References of "Geophysical Research Abstracts"
     in
Bookmark and Share    
Full Text
See detailExtension of the long-term total column time series of atmospheric methane above the Jungfraujoch station: analysis of grating infrared spectra between 1976 and 1989
Bader, Whitney ULiege; Lejeune, Bernard ULiege; Demoulin, Philippe ULiege et al

in Geophysical Research Abstracts (2011), 13

Methane (CH4) is one of the most abundant greenhouse gases in the Earth’s atmosphere, with current mean volume mixing ratio close to 1800 ppb. Since methane has a global warming potential of 25 (100-yr ... [more ▼]

Methane (CH4) is one of the most abundant greenhouse gases in the Earth’s atmosphere, with current mean volume mixing ratio close to 1800 ppb. Since methane has a global warming potential of 25 (100-yr horizon) and an atmospheric lifetime of 12 years, the Kyoto Protocol has included it among the species to be regulated to limit global warming. Anthropogenic sources of methane are mainly energy production (coal and leaks) and agriculture while main natural sources are swamps and biomass waste. The main sink of methane is oxidation in the troposphere, primarily by reaction with the hydroxyl radical. Methane trends have exhibited significant changes during the last twenty-five years. For instance, long-term monitoring of its vertical total column above the high-altitude station of the Jungfraujoch (46.5°N, 8°E, 3580 m asl) has indeed allowed to derive column changes ranging from +0.72% in 1987-1988 to +0.14% in 1999-2000 (Zander et al., 2002), relative to 1988 and 2000, respectively. More recently and for the same site, Duchatelet et al. (2010) have even reported a significant slowdown of -0.02%/yr between years 2000 and 2005. This study also showed that since then, CH4 is on the rise again, at a rate close to +0.30%/yr. While the numbers reported here above have been derived from the Fourier Transform Infrared (FTIR) data set starting in 1984, earlier pioneering observations have been collected at the Jungfraujoch since 1950, using grating spectrometers. During the 1958-1975 period, the main objectives has been the study of the solar photosphere in the visible and the near infrared and the publication of high-resolution solar atlases. From 1976 to 1989, narrow-band IR solar absorption observations achieving a spectral resolution of about 0.02 cm-1 have been recorded with the high-performance double-pass grating spectrometer. Analysis of these historical spectra provides a unique opportunity to extend the Jungfraujoch’s total column time series of important atmospheric gases, including methane, by nearly 10 years. The aim of this contribution is to present the inversion strategy adopted to derive CH4 from the grating spectra, using the SFIT-2 algorithm (v3.91) We will evaluate the impact of resolution, spectroscopic parameters (from the EU HYMN project -see www.knmi.nl/samen/hymn-, and from HITRAN 2004), atmospheric pressure and temperature profiles on the error budget. The 1976-1989 total column time series produced will be presented and critically discussed. In particular, we will identify and correct for possible biases between double-pass grating spectrometer measurements and more recent FTIR total columns. The harmonized and consolidated time series will be investigated to characterize the long-term trend of methane for the 1976-2010 time period. Comparisons with synthetic data produced by the CHASER 3-dimensional chemical transport model will also be presented and analyzed. [less ▲]

Detailed reference viewed: 310 (48 ULiège)
Full Text
Peer Reviewed
See detailDune rehabilitation using a mechanical fixation technique : effect on sediment fluxes and on the quantitative and qualitative recovery of the herbaceous groundcover.
Tidjani, Adamou Didier; Bielders, Charles; Ambouta, Karimou et al

in Geophysical Research Abstracts (2011)

This research shows the potentialities of mechanical windbreaks to rapidly stabilize dunes in the north sahelian area (Niger) thanks to the recovery of herbaceous groundcover.

Detailed reference viewed: 67 (4 ULiège)
Full Text
Peer Reviewed
See detailSoil redistribution in rural catchment: how fifty years old soil survey can help model improvement
Legrain, Xavier ULiege; Colard, François ULiege; Colinet, Gilles ULiege et al

in Geophysical Research Abstracts (2011), 13

In a context of high urbanization’s pressure in rural zones, landscape modelling of erosion opens interesting perspectives in land use planning. In most cases, validation data are the weak point. In this ... [more ▼]

In a context of high urbanization’s pressure in rural zones, landscape modelling of erosion opens interesting perspectives in land use planning. In most cases, validation data are the weak point. In this study, we present how fifty years old soil observations can help progressing towards a more accurate validation of such modelling in rural areas. As of 1947, a comprehensive systematic survey of the Belgian soil cover was initiated. Field observations were done every 75 meters by soil auger to a standard depth of 125cm (if possible). Map units were delineated on cadastral field survey maps at scale 1:5,000, based on auger observations and landscape context, then generalised on the 1:10,000 topographic base map for a publication at 1:20,000 scale. The legend of the map includes more than 6,000 different soil types and variants. More recently, the Walloon part of this map was digitalised to produce the Digital Soil Map of Wallonia (DSMW). A 10m resolution DEM was build up in 2009. Its RMSE is 0.8m. Soil erodibility and runoff production maps were derived at the same resolution. A land use map exists at 1:10,000 scale since 2005 and is updated yearly. We applied the USPED model (Unit Stream Power - based Erosion Deposition) (Moore and Burch, 1986) in a small watershed where first soil observations took place in 1956. New soil observations were done in 2010. The watershed is completely included in a cultivated area. The model was applied considering a transport capacity limitation proposed by Mitasova and Mitas (1996). Furthermore, we slightly modified it, in order to take into account recent advances in RUSLE factors computations like LS computation proposed by Desmet and Govers (1996) and Nearing (1997). The spatial distribution of erosion and deposition area produced by the model on the basis of the current DEM is consistent with a comparison between old and recent pedological observations. Furthermore, a comparison between horizons’ thickness in 1956 and 2010 gives spatially distributed quantitative information on erosion and deposition. Nevertheless, some uncertainties remain since the pedological descriptions are based on thickness classes, due to the variability of soil cartographical units, and since the current DEM is itself affected by an uncertainty on the elevation value. Future research will then focus on more accurate elevation data as starting point and then it will become conceivable to model the evolution of watershed elevation including land use and other local anthropogenic structures like hedgerows, ditches or grass strips. [less ▲]

