Water Drop Penetration Time Test Research Articles

Biological soil crusts cause subcritical water repellency in a sand dune ecosystem located along a rainfall gradient in the NW Negev desert, Israel

Abstract The biological soil crusts (BSCs) in the NW Negev cause local water redistribution by increasing surface runoff. The effects of pore clogging and swelling of organic and inorganic crust components were intensively investigated in earlier studies. However, the effect of water repellency (WR) was not addressed systematically yet. This study investigates subcritical WR of BSCs in three different study sites in the NW Negev. For this purpose, three common methods to determine soil WR were used: (i) the repellency index (RI) method (ii) the water drop penetration time (WDPT) test and (iii) the Wilhelmy plate method (WPM). Furthermore, the potential influence of WR on local water redistribution is discussed and the applied methods are compared. We found the BSC to be subcritically water repellent. The degree of WR may only affect water redistribution on a microscale and has little influence on the ecosystem as a whole. The RI method was clearly the most appropriate to use, whereas the WDPT and the WPM failed to detect subcritical WR.

Occurrence and distribution of water repellency in size fractionated coastal dune sand in Sri Lanka under Casuarina shelterbelt

The occurrence of water repellency in Sri Lankan soils has not been extensively studied and reported so far. Although most Sri Lankan soils are readily wettable, some soils show water repellent conditions. In this study, we examined a dune sand under Casuarina equisetifolia shelterbelt in Sri Lanka. The objectives were to ascertain the occurrence and the distribution of water repellency along the particle size fractions and to perceive the role of intermixed organic material in producing water repellency. The soil type is locally known to be sandy Regosols (USDA classification: Ustic Quartzipsamments). Bulk soil samples were taken from 0–5, 5–10, and 10–15 cm depths. Soil samples were separated into six size fractions by passing through a set of sieves and further separated into two sets as washed (with water) and untreated. Soil organic matter (SOM) content and the water repellency were discretely determined in untreated and washed samples. Water drop penetration time (WDPT) test and modified sessile drop method were used to estimate the water repellency. Despite the low SOM content (<2%), water repellency was extreme (contact angle >118°; WDPT >3600 s) on the topmost layer (0–5 cm) and decreased with increasing soil depth. All the size fractions of the topmost layer showed significantly high repellency compared with those of the other two layers. Both the water repellency and the SOM content showed the highest values in the finest fraction and the lowest in the coarsest fraction. Washing samples with water decreased the WDPT in >96% of all size fractions in the 0–5 cm layer, and by lower proportions in the lower layers. Washing samples significantly decreased the contact angle of most size fractions although the decline did not exceed 15%. Washing with water removed a considerable portion of organic material. Despite the obvious declining through washing, both the SOM content and the water repellency showed the highest values in the smallest particle size fraction. Water repellency in both washed and untreated samples showed a considerably strong positive linear correlation with the SOM content suggesting that water-insoluble hydrophobic organic coatings might not be the dominant factor triggering water repellency in the tested sandy soils.

Does drought alter hydrological functions in forest soils?

Abstract. Climate change is expected to impact the water cycle and severely affect precipitation patterns across central Europe and in other parts of the world, leading to more frequent and severe droughts. Usually when projecting drought impacts on hydrological systems, it is assumed that system properties, like soil properties, remain stable and will not be affected by drought events. To study if this assumption is appropriate, we address the effects of drought on the infiltration behavior of forest soils using dye tracer experiments on six sites in three regions across Germany, which were forced into drought conditions. The sites cover clayey-, loamy- and sandy-textured soils. In each region, we compared a deciduous and a coniferous forest stand to address differences between the main tree species. The results of the dye tracer experiments show clear evidence for changes in infiltration behavior at the sites. The infiltration changed at the clayey plots from regular and homogeneous flow to fast preferential flow. Similar behavior was observed at the loamy plots, where large areas in the upper layers remained dry, displaying signs of strong water repellency. This was confirmed by water drop penetration time (WDPT) tests, which revealed, in all except one plot, moderate to severe water repellency. Water repellency was also accountable for the change of regular infiltration to fingered flow in the sandy soils. The results of this study suggest that the drought history or, more generally, the climatic conditions of a soil in the past are more important than the actual antecedent soil moisture status regarding hydrophobicity and infiltration behavior; furthermore, drought effects on infiltration need to be considered in hydrological models to obtain realistic predictions concerning water quality and quantity in runoff and groundwater recharge.

