Molarity Of An Ethanol Droplet Research Articles

Post-Fire Impacts of Vegetation Burning on Soil Properties and Water Repellency in a Pine Forest, South Korea

Forest fires can have a direct and immediate impact on soil properties, particularly soil water repellency. This study investigated the direct impacts of the Gangneung forest fire of 2019 on soil properties and the spatial variability of soil water repellency with vegetation burn severity in the Korean red pine (Pinus densiflora Siebold and Zucc) forest of South Korea. A total of 36 soil samples were collected at depth intervals of 0–5 cm, 10–15 cm, and 20–25 cm from three burned sites, representing surface-fuel consumption (SC), foliage necrosis (FN), and crown-fuel consumption (CC), respectively. An unburned site was also used as a control. Soil properties such as soil texture, pH, bulk density, electrical conductivity (EC), total organic carbon (TOC), and cation exchange capacity (CEC) were analyzed in the laboratory. The increase in the sand fraction near the soil surface after a fire was associated with changes in silt and clay fractions. Moderate to high vegetation burn severity at the FN and CC sites caused a decrease in soil pH due to the thermal destruction of kaolinite mineral structure, but organic matter combustion on the soil surface increased soil pH at the SC site. Forest fires led to increases in total organic carbon at the FN and SC sites, owing to the external input of heat damaged foliage and burnt materials. Molarity of an ethanol droplet (MED) tests were also conducted to measure the presence and intensity of soil water repellency from different locations and soil depths. MED tests showed that vegetation burn severity was important for determining the strength of water repellency, because severely burned sites tended to have stronger water repellency of soil than slightly burned sites. Unburned soils had very hydrophilic characteristics across soil depths, but a considerably thick hydrophobic layer was found in severely burned sites. The soil water repellency tended to be stronger on steep (>30°) slopes than on gentle (<15°) slopes.

Do wildfire and slope aspect affect soil water repellency in eucalypt plantations? – A two-year high resolution temporal dataset

Wildfire can induce, enhance, or destroy natural soil water repellency (SWR) depending mostly on soil burnt severity, soil and vegetation types, soil organic matter (SOM) content and type, soil texture, peak soil temperature and remaining time and, antecedent soil moisture content (SMC). In Portugal, SWR in eucalypt stands has been shown to increase surface runoff under dry antecedent SMCs in both burnt and unburnt sites, thereby enhancing soil erosion. Despite the many hydrological implications of SWR, intensive field monitoring under a wide range of SMCs is still limited. This study aimed to improve our understanding of the role of slope aspect and wildfire on the temporal evolution of SWR in Mediterranean environments, which will be helpful for hydrological modelling and post-fire soil erosion risk assessment. In this sense, two burnt eucalypt plantations (wildfire of July 2010) were selected along a neighbouring, long-unburnt eucalypt stand, in north-central Portugal, in order to assess SWR and SMC’s in a high temporal resolution. During the first two post-fire years, at 1–2 week intervals, surface (2.5 cm depth) SWR and SMC were monitored using the “molarity of an ethanol droplet” (MED) test and a soil probe, respectively. The results showed that the observed annual SWR was very high to extreme (80% of measurements ≥ 6 ethanol class) for all three sites. Wildfire reduced the annual median SWR by one class (extreme to very high) for both post-fire years. During prolonged dry periods the unburnt site was significantly more repellent (P < 0.01) and less spatially variable (P < 0.01) than the recently burnt site, while in the wet periods there was no significant difference. After burning, slope aspect did not have a strong impact on the annual median SWR class. However, the north-west aspect site recorded significantly (p < 0.01) greater annual frequencies of extreme repellency than the south-east aspect site (70–73% vs. 51–55%).

