1. Computing the net primary productivity for a Savanna- Dominated ecosystem using stable isotopes: A case study of the Volta River Basin

Abstract

The hydrologic systems and the terrestrial ecosystem of the Volta river basin in West Africa, play important role in the carbon cycle. This is so because of the coupling of water vapour release and CO₂ uptake during photosynthesis, expressed as water use efficiency or transpiration ratio. Hydrologic and land-cover data, together with stable isotope ratio measurements of δ¹⁸O and δD, and data from the global network of isotopes in precipitation (GNIP) are used to determine the net primary productivity (NPP) of the savanna-dominated ecosystem. The δ¹⁸O and δD values in the Volta rivers range from -4.72 to 2.37 mm^-l and from -35.28 to 9.30 mm^-1 SMOW, respectively. The results indicate that the vegetation is supported by 380 km³ of rainfall, out of which 50% is returned to the atmosphere via plant transpiration. Associated with annual transpiration is the NPP of 0.170 × 10¹⁵gCyr^–1 or 428 gCm^-2 from the terrestrial ecosystem. Modelled estimates of heterotrophic soil respiration in this study slightly exceeded the NPP estimates, implying a small source of CO₂ to the atmosphere. This condition does not favour the postulated existence of a major sink of atmospheric CO₂ in the Volta basin.