Effects of relative humidity on the water repellency of fire-affected soils

Soil water repellency (SWR) is a common feature in unburned and particularly in fire-affected soils, and can enhance several environmental risks. It can be affected by many factors such as vegetation cover, moisture content and, in fire-affected areas, the degree of heating during burning. In addition, experiments using unburned soils have shown that atmospheric relative humidity can affect their water repellency. The purpose of this laboratory study was to examine how ambient relative humidity (RH) affects SWR of burned soils, and to explore its implications for fire-affected regions. Soil samples were taken from under fire-prone, but long unburned Pinus halepensis and a shrub site in Gorga, Alicante (SE Spain). In order to simulate different fire severities, samples were heated for 20min at different temperatures (50, 100, 150, 200, 250, 300 and 350°C). Samples were then equilibrated at different RHs (30, 50, 70 and 95%) in a sealed climate chamber at a constant temperature of 20°C. The water drop penetration time (WDPT) test, molarity of ethanol droplet (MED) test, and advancing contact angle (CA) measurements were performed inside the sealed climate chamber to assess SWR for each sample and treatment. Overall, increasing heat treatments enhanced SWR, which in turn was enhanced further following exposure to high RHs. The WDPT test showed that soils under pine were water repellent at the lowest heating temperature and became strongly water repellent at the higher heating temperatures and near saturation (95% RH). Shrubland soils were mostly wettable at the onset and remained so at every RH level studied except being slightly SWR at 95% RH. A similar trend was found after MED and CA measurements. The results demonstrate that high RH contributes to enhanced SWR also in burned soils, where high temperatures had already led to a substantial enhancement of SWR. These findings suggest that SWR levels determined for fire affected areas ambient under field or laboratory conditions may underestimate the apparent SWR levels present at the high RH levels that often precede major storm events. This in turn has implications for predicting post-fire runoff and erosion events.

Soil-water repellency characteristic curves for soil profiles with organic carbon gradients

Soil water repellency (SWR) of soils is a property with significant consequences for agricultural water management, water infiltration, contaminant transport, and for soil erosion. It is caused by the presence of hydrophobic agents on mineral grain surfaces. Soils were samples in different depths at three forest sites in Japan and three pasture sites in New Zealand, covering soil organic carbon (SOC) contents between 1 and 26%. The SWR was measured over a range of water contents by three common methods; the water drop penetration time (WDPT) test, the molarity of an ethanol droplet (MED) method, and the sessile drop method (SDM). The aim to (i) compare the methods, (ii) characterize the soil-water repellency characteristic curves (SWRCC) being SWR as a function of the volumetric soil-water content (θ) or matric potential (ψ), and (iii) find relationships between SWRCC parameters and SOC content. The WDPT, MED, and SDM generally agreed well in predicting the θ range where SWR occurred, and there was close agreement between SWR results determined by average MED and SDM at similar θ. Generally, SWR was only found within the top 20cm of the soil profiles. Six SWR parameters were introduced: (i) the area under the curve (SWR(θ)); (ii) θ at the maximum SWR (θWR-max), (iii) θ where SWR ceased (θnon-WR), (iv) the maximum SWR (CAi-max), (v) pF at the maximum SWR (pFWR-max) and (vi) pF where SWR ceased (pFnon-WR). The relationship between the first three parameters and SOC content were best described with Langmuir type equations (r2 of 0.5–0.7), while the other three parameters changed linearly with SOC contents.

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.

Basic soil properties as a factor controlling the occurrence and intensity of water repellency in rankers of the White Carpathians

Abstract Water repellency in soils is controlled by many different factors, basic physical and chemical properties might be considered the crucial ones. For the purpose of this study, 12 sites were selected and sampled (0–20 cm depth) in the White Carpathians. Repellency tests were conducted under laboratory conditions in triplicate using water drop penetration time (WDPT) test and the molarity of ethanol droplet (MED) test. Results of WDPT measurements showed that three samples were marked by slight to extreme water repellency. Regarding the relationship between WDPT/MED and tested soil properties, the highest value of correlation coefficient was calculated for soil organic carbon (r = 0.706; p &lt; 0.05), suggesting there is a positive, statistically significant correlation between repellency severity and total carbon content. A negative relationship between repellency and soil reaction/silt/silt + clay contents of studied soils was found. Samples taken from the surface horizon of arable soils showed no repellency.

