Indian Journal of Science and Technology Vol. 5 No. 5 (May 2012) ISSN: 0974- 6846 Research article “Rainfall runoff modeling” A.A.Adagbola & J.K.Jolayemi Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol. 2725 Historical rainfall-runoff modeling of river Ogunpa, Ibadan, Nigeria A.A. Adegbola * and J.K. Jolayemi 1 Department of Civil Engineering, Ladoke Akintola University of Technology, Ogbomoso, 2 dayomos2002@yahoo.com*, jolayemijoshua@yahoo.com Abstract Flooding in major cities like the city of Ibadan, the largest urban centre south of the Sahara, Africa, is a common phenomena. Flooding occurred in several areas of the city each time when Ogunpa River overflowed its banks. Flood damage mitigation measures were necessitated by increased runoff due to rapid urbanization of the catchment area coupled with inadequate runoff data along the river course. Ogunpa River gained its national and international notoriety when many lives and properties worth billions of Naira were lost in the floods of 1960, 1963, 1978, 1980 and 2011. This study was conducted at Queen Elizabeth gauging station to develop an historical rainfall-runoff model for River Ogunpa. The model developed was a linear regression approach considering the effects of previous and current rainfall on the flow of the effluent streams. Average daily net rainfall data and average daily rainfall were regressed against average daily runoff data. Using linear regression method Net rainfall values (R') and rainfall values (R) were regressed against the corresponding discharge (Q) arrived at correlation coefficients of 0.66975 and 0.71191989 respectively. Utilizing 101 years of rainfall records for Ibadan City, runoff data for Ogunpa River were derived. This data could serve as a veritable hydrologic input in the design of embankment flood mitigation structures for River Ogunpa. It is recommended that to find a lasting solution to the menacing frequent flooding more runoff gauging stations be provided along the river course of Ogunpa River within Ibadan metropolis. Keywords: Rainfall run-off model, River Ogunpa, Nigeria Introduction Ibadan city, Nigeria, a large city in Africa, lies within longitudes 3 0 45' and 4 0 05' East and latitudes 7 0 10' and 7 0 30' North (Adegbola, 2006). The city of Ibadan has a population of slightly over five million, spread over an area of about 400km 2 .The city is naturally drained by four rivers with many tributaries viz: Ona River in the North and West; Ogbere River towards the East; Ogunpa River flowing through the city and Kudeti River in the Central part of the metropolis. The maps of Ibadan Metropolis and drainage basins are presented in Fig.1. & 2. There are four major streams in the Ogunpa River basin with pereinial flow, viz: Ogunpa, Labelabe, Gege and Kudeti. Ogunpa River has a catchment area of about 54.02km 2 , with a length of 12.76km. It originates at Asi/Bodija Area and flows through the city towards the Lagos Ibadan Expressway. Ogunpa River, from its source to Queen Elizabeth Road gauging-station, has a catchment area and length of about 17.89km 2 and 4.23km, respectively. Ogunpa basin rises in the north- east section of the city near Agodi and Bodija, and eventually flows southwards passing through Oyo State Secretariat to Ogunpa Lake. A dam built across the Ogunpa created the lake for storage purposes to augment water supply. Almost fifty percent of the Ogunpa basin north of the confluence with the Kudeti is tributary to the Ogunpa Lake. Downstream of the Lake, the Ogunpa flows southwards through the western part of the inner core and past the south-eastern end of the Gbagi Market, a business district. From this point, the Ogunpa flows generally southwards through an area subjected to very intensive development at Oke- Foko, Oke-Ado, Molete and Isale-Ijebu and Bode. South of Ibuko Market, the Ogunpa is joined by the Kudeti stream which emanates from Oke-Irefin area of the metropolis, and both flow towards the Lagos Ibadan Expressway. The Labelabe stream originates from the north end of the Golf Course and flows southwards through Onireke and Ekotedo before crossing the Ibadan-Oyo Road. From Ibadan- Oyo Road, it flows through the northern boundary of Fig.1. Map of Ibadan City (Ajibade et al., 2010)