Persistence Of Water Repellency Research Articles

Intensity and persistence of water repellency at different soil moisture contents and depths after a forest wildfire

Abstract The Mediterranean mixed coniferous and broad-leaved forest of Moarda (Palermo) was affected by a large wildfire in summer 2020. In spring 2021, burned and unburned loam soil sites were sampled and the water drop penetration time (WDPT) and ethanol percentage (EP) tests applied to assess the influence of wetting-drying processes and soil water content on post-fire soil water repellency (SWR) as well as its vertical distribution. According to the WDPT test, the surface layer of the natural unburned soils was severely hydrophobic at intermediate soil water contents roughly corresponding to wilting point and SWR reduced either for very dry conditions (air- or oven-dried conditions) or wetter conditions close to field capacity. For these soils, EP test yielded results in agreement with WDPT. An influence of the wetting/drying cycle was detected as, for a given soil water content, WDPT was generally higher for the drying than the wetting process. The surface of burned soils was always wettable independently of the soil water content. The vertical distribution of SWR was modified by wildfire and the maximum hydrophobicity layer, that was located at the surface of the unburned soils, moved to a depth of 2–4 cm in the soils of burned sites. The results confirmed that wildfire can induce destruction of soil water repellency (SWR) naturally occurring at the surface of forest soils and create a shallow hydrophobic layer that may increase overland flow and erosion risk.

Capturing water repellency cessation time by means of characteristic time method

Persistence of water repellency is a significant physical key factor that governs water infiltration into unsaturated soil. We investigated water and ethanol infiltration using the characteristic time method (CTM), repellency index (RI), and the contact angle (CA) behavior for a comprehensive assessment on soil water repellency (SWR). We analyzed the impact of soil structure and thermal and wetting–drying treatments on water and ethanol infiltration. The first objective was to evaluate the CTM for water-repellent soils by partitioning characteristic and gravity infiltration behavior (Ichar and Igrav) and to evaluate parameters like infiltration beginning (tb) and gravity times (tchar and tgrav). The second objective was to characterize the CTM estimated water sorptivity (Sw), either as the Sww (the hydrophilic state water sorptivity) or the Swh (the hydrophobic state water sorptivity) to improve the calculation of water repellency cessation time (WRCT). Three soils with initial contact angles (CAi) of 18°, 60°, and 90° (20 °C ± 3 °C) were additionally heated to temperatures of 40 °C and 60 °C resulting in CAT40: of 23.6°, 56.4°, and 97.3° and CAT60 of 18.5°, 88.0°, and 126.6°. The wetted state CA was determined for a wetting- and a rewetting-infiltration cycle (CAwe and CArewe) under –2 cm tension, followed by air-drying and further CA measurements (CAair-dried). There was significant agreement (R2 > 0.95) between the Sw evaluated by CTM and the Swh, and excellent correspondence between tchar and the tb. The relations between Se, RI, CAwe, CArewe, CAair-dried, and WRCT clearly showed dynamics and reversibility of SWR and also its dependence on persistence, even for the air-dried 20 °C soil with small CAi. Persistence of SWR as characterized by the time components resulted in a long flow transition state (i.e., time development and respective changing relevance of capillary and gravity forces). Hence, missing experimental data has to be considered as the main barrier for modeling approaches. Further research is necessary to improve flexibility of the CTM code to reliably estimate Sw and Swh with respect to persistence of SWR.

