Nigeria as a whole is well endowed with water resources. The country is well drained with a reasonably close network of rivers and streams. Some of these rivers, particularly the smaller ones, are, however seasonal, especially in the northern parts of the country where the rainy season is only three or four months in duration. In addition, there are natural water bodies like lakes, ponds as well as lagoons, particularly in the coastal areas. The country's water problems arise from (i) inadequate planning and management of the water resources and (2) poor distribution of water in time and space in relation to man's needs. Since the ultimate source of water found on the earth's surface is precipitation, usually the water resources of a given area are closely related to the rainfall over that area. Of the rain that falls over an area, some percolates to replenish soil moisture and groundwater storage, some runs off to join streams and rivers. A substantial amount is lost through the evaporation processes. An accurate assessment of the water resources of any area there- fore requires knowledge of not only the magnitude of rainfall and its spatial and temporal distribution but also the nature and magnitude of water losses by evaporation. The rates of water loss by evaporation in a tropical country like Nigeria are enormous so the effectiveness of rainfall is considerably reduced especially in the drier northern parts of the country. Rainfall constitutes the major input into the hydrological cycle of any area. In Nigeria, the mean annual rainfall decreases northward from over 100 in. in the Niger Delta to less than 20 in. in the extreme north-eastern parts of the country . In the southern part of the country, the rainfall tends to decrease towards the west due to differences in coastal alignment between the southeast and the southwest and the occurrence of the 'little dry season' in July-August in the latter area. The Jos Plateau area constitutes an outlier of moderately high rainfall area in the central part of the country. Nigeria's climate is characterized by two distinct seasons-wet and dry. The duration of the wet season generally decreases from south to north, thus rainfall is highly seasonal in the north. In fact, in the extreme northern parts of the country the wet season is as short as 3 months compared with 9 or 10 months in the Niger Delta area of the south .