Effet des facteurs climatiques et anthropiques dans l'évolution récente des écosystèmes tropicaux (modélisation spatialisée du bilan hydrique d'un petit bassin versant sahélien)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Association between subterranean termites and grasses in a West African savanna: spatial pattern analysis shows a significant role for Odontotermes n. pauperans.

International audienc

Hydrologic and land use impacts on vegetation growth and NPP at the watershed scale in a semi-arid environment.

International audienc

Functional convergence in ecosystem carbon exchange in adjacent Savanna vegetation types of the Kruger National Park, South Africa

Approximately one-eighth of the global land surface is covered by Savannas and open tropical woodlands. Of that total, about 60% is in Africa, where Savannas are the dominant land cover south of the Sahara (Scholes and Hall, 1996). Savannas are characterized by the coexistence of a tree or shrub canopy of varying density with a lower canopy of annual or perennial herbs, often composed primarily of grasses, with variable contribution by forbs (Sankaran et al., 2004). In Africa, the Savanna regions are of great economic and ecological importance to the human populations of the region, being the home and primary subsistence resource of the majority of the population. Future changes in land use and management practices (agricultural and grazing systems, fire frequency, and wood harvest) may result in the partial or complete modification of extensive areas of the remaining seasonal Savannas, with direct impact on ecosystem functioning, water, and energy balance. Changes in weather patterns or increased variability associated with global climate change may also impact the productivity and sustainability of the Savanna areas. Climate change could also modify tree-grass interactions sufficient to induce changes in community structure and the associated carbon, water, and energy relations of the vegetation (Bond, 2008)

Land clearance and long-term changes in the water balance in semiarid Niger 1 – evidence of increased runoff using aerial photographs

In the rain-fed agricultural belt of the Sahel, for the past decades, a la sting rainfall deficit and one of the world's highest population growths have resulted in a dramatic land cover change. Using a normalised mosaics set of aerial photographs dating back from 1950 over a 500 km2\ud area in southwest Niger, land cover and hydrological changes were quantified. For the 1950 - 1992 period, the whole landscape was significantly cleared, first to extend the millet field area and secondly for firewood supply. By 1975 and 1992, land clearance had affected respectively 23% and 87% of the sandy slopes, with similar figures obtained for the other landscape units. These observations indicate an accelerated loss in the woody savannah that could not be recovered on the short term. For the same period, aerial photographs show a 157% increase in the drainage density with the development of larger drainage systems and\ud new ponds. These changes highlight a long-term increase in surface runoff production, and indicate that land clearance had a stronger effect on the water balance than the rainfall deficit. The drainage network density appears to respond quasi immediately to land clearance; however a time-lag of a few decades is observed for the connectivity of the drainage network. As the annual rate of land clearance increased for the past century, its main hydrological effects may not yet be fully perceptible. These results imply a long-term effect of land clearance on several key parameters (e.g., albedo, LAI) known for their possible feedback on the West African monsoon dynamics

Is water availability really the main environmental factor controlling the phenology of woody vegetation in the central Sahel ?

Rainfall distribution and the soil moisture regime have been recognized to be the key drivers of the phenological rhythms in Sahelian woody plants, although different climate triggers have been assumed to be involved in determining the date of the onset of the phenophase. However, almost no comparisons have been made of the actual relative predictive power of these environmental factors. The aim of our study was to quantify the ability of several factors to predict phenophase occurrence in the dominant woody populations of northern Mali. Canopy leafing, flowering and fruiting were monitored from May 2005 to July 2007. Multiple logistic regressions were used to test the predictive power of cumulative rainfall, soil moisture, air temperature, air humidity and day length, with time lags of up to 2 months. Artificial variables derived from time lags observed in phenophases were included as predictors to account for possible auto-correlation and cross-correlation among phenophases. Surprisingly, a decrease in temperature associated with different time lags was most often found to be the strongest predictor of both leafing and reproductive phenophases. In Sahelian shrubs, morphological and physiological adaptations strongly contribute to the relative independence of their activity from water availability, leaf phenology being a way to adjust the plant water balance to current water availability and atmospheric water content. This study provides insight towards the development of a mechanistic understanding of phenological control in the Sahel, which is becoming increasingly important in the context of expected climate changes

The AMMA Surface Flux network: Measurements, preliminary data and modelling

The surface flux network and allied instrumentation is a pivotal part of the AMMA programme providing the LOP, SOP's, satellite, aircraft and modellers with 30 minute averages of long-term measurements of the soil-vegetation-atmosphere interface. The location and instrumentation of the surface flux network is summarised, with the emphasis on the instrumentation provided through AMMA-EU and AMMA-UK funding. This network is a mixture of full C02/H20lEnergy eddy correlation systems and Sensible heat flux/momentum eddy correlation systems, both systems augmented by surface radiation, micrometeorological and soil physics instrumentation. These are deployed at the AMMA supersites to fully sample the latitudinal rainfall gradient and seasonality and the typical vegetated surfaces within that gradient. The range of sites, the instrumentation and some preliminary data from the sites are presented. An initial comparison with flux estimates simulated for these sites using the JULES land-surface model is also presented. [no pdf

Spatio-temporal soil heat flux estimates from satellite data; results for the AMMA experiment, Fakara supersite.