Methods for Determining Lignocellulosic Biochar Wettability

It was shown that many various biochar types are hydrophobic in low temperature ( 500 °C) has a tendency of increasing hydrophilicity. Hydrophobicity of low temperature biochar is the most often referred to the effect of pores clogging with tars, since aliphatic compounds vapour only at high temperatures. This research used various methods for the evaluation of biochar wettability, such as the water drop penetration time (WDPT) test, the molarity of an ethanol droplet (MED) test and optical determination of a contact angle (CA). Selection of the best method is difficult because of the fact that many studies used different methods, they have been compared with each other, they refer to different theoretical assumptions and they differ technically. The aim of this article is to compare the three methods used for the evaluation of biochar wettability. The chosen techniques include WDPT, MED and CA measurement. Two different temperatures (450 °C and 700 °C) and two different holding times (1 h and 2 h) were used for the production of different wettability biochar types from five different lignocellulosic feedstocks (pine bark, birch bark, pine wood, birch wood and hemps). Results of wettability and all used techniques overlapped; and this means that it had the same tendency in characterising biochar wettability. Mechanical and practical arguments have been made for the selection of the most suitable method for determination of biochar wettability.

Compatibility of methods used for soil water repellency determination for organic and organo-mineral soils

Soil water repellency (i.e. hydrophobicity, SWR) is a common soil phenomenon inhibiting water infiltration and water movement in the soil. SWR has significant hydrological implications for enhanced overland and preferential water flows and erosion. Several methods are used to determine the degree of SWR. The methods are typically chosen based on their suitability for field or laboratory work, as well as time and resources availability. Unfortunately, each measurement method has a different analytical approach, hence the direct comparison between results from different methods is not possible. A faster and statistically sound technique for converting results is needed, especially to convert results from field applicable techniques to contact angle (CA) value, which is a valuable parameter for soil hydraulic modelling. The aim of this paper is to define a reliable compliance between methods defined on a statistical approach basis (weighted kappa coefficient κw), which will allow to determine the CA value based on straightforward tests, such as water drop penetration time (WDPT) and molarity of an ethanol droplet (MED). For this purpose, we measured SWR in 106 organic and organo-mineral soils collected from different locations in North East Poland using four common methods. The sessile drop and Wilhelmy plate laboratory-based methods were used to determine the CA between water and the solid phase. The other two tests are common field methods for assessing SWR by measuring water infiltration time (WDPT) and the highest surface tension of ethanol-water droplet infiltration into the soil (MED). The results revealed that the weighted kappa coefficient, when assumed as a measurement of an observer's compliance, indicates a strong relationship (κw=0.84) between the average CA (CAav), measured with the sessile drop method, and the median value of the WDPT (WDPTme). Based on the results, we can conclude that hydrophilic samples with WDPT<5s have the average CA values below 40°, while extremely hydrophobic samples with WDPT above 3600s have CA values higher than 130°. This is a proof that these tests can be a good estimator of CA value for SWR determination in the laboratory or the field.

Erratum to: Unimodal pattern of soil hydrophobicity along an altitudinal gradient encompassing Mediterranean, temperate, and alpine ecosystems

Soil water repellency (SWR, i.e. the reduced affinity for water due to the presence of hydrophobic coatings on soil particles) has relevant hydrological implications on the rate of water infiltration, surface runoff, and overland flow. Here, we test how SWR varies along a 2490 m altitudinal gradient encompassing six ecosystems including Mediterranean, Temperate, and Alpine vegetation types. Water repellency, measured by the Molarity of an Ethanol Droplet (MED) test, was quantified in 80 soil samples collected for 16 different elevations. Soil quality was assessed by measuring soil texture, pH, organic carbon, salinity, and nutrient availability. SWR showed a unimodal pattern along the 2490 m transect, peaking at intermediate elevations. Unexpectedly, SWR was the highest under broad-leaf deciduous forests, and the lowest under evergreen, sclerophyllous Mediterranean vegetation types. The soil organic carbon content, and the pH were the main determinants of water repellency, showing respectively a positive, and a negative correlation with the SWR. In contrast, soil texture and salinity resulted unrelated to the SWR. With this study we demonstrated a linkage between SWR, vegetation type and soil pH and organic carbon content along the elevation gradient. Further studies are needed to explicitly evaluate the impact SRW on erosion risk at catchment scale in the context of climatic change.

