Dendrogeomorphology has been used since the 1960s to estimate sheet erosion rates. To date, most ... more Dendrogeomorphology has been used since the 1960s to estimate sheet erosion rates. To date, most efforts have fo-cused on accurately determining the first year of root exposure. However, an adequate methodological approach that takes into consideration the microtopography of the ground surface when estimating sheet erosion rates using dendrogeomorphology has not been proposed. In this study, terrestrial laser scanning (TLS) was used for the first time to examine how changes in microtopography determine the level of certainty in estimates. To this end, highly accurate TLS-based digital elevation models representing exposed roots and their immediate vicinity were analysed using geographic information system tools. The results indicate that erosion rates calculated using the standard dendrogeomorphic method have been underestimated by up to 29% because the method does not take into account changes to the microtopography caused by the axial and radial pressure of the roots. Another source of uncertainty, which we estimate to be 50%, was also found and is the result of changes in the ground surface microtopography caused by variations in soil roughness. These findings do not invalidate the usefulness of dendrogeomorphology for assessing soil erosion, although they do show the need for correct characterization of the microtopography to guarantee reliability.
Aims This paper aims to analyze the reliability of exposed roots oriented perpendicular to the sl... more Aims This paper aims to analyze the reliability of exposed roots oriented perpendicular to the slope to quantify sheet erosion rates based on accurate reconstruction of topography using terrestrial laser scanner (TLS). Methods The study was performed in an experimental sandy badland located in Central Spain. Sampling procedures were conducted in three different homogenous hydrological response units (HRU). We derived eroded soil thickness from by obtaining accurate microtopographic data using TLS. In addition, dendrogeomorphic procedures , based on anatomical changes in root rings, were used to determine the first year of exposure of 46 Pinus pinaster roots. Results Results indicate that medium-term (±30 year) erosion rates obtained from roots growing perpendicular to the slope were significantly different from those obtained from exposed roots growing parallel to the slope (p-value <0.05). However, at short term (up to 5 years), result agree with those obtained from erosion pin monitoring at the study site, which confirms the potential of reconstructions based on perpendicular roots. Conclusion The utility of exposed perpendicular roots coupled with accurate eroded soil estimation has been proved. It allows the extension of the applicability of dendrogeomorphic approaches, particularly for ungauged badlands where instrumental data is scarce or completely missing.
s u m m a r y Paleoflood hydrology typically deals with the reconstruction of floods in ungauged ... more s u m m a r y Paleoflood hydrology typically deals with the reconstruction of floods in ungauged and poorly gauged basins by combining different sources of indirect evidence. Botanical indicators have been used repeatedly in the past, mostly through the study of scars in trees or germination dates of plants on newly created surfaces. In this paper we test the hypothesis that the inclination of trees – as induced by floods – can provide information on flood magnitude, and that this source of information can therefore be used for flood reconstructions. We used a mechanical root-plate rotational stiffness model in three gauged river reaches in Central Spain to test our hypothesis and combine approaches typically applied in dendrogeomorphic, dendrometric, mechanical structure, and hydraulic research. Results show a correlation between modeled and observed deformation at the stem base of trees induced by floods (coefficient of correlation 0.58 for all observations). However they also point to a clear underestimation of peak discharge reconstructions. We used different efficiency criteria to test the reliability of results and differences between river reaches. In addition, we carried out a sensitivity analysis and discussed sources of uncertainties which may reach up to 112%, mainly due to difficulties to determine the rotational stiffness of the root plate system a posteriori. The approach presented here is promising, but more research is clearly required to improve the quality of peak discharge estimations based on stem tilting.
s u m m a r y In many regions of the world flood events in mountain basins are one of the greates... more s u m m a r y In many regions of the world flood events in mountain basins are one of the greatest risks to the local population, due to the pressure placed on land use by social and economic development. Conventional hydrologic–hydraulic methodological approaches are not usually feasible in mountainous basins because they are not gauged at all or, in the best-case scenario, are poorly gauged. In this context, palaeohydro-logical research offers a valuable alternative to the above approaches. However, many palaeohydrological data sources and associated methods have been proposed and initially used in large basins with extensive floodplains. As a result, when they are used in mountainous areas they must be adapted to include different techniques, since the problems to be addressed are different and less data is usually available. In this paper, we review classic data sources and different analytical methods and discuss their advantages and shortcomings with particular attention to mountain basins. For this purpose, examples are provided where improvements in the palaeohydrologic methods are proposed by incorporating uncertainties, describing sources of error or putting forward hypotheses for hydraulic calculation to make palaeoflood hydrology more objective and useful in risk assessment.
