A. J. D. Ferreira

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.

Hydrological implications of soil water-repellency in Eucalyptus globulus forests, north-central Portugal

Soil water-repellency (hydrophobicity) is a widespread property of Eucalyptus globulus and Pinus pinaster forest soils in central and north littoral Portugal and is particularly severe during the summer dry conditions. This paper attempts to assess the impact of water repellency on overland flow and runoff generation at plot and catchment scales for two types of terrain with differing land management and degree of soil hydrophobicity: (i) highly hydrophobic land with regenerating eucalyptus forest following fire; and (ii) largely hydrophilic land on which deep-ploughing prior to planting eucalyptus seedlings had eliminated hydrophobicity. Overland flow responses were monitored over a 40-month period for two 8 m × 2 m plots and streamflow was recorded continuously at gauging stations for two small catchments of predominantly regrowth eucalyptus and ploughed/ planted eucalyptus, respectively. Soil hydrophobicity was assessed using the Water Drop Penetration Time (WDPT) test. Seasonal variations in the factors influencing plot overland flow response were assessed for each land management type using multivariate analysis. For the regrowth eucalyptus plot, overland flow generation was found to be negatively correlated with antecedent soil moisture in summer (suggesting that hydrophobicity-linked Hortonian overland flow is then dominant), but positively related to throughflow in winter (suggesting that saturation overland flow generation in a hydrophilic-phase soil was at that time the dominant mechanism). In the ploughed/planted areas, negative correlations with soil moisture were found neither in summer nor winter. Rainfall amount (and in winter also antecedent precipitation) were found to be the variables most strongly and positively related to overland flow volume. The plot results are compared with streamflow responses for the small catchments.

Temporal dynamics of water repellency and soil moisture in eucalypt plantations, Portugal

This paper investigates water repellency and soil moisture under 4 different Eucalyptus globulus plantations in Portugal. On 8 occasions over a 16-month period, measurements were made at 3 depths (surface, 0.10 and 0.20 m) at 60 points on four 10 by 18 m grids. The main results are: (i) at all sites and depths, spatial frequency of repellency (defined as percentage of repellent grid points) followed a moisture-related seasonal cycle, its amplitude being greatest for the longest established site, where surface repellency was contiguous in dry late-summer conditions, but was entirely absent after wet winter conditions; (ii) at a few points at 2 sites, repellency persisted during winter; (iii) repellency severity was dichotomously distributed regardless of season (i.e. soils were generally either wettable or highly repellent); and (iv) at the longest established site, when soil moisture was 27% soils were wettable. This may either support the existence of a ‘transition zone’, or be an artefact of the different scales of repellency and soil moisture assessments. Reasons for the observed changes in repellency and their relationship with soil moisture and antecedent rainfall are explored and soil hydrological implications discussed.

Overland flow generation processes, erosion yields and solute loss following different intensity fires

Fire induces important changes in vegetation and soil structure, which can have major impacts on overland flow generation processes, runoff amounts and erosion yields. This paper presents the results of an investigation into the impact of different fire intensities on water, sediment and solute yields. Wildfire, prescribed fire and experimental fire impacts were investigated at two scales: (1) a temporary circular bounded plot of 0.24 m2 (rainfall simulation plot); (2) a permanent bounded runoff plot of 16 m2 (natural rainfall conditions). In addition, the spatial patterns of the soil water repellency, vegetation, litter cover and stone cover for each land use and fire intensity were assessed using a transect system.The results of this study show a significant difference in the spatial distribution patterns of soil water repellency. Wildfire sites were characterized by more intense and by more spatially contiguous repellency than the experimental and prescribed fire sites. These differences have important effects on overland flow generation processes and runoff amounts. At the prescribed and experimental fire sites, the discontinuous distribution of repellency provides greater potential for water to infiltrate via non-repellent soils, whereas there are far fewer infiltration pathways at the wildfire sites because of their spatially contiguous repellency. This spatial discontinuity (alternation between hydrophobic and hydrophilic soil patches) is therefore responsible for the lower overland flow amounts, smaller erosion rates and nutrient yields at broader scales, when compared with the wildfire burned areas where soil water repellence is more intense and spatially contiguously distributed.