The Influence of Plant Litter on Soil Water Repellency: Insight from 13C NMR Spectroscopy.

Soil water repellency (SWR, i.e. reduced affinity for water owing to the presence of organic hydrophobic coatings on soil particles) has relevant hydrological implications because low rates of infiltration enhance water runoff, and untargeted diffusion of fertilizers and pesticides. Previous studies investigated the occurrence of SWR in ecosystems with different vegetation cover but did not clarify its relationships with litter biochemical quality. Here, we investigated the capability of different plant litter types to induce SWR by using fresh and decomposed leaf materials from 12 species, to amend a model sandy soil over a year-long microcosm experiment. Water repellency, measured by the Molarity of an Ethanol Droplet (MED) test, was tested for the effects of litter species and age, and compared with litter quality assessed by 13C-CPMAS NMR in solid state and elemental chemical parameters. All litter types were highly water repellent, with MED values of 18% or higher. In contrast, when litter was incorporated into the soil, only undecomposed materials induced SWR, but with a large variability of onset and peak dynamics among litter types. Surprisingly, SWR induced by litter addition was unrelated to the aliphatic fraction of litter. In contrast, lignin-poor but labile C-rich litter, as defined by O-alkyl C and N-alkyl and methoxyl C of 13C-CPMAS NMR spectral regions, respectively, induced a stronger SWR. This study suggests that biochemical quality of plant litter is a major controlling factor of SWR and, by defining litter quality with 13C-CPMAS NMR, our results provide a significant novel contribution towards a full understanding of the relationships between plant litter biochemistry and SWR.

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.

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.

Soil Water Repellency Dynamics in Pine and Eucalypt Plantations in Portugal – A High‐resolution Time Series

AbstractDuring the 20th century, afforestation resulted in plantations of Pine and Eucalypt becoming the main crops in north‐central Portugal with associated and well‐known soil water repellency (SWR). The aim of this study was to improve the insights in the temporal dynamics and abrupt transitions in water repellency of the topsoil and the mechanism that determine the behaviour of SWR. Topsoil water repellency was monitored in the Caramulo Mountains (north‐central Portugal) between July 2011 and June 2012. The intensity of SWR was measured in situ at soil depths of 0, 2.5 and 7.5 cm using the ‘molarity of an ethanol droplet’ test. Volumetric soil moisture content was monitored in situ using a Decagon ECH2O EC‐5 probe. SWR behaviour broadly followed five alternating dry and wet periods during the 12‐month period, with more pronounced differences in the Pine site than in the Eucalypt site. SWR under Eucalypt was substantially more temporally dynamic than under Pine, with double the number of moderate and large SWR changes at the 7.5 cm depth. Soil moisture content and antecedent rainfall were better correlated to SWR under Pine than under Eucalypt, although in both cases insufficient to predict the temporal variations. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Soil water repellency in rangelands of Extremadura (Spain) and its relationship with land management

Abstract Soil water repellency reduces infiltration capacity, enhancing overland flow and even runoff production, and may produce patchiness in water infiltration at the hillslope scale. Knowledge about hydrophobicity in rangelands of Mediterranean type climate and its relation with vegetation cover and land management is sparse. The objectives of the present work are to determine the degree and spatial occurrence of soil water repellency and to define its relationship with site characteristics, such as soil, vegetation and land management in rangelands of SW Spain. Field work was conducted in September 2009 in 22 environmental units belonging to 10 farms, distributed throughout the region of Extremadura. The Molarity of an Ethanol Droplet (MED) method was used for measuring water repellency of the soil surface. A total of 725 points were sampled and the study was combined with a detailed vegetation survey, the determination of soil properties and of land use and management characteristics. More than 70% of the measurement points were hydrophilic, however differences between farm units were found. The highest values of hydrophobicity were observed on soil surfaces below the canopy of holm oaks, as compared to the low values found below shrub canopies ( Retama sphaerocarpa ) and in open areas. With respect to ground cover, water repellency was highest at sites covered by holm oak litter. At soil surfaces covered by cork oak litter, dry grass and mosses the degree of repellency was lower. Almost all sites with a bare soil surface were hydrophilic, independent of whether these were located below a tree canopy or in open areas. A significant positive relationship between livestock density and the degree of bare soil existed, and consequently, areas with high animal numbers were dominantly hydrophilic. No significant relationships were encountered between the degree of soil water repellency and soil properties.