Detailed reference viewed: 85 (31 ULiège)
Full Text
See detailTrend evolution and seasonal variation of tropospheric and stratospheric carbonyl sulfide (OCS) above Jungfraujoch
Lejeune, Bernard ULiege; Mahieu, Emmanuel ULiege; Duchatelet, Pierre ULiege et al

in Geophysical Research Abstracts (2011), 13

Carbonyl sulfide (OCS) is the most abundant sulfur-containing trace gas in the atmosphere and accounts for a substantial portion of the sulfur in the stratospheric aerosol layer which influences the ... [more ▼]

Carbonyl sulfide (OCS) is the most abundant sulfur-containing trace gas in the atmosphere and accounts for a substantial portion of the sulfur in the stratospheric aerosol layer which influences the Earth’s radiation budget and stratospheric ozone chemistry. The major identified OCS sources are oceans and anthropogenic emissions, while atmospheric loss and uptake by vegetation and soils constitute the main OCS sinks. The uptake by vegetation strongly influences the distribution and seasonality of OCS throughout most of the Northern Hemisphere, just like for CO2. Montzka et al. (2007) pointed that atmospheric OCS measurements have the potential to constrain the biomass Gross Primary Production (GPP). Unfortunately, there remain large uncertainties on some components strengths of the atmospheric OCS budget. A recent work by Suntharalingam et al. (2008) showed that uptake by plants has been strongly underestimated in actual balanced budgets, suggesting that additional significant OCS sources have still to be identified. In order to improve our understanding of the different processes governing seasonal and inter-annual OCS variability, a new approach has been developed and optimized, using the SFIT-2 algorithm, to retrieve atmospheric abundance of OCS from high-resolution ground-based infrared solar spectra. Our observations are recorded on a regular basis with Fourier Transform Infrared spectrometers (FTIRs), under clear-sky conditions, at the NDACC site (Network for the Detection of Atmospheric Composition Change, http://www.ndacc.org) of the International Scientific Station of the Jungfraujoch (Swiss Alps, 46.5°N, 8.0°E, 3580m asl). Information content analysis of the retrieved OCS products shows us that we are able to distinguish between tropospheric and stratospheric partial column contributions for this species. Thanks to our unique observational database, we have produced an updated OCS long-term trend from 1995 to 2010, representative for both the troposphere and stratosphere at northern mid-latitudes. In this contribution, we will present and critically discuss the recent OCS trend evolution, in particular the end of the slow decline of its abundance observed in 2002 and the maximum reached in 2008. In addition to the OCS inter-annual variations, we will analyze the OCS seasonal cycle during the 15 last years. We will also compare our results with simulations of seasonal OCS variations issued from a 3D global atmospheric chemical transport model (CTM), in order to try to quantify the individual contribution of the various processes playing a role in the Jungfraujoch OCS variability and influencing its atmospheric abundance. [less ▲]

Detailed reference viewed: 133 (19 ULiège)
Full Text
See detailFirst retrievals of carbon tetrafluoride (CF4) from ground-based FTIR measurements: production and analysis of the two-decadal time series above the Jungfraujoch
Duchatelet, Pierre ULiege; Zander, Rodolphe ULiege; Mahieu, Emmanuel ULiege et al

in Geophysical Research Abstracts (2011)

Carbon tetrafluoride (CF4 or PFC-14) is a potent greenhouse gas that is almost 7400 times more effective (100-yr horizon) than CO2 on a per molecule basis (IPCC, 2007). This high global warming potential ... [more ▼]

