Changes In Soil Water Repellency Research Articles

Ground Fire Legacy Effects on Water-Dynamics of Volcanic Tropical Soils

The forest floor is a critical component in maintaining the life cycles of forest ecosystems. It normally includes organic soil horizons, known as duff and litter, which are prone to be rapidly consumed after flaming and smoldering fires. This work aims to understand the legacy effects of surface and ground fires on the infiltration capacity of a volcanic forest soil. We studied five sites with fires recorded in the last 20 years. All of them are located in pine-oak forests of the volcanic mountain region in central Mexico with a temperate climate and Andic soil properties. Tension-infiltration tests were carried out to determine hydraulic conductivity and the number of active macropores. After each test, cores were taken to evaluate in a laboratory setting, where soil water repellency at different moisture concentrations and the integrative dynamic repellency index were determined. Field-saturated hydraulic conductivity was moderately high in all sites, with mean values of 13 and 42 mm·h−1 for burned and control plots, respectively. A non-linear relationship was found between recurrence and type of fires with the concentration of active pores and several dynamic water repellency parameters. This work confirmed the presence of latent combustion in these temperate neotropical forests. The changes in soil water repellency and hydraulic conductivity detected do not necessarily imply an exceeded soil infiltration capacity. However, many of the fires in this region are associated with increasing agricultural activities, so further studies are needed to determine if higher fire frequencies could exceed the resilience capacity of the soils triggering land degradation.

Testing soil water repellency in a Sicilian area two years after a fire

The water drop penetration time (WDPT) technique was applied in 2018 to check persistence of soil water repellency (SWR) in a Sicilian mountain area affected by a wildfire on June 2016. A total of four sites, that were severely water repellent immediately after burning, were sampled. Depending on the site, wettable soil conditions, less SWR and maintenance of a noticeable SWR were detected two years later. At the site showing a near-constant SWR, WDPTs were particularly high in the top soil layer (0-0.03 m) and they appreciably decreased more in depth. Signs of decreasing SWR in drier soil conditions and in association with coarser soil particles were also detected at this site. High gradients of the WDPT can occur at very small vertical distances and a depth increment of approximately 0.01 m should be appropriate to capture small-scale vertical changes in SWR, especially close to the soil surface. Occurrence of SWR phenomena is easily perceivable and explainable if an inverse relationship between WDPTs and antecedent soil water content is obtained. A direct relationship between these two variables is more difficult to interpret because infiltration times that increase in wetter soil are expected according to the classical infiltration theory. A hypothesis that should be tested in the future is to verify if WDPTs that decrease in drier soil conditions signal less SWR as a consequence of a reduced biological activity of the soil. Finally, long-term monitoring projects on longevity of fire effects on SWR should be developed, even because an in depth knowledge of the involved processes is relevant for the civil protection system.

Soil water repellency after slurry fertilization in a dryland agricultural system

In the context of the circular economy, the use of animal excrements as fertilizers is encouraged, but the addition of such organic materials can develop soil water repellency (SWR) or hydrophobicity. There is a lack of consensus about how to assess SWR in agricultural soils. This work evaluated SWR when pig slurries were applied onto soil using the two most common testing methods: the water drop penetration time (WDPT) and the molarity of ethanol droplet (MED) tests. The experiment consisted of five different slurry treatments plus a control (no slurry added). At sowing, slurry from fattening pigs (SF) or no slurry application was combined with a second application at the cereal tillering stage, in which SF, slurry from sows (SS) or no-slurry were the treatments. Soil water repellency was tested at cereal tillering before slurry application and then during the following 47 days. At each sampling date, hydrophobicity was tested in undisturbed samples at field-moist and after 25 °C, 65 °C, and 105 °C oven drying. Disturbed samples were tested after 40 °C oven drying. Soil disturbance removed SWR. Under field conditions, undisturbed samples attained the maximum SWR expression 7d from pig slurry application; moderate and very severe scores using WDPT and MED were respectively attained. From 14d to the end of the experiment, the highest SWR was observed after 105 °C oven drying and when SF applications at sowing (900 kg TOC ha−1) had been combined with SS applications at cereal tillering (1894 kg TOC ha−1). Slurry hydrophobic compounds rather than slurry dry matter influenced SWR expression which is enhanced as the soil dries. The persistence of repellency (WDPT test) was more sensitive to detecting changes in SWR between treatments than the changes in its severity (MED test). The importance of the SWR described hereafter slurry application will depend on plant cover over the soil and the effects in slaking prevention in order to avoid superficial water runoff.