Effects of wildfire and laboratory heating on soil aggregate stability of pine forests in Galicia: The role of lithology, soil organic matter content and water repellency

Abstract The present work aims to assess the effects of wildfires on soil aggregate stability and the role therein of contrasting lithologies as well as of fire-induced changes in organic matter content and soil water repellency. To this end, a pair-wise comparison of neighbouring burned and unburned soils was carried out and complemented by laboratory heating experiments to clarify the role of fire intensity. In total, 18 pairs of adjacent burned and unburned pine forest soils were sampled within one month after wildfire. At each site, five samples were collected of the top 5 cm of the A horizon at randomly selected sample points and were mixed in the field to obtain one composite sample per site. Three additional samples were collected at each site but stored separately, and those of three sites were selected for the laboratory heating experiments. Laboratory heating involved five different temperatures ranging from 25 to 460 °C. Aggregate stability of the field and laboratory samples was determined using the water drop impact test, organic carbon content using a modified Sauerlandt method and soil water repellency using the ‘Molarity of an Ethanol Droplet’ test. The wildfire effects on field aggregate stability were highly variable and results indicated that these changes depend primarily on organic matter combustion and, thus, fire intensity. Controlled heating up to 220 °C either did not alter aggregate stability or increased it with increasing temperature, possibly due to the development of a protective coating of organic compounds inducing water repellency. Heating at 380 and 460 °C, by contrast, produced considerable to massive combustion of organic matter and, thereby, very pronounced reduction of aggregate stability as well as water repellency.

Water repellency of post-boggy soils with a various content of organic matter

The objective of this study was to estimate the water repellency of post-boggy soils in north-eastern Poland. Potential water repellency was determined based on the water drop penetration time (WDPT) test and the molarity of an ethanol droplet (MED) test. A total of 276 soil samples with a varied organic carbon (OC) content, ranging from trace amounts in sandy subsoils to 44.4% in organic soils, were analyzed. The investigated material represents peat-muck soils (Eutri-Sapric Histsols) and muck-like soils (Arenic Gleysols, Areni-Humic Gleysols, Gleyic Arenosols). The mineral matter of the analyzed soils comprised loose sand. The obtained results indicate that peat soil formations are marked by higher potential water repellency than muck soil formations. The highest WDPT values (16 390 s) were reported in respect of an alder peat sample with 41.9% OC content, collected at a depth of 55–60 cm. In the group of muck soils, a sample with 36.7% OC content, collected at a depth of 15–20 cm, was marked by the highest water repellency (WDPT 10 492 s). The water repellency of the studied soils is dependent on organic matter content, and it is manifested only when organic matter content is higher than 20%. Soils with OC content of up to 12% show low water repellency or are hydrophilic. Organic soil formations (>12% OC) are characterized by a varied degree of water repellency, but WDPT values in excess of 2000 s are reported only in respect of soils containing more than 35% OC. A significant positive correlation between the content of organic matter, organic carbon, total nitrogen and water repellency was observed in the entire studied population (n = 276). A significant positive correlation was also found between WDPT values and the C:N ratio, while a significant negative correlation was reported in respect of \( pH_{H_2 O} \).

