Water Resources Systems—Hydrological Risk, Memagement and Development (Proceedings of symposium IIS02b held during 1UGG2003 at Sapporo, July 2003). IAHS Publ. no. 281, 2003. 287 Effects of climate change on irrigation activities: a case study of the Benin-Owena River basin irrigation projects CATHERINE I. IKHILE Department of Geography and Regional Planning, University of Benin, PMB 1154, Benin City, Nigeria [email protected] GEORGE U. IKHILE c/o Scripture Union (Nigeria), PO Box 743, Benin City, Nigeria Abstract The effects of climate change on irrigation activities in the Benin- Owena River Basin (BORB), southwestern Nigeria, were investigated, using rainfall and temperature data from 1961 to 2000 with irrigation activity data from 1990 to 2000. Climate records showed a large variation in annual rainfall between 1961-1980. The rainfall variation between 1980-2000 was less pronounced, and rainfall was much lower. The period 1982-1986 exhibited the greatest downward trend in rainfall. The decadal range of temperature showed a gradual warming of the environment. Increasing temperatures and decreasing rainfall, coupled with an increasing population, led farmers and policy makers in Nigeria to embark on massive irrigation activities from the 1990s to 2000. Farm productivity levels and irrigations activities in Edo State are discussed. Key words Benin-Owena River basin; climate change; irrigation; Nigeria; rainfall; temperature INTRODUCTION Over the years, the Federal Government of Nigeria has launched several agricultural programmes (Sanda et ai, 1994; Imoudu et al., 2001) to adopt irrigation agriculture as a means of achieving the government's objective of self-sufficiency in food and fibre production. The history of irrigation activities in Edo State is attributable to the sub- Saharan West African drought and desertification of the late 1970s and early 1980s (Oyebande et al., 1978; Nicolson, 1980; Lamb, 1982; Olaniran et al, 1991). Climate change is now a global concern. It is becoming increasingly clear that it has serious implications on food production and other agricultural practices in Africa (Olufayo et ai, 1998). The IPCC (1996) had earlier reported that climate change that will accompany higher temperatures will intensify the hydrological cycle, thereby more precipitation and evaporation will be experienced in different regions of the world and this will invariably have different impacts on major river basins. It is in the light of the above studies that this paper examines the effects of climate change indexed by rainfall and temperature characteristics, vis-à-vis the influence on the basin hydrology as it affects irrigation practices in Edo State, Nigeria. 288 Catherine I. Ikhile & George U. Ikhile THE STUDY AREA The study sites are located in Edo State (S^OTM to 7°4(W and 5°00'E to 6°40'E, Fig. 1). Physiographically, the basin is well drained with an average surface slope of 0-6%. Geologically, the state can be divided into two broad parts (the basement complex rocks of the north with little or no prospect for exploitable groundwater and the sedi­ mentary rocks of the south with relatively high groundwater potential). The soils are Oxisols in the south and Utisols in the north. The hydrology is a reflection of the geology and is dominated by the extensive River Niger flood plain in the east and other rivers such as the Ubo, Edion, Orle, Okhuamahun and Owena. These are amongst the numerous sources for irrigation in the State. Their flow patterns are similar: minimum flows occur in March/April and the maximum flows occur in September. Most of the irrigation projects are developed using boreholes and aquifers in addition to the numerous water sources. In some cases, earth dams have been constructed for water storage. The average monthly rainfall of the Benin City synoptic station ranges from 27 mm in December to 365 mm in July, with a mean annual rainfall of 2165 mm. Over 90% of the rain falls between June and October, during the southwestern monsoon. Benin City experiences double maxima of rain, June-July and September—October, characteristic of the tropical continental climate with alternation of wet and dry seasons of varying duration. The mean monthly minimum temperature varies from 22.3°C in January to 22.7°C in June; and the mean maximum from 32.6°C in January to 30.3°C in July. The average daily pan evaporation varies from 3.8 ml in January to 1.7 ml in July. The average annual wind speed, sunshine and relative humidity are 3 knots, 5.1 h and 75%>, respectively. MAJOR TOWNS & Other Towns Roads Fig. 1 Edo State showing study sites. Effects of climate change on irrigation activities: Benin-Owena River basin case study 289 Edo State is densely populated with a population of 2.6 million (1991 Census). The major towns are Benin City, Auchi, Ekpoma, Uromi, and Igarra. METHODS Rainfall was measured with a Stevenson automatic raingauge installed at the Benin City Synoptic