BEST-WR: An adapted algorithm for the hydraulic characterization of hydrophilic and water-repellent soils

Water-repellent soils usually experience water flow impedance during the early stage of a wetting process followed by progressive increase of infiltration rate. Current infiltration models are not formulated to describe this peculiar process. Similarly, simplified methods of soil hydraulic characterization (e.g., BEST) are not equipped to handle water-repellent soils. Here, we present an adaptation of the BEST method, named BEST-WR, for the hydraulic characterization of soils at any stage of water-repellency. We modified the Haverkamp explicit transient infiltration model, included in BEST for modeling infiltration data, by embedding a scaling factor describing the rate of attenuation of infiltration rate due to water repellency. The new model was validated using analytically generated data, involving soils with different texture and a dataset that included data from 60 single-ring infiltration tests. The scaling factor was used as a new index to assess soil water repellency in a Mediterranean wooded grassland, where the scattered evergreen oak trees induced more noticeable water repellency under the canopies as compared to the open spaces. The new index produced results in line with those obtained using the water drop penetration time test, which is one of the most widely test applied for quantifying soil water repellency persistence. Finally, we used BEST-WR to determine the hydraulic characteristic curves under both hydrophilic and hydrophobic conditions.

Contributions of soil organic carbon to soil water repellency persistence: Characterization and modelling

Soil water repellency (SWR) affects hydrological processes such as water infiltration, preferential flow, run-off, and evapotranspiration. Understanding the relationship between SWR and volumetric soil water content (θ) is critical for predicting hydrological processes in hydrophobic soils. In this study, water droplet penetration time (WDPT) was used to investigate the SWR persistence (SWRP) in three soil types with contrasting drainage levels, namely a well-drained Lismore (LIS) stony silt loam, a moderately well-drained Templeton (TEM) silt loam, and a poorly drained Waterton/Temuka (WAT) clay loam under three long-term (>20 years) land uses (irrigated pasture [IP], dryland pasture [DP], and irrigated crop [IC]) in Canterbury, New Zealand. The three soils belong to different soil orders (i.e., Brown, Pallic, and Gley, respectively) in the New Zealand soil classification. Within each land use, 9, 12, and 9 sites were sampled in the LIS, TEM, and WAT soils respectively. There was strong evidence of SWR in all three soil types under three land uses. The maximum SWRP (WDPTmax), ranging from 6 to 1470 s with the median value of 56 s, differed with land use by following descending order of IP > DP > IC. The effects of land use on SWRP differed with soil type. Compared with other two soils, LIS soil had higher differences in WDPTmax between IP and DP, but lower differences in WDPTmax between DP and IC. Most of the SWRP parameters were significantly correlated with soil organic carbon (SOC) content. In soils with higher SOC content, SWR usually developed under drier conditions and declined as soils were maintained under wetter conditions. This study suggests that the WDPTmax appears at soil water contents of 0.17 to 0.19 cm3 cm−3 (equivalent to 0.15 to 0.16 g g−1). Soil water repellency persistence was positively related to SOC content. Soils with SOC content < 2% tended to be wettable, while soils with SOC content > 4% tended to be water-repellent. A Gaussian model with three physically meaningful parameters was developed to successfully fit the SWRP curve (SWRPC, LnWDPT as a function of θ). Our study also showed the possibility to predict the model parameters and SWRPC using SOC content. The model needs to be tested across other soil types, land uses, and climates before it can be widely applied to predict the SWR occurrence from SOC data. Future work should also focus on the impacts of SOC composition on SWRP parameters and other hydrological properties for predicting soil water processes in water-repellent soils.

The Relationship between Soil Moisture and Soil Water Repellency Persistence in Hydrophobic Soils

In this work, we modelled the response of soil water repellency (SWR) persistence to the decrease in moisture in drying soils, and we explored the implication of soil particle size distribution and specific surface area on the SWR severity and persistence. A new equation for the relationship between SWR persistence and soil moisture (θ) is described in this paper. The persistence of SWR was measured on ten different hydrophobic soils using water drop penetration time (WDPT) at decreasing levels of gravimetric water content. The actual repellency persistence showed a sigmoidal response to soil moisture decrease, where Ra(θ)=Rp/1+eδ(θ−θc). The suggested equation enables one to model the actual SWR persistence (Ra) using θ, the potential repellency (Rp) and two characteristic parameters related to the shape of the response curve. The two parameters are the critical soil moisture θc, where the Ra increase rate reaches its maximum, and the parameter δ affecting the steepness of the curve at the inflexion point of the sigmoidal curve. Data shows that both soil carbon and texture are controlling the potential SWR in New Zealand pastures.

