DERIVING FLASH FLOODS IN THE CASE OF SIMULATED PRECIPITATIONS

Deriving flash floods in the case of simulated precipitations. The present study sets to determine the historical flash-floods on the Petriș watershed making use of the implemented mathematical models in a Hydrological Modeling System. In order to simulate the 1%, 2% and 10% exceedance probability flash-floods, the Intensity-Frequency-Duration (IDF) curves have been built on the basis of 22 years of precipitation records (1988-2009). They serve as input data together with the hydrological parameters identified by the means of a Geographic Information Systems. The results, computed mainly on the basis of the Soil Conservation Service (SCS) method, consist of hyetographs and hydrographs at the outlet of the main watershed, as well as to that of the ungauged tributaries of the Petriș River

A spatially explicit database of wind disturbances in European forests over the period 2000-2018

Strong winds may uproot and break trees and represent a major natural disturbance for European forests. Wind disturbances have intensified over the last decades globally and are expected to further rise in view of the effects of climate change. Despite the importance of such natural disturbances, there are currently no spatially explicit databases of wind-related impact at a pan-European scale. Here, we present a new database of wind disturbances in European forests (FORWIND). FORWIND is comprised of more than 80 000 spatially delineated areas in Europe that were disturbed by wind in the period 2000-2018 and describes them in a harmonized and consistent geographical vector format. The database includes all major windstorms that occurred over the observational period (e.g. Gudrun, Kyrill, Klaus, Xynthia and Vaia) and represents approximately 30% of the reported damaging wind events in Europe. Correlation analyses between the areas in FORWIND and land cover changes retrieved from the Landsat-based Global Forest Change dataset and the MODIS Global Disturbance Index corroborate the robustness of FORWIND. Spearman rank coefficients range between 0.27 and 0.48 (p value < 0.05). When recorded forest areas are rescaled based on their damage degree, correlation increases to 0.54. Wind-damaged growing stock volumes reported in national inventories (FORESTORM dataset) are generally higher than analogous metrics provided by FORWIND in combination with satellite-based biomass and country-scale statistics of growing stock volume. The potential of FORWIND is explored for a range of challenging topics and scientific fields, including scaling relations of wind damage, forest vulnerability modelling, remote sensing monitoring of forest disturbance, representation of uprooting and breakage of trees in large-scale land surface models, and hydrogeological risks following wind damage. Overall, FORWIND represents an essential and open-access spatial source that can be used to improve the understanding, detection and prediction of wind disturbances and the consequent impacts on forest ecosystems and the land-atmosphere system. Data sharing is encouraged in order to continuously update and improve FORWIND

A stochastic vision of the paleoclimate. Modelling and predictibility

The objective of the paper is related to the use of stochastic methods to appreciate if the recent climate trend is similar to the decennial trends of the last thousands years.If not, it means that the present climate change is unprecedented.If yes, it means that the extraordinary climatic events of the recent years are a natural expression of climate variability, but the memory of several generations and of instrumental series, more than 100 years old, cannot point it out. The first hypothesis is an “alarming” possibility, while the latter is a “relaxing” one.</p

Évolution du ruissellement et du volume d'eau ruisselé en surface urbaine. Étude de cas : Bordeaux 1984-2014, France

Cette étude a visé l'identification des tendances de modification des cheminements naturels des eaux et du volume d'eau ruisselé en surface urbaine. L'identification des réseaux d'écoulement en surface couverte, respectivement non couverte, sur la base des Modèle Numérique de Terrain - MNT et des Modèle Numérique de Surface - MNS, a mis en évidence la manière dont le cheminement naturel de l'eau a été modifié par les obstacles existants à la surface du terrain. Les Surfaces Totales Imperméables - STI pour les années 1984, 1991, 2002 et 2014 ont été extraites sur la base des images Landsat. La tendance du ruissellement de l'eau pour la période 1984-2014, pour un sous-bassin versant de Bordeaux, a été démontrée à travers la modélisation hydrologique HEC-HSM sur la base des pourcentages des Surfaces Effectives Imperméables - SEI calculés. Les résultats ont mis en évidence la modification des cheminements naturels dans l'aire étudiée et une tendance à l'augmentation du volume d'eau ruisselé. Ces tendances sont le résultat de l'urbanisation et de l'industrialisation et requièrent la mise en place de mesures pour leur atténuation à l'avenir

Spatial effect of anti-COVID measures on land surface temperature (LST) in urban areas: A case study of a medium-sized city

