept, the agreement between measured and model simulation data considerably improved.

Changes in soil properties and crop performance on stubble-burned and cultivated water-repellent soils can take many years following reversion to no-till and stubble retention

No-tillage and stubble retention have been widely adopted in south-western Australia to protect agricultural soils from erosion, but potentially exacerbate repellency. In a 10-year (2008–2017) field experiment, 4 combinations of tillage (no-till vs cultivation) and stubble management (retention or burning) were applied annually from 2008 to 2011. In 2012, all plots were returned to no-till and stubble retention to determine the recovery of crop performance and soil properties from stubble burning and/or cultivation, and this recovery phase is reported here. Changes to ground cover, soil water repellency, soil water content, soil carbon and crop performance during a 6-year period (2012–2017) were compared with plots where no-tillage and stubble retention had been practised for several decades. At the conclusion of monitoring in 2017, ground cover (average 55%) and soil carbon (average 1.2%) in the previously burned treatments remained significantly (P < 0.05) less than in treatments where stubble had always been retained (average ground cover 68% and soil carbon 1.7%). Soil water repellency (Molarity of Ethanol Drop (MED)) varied with seasonal conditions but was always highest (P < 0.001) in the treatment which had been continuously no-till, stubble retained (MED 1.7 in 2017) compared with the treatment that was previously burned and cultivated (MED 1.0 in 2017), but this did not influence soil water contents which were generally higher (P < 0.05) in the more repellent soils. The effect of previous tillage treatment on soil water contents resolved quickly after the reversion to no-tillage, likely due to the rapid establishment and preservation of root pathways to aid water infiltration. However, the effect of stubble burning on soil water contents remained significant (P < 0.001) with 2% v/v less water in 2017 in previously burned treatments compared with those where stubble had always been retained. Average grain yields where stubble had always been retained were significantly (P < 0.001) greater by 22–33% than where stubbles had previously been burned in 4 of the 6 seasons. The poor recovery in crop performance in the previously stubble-burned treatments was attributed to the slow build-up of soil carbon to support water- and nutrient-holding capacity, and crop residue ground cover to protect soils from water loss and erosion.

Remediating the Adverse Effects of Treated Wastewater Irrigation by Repeated On‐Surface Surfactant Application

AbstractThe use of treated wastewater (TWW) has gained recognition as an alternative source for freshwater irrigation, and is steadily expanding worldwide. Despite the benefits of freshwater conservation and nutrient richness, there is mounting evidence of TWW adverse effects on soil, yield, and the environment. Irrigation using TWW has resulted in soil water repellency, in which preferential flow pathways and uneven soil water and chemical distribution occur. These increase deep water percolation and chemical leaching, which can lead to soil and groundwater pollution. This study was conducted in a commercial citrus orchard grown on sandy‐loam soil in central Israel and irrigated with TWW, with the aim of investigating the remediation of these adverse effects, by repeatedly spraying a nonionic surfactant on the soil surface. The surfactant application succeeded to turn the soil wettable, diminishing the preferential flow pathways, and rendering the soil water and dissolved chemicals uniformly distributed. The overall water content in the 0–40 cm layer increased, and deep percolation and chemical leaching substantially decreased. The grapefruit yield increase during the two‐year study period increased the water use efficiency. Electrical resistance tomography scans executed during and after irrigation events for two subsequent years revealed that a “soil memory” phenomenon has been developed for water repellent soils, where water flow takes place through previously developed preferential flow pathways in such soils. This study demonstrates that recurrent surfactant application enables a continuous use of TWW, while eliminating most of its prejudicial effects.

Projection of the climate change effects on soil water dynamics of summer maize grown in water repellent soils using APSIM and HYDRUS-1D models