Water repellency in Japanese coniferous forest soils as affected by drying temperature and moisture

Soil water repellency occurs where the attraction between solid and liquid phases is weak. The objective of this study was to examine the occurrence and behavior of water repellency in Japanese coniferous forest soils as affected by drying temperature and moisture. Soil samples were obtained from forest soils under Chamaecyparis obtusa (CYP) and Cryptomeria japonica (CED), and an uncultivated land (weed and grass cover) in the Field Science Center of Ibaraki University (FSC) in Ibaraki prefecture, Japan. The persistence of soil water repellency (SWR) was estimated using the water drop penetration time (WDPT) test and the severity of SWR using the molarity of ethanol droplet (MED) test. In Experiment 1, soil samples were exposed to 25 °C and 35 °C temperatures for a period of 30 d. SWR and moisture contents were measured at weekly intervals. In Experiment 2, moisture-dependent repellency was examined in a drying process. CED soil was extremely repellent (WDPT>3600 s; contact angle>106°) and FSC soil was non-repellent (WDPT<1 s). The CYP soils were strongly (35 °C) to severely (25 °C) repellent compared with CED soils, and more prone to significant changes caused by temperature and duration. In CED soil, the changes in contact angles were slight while moisture content variations were significant. In CYP soils, the changes in repellency were significant while moisture content variations were slight. The integrated area below the water-dependent repellency curve (SWR) was greater in CED soil (~140 log s g 100 g−1) than in the CYP soils (~60 log s g 100 g−1). Slight changes in moisture content may or may not cause severe changes in SWR depending on the level of moisture content. These alterations in moisture content in the surface soils may occur as a result seasonal environmental changes and contribute to topsoil loss in mountain areas with high slopes.

Changes in soil water repellency after prescribed burnings in three different Mediterranean forest ecosystems

Prescribed burnings reduce the biomass and the risk of wildfires but can also alter soil water repellency. The objective of this study is to evaluate the impact of several prescribed burnings in soil water repellency (SWR). In spring 2016, prescribed burns were carried out at three forest sites located in: (i) Beteta in a pure forest of Pinus nigra Arnold ssp. salzmannii; (ii) El Pozuelo in mixed forest stands of Pinus pinaster Aiton and Pinus nigra Arnold ssp. salzmannii; (iii) Lezuza in mixed forest stands of Pinus halepensis Miller and Pinus pinaster Aiton. Six plots were established in each study area: three burned and three unburned. SWR was measured before and immediately after prescribed burns following a 1-year periodic evaluation. There were seven sampling dates at Beteta and El Pozuelo and nine at Lezuza with six plots and six measurement transects in each plot (36 measurement transects on each date). Soil water content (SWC), soil temperature (ST) and soil organic matter (SOM) were also measured. Our results showed that SWR increased after burning to quickly return to normal values in Lezuza, after 1 month in El Pozuelo and after 1 year in Beteta. Moreover, a significant positive relationship between SWR and both SOM and ST, but a negative one with SWC, were observed, which led SWR to increase after fire passage, also in the summer months. Continuous monitoring of these study sites is recommended to determine if low-intensity burnings promote mid- to long-term changes in soil characteristics.

Developing generalized parameters for post-fire erosion risk assessment using the revised Morgan-Morgan-Finney model: A test for north-central Portuguese pine stands

Models can be useful for predicting the hydrological impacts of natural phenomenon such as wildfires and to help implement effective post-fire land management options. In this research, the revised Morgan–Morgan–Finney (MMF) model was used to simulate runoff and soil erosion in recently burned maritime pine plantations with contrasting fire regimes, in a wet Mediterranean region of north-central Portugal. The MMF model was adapted for burnt areas by implementing seasonal changes in model parameters in order to accommodate seasonal patterns in runoff and soil erosion, attributed to changes in soil water repellency and vegetation recovery. The model was then evaluated by applying it for a total of 18 experimental micro-plots (0.25 m2) at 9 once burned and 9 four times burned slopes, using both previously published and newly calibrated parameters, with observed data used to evaluate the robustness and wider applicability of each parameterization. The prediction of erosion was more accurate than that of runoff, with an overall Nash-Sutcliffe efficiency of 0.54. Slope angle and the soils' effective hydrological depth (which depends on vegetation and/or crop cover) were found to be the main parameters improving model outcomes, and different parameters were needed to differentiate between the two contrasting fire regimes. This case study showed that most existing benchmark parameters can be used to apply MMF in burned pine forest areas with moderate severity fires to support post-fire management, but indicated that further efforts should focus on mapping soil depth and vegetation cover to improve these assessments.