Cokriging Spatial Interpolation of Soil Water Dependent Repellency Parameters Determined with Two Different Tests

The nonlinear behavior of water repellency (WR) with respect to soil water content (θg) may be described by curve parameters derived from two widespread methodologies such as the water drop penetration time (WDPT) and molarity of an ethanol droplet (MED) tests. While the former measures WR persistence in terms of the infiltration time of a water drop sitting on the soil surface, the latter quantifies the repellency degree as contact angle or 90° surface tension, γ90° From a hydrologic point of view, the WDPT test may be considered more closely related to the natural infiltration process of the soil, but the infiltration times become prohibitively large (on the order of hours) in highly repellent soils; thus, the MED test is more convenient. This study was conducted to determine whether the spatial interpolation of time‐consuming WDPT measurements may be complemented with less time‐demanding WR parameters derived from the MED test. Measurements from saturation to oven dryness using both MED and WDPT were performed on 140 topsoil samples from a humid evergreen forest watershed of the Garajonay National Park (Canary Islands). We found useful links between curve parameters derived from the MED and WDPT tests. The area below the WR–θg curve measured with the WDPT test (SWDPT) was found to be correlated with that measured with the MED test (SMED) and with the θg at which repellency is triggered (θg‐min). By contrast, previously published relations that established a logarithmic relationship between 1/WDPT and γ90° were valid only in the dry soil state, but must be ruled out if soil water content is taken into account. We took advantage of the existence of such parameter correlations to show how cokriging geostatistical tools may be applied to improve the spatial interpolation of SWDPT, with SMED or θg‐min as subsidiary variables. In general, cokriging was superior to kriging in terms of mean variance minimization with little extra labor cost. A minimal‐effort spatial sampling strategy may be designed taking into account these considerations.

USEFUL SOIL-WATER REPELLENCY INDICES

Water repellency (WR) has been classically characterized at fixed (usually oven-dry) soil water content (θ g ) in terms of the soil water contact angle (CA), a. However, a has been previously reported to depend upon θ g in a nonlinear fashion, such that WR increases from a wettable state close to saturation (θ g-min ) up to a maximum, α max , decreasing afterward either monotonically or rising again to a second local or absolute a maximum nearby the dried soil state. Hence, a CA versus water content (α-θ g ) curve may be described in terms of different WR parameters, such as θ g-min , θ g-max , α max , or the integrated area below the α-θ g curve, S. Based on previous α-θ g measurements carried out with the molarity of an ethanol droplet (MED) test, both in mineral and volcanic soils from different world regions, including cultivated and natural forest soils, and textures ranging from clay-loam to sandy, we confirm here the usefulness of the integrated area below the α-θ g curve (S) as a WR describing index for a large variety of α-θ g curve shapes. We found a simple relationship between S and the soil water content at which WR is triggered, θ g-min , such that S = 16.903 θ g-min (R 2 = 0.946), which provides an easy method for the rapid characterization of the overall WR degree of soils. S was also linearly correlated with the soil organic matter (SOM) content (R 2 = 0.817) for 1 g (100 g) -1 < SOM < 88 g (100 g) -1 , such that the best estimate of S was that obtained by combining linearly both θ g-min and the SOM content (R 2 = 0.990). Linear correlations were also found between θ g-max , that is, the soil water content at which a is maximum, and S (R 2 = 0.834) or the SOM content (R 2 = 0.705), and consequently between θ g-max and θ g-min (R 2 = 0.830). In addition, both θ g-min and θ g-max were found to depend linearly upon the soil water content at -33 kPa and -1500 kPa, respectively. Finally, a mean soil WR may be defined as the ratio S/θ g-min . We found that the maximum CA, α max , and the mean soil WR S/θ g-min were positively correlated (R 2 = 0.780), such that a particular soil with high (low) values of maximum CA is expected to exhibit a high (low) WR degree on average across the whole water regimen from -33 kPa down to oven-dry moisture. Such an estimate of the mean WR index S/θ g-min was further improved if both α max and the SOM content were available (R 2 = 0.825).