The use of high resolution ground-based light detection and ranging (LiDAR) datasets provides spa... more The use of high resolution ground-based light detection and ranging (LiDAR) datasets provides spatial density and vertical precision for obtaining highly accurate Digital Surface Models (DSMs). As a result, the reliability of flood damage analysis has improved significantly, owing to the increased accuracy of hydrodynamic models. In addition, considerable error reduction has been achieved in the estimation of first floor elevation, which is a critical parameter for determining structural and content damages in buildings. However, as with any discrete measurement technique, LiDAR data contain object space ambiguities, especially in urban areas where the presence of buildings and the floodplain gives rise to a highly complex landscape that is largely corrected by using ancillary information based on the addition of breaklines to a triangulated irregular network (TIN). The present study provides a methodological approach for assessing uncertainty regarding first floor elevation. This is based on: (i) generation an urban TIN from LiDAR data with a density of 0.5 points¨m´2 , complemented with the river bathymetry obtained from a field survey with a density of 0.3 points¨m´2. The TIN was subsequently improved by adding breaklines and was finally transformed to a raster with a spatial resolution of 2 m; (ii) implementation of a two-dimensional (2D) hydrodynamic model based on the 500-year flood return period. The high resolution DSM obtained in the previous step, facilitated addressing the modelling, since it represented suitable urban features influencing hydraulics (e.g., streets and buildings); and (iii) determination of first floor elevation uncertainty within the 500-year flood zone by performing Monte Carlo simulations based on geostatistics and 1997 control elevation points in order to assess error. Deviations in first floor elevation (average: 0.56 m and standard deviation: 0.33 m) show that this parameter has to be neatly characterized in order to obtain reliable assessments of flood damage assessments and implement realistic risk management.
The Pampas region has recently experienced an expansion of agriculture towards more fragile envir... more The Pampas region has recently experienced an expansion of agriculture towards more fragile environments, which has been associated with an increase in the frequency of droughts affecting the whole region. In the present study, we obtained a long-term record of soil water content (SWC) using a Soil and Water Assessment Tool (SWAT) model, for the first time in Argentina. The reliability of this model was contrasted with the temporal variation of the Standard Precipitation Index (SPI) and the Normalized Difference Vegetation Index (NDVI) to characterise episodes of drought. We also estimated the correlation between SWC anomaly (aSWC) and SPI, as well as the correlation between NDVI anomaly (aNDVI) and SPI. The model performance was reasonably satisfactory. The model calibration showed determination coefficient (R 2) and Nash–Sutcliffe coefficient (NS) values of 0.70 and 0.59, respectively, and the model validation showed R 2 and NS values of 0.77 and 0.75, respectively. The aNDVI showed a relatively low correlation with aSWC (0.26 ≤ r ≤ 0.45). In contrast, the SPI presented significantly positive correlations with aSWC (0.67 ≤ r ≤ 0.83). This work showed that SWAT is a suitable tool to measure SWC in poorly gauged geographical areas such as the Pampas region. Additionally, our approach could be applied to other systems resembling that studied here, without any significant reduction in performance.
Laser disdrometers measure the particle size distribution (PSD) of hydrometeors through a small c... more Laser disdrometers measure the particle size distribution (PSD) of hydrometeors through a small cross-sectional (tens of square centimeters) surface. Such a limited area induces a sampling effect in the estimates of the PSD, which translates to error in the reflectivity–rain-rate (Z–R) relationship used for ground radar estimates of rainfall, estimates of kinetic energy of precipitation, and any other hydrometeorological application relying on particle size information. Here, the results of a dedicated experiment to estimate the extent of the effect of limited area sampling of rainfall are presented. Using 14 Parsivel, version 1 (Parsivel-1), disdrometers placed within 6 m 2 , it was found that the combined area of at least seven disdrometers is required for the estimates to start converging to a stable value. The results can be used to quantify the degree of over-/underestimation of precipitation parameters for a single instrument due to the limited collecting area effect. It has been found that a single disdrometer may underestimate instantaneous rain rate by 70%.
Badlands and gullied areas are among those geomorphic environments with the highest erosion rates... more Badlands and gullied areas are among those geomorphic environments with the highest erosion rates worldwide. Nevertheless, records of their evolution and their relations with anthropogenic land transformation are scarcer. Here we combine historical data with aerial photographs and tree-ring records to reconstruct the evolution of a badland in a Mediterranean environment of Central Spain. Historical sources suggest an anthropogenic origin of this badland landscape, caused by intense quarrying activities during the 18 th century. Aerial photographs allowed detection of dramatic geomorphic changes and the evolution of an emerging vegetation cover since the 1960s, due to widespread reforestation. Finally, tree-ring analyses of exposed roots allowed quantification of recent channel incision of the main gully, and sheet erosion processes. Our results suggest that reforestation practices have influenced the initiation of an episode of incision in the main channel in the 1980s, through the hypothesized creation of disequilibrium in water-sediment balance following decoupling of hillslopes from channel processes. These findings imply an asymmetry in the geomorphic response of badlands to erosion such that in the early evolution stages, vegetation removal results in gullying, but that reforestation alone does not necessarily stabilize the landforms and may even promote renewed incision. Badlands and gullied areas have some of the highest erosion rates worldwide 1. Nevertheless, records of their historical and recent evolution are scarce and often limited to comparisons of presumed initial conditions and the known present state 2. Badland morphologies typically develop in horizontally stratified and relatively impermeable lithologies (e.g., marls), although they also form on poorly consolidated sands. Areas prone to badland formation cover a wide range of climatic