Carbon tetrafluoride (CF4 or PFC-14) is a potent greenhouse gas that is almost 7400 times more effective (100-yr horizon) than CO2 on a per molecule basis (IPCC, 2007). This high global warming potential, coming from its medium absorbance combined with a very long atmospheric lifetime (>50000 years; Ravishankara et al., 1993), makes CF4 a key species among the various greenhouse gases targeted by the Kyoto Protocol. In the Northern hemisphere, current atmospheric CF4 concentrations are close to 78 pptv, with a large fraction (around 35 pptv, Mühle et al., 2010) coming from natural processes like lithospheric emissions (Harnisch and Eisenhauer, 1998). In addition, CF4 has been used increasingly since the eighties in electronic and semiconductors industry. The primary aluminum production processes have also been clearly identified as an important anthropogenic source of CF4 emissions. The partitioning between these two main sources is however problematic, principally due to lacking or incomplete CF4 emission factors from inventories performed in industrial fields (e.g. International Aluminum Institute, 2009). Recent in situ ground level measurements of CF4 in the Northern hemisphere (Khalil et al., 2003; Mühle et al., 2010) or remotely from space (Rinsland et al., 2006) have indicated a significant slowdown in the increase rate of atmospheric CF4. This probably results from measures adopted by the aluminum industry aiming at the reduction of the frequency and duration of “anode effects” and therefore of related PFCs emissions (International Aluminum Institute, 2009). The present contribution reports on the long-term evolution (1990-2010) of the atmospheric carbon tetrafluoride total vertical abundance derived from ground-based Fourier transform infrared (FTIR) solar spectroscopy observations around 1285 cm-1 at the Jungfraujoch (46.5°N, 8.0°E, 3580m asl) and compares our findings with results available in the literature. To our knowledge, no equivalent time series (i.e. based on ground-based FTIR technique) has been published to date. [less ▲]

Detailed reference viewed: 102 (14 ULiège)
Full Text
See detailFirst retrievals of methyl chloride from ground-based high-resolution FTIR solar observations
Mahieu, Emmanuel ULiege; Harrison, Jeremy; Bernath, Peter F. et al

in Geophysical Research Abstracts (2011), 13

Methyl chloride (CH3Cl) is one of the most abundant chlorine-bearing gas in the Earth’s troposphere and a significant contributor to the organic chlorine budget. Measurements by in situ networks indicate ... [more ▼]

Methyl chloride (CH3Cl) is one of the most abundant chlorine-bearing gas in the Earth’s troposphere and a significant contributor to the organic chlorine budget. Measurements by in situ networks indicate a mean volume mixing ratio of 550 pptv, with a significant seasonal cycle of about 80 pptv, peak to peak. This species also exhibits inter-annual variability, but no long-term trend. Major sources are from tropical and sub-tropical plants and dead leaves, the oceans and biomass burning. Some industrial processes and waste incineration further add to the emissions. Oxidation by the hydroxyl radical is by far the largest CH3Cl sink, followed by soil uptake. Although balanced, its atmospheric budget is still affected by large uncertainties and contributions from unidentified sources and sinks cannot be ruled out. Methyl chloride has an atmospheric lifetime of 1 year, a global warming potential of 13 (100-yr horizon) and an ozone depleting potential of 0.02. The retrieval of methyl chloride from ground-based infrared (IR) spectra is very challenging. Indeed, numerous interferences by strong water vapor and methane lines complicate the detection of small CH3Cl absorptions, close to 1%, near 3 microns. In addition, and although weak, ethane features contribute to the difficulty, in particular since a significant number of ethane branches were absent until very recently from official HITRAN compilations. Therefore, the scientific literature does not report thus far about any investigations of CH3Cl from ground-based remote sensing observations. In this contribution, we will present first CH3Cl total column retrievals, using the SFIT-2 algorithm (v3.94) and high-resolution Fourier Transform Infrared (FTIR) solar absorption observations recorded with a Bruker 120HR instrument, at the high altitude station of the Jungfraujoch (46.5°N, 8°E, 3580 m asl), within the framework of the Network for the Detection of Atmospheric Composition Change (NDACC, visit http://www.ndacc.org). In our retrievals, we use new ethane absorption cross sections recorded at the Molecular Spectroscopy Facility of the Rutherford Appleton Laboratory (Harrison et al., 2010). They were calibrated in intensity by using reference low-resolution spectra from the Pacific Northwest National Laboratory (PNNL) IR database. These new cross sections were recently released as a HITRAN update (see http://www.hitran.com). Pseudoline parameters fitted to these ethane spectra have been combined with HITRAN 2004 line parameters (including all the 2006 updates) for all other species encompassed in the selected microwindows, including our target CH3Cl. We will evaluate the improvement brought by the new ethane line parameters on the fitting residuals, and characterize the quality, the precision and the reliability of the retrieved product. If successful, a long-term CH3Cl total column time series will be produced using the Jungfraujoch observational database, and we will perform preliminary investigations of the seasonal and inter-annual variations of methyl chloride total columns at northern mid-latitudes. [less ▲]

Detailed reference viewed: 191 (15 ULiège)
Full Text
See detailThe evolution of the inorganic fluorine budget since the mid-1980s based on FTIR measurements at northern mid-latitudes
Duchatelet, Pierre ULiege; Feng, Wuhu; Chipperfield, Martyn et al

in Geophysical Research Abstracts (2011)

Fluorine enters the stratosphere principally in the form of CFCs (chlorofluorocarbons, mainly CFC-12 and CFC-11), HCFCs (hydrochlorofluorocarbons, mainly HCFC-22) and HFCs (hydrofluorocarbons, mainly HFC ... [more ▼]