Seasonal changes in soil water repellency of different land use types in Inner Mongolia grassland

Soil water repellency (SWR) is a physical phenomenon in which the rate of wetting is restricted and water beads are formed on the soil surface where the soil is called water-repellent soil. SWR analysis is necessary to interpret nonuniform wetting and preferential flow for site-specific ecosystems. This study aims to investigate the effects of soil water conditions and land use types on SWR in Leymus chinensis steppe soil in Xilinhot, Inner Mongolia, China. The main objective is to determine how SWR in soils in Inner Mongolia is influenced by different grassland utilization types, including no grazing since 1979 and 1999, winter grazing, and continuous grazing, by using water drop penetration time (WDPT). Results show that a slightly hydrophobic or hydrophilic soil behavior was often observed, and SWR was most evident in the months with the highest evaporation despite heavy rainfall. Changes in SWR in different seasons were affected by grazing intensity. The highest number of hydrophobic soil samples was found in the continuous grazing sites, followed by that in the winter grazing sites. Few hydrophobic soil samples were detected in the ungrazed sites since 1979, and the lowest number of samples was observed in the ungrazed sites since 1999. The SWR of different particle size classes decreased with increasing particle size. Therefore, SWR was significantly correlated with soil particle size class at 0–0.05 and 0.05–0.1mm. These results emphasized the need to consider SWR in soil utilization because grazing intensity affects the soil repellent properties in steppe soil and provided a basis for the development of grassland management.

Temporal changes in soil water repellency after a forest fire in a Mediterranean calcareous soil: Influence of ash and different vegetation type

Forest fires usually modify soil water repellency (SWR), and its persistence and intensity show a high variability both in space and time. This research studies the evolution of SWR in a Mediterranean calcareous soil affected by a forest fire, which occurred in Gorga (SE Spain) in July 2011, comparing the effect of the main vegetation cover between pine (Pinus halepensis) and shrubs species (Quercus coccifera, Rosmarinus officinalis, Cistus albidus, Erica arborea and Brachypodium retusum) and the relationship with soil moisture content (SMC). Also the study analyzed the effect of ash on SWR dynamics under field conditions. Six plots were established on the fire-affected area and the unburned–control–adjacent area to monitoring SWR with the water drop penetration time (WDPT) test, SMC through moist sensors (5cm depth) and three different ash treatments: ash presence, ash absence and incorporation of ash into the soil. An immediate increase of SWR was observed in the fire-affected area, mainly in pine plots. SWR changes in control (unburned) plots were quite similar between different types of vegetation influence, despite higher SWR values being observed on pine plots during the study period. A noticeable decrease of SWR was observed during the first months after fire in the affected areas, especially after the first rainy period, both in pine and shrubs plots. SWR increase was registered in all plots, and the highest levels were in March 2012 in burned pine plots. SWR decrease was higher in plots where ash was removed. Fire-affected soils became wettable 1year and a half after the fire.

Patterns of soil water repellency change with wetting and drying: the influence of cracks, roots and drainage conditions

