Spatial and temporal variations in net carbon flux during HAPEX-Sahel

doi:10.1016/S0022-1694(96)03193-9

Journal of Hydrology

Volumes 188–189, February 1997, Pages 563-588

https://doi.org/10.1016/S0022-1694(96)03193-9 Get rights and content

Abstract

Micrometeorological measurements of the surface flux of carbon dioxide were made at a number of spatially separate sites within the HAPEX-Sahel experimental area. Differences in the timing of plant development caused by differences in rainfall (both quantity and frequency) over the experimental area exert a major influence on the absolute magnitude of the surface flux of carbon made simultaneously over the experimental area. Surface fluxes are presented for the three major types of vegetation in the Sahelian landscape, namely, millet, savannah and tiger bush. The atmospheric and surface controls on the surface flux of CO₂ are described for these three land surfaces and the fluxes are compared between sites. The comparison shows that modellers need to take into account the heterogeneity not only of the type of surface vegetation but also the variability in the life cycle stage of the vegetation, when scaling up to larger areas.

References (19)

D. Baldocchi
A comparative study of mass and energy exchange rates over a closed C3 (wheat) and an open C4 (corn) crop: II. CO₂ exchange and water use efficiency
Agric. For. Meteorol.
(1994)
A. Bégué et al.
Radiation use efficiency of pearl millet in the Sahelian zone
Agric. For. Meteorol.
(1991)
S.D. Prince et al.
Geographical, biological and remote sensing aspects of the Hydrologic Atmospheric Pilot Experiment in the Sahell (HAPEX-Sahel)
Remote Sens. Environ.
(1995)
H. Soegaard et al.
Estimation of evapotranspiration from a millet crop in the Sahel combining sap flow, leaf area index and eddy correlation technique
J. Hydrol.
(1995)
A. Verhoef et al.
Fluxes of CO₂ and water vapour from a Sahelian savannah
Agric. For. Meteorol.
(1996)
J.-P. Goutorbe et al.
HAPEX-Sahel: a large scale study of land-atmosphere interactions in the semi-arid tropics
Ann. Geophys.
(1994)
J.P. Goutorbe et al.
An overview of HAPEX-Sahel: a study in climate and desertification
J. Hydrol.
(1997)
A. Haverkort et al.
Fysiologisch aanpassing aan droogte: perspectieven voor verbetering van droogtetolerantie
D. Hayward et al.
Climatology of West Africa

There are more references available in the full text version of this article.

Cited by (44)