Characterizing water repellency indices: Contact angle and water drop penetration time of hydrophobized sand

The accuracy and the reproducibility of water repellency measurements can be improved with the use of proper measurement techniques. The purpose of this study is to compare the contact angles obtained using three different methods and to examine the relationship between contact angle and water drop penetration time (WDPT) using non-repellent to extremely repellent sands. Fine silica sand, artificially hydrophobized with stearic acid, was used for the measurements. Contact angles were estimated using the molarity of an ethanol droplet (MED) test, the capillary rise method (CRM) and the sessile drop method (SDM). The CRM was effective for soils with contact angles < 90°. The MED test was suitable for soils with contact angles ≥ 90° using 10 s as the ethanol drop penetration time. The SDM was effective for soils with contact angles ranging from very low to very high (11–105°). Directly measured contact angles using the SDM were in good agreement with indirectly obtained contact angles using the MED test and the CRM. The WDPT was < 1 s for contact angles ranging from 11 to 69°, gradually increased from 69 to 93°, and exceeded 3600 s beyond that. The WDPT was most sensitive for contact angles within the range of 88–93°. Extreme repellency was observed where the solid surface free energy was ≤ 45 mN m−1.

Temporal variation in topsoil water repellency in two recently burnt eucalypt stands in north-central Portugal

The aim of this study is to improve our knowledge of the temporal and spatial variations of soil water repellency following wildfire, in particular for the eucalypt stands that now dominate the landscape of north-central Portugal. Topsoil water repellency was monitored on 21 occasions over a 10-month period, starting in September 2005, six weeks after a moderately severe wildfire. This was done, on mostly alternating dates, in two neighbouring commercial eucalypt plantations, one with an undisturbed and one with a ploughed soil profile, in the foothills of the Gralheira Massif in north-central Portugal. Water repellency severity was measured in situ at soil depths of 2-3 and 7-8 cm using the 'Molarity of an Ethanol Droplet' (MED) test, and accompanied by soil moisture measurements using a ThetaProbe(TM) or, at a few occasions, sample analysis in the laboratory for gravimetric content. The results show a broadly seasonal pattern of overall very high water repellency in dry periods and reduced or no repellency following prolonged rainfall. This was more pronounced at the undisturbed compared to the ploughed site, as the latter exhibited strong to extreme water repellency at almost all sampling dates. Significant changes in repellency severity, including major increases, occurred within periods as short as 6-7 days, suggesting that the sampling intervals used here may have not captured the full dynamics of topsoil repellency. Repellency severity was consistently lower at greater soil depth, in particular when considering the whole Study period. Soil moisture Was found to relate to the temporal variations in repellency. As found in previous Studies, however, soil moisture alone was not sufficient to predict the temporal variations in water repellency. (C) 2008 Elsevier B.V. All rights reserved.

Temporal and spatial variations in topsoil water repellency throughout a crop‐rotation cycle on sandy soil in north‐central Portugal

AbstractThe aim of this study is to improve our understanding of the temporal and spatial variations of soil water repellency, in particular those of agricultural lands where its economic and environmental impacts can be particularly severe. Variations in water repellency severity were studied with particular emphasis as to the role therein of the micro‐topographic differences between furrows and ridges.Topsoil water repellency was monitored over a 1‐year period at about monthly intervals and on some additional occasions in an agricultural field involving three land‐use phases (potatoes, maize, fallow) in the Dunas de Vagos area, north‐central Portugal. Water repellency severity was measured in situ at soil depths of 2–3 and 7–8 cm using the molarity of an ethanol droplet (MED) test, and accompanied by soil moisture measurements using a ThetaProbe. Under potatoes as well as maize, these measurements were carried out on ridges and in furrows separately.The results confirmed the transience of soil water repellency, which changed markedly within periods as short as 2 weeks. Although substantial changes in water repellency severity occurred, repellency was present over a large part of the monitoring period under potatoes, maize and fallow. Repellency severity was found to regularly differ significantly between furrows and ridges as well as between 2–3 and 7–8 cm depth. Soil moisture content was found to be an important factor, but by itself not sufficient to account for the observed temporal variations. Other temporally variable factors (e.g. microbial and root activity) might contribute to the variations in water repellency at this agricultural site under rotating crops. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Geostatistical Tools for Characterizing the Spatial Variability of Soil Water Repellency Parameters in a Laurel Forest Watershed