Soil stabilisation by water repellency under no-till management for soils with contrasting mineralogy and carbon quality

No-till soil management is common around the globe, but the impacts on soil structural quality varies depending on cropping practice and inherent soil properties. This study explored water repellency as a driver of soil stabilization, as affected by soil mineralogy, granulometry and organic carbon quality in three Mollisols and one Vertisol under no-till management and with different levels of cropping intensity. The studied soils were located along a west-east textural gradient in the northern part of the Pampean region of Argentina. Cropping intensity treatments evaluated in each one of the soils were: Poor Agricultural Practices (PAP) close to a monoculture, Good Agricultural Practices (GAP) involving a diverse crop rotation and more targeted inputs, and the soil in the surrounding natural environment (NE) as a reference. NE had the greatest aggregate stability (MWD) of all cropping intensities, with GAP being more stable than PAP for Mollisols and PAP being greater than GAP for the Vertisol. This trend matched the Repellency Index (Rindex), with greater Rindex associated with greater MWD, including the difference between the Mollisols and Vertisol. However, the persistence of water repellency, measured by the Water Drop Penetration Time (WDPT) test followed the trend NE > GAP>PAP regardless of soil type. The increases in Rindex and MWD were related to higher intensification as measured by the Crop Sequence Index, and decreased with greater soybean occurrence in the sequence. Both WDPT and Rindex were closely related to aggregate stability, particularly for Mollisols. These results highlight the importance of considering the inherent soil characteristics texture and mineralogy to understand aggregate stabilization mediated by water repellency. Good correlations between soil water repellency, organic carbon fractions and aggregate stability were found. Under no-till, crop rotations can be altered to increase soil stability by inducing greater water repellency in the soils. The findings suggest that water repellency is a major property influencing soil structure stabilization, thus providing a useful quality indicator.

Persistence of water repellency in coarse-textured soils under various types of forests in NW Spain

Abstract The primary purpose of this work was to assess the persistence of water repellency in the surface horizon of coarse-textured soils under natural Quercus robur ecosystems, and Pinus pinaster and Eucalyptus globulus plantations, in the northwest of the Iberian Peninsula. Water repellency was determined by applying the water drop penetration test (WDPT) to soil samples collected from variable depths (0–40 cm). Measurements were made on field-moist samples obtained at the end of the dry period and on samples dried at 25ºC in the air. All soils exhibited very high (severe to extreme) water repellency in the topmost soil layer (0–5 cm) but no significant differences among the three plant species studied. Extreme persistence was observed down to 20 cm in the soils under eucalyptus and down to 10 cm in those under pine. The soils under oak were those exhibiting the highest variability in water repellency and the greatest decrease in it with increasing depth (especially in relation to soils under eucalyptus). Water repellency exhibited significant positive correlation with the C content and C/N ratio of the soils. Soil water repellency was similar in the air-dried samples and field-moist samples.

Dye Tracer Visualization of Infiltration Patterns in Soils on Relict Charcoal Hearths