Fluorine enters the stratosphere principally in the form of CFCs (chlorofluorocarbons, mainly CFC-12 and CFC-11), HCFCs (hydrochlorofluorocarbons, mainly HCFC-22) and HFCs (hydrofluorocarbons, mainly HFC-134a) which have been (or are still) widely emitted at ground level by human activities. In the lower stratosphere, the photolysis of these halogenated source gases leads to the formation of the two fluorine reservoirs COClF and COF2. The subsequent photolysis of these two compounds frees F atoms, which principally react with CH4 and H2O to form the extremely stable HF gas, by far the dominant fluorine reservoir in the middle and upper stratosphere. Despite the fact that fluorine does not significantly contribute in stratospheric ozone depletion, measurements of the concentrations of individual F-containing species in different altitude ranges of the atmosphere are important as they reflect the amounts of anthropogenic gases transported into the middle atmosphere as well as their decomposition. Such measurements also provide insight into the partitioning between major fluorine source gases (which are potent greenhouse gases) and reservoirs and allows a global inventory of organic (CFy), inorganic (Fy) and total (FTOT) fluorine burdens to be monitored as a function of time. Indeed, regular updates of such inventories are important as the partitioning between F-containing gases in the stratosphere is continually evolving as emissions of anthropogenic gases from the surface change, principally as a consequence of the progressive ban on the production of CFCs and HCFCs adopted by the Montreal Protocol and its subsequent Amendments and Adjustments. To complement recent studies regarding fluorine species (Duchatelet et al., 2009, 2010, 2011; Mahieu et al., 2011), this communication presents the time series of the inorganic fluorine budget Fy over the last twenty-five years, based on HF and COF2 total column amounts derived from high resolution Fourier transform infrared (FTIR) solar spectra recorded at Jungfraujoch (46.5°N, 8.0°E, 3580m asl). A trend analysis of our HF, COF2 and Fy time series is performed and discussed in the context of past and current emissions of halogenated source gases. Comparisons with model and space data are also included. [less ▲]

Detailed reference viewed: 42 (9 ULiège)
Full Text
See detailTrend and lifetime of sulfur hexafluoride at mid-latitudes deduced from ACE-FTS occultation measurements
Mahieu, Emmanuel ULiege; Duchatelet, Pierre ULiege; Zander, Rodolphe ULiege et al

in Geophysical Research Abstracts (2011), 13

Sulfur hexafluoride (SF6) is one of the strongest greenhouse gases on a per molecule basis, with a global warming potential of 22800 (100-yr horizon). This is an extremely stable gas in the atmosphere ... [more ▼]

Sulfur hexafluoride (SF6) is one of the strongest greenhouse gases on a per molecule basis, with a global warming potential of 22800 (100-yr horizon). This is an extremely stable gas in the atmosphere, which results in a very long lifetime, with large uncertainties. The value adopted by IPCC is 3200 years, but some studies suggest shorter lifetimes, as low as 800 years. Surface concentrations are now about 7 ppt, with reported trends indicating a steady and strong increase of 0.3 ppt/yr. Most emissions are of anthropogenic origin, related to its use as an insulator in high-voltage electrical installations. Secondary contributions result from magnesium and aluminum production as well as from the manufacturing of semiconductors (see e.g. Levin et al., 2010; Rigby et al., 2010 and references therein). In this contribution, we use occultation measurements performed by the ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) instrument, launched in August 2003 onboard the Canadian SCISAT satellite (Bernath et al., 2005). ACE-FTS is still in operation to date, with no significant degradation in its performance. This spectrometer achieves a spectral resolution of 0.02 cm-1 in the broad 750-4400 cm-1 range which covers the unresolved nu-3 band Q branch of SF6 centered at 947.9 cm-1. Signal-to-noise ratios of 200-300 are typically obtained in the spectral region of interest. Version 3 retrievals performed by University of Waterloo give volume mixing ratio profiles of SF6 in the 11-32 km altitude range. We consider all available sunrise and sunset occultation measurements obtained at midlatitudes in both hemispheres to derive the trend of SF6 in the lower stratosphere, from late February 2004 onwards. Consistency between both hemispheres will be investigated. In addition, concurrent N2O measurements are used to evaluate the atmospheric lifetime of SF6, following a method used previously for other long-lived gases (e.g. Zander et al, 1996). Comparisons with trends derived from in situ surface measurements or from ground-based remote-sensing observations (e.g. at the Jungfraujoch station, 46.5ºN) are also included. [less ▲]

Detailed reference viewed: 132 (5 ULiège)
Full Text
Peer Reviewed
See detailImpact of thresholding techniques on X-ray soil microtomogram analyses
Beckers, Eléonore ULiege; Léonard, Angélique ULiege; Beekkerk van Ruth, Jöran ULiege et al

in Geophysical Research Abstracts (2011), 13

Using X-ray microtomography, 3D soil structure can be visualised and analysed through useful factors like pore size distribution, shape, connectivity, orientation, etc. X-ray scans yield grey-level 2D ... [more ▼]