Station. Rainfall data for the period was recorded on monthly charts. Monthly and annual totals and decadal total rainfall figures were used. Temperature was measured using minimum and maximum thermometers. Mean monthly and mean annual minima, mean monthly and mean annual maxima, annual mean and decadal mean temperatures were used. These data were obtained from the unpublished climatological data monitored by the Federal Ministry of Aviation, Meteorological Services Department (FMAMS) in Benin City. Information on irrigation activities were gathered from the BORB Development Authority (BORBDA) and the Edo State Agricultural Development Programme (EDADP) in Benin City. The irrigation schemes used include the Illushi/Ega/Oria, Obayantor, Elobi, Ukpoke and Ukhun/Era Dams maintained by BORBDA and the Irrigation Projects maintained by EDADP. RESULTS AND DISCUSSION Rainfall Rainfall in the decades 1961-2000 exhibited distinct patterns (Fig. 2). The annual total for the decade 1961-1970 ranged between 1985 mm and 3049 mm (1964 and 1965, respectively). January was particularly dry in 1964, 1967 and 1969, with zero values of mean monthly rainfall, while it was only 1970 that had no rainfall in the month of December. The decade 1971-1980 showed a different pattern (Fig. 2). The total annual rainfall ranged between 1702 mm (1972) and 2585 mm (1980). In this decade, most January and December months were very dry with zero rainfall values for up to five years (1973-1976, 1979). This decade was drier than the previous one. The total annual rainfall for the third decade, 1981-1990, ranged between 1227 mm and 2461 mm (1986 and 1990, respectively). Between 1985-1989, Decembers were completely dry with zero rainfall values and January was also dry for many years. This decade was 4QQQC0 3000.00 200000 1 tCOQCO 0.00 Years( 1960-2000) Fig. 2 Rainfall distribution pattern over Benin City (1961-2000). 290 Catherine I. Ikhile & George U. Ikhile apparently the driest of the record. A similar observation was made by Olaniran et al. (1991). December 1990 recorded an unusually high mean monthly total of 168.6 mm, and this was the highest ever recorded rainfall in December between 1961-2000. The total annual rainfall for 1991-2000 ranged between 1860 mm and 2776 mm (1993 and 1992, respectively). December and January were not as dry as previously (Ojo, 1987). The rains tended to fall in all months of the year, with the exception of 1992 and 1994 when the Decembers were dry. Generally, the first three decades showed double maxima of rainfall with an August break. In the fourth decade (1991-2000), the August break was absent with the August months recording generally higher rainfall values than in all the other decades. This means that the normal pattern was reversed in this decade. Even the December months were very wet. January was generally the driest month with a total of 682 mm of rain between 1961-2000. July was the wettest month with a total of 14 975 mm of rainfall between 1961-2000. Air temperatures The temperature conditions for Benin City are presented in Table 1. The temperature distribution pattern for the period 1961-2000 showed a wide variation and a gradual warming of the environment. For the decade 1961-1970, the annual minimum ranged from 19.2°C to 24.2°C (January 1967 and February 1970) and the minima slightly increased over the following decades (Table 2). The 1981-1990 decade showed the greatest extremes of temperature, recording the lowest and highest annual minimum temperatures for the whole period of study. The mean annual minimum followed a similar pattern. The annual maximum for 1961-1970 ranged from 27.2°C to 36.7°C (July 1961 and March 1962) and remained approximately constant over the following decades (Table 2). The year 1998 recorded the highest temperature of 37.0°C during the 40-year period. This is in line with Odjugo (2000). The annual mean maximum followed a similar pattern. The annual mean temperatures increased steadily from 1961 to 2000. During the 1961-1970 decade, it was 25.8°C to 27.0°C (decadal range: 1.2°C). During the 1971- 1980 decade, it was 26.3°C to 27.5°C (decadal range: 1.2°C). During the 1981-1990 decade, it was 27.1°C to 27.6°C (decadal range: 0.5°C). In this decade, the climate of Nigeria was described as playing a climatic drama (Ojo, 1987). The climate fluctuations Table 1 Temperature condition of Benin City (1961-2000) (°C) (Ranges). Source: Ikhile (2002). Decade Annual Mean annual Annual Mean annual Annual mean Decadal minimum minimum maximum maximum range 1961- 19.2-24.2 21.9-22.8 27.2-36.7 31.1-31.3 25.8-27.0 1.2 1970 1971- 21.0-24.9 22.4-23.3 27.0-34.7 30.2-31.7 26.3-27.5 1.2 1980 1981- 18.4-26.3 22.2-23.8 27.6-36.0 31.3-32.1 27.1-27.6 0.5 1990 1991- 21.0-25.5 23.3-23.9 27.4-37.0 31.2-31.8 27.2-28.2 1.0 2000 Effects of climate change on irrigation activities: Benin-Owena River basin case study 291 were larger and the weather became more unpredictable.