Variability Of Water Repellency Research Articles

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.

Does fire alter soil water repellency in subtropical coastal sandy environments?

AbstractIrregular wetting, water repellency, and preferential flow are well‐documented properties of coastal sandy podzols, though little is known about the effect of fire on unsaturated zone processes in this environment. This study investigates water repellency at and below the soil surface in two coastal sandy podzols following bushfire. Water drop penetration time tests were applied to burned and unburned soils at a high dune field site in South East Queensland, Australia. It was found that the mean water drop penetration time of the burned soil was four times that of the unburned soil, but both soils were largely non‐repellent. Post‐fire repellency peaked below the surface in a patchy layer, in contrast to the laterally extensive layer reported in other studies, and high organic matter content in the soil did not appear to significantly influence repellency post‐burn. Non‐parametric statistics were used to quantify the high spatial variability in water repellency, which was ultimately insufficiently captured by atypically large (n = 1000 drop) datasets. This study confirms the presence of naturally occurring repellency and patchy infiltration in sandy soils while demonstrating that conclusively describing the influence of fire is challenging in a soil with heterogeneous infiltration characteristics. With respect to this uncertainty, it appears that fire does not increase soil water repellency such that infiltration and runoff processes due to fire‐induced water repellency would differ post‐burn.

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.

Spatial and temporal variability of surface water repellency in sandy loam soils of NW Spain under Pinus pinaster and Eucalyptus globulus plantations

AbstractThis work was undertaken for two main purposes. One was to examine spatial and temporal variability in surface water repellency under field conditions in sandy loam forest soils of NW Spain, and its relationship to weather and soil moisture conditions. The other purpose was to get further inside in the dynamics of soil water repellency by studying a wetting–drying cycle under controlled laboratory conditions. Both for the field and laboratory study, water repellency was determined using the Water Drop Penetration Time test. Soil water repellency under field conditions was found to exhibit a seasonal pattern, i.e. it peaked during the summer and was absent between November and May. The time required for repellency to become re‐established during the spring was shorter under eucalyptus than under pine. Spatial variability peaked at an early stage of soil drying and was minimal during the wet period when soils were hydrophilic as well as at the end of the summer, when repellency was strongest. Spatial and temporal variability in water repellency was found to be negatively correlated with soil moisture and, to a lesser extent, with antecedent rainfall. The moisture range of the so‐called transition zone (below which the soil is hydrophobic and also above which it is hydrophobic) differed for the pine (21–50%) and eucalyptus plantations (17–36%). The lower and upper bounds of the transition zone agreed well with the soil moisture contents at the permanent wilting point and at field capacity, respectively. The laboratory results with samples in the wetting phase confirmed those of the field tests. Water repellency increased slightly during the drying phase, but not so much as in the field. Copyright © 2011 John Wiley & Sons, Ltd.

Determination of Small‐Scale Spatial Heterogeneity of Water Repellency in Sandy Soils

The degree of water repellency in soils is commonly characterized by the contact angle (CA). For its determination, disturbed and homogenized soil samples are used. Thus, information about the small‐scale spatial heterogeneity of soil water repellency is lost. We modified the sampling technique of the sessile drop method to allow the determination of the CA on particles located at their original positions, reflecting the natural wetting properties of the site. The proposed preparation technique further allows the determination of small‐scale variability of water repellency (millimeter to centimeter scales) along transects with a length of approximately 1 m or more. Hence, the small‐scale spatial distribution of water repellency within a soil profile or transect can be determined and its impact on physical, biological, or chemical processes in soil can be evaluated.

Variability of water repellency in sandy forest soils under broadleaves and conifers in north-western Jutland/Denmark

Soil water repellency has important consequences for ecological and hydrological properties of soils and usually retards infiltration capacity and induces preferential flow. This phenomenon has been known to occur on a wide range of sites under a variety of climatic conditions. The objective of this study was to investigate and characterize soil water repellency on forest sites with identical substrate and climatic conditions, differing in tree age and species. In the Vester Torup Klitplantage, an area comprising a conifer dominated forest plantation stocking on sandy deposits in a coastal setting near the Jammer Bay in north-western Jutland/Denmark, four different forest plots were investigated for water repellency effects four times in 2005. To measure soil water repellency, the water drop penetration time test and the critical surface tension test were carried out. Both tests revealed a seasonal variability in water repellency, exhibiting the highest water repellency for the upper 10 cm of the soil during the summer months, whereas the variability between the different plots seems to be less significant. There was no coherence between humus forms, thickness of litter layer and water repellency.

Soil water repellency and infiltration in coarse-textured soils of burned and unburned sagebrush ecosystems

Millions of dollars are spent each year in the United States to mitigate the effects of wildfires and reduce the risk of flash floods and debris flows. Research from forested, chaparral, and rangeland communities indicate that severe wildfires can cause significant increases in soil water repellency resulting in increased runoff and erosion. Few data are available to document the effects of fire on the spatial and temporal variability in soil water repellency and potential impacts on infiltration and runoff on sagebrush-dominated landscapes. Soil water repellency, infiltration and runoff were assessed after two wildfires and one prescribed fire in three steep, sagebrush-dominated watersheds with coarse-textured soils. Water repellency was generally greater on unburned hillslopes and annual variability in water repellency had a greater impact on infiltration capacity than fire effects. The most significant impact of fire was canopy and ground cover removal on coppice microsites. Infiltration rates decreased on coppice microsites after fire even though soil water repellency was reduced. Fire-induced reduction in infiltration resulted from the combined effect of canopy and ground cover removal and the presence of naturally strong water repellent soils. Removal of ground cover likely increased the spatial connectivity of runoff areas from strongly water repellent soils. The results indicate that for coarse-textured sagebrush landscapes with high pre-fire soil water repellency, post-fire increases in runoff are more influenced by fire removal of ground and canopy cover than fire effects on soil water repellency and that the degree of these impacts may be significantly influenced by short-term fluctuations in water repellent soil conditions.