Spatiotemporal variability in carbon exchange fluxes across the Sahel
2016, Agricultural and Forest Meteorology
Citation Excerpt :
Merbold et al. (2009) explained the variability in CO2 exchange across the African biomes with annual sums of rainfall and Brümmer et al. (2008) have shown the importance of water availability and rainfall distribution for inter-annual variation in C budgets for a Soudanian savanna ecosystem. Moncrieff et al. (1997) showed that phenology was the main factor determining CO2 flux variability, both in space and in time, which in turn was mainly determined by the timing of the start of the rainy season. Rockström and de Rouw (1997) on the other hand showed that short periods of intra-seasonal drought have a larger effect on spatiotemporal variability in grain yield than the annual sums of rainfall for a millet farm in the Sahel.
Semi-arid regions play an increasingly important role as a sink within the global carbon (C) cycle and is the main biome driving inter-annual variability in carbon dioxide (CO₂) uptake by terrestrial ecosystems. This indicates the need for detailed studies of spatiotemporal variability in C cycling for semi-arid ecosystems. We have synthesized data on the land-atmosphere exchange of CO₂ measured with the eddy covariance technique from the six existing sites across the Sahel, one of the largest semi-arid regions in the world. The overall aim of the study is to analyse and quantify the spatiotemporal variability in these fluxes and to analyse to which degree spatiotemporal variation can be explained by hydrological, climatic, edaphic and vegetation variables. All ecosystems were C sinks (average ± total error −162 ± 48 g C m⁻² y⁻¹), but were smaller when strongly impacted by anthropogenic influences. Spatial and inter-annual variability in the C flux processes indicated a strong resilience to dry conditions, and were correlated with phenological metrics. Gross primary productivity (GPP) was the most important flux process affecting the sink strength, and diurnal variability in GPP was regulated by incoming radiation, whereas seasonal dynamics was closely coupled with phenology, and soil water content. Diurnal variability in ecosystem respiration was regulated by GPP, whereas seasonal variability was strongly coupled to phenology and GPP. A budget for the entire Sahel indicated a strong C sink mitigating the global anthropogenic C emissions. Global circulation models project an increase in temperature, whereas rainfall is projected to decrease for western Sahel and increase for the eastern part, indicating that the C sink will possibly decrease and increase for the western and eastern Sahel, respectively.
Dynamics in carbon exchange fluxes for a grazed semi-arid savanna ecosystem in West Africa
2015, Agriculture, Ecosystems and Environment
Citation Excerpt :
The seasonal dynamics in GPP were as well strongly affected by soil moisture and VPD. It has also previously been shown that photosynthesis and productivity in semi-arid savanna ecosystems are highly governed by water availability, and VPD additionally governs the air’s ability to extract water from the plants (Moncrieff et al., 1997b; Kutsch et al., 2008; Merbold et al., 2009). The main factor determining the seasonal variation in all CO2 exchange fluxes (NEE, GPP and Reco) was NDVI, i.e. the phenology of the vegetation, indicating the importance of vegetation parameters for the CO2 exchange fluxes (Moncrieff et al., 1997b).
The main aim of this paper is to study land–atmosphere exchange of carbon dioxide (CO₂) for semi-arid savanna ecosystems of the Sahel region and its response to climatic and environmental change. A subsidiary aim is to study and quantify the seasonal dynamics in light use efficiency (ϵ) being a key variable in scaling carbon fluxes from ground observations using earth observation data. The net ecosystem exchange of carbon dioxide (NEE) 2010–2013 was measured using the eddy covariance technique at a grazed semi-arid savanna site in Senegal, West Africa. Night-time NEE was not related to temperature, confirming that care should be taken before applying temperature response curves for hot dry semi-arid regions when partitioning NEE into gross primary productivity (GPP) and ecosystem respiration (R_eco). Partitioning was instead done using light response curves. The values of ϵ ranged between 0.02 g carbon (C) MJ⁻¹ for the dry season and 2.27 g C MJ⁻¹ for the peak of the rainy season, and its seasonal dynamics was governed by vegetation phenology, photosynthetically active radiation, soil moisture and vapor pressure deficit (VPD). The CO₂ exchange fluxes were very high in comparison to other semi-arid savanna sites; half-hourly GPP and R_eco peaked at −43 μmol CO₂ m⁻² s⁻¹ and 20 μmol CO₂ m⁻² s⁻¹, and daily GPP and R_eco peaked at −15 g C m⁻² and 12 g C m⁻², respectively. Possible explanations for the high CO₂ fluxes are a high fraction of C4 species, alleviated water stress conditions, and a strong grazing pressure that results in compensatory growth and fertilization effects. We also conclude that vegetation phenology, soil moisture, radiation, VPD and temperature were major components in determining the seasonal dynamics of CO₂ fluxes. Despite the height of the peak of the growing season CO₂ fluxes, the annual C budget (average NEE: −271 g C m⁻²) were similar to that in other semi-arid ecosystems because the short rainy season resulted in a short growing season. Global circulation models project a decrease in rainfall, an increase in temperature and a shorter growing season for the western Sahel region, and the productivity and the sink function of this semi-arid ecosystem may thus be lower in the future.
Long term observations of carbon dioxide exchange over cultivated savanna under a Sudanian climate in Benin (West Africa)
2014, Agricultural and Forest Meteorology
Citation Excerpt :
This suggests a partial stomatal closure impact besides the control by the solar radiation (Yang et al., 2010). Similar daytime WUE behaviors were found for similar ecosystems in Niger in West Africa (Boulain et al., 2009; Moncrieff et al., 1997b; Friborg et al., 1997; Verhoef et al., 1996) and in Botswana and Zambia in South Africa region (Williams and Albertson, 2004). Overall, the daytime WUE values were observed also in the same order.
Turbulent CO₂ exchanges between a cultivated Sudanian savanna and the atmosphere were measured during 29 months (August 2007–December 2009) by an eddy-covariance system in North-Western Benin, West Africa. The site (Lat 9.74° N, Long 1.60° E, Alt: 449 m) is the one of three sites fitted out by the international AMMA-CATCH program. The flux station footprint area is mainly composed of herbs and crops with some sparse trees and shrubs. Fluxes data were completed by an inventory of dominating species around the tower and the meteorological measurements. Flux response to climatic and edaphic factors was studied. Water was found the main controlling factor of ecosystem dynamics: much larger uptake was found in wet than dry season. During wet season, a very clear answer of net CO₂ fluxes to photosynthetic photon fluxes density (PPFD) was observed. A low limitation in response to saturation deficit and soil water variability was however observed. The total ecosystem respiration (TER) was found highly dependent on soil moisture below 0.1 m³ m⁻³, but saturates above this threshold. The average annual carbon sequestration was 232 ± 27 gC m⁻² with its inter-annual variability mainly controlled by TER. Finally, the ecosystem appeared more efficient during morning and wet season than during afternoon and dry period.