Soil water repellency is recognized to be a widespread phenomenon which may affect a wide range of spatially dependent hydrological processes that take place in the vadose zone such as infiltration, preferential flow, and soil water distribution. Despite this, the spatial structure of soil repellency has received almost no attention in the past. The objective of this study is to investigate the spatial variability of water repellency in the top horizon of a laurel forest watershed. Water repellency was measured with the molarity of an ethanol droplet (MED) test, from saturation to oven‐dry, in 140 soil samples taken in a nested structure that encompasses both short (centimeter scale) and long (meter scale) distances. Geostatistical tools such as correlograms and kriging were used to quantify the spatial structure of soil organic matter (SOM) content and the parameters that characterize the water repellency curve. Both showed characteristic spatial trends, and, in general, spatial correlation followed a spherical, exponential, or gaussian model. All parameters investigated exhibit scale dependence, in particular, variability of SOM and the area below the repellency curve point toward a self‐similar fractal scaling. Cross‐correlation of some of the repellency parameters and SOM was not random but showed spatial structure.

Characterizing Water Dependent Soil Repellency with Minimal Parameter Requirement

Soil water repellency varies nonlinearly with soil moisture, describing a repellency versus water content curve. Despite this water dependence, soil repellency is usually characterized by repellency indexes measured at fixed soil water content. We provide a statistically robust yet simple method to determine water repellency at a range of soil water contents, by proposing alternative water-dependent soil repellency parameters derived from the molarity of an ethanol droplet (MED) test. Useful correlations are found between some of these proposed repellency parameters, such is the case of'the integrated area below the repellency curve, which is found to be strongly correlated with the soil water content at minimum repellency, Consequently, an efficient strategy for describing the repellency versus water content curve is designed based on a combination of parameter interdependence and a minimum number of determinations necessary. Following this strategy, only 27 samples are sufficient (p < 0.05) to characterize the water-dependent repellency of the studied soil. The integrated area below the repellency curve is proposed as the key Index for repellency characterization. Furthermore, a new combined parameter IRDI (Integrative Repellency Dynamic Index) is defined as a measure of the average water dependent repellency. This allows simple comparisons of the whole water repellency behavior among different soils by coalescing the repellency curve into a single characteristic value.

Characterizing Water Dependent Soil Repellency with Minimal Parameter Requirement

Soil water repellency varies nonlinearly with soil moisture, describing a repellency versus water content curve. Despite this water dependence, soil repellency is usually characterized by repellency indexes measured at fixed soil water content. We provide a statistically robust yet simple method to determine water repellency at a range of soil water contents, by proposing alternative water‐dependent soil repellency parameters derived from the molarity of an ethanol droplet (MED) test. Useful correlations are found between some of these proposed repellency parameters, such is the case of the integrated area below the repellency curve, which is found to be strongly correlated with the soil water content at minimum repellency. Consequently, an efficient strategy for describing the repellency versus water content curve is designed based on a combination of parameter interdependence and a minimum number of determinations necessary. Following this strategy, only 27 samples are sufficient (p &lt; 0.05) to characterize the water‐dependent repellency of the studied soil. The integrated area below the repellency curve is proposed as the key index for repellency characterization. Furthermore, a new combined parameter IRDI (Integrative Repellency Dynamic Index) is defined as a measure of the average water dependent repellency. This allows simple comparisons of the whole water repellency behavior among different soils by coalescing the repellency curve into a single characteristic value.