Anthropogenically modified soils are often characterized by a high heterogeneity of substrates and show unique patterns of water infiltration. Such effects are not limited to intensively used or disturbed agricultural and technogenic soils, but can also occur as legacies of former land use in forested areas. The remains of historic charcoal hearths represent a widespread legacy of historic land use. Soils at relict charcoal hearths (RCHs) are most prominently altered by the deposition of a layer of charcoal-rich substrate on top of the natural soil surface. The presence of such a technogenic layer can considerably influence infiltration and soil wetness patterns on the sites. This study describes the spatial patterns of infiltration and soil wetness at charcoal hearth sites compared with undisturbed sandy forest soils for a historic charcoal production area north of Cottbus, Germany. We characterized six plots on RCH and reference soils under pine, oak and mixed forest by visualizing preferential flow patterns of infiltrating water in dye tracer experiments. Additionally, we characterized bulk density, soil organic matter contents and water repellency, using water drop penetration time (wdpt) tests, of the RCH and reference soil horizons. The results reflect that the persistence of water repellency of both the technogenic substrates and the natural topsoils is extremely high under dry conditions, but is drastically reduced after wet antecedent conditions. The dye tracer experiments reflect increased preferential flow on the RCHs for dry soil conditions, for which infiltration is limited to very few flow paths in the technogenic substrate layer. Differences between RCH and reference soils are less clear for higher antecedent soil wetness, for which the results indicate more uniform wetting of the technogenic substrates. We conclude that the structural properties of the additional technogenic substrate layer of RCHs have characteristic effects on water infiltration, causing a high temporal variation of preferential flow in relation to antecedent soil moisture conditions. These effects can result in high heterogeneity of soil moisture for dry conditions, and generally in a high temporal variation of soil wetness in RCHs soils.

Changes in Topsoil Properties after Centennial Scots Pine Afforestation in a European Beech Forest (NE Spain)

In this work, we studied the effects of centenary Scots pine (Pinus sylvestris L.) afforestation on topsoil properties conducted in a deforested area that was previously occupied by a natural European beech (Fagus sylvatica L.) forest. Organic layers and topsoil Ah mineral horizons (0–10 cm) were sampled in the Scots pine and European beech forests of Moncayo Natural Park (north-eastern Spain). The physical (stoniness, aggregate stability, and water repellency persistence and intensity), chemical (total organic C, total N, C/N, pH, and exchangeable Ca2+, Mg2+, Na+, K+, Al3+, and Fe3+), and physicochemical (cation exchange capacity) properties of soil were analyzed. Total organic C and N were also obtained for litter samples. The studied topsoils shared a series of common properties, such as a high stoniness and aggregate stability, very low base content, high cation exchange capacity, and extreme acidity. Soils that developed under the pinewood showed a higher soil water repellency intensity. However, K+ content was significantly higher in the beechwood soil. In both forest types, total organic C and N were similar in topsoil and litter (Hemimoder type), although C and N were pooled in different O-layers. Results indicate that pine afforestation in a deforested area was an adequate measure for soil protection since it did not show significant differences in the long term (ca. 100 years) compared to the nearby natural beech stands.

Extent and persistence of water repellency in two Iranian soils

AbstractSoil water repellency (SWR) can affect the hydrophysical properties of soils. The objective of this study was to evaluate a new approach, which allows estimating both the extent (the modified soil water repellency index,

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.

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.

DESARROLLO DE REPELENCIA AL AGUA EN SUELOS AFECTADOS POR INCENDIOS, EN UNA ZONA DEL PARQUE METROPOLITANO DEL DISTRITO METROPOLITANO DE QUITO

&lt;p&gt;La exposición del suelo a temperaturas altas durante la ocurrencia de un incendio forestal, puede inducir el desarrollo de repelencia al agua y otros efectos que alteran la calidad del suelo, entre los que destacan: cambios en el contenido de carbono y de materia orgánica, pérdida de suelo, aparecimiento de flujos preferenciales y erosión. En este trabajo se determinó la severidad y persistencia de la repelencia al agua alcanzada por un suelo previamente quemado dentro de una zona de 13,4 has en el Parque Metropolitano en la Ciudad de Quito. La determinación de la persistencia de la repelencia al agua del suelo se realizó utilizando la metodología de tiempo de penetración de una gota de agua (WDPT) y la severidad de la repelencia se determinó utilizando el test de molaridad de etanol (MED). Los valores más altos de persistencia a la repelencia al agua, fueron determinados a los cuatro meses de haber ocurrido el incendio y permitieron clasificar al suelo afectado por fuego como “Fuertemente Repelente”. Se observó que la persistencia de la repelencia inducida por el fuego fue transitoria y disminuyó con relación al tiempo, determinándose un decremento de la repelencia en las muestras analizadas a los siete y nueve meses. No se encontró una relación definitiva entre el incremento de la repelencia al agua y el aumento de la materia orgánica, que presentó el promedio de concentración más alta a los cuatro meses de ocurrido el incendio (5,88 %).&lt;/p&gt;

