Use of limited hydrological data and mathematical parameters for catchment regionalization: a case study of the Osun Drainage Basin, Nigeria

Abstract

A protocol is proposed for the regionalization and subdivision of catchments based on hydrometric parameters. Such catchment regionalization may assist the development of appropriate catchment management strategies and policies. As a case study, the trends of variations in daily stage and discharge of seven gauging stations located in the 9,900 km² Osun Drainage Basin (South West Nigeria) were investigated. Linear regression models for all stations show the expected strong positive association of stage and discharge. The estimated daily changes explain only 1.44% of variations in stage, 0.25% variation in discharge and 99.5% in stage-discharge for station 5, 5.5% variation in stage, 0.7% variation in discharge and 99.7% in stage-discharge at station 25, and 10% variation in stage, 8.9% variation in discharge and 100% in stage-discharge at station 27. For the other studied stations, R² estimated from daily stage and daily discharge give widely varying patterns. R² estimated from daily stage and daily discharge is non-significant, but is significant for the daily stage-discharge relationship. The derived daily-stage and daily-discharge equations for the seven stations and their corresponding coefficients of determination can be used to classify the basin into three distinct zones. These are Zone I (coefficient of determination within the range of 0-6% for the daily-stage and daily-discharge), Zone II (coefficient of determination within the range of 7-10.5% for the daily-stage and daily-discharge), and Zone III (coefficient of determination within the range of 11-22% for the daily-stage and daily-discharge). The exponents of the stage-discharge equation can also be used for spatial classification. Zone A exponent is in the range of 1.3-1.7, Zone B exponent is in the range 2.2-2.3 and Zone C exponent is in the range 4.0-4.7. These can be combined to produce three hydrometric regions. It is proposed this regionalization protocol could be used as an initial step in dividing complex catchment systems into more homogeneous subunits, to assist subsequent catchment management and planning. The hydrometric regionalization protocol is now being evaluated on the Osun and other drainage basins in Nigeria.

Keyword: Derivation, Gauged and Ungauged Rivers, Drainage Basin, Power Equations