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Rheinischen Friedrich-Wihelm-Universität Bonn (2007)

Modeling of vegetation dynamics and its contribution to the water balance in semi-arid lands of West Africa

Tia, Lazare

Titre : Modeling of vegetation dynamics and its contribution to the water balance in semi-arid lands of West Africa

Auteur : Tia, Lazare

Université de soutenance : Rheinischen Friedrich-Wihelm-Universität Bonn

Doktorgrade : Dr. re. nat. 2007

Initially considered as merely a spectator in the functioning of the Earth system, vegetation is now recognized as an important component of the global climate system through its control of energy fluxes over substantial portions of the land surface. Moreover, it has been shown that vegetation-atmosphere interactions regulate local weather and hydrological balances and regional climate. Thus, the vegetation-climate feedback is deemed essential to be included in climate change studies. This study focuses on the estimation of the contribution of tree stands to the surface water balance by means of transpiration and actual evapotranspiration (ETa). The study used the nature reserve of Bontioli (NRB) in southwestern Burkina Faso, from which it scaled up to larger areas within the Volta Basin. This was achieved by modeling tree density and estimating daily whole-tree water use rates. The method of research encompasses two components : (1) estimation of climate, tree species and water-related variables, and (2) remote sensing and GIS studies. The quantification of tree water uptake was achieved by the xylem Heat-Balance method to assess sap flow rates of 17 tree species measured continuously from April 2005 to December 2006. During this period, weather data were recorded by Eddy Correlation and microclimate stations. Tree parameters were collected on the ground according to biometric standard methods in order to determine the phytosociology and the physiognomy of the vegetation cover. In addition, the LAI-SEB model, based on remotely sensed spectral vegetation indices and the surface energy balance outputs derived from the Aster image of November 2006, was developed to accurately estimate tree densities. The results reveal that the NRB is useful for biodiversity conservation because it provides a habitat for 71 (± 2) tree species representing 19 families, typical for tree savanna (33 species), shrub savanna (39) and gallery forest (10). The LAI-SEB model produced validated large-scale maps of (1) tree density by stem count, (2) tree density by DBH estimates and (3) tree density by crown cover estimates. The reliability of the model was proven through the comparison between the mean absolute tree density (331 ± 4 stems ha-1) and the mean predicted tree density (325 ± 87 stems ha-1). In the study area, the mean annual ETa was 94 % of the rainfall in the dry year 2005 and 80 % in the wet year 2006. The mean daily whole-tree water use rates ranged from 10.1 kg day-1 for Crossopteryx febrifuga to 492 kg day-1 for Pterocarpus erinaceus. Those rates were influenced and regulated by weather conditions, specifically by solar energy. Moreover, the field-specific mean daily tree stand transpiration was 0.7 mm day-1 ; transpiration rates increased from dry to rainy seasons with highs between mid-June and mid-September. The predicted tree stand transpiration map obtained by means of the LAI-SEB model shows that 62.1 % of the map surface were not contributing to transpiration, whereas 34.3 % of its surface transpired between 0 and 1 mm day-1. The mean daily ETa was 3.6 mm day-1. The final analysis highlight that the contribution of large trees (DBH > 5 cm) to the water balance ranges from 9 to 20 % of rainfall, depending on the vegetation type and the weather conditions. These results demonstrate the importance of trees in the functioning of the surface water balance and climate regulation through the maintenance of evapotranspiration in semiarid regions, particularly during dry seasons. Consequently, to mitigate drought, water scarcity, poverty, threats to livelihood and food security one should start by preventing the destructive effects of anthropogenic activities on vegetation covers. In this context, decision-makers should advocate regional and concerted efforts to restore natural vegetation through reforestation campaigns.


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