Influence of burning intensity on water repellency and hydrological processes at forest and shrub sites in Portugal

In addition to the incineration of vegetation and litter layer, fires are also responsible for the formation of a water repellent layer with significantly different severity and spatial distribution patterns following different burning intensities. Those spatial distribution patterns have an enormous influence on soil wetting patterns, and on hydrological processes at different scales. This study attempts to understand the role of water repellence severity and spatial distribution patterns on soil, slope, and catchment water processes, and on the transmission of hydrological processes between different scales. The comparison between microplot (0.24 m2), plot (16 m2), and catchment (<1.2 km2) scales shows that water repellence spatial homogeneity enhances water fluxes transfer between the different scales. In fact, the more intense the fires, the more severe and spatially uniform the soil water repellency became. For burned areas with heterogeneous soil water repellency, overland flow produced in water repellent patches infiltrated downslope at hydrophilic sites, thereby reducing superficial water fluxes at wider scales. For the more severe and homogeneous water repellent areas following forest wildfires, overland flow was enhanced downslope, increasing fast superficial water fluxes at wider scales.

Temporal patterns of solute loss following wildfires in Central Portugal

The present paper studies the hydrological implications of forest fire and the associated export of nutrients as solutes at the micro-plot, plot and catchment scales immediately after fire. The use of three different spatial scales provides improved insights into the mechanisms that drive hydrological and solute movement processes as they change with time following forest fire. Rainfall simulations were performed for 1 h, right after forest fires. Plots were monitored during and after rainfall events, and catchments were instrumented continuously with water level recorders. Samples were collected weekly to determine some of the main nutrients present over a period of 14 months. At all scales, the main hydrological processes were assessed and samples collected for chemical analysis. Measurements were made at burned Pinus pinaster locations in the central region of Portugal. These are ordinarily placed in poor, shallow Humic Cambisol soils located in steep slopes. The results show that there was a rapid and widespread export of nutrients during the first 4 months following the wildfire. The amount of nutrients lost decreased gradually over those 4 months in response to the exhaustion of the ash source. After this period, nutrient peak losses occurred only in response to extreme rainfall events.

The role of soil water repellency in overland flow generation in pine and eucalypt forest stands in coastal Portugal

Soil water repellency is now known to occur in diverse soils in various parts of the world. One of the possible adverse effects of soil water repellency is that it can reduce infiltration capacity and hence, on sloping terrain, enhance overland flow and soil erosion. The main aim of the present work is to assess the effects of soil water repellency on surface runoff production in the inner coastal dune areas of central Portugal. This was done for a pine and a eucalypt forest stand and, within each stand, for 2 slopes with contrasting aspect and somewhat different slope angles. Overland flow was measured for 4 pairs of unbounded plots of about 5 m2 at fortnightly intervals from February to October 2001. Over the same period, soil water repellency at and immediately below the soil surface was measured next to the plots at monthly intervals. The runoff–repellency relationship was also studied by carrying out rainfall simulation experiments on 0.24-m2 plots and associated repellency measurements. The effect of soil water repellency was most clearly demonstrated by statistically significant higher runoff coefficients under strong-to-extremely than under none-to-slightly hydrophobic conditions immediately below the soil surface. Such a difference in runoff over the measurement period was, however, restricted to 2 unbounded plots, both of which were located on the eucalypt slope with a southerly aspect and the greater slope angle. At the scale of these plots, the increase in runoff coefficient due to soil water repellency is moderate, when integrated over the entire period of strong–extremely repellent conditions, but can be quite substantial for individual 2-weekly periods. With respect to the observed differences in runoff between plots, be it plots on the same slope or not, it has proved difficult to distinguish the effect of soil water repellency from that of other factors likely to affect overland flow generation.