Formation of soil–water repellency in olive orchards and its influence on infiltration pattern

Water-repellent (hydrophobic) soils do not wet instantaneously, but only after some time (a few seconds to hours) of soil-particle contact with water. Some plant species can render soils hydrophobic but in this respect, olive trees have scarcely been examined. Measurements of water repellency in olive orchards of different ages in different locations in Israel using the water drop penetration time (WDPT) test have shown that soils tend to become hydrophobic, regardless of texture and structure. A comprehensive study was then performed for an irrigated young and mature olive grove and nearby uncultivated bare soil in the southern part of Israel. The study included intensive WDPT measurements, initial (repellency intensity) and rate of decrease (repellency persistence) for sessile drops placed on the soil surface, cumulative infiltration using tension disc infiltrometer, and monitoring flow in a transparent flow chamber packed with soils from the different plots. The soil from the mature olive plot was noticeably more water repellent than the young plot's soil, and both differed from the uncultivated soil that was fully wettable. The contact angle of a drop placed on the surface of a single layer of soil particles decreased exponentially with time, with a lower decay rate for the mature orchard soil. Cumulative infiltration had a convex pattern for wettable soils and a concave pattern for water-repellent ones. The difference in infiltration pattern was attributed to water/pressure buildup behind the wetting front as a result of the dynamic contact-angle-induced pore resistivity to wetting. The supplemental pressure, also known as dynamic water-entry pressure, increases the infiltration rate beyond that obtained by the capillary pressure per se. The significant correlation between soil sorptivity and the asymptotic infiltration rate, both calculated from the cumulative infiltration curves, and the WDPT substantiates the dependence of pressure overshoot and the rate at which the contact angle decreases prior to pore wetting. The considerable differences in plume shape, size, and internal saturation distribution between the wettable and water-repellent soils, indicating unstable flow in the latter, were also explained by the wettability-dependent water-entry pressure. The outcome of this study indirectly supports the findings that higher surface runoff and erosion are associated with no-till farming in olive orchards, due to the combination of no-till cropping and the near-surface accumulation of hydrophobic organic carbon compounds.

Identifying the origin of soil water repellency at regional level using multiple soil characteristics: The White Carpathians and Myjavska pahorkatina Upland case study

This paper evaluates the relationship between water repellency and multiple characteristics of topsoil samples belonging to seven Reference Soil Groups, taken from the area of the White Carpathians and the Myjavska pahorkatina Upland. In order to quantify water repellency, the Water Drop Penetration Time test and the Molarity of an Ethanol Droplet test were performed on 210 soil samples. The water repellency data were confronted with a number of categorical and numerical soil variables. It was observed that the particular land-use type and the nature of soil parent material, both are related towards detected water repellency of soil samples. All samples taken from the agricultural (tilled) and grassland soils were wettable. On the contrary, all samples which exhibited water repellency, belonged to the group of forest soils, although, not all forest soils were water repellent. Samples which showed considerable repellency were soils developed either on consolidated sedimentary rocks (sandstones, limestone-dolomitic rocks, flysch) or unconsolidated sediments of aeolic or polygenetic origin. On the other hand, the great majority of soils developed on recent alluvial deposits were clearly wettable. Correlation and regression analyses showed that susceptibility of forest topsoil to exhibit water repellency generally increases with increasing sand and organic carbon contents, and with a simultaneous decrease of soil pH value. An interesting observation came out regarding CaCO 3 and water repellency relation. Although certain soils with higher CaCO 3 exhibited water repellency (Rendzic Leptosols and Cambisols), all soils that developed on loose sediments and contained CaCO 3 were wettable.

