Computing the net primary productivity
for a savanna-dominated ecosystem
using stable isotopes : A case study of
the Volta River Basin.
Dr Ebenezer Kofi Hayford
Department of Geology, University of Ghana,
Associate Research Fellow - Volta Basin
Terrestrial nature of the watershed.
Experimental work : DH2O ‘ 18OH2O ,
13CDIC and dissolved 87Sr/86Sr.
Chemical characteristics of water in the
Volta River System.
Objectives of the Study.
To provide data on the Chemical dynamics of
the Volta River System.
To quantify the water budget and the net
primary productivity of the Volta River System.
To extrapolate computed data to assist in the
solution of some related problems in Ghana.
Bird’s view of the Volta Lake.
Southern part of the Volta Lake:
Contact point to the Turbines.
An outcrop on the Western side of the Volta
Lake showing multiple foliations.
Political nature of the basin.
Burkina Faso (42%)
Ivory Coast (4.0%)
GLC2000 from SPOT-4 Satellite
B. V. 156900km2
W. V. 111800km2
Lower V. 58200km2
Lake V. 8500km2
The catchments area in
the North, is dominated by the Precambrian
formation of the West African craton (the
Baoule-Mossi block) dating from 2.6Ga-
1.6Ga (Sattran et al.,2002).
1) The Voltaian sediments
2) The Dahomeyan system
3) The Togo series.
Muscovite - KAl3Si3O10(OH,F)2
Biotite – K(Mg,Fe)3AlSi3O10(OH,F)2
Hornblende – NaCa2(Mg,Fe)4AlSi6Al2O22(OH,F)2
Feldspar Group- NaAlSi3O8-KAlSi3O8-CaAl2Si2O8
Quartz - SiO2
Calcium Carbonate – CaCO3
Tremolite – Ca2Mg5Si8O22(OH)2
5 Sample Stations
Samples from GNIP (in Kano, Bamako,
Borogo and Niamey)
Samples from Lake Volta: Surface (1m),
Analysis :Samples for major cation and
Strontium analysis were acidified with
conc. HNO3 and analyzed using Thermo
Jarrell Ash Atom-scan 25 Argon ICP.
The 13C/12C : Samples for isotopes of
carbon were poisoned with HgCl2.
and conc. H3PO4 was added to the
compositions under vacuum to liberate.
ISOTOPES: Dissolved DH2O , 18ODO,
13CDIC and 87Sr/86Sr were determined as
METHOD: Samples were converted to
hydrogen and oxygen with hot zinc. The
18O/16O ratio was determined by
equilibrating a small amount of CO2 with
3mL of sample and analyzing the resulting
solution on a triple collector VG SIRA 12
DH2O , 18ODO, 13CDIC are reported
relative to SMOW.
Calculations for fluxes in the isotope
systematic for the Volta River is
based on equations put forward by
Net Ecosystem Productivity
Sub-Basin Trans. NPP ET Precipi. Int Discharge
(km3) g Cm2yr-1 km3 km3 km3 km3
Black Volta 75 437 133 140 34 7
White Volta 58 461 94 101 24 7
Oti River 34 389 73 84 19 11
Lower Volta 23 400 41 47 10 6
from SPOT-4 (NASA-Satellite)
Sub-Basin Trees Shrubs Grass Crops C4 WUE
% % % % % (molH2Oper mol
Black Volta 18.0 16.7 44.0 21.3 39 730
White Volta 18.8 17.6 40.0 23.6 35 750
Oti River 22.3 21.4 42.5 13.7 37 740
Lower Volta 25.9 23.6 40.4 14.4 35 750
Sr and 87Sr/86Sr
Concentration in um/l
6 Lake Volata 87Sr/86Sr
Distance in kilometres
Geogene and Anthropogenic influences
Concentration in umol/l
1500 C l l-
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Empirical Data on Evapotranspiration in
the Volta Basin.
Projected attempts to reduce
Evaporation on Lake Volta.
Development of a concept of
Regular controls of radiogenic
elements in the Lake and Rivers.
This Project was financed by
1) The National Science Research Council, Canada.
2) Canadian Institute for Advanced Research.
Research Team :
• Prof. J. Veizer : Environmental Geoscience Center,
University of Ottawa, Canada; and
Member of NASA Research Council.
• Dr. Heiko Freitag : Isotope Laboratory, Dept. of
Chemistry University of Bochum,
• Dr. E.K.Hayford : Dept. of Geology, University of