Stefan H. Doerr

Soil water repellency: its causes, characteristics and hydro-geomorphological significance

Abstract Water repellency (hydrophobicity) of soils is a property with major repercussions for plant growth, surface and subsurface hydrology, and for soil erosion. Important advances have been made since the late 1980s in identifying the range of environments affected by water repellency, its characteristics and its hydro-geomorphological impacts. This review summarises earlier work, but focusses particularly on these recent advances and identifies remaining research gaps. The associations of water repellency with (a) soils other than coarse-textured ones, (b) an expanding list of plant species, and (c) a widening range of climates other than seasonally dry types have been recurrent themes emphasised in recent literature. Nevertheless, knowledge about the extent of water repellency amongst world soils is still comparatively sparse. Its origin by the accumulation of long-chained organic compounds on or between soil particles is now widely accepted, but understanding of their exact chemical composition and means of attachment to particle surfaces remains incomplete. The transient nature of water repellency has been found to be mainly associated with fluctuations in soil moisture, but the precise processes and required conditions for the changes from hydrophobic to hydrophilic and vice versa are so far only poorly understood. Significant advances relating to the hydro-geomorphological impacts of hydrophobic layers have been made since the late 1980s in identifying and separating the various effects of such layers on surface and subsurface water flow. It has become evident that these effects in turn are influenced by variables such as the frequency and effectiveness of flow pathways through hydrophobic layers as well as their position and transitory behaviour. Recent literature has continued to highlight the role of water repellency in promoting soil erosion and it is now recognised that it can promote rainsplash detachment and soil loss not only by water, but also by wind. Major research gaps, however, remain in (a) isolating the erosional impact of water repellency from other factors, and (b) identifying the exact role of, and the interactions between the different variables controlling development and effectiveness of flow pathways through hydrophobic soil. Improved understanding of the effects of soil water repellency will enable its overall role in surface and subsurface hydrological and erosional processes to become more clearly defined.

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

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

On standardizing the ‘Water Drop Penetration Time’ and the ‘Molarity of an Ethanol Droplet’ techniques to classify soil hydrophobicity: A case study using medium textured soils

Two common methods to assess soil hydrophobicity are the ‘Water Drop Penetration Time’ and ‘Molarity of an Ethanol Droplet’ techniques. For these, uncertainty exists regarding the representativeness of laboratory tests reflecting field conditions, their replicability, and the comparability of results between the two techniques. Using air-dried soils with a broad particle size and hydrophobicity range, this study shows that a high representativeness and replicability of results can be achieved. A close relationship between the two tests was found for highly, but not for moderately hydrophobic soils. Guidelines are suggested to increase representativeness, replicability and comparability of results in future studies.

Hydrophobicity and aggregate stability in calcareous topsoils from fire-affected pine forests in southeastern Spain

Soil hydrophobicity is known to enhance runoff responses to rainstorms and to increase soil aggregate stability (AS). It has been widely reported for acidic soils particularly under burnt, but also unburnt pine forests following dry periods. Few studies have reported hydrophobicity from alkaline soils, but they have not established whether hydrophobicity also occurs in burnt or unburnt pine forests on alkaline soil. This study examines the wettability and stability of air-dry aggregates and their size fractions ( 30-year-old Aleppo pine {Pinus halepensis} and associated shrub community), geology (limestone), soil type (Lithic Xerorthents), slope angle and aspect (5–8°SW). Included were three sites (A, B, C) burned, respectively, in 1998, 1999 and 2000, and one unburnt for >30 years (D). Hydrophobicity was detected in samples from all sites. Both spatial frequency and persistence of hydrophobicity (Water Drop Penetration Times (WDPT) ranged from 10 to 600 s), however, was lower than reported from studies of acidic soils under pine. This might be associated with a lower susceptibility of alkaline soils to hydrophobicity development and/or the comparatively low biomass production in the region. Probably because it had been most recently affected by severe fire, spatial frequency of hydrophobicity was higher at site B (53% of samples), compared to A, C and D (6%, 33% and 10% of samples, respectively). In contrast to some previous studies, the finest size fraction of the samples consistently had the highest degree of hydrophobicity. Degree of hydrophobicity was positively correlated with organic matter (OM) content (r=0.714). It is speculated that fine, interstitial hydrophobic organic matter accumulating in the finest sieve fraction contributes to this enhanced hydrophobicity. As shown in previous studies on acidic soils, aggregate stability increased with hydrophobicity (r=0.897 in the fraction 0.25–2 mm) for the samples investigated here. This elevated stability occurs despite an already relatively high level of aggregate stability amongst all samples investigated. Hydrophobicity observed at the study sites was not spatially contiguous and it may therefore enhance overland flow and slope wash over only short distances for most, except the very high intensity rainstorms that occur in the region. The increased stability of hydrophobic soil aggregates against slaking, however, may counter an otherwise enhanced susceptibility to erosion.