Water repellency under coniferous and deciduous forest — Experimental assessment and impact on overland flow

Current climate change makes it necessary to gain a deeper understanding of the runoff generation processes in Central European forests. A changing climate might affect soil water repellency (SWR) which can be seen as an import trigger for overland flow generation in forested areas. In this study the differences between coniferous and deciduous forest concerning SWR and overland flow generation were investigated in a small catchment in the Hunsrück low mountain range, Rhineland-Palatinate, Germany.To investigate the occurrence and persistence of SWR and its influence on overland flow generation, a combination of two experimental methods was applied: water drop penetration time (WDPT) test, and rainfall experiments. The field WDPT test results ranged from wettable (WDPT<5s) up to more than 900second persistence of water repellency in both forest types. The median WDPT was 30s for the coniferous forest and 1s for the deciduous forest sites. On the deciduous forest soils, only the Of-horizon showed considerable water repellency. Rainfall experiments with 40mmh−1 rainfall intensity yielded runoff coefficients between 0% and 63%. The lowest measured infiltration rate of the rainfall experiments was 11.6mmh−1. The highest runoff coefficients were measured on water repellent (WDPT>300s) coniferous forest sites. The overland flow starts significantly earlier with water repellent soil conditions. The median runoff rate for the wettable forest soils is 2.7%, whereas the water repellent sites show a median runoff coefficient of 11.4%.The results suggest that the occurrence of SWR can lead to considerable overland flow generation under forest.

Environmental Factors Governing Soil Water Repellency Dynamics in a Pinus Pinaster Plantation in NW Spain

AbstractSoil water repellency (SWR) is a dynamic property that changes throughout the year. The objective of this work was to identify the environmental factors governing the temporal patterns in SWR in a pine plantation in northwest Spain with a view of predicting its occurrence and persistence. For this purpose, 24 samples were collected from the soil surface (0–5 cm) at 25 different times over a 1‐year period and analysed for SWR by using water drop penetration time test and soil moisture measurements. Temporal variations in SWR exhibited a well‐defined seasonal pattern. The soil surface was largely wettable from late autumn to early spring and extremely water repellent during summer and early autumn. Repellency persistence was rather variable during spring. There was highly significant correlation between SWR and soil moisture content. The moisture range defining the presence or absence of repellency under field conditions was 22–57%. There were also significant correlations with the target variables (maximum temperature, minimum temperature, precipitation and water balance during variably long antecedent periods), with coefficients that increased with increasing length of the antecedent period considered. The moisture content of soil at the time of sampling and the average maximum temperature for the 28 days before sampling are the best predictors of occurrence of SWR and its persistence in different seasons. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Initial water repellency affected organic matter depletion rates of manure amended soils in Sri Lanka

Abstract The wetting rate of soil is a measure of water repellency, which is a property of soils that prevents water from wetting or penetrating into dry soil. The objective of the present research was to examine the initial water repellency of organic manure amended soil, and its relation to the soil organic matter (SOM) depletion rates in the laboratory. Soil collected from the Wilpita natural forest, Sri Lanka, was mixed with organic manure to prepare soil samples with 0, 5, 10, 25, and 50% organic manure contents. Locally available cattle manure (CM), goat manure (GM), and Casuarina equisetifolia leaves (CE) were used as the organic manure amendments. Organic matter content of soils was measured in 1, 3, 7, 14, and 30 days intervals under the laboratory conditions with 74±5% relative humidity at 28±1°C. Initial water repellency of soil samples was measured as the wetting rates using the water drop penetration time (WDPT) test. Initial water repellency increased with increasing SOM content showing higher increasing rate for hydrophobic CE amended samples compared with those amended with CM and GM. The relation between water repellency and SOM content was considered to be governed by the original hydrophobicities of added manures. The SOM contents of all the soil samples decreased with the time to reach almost steady level at about 30 d. The initial SOM depletion rates were negatively related with the initial water repellency. However, all the CE amended samples initially showed prominent low SOM depletion rates, which were not significantly differed with the amended manure content or the difference in initial water repellency. It is explicable that the original hydrophobicity of the manure as well has a potentially important effect on initiation of SOM decomposition. In contrast, the overall SOM depletion rate can be attributed to the initial water repellency of the manure amended sample, however, not to the original hydrophobicity of the amended manure. Hydrophobic protection may prevent rapid microbial decomposition of SOM and it is conceivable that hydrophobic substances in appropriate composition may reduce organic matter mineralization in soil. These results suggest the contribution of hydrophobic organic substances in bioresistance of SOM and their long-term accumulation in soils