zones, but are typically found in semi-arid environments with marked seasonal contrasts and to a lesser extent in sub-humid and humid regions 1,3,4. Gully formation is usually related to changes in the base level 5 and alteration of vegetation cover on soils developed on highly erodible geological material 6. This alteration may be induced by climatic changes and the alternating occurrence of severe droughts and extreme rainfall 6. Inappropriate agricultural practices, overgrazing, logging, quarrying or dumping of spoil heap deposits may result in alteration of the vegetation cover and thereby enhance erosion 7,8. Because of such actions, anthropogenic activities are the main geomorphic agent of current land degradation leading to gullying 9. As a consequence, many landforms and landscapes over large areas of Earth's surface, especially in the Old World, cannot be understood without knowledge of land surface transformation by human activities 9,10. Mining, plowing, or infrastructure expansion has moved huge amounts of earth and led to accelerated erosion. The scars of old mining activities can evolve to gullied landscapes 11 such that eroded sediments end up as colluvium on hillslopes, as alluvial cones on piedmonts, and as alluvium in floodplains, in ways that shape what we see now and interpret in geomorphic terms. Human activities involving earth movement can act as a main landscape
Four alluvial floodplains were selected in the European southwestern lowland area: the rivers Ebr... more Four alluvial floodplains were selected in the European southwestern lowland area: the rivers Ebro, Bidasoa, Tagus (Spain) and the Garonne (France) were chosen. They have some common characteristics (alluvial aquifers, connectivity with the river, nitrate pollution) but differ in other important aspects (water table fluctuations, flooding dynamics, landscape control, land use, agricultural practices, climate). A comparative study considering nitrogen and carbon sources and the way they interact in the riparian zone was conducted in order to test a simple approach to delimitate permanent nitrate attenuation zones and to evidence the importance of site-specific attributes. The observation was based on a detailed monthly monitoring during a year ensuring data at high and low water periods. The tested approach results in a useful tool to spatially delimitate the attenuation zones. However, this approach is difficult to apply in areas where pollution sources are very diverse in type and in both time and space. This leads us to conclude that a general conceptual model cannot capture all the factors needed to understand the nitrate removal dynamics of a riparian zone. Hence, the study combines observation-conceptualization framework with river-riparian-upland connectivity and source-pathway-target continuity.
Modification of the hydrological regime by human actions can reduce the capacity of wetlands to i... more Modification of the hydrological regime by human actions can reduce the capacity of wetlands to improve water quality. For the first time, a study was conducted of the inherent pollution attenuation capacity of wetlands associated with the highly regulated Spanish Tagus River. We determined the natural purification capacity of a floodplain where irrigated agriculture predominates and hydraulic connection with the river is virtually nonexistent. To this end, continuous hydrophysical measurements and sediment and water samples were taken during the period extending from April 2013 to March 2014. A multi-parametric dataset including hydraulic, physico-chemical, bacterial and macroinvertebrate indicators was collected from ten piezometers located within a meander, together with two additional sampling points in the river. Sampling was performed monthly to measure hydraulic and physico-chemical parameters and quarterly for bacteria and macroinvertebrates. The data enabled us to implement different but complementary methodological approaches to characterize denitrification. Specifically, we performed: (i) end-member mixing analysis (EMMA); (ii) macroinvertebrate characterization; (iii) denitrification potential analysis; (iv) bacterial assemblage structure analysis; and (v) hydrological modeling of the current and different future management scenarios. All the approaches except EMMA indicated the same conclusion: denitrification is almost nonexistent due to the fact that the study site does not present the hydric soil and oxygen-limited conditions required to enable denitrification. The EMMA analysis showed that theoretical nitrate concentrations were lower than expected in some areas during the summer months (e.g. NO 3 = −41 in August 2013), which may have been because irrigation intensity was spatio-temporal variable at the study site. Our results show that floodplain denitrification has been drastically reduced due to the suppression of flood pulses. In this context, restoration of the hydrological regime in riverine wetlands would lead to the decay of nitrates whose dynamic evolution increases with flooding, as scenarios tested by the MOHID hydro-biogeochemical model have demonstrated.
Among the natural hazards, flash flooding is the leading cause of weather-related deaths. Flood r... more Among the natural hazards, flash flooding is the leading cause of weather-related deaths. Flood risk management (FRM) in this context requires a comprehensive assessment of the social risk component. In this regard, integrated social vulnerability (ISV) can incorporate spatial distribution and contribution and the combined effect of exposure, sensitivity and resilience to total vulnerability, although these components are often disregarded. ISV is defined by the demographic and socioeconomic characteristics that condition a population's capacity to cope with, resist and recover from risk and can be expressed as the integrated social vulnerability index (ISVI). This study describes a methodological approach towards constructing the ISVI in urban areas prone to flash flooding in Castilla y León (Castile and León, northern central Spain, 94 223 km 2 , 2 478 376 inhabitants). A hierarchical segmentation analysis (HSA) was performed prior to the principal components analysis (PCA), which helped to overcome the sample size limitation inherent in PCA. ISVI was obtained from weighting vulnerability factors based on the tolerance statistic. In addition, latent class cluster analysis (LCCA) was carried out to identify spatial patterns of vulnerability within the study area. Our results show that the ISVI has high spatial variability. Moreover, the source of vulnerability in each urban area cluster can be identified from LCCA. These findings make it possible to design tailor-made strategies for FRM, thereby increasing the efficiency of plans and policies and helping to reduce the cost of mitigation measures .
