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Wildfires can have important impacts on hydrological processes and soil erosion in forest catchments, due to the destruction of vegetation cover and changes to soil properties. However, the processes involved are non-linear and not fully understood. This has severely limited the understanding on the impacts of wildfires, especially in the up-scaling from hillslopes to catchments; in consequence, current models are poorly adapted for burnt forest conditions. The objective of this presentation is to give an overview of the hydrological response and sediment yield from the micro-plot to slope scale, in the first year following a wildfire (2008/2009) that burnt an entire catchment nearby the Colmeal village, central Portugal. The overview will focus on three slopes inside the catchment, with samples including: • Runoff at micro-plot scale (12 bounded plots) and slope scale (12 open plots); • Sediments and Organic Matter loss at micro-plot scale (12 bounded plots) and slope scale (12 open plots plus 3 Sediment fences); • Rainfall and Soil moisture data; • Soil Water Repellency and Ground Cover data. The analysis of the first year following the wildfire clearly shows the complexity of runoff generation and the associated sediment transport in recently burnt areas, with pronounced differences between hillslopes and across spatial scales as well as with marked variations through time. This work was performed in the framework of the EROSFIRE-II project (PTDC/AGR-CFL/70968/2006) which has as overall aim to predict soil erosion risk in recently burnt forest areas, including common post-fire forest management practices; the project focuses on the simultaneous measurement of runoff and soil erosion at multiple spatial scales.The results to be presented in this session are expected to show how sediment is generated, transported and exported in the Colmeal watershed; and contribute to understand and simulate erosion processes in burnt catchments, including for model development and evaluation.

The EROSFIRE-II project (PTDC/AGR-CFL/70968/2006) has as overall aim to predict soil erosion risk in recently burnt forest areas, including following common post-fire forest management practices. Although the project’s main focus is on onsite erosion processes, also the export of sediments out of small catchments is addressed. To this end, a study area of about 60 ha located near to the village of Colmeal (municipality of Góis, central Portugal) was instrumented extensively following a wildfire during August 2008, and has been monitored intensively afterwards.(...)

Post-fire erosion is a major concern because of its potential effects on soil and water resources. Wildfire is frequently observed to increases soil erosion rates (Shakesby and Doerr, 2006). Many studies have emphasized the role of the partial or entire elimination of vegetation and ground cover (thereby exposing the mineral soil to direct impact of raindrops and overland flow) as a key factor in the increased soil losses. Also fire-induced changes in topsoil properties like total porosity (e.g. Neary et al., 2005) and water repellency (e.g. Keizer et al., 2008a) can enhance overland flow and the associated transport of soil particles. Existing process-based erosion models like e.g. WEPP (Nearing et al., 1989) and EUROSEM (Morgan et al., 1998) can successfully simulate the effect of vegetation on soil losses in individual storms but their complexity often limits their usefulness for non-academic purposes. Therefore, empirical models such as USLE (Wischmeier and Smith, 1978) continue to be widely used. For example, the Portuguese national Water Institute (INAG) applied USLE to the 300.000 ha of rural areas affected by wildfires during the summer of 2003. However, the model’s suitability for burnt areas (as opposed to the agricultural areas for which USLE was originally developed) was not supported by field observations. The present work concerns the EROSFIRE-II project (PTDC/AGR-CFL/70968/2006), which has as overall aim to assess the suitability of selected models for predicting soil erosion in recently burnt forest areas in Portugal. The proposed presentation will focus on the evaluation of the revised Morgan-Morgan-Finney model (MMF; Morgan, 2001) against slope-scale measurements of overland flow and/or erosion during the first year after fire. Earlier studies reported this model to perform satisfactorily in forests and shrublands in the humid areas of the north-eastern Iberian Peninsula (Keizer et al., 2008b; Fernández et al., 2010). The hydrological and erosion response at the catchment scale will be modeled with MEFIDIS (Nunes et al., 2005) but that is beyond the present scope.