Organic Matter Fractions Controlling Soil Water Repellency in Sandy Soils From the Doñana National Park (Southwestern Spain)

AbstractThe relationship between soil organic matter (SOM) content and organic molecular assemblages (n‐alkane/alkenes pairs andn‐fatty acids) with soil water repellency (WR) in different sieve fractions (1–2, 0·25–1, 0·05–0·5 and &lt;0·05 mm) was studied in sandy soils from the Doñana National Park (SW Spain) under four vegetation species:Quercus suber(QS),Pteridium aquilinum(PA),Pinus pinea(PP) andHalimium halimifolium(HH). The severity of soil WR [assessed according to the water drop penetration time (WDPT) test] was ordered according to the sequence QS &gt; PA &gt; PP &gt; HH. A positive correlation was found between soil WR from each sieve fraction and SOM content. Most severe soil WR was detected in QS for all sieve fractions, followed by the finer fractions (&lt;0·05 mm) from PA, PP and HH samples. The finer fraction had the highest SOM content, ranging between 20·9% (PP) and 46·9% (QS). Coarser PA, PP and HH soil fractions (1–2 mm) showed the highest long‐chain‐even C numbered fatty acids in the order PP &gt; PA &gt; HH. A significant correlation was observed between SOM content and the severity of soil WR in QS samples and finer fractions of soil under other vegetation (PA, PP and HH). Coarser fractions (1–2 mm) from PP, PA and HH 1–2 mm sieve fractions showed high severity of WR and relatively low SOM contents that could be explained by a low degree of evolution of the organic residues as seen by analytical pyrolysis, that is, higher alkane/alkene carbon preference index values and the presence of a higher diversity of fatty acid structures. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Small variations of soil properties control fire-induced water repellency .

Fire induced soil water repellency (WR) is controlled by many different factors (temperature reached, amount and type of fuel, etc.). Soil properties may determine the occurrence and intensity of this property in burned soils. The objectives of this paper are to make advances in the study of soil properties as key factors controlling the behaviour of fire-induced WR, and to study the impact of pre-fire SOM content and SOM quality in fire-induced soil WR. In this research, experimental laboratory burnings were carried out using soil samples from different sites with different lithologies, soil types and plant species. Soil samples taken from the same site differ only in quantity and quality of soil organic matter, as they were collected from under different plant species. All soil samples were heated in a muffle furnace at 200, 250, 300 and 350 ºC without the addition of any fuel load. WR was measured using the water drop penetration time test (WDPT). The results showed significant differences between soil types and plant species, indicating that small differences in soil properties may act as key factors controlling the development and persistence of WR reached, with burned soil samples ranging from wettable to extremely water repellent. The main soil property controlling the response was texture, specifically sand content. The quality of organic matter was also observed to have an effect, since soil samples from the same site with similar organic matter contents, but taken from beneath different plant species, showed different WR values after burning.

Effects of Soil Water Repellency on Moisture Patterns in a Degraded Sapric Histosol

AbstractAn understanding of soil moisture content variability is fundamental in hydrological studies of peat soils, whose preservation depend on water‐related processes. Dehydration of fens and adapting them for agricultural production have contributed to the degradation of peat soils. The goal of this study was to determine how the critical soil moisture content (CSMC) and soil water repellency (SWR) affect soil moisture patterns in a degraded peat‐muck soil profile. SWR was measured under laboratory conditions using the water drop penetration time test, and then the CSMC was assessed. An investigation of moisture patterns was based on soil moisture data collected over short distances in a grass‐covered peat‐muck soil profile on seven dates. Observed differences in moisture patterns demonstrate that the CSMC can be used for the prediction of preferential flow occurrences in peat‐muck soils. Lower values of the CSMC and lower levels of SWR persistence in muck layers than in peat layers indicate that degradation of peat soils improves their wettability. The relatively low values of CSMC and the low shrinkage potential in the muck layer suggest that preferential water flow in the degraded organic soils can occur when heavy rains are preceded by long periods of summer drought. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Organic matter and wettability characteristics of wildfire ash from Mediterranean conifer forests