International audienceThe soil heat flux, G, is an important component in the energy balance, especially for sparsely vegetated (semi)-arid regions. In order to obtain large-scale estimates of this flux, for example for land surface model (e.g. GCM) verification, scientists have to rely on remote sensing data. Unfortunately, in these cases G is often estimated using highly empirical methods. Examples are relationships between the ratio of G and net radiation, Rn, and surface variables such as leaf area index (LAI). Other approaches use surface temperature observations to get maximum G/Rn values. However, such approaches are not universal. In Murray and Verhoef 2007a&b we proposed to use a standard physical equation, involving a harmonic analysis of surface temperatures for the estimation of G, in combination with a simple, but theoretically derived, equation for soil thermal inertia (TI). This method does not require in situ instrumentation. Moreover, such an approach ensures a more universally applicable method than those derived from purely empirical studies. This method requires knowledge of soil texture, in combination with an estimate of near surface soil moisture content, SM, to obtain spatio-temporal variation in thermal inertia. To get the diurnal and seasonal shape of G we ideally need time series of soil surface temperature, Ts. However, when vegetation obscures the surface these are not available through remote sensing. Therefore, a direct relationship between the harmonic analysis of Ts (Hs) and the harmonic analysis (Hb) of the remotely observed brightness temperature, Tb, obtained from remote sensing equipment was used instead. This relationship was tested for 4 different UK crops in Murray and Verhoef (2007b). Knowledge of LAI, canopy extinction coefficient and IR sensor view angle is required to go from Hb to Hs. To account for phase lag differences between Hs and Hb a time delay of 1.5 hrs was used. Here, the method is used to calculate spatiotemporal soil heat fluxes for the Fakara supersite domain in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) program. MSG-SEVIRI land surface temperature (spatial resolution at nadir 3 x 3 km, temporal resolution 15 mins, averaged to half-hours, time period 13 July to 31 December 2005) and ENVISAT-ASAR soil moisture products were used to derive Tb and SM. The seasonal evolution of LAI was derived from SPOT-HRV. Soil texture was obtained from existing soil maps. Verification soil heat fluxes were provided by a meteorological station installed at the Fakara experimental site, called Wankama (Ramier et al., 2009). The MSG file contains 266 records (grid points over the super site domain, although only 190 were left once those pixels containing significant water bodies were removed) containing each 16512 values of temperature (172 (days)x 24 x 4 time steps). Soil moisture and LAI images over the same domain were available on 7-8 (non-corresponding) occasions between 13 July and 31 December 2005. Daily estimates of LAI and SM were obtained from interpolation. Results revealed a large spatial variability in soil heat flux, mainly dependent on the relative fraction of surface components at each pixel (e.g. plateaus bare soil, plateaus vegetation, crops, recent and old fallows) and corresponding pixel-averaged LAI. SM affected thermal inertia, but estimates of SM, and hence TI appeared to be on the low side, thereby probably underestimating G, although estimated and measured G compared relatively well. Overall, the method appeared promising and work is underway to determine G over the entire AMMA domain in the context of the ALMIP project. • Murray, T., and A. Verhoef, 2007a. Moving towards a more mechanistic approach in the determination of soil heat flux from remote measurements. I. A universal approach to calculate thermal inertia. Agricultural and Forest Meteorology 147: 80-87. • Murray, T., and A. Verhoef, 2007b. Moving towards a more mechanistic approach in the determination of soil heat flux from remote measurements. II. Diurnal shape of soil heat flux. Agricultural and Forest Meteorology 147: 88-97. • Ramier, D, Boulain N., Cappelaere B., Timouk F., Rabanit M., Lloyd C.R., Boubkraoui, S., Métayer F., Descroix L., Wawrzyniak V., 2009. Towards an understanding of coupled physical and biological processes in the cultivated Sahel - 1. Energy and water. Journal of Hydrology, 375 (1-2), 204-216. • Saux-Picart, S., C. Ottlé, B. Decharme, C. André, M. Zribi, A. Perrier, B. Coudert, N. Boulain, B. Cappelaere, L. Descroix, D. Ramier, 2009. Water and energy budgets simulation over the AMMA-Niger super-site spatially constrained with remote sensing data. Journal of Hydrology 375, 287-295

Spatio-temporal soil heat flux estimates from satellite data; results for the AMMA experiment, Fakara supersite.