Soil water repellency greatly affects crop growth and soil water movement. The aim of this study was to estimate dynamics of soil water storage (SWS), actual evapotranspiration (ETa), root water uptake (RWU) and actual evaporation (Ea) under an annual crop grown in water repellent (WR) soils at future climate scenarios. The soil hydraulic parameters were calibrated and validated for HYDRUS-1D based on the experimental data in 2016 and 2017. The summer maize growth periods and irrigation schedules were generated with Agricultural Production Systems Simulator (APSIM). The daily SWS, ETa, RWU and Ea values from five water repellent treatments were simulated for summer maize growth periods during 1981–2000, 2030–2059 and 2060–2089 using eight selected global climate models under two representative concentration pathways (RCP 4.5 and RCP 8.5). Due to the increased temperature, the growth period reduced by 12–27 days, the total SWS, ETa, RWU and Ea decreased by 8.1%-21.1%, 2.2%-11.1%, 0.5%-9.7% and 0.8%-9.6% compared to the baseline period, respectively. Changes of total SWS, ETa, RWU and Ea during the whole summer maize growth periods under RCP 4.5 were greater than RCP 8.5 during the same period. Values of total SWS, ETa, RWU and Ea in 2030–2059 were higher than 2060–2089 for the same RCP scenario. With increasing initial water droplet penetration time, total SWS and Ea increased, while ETa and RWU decreased. The global circulation model (GCM) and Period contributed greatly to uncertainty. The results implied that it is necessary to adjust the planting date of summer maize.

The applicability of HYDRUS‐1D to infiltration of water‐repellent soil at different depths

AbstractHYDRUS‐1D has been extensively applied to the simulation of wettable soil infiltration, but very few studies have explored its applicability to the infiltration of water‐repellent soil (WRS) at different depths. To this end, HYDRUS‐1D was used to simulate water movement of layered soil with a surface repellent thickness of 10 cm. The parameters of the van Genuchten model were calibrated and validated using cumulative infiltration (CI), distance of the wetting front (Zf) and volumetric water content (θv) through five respective treatments. Another 10 tilled scenarios were selected to simulate water movement of tilled WRS. RRMSE (relative root mean square error) and CRM (coefficient of residual mass) were adopted to evaluate simulation accuracy. For the calibration and validation, the RRMSE values ranged from 0.04 to 0.251 and 0.042 to 0.101, respectively, indicating that HYDRUS‐1D could effectively simulate water movement of WRS. Furthermore, for layered treatments, the simulation accuracy of θv, CI and Zf decreased with the increase of the degree of repellency and thickness. The treatments with the repellent thickness of 10 cm had higher simulation accuracy than that with repellent thickness of 5 cm when the top layer had the same WRS. The results revealed HYDRUS‐1D was effective in the simulation of water movement in repellent clay loam with different degrees of repellency and depths.Highlights The applicability of HYDRUS‐1D to the infiltration of water‐repellent soil (WRS) at different depths was explored. The simulation of different degrees of repellency and depths was first conducted. Simulation accuracy decreased with the increase of degree of repellency and thickness. HYDRUS‐1D was effective in simulating water movement of repellent soil.

A Simple Correction Term to Model Infiltration in Water‐Repellent Soils

AbstractSoil water repellency can substantially alter hydrologic processes, particularly the ability of soils to infiltrate water. Water repellency often changes through time, making it difficult to simulate infiltration behaviors of water‐repellent soils using standard models. Here, we propose a simple rate‐based correction term that starts with a value of zero at the beginning of the infiltration process (t = 0) and asymptotically approaches 1 as time increases, thus simulating decreasing soil water repellency through time. The correction term can be used with any infiltration model. For this study, we selected a simple two‐term infiltration equation and then, using two data sets of infiltration measurements conducted in soils with varying water repellency, compared model error with versus without the added term. The correction substantially reduced model error, particularly in more repellent soils. At the same time, the rate constant parameter introduced in the new model may be useful to better understand dynamics of soil water repellency and to provide more consistent interpretations of hydraulic properties in water‐repellent soils.

Reduced root water uptake of summer maize grown in water-repellent soils simulated by HYDRUS-1D