How severe and subcritical water repellency determines the seasonal infiltration in natural and cultivated sandy soils

Infiltration into meadow and fallow (cultivated) sandy soils was evaluated after several prolonged rainy and dry spells during the years 2005–2011. Both soils evolved on fluvio-eluvial sandy sediments planted with pine forest, were originally strongly water repellent, but their management after deforestation was different. The fallow soil was intensively cultivated since 1950 while the other was left for natural grassland succession. For comparison, the perfectly wettable bare sediment of similar origin and texture was taken as reference material.We focused on soil porosity, hydraulic conductivity (k(−20mm)) and sorptivity (S) estimated by mini-disc infiltrometer, water drop penetration time (WDPT), and water repellency index (R).The results indicate that cultivation (mainly liming) the fallow soil alleviated the water repellency to its subcritical level, what is also the main explanation for different water repellency persistence levels in fallow versus meadow soil. Notwithstanding, cultivation has not substantially increased water infiltration properties confirming the hypothesis that subcritical water repellency may still retard water infiltration. Some stability of wetting patterns observed in the meadow and fallow soils resulted in only insignificant increase of k(−20mm) during the rainy periods.Long dry spells enhanced the infiltration capacity in wettable reference material because of sorptivity increase. Sorptivities of water repellent meadow and fallow soils, however, remained restrained during both, the rainy and dry spells due to higher water content (when wet) and to stronger water repellency (when dry). As a result, only small seasonal variability in infiltration rates was observed in both water repellent soils.

Soil water repellency persistence after recurrent forest fires on Mount Carmel, Israel

Variations in forest fires regime affect: (1) the natural patterns of community structure and vegetation; (2) the physico-chemical properties of soils and consequently (3) runoff, erosion and sediment yield. In recent decades the Mediterranean ecosystem of Mount Carmel, north-western Israel, is subjected to an increasing number of forest fires, thus, the objectives of the study were to evaluate the long-term effects of single and recurrent fires on soil water repellency (WR) and organic matter (OM) content. Water repellency was studied by applying water drop penetration time (WDPT) tests at sites burnt by single-fire, two fires, three fires and unburnt control sites. Water repellency in the burnt sites was significantly lower than in the unburnt control sites, and the soil maintained its wettability for more than 2 decades, whereas after recurrent fires, the rehabilitation was more complicated and protracted. The OM content was significantly lower after recurrent than after a single fire, causing a clear proportional decrease in WR. The rehabilitation of WR to natural values is highly dependent on restoration of organic matter and revegetation. Recurrent fires may cause a delay in recovery and reduced productivity of the soil for a long period.

Spatial and temporal variations of water repellency and probability of its occurrence in calcareous Mediterranean rangeland soils affected by fires

Abstract Water repellency (WR) is a common soil property in many fire-affected ecosystems, but it also occurs in long-unburned terrain. It can vary in space at different scales (between point and pedon or slope and catchment) and time (during the same day, between seasons or years, or with a post-fire recovery period). This paper: i) reports on the occurrence and persistence of WR in fire-affected calcareous forest soils under Mediterranean climatic conditions, examining its spatial variability at macro-, meso- and micro-scales, and monthly changes with soil moisture content; and ii) develops exploratory models to estimate the probability of the natural background (not fire-induced) WR to occur through a Mixed-Effect Logistic Regression Model. Four sites with comparable soil and vegetation types were studied, all of them burned in 1979, the third again 1999, and the fourth in April 2008. All sites were sampled immediately after the fire of 2008, a further 17 times until July 2009, and once more in August 2011. At each site, 5 random plots (10 cm × 10 cm) were selected for each of the vegetation types: Pinus halepensis, Quercus coccifera , Rosmarinus officinalis and bare soil. In each plot we carried out 10 Water Drop Penetration Time measurements at the surface and at 1 cm depth. WR was detected in samples from all sites, although WR was most frequent at the soil surface at the sites last burnt in 1979. The recently burned site had similar WR to the long-unburned ones in surface but greater at 1 cm depth, although for both depths WR was reduced by the following year. WR was still very low in the site burned 10 years before. Variability of WR between different vegetation types was as high as within the same type at the same site, and similarly high at within sites (1 ha) and at 10 cm × 10 cm plot-scales. The lowest variability was found in bare soil plots because they were mostly wettable. An exploratory model to estimate the probability of WR occurrence was derived for each unburned site. The most powerful explanatory variables for the probability of WR to occur (P P. halepensis (associated to vegetation type) for the other. The model input parameters are straightforward to obtain and the model may be a useful tool in estimating occurrence and fluctuation of soil WR.