Recreational activities may impose adverse impacts on the environment of natural landscapes and p... more Recreational activities may impose adverse impacts on the environment of natural landscapes and protected areas owing to persistent tourist influx. Here, we use a dendrogeomorphic approach to estimate soil erosion induced by hikers at trails in the Ordesa and Monte Perdido National Park (northeastern Spain). For the first time, exposed roots of Pinus uncinata Ramond ex DC and Fagus sylvatica L. were used on the Iberian Peninsula to reconstruct the timing and amount of soil erosion induced by hikers based on dendrogeomorphology. In addition, we propose a new characterization of ground microtopography using a microtopographic profile gauge and validate results of this approach with 3D point clouds derived from terrestrial laser scanning. Determination of the first year of root exposure was based on the analysis of changes in roots, at both the macroscopic and tissue levels. Analysis shows that a distinctive footprint is observable at macroscopic and microscopic scales following initial exposure and thus confirms results of previous work realized with roots of other tree species (e.g. Pinus sylvestris L.). Our results also indicate that a characterization of erosion based on microtopographic profiles can replace terrestrial laser scanning measurements, which are often difficult to obtain in remote areas. Estimates of soil erosion ranged between 3·1 ± 1·5 and 8·9 ± 4·3 mm y À1 (or 52·7 ± 25·5 to 151·3 ± 73·1 t ha À1 y À1). The approach deployed here could help improve management of and access to natural protected areas and thus reduce the potentially negative impacts of recreational activities on these sensitive environments. Copyright
In the dynamics and evolution of large river floodplains, alluvial fans and debris cones, avulsio... more In the dynamics and evolution of large river floodplains, alluvial fans and debris cones, avulsion phenomena have been considered very important. However, traditionally, in mountain streams, avulsion phenomena have been underestimated because they are not very frequent, and usually they are restricted to little chute cut-off. Nevertheless, there are mountainous areas where the valley bottom gets wider, and the alluvial filling works like an elongated debris cone. The Cabrera Stream in the Venero Claro reach (Spanish Central System) is one of these special areas. In this case, avulsion phenomena took place triggered by flash floods, and they were usually associated with hyperconcentrated flows, which exceeded the upper level of levees and banks, redefining the sedimentary architecture of the alluvial filling. In this study, an analysis of geomorphologic and sedymentological evolution of the alluvial filling in Venero Claro have been carried out. On this way, cartography made from the...
ABSTRACT Flash flood analysis in mountain areas presents specific challenges due to a lack of spa... more ABSTRACT Flash flood analysis in mountain areas presents specific challenges due to a lack of spatially well distributed rain and stream gauges with long enough records. In this case, dendrogeomorphology allows improvement of the understanding of the spatial and temporal occurrence of past events. Flash floods may influence trees in different ways and trees react to these disturbances with specific growth and anatomical changes. In this study, we sampled 90 cross sections and 265 increment cores of trees heavily affected by past flash floods to date past events in two ungauged basins in the Spanish Central System. The first study site is located along the Pelayo, a torrent in natural conditions. Based on the external disturbances of trees and their geomorphic position, 114 Pinus pinaster (Ait.) (265 increment corers) were sampled using an increment borer. Based on the number of trees showing disturbances, the quantification of disturbance intensity and the spatial distribution of these trees in the field, seven flash flood events were dated for the last 50 years: 2005, 2000, 1996, 1976, 1973, 1966 and 1963. The second study site is a reach of 2 km length along the Arenal River, where the stream is channelized. Here stumps from previously felled trees could be analyzed directly in the field: 100 Alnus glutinosa (L.) Gaertn. and Fraxinus angustifolia (Vahl.) cross sections were directly investigated in the field and 22 cross sections sampled and analyzed in the laboratory. Based on the injuries dated in the field and in the laboratory, and based on the location of the trees, eight events were dated for the last 30 years: 2005, 2003, 2000, 1998, 1997, 1995, 1993 and 1978. The strength of dendrogeomorphology in flood analysis has been demonstrated, especially in areas where the lack of historical documents, rainfall and flow data limits the use of traditional methods.
Dendrogeomorphology has been used since the 1960s to estimate sheet erosion rates. To date, most ... more Dendrogeomorphology has been used since the 1960s to estimate sheet erosion rates. To date, most efforts have fo-cused on accurately determining the first year of root exposure. However, an adequate methodological approach that takes into consideration the microtopography of the ground surface when estimating sheet erosion rates using dendrogeomorphology has not been proposed. In this study, terrestrial laser scanning (TLS) was used for the first time to examine how changes in microtopography determine the level of certainty in estimates. To this end, highly accurate TLS-based digital elevation models representing exposed roots and their immediate vicinity were analysed using geographic information system tools. The results indicate that erosion rates calculated using the standard dendrogeomorphic method have been underestimated by up to 29% because the method does not take into account changes to the microtopography caused by the axial and radial pressure of the roots. Another source of uncertainty, which we estimate to be 50%, was also found and is the result of changes in the ground surface microtopography caused by variations in soil roughness. These findings do not invalidate the usefulness of dendrogeomorphology for assessing soil erosion, although they do show the need for correct characterization of the microtopography to guarantee reliability.