International audience⎯ This case study investigates the magnitude and nature of the spatial effect generated by the anti-COVID measures on land surface temperature (LST) in the city of Târgu Mureș (Marosvásárhely), Romania. The measures were taken by the Romanian government during the state of emergency (March 16-May 14, 2020) due to the SARS-CoV-2 coronavirus pandemic. The study shows that-contrary to previous studies carried out on cities in China and India-in most of the urban areas of Marosvásárhely LST has increased in the period of health emergency in 2020 concerning the large average of the years 2000-2019. Remote sensing data from the MODIS and the Landsat satellites show, that MODIS data, having a moderate spatial (approximately 1 km) but good temporal resolution (daily measurements), show a temperature increase of +0.78 °C, while Landsat data, having better spatial (30 m) but lower temporal resolution, show an even greater increase, +2.36 °C in the built-up areas. The difference in temperature increase is mainly due to the spatial resolution difference between the two TIR band sensors. The LST anomaly analysis performed with MODIS data also shows a positive anomaly increase of 1 °C. However, despite this increase, with the help of the hotspot-coldspot analysis of the Getis-Ord Gi* statistic we were able to identify 46 significant coldspots that showed a 1-2 °C decrease of LST in April 2020 compared to the average of the previous years in April. Most of these coldspots correspond to factory areas, public transport epicenters, shopping centers, industrial polygons, and non-residential areas. This shows that anti-COVID measures in the medium-sized city of Marosvásárhely had many effects on LST in particular areas that have links to the local economy, trade, and transport. Paired sample t-test for areas identified with LST decrease shows that there is a statistically significant difference in the average LST observed before and after anti-COVID measures were applied. MODIS-based LST is satisfactory for recognizing patterns and trends at large or moderate geographical scales. However, for a hotspot-coldspot analysis of the urban heat islands, it is more suitable to use Landsat data

Ponds delineation in various urban landforms. Case study for Cluj-Napoca, Romania

International audienceUrbanization and industrialization have led to the increase in impervious surfaces, the reduction of water seepage into soil and the increase of runoff discharges in urban areas. The overland flow may be carried over long distances along the streets and sidewalks depending on the land slope and morphology, while in lowland, water tends to accumulate in ponds. In this research, we aim to identify these ponds in the floodplain, slope and urbanized interfluves in Cluj-Napoca. The ponds outline the locations that are most prone to urban floods. The preferential flowpathways were identified based on the flow direction and flow accumulation determined using the D8 method, while the ponds were automatically extracted using an algorithm developed based on the Contour Polygon Screening method. Based on four digital elevation models (DEM) of different resolutions (DEM 5 m, DEM 20 m, ASTER DEM 30 m and STRTM 90 m) and one Digital Surface Model (DSM) incorporating urban buildings, we highlighted the effect of the spatial resolution and buildings on the water flow delineation in urban areas. Our results revealed an increase in the number of flow pathways and ponds with the increase in DEM resolution and a more realistic outline of flow pathways and ponds delineated based on the DSM. In the urbanized floodplain, the ponds have distinctive features, while on the urbanized slope and interfluves these are temporary. The identification of ponds in a varied and urbanized relief provides useful information to support local authorities in the overland flow management and flash flooding in urban areas

Synoptic Processes Generating Windthrow. A Case Study for Apuseni Mountains (Romania)

International audienceWindstorms are among the main factors causing damages to forest ecosystems. These meteorological phenomena cannot be predicted, prevented or controlled. They are occurring rapidly and take place in a meteorological context characterized by high velocities of the air currents. This paper analyses the characteristics of the severe meteorological events on 20 July 2011 which have led to windthrows on extended areas within the Apuseni Mountains in the Romanian Carpathians. The study highlights the evolution of the synoptic processes both at regional scale and at mesoscale. The meteorological analysis is carried out based on synoptic maps and by means of the data coming from the doppler WSR-98D radar in Bobohalma. The main results indicate the occurrence of the severe meteorological phenomena against the background of alternating atmospheric circulation types. The main cause is the transition of the zonal circulation into maritime tropical circulation. This contributed to the destabilizing of the warm and moist air masses that have generated strong storms at national leve

Gis module for the estimation of the hillslope torrential peak flow

International audienceIn rural regions, characterized by a large number of small basins, some of them torrential and others without permanent flow, high intensity convective rainfall generate overland flow, not always on the same path. Overland hillslope flow, followed by a dangerous peak discharge, can happen almost anywhere at any time. The purpose of this paper is to present a GIS module for computation of possible peak flow in any point of a hillslope. The workflow is based on the rational method, but it is applied rather differently by avoiding the previous delineation of watersheds and working on a raster cell level. The module that we present is more than just an automation of known GIS procedures because the result of this model also includes the path of the maximum flow and the peak flows, which would otherwise remain unknown. There is a possibility of another path for the peak flow under different rainfall and antecedent moisture conditions. In this case, the module needs to be rerun for the new conditions when a new torrential rainfall event occurs. The module provides an easy-to-understand graphical representation of peak flow paths, maximum flows and associated maps that can be useful for further decisions and prevention measures

Detection of old scattered windthrow using low cost resources. The case of Storm Xynthia in the Vosges Mountains, 28 February 2010

Unlike the contiguous windthrows, the scattered windthrows occurring as a result of wind gusts of lower speed (100-140 km/h) than in the first case (>140 km/h) are much more difficult to detect due to their much smaller areas and due to their very large number (several hundred in the wooded Vosges Mountains). The objective of this research is to present a rapid procedure for the detection of the scattered windthrows based on low cost, Landsat type images, knowing that certain sensors cannot be accessed without significant investments. Our application is based on the study of effects caused by the Storm Xynthia in the Vosges Mountains in the North-East of France, on 28 February 2010. Thus, based on two sets of Landsat satellite images, we used the “dark object” approach and the Disturbance Index, as well as the image classification before and after the storm, resulting in a map of changes. Following the detection process, 257 scattered polygons were detected, totalling 229 ha. For validation purposes, high-resolution images and orthophotoplans taken before and after storm were used. The error matrix was calculated, achieving an overall accuracy of 86%, which confirmed the quality of our analysis and supported this procedure for detecting scattered windthrow based on low cost resources