Using X-ray microtomography, 3D soil structure can be visualised and analysed through useful factors like pore size distribution, shape, connectivity, orientation, etc. X-ray scans yield grey-level 2D images, which can be recombined to form 3D structures. Treatments of the grey-level images can consist of either a binarization -distinguishing soil and pores- or a permeability level attribution directly linked to the grey-level values. The latter leads to problems in order to assign a permeability for each point (in soil a same grey level value can be assigned to points with a different permeability), as well as to analyse the 3D structure. On the other hand, treatment of black and white 3D structures is well-handled. However, the impact of the choice of one thresholding technique on the resulting images has already been demonstrated. Moreover, thresholding methods are various and numerous. Many of them are based on the image histogram analysis. But because of the soil complexity, the relevance of these techniques becomes debatable, with a risk of producing non reliable images. We propose to palliate this with a simple new algorithm based on physical measurements: it uses a loop fixing the threshold value in order to match the measured porosity for each sample. In this communication, our point is to highlight the impact of different thresholding techniques on the analysis and interpretation of our soil microtomograms. The underlying questions could be: Does the thresholding method influence our conclusions? Are the results depending on the sample itself or on the methodology? In order to answer these questions we tested the Otsu technique and our physically based algorithm. Soils samples were removed from the upper layer (Ap horizon) of a silty soil (Gentinnes, Brabant Walloon, Belgium) in plots with different management practices. In fact, since 2004, the field has been cultivated in conventional tillage (CT) or reduced tillage (RT). In order to empty the meso- and macroporosity, samples were placed under a 1.5 MPa pressure (Richards apparatus). Samples were then scanned by X-ray microtomography using a Skyscan-1172 high-resolution desktop micro-CT system (Skyscan, Kontich, Belgium). The cone beam source operated at 100 kV, using an aluminium filter. The detector configuration, i.e. 1048x2000 pixels with a 16-bit X-ray camera, and the distance source-object-camera were adjusted to produce images with a pixel size of 17 µm. Porosity was measured for each scanned sample. Then the threshold methods -the Otsu technique on one hand and our developed algorithm on the other - were applied, and morphological factors were calculated for both methods. A comparison of the first results shows a threshold influence on average porosity and number of pores, but also on connectivity factors and size distribution. The apparent porosity of the images, as well as connectivity, is underestimated with the Otsu technique. Despite the fact that the changes induced by thresholding are more important for RT than CT, global conclusions about the comparison of these agricultural practices are approximately the same in this case. However, differences between tillage systems are less important with the Otsu method, confirming the impact of choosing the adapted threshold method. [less ▲]

Detailed reference viewed: 102 (36 ULiège)
Full Text
Peer Reviewed
See detailStatistical analysis of low-flow based on short time series. The case of Wallonia
Verstraete, Arnaud ULiege; Gailliez, Sébastien; Degre, Aurore ULiege

in Geophysical Research Abstracts (2011), 13

The floods have always been the main concern resulting of extreme weather conditions. Now droughts and low flows are more and more recognized as risk situations due to the huge consequences of water ... [more ▼]

The floods have always been the main concern resulting of extreme weather conditions. Now droughts and low flows are more and more recognized as risk situations due to the huge consequences of water shortage. Furthermore, the changing climate context constitutes a new threat even if the uncertainty in low-flows evolution remains high. In Wallonia (Southern part of Belgium), a knowledge gap remains on statistical analysis of low-flows. In this paper, we present a study of historical data in Walloon Region and the first steps of their statistical analysis. Wallonia is one of the three regions of Belgian federal state. It covers an area of 17000 km² and spreads on 4 districts (Meuse, Escaut, Rhin, Senne). The watercourses are divided in navigable watercourses (700km), not navigable watercourses that are listed in 3 categories (14300km) and streams that are not navigable and not listed (4000km). Hydrological monitoring has a short history in Wallonia. The first monitoring site was installed in 1960. It consisted in a limnimetric scale and daily manual readings. Since 1974, hourly data are recorded. The number of measurement sites reached 244 stations in 2011. About 60 % of the monitoring sites have less than 20 years of hourly data. A qualitative analysis of monitoring stations led us to disregard 184 stations. The main quality problems were important discharge rating curve extrapolation, algae development in summer or low flow inferior to 5l/s. In preparation for a statistical analysis of low-flows, the old manual daily readings were analyzed. Unusable in high flow situations, they still hold practical and usable information during drought. This analysis allowed us to extend the registration period up of 7 stations and to recover 16 stations. There were lots of missing data during the 1960-1994 period, due to a poor management of the monitoring network. A yearly hydrograph analysis leads us to keep years of partial measurement when the gaps were found to be out of the low-flow period. The monitoring sites presenting more than 20 years of readings were selected for the statistical analysis. A homogeneity test was performed. Finally 64 out of 244 monitoring sites are kept for the frequency analysis. The indicators used to characterize low-flows are the popular Q95 and MAM7. Five below bounded distributions are tested with the HYFRAN software: Weibull (2 parameters), log-normal 2 parameters and 3 parameters, Gamma and Pearson type III. The parameters of the laws are estimated by the maximum likelihood estimation. The selection of the three best laws is performed for each site thanks to three Bayesian criterions proposed by HYFRAN. Then the distribution that fits the best the data is visually chosen. The results of the adjustment method are the same for the two indicators. The Gamma distribution is the most used followed by the lognormal with 2 parameters. However in some cases a law of three parameters is more appropriate. This preliminary work gives a first analysis of low-flows statistics in Wallonia. Yet a lot of missing data or short recording duration still limits our knowledge, this analysis allows us to progress towards best management practices in rivers and watersheds. [less ▲]