Soil water repellency (SWR) is an ubiquitous soil property, that has major effects on surface and subsurface water flow, soil erosion, and therefore also affects plant growth and development. Soil water repellency has been recently observed to decrease summer maize growth, especially when the initial persistence of soil water repellency was high. However, mechanics implicated in maize biomass limitations are still unclear due to limited field observations and measurements of plant water use. This research aims to explain the possible mechanisms that impede the growth of summer maize in water repellent soils by examining soil water content, soil water evaporation and root water uptake (RWU) and comparing the measurements to simulations using HYDRUS-1D. Data from five increasing levels of SWR (CK, WR1, WR2, WR3 and WR4) from a two-year experiment were used. The soil hydraulic parameters described with the van-Genuchten model were calibrated inversely based on the observed volumetric soil water content at 26, 47 and 68 days after sowing (DAS) in 2016 and validated by water content values on 33, 62 and 91 DAS in 2017. The results showed the daily and cumulative RWU values ranked in an order of CK > WR1 > WR2 > WR3 > WR4. The RWU rates and cumulative RWU decreased with an increase of initial water droplet penetration time (persistence), which indicated weaker RWU ability of summer maize in the water repellent treatments. Thirty days after sowing, higher evaporation was observed in water repellent soils compared to CK. Weak RWU of summer maize grown in the water repellent soils and strong soil water evaporation were found. This research demonstrates the possible mechanisms that impede summer maize growth in water repellent soils could include weak RWU and high evaporation.

Seeding next to previous year’s crop row (near-row sowing) can increase grain yields on water repellent soils

Context The combination of no-till and stubble retention has been shown to preserve old crop roots, which behave as pathways for water infiltration into water repellent soil, by-passing repellent surface soil layers. Aim To evaluate the benefits to soil properties and crop performance of seeding close to the previous season’s crop rows (near-row sowing) compared with inter-row sowing on water repellent soils. Methods At four field sites, near Moora, Pingrup and Calingiri in Western Australia and Wanilla in South Australia, measurements were made of: (1) crop performance of near- and inter-row sown crops (Moora and Wanilla); and (2) differences in soil properties between the crop row and inter-row at Wanilla, Calingiri and Pingrup. Key results Biomass accumulation (Moora) and grain yields (Moora and Wanilla) were significantly improved by near-row sowing compared with inter-row sowing, particularly under no-till and stubble retention, but these differences were reduced after cultivation, which either buried repellent surface soils or disrupted root pathways. At Calingiri and Pingrup, where near-row sowing had been practised for ≥4 years, and at Wanilla, soil water contents were higher in the crop row than the inter-row by up to 4% v/v, and this was associated with significantly reduced repellency (Calingiri and Pingrup) and larger communities of wax-degrading bacteria (Pingrup). Conclusions Near-row sowing may enhance crop production directly through improved water infiltration down root pathways, and indirectly by reduced soil water repellency in the row. Implications Near-row sowing is potentially a low-cost management for enhanced crop production on water repellent soils.

Increased colluvial hollow discharge and subsequent recovery after a low intensity wildfire in the Blue Ridge Mountains, USA

AbstractWildfires in mountainous regions have been documented to enhance water repellent soils which can increase runoff, erosion, and sedimentation during subsequent rain events. However, the extent of soil hydrophobicity and water repellency varies significantly with burn severity and between ecosystems, and the southern Appalachians remain an understudied region. Here we examine the impact of the low severity Chestnut Knob Fire, which occurred in the fall 2016, on soil properties and runoff in South Mountains State Park. To examine these impacts, we installed crest‐stage gauges in burned (n = 10) and unburned (n = 8) colluvial hollows to compare peak runoff. Results from the 2017 field season indicated that burned locations produced significantly higher peak discharges than unburned sites. From July 2019 to January 2020, we repeated the experiment and found that burned areas produced runoff comparable to unburned areas. Examination of soil profiles during the summer of 2017 found high variability in hydrophobicity in both the burned (n = 10) and unburned (n = 2) soils. Further, we found that burned soils had significantly deflated organic surface horizons compared with unburned soils. We interpret the differences in runoff in 2017 to be the result of a combination of increased hydrophobicity and decreased soil moisture storage capacity in organic rich surface soils. While the recovery we observed here was relatively fast, it is important to understand that increased runoff immediately after a fire likely increases the chances of sediment mobilization and debris flow occurrence.

Quantifying the Effect of Subcritical Water Repellency on Sorptivity: A Physically Based Model

AbstractSoil water wettability or water repellency is a phenomenon that can affect infiltration and, ultimately, runoff. Thus, there is a need to develop a model that can quantitatively capture the influence of water repellency on infiltration in a physically meaningful way and within the framework of existing infiltration theory. The analytical model developed in this study relates soil sorptivity (an infiltration parameter) with contact angle (a direct measure of water repellency) for variably saturated media. The model was validated with laboratory experiments using a silica sand of known properties treated to produce controlled degrees of water repellency. The measured contact angle and sorptivity values closely matched the model‐predicted values. Further, the relationship between the frequently used water drop penetration time test (used to assess water repellency) and sorptivity was illustrated. Finally, the direct impact of water repellency on saturated hydraulic conductivity was investigated due to its role in infiltration equations and to shed light on inconsistent field observations. It was found that water repellency had minimal effect on the saturated hydraulic conductivity of structureless sand. A quantitative model for infiltration incorporating the effect of water repellency is particularly important for post‐fire hydrologic modeling of burned areas exhibiting water repellent soils.