Plant communities display a vertical structure based on the size and growth pattern of the dominant species. To a large extent, this pattern, called vertical stratification, depends on the climatic zone. Vertical structure analysis consists in detecting the number of layers and their limits within a forest stand. So far, there is a lack of robust approaches applied to airborne laser scanning (ALS) data that properly segment the different strata of forests having complex structures. In this study, we propose a procedure to characterize vertical forest stratification based on the mean shift (MS) algorithm. The MS is a non-linear filter that searches for local density maxima (modes). It is a non-parametric and unsupervised approach, which only requires a single criterion, the kernel bandwidth. Since the forest point cloud is a multi-modal distribution, the MS is used to find the modes which are supposed to be the barycenters of vegetation features. Once achieved, the modes are grouped together according to height range and the corresponding ALS points are assigned to each vegetation strata. Due to their complex pattern, using a single scale over the whole space is not recommended for the analysis of such environments. On this basis, the modes are computed using a variable kernel bandwidth according to the forest pattern. To depict such a pattern, we propose a new technique that segments the main forest layers at the plot level: overstory, understory, and surface vegetation. The procedure has been carried out on 45 plots of a Portuguese forest mainly composed of eucalyptus (Eucalyptus globulus) and pine (Pinus pinaster) trees that can be strongly populated by understory and surface vegetation.

Airborne Laser Scanning is an established technique to produce Digital Terrain Models (DTM). In spite of its widespread use, the extraction of a DTM in urban areas still poses problems because the filtering process often fails to separate some of the objects on the terrain surface from the terrain itself. This happens because only one kind of information – geometric information – is used in the process. To overcome this problem a methodology is proposed that integrates aerial laser scanning data with high-resolution orto-rectified CIR images to extract reliable information of the terrain and of the objects on it (like buildings and trees). The methodology was tested in an urban area located in north Portugal. The obtained results show that in complex urban areas the proposed methodology has a superior performance than the conventional one.

Wildfires are a common phenomenon in Portugal, affecting on average 100.000 ha of rural areas per year and up to 400.000 ha in dramatic years like 2003 and 2005. Wildfires can strongly enhance the hydrological response and associated sediment losses in recently burnt forest catchments and, thereby, negatively affect land-use sustain- ability of the affected terrains as well as ecosystem functioning of downstream aquatic habitats. Therefore, the EROSFIRE-I and –II projects aim at developing a GIS-tool for predicting soil erosion hazard following wildfire and, ultimately, for assessing the implications of alternative post-fire land management practices. Assessment of runoff and soil erosion rates critically depends on accurate estimates of the corresponding runoff areas. In the case of catchments as well as unbounded erosion plots (arguably, the only practical solution for slope-scale measurements), delineation of runoff area requires a Digital Terrain Model (DTM) with an adequate resolution and accuracy. The DTM that was available for the Colmeal study area, localized in the mountain range of Lousã, in the central part of Portugal, of EROSFIRE-II project is that of the 1:25.000 topographic map produced by the Military Geographic Institute. Since the Colmeal area involves a rather small experimental catchment of roughly 10 ha and relatively short study slopes of less than 100 m long, two different data acquisition techniques were used to produce high-resolution and high-accuracy DTM. One of the data acquisition techniques is aerial photogrammetry whilst the other is terrestrial laser scanning. In order to produce a DTM by photogrammetric means, a dedicated digital aerial photography mission was carried out. The images have a pixel size of 10 cm. Manual measurements permitted to measure breaklines and were complemented by automatic measurements. In this way, a DTM in a TIN format was produced. This was further converted to grid format using the ArcGIS software system. Signalized control points allowed obtaining the DTM in the same global reference system as that employed for terrestrial laser scanning. The terrestrial laser scanning was done using a Riegl LMS Z360I, stationed in 8 points within the area to provide a complete coverage. The resulting dense cloud of points was filtered – by the company carrying out the scanning mission - to remove the non-terrain points (in particular vegetation). Several grids of different sizes were produced (0.10 x 0.10, 0.20 x 0.20, 0.50 x 0.50, 1 x 1 and 2 x 2 m2). This work will study the effect on runoff and erosion rates at the slope- and catchment-scale of DTM with differ- ent resolution, but produced with data collected with the same acquisition technique, and of DTM with the same resolution, but produced with data collected with the two different acquisition techniques. The study is being carried out in ArcGIS using DTM in a grid format. Preliminary results suggest that the conver- sion of TIN-to-grid in ArcGIS produces results that depend on the procedure being applied. Therefore, the different algorithms available at ArcGIS for TIN-to-grid conversion are currently being tested, using an artificially produced DTM. This testing includes various interpolation techniques for grid generation, and will be extended to different algorithms for computation of drainage flow direction.