Aims This paper aims to analyze the reliability of exposed roots oriented perpendicular to the sl... more Aims This paper aims to analyze the reliability of exposed roots oriented perpendicular to the slope to quantify sheet erosion rates based on accurate reconstruction of topography using terrestrial laser scanner (TLS). Methods The study was performed in an experimental sandy badland located in Central Spain. Sampling procedures were conducted in three different homogenous hydrological response units (HRU). We derived eroded soil thickness from by obtaining accurate microtopographic data using TLS. In addition, dendrogeomorphic procedures , based on anatomical changes in root rings, were used to determine the first year of exposure of 46 Pinus pinaster roots. Results Results indicate that medium-term (±30 year) erosion rates obtained from roots growing perpendicular to the slope were significantly different from those obtained from exposed roots growing parallel to the slope (p-value <0.05). However, at short term (up to 5 years), result agree with those obtained from erosion pin monitoring at the study site, which confirms the potential of reconstructions based on perpendicular roots. Conclusion The utility of exposed perpendicular roots coupled with accurate eroded soil estimation has been proved. It allows the extension of the applicability of dendrogeomorphic approaches, particularly for ungauged badlands where instrumental data is scarce or completely missing.
s u m m a r y Paleoflood hydrology typically deals with the reconstruction of floods in ungauged ... more s u m m a r y Paleoflood hydrology typically deals with the reconstruction of floods in ungauged and poorly gauged basins by combining different sources of indirect evidence. Botanical indicators have been used repeatedly in the past, mostly through the study of scars in trees or germination dates of plants on newly created surfaces. In this paper we test the hypothesis that the inclination of trees – as induced by floods – can provide information on flood magnitude, and that this source of information can therefore be used for flood reconstructions. We used a mechanical root-plate rotational stiffness model in three gauged river reaches in Central Spain to test our hypothesis and combine approaches typically applied in dendrogeomorphic, dendrometric, mechanical structure, and hydraulic research. Results show a correlation between modeled and observed deformation at the stem base of trees induced by floods (coefficient of correlation 0.58 for all observations). However they also point to a clear underestimation of peak discharge reconstructions. We used different efficiency criteria to test the reliability of results and differences between river reaches. In addition, we carried out a sensitivity analysis and discussed sources of uncertainties which may reach up to 112%, mainly due to difficulties to determine the rotational stiffness of the root plate system a posteriori. The approach presented here is promising, but more research is clearly required to improve the quality of peak discharge estimations based on stem tilting.
s u m m a r y In many regions of the world flood events in mountain basins are one of the greates... more s u m m a r y In many regions of the world flood events in mountain basins are one of the greatest risks to the local population, due to the pressure placed on land use by social and economic development. Conventional hydrologic–hydraulic methodological approaches are not usually feasible in mountainous basins because they are not gauged at all or, in the best-case scenario, are poorly gauged. In this context, palaeohydro-logical research offers a valuable alternative to the above approaches. However, many palaeohydrological data sources and associated methods have been proposed and initially used in large basins with extensive floodplains. As a result, when they are used in mountainous areas they must be adapted to include different techniques, since the problems to be addressed are different and less data is usually available. In this paper, we review classic data sources and different analytical methods and discuss their advantages and shortcomings with particular attention to mountain basins. For this purpose, examples are provided where improvements in the palaeohydrologic methods are proposed by incorporating uncertainties, describing sources of error or putting forward hypotheses for hydraulic calculation to make palaeoflood hydrology more objective and useful in risk assessment.
The use of high resolution ground-based light detection and ranging (LiDAR) datasets provides spa... more The use of high resolution ground-based light detection and ranging (LiDAR) datasets provides spatial density and vertical precision for obtaining highly accurate Digital Surface Models (DSMs). As a result, the reliability of flood damage analysis has improved significantly, owing to the increased accuracy of hydrodynamic models. In addition, considerable error reduction has been achieved in the estimation of first floor elevation, which is a critical parameter for determining structural and content damages in buildings. However, as with any discrete measurement technique, LiDAR data contain object space ambiguities, especially in urban areas where the presence of buildings and the floodplain gives rise to a highly complex landscape that is largely corrected by using ancillary information based on the addition of breaklines to a triangulated irregular network (TIN). The present study provides a methodological approach for assessing uncertainty regarding first floor elevation. This is based on: (i) generation an urban TIN from LiDAR data with a density of 0.5 points¨m´2 , complemented with the river bathymetry obtained from a field survey with a density of 0.3 points¨m´2. The TIN was subsequently improved by adding breaklines and was finally transformed to a raster with a spatial resolution of 2 m; (ii) implementation of a two-dimensional (2D) hydrodynamic model based on the 500-year flood return period. The high resolution DSM obtained in the previous step, facilitated addressing the modelling, since it represented suitable urban features influencing hydraulics (e.g., streets and buildings); and (iii) determination of first floor elevation uncertainty within the 500-year flood zone by performing Monte Carlo simulations based on geostatistics and 1997 control elevation points in order to assess error. Deviations in first floor elevation (average: 0.56 m and standard deviation: 0.33 m) show that this parameter has to be neatly characterized in order to obtain reliable assessments of flood damage assessments and implement realistic risk management.
