Sizeable aquifers in the Basement Complex are rare. Shallow groundwater is found in localized fractured strata or faults and in places where the impervious subsoil or bedrock forms a trough. Streams and creeks are fed by direct rainfall, interflow and runoff. IITA is situated in the ecological zone of moist lowland tropical forest. The climax vegetation is moist semideciduous forest. The vegetation at the experimental site was a secondary regrowth of about five years that was cleared in February-March 1982 to establish field runoff plots.