Bazie P., Dieng B. and P. Ackerer (1995) Water balance in
Unsaturated Soil under a Sundano-Sahelian Climate: Estimation of the Groundwater
Sci. Eau 8 (2) : 237-260. [article in French]
Original title: Bilan des transferts verticaux d'eau en zone non-saturée
sous climat soudano-sahélien: application à l'estimation de
la recharge des nappes.
A reliable estimation of the recharge rate of an aquifer is a prerequisite
for the efficient management of the groundwater resources. A variety of methods
for recharge estimation in Sudano-Sahelian areas are available, but most of
them do not take into account all mechanisms of water transfer in the unsaturated
zone; as a result, the estimated recharge values for a given region are often
Field equipment was used to measure water content and capillary tension at
various depths in the unsaturated zone at an experimental site at Ouagadougou,
Burkina Faso, in order to study infiltration and evaporation processes. Several
water content profiles and water tension profiles were used to determine mass
balances and water fluxes. The calculation of the head gradients allows the
determination of flow direction.
This study, carried out both in deep lysimeters (7 m) and on undisturbed soil
over two years, has allowed:
- A description of water dynamics at different soil depths.
- The soil can be divided into three parts. The water movement (infiltration
and evaporation) takes place in the upper part of the soil. Its extension
reaches 2.5 m depth. Drainage at this depth could not be measured and
can be neglected. Below this zone, the water movements are very slow.
The water content is constant over time and is equal to about 22%.
The third zone, which lies between 5.5 m and groundwater table (7 m
depth) is influenced by the groundwater table fluctuations.
- An estimation of the depth at which water can be taken up by evapotranspiration
and of the amount of this water at different depths.
- This depth is equal to 2.5 to 3 m under our experimental conditions
(soil and climatic conditions). At the end of the dry period, about
180 mm water has been extracted from the soil by evaporation: 50 mm
from the first meter, 100 mm between 1 m and 2 m depth and 20 mm for
the last 50 cm.
- A reevaluation of the effective minimum soil capacity by taking into account
the soil-plant- atmosphere exchange depth.
- The results show that this effective minimum soil capacity, usually
calculated on an agricultural basis, is under-estimated for hydrogeologic
The hydrodynamic behavior of the lysimeters and the undisturbed soil are similar.
Under the studied climate conditions and soil hydrodynamic properties, groundwater
recharge by direct infiltration can be neglected.
Infiltration, groundwater recharge, unsaturated soil, Sahara, Burkina Faso
P Bazie, École Inter-États d'Ingénieurs
l'Équipement Rural, Ouagadougou, Burkina Faso