AbstractLaboratory experiments were used to investigate the influence of simulated cracks and roots on soil water repellency (SWR) dynamics with and without basal drainage impedance in wetting–drying cycles. Observations and measurements were taken following water application equivalent to 9.2‐mm rainfall and then periodically during 80 h of drying. In total, 180 experiments were carried out using 60 samples of three homogeneous, reconstituted soils with different organic matter contents and textures, but of similar initial severity of SWR [18% molarity of an ethanol droplet (MED)]. Water flowing down the cracks and roots left the soil matrix largely dry and water repellent except for vertical zones adjacent to them and a shallow surface layer. A hydrophilic shallow basal layer was produced in experiments where basal drainage was impeded. During drying, changes in SWR were largely confined to the zones that had been wetted. Soil that had remained dry retained the initial severity of SWR, while wetted soil re‐established either the same or slightly lower severity of SWR. In organic‐rich soil, the scale of recovery to pre‐wetting MED levels was much higher, perhaps associated with temporarily raised levels (up to 36% MED) of SWR recorded during drying of these soils. With all three soils, the re‐establishment of the original SWR level was less widespread for surface than subsurface soil and with impeded than unimpeded basal drainage.Key findings are that as follows: (1) with unimpeded basal drainage, the soils remained at pre‐wetting repellency levels except for a wettable thin surface layer and zones close to roots and cracks, (2) basal drainage impedance produced hydrophilic basal and surface layers, (3) thorough wetting delayed a return to water‐repellent conditions on drying, and (4) temporarily enhanced SWR occurred in organic‐rich soils at intermediate moisture levels during drying. Hydrological implications are discussed, and the roles of cracks and roots are placed into context with other influences on preferential flow and SWR under field conditions. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Assessing the role of pre-fire ground preparation operations and soil water repellency in post-fire runoff and inter-rill erosion by repeated rainfall simulation experiments in Portuguese eucalypt plantations

Abstract This study addressed the impacts of contrasting pre-fire ground preparation operations on post-fire runoff and inter-rill erosion in six eucalypt plantations in north-central Portugal, with a special emphasis on the role of soil water repellency in the seasonal patterns of overland flow generation. To this end, a down slope ploughed, a contour ploughed and a terraced site were compared with three unploughed sites. Runoff and erosion data were collected in the field by carrying out rainfall simulation experiments (RSEs) with two intensities (45–50 and 80–85 mm h− 1) at six occasions during the first one to two years following wildfires in 2005 and 2006. Overall runoff coefficients varied markedly amongst the six study sites and between the two intensities (7 to 55%). While runoff figures were comparable to those of prior RSE studies in recently burnt areas, overall sediment losses were comparatively low (7–155 g m− 2) but contained a substantial organic matter fraction (29–74%). Apparently, the inter-rill erosion rates were essentially sediment-limited, fitting in with the long history of intensive land use that is typical in the Mediterranean Basin. The hydrological and erosion impacts of the three pre-fire ground preparation operations were minor, probably because these operations took place several years before the latest wildfire. Overall, the two rainfall intensities produced the expected differences but this effect was only statistically significant for simultaneous RSEs. Furthermore, the effect of rainfall intensity varied markedly between the study sites and, occasionally, between the two plot pairs at the same site. This impeded an erosion risk ranking of the six study sites that was consistent for both rainfall intensities. Runoff and erosion rates did not decrease in a simple or pronounced manner with time-since-fire. These temporal patterns could in part be attributed to changes in soil water repellency but other factors were involved as well. Removal of the protective soil cover by litter in particular appeared to play a key role in the increase in sediment losses following logging and wood extraction.

Fire‐induced changes in soil water repellency increased fingered flow and runoff rates following the 2004 Huelva wildfire

AbstractWater repellency (WR) from fire‐affected soils can affect infiltration processes and increase runoff rates. We investigated the effects of fire‐induced changes in soil WR and the related soil hydrological response after one of the largest wildfires in Spain in recent years. The vertical distribution of WR in soil profiles was studied under oak and pine forests and the wetting pattern was analysed after rainfall simulations (85 mm h−1 during 60 min). After burning, the persistence of WR in soils under oaks increased in the upper 0–5 cm of soil in comparison with pre‐fire WR, but no significant changes were observed under pines. After a fire, WR was stronger and the thickness of the water‐repellent layer increased in soils under pines in the upper 0–16 cm of soil. The hydrophobic layer was thinner under oaks, where no strong to extremely water‐repellent samples were observed below 12 (in burnt soils) and 8 cm (in unburnt soils). Uniform wetting was observed through soil depth in burnt and unburnt soils under oaks, as a consequence of the prevailing matrix flux infiltration. Water was mostly stored in the upper few centimetres and soil became rapidly saturated, favouring a continuous rise in the runoff rate during the experiments. Moisture profiles under pines showed a heterogeneous wetting pattern, with highly irregular wetting fronts, as a result of wettable and water‐repellent three‐dimensional soil patches. In this case, runoff rates on burnt plots increased in relation to unburnt plots, but runoff generation reached a steady state after 25–30 min of simulated rainfall at an intensity of 85 mm h−1. Rainfall water infiltrated over a small part of the ponded area, where the vertical pressure of the water column overcame the WR. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Reaction of soil water repellency to artificially induced changes in soil pH