International audienceThe soil heat flux, G, is an important component in the energy balance, especially for sparsely vegetated (semi)-arid regions. In order to obtain large-scale estimates of this flux, for example for land surface model (e.g. GCM) verification, scientists have to rely on remote sensing data. Unfortunately, in these cases G is often estimated using highly empirical methods. Examples are relationships between the ratio of G and net radiation, Rn, and surface variables such as leaf area index (LAI). Other approaches use surface temperature observations to get maximum G/Rn values. However, such approaches are not universal. In Murray and Verhoef 2007a&b we proposed to use a standard physical equation, involving a harmonic analysis of surface temperatures for the estimation of G, in combination with a simple, but theoretically derived, equation for soil thermal inertia (TI). This method does not require in situ instrumentation. Moreover, such an approach ensures a more universally applicable method than those derived from purely empirical studies. This method requires knowledge of soil texture, in combination with an estimate of near surface soil moisture content, SM, to obtain spatio-temporal variation in thermal inertia. To get the diurnal and seasonal shape of G we ideally need time series of soil surface temperature, Ts. However, when vegetation obscures the surface these are not available through remote sensing. Therefore, a direct relationship between the harmonic analysis of Ts (Hs) and the harmonic analysis (Hb) of the remotely observed brightness temperature, Tb, obtained from remote sensing equipment was used instead. This relationship was tested for 4 different UK crops in Murray and Verhoef (2007b). Knowledge of LAI, canopy extinction coefficient and IR sensor view angle is required to go from Hb to Hs. To account for phase lag differences between Hs and Hb a time delay of 1.5 hrs was used. Here, the method is used to calculate spatiotemporal soil heat fluxes for the Fakara supersite domain in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) program. MSG-SEVIRI land surface temperature (spatial resolution at nadir 3 x 3 km, temporal resolution 15 mins, averaged to half-hours, time period 13 July to 31 December 2005) and ENVISAT-ASAR soil moisture products were used to derive Tb and SM. The seasonal evolution of LAI was derived from SPOT-HRV. Soil texture was obtained from existing soil maps. Verification soil heat fluxes were provided by a meteorological station installed at the Fakara experimental site, called Wankama (Ramier et al., 2009). The MSG file contains 266 records (grid points over the super site domain, although only 190 were left once those pixels containing significant water bodies were removed) containing each 16512 values of temperature (172 (days)x 24 x 4 time steps). Soil moisture and LAI images over the same domain were available on 7-8 (non-corresponding) occasions between 13 July and 31 December 2005. Daily estimates of LAI and SM were obtained from interpolation. Results revealed a large spatial variability in soil heat flux, mainly dependent on the relative fraction of surface components at each pixel (e.g. plateaus bare soil, plateaus vegetation, crops, recent and old fallows) and corresponding pixel-averaged LAI. SM affected thermal inertia, but estimates of SM, and hence TI appeared to be on the low side, thereby probably underestimating G, although estimated and measured G compared relatively well. Overall, the method appeared promising and work is underway to determine G over the entire AMMA domain in the context of the ALMIP project. • Murray, T., and A. Verhoef, 2007a. Moving towards a more mechanistic approach in the determination of soil heat flux from remote measurements. I. A universal approach to calculate thermal inertia. Agricultural and Forest Meteorology 147: 80-87. • Murray, T., and A. Verhoef, 2007b. Moving towards a more mechanistic approach in the determination of soil heat flux from remote measurements. II. Diurnal shape of soil heat flux. Agricultural and Forest Meteorology 147: 88-97. • Ramier, D, Boulain N., Cappelaere B., Timouk F., Rabanit M., Lloyd C.R., Boubkraoui, S., Métayer F., Descroix L., Wawrzyniak V., 2009. Towards an understanding of coupled physical and biological processes in the cultivated Sahel - 1. Energy and water. Journal of Hydrology, 375 (1-2), 204-216. • Saux-Picart, S., C. Ottlé, B. Decharme, C. André, M. Zribi, A. Perrier, B. Coudert, N. Boulain, B. Cappelaere, L. Descroix, D. Ramier, 2009. Water and energy budgets simulation over the AMMA-Niger super-site spatially constrained with remote sensing data. Journal of Hydrology 375, 287-295