Computation of River Discharge from Formulated Rating Equation in Hydrological Catchments with Inadequate Data: River Omi, South Western Nigeria, as Case Study

River discharge is an important hydrological parameter in any water resources management. Most rivers in Nigeria, with sizeable catchment areas, are poorly gauged and are noted to have histories of frequent flooding experience. Since Stage or Gauge, which is the height of water surface at a relative datum, can be easily measured, it is possible to formulate a rating equation which can predict discharge with a known gauge-depth. This Paper develops a mathematical relationship between gauge heights and discharge, using River Omi, South Western Nigeria, as a case study. Series of gauge-discharge relationships were established on River Omi from measured values and past records. The water velocities were measured using ultrasonic current meter and geometrical parameters were measured at the gauging station. The stage-discharge relationship, over a period of time, was plotted and a rating equation generated. The generated rating equation: Q = 0.1828*(G-0.14) 0.78, has a coefficient of regression of 97%. The formulated equation was calibrated and validated with discharge data obtained from 2007 to 2012. The rating equation performed better at depth below 0.6 m with less than 1% variation between the simulated and measured discharge. This formulated equation can be adapted to other river catchments with similar hydrological characteristics. It should be noted that below gauging depth of 0.14 m, the rating equation cannot be used. Keywords : River Discharge, Rating Equation, Gauge Depth, River Omi, Regression