Soil water repellency (SWR) is widely thought to be influenced by soil pH, however, few studies have systematically investigated the relationship between these variables. Specifically the hypothesis that the pH may control repellency via changes in the variable surface charge of soil material has not yet been tested. In previous work, methods for changing soil pH have also involved changes in soil moisture, but it has been argued that the potential influence of soil moisture changes needs to be eliminated before the actual relationship between pH and SWR can be isolated. The paper addresses this research gap using a new method, which enables adjustment of the pH of soils with low moisture content via the gas phase and thus involves minimal change in moisture content, allowing the response of SWR to pH changes to be evaluated. The method was applied to 14 soil samples from Germany, The Netherlands, the UK, and Australia, using the water drop penetration time ( WDPT) as the indicator of SWR. Additionally, sessile drop contact angles (Θ sess ) were measured on the four samples from Germany and the titratable surface charge of these four soils was measured at selected pH values using a particle charge detector (PCD). Changes in SWR with soil pH were found to be influenced by the density and type of sites able to interact with protons at the available surfaces of organic and mineral materials in soil. The maximum SWR occurred for soil at natural pH and where the charge density was minimal. As pH increased, negative surface charge increased due to deprotonation of sites and WDPT decreased. Two types of behaviour were observed: i) a decrease in repellency with decreasing pH, probably because of a sufficient number of proton accepting surface sites with a significant amount of positive surface charge, ii) no decrease in repellency with decreasing pH in soils with insufficient proton accepting surface sites to develop significant positive surface charge. The data suggest that the availability and relative abundance of proton active sites at mineral surfaces, and those at organic functional groups influence the response of the soil samples to changes in pH. The variety of geographic origins and histories of the soils examined provides a distinction between site specific and more widely applicable soil characteristics of pH dependent SWR.

Intact ash and charred litter reduces susceptibility to rain splash erosion post‐wildfire

AbstractThis paper describes the changes in soil water repellency and soil hydrological and erosional responses to rainfall at small‐plot scale, arising from a prescribed fire immediately following burning and one year later in a Mediterranean heathland in the area of the Strait of Gibraltar (southern Spain). Very little research has been carried out about the modifications on the ground surface after fire immediately after burning. A prescribed fire was conducted to study short‐term changes of the ground surface immediately and one year following burning. After a prescribed fire, a homogeneous charred litter layer and ash‐bed covered the mineral soil surface. This cover stayed stable on the soil surface during a period of seven days, until strong winds redistributed litter and ashes. The hydrophobicity of the exposed surface (litter and ashes) decreased considerably in relation with the litter layer properties before the fire. Ponding, runoff coefficients and soil loss were determined using simulated rainfall over the litter layer, the ash‐bed and the bare soil. Significant differences were not detected between pre‐ and post‐fire soil loss rates while a charred litter and thick ash layer were present on the ground surface. Runoff and erosion rates increased and time to ponding and runoff decreased when the charred litter and ash layers were artificially removed and the bare soil was exposed. Although wildfires will increase soil erodibility, the trends observed in this study suggest that this increased susceptibility to erosion from rainsplash processes may be limited to some degree while an intact ash and charred litter layer is still present. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Fire effects and short-term changes in soil water repellency – Mt. Carmel, Israel

Abstract The Mediterranean ecosystem of the Carmel Mountain ridge in Israel is subjected to an increasing number of forest fires of various extents and severities due to intense human activities in the region. On 8 April 2005, a low-moderate severity forest fire occurred at the northwestern part of the ridge and burned more than 150 ha of natural vegetation. Soil water repellency (WR) is a property usually modified by the litter and soil organic matter combustion as a consequence of fire, which has implications for the hydrological balance in the affected soils. A field study was conducted with the following objectives: 1) to investigate in situ WR changes at three soil depths as a consequence of the fire, 2) to evaluate the short-term evolution of WR under field conditions, and 3) to study the relationship between pre-fire vegetation type and slope aspect on the persistence of WR in the burned area. Soil WR was measured by the Water Drop Penetration Time (WDPT) test. Measurements were conducted monthly at 31 field sites within the burned area over a period of seven months (April 2005–November 2005), and compared to adjacent unburned areas. Soil WR measurements included more than 3400 WDPT tests at soil surface and at 5 and 10 cm depths. The results indicate that fire induced WR in previously wettable soils exhibited high levels of persistence at the soil surface during the first six weeks after the fire, while at 5 cm depth WR persistence was lower. At 10 cm depth soil was mostly wettable. After six weeks the frequency of WR occurrence diminished at the soil surface and at 5 cm depth. In addition, WR was found to be highly related with the pre-fire vegetation type and with slope aspect.