The Pampas region has recently experienced an expansion of agriculture towards more fragile envir... more The Pampas region has recently experienced an expansion of agriculture towards more fragile environments, which has been associated with an increase in the frequency of droughts affecting the whole region. In the present study, we obtained a long-term record of soil water content (SWC) using a Soil and Water Assessment Tool (SWAT) model, for the first time in Argentina. The reliability of this model was contrasted with the temporal variation of the Standard Precipitation Index (SPI) and the Normalized Difference Vegetation Index (NDVI) to characterise episodes of drought. We also estimated the correlation between SWC anomaly (aSWC) and SPI, as well as the correlation between NDVI anomaly (aNDVI) and SPI. The model performance was reasonably satisfactory. The model calibration showed determination coefficient (R 2) and Nash–Sutcliffe coefficient (NS) values of 0.70 and 0.59, respectively, and the model validation showed R 2 and NS values of 0.77 and 0.75, respectively. The aNDVI showed a relatively low correlation with aSWC (0.26 ≤ r ≤ 0.45). In contrast, the SPI presented significantly positive correlations with aSWC (0.67 ≤ r ≤ 0.83). This work showed that SWAT is a suitable tool to measure SWC in poorly gauged geographical areas such as the Pampas region. Additionally, our approach could be applied to other systems resembling that studied here, without any significant reduction in performance.
Laser disdrometers measure the particle size distribution (PSD) of hydrometeors through a small c... more Laser disdrometers measure the particle size distribution (PSD) of hydrometeors through a small cross-sectional (tens of square centimeters) surface. Such a limited area induces a sampling effect in the estimates of the PSD, which translates to error in the reflectivity–rain-rate (Z–R) relationship used for ground radar estimates of rainfall, estimates of kinetic energy of precipitation, and any other hydrometeorological application relying on particle size information. Here, the results of a dedicated experiment to estimate the extent of the effect of limited area sampling of rainfall are presented. Using 14 Parsivel, version 1 (Parsivel-1), disdrometers placed within 6 m 2 , it was found that the combined area of at least seven disdrometers is required for the estimates to start converging to a stable value. The results can be used to quantify the degree of over-/underestimation of precipitation parameters for a single instrument due to the limited collecting area effect. It has been found that a single disdrometer may underestimate instantaneous rain rate by 70%.
Badlands and gullied areas are among those geomorphic environments with the highest erosion rates... more Badlands and gullied areas are among those geomorphic environments with the highest erosion rates worldwide. Nevertheless, records of their evolution and their relations with anthropogenic land transformation are scarcer. Here we combine historical data with aerial photographs and tree-ring records to reconstruct the evolution of a badland in a Mediterranean environment of Central Spain. Historical sources suggest an anthropogenic origin of this badland landscape, caused by intense quarrying activities during the 18 th century. Aerial photographs allowed detection of dramatic geomorphic changes and the evolution of an emerging vegetation cover since the 1960s, due to widespread reforestation. Finally, tree-ring analyses of exposed roots allowed quantification of recent channel incision of the main gully, and sheet erosion processes. Our results suggest that reforestation practices have influenced the initiation of an episode of incision in the main channel in the 1980s, through the hypothesized creation of disequilibrium in water-sediment balance following decoupling of hillslopes from channel processes. These findings imply an asymmetry in the geomorphic response of badlands to erosion such that in the early evolution stages, vegetation removal results in gullying, but that reforestation alone does not necessarily stabilize the landforms and may even promote renewed incision. Badlands and gullied areas have some of the highest erosion rates worldwide 1. Nevertheless, records of their historical and recent evolution are scarce and often limited to comparisons of presumed initial conditions and the known present state 2. Badland morphologies typically develop in horizontally stratified and relatively impermeable lithologies (e.g., marls), although they also form on poorly consolidated sands. Areas prone to badland formation cover a wide range of climatic zones, but are typically found in semi-arid environments with marked seasonal contrasts and to a lesser extent in sub-humid and humid regions 1,3,4. Gully formation is usually related to changes in the base level 5 and alteration of vegetation cover on soils developed on highly erodible geological material 6. This alteration may be induced by climatic changes and the alternating occurrence of severe droughts and extreme rainfall 6. Inappropriate agricultural practices, overgrazing, logging, quarrying or dumping of spoil heap deposits may result in alteration of the vegetation cover and thereby enhance erosion 7,8. Because of such actions, anthropogenic activities are the main geomorphic agent of current land degradation leading to gullying 9. As a consequence, many landforms and landscapes over large areas of Earth's surface, especially in the Old World, cannot be understood without knowledge of land surface transformation by human activities 9,10. Mining, plowing, or infrastructure expansion has moved huge amounts of earth and led to accelerated erosion. The scars of old mining activities can evolve to gullied landscapes 11 such that eroded sediments end up as colluvium on hillslopes, as alluvial cones on piedmonts, and as alluvium in floodplains, in ways that shape what we see now and interpret in geomorphic terms. Human activities involving earth movement can act as a main landscape
Four alluvial floodplains were selected in the European southwestern lowland area: the rivers Ebr... more Four alluvial floodplains were selected in the European southwestern lowland area: the rivers Ebro, Bidasoa, Tagus (Spain) and the Garonne (France) were chosen. They have some common characteristics (alluvial aquifers, connectivity with the river, nitrate pollution) but differ in other important aspects (water table fluctuations, flooding dynamics, landscape control, land use, agricultural practices, climate). A comparative study considering nitrogen and carbon sources and the way they interact in the riparian zone was conducted in order to test a simple approach to delimitate permanent nitrate attenuation zones and to evidence the importance of site-specific attributes. The observation was based on a detailed monthly monitoring during a year ensuring data at high and low water periods. The tested approach results in a useful tool to spatially delimitate the attenuation zones. However, this approach is difficult to apply in areas where pollution sources are very diverse in type and in both time and space. This leads us to conclude that a general conceptual model cannot capture all the factors needed to understand the nitrate removal dynamics of a riparian zone. Hence, the study combines observation-conceptualization framework with river-riparian-upland connectivity and source-pathway-target continuity.