Soil water repellency under dry and wet antecedent weather conditions for selected land-cover types in the coastal zone of central Portugal

This paper reports on the first systematic inventory of soil water repellency in Portuguese coastal dune sand areas. Since water repellency is widely associated with certain vegetation types or individual plant species, this inventory concerned arable land as well as 6 natural and semi-natural land-cover types representative for the vegetation zonation in the study area. Since water repellency further is a feature that commonly varies through time, disappearing when soils become wet, initial sampling was carried out during late summer 2000 and later repeated, at 1 of the 2 sites per land-cover type, during early spring 2001. Water repellency was principally measured in the field using the Molarity of an Ethanol Droplet (MED) test. Under the dry summer conditions, water repellency was a widespread phenomenon at and immediately below, the soil surface and numerous significant differences in ethanol classes existed between the land-cover types. The transient nature of water repellency was confirmed by many instances of significantly lower spring than summer ethanol classes. These significant differences were in general accompanied by a significant negative correlation of the summer and spring ethanol classes with volumetric soil moisture content. The sites’ overall repellency levels under dry antecedent weather conditions were significantly correlated with their overall levels of soil organic matter.

Sediment and solute yield in forest ecosystems affected by fire and rip-ploughing techniques, central portugal: A plot and catchment analysis approach

Abstract In the last two decades, Portuguese forests have undergone important changes in land use and management practices, some of which are posing significant problems as regards long-term sustainability of forestry. This paper examines the degradation processes associated with two events: deep ploughing prior to planting (termed ‘ripploughing’) and forest fire. The research design comprised plot and catchment approaches in which fluxes of water, sediment and solutes for rip-ploughed and burnt pine forest were monitored for the first year after ploughing and fire respectively. Although rip-ploughed areas experienced very high erosion rates at the plot scale (up to 51 t ha−1 yr−1) and enhanced catchment runoff due to increased overland flow (up to 50% of rainfall), catchment sediment yield was small because of sediment storage at the bases of slopes and in channel heads. For burned pine, the destruction of the vegetation and litter layer led to sharp increases in catchment runoff and nutrient losses both as dissolved and suspended sediments. Despite sediment loss at the plot scale being below 3 t ha−1 in the first year after the fire, the catchment suspended sediment loss in storm events was several orders of magnitude higher than for the partly rip-ploughed catchment.

The impact of soil water repellency on soil hydrological and erosional processes under Eucalyptus and evergreen Quercus forests in the Western Mediterranean

Forest areas of the Mediterranean regions of Portugal, Morocco, and Tunisia are suffering major land use changes, with the replacement of traditional evergreen Quercus species (i.e. Quercus suber and Quercus ilex) by fast-growing Eucalyptus species. Since Eucalyptus species are amongst those with a higher impact on soil water repellency, this study examined the effect of the replacement on soil properties, water repellency, and on soil hydrological processes and erosion rates. Measurements were performed in areas that correspond to the climatic distribution of evergreen Quercus suber: at Macao and Portel in Portugal; Ben Slimane in Morocco; and Cap Bon, Sousse, and Ain Snoussi in Tunisia. Soil superficial characteristics, including vegetation and litter cover, organic matter content, soil compaction and shear strength, and water repellency were measured for evergreen oak and Eucalyptus stands and related to soil erosion rates and soil hydrological processes. The data are based on the spatial distribution of properties assessed through the use of intensive spatial sampling and on rainfall simulations to address soil hydrological and erosional processes. The results show very different wetting patterns for some of the Eucalyptus stands during dry and moist periods, as a result of strong hydrophobic characteristics following dry spells. Nevertheless, the Eucalyptus stands in semi-arid climate show no sign of water repellency, which contradicts the theory that water repellency is purely a result of dry conditions. The experiments show no significant increases on overland flow amounts and erosion rates as direct result of soil water repellence (hydrophobicity) characteristics.