Thermogravimetric analysis (TGA) and water repellency (wettability) determination of ash samples collected following wildfires at four different Mediterranean conifer forest sites in eastern Spain were conducted to provide insights into variations in key chemical and physical characteristics of ash. TGA allowed the evaluation of organic matter (OM) and inorganic carbonate (IC) characteristics and their role in ash wettability. Thermogravimetric profiles were recorded for 40 ash samples (ten from each site). The persistence of water repellency of all ash samples was assessed by the Water Drop Penetration Time (WDPT) test. OM contents decreased, whilst OM stability indices and IC contents increased with increasing combustion completeness. The highest carbon fixation in IC was determined at the site that had experienced the most severe fire. WDPT values were positively correlated with OM and negatively with IC contents. All strongly to severely water repellent ash samples had OM/IC ratio above 2. Pools of OM in ash formed during wildfires represent a more resistant pool of C compared to their unburned precursors. Together with the new IC pool formed in ash, both pools could represent a carbon sink triggered by wildfire.

The use of visible and near‐infrared spectroscopy for the analysis of soil water repellency

SummaryThis study investigated the potential of visible/near‐infrared reflectance spectroscopy (Vis‐NIRS) to predict soil water repellency (SWR). The top 40 mm of soils (n = 288) across 48 sites under pastoral land‐use in the North Island of New Zealand, which represented 10 soil orders and covered five classes of drought proneness, were analysed by standard laboratory methods and Vis‐NIRS. Soil WR was measured by using the molarity of ethanol droplet (MED) and the water drop penetration time (WDPT) tests. Soil organic carbon content (%C) was also measured to examine a possible relationship with SWR. A partial least squares regression (PLSR) model was developed by using Vis‐NIRS spectral data and the reference laboratory data. In addition, we explored the power of discrimination based on WDPT classes using partial least squares discriminant analysis (PLS‐DA). The PLSR of the processed spectra produced moderately accurate prediction for MED (R2val = 0.61, RPDval = 1.60, RMSEval = 0.59) and good prediction for %C (R2val = 0.82, RPDval = 2.30, RMSEval = 2.72). When the data from the 10 soil orders were considered separately and based on soil order rather than being grouped, the prediction of MED was further improved except for the Allophanic, Brown, Organic and Ultic soil orders. The PLS‐DA was successful in classifying 60% of soil samples into the correct WDPT classes. Our results indicate clearly that Vis‐NIRS has the potential to predict SWR. Further improvement in the prediction accuracy of SWR is envisaged by increasing the understanding of the relationship between Vis‐NIRS and the SWR of all New Zealand soil orders as a function of their physical properties and chemical constituents such as hydrophobic compounds.

Burned and unburned peat water repellency: Implications for peatland evaporation following wildfire

Summary Water repellency alters soil hydrology after periods of wildfire, potentially modifying the ecosystem recovery to such disturbance. Despite this potential importance, the extent and severity of water repellency within burned peatlands and its importance in regulating peatland recovery to wildfire disturbance remains poorly understood. We characterised the water repellency of peat in a burned (one year post-fire) and unburned peatland in the Western Boreal Plain utilising the water drop penetration time and ethanol droplet molarity tests. Burned Sphagnum moss and feather moss sites had a more severe degree of water repellency than unburned sites, with differences being more pronounced between burned and unburned feather moss sites. Burned feather moss exhibited the most extreme water repellency, followed by unburned feather moss, and burned Sphagnum. The severity of water repellency varied with depth through the near surface of the moss/peat profile. This was most evident within the burned feathermoss where more extreme water repellency was observed at the near-surface compared to the surface, with the most extreme water repellency found at 1 and 5 cm depths. Unburned Sphagnum was completely hydrophilic at all depths. We suggest that the extreme water repellency in near-surface feather moss peat acts as a barrier that impedes the supply of water to the surface that replaces that lost via evaporation. This leads to drying of the near-surface vadose zone within feather moss areas and a concomitantly large decrease in peatland evaporation within feather moss dominated peatlands. This negative feedback mechanism likely enhances the resilience of such peatland to wildfire disturbance, maintaining a high water table position, thereby limiting peat decomposition. In comparison, such a feedback is not observed strongly within Sphagnum, leaving Sphagnum dominated peatlands potentially vulnerable to low water table positions post disturbance.