The Impact of Soil Water Repellency and Slope upon Runoff and Erosion

Soil water repellency (SWR) increases the amount of runoff and erosion from soils. Previous experiments on water-repellent soils had shown intricate runoff movement and high rates of water and soil losses; our aim was to observe these processes, noting their influences and quantifying impacts. The experiment used a 60 × 60 cm rainfall simulation plot setup to represent an agricultural plot with packed water repellent soil and no interference of plants. We measured inputs, and monitored surface flows and erosion on 3°, 6° and 9° slopes. A surfactant treated plot was used as a control. Eroded sediment samples were collected and measured for particle size, and together with runoff analysed for organic carbon, nitrogen and macronutrients. Runoff coefficients were high (0.53 to 0.78) for untreated soil with erosion decreasing over the course of the experiment (1.41 t ha-1 to 0.74 t ha-1 over five 2 mm rainfall events, of 2 min duration, on the 9° slope). This resulted in cumulative erosion of up to 5.35 t ha-1 after a total of 10 mm of rainfall (5 × 2 mm). Importantly, silt and clay were preferentially eroded (up to 12 × higher in eroded soil than the baseline composition) and this correlated with the loss of organic carbon, nitrogen, and macronutrients. As expected, the steepest slope (9°) caused greater runoff and erosion than the lowest (3°), however, there was no significant difference in runoff and erosion between the 6° and 9° slope. Surfactant significantly decreased runoff coefficients by two orders of magnitude compared to the untreated soil. Surface flow was initially characterised by beading and rivulet formation, minimising soil contact. Over time, however, a perched, protective water layer formed over the surface, allowing subsequent water flow with reduced soil interaction. The runoff mechanisms for these small rain events were only observed on the water-repellent soils and likely reduced the degree of erosion given the high runoff coefficients. Initial water flow also promoted armouring of the soil by the attraction of smaller particles, leading to increased loss of organic carbon and nitrogen as well as macronutrients.

Measuring water repellency of individual particles: The new “micro-Wilhelmy Plate Method” and its applicability to soil

Water repellency (hydrophobicity) of granular materials such as soil is usually assessed on bulk samples or arrays of grains, with their wettability being influenced by the often-variable properties of individual particles. Numerous methods exist to assess the wetting behaviour of granular bulk materials, whereas methods for determining the wettability of individual grains are scarce. Here we introduce a new technique, based on the Wilhelmy plate method and termed “micro-Wilhelmy plate method” (mWPM) that allows quantification of the water repellency of an individual particle.We developed two complementary variants of the methods, which involve the rupture of a water lamella after a particle has been brought into contact with water and then withdrawn from it. They were applied to individual wettable or water repellent spherical glass of diameter 120 and 270 µm and polymer particles (270 µm), as well as those of both wettable and natural water repellent particles (of similar size i.e. 120 to 270 µm) from sandy soils of the UK and the Netherlands. Spherical glass and polymer particles were examined in their native condition and following treatment with a hydrophobic (silicone-based water proofing) agent. In one method the break point of lamella was determined gravimetrically (g-mWPM, using an electronic 5-digit balance) and in the other it was determined optically (o-mWPM) from a video sequence obtained from a contact angle goniometer as the distance between the particle and the water surface at the point of lamella rupture. The latter required the use of image analysis and computation to estimate the potential energy of the water lamella.Both methods provided meaningful assessments of particle wettability. Man-made particles showed limited variability, whereas those drawn from naturally wettable or water repellent soils exhibited substantial variability, indicating that wettable soils contain water repellent particles and vice versa. Both methods introduced here offer a relatively quick examination of the wettability of individual particles. The o-mWPM is a particularly simple method only requiring a video camera, stepper-motor driven sample holder and a low magnification optical system. Additionally, it offers the possibility to investigate particle shape.