Modification of the hydrological regime by human actions can reduce the capacity of wetlands to i... more Modification of the hydrological regime by human actions can reduce the capacity of wetlands to improve water quality. For the first time, a study was conducted of the inherent pollution attenuation capacity of wetlands associated with the highly regulated Spanish Tagus River. We determined the natural purification capacity of a floodplain where irrigated agriculture predominates and hydraulic connection with the river is virtually nonexistent. To this end, continuous hydrophysical measurements and sediment and water samples were taken during the period extending from April 2013 to March 2014. A multi-parametric dataset including hydraulic, physico-chemical, bacterial and macroinvertebrate indicators was collected from ten piezometers located within a meander, together with two additional sampling points in the river. Sampling was performed monthly to measure hydraulic and physico-chemical parameters and quarterly for bacteria and macroinvertebrates. The data enabled us to implement different but complementary methodological approaches to characterize denitrification. Specifically, we performed: (i) end-member mixing analysis (EMMA); (ii) macroinvertebrate characterization; (iii) denitrification potential analysis; (iv) bacterial assemblage structure analysis; and (v) hydrological modeling of the current and different future management scenarios. All the approaches except EMMA indicated the same conclusion: denitrification is almost nonexistent due to the fact that the study site does not present the hydric soil and oxygen-limited conditions required to enable denitrification. The EMMA analysis showed that theoretical nitrate concentrations were lower than expected in some areas during the summer months (e.g. NO 3 = −41 in August 2013), which may have been because irrigation intensity was spatio-temporal variable at the study site. Our results show that floodplain denitrification has been drastically reduced due to the suppression of flood pulses. In this context, restoration of the hydrological regime in riverine wetlands would lead to the decay of nitrates whose dynamic evolution increases with flooding, as scenarios tested by the MOHID hydro-biogeochemical model have demonstrated.
Among the natural hazards, flash flooding is the leading cause of weather-related deaths. Flood r... more Among the natural hazards, flash flooding is the leading cause of weather-related deaths. Flood risk management (FRM) in this context requires a comprehensive assessment of the social risk component. In this regard, integrated social vulnerability (ISV) can incorporate spatial distribution and contribution and the combined effect of exposure, sensitivity and resilience to total vulnerability, although these components are often disregarded. ISV is defined by the demographic and socioeconomic characteristics that condition a population's capacity to cope with, resist and recover from risk and can be expressed as the integrated social vulnerability index (ISVI). This study describes a methodological approach towards constructing the ISVI in urban areas prone to flash flooding in Castilla y León (Castile and León, northern central Spain, 94 223 km 2 , 2 478 376 inhabitants). A hierarchical segmentation analysis (HSA) was performed prior to the principal components analysis (PCA), which helped to overcome the sample size limitation inherent in PCA. ISVI was obtained from weighting vulnerability factors based on the tolerance statistic. In addition, latent class cluster analysis (LCCA) was carried out to identify spatial patterns of vulnerability within the study area. Our results show that the ISVI has high spatial variability. Moreover, the source of vulnerability in each urban area cluster can be identified from LCCA. These findings make it possible to design tailor-made strategies for FRM, thereby increasing the efficiency of plans and policies and helping to reduce the cost of mitigation measures .