Detailed reference viewed: 84 (27 ULiège)
Full Text
Peer Reviewed
See detailErosion and rainfall erosivity under climate change: rainfall simulation and soil losses measurement at field scale
Kummert, Nora ULiege; Beckers, Eléonore ULiege; Degre, Aurore ULiege

in Geophysical Research Abstracts (2011), 13

Soil and water conservation is a big issue of this century. The soil is a non renewable resource. As we know, the change in climate brings more short erosive rainfall with a high capacity to take away the ... [more ▼]

Soil and water conservation is a big issue of this century. The soil is a non renewable resource. As we know, the change in climate brings more short erosive rainfall with a high capacity to take away the topsoil. Moreover, topsoil contains all the nutrients the plants needs. It is now essential that we found a new balance between productivity and durability. Impacts of new agricultural techniques on soil structure are already studied in different countries (i.e. Beckers et al., 2010; Walh et al., 2004; Malone et al., 2003). But what are the impacts on erosion? That is a question with few answers. And this is where our experiment comes, in order to link erosion and future erosion with management practices. Two ways exist to act against soil losses: enhancing soil structure or increasing vegetation cover. Our study aims at measuring soil losses and runoff under different practices and for a future scenario of climate change. This study explores new practices and measures their effects on erosion and runoff under a future rainfall. We focus on two cultures: sugar beet and maize. Each is tested under three different systems. For sugar beet soil structure impact is monitored: three tillage systems are tested: winter ploughing, fall ploughing and fall topsoiling. For maize vegetation cover impact is monitored: three seeding systems are studied: classical seeding (75 cm interrow), classical seeding with Ray-grass seeding in the interrows, and distributed seeding (obtained with a grains seeder). Rainfall simulation has been chosen for the study so the impacts of climate change can also be tested. A future rainfall was calculated based on a climate change scenario for Belgium (CCI-HYDR project, Willems, 2006-2010). A basic current rainfall of 100 years return period and 30 minutes duration (correspondent intensity: 70 mm/h) entered into the model gives the new rainfall. After the application of the scenario, the new rainfall has an intensity of 80 mm/h. This is our future rainfall used in this experiment. The simulations of this rainfall were carried on during the main crop season (between June and August). Three simulations were performed on sugar beet and two on maize on plots with the dimensions: 3 m length and 90 cm and 120 cm width respectively for sugar beet and maize (corresponding to two rows of the main culture). During each simulation soil losses and runoff quantities were measured. From the first year experiment, some tendencies can be observed. The topsoiling on sugar beet culture seems to produce less soil losses when the winter ploughing gives the lower quantities of runoff. The distributed seeding for the maize culture gives the lower rates for both soil losses and runoff quantities. Our experiment will be repeated at least for the next two years with new future rainfall to be tested. The climatic conditions are an important factor which can modify the behavior of soil response under rainfall event. More research has to be done in order to improve our knowledge of runoff and erosion phenomenon at smaller scale. [less ▲]

Detailed reference viewed: 144 (44 ULiège)
Full Text
Peer Reviewed
See detailA methodology to assess the exactness of Stream Network modeling process on agricultural watersheds
Ouedraogo, Mohamar ULiege; Degre, Aurore ULiege; Beckers, Eléonore ULiege et al

in Geophysical Research Abstracts (2011)

The goal of our study is to evaluate the exactness of stream network modeling process on agricultural watersheds. Agricultural watersheds topography is always changing, making it difficult to be modeled ... [more ▼]

The goal of our study is to evaluate the exactness of stream network modeling process on agricultural watersheds. Agricultural watersheds topography is always changing, making it difficult to be modeled. According to the standard ISO-7078 (ISO-7078, 1985) of the Inernational Organization of Standardization, the exactness of a measurement process or a modeled process can be defined as the difference between results obtained from the measurement process and a reference accepted as the «true value». A small watershed of a dozen hectares size has been surveyed by terrestrial LiDAR (Light Detecting And Ranging) scanner and photogrammetrical techniques to produce a row data of 30 cm resolution. Three interpolation techniques i.e. natural neighbourg, multiquadratic radial basis function and inverse distance weighted have been applied on the original data to create original digital elevation models (DEM) of 1 m resolution. RTK (Real Time Kinematics) GPS (Global Positionning System) ground control points have been surveyed on the watershed to evaluate DEM errors and fit a variogram that is used by a conditional sequencial gaussian simulation model to generate error maps. However, ground control point’s elevations accuracy is depending on the microtopography of parcels in an agricultural watershed. Depending on the crop that is planned by the farmer, the soil tillage will be different, and soil structure and roughness can considerably influence ground control point’s elevation. Analysis of variance and geostatistical methods have been applied on total station and RTK GPS data to estimate intervals in which, ground control points elevations vary. These intervals have been estimated for two parcels that soils are tilled in different ways. These errors are added to the generated errors maps to create final error maps. The final errors maps are added to the original DEM to create likely DEM realizations for the watershed (Temme and al., 2007). Then, two spurious sinks filtering methods (Colson 2006; Lindsay and Creed, 2005) and one flat area treatment method (Jenson and Domingue, 1988) are applied on each DEM realization for preprocessing. Finally, the three common flow direction extraction methods (D8, D-infity and Multiple Flow Direction) are applied on each preprocessed DEM to extract stream Network. The extracted stream network is overlapped with RTK GPS field positioned stream network i.e. a polyline format data. To estimate the exactness of the stream extraction methods, the polyline format is converted in raster format. That allows to compute for each pixel of the observed stream network, the distance to the extracted stream network. Then, for each pixel the mean distance can be calculated, and can be represented through the stream network. LiDAR technology is becoming useful for environment modeling because of his accuracy. Such quantity of data is not free of errors. This research will allow us to estimate the uncertainty of stream network modeled from agricultural watersheds by considering the main sources of errors that are propagated through computing processes. [less ▲]