The erosional impact of soil hydrophobicity: current problems and future research directions.

Abstract Soil hydrophobicity affects the susceptibility of soils to erosion in a variety of ways (e.g. increased aggregate stability, reduced infiltration capacity, enhanced overland flow), but there are problems concerning the overall assessment of its erosional impact. Three current problems are discussed: (1) poor isolation of hydrophobicity from other effects; and poor understanding of its overall impact; (2) areally, at slope and catchment scales; and (3) temporally, over periods of months or years rather than on a storm basis. These problems are highlighted with reference to the literature and to research in Portugal on highly hydrophobic forest soils. A conceptual model relating erosion risk to three key aspects of soil hydrophobicity (temporal contiguity, spatial contiguity and the thickness of any overlying hydrophilic soil) is presented in order to provide a framework for future research into hydrophobicity–soil erosion links.

Influence of vegetation recovery on soil hydrology and erodibility following fire: an 11-year investigation

The present study investigates long-term changes in soil hydrological properties and erodibility during the regrowth of different types and densities of vegetation following a severe wildfire in the Serra Grossa Range, eastern Spain. Twelve plots of similar slope and soil characteristics, naturally recolonized by four different plant species (trees, herbs, shrubs and dwarf shrubs) were examined using rainfall simulations during an 11-year period. The mean erosion rate was 80 g m−2 h−1, 6 months after the fire under wet-winter conditions, declining to 30 g m−2 h−1 in the following summer and reaching <10 g m−2 h−1 after 2 years. Considerable variation under the different vegetation types was observed. Herbs and shrubs reduced erosion and overland flow coefficients to negligible values 2 years after fire, whereas under trees and dwarf shrubs, appreciable overland flow and soil loss still occurred after 5 years. On tree-covered plots (Pinus halepensis), overland flow actually increased over time in association with the development of topsoil hydrophobicity, reaching a coefficient of 27% 10 years after burning. Rates of post-fire overland flow and erosion reduction were strongly influenced not only by vegetation coverage but also by the type of cover and its effects on soil hydrophobicity.

Spatial variability of soil hydrophobicity in fire-prone eucalyptus and pine forests, Portugal

Because of its implications for slope hydrology and soil erosion in the region and the lack of previous work on (i) spatial variability of hydrophobicity and (ii) hydrophobicity in a wet Mediterranean environment, this paper assesses the in situ severity and spatial variability of hydrophobicity of surface soils in dry summer conditions in burnt and unburnt Pinus pinaster and Eucalyptus globulus forests in north-central Portugal. Results of experiments to explore the origin of hydrophobicity are also reported. The molarity of ethanol droplet (MED) technique was employed. The average severity of hydrophobicity (MED > 24%) in both long-unburnt and recently burnt forests is among the highest recorded. In contrast to other studies, spatial variability of hydrophobicity is generally low for all land types. This is thought to be caused by a comparatively high release rate and thorough distribution of hydrophobic substances aided by the relatively wet climate combined with the fairly uniform character of the commercial forest stands investigated. Although forest fires are usually thought either to increase (for low ground temperatures) or to destroy (for high ground temperatures) surface soil hydrophobicity, burning in the study area had little impact on surface hydrophobicity. This is attributed to (i) preburn hydrophobicity already so severe that the organic compounds released from the litter during burning contribute no detectable additional hydrophobic effects and (ii) fire temperatures insufficient to destroy surface hydrophobicity. The results suggest that the relative spatial uniformity of hydrophobicity in the study area is induced by the planting of E. globulus or P. pinaster. The litter layers of both species, and the root zone in the case of E. globulus, are identified as sources of hydrophobic substances. Extreme hydrophobicity in E. globulus stands is found to develop within 2 years of planting on previously hydrophilic plowed terrain.