Wildfires can have important impacts on hydrological and soil erosion processes, due to the destruction of vegetation cover and changes to soil properties. According to Shakesby and Doerr (2006), these wildfire effects are: i) much better known at small spatial scales (especially erosion plots) than at the scale of catchments; ii) much better studied with respect to overland flow and streamflow (and, then, especially peak discharges) than to soil erosion. Following up on a precursor project studying runoff generation and the associated soil losses from micro-plot to slope-scale in Portuguese eucalypt forests, the EROSFIRE-II project addresses the connectivity of these processes across hillslopes as well as within the channel network. This is done in the Colmeal study area in central Portugal, where the outlet of an entirely burnt catchment of roughly 10 ha was instrumented with a gauging station continuously recording water level and tubidity, and five slopes were each equipped with 4 runoff plots of < 0,5 m2 (“micro-plot”) and 4 slope-scale plots as well as 1 slope-scale sediment fence. Starting one month after the August 2008 wildfire, the plots were monitored at 1- to 2-weekly intervals, depending on the occurrence of rainfall. The gauging station became operational at the end of November 2008, since the in-situ construction of an H-flume required several weeks. A preliminary analysis of the data collected till the end of 2008, focusing on two slopes with contrasting slope lengths as well as the gauging station: revealed clear differences in runoff and erosion between: (i) the micro-plot and slope-scale plots on the same hillslope; (ii) the two slopes; (iii) an initial dry period and a subsequent much wetter period; (iv) the slopes and the catchment-scale, also depending on the sampling period. These results suggest that the different processes govern the hydrological and erosion response at different spatial scales as well as for different periods, with soil water repellency playing a role during the initial post-fire period. The current presentation will review these preliminary results based on the data collected during the first year after the wildfire.

A Digital Terrain Model (DTM) is fundamental for extracting several forest canopy structure metrics from data acquired with small-footprint airborne laser scanning (ALS). This modern remote sensing technology is based on laser measurements from a laser system mounted on an aircraft and integrated with a geodetic GNSS receiver and an inertial measurement unit (IMU) or inertia navigation system (INS). In the context of a research project for deriving forest inventory parameters and fuel variables under eucalypt stands in Mediterranean climates, the vertical precision of the DTM obtained by automatic filtering of full-waveform ALS data had to be evaluated. The DTM accuracy estimation on a study area with peculiar characteristics, which are often avoided in related studies, will also allow verifying the performance of full- waveform ALS systems. The accuracy estimation is carried out in a novel way. By novel way, it is meant an exhaustive, well-planned collection of reliable control data in forest environment. The collection of the control data involves the production of DTM on 43 circular plots (radius = 11.28m) using total stations and geodetic GNSS receivers. These DTM, with a total of 3356 points, allowed one to evaluate consistently and reliably the vertical accuracy of the terrain surface produced with ALS under a eucalypt forest. This global accuracy, expressed by the Root Mean Square Error (RMSE) of the vertical differences between the field surveyed surface and the ALS derived DTM surface is 0.15m (mean=0.08m and std=0.09m). This impressive value indicates that, for an ALS point cloud density of 10pts/m2 and footprint of 20 cm, the methodology used to extract the DTM from full- waveform ALS data under an unstructured eucalypt forest is very accurate. In this article it is addressed both the strategy adopted to collect the control data and the quality assessment of the DTM produced by means of the ALS data.

In this study the strengths and weaknesses of eight filtering algorithms are evaluated by using the mean, standard deviation and RMSE metrics. Seven of these algorithms are implemented in the freeware software ALDPAT (Airborne LiDAR Data Processing and Analysis Tools) and the eighth, known as the Axelsson filter, in the commercial software Terrascan. The referred metrics are calculated by using DTM of topographic surfaces with quite different morphologies and vegetation covers. Forty-three of these surfaces, on circular plots of 400 m2 each, are covered by brushwood and unmanaged eucalypt forest with different stand characteristics. The mean tree density is around 1600 trees per hectare. The reference DTM for assessing the DTM produced by filtering full-waveform LiDAR data using the eight filtering algorithms are created with the help of a total station and geodetic GNSS receivers. The results show that the Axelsson and the so-called Polynomial Two Surface Fitting filters give the best results in terms of RMSE. Nonetheless, the results also show that all the tested filters are suitable for the filtering of full-waveform LiDAR data used in forestry related work, and collected over areas with great amount and high brushwood, chaotic eucalypt tree distribution and high tree density. The results obtained for a forest area with such characteristics – among which it should be mentioned a RMSE of 15 cm - are quite surprising.