Detailed reference viewed: 94 (14 ULiège)
Full Text
Peer Reviewed
See detailAnnual-scale adaptation of a soil heterotrophic respiration model to three agricultural sites in Belgium and South-Western France.
Buysse, Pauline ULiege; Le Dantec, Valérie; Mordelet, Patrick et al

in Geophysical Research Abstracts (2010, May 06), 12

Detailed reference viewed: 25 (8 ULiège)
Full Text
Peer Reviewed
See detailShort-term temperature impacts on soil CO2 fluxes: An incubation experiment with agricultural soil samples.
Buysse, Pauline ULiege; Goffin, Stéphanie ULiege; Carnol, Monique ULiege et al

in Geophysical Research Abstracts (2010, May 06), 12

Detailed reference viewed: 39 (21 ULiège)
Full Text
See detailThe earthquake sedimentary record of the Marmara Sea, Turkey
Drab, Laureen; Hubert, Aurelia ULiege; Schmidt, Sabine et al

in Geophysical Research Abstracts (2010, May)

Detailed reference viewed: 23 (3 ULiège)
Full Text
Peer Reviewed
See detailComparison of agricultural soils' structure depending on tillage system using X-ray microtomography
Beckers, Eléonore ULiege; Ly, Sarann ULiege; Léonard, Angélique ULiege et al

in Geophysical Research Abstracts (2010, May), 12

This study aims at characterizing agricultural soils’ structure depending on the tillage system: conventional tillage or conservational tillage. Tillage reduction is an increasing practice, but the micro ... [more ▼]

This study aims at characterizing agricultural soils’ structure depending on the tillage system: conventional tillage or conservational tillage. Tillage reduction is an increasing practice, but the micro-structural effects on soils and on their hydrodynamic parameters are still not well described. Recent research shows non-converging results. Our point is to highlight fundamental differences in structure through characterization of soils porosity’s parameters using X-ray microtomography measurements coupled to image analysis. This attempt is in line with a more integrated experiment of which the aim is to quantify the effects of tillage intensity on lateral flow production, and finally on global water balance. Parameters’ measurements consist in a combined approach, based on two different space-time scales of exploration: fundamental scale, with soil sampling campaign for microtomography analysis, and field scale, with continuous flow measurements (plots’ dimensions: 18*28 m). For their part, parameters for water balance determination (precipitation, evapotranspiration…) are monitored on the field. All of these measurements have the main objective of hydrological modeling enhancement by taking into account a better lateral flow description. Discussion in this paper will focus on the first results obtained by X-ray microtomography measurements. Our experiment takes place in Gentinnes (Walloon Brabant, Belgium), on a field organized in a split-plot scheme. Since 2004, plots have been cultivated in conventional tillage or in reduced tillage. The latter consists in sowing after stubble ploughing of about 10cm. The crop rotation is sugar beet followed by winter wheat. The soil is mainly composed of silt loam. Soils samples, with a 3 cm diameter and a 5 cm height, were removed from the upper layer (Ap horizon) for both management practices. Samples are scanned by X-ray microtomography using a Skyscan-1172 high-resolution desk-top micro-CT system (Skyscan, Kontich, Belgium). The cone beam source operated at 100 kV, using an aluminium filter. The detector configuration, i.e. 1048×2000 pixels 16-bit X-ray camera, and the distance source-object-camera were adjusted to produce images with a pixel size of 17 µm. This resolution allows us to visualize both meso- and macro- porosity. In this study, half the samples were placed under a 15000 kPa pressure (corresponding pressure for the wilting point) in Richards’ apparatus in order to empty the meso- and macro-porosity. To determine a priori the class of porosity for the samples, relations between water retention and pressure head can be plotted using this apparatus. Scanning results consist in 2D images. The 2D images are recombined to form 3D structure. Then the pore network can be analyzed through useful factors like size distribution, shape, connectivity, orientation, tortuosity etc. [less ▲]

Detailed reference viewed: 137 (65 ULiège)
Full Text
See detailMicrometeorological survey of air-sea ice CO2 fluxes in arctic coastal waters
Heinesch, Bernard ULiege; Tison, Jean-Louis; Carnat, Gauthier et al

in Geophysical Research Abstracts (2010), 12(EGU2010-10570),

Detailed reference viewed: 29 (11 ULiège)
Full Text
Peer Reviewed
See detailElectrical resistivity tomography and self-potential case studies for fractured aquifer characterization and monitoring
Robert, Tanguy ULiege; Dassargues, Alain ULiege; Nguyen, Frédéric ULiege

in Geophysical Research Abstracts (2010), 12

Electrical resistivity tomography (ERT) and self-potential (SP) investigations have been conducted in complex carboniferous limestones aquifers in Belgium (synclinorium structures). The aims of this study ... [more ▼]