Soil hydrophobicity variations with depth and particle size fraction in burned and unburned Eucalyptus globulus and Pinus pinaster forest terrain in the Águeda Basin, Portugal

A laboratory programme of water drop penetration tests is used to investigate the nature of hydrophobicity of soils in burnt and unburnt Eucalyptus globulus and Pinus pinaster forest areas of northern Portugal. Variations in hydrophobicity of air-dried soil with soil depth, soil particle size fraction, land use and burn history are assessed. Results differ from those found by many other studies in several respects: (1) fire was not found to enhance hydrophobicity, as unburnt and old-burn soils are as hydrophobic as newly-burnt ones; (2) hydrophobicity was found to be characteristic of soils from the surface down to the weathered (Cw) horizon rather than confined to a near-surface layer, (3) it is also associated with the finer rather than the coarser size fractions of the soils. Soils under E. globulus are distinctly more hydrophobic than those under P. pinaster. Implications for the generation of overland flow are briefly explored.

Extraction of compounds associated with water repellency in sandy soils of different origin

After an initial evaluation of several solvents, the efficiency of Soxhlet extractions with isopropanol/ammonia (s.g. 0.88) (70 : 30 v : v; 24 h) in extracting compounds associated with water repellency in sandy soils was examined using a range of repellent and wettable control soils (n = 15 and 4) from Australia, Greece, Portugal, The Netherlands, and the UK. Extraction efficiency and the role of the extracts in causing soil water repellency was examined by determining extract mass, sample organic carbon content and water repellency (after drying at 20 ◦ C and 105 ◦ C) pre- and post-extraction, and amounts of aliphatic C-H removed using DRIFT, and by assessing the ability of extracts to cause repellency in acid-washed sand (AWS). Key findings are: (i) none of organic carbon content, amount of aliphatic C-H, or amount of material extracted give any significant correlation with repellency for this diverse range of soils; (ii) sample drying at 105 ◦ Ci s not necessarily useful before extraction, but may provide additional information on extraction effectiveness when used after extraction; (iii) the extraction removed repellency completely from 13 of the 15 repellent samples; (iv) extracts from all repellent and wettable control soils were capable of inducing repellency in AWS. The findings suggest that compounds responsible for repellency represent only a fraction of the extract composition and that their presence does not necessarily always cause repellency.

Heating effects on water repellency in Australian eucalypt forest soils and their value in estimating wildfire soil temperatures

Wildfires can induce or enhance soil water repellency under a range of vegetation communities. Accord- ing to mainly USA-based laboratory studies, repellency is eliminated at a maximum soil temperature (T )o f 280-400 ◦ C. Knowledge of T reached during a wildfire is important in evaluating post-fire soil physical prop- erties, fertility and seedbed status. T is, however, notoriously difficult to ascertain retrospectively and often based on indicative observations with a large potential error. Soils under fire-prone Australian eucalypt forests tend to be water repellent when dry or moderately moist even if long unburnt. This study aims to quantify the temperature of water repellency destruction for Australian topsoil material sampled under three sites with contrasting eucalypt cover (Eucalyptus sieberi, E. ovata and E. baxteri). Soil water repellency was present prior to heating in all sam- ples, increased during heating, but was abruptly eliminated at a specific T between 260 and 340 ◦ C. Elimination temperature varied somewhat between samples, but was found to be dependent on heating duration, with longest duration resulting in lowest elimination temperature. Results suggest that post-fire water repellency may be used as an aid in hindcasting soil temperature reached during the passage of a fire within repellency-prone environments.