FT-IR spectroscopy reveals that ash water repellency is highly dependent on ash chemical composition

After a fire, an ash layer is commonly present, which influences soil properties and hillslope hydrology. The wettability of ash, which can vary from highly absorbent to water repellent, is an important characteristic in this context. Recent work has suggested that ash wettability is related to its degree of combustion, which in turn, can be expected to determine ash chemical composition. In this paper we therefore examine the relationship between ash water repellency and ash chemical composition. Ten ash samples with different wettability were each taken from four burned Mediterranean forest sites located at Albaida (A), Llíber (L), Navalón (N), and Pinoso (P), in the east of Spain. The persistence of water repellency of samples was assessed by the Water Drop Penetration Time (WDPT) test and Fourier Transform-Infrared (FT-IR) analysis was applied to characterise sample chemical composition. Ash water repellency varied from wettable to severely water repellent and differed in terms of maximum WDPT and number of water repellent samples within the four locations. In all FT-IR spectra obtained, the absorbance bands assigned to organic matter and carbonates were dominant. They are subject to change during progressive combustion. The ratios of relevant peak areas were calculated, i.e. between aliphatic bands in the 3000–2800 cm− 1 region, other organic matter bands in the 1800–1200 cm− 1 region, and of the calcite band at 875 cm− 1. These ratios are related to organic matter and calcite contents in ash samples and proved to be closely linked to the measured WDPT values through an exponential relationship. Organic matter content in all water repellent ash samples was more than twice the carbonate content. From the results obtained it is concluded that the water repellency or wettability of wildfire ash is driven by the ratio of remaining organic matter to emerging carbonates, which in turn, is related to fire severity.

Sieving effect on the intensity and persistence of water repellency at different soil depths and soil types from NE-Spain

Abstract Soil water repellency has been studied for different soil types and plant covers, but uncertainty exists regarding the representativeness of water repellency measured on unaltered soil surfaces compared to sieved samples. Two different methods are used to characterize water repellency, which provide a certain degree of different information: the persistence of soil water repellency, measured by the water drop penetration time (WDPT) test; and the intensity of soil water repellency, measured by the ethanol percentage (EP) test. The main objectives of this research were to analyze the relationships between soil water repellency measured on undisturbed soil blocks and 2-mm hand sieved samples, and to compare the results between both methods (WDPT and EP tests). Water repellency was measured on the surface and at soil depths of 2 cm and 5 cm for three fine textured soil types (Rendzic Phaeozem, Hypercalcic Calcisol, and Eutric Cambisol) developed below different plant covers (mossy pine forest, evergreen oak forest, and meadow, respectively). At the class level, WDPT and EP tests offered similar results for unaltered and sieved samples. The EP test tends to slightly overestimate values on sieved samples, whereas the WDPT test tends to slightly underestimate measurements on sieved samples. Based on the good relationship between the two types of soil samples used (unaltered and sieved), conversion formulas for both tests have been obtained: a linear regression for EP test (R 2 = 0.862; p 2 = 0.913; p Differences in the wettability has been observed between soil depths and soil types: Rendzic Phaeozem under pinewood is very strongly hydrophobic and consistently severely water repellent at any soil depth; Hypercalcic Calcisol under oakwood has the same behavior on the surface, but water repellency decreases progressively with depth; finally, Eutric Cambisol under meadows is hydrophilic and mainly wettable for all depths.