Electrical resistivity tomography (ERT) and self-potential (SP) investigations have been conducted in complex carboniferous limestones aquifers in Belgium (synclinorium structures). The aims of this study were (1) to detect and characterize fractured zones in hard rock aquifers, (2) to monitor groundwater flow/water content in these fractured or karstic areas and (3) to use geophysical data to support groundwater flow model set-up and calibration. The investigated areas lie in calcareous synclines. Electrical images allowed us to detect and characterize (in terms of direction, width and depth) several less resistive anomalies, which are interpreted in terms of fractured and/or karstic zones. To interpret the ERT images, data errors as well as image appraisal indicators (resolution matrix, sensitivity matrix and DOI index) were analysed and compared. This allowed us to determine the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Inversions based on focusing scheme are tested against smoothness-constraint inversion on these field data to provide more realistic images on the basis of prior geological knowledge. Self-potential measurements were performed along the electrical profiles and allowed us to find negative anomalies possibly related with groundwater preferential flow pathways. By taking the assumption that only the electrokinetic effect plays a role in the SP signals, we were able to estimate a first distribution of the water table along our profiles. The SP data showed that in this particular tectonic structure, two perpendicular hydraulic gradients are present. The first gradient is related with the main fold axis direction and is the major drainage system. The second hydraulic gradient is related with the flanks of the calcareous valley. Geophysical data concurrently with ‘ground truth’ geological and hydrogeological data allowed us to better understand the groundwater flow in these calcareous synclines and to verify the conceptual groundwater flow model. [less ▲]

Detailed reference viewed: 216 (47 ULiège)
Full Text
Peer Reviewed
See detailModelling climate control on cropland and grassland development using phenologically tuned variables
Horion, Stéphanie; Tychon, Bernard ULiege; Cornet, Yves ULiege

in Geophysical Research Abstracts (2010), 12

Many studies already investigated the impact of climate change and climate variability on vegetation at global and continental scales. Using time series of remote sensing and climate data, Nemani et al ... [more ▼]

Many studies already investigated the impact of climate change and climate variability on vegetation at global and continental scales. Using time series of remote sensing and climate data, Nemani et al. (2003) analyzed trends in Net Primary Production in relation with changes in climate and showed that, between 1982 and 1999, primary productivity increased by 6% globally in response to climate change. This study also stressed the need to take into account the spatial variability of climatic constraints to plant growth when analyzing the climate change impact on vegetation. Others authors described different phenomenon linked with climate change such as increases of seasonal NDVI amplitude and growing season duration in the Northern high latitude or changes in circumpolar photosynthetic activities. Understanding the interactions between climate and vegetation is also a key issue in our PhD research. Our objective is to identify the meteorological factors which limit the development of croplands and grasslands in relation with their geographical localization. For that purpose, we acquired 10-daily time series of the Normalized Difference Vegetation Index, NDVI, derived from SPOT-VEGETATION and 7 meteorological parameters (Tmean, Tmin, Tmax, Rain, Rad, ETP, Rain-ETP) derived from ERA40 re-analyses and the operational ECMWF (European Centre for Medium-Range Weather Forecast) atmospheric model. Cross-correlations between NDVI and each one of the meteorological parameters were analysed for a set of 25 regions over Europe and Africa: 15 agricultural regions and 10 regions covered by grassland or savannas. Unlike others studies, we did not consider the vegetation globally but we focussed on two types of vegetation: croplands and grasslands. This is quite important considering the role of phenology on the vegetation cycle and its relation with climate. Moreover the analysis was not realised using yearly estimates but using 10-daily products. In order to avoid stationarity related issue, a specific methodology was developed taking into account the phenological cycle of the vegetation under consideration. Preliminary results showed that the relation between a meteorological limiting factor, e.g. precipitation, and NDVI can not be considered as linear during the year or even during the growing season. Interactions must to be studied at a smaller time scale than the growing season in order to identify properly the limiting factors to plant growth taking into account its phenology. Moreover the main limiting factors are variable from a region to another. In our analysis we also considered the possibility of a delayed response of the vegetation or a cumulated effect of meteorological events (up to 3 months). Our methodology will be presented during the conference and results will be discussed and illustrated by some test cases. [less ▲]

Detailed reference viewed: 122 (12 ULiège)
Full Text
Peer Reviewed
See detailCombining remote sensing imagery of both fine and coarse spatial resolution to estimate crop evapotranspiration and quantifying its influence on crop growth monitoring
Sepulcre-Canto, Guadalupe; Gellens-Meulenberghs, Françoise; Arboleda, Alirio et al

in Geophysical Research Abstracts (2010)

This study has been carried out in the framework of the GLOBAM -Global Agricultural Monitoring system by integration of earth observation and modeling techniques- project whose objective is to fill the ... [more ▼]

This study has been carried out in the framework of the GLOBAM -Global Agricultural Monitoring system by integration of earth observation and modeling techniques- project whose objective is to fill the methodological gap between the state of the art of local crop monitoring and the operational requirements of the global monitoring system programs. To achieve this goal, the research aims to develop an integrated approach using remote sensing and crop growth modeling. This paper concerns the use of MSG geostationnary satellite data for the calculation of Actual Evapotranspiration and its integration into a crop growth model. [less ▲]

Detailed reference viewed: 71 (1 ULiège)