Changes in soil water repellency in a pine–beech forest transformation chronosequence: Influence of antecedent rainfall and air temperatures

Soil water repellency often exhibits pronounced temporal variation, which can hamper the assessment of repellency levels at a site when samples taken on only one date are evaluated. These temporal variations are often correlated to the season of the year, although the factors which govern seasonal repellency trends and the pertinent time scales are often unknown. Our objective has been to elucidate the year-to-year temporal variability of soil water repellency and the relationships of soil water repellency to soil moisture, antecedent rainfall, and temperatures prior to sampling at a forest transformation chronosequence in the climatic conditions of Central Europe. The forest transformation practice of underplanting beech trees into monospecies Scots pine forests is interpreted as a “disturbance” of the established pine ecosystem. Soil water repellency of samples taken in two consecutive years during summer was assessed using the water drop penetration time and the ethanol percentage methods. The results indicated extremely high repellency, but also pronounced differences for both years. Much higher levels of repellency were observed in 2001 than in 2002. This difference could only partly be ascribed to higher soil water content in 2001. In the 5 days before sampling, rainfall and maximum daily temperatures were clearly higher in 2001. However, no differences in averaged weather conditions between the 2 years were observed for the 60 days before sampling. This suggests that the time scale pertinent to changes in repellency levels could be short, i.e. on the order of a few days.

Post‐fire Soil Water Repellency

Fire‐induced soil water repellency is a key control on post‐fire runoff and erosion rates, but there are few data on the persistence of soil water repellency and the soil moisture threshold at which water repellent soils become hydrophilic. This study used repeated sampling to quantify changes in soil water repellency over time and identify soil moisture thresholds for the loss of soil water repellency. The study area was a wildfire in the northern Colorado Front Range that burned 43 km2 of ponderosa and lodgepole pine forests in June 2000. Soil water repellency and soil moisture were measured periodically from June 2000 through June 2001 at 36 sites stratified by burn severity and nine unburned sites. Water repellency was assessed in the field at depths of 0 to 18 cm using the critical surface tension (CST) test. Soil water repellency was strongest in sites burned at high and moderate severity, decreased with increasing depth, and was spatially highly variable. The fire‐induced soil water repellency progressively weakened and became statistically nondetectable by 1 yr after burning. The effect of time since burning on soil water repellency was increasingly significant with increasing burn severity and progressively less important with increasing soil depth. The soil moisture thresholds at which water repellent soils become hydrophilic apparently increase with increasing burn severity. The data suggest soil moisture thresholds of approximately 10% for unburned sites, 13% for sites burned at low severity, and no less than 26% for sites burned at moderate and high severity.

The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal

Water repellency (hydrophobicity) is known to be temporally variable. Most studies indicate that soils are most repellent when dry and least repellent or non-repellent (hydrophilic) when moist. In several studies, attempts have been made to establish a critical soil moisture threshold, demarcating water-repellent and non-repellent conditions. The reported thresholds vary widely and the exact relationship between hydrophobicity and soil moisture remains far from being understood. Using field and laboratory measurements, this study explores the effect of soil moisture on water repellency for Portuguese sandy loam and loamy sand forest soils. The results indicate that for these soils, repellency is absent when soil moisture exceeds 28%, but show that after wetting, repellency is not necessarily re-established when soils become dry again. It is thought that short-term and seasonal changes in soil water repellency are not simply a function of variations in soil moisture as indicated in the literature. It is suggested that, after wetting, re-establishment of repellency may also require a fresh input of water-repellent substances. The mechanisms of wetting and drying in water-repellent soils are discussed and associated hydrological implications are explored.