Exploring the link between clouds, radiation, and canopy productivity of tropical savannas
2013, Agricultural and Forest Meteorology
Citation Excerpt :
This model is widely used, simple and can reproduce the saturating behaviour of leaf and canopy level photosynthesis as a function of PAR (Hollinger et al., 1994). It has also been used in other savanna ecosystems to describe the relationship between canopy photosynthesis and light (Le Roux and Mordelet, 1995; Moncrieff et al., 1997; Scanlon and Albertson, 2004). Only daytime (sunrise to sunset) observations were used in the analysis.
The control of clouds on the canopy gross primary productivity (GPP) was examined at Howard Springs, a tropical savanna site in the Northern Territory, Australia. It was demonstrated in this study that cloudiness can increase the initial canopy quantum efficiency (α), midday light use efficiency (LUE) and water use efficiency (WUE), but decrease GPP in savannas. Thick clouds (clearness index of 0–0.3 in the wet season produced much more diffuse fraction of Photosynthetically Active Radiation (fDPAR > 80%), which caused increases in α by 24% and 62% compared to thin clouds (fDPAR between 30% and 80%) and clear sky (fDPAR < 30%) conditions respectively. The influence of environmental conditions shows that under similar vapour pressure deficit, temperature and soil water content classes, α values were significantly higher under thick clouds compared to thin clouds or clear skies. This indicates the importance of diffuse radiation in enhancing LUE even within similar environmental conditions. However, the enhanced LUE under cloudy skies is insufficient to increase GPP due to the dramatic decline in total radiation. Therefore, it can be concluded that the quantity of solar radiation is more critical than the quality of radiation in savannas. However, savanna ecosystems appear to be well adapted to the environment where a 63% decrease in PAR only reduced GPP by 26%. These findings highlight the importance of clouds as a critical factor in determining savanna productivity that has implications for savannas carbon cycle.
The AMMA-CATCH experiment in the cultivated Sahelian area of south-west Niger - Investigating water cycle response to a fluctuating climate and changing environment
2009, Journal of Hydrology
Among the three sites distributed along the West African latitudinal gradient in the AMMA-CATCH observation system, the experimental setup in the Niamey area of south-west Niger samples the cultivated Sahel environment, for hydrological, vegetation and land surface processes. The objective is to investigate relationships between climate, land cover, and the water cycle, in a rapidly changing semiarid environment. This paper first presents the main characteristics of the area, where previous research, including the EPSAT and HAPEX-Sahel experiments, had evidenced a widespread decadal increase in water resources, concurrently with severe drought conditions. The specifics of AMMA-CATCH research and data acquisition at this site, over the long-term (∼2001–2010) and enhanced (∼2005–2008) observation periods, are introduced. Objectives and observation strategy are explained, and the main characteristics of instrument deployment are detailed. A very large number of parameters – covering rainfall, vegetation ecophysiology, phenology and production, surface fluxes of energy, water vapour and CO₂, runoff and sediment, pond water, soil moisture, and groundwater – were monitored at local to meso scales in a nested structure of sites. The current state of knowledge is summarized, connecting processes and patterns of variation for rainfall, vegetation/land cover, and the terrestrial hydrologic cycle. The central role of land use and of its spectacular change in recent decades is highlighted. This paper provides substantial background information that sets the context for papers relating to the south-west Niger site in this AMMA-CATCH special issue.
AMMA-CATCH studies in the Sahelian region of West-Africa: An overview
2009, Journal of Hydrology
The African Monsoon Multidisciplinary Analysis (AMMA) is an international and interdisciplinary experiment designed to investigate the interactions between atmospheric, oceanic and terrestrial systems and their joint controls on tropical monsoon dynamics in West Africa. This special issue reports results from a group of AMMA studies regrouped in the component “Couplage de l’Atmosphère Tropicale et du Cycle Hydrologique” (CATCH). AMMA-CATCH studies focus on measuring and understanding land surface properties and processes in West Africa, the role of terrestrial systems in altering boundary layer dynamics, and thus the potential that surface hydrology and biology, and human land use practices, may directly or indirectly affect monsoon dynamics and rainfall in the region. AMMA-CATCH studies focus on three intensively instrumented mesoscale sites in Mali, Niger and Benin that sample across the 100–1300 mm/annum rainfall gradient of the Sahel, Sudan and North-Guinean bioclimatic zones. Studies report on: (i) surface–boundary layer interactions that may influence atmospheric convergence and convective processes and thus rainfall type, timing and amount; (ii) vegetation dynamics at seasonal to decadal time-scales that may respond to, and alter, atmospheric processes; (iii) surface–atmosphere fluxes of heat, water and carbon dioxide that directly influence the atmosphere; (iv) soil moisture variability in space and time that provide the proximate control on vegetation activity, evapotranspiration and energy balance; and (v) local and mesoscale modeling of hydrology and land surface–atmosphere exchanges to assess their role in the hydrological, atmospheric and rainfall dynamics of West Africa. The AMMA-CATCH research reported in this issue will be extended in future years as measurements and analysis continue and are concluded within the context of both CATCH and the wider AMMA study. This body of research will contribute to an improved understanding of the functioning of the coupled West African system, and enhance our ability to model and predict rainfall, vegetation and biogeochemical dynamics across time-scales (day, year, decade, and century), and in response to changing climate and land use. Such information is vital for policy makers and managers in planning for future economic development, sustainability and livelihoods of the growing populations of the region.

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Spatial and temporal variations in net carbon flux during HAPEX-Sahel

Abstract

Agric. For. Meteorol.

Agric. For. Meteorol.

Remote Sens. Environ.

J. Hydrol.

Agric. For. Meteorol.

HAPEX-Sahel: a large scale study of land-atmosphere interactions in the semi-arid tropics

Ann. Geophys.

An overview of HAPEX-Sahel: a study in climate and desertification

J. Hydrol.

Fysiologisch aanpassing aan droogte: perspectieven voor verbetering van droogtetolerantie

Climatology of West Africa