A. K. Boulet

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 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.