Natural soil water repellency in different types of Mediterranean woodlands

Water repellency (WR) is a property of some soils that reduces infiltration rates, enhances runoff generation and increases soil erosion risk. Although wildfires are considered a triggering factor, the characteristics of plant residues and soil properties may contribute to the development of soil WR. Because of its impacts, soil WR must be considered when modeling soil erosion risk and hydrological processes. Although many studies on WR from Mediterranean soils exist, relatively few studies have contributed to the knowledge of the natural baseline of soil WR in wide areas. The objective of this paper is to study the natural background soil WR in Mediterranean soils from south-western Spain under three representative forest types (pines, eucalypts and holm oaks) and its relation with plant cover (trees, shrubs and herbaceous plants) and soil properties. Field sampling was carried out in August 2013 in 15 areas from Huelva (SW Spain) under the studied forest types. Vegetation cover (trees, shrubs and herbaceous plants) was determined using transects at each case. The water drop penetration time test was used for assessing soil WR, and main soil properties were determined (texture, pH, organic C, N, Extractable P, exchangeable base cations – Ca2+, Mg2+, K+ and Na+ – and cation exchange capacity). According to results, soil WR was observed in all areas, increasing according to the sequence soils under holm oaks<eucalypts<pines. The severity of soil WR was always higher under the canopy of trees and usually decreased in bare areas, although bare soils under pine forest showed a proportion of slight to strong water-repellent cases. Severity of WR from soils under eucalypts and holm oaks increased with the presence of shrubs and herbaceous plants, but similar levels were not reached out of the tree-covered areas. Soils under vegetation in pine forests were always water-repellent, and wettability was observed only in some bare areas, suggesting a high potential of pines for induction of WR. Soil pH and the proportion of clay showed negative correlations with soil WR. A negative correlation between WR class and the proportion of exchangeable K+ was found, suggesting that K deficiency for trees and shrubs restricts the input of hydrophobic substances in soil.

Effects of controlled fire on hydrology and erosion under simulated rainfall

Mediterranean forests are frequently subject to wildfires, inducing risks of high runoff and loss of nutrient-rich topsoil. The mechanisms influencing these post-fire effects are spatially variable due to differences in vegetation density, litter composition, soil texture and structure, and fire intensity, and therefore difficult to separate. The characteristics of the soil surface in the immediate post-fire period are of critical importance to the hydrological response and erosion susceptibility of the burnt hillslope and catchment. In laboratory experiments, uniform soil, controlled temperature and rain regimes were used to create a controlled environment to isolate the impact of certain influencing parameters. In this study, for a Rendzina soil, we investigated the post-fire impacts of laboratory fire and two successive rainfall (rotating disktype) simulation experiments to evaluate short-term effects of fire on soil hydrological and erodibility parameters by examining (i) soil water repellency (WR) levels and distribution, (ii) surface cover features, and (iii) infiltration, runoff and erosion responses to simulated rain on control (bare and pineneedle covered) and burnt (with and without ash cover) samples. Fire-induced surface WR tested in the laboratory by grid-wise Water Drop Penetration Time tests (WDPT), revealed moderate WR, which decreased for all treatments after rainfall. The response to rain (33 mmh-1) differed for the two simulation runs. The rates of drainage and runoff of the burnt samples in the first run varied between the values of cover (low runoff, high infiltration) and bare (high runoff, low infiltration). Drainage in the ash-covered samples was twice as high as ashremoved samples. In the second run, both samples showed a similar response compared to bare conditions. These laboratory observations suggest that WR and protection by ash are factors to consider in assessing the erosion susceptibility of a burnt forest soil. Furthermore, possible management implications based upon this research are that: 1) ash can have several important roles immediately after a forest fire by protecting the forest soil from rain splash erosion, and with its high water holding capacity, absorbs rainfall, thereby reducing runoff; and 2) ash has no negative influence upon soil infiltration demonstrating the important benefits of ensuring the longer term maintenance of post-fire ash within the burnt landscape. Finally, management actions including mulching further enhance soil stability and minimize soil erosion.