Recreational activities may impose adverse impacts on the environment of natural landscapes and p... more Recreational activities may impose adverse impacts on the environment of natural landscapes and protected areas owing to persistent tourist influx. Here, we use a dendrogeomorphic approach to estimate soil erosion induced by hikers at trails in the Ordesa and Monte Perdido National Park (northeastern Spain). For the first time, exposed roots of Pinus uncinata Ramond ex DC and Fagus sylvatica L. were used on the Iberian Peninsula to reconstruct the timing and amount of soil erosion induced by hikers based on dendrogeomorphology. In addition, we propose a new characterization of ground microtopography using a microtopographic profile gauge and validate results of this approach with 3D point clouds derived from terrestrial laser scanning. Determination of the first year of root exposure was based on the analysis of changes in roots, at both the macroscopic and tissue levels. Analysis shows that a distinctive footprint is observable at macroscopic and microscopic scales following initial exposure and thus confirms results of previous work realized with roots of other tree species (e.g. Pinus sylvestris L.). Our results also indicate that a characterization of erosion based on microtopographic profiles can replace terrestrial laser scanning measurements, which are often difficult to obtain in remote areas. Estimates of soil erosion ranged between 3·1 ± 1·5 and 8·9 ± 4·3 mm y À1 (or 52·7 ± 25·5 to 151·3 ± 73·1 t ha À1 y À1). The approach deployed here could help improve management of and access to natural protected areas and thus reduce the potentially negative impacts of recreational activities on these sensitive environments. Copyright
In the dynamics and evolution of large river floodplains, alluvial fans and debris cones, avulsio... more In the dynamics and evolution of large river floodplains, alluvial fans and debris cones, avulsion phenomena have been considered very important. However, traditionally, in mountain streams, avulsion phenomena have been underestimated because they are not very frequent, and usually they are restricted to little chute cut-off. Nevertheless, there are mountainous areas where the valley bottom gets wider, and the alluvial filling works like an elongated debris cone. The Cabrera Stream in the Venero Claro reach (Spanish Central System) is one of these special areas. In this case, avulsion phenomena took place triggered by flash floods, and they were usually associated with hyperconcentrated flows, which exceeded the upper level of levees and banks, redefining the sedimentary architecture of the alluvial filling. In this study, an analysis of geomorphologic and sedymentological evolution of the alluvial filling in Venero Claro have been carried out. On this way, cartography made from the...
ABSTRACT Flash flood analysis in mountain areas presents specific challenges due to a lack of spa... more ABSTRACT Flash flood analysis in mountain areas presents specific challenges due to a lack of spatially well distributed rain and stream gauges with long enough records. In this case, dendrogeomorphology allows improvement of the understanding of the spatial and temporal occurrence of past events. Flash floods may influence trees in different ways and trees react to these disturbances with specific growth and anatomical changes. In this study, we sampled 90 cross sections and 265 increment cores of trees heavily affected by past flash floods to date past events in two ungauged basins in the Spanish Central System. The first study site is located along the Pelayo, a torrent in natural conditions. Based on the external disturbances of trees and their geomorphic position, 114 Pinus pinaster (Ait.) (265 increment corers) were sampled using an increment borer. Based on the number of trees showing disturbances, the quantification of disturbance intensity and the spatial distribution of these trees in the field, seven flash flood events were dated for the last 50 years: 2005, 2000, 1996, 1976, 1973, 1966 and 1963. The second study site is a reach of 2 km length along the Arenal River, where the stream is channelized. Here stumps from previously felled trees could be analyzed directly in the field: 100 Alnus glutinosa (L.) Gaertn. and Fraxinus angustifolia (Vahl.) cross sections were directly investigated in the field and 22 cross sections sampled and analyzed in the laboratory. Based on the injuries dated in the field and in the laboratory, and based on the location of the trees, eight events were dated for the last 30 years: 2005, 2003, 2000, 1998, 1997, 1995, 1993 and 1978. The strength of dendrogeomorphology in flood analysis has been demonstrated, especially in areas where the lack of historical documents, rainfall and flow data limits the use of traditional methods.
s u m m a r y In urban areas prone to flash floods, characterization of social resilience is crit... more s u m m a r y In urban areas prone to flash floods, characterization of social resilience is critical to guarantee the success of emergency management plans. In this study, we present the methodological approach that led to the submission and subsequent approval of the Civil Protection Plan of Navaluenga (Central Spain), in which the first phase was to analyse flood hazard by combining the Hydrological Modelling System (HEC-HMS) and the Iber 2D hydrodynamic model. We then analysed social vulnerability and designed measures to put into practice within the framework of the Civil Protection Plan. At a later phase, we assessed citizens' flash-flood risk perception and level of awareness regarding some key variables of the Civil Protection Plan. To this end, 254 adults representing roughly 12% of the population census were interviewed. Responses were analysed descriptively, comparing awareness regarding preparedness and response actions with the corresponding information and behaviours previously defined in the Civil Protection Plan. In addition, we carried out a latent class cluster analysis aimed at identifying the different groups present among the interviewees. Our results showed that risk perception is low. Specifically, 60.8% of the interviewees showed low risk perception and low awareness (cluster 1); 24.4% had high risk perception and low awareness (cluster 2), while the remaining 14.8% presented high long-term risk perception and high awareness (cluster 3). These findings suggest the need for integrating these key variables of social risk perception and local tailored information in emergency management plans, especially in urban areas prone to flash-floods where response times are limited.
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Papers by Jose Bodoque