Natural severity of water repellency in pine forest soils from NW Spain and influence of wildfire severity on its persistence

Soil water repellency is a widespread property of Pinus pinaster forest soils in Galicia (NW Spain) that can be intensified by soil heating during fires. Fire-induced or -enhanced soil water repellency, in combination with reduced vegetation and litter cover, is widely considered to be one of the main causes of increased overland flow and accelerated soil erosion on recently burnt hill slopes.In this work, we assessed the natural severity of water repellency in forest soils of different texture under P. pinaster in Galicia (NW Spain) and we examined the specific effect of a forest fire in an area under granite substrate and P. pinaster plantation on the severity and persistence of soil water repellency. Because the burnt area included zones that were affected to a different extent by the fire, we focused on the effect of its severity on soil water repellency from surface level down to a depth of 40cm immediately after the fire and 15months later. Persistency in the soil water repellency was assessed with the water drop penetration time test (WDPT).Water repellency in the sampled soils was found to be extremely persistent in the surface layer (0–5cm). Also, the soils of a coarser texture, exhibit more persistence repellency and at greater depths than those of a finer texture.Fire severity was found to influence the persistence of water repellency and the thickness of the hydrophobic layer in the burnt zones. Such persistence in the soil affected by low-severity fire not differed significantly from that in unburnt soil; also, repellency remained severe to extreme down to 10cm. On the other hand, soil water repellency in areas subject to fire of medium–high severity was completely suppressed within the top 2cm of soil, but increased with depth (extreme water repellency between 2 and 20cm). Fifteen months after the fire, both burnt and unburnt zones exhibited water repellency throughout the sampled profile (0–40cm); however, the burnt zone affected by fire of medium–high severity exhibited decreased water repellency in the surface layer but increased at depths of 2–40cm than it was both in the unburnt zone and in the zone burnt by low-severity fire.

Grass Cover Influences Hydrophysical Parameters and Heterogeneity of Water Flow in a Sandy Soil

Vegetation cover has a major effect on water flow in soils. Two sites, separated by distance of about 50 m, were selected to quantify the influence of grass cover on hydrophysical parameters and heterogeneity of water flow in a sandy soil emerging during a heavy rain following a long hot, dry period. A control soil (pure sand) with limited impact of vegetation or organic matter was obtained by sampling at 50 cm depth beneath a glade area, and a grassland soil was covered in a 10 cm thick humic layer and colonised by grasses. The persistence of water repellency was measured using the water drop penetration time test, sorptivity and unsaturated hydraulic conductivity using a mini disk infiltrometer, and saturated hydraulic conductivity using a double-ring infiltrometer. Dye tracer experiments were used to assess the heterogeneity of water flow, and both the modified method for estimating effective cross section and an original method for assessing the degree of preferential flow were used to quantify this heterogeneity from the images of dyed soil profiles. Most hydrophysical parameters were substantially different between the two surfaces. The grassland soil had an index of water repellency about 10 times that of pure sand and the persistence of water repellency almost 350 times that of pure sand. Water and ethanol sorptivities in the grassland soil were 7% and 43%, respectively, of those of the pure sand. Hydraulic conductivity and saturated hydraulic conductivities in the grassland soil were 5% and 16% of those of the pure sand, respectively. Dye tracer experiments revealed a stable flow with “air-draining” condition in pure sand and well-developed preferential flow in grassland soil, corresponding to individual grass tussocks and small micro-depressions. The grassland soil was substantially more water repellent and had 3 times the degree of preferential flow compared to pure sand. The results of this study reinforce our view that the consequences of any change in climate, which will ultimately influence hydrology, will be markedly different between grasslands and bare soils.