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RAINFALL TRENDS AND VARIABILITY
1.1 BACKGROUND TO THE STUDY
Rain is liquid water in form of droplets that have condensed from atmospheric water vapor and then precipitated that is, become heavy enough to fall under gravity (Mordurch, 1995; Sala and Lauenroth, 1982). Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides suitable conditions for many types of ecosystems, as well as water for hydroelectric power plants and crop irrigation. Rainfall is a determinant factor of many natural occurrences. Vegetation distribution and type of land masses are as a result of rainfall (Ronen and Avinoam, 1999;Tielborger and Kadmon,2000; Shukla et al, 1990). Animals breeding periods synchronize with rainfall periods (Radford and Du plessis 2003). Crop planting, yields and harvests are influenced by rainfall and are practiced in accordance with their respective enabling seasons to ensure improved productivity (Laux et al, 2010; Mudita et al, 2008;Omokhafe and Emuedo 2006). Similarly, the gaseous content of the soil composition from time to time is much affected by rainfall (Lee et al, 2002).
Rainfall trend is the significant change in the spatial and temporal patterns of rainfall. Rainfall trend in other words can be said to be the general tendency, movement, or direction and pattern in which rainfall takes. In the global scene for example, rainfall trend analyses, on different spatial and temporal scales, has been of great concern during the past century because of the attention given to global climate change from the scientific community: they indicate a small positive global trend, even though large areas are instead characterized by negative trends (IPCC, 1996). Murphy and Timbal (2007) reported that most of the rainfall decline (61%) has occurred in autumn (March–May) in southeastern Australia. A similar rainfall decline occurred in the southwest of Western Australia around 1970 that has many common features with the southeastern Australia decline. However, in the regional level Nicholson (2000) observed that one of the most important contrasts in rainfall is the multi-decadal persistence of anomalies over northern Africa. Nicholson and Grist (2001) had identified several changes in the general atmospheric circulation that have accompanied the shift to drier conditions in West African Sahel. Rotstayn and Lohmann (2002) showed a prominent feature is the drying of the Sahel in North Africa and suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend (Akinremi et al 2001).
Rainfall variability on the other hand is the degree to which rainfall amounts vary across an area or through time. Variability of rainfall can be used to characterize the climate of a region. Rainfall in Nigeria is subjected to wide variability both in time and space. This variability has assumed a more pronounced dimension as a result of climate change. According to Chidozie et al, rainfall variability increases from the northwest to the southwest, while between-year (yearly) rainfall variability increases from the north central to the southeast. This study further confirms that rainfall variability over time follows a spatial trend within a certain arbitrary boundary (Laux et al, 2010, Mudita et al 2008).
The major cause of rain production is moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts. If enough moisture and upward motion is present, precipitation falls from convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds) which can organize into narrow rain bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation which forces moist air to condense and fall out as rainfall along the sides of the mountains. On the leeward side of mountains, desert climate can exist due to the dry air caused by downslope flow which causes heating and drying of the air mass. The movement of the monsoon trough or inter-tropical convergence zone, brings rainy seasons to savannah climes (Laux et al, 2010, Mudita et al 2008). The urban heat island effect leads to increased rainfall, both in amounts and intensity, downwind of cities. Global warming is also causing changes in the precipitation pattern globally, including wetter conditions across Eastern North America and drier conditions in the tropics.
Rainfall characteristics in Nigeria have been examined for dominant trend notably by Olaniran (1990, 1992) and by Olaniran and summer (1989, 1990). They showed that there has been a progressive early retreat of rainfall over the whole country, and consistent with this pattern, they reported a significant decline of rainfall frequency in September and October which, respectively coincide with the end of the rainy season in the northern and central parts of the country. The pattern of rainfall in northern Nigeria is highly variable in spatial and temporal dimensions with inter-annual variability of between 15 and 20% (Oladipo, 1993). As a result of the large inter-annual variability of rainfall, it often results in climate hazards, especially floods and severe and droughts with their devastating effects on food production and associated calamities and sufferings (Oladipo, 1993; Okorie, 2003; Adejuwon, 2004). Rainfall is one of the key climatic resources of Nigeria. Crops and animals derived their water resources largely from rainfall. It is considered as the main determinant of the types of crops that can be grown in the area and also the period of cultivation of such crops and the farming systems that can be practiced.
1.2 STATEMENT OF THE PROBLEM
Variations in rainfall trends are still burning issues in the research frontiers. Global warming and climate change have been identified as the major factors influencing rainfall trend and variability. Climate change in particular constitute a major menace to rainfall patterns which will directly or indirectly affects the ecosystems. According to the Department of Ecology 2015, state of Washington, climate change leads to rising levels of carbon dioxide and other heat-trapping gases in the atmosphere have warmed the Earth and are causing wide-ranging impacts, including rising sea levels; melting snow and ice; more extreme heat events, fires and drought; and more extreme storms, rainfall and floods.
Furthermore, scientists project that variation in rainfall trends will continue to accelerate, posing significant risks to human health, our forests, agriculture, freshwater supplies, coastlines, and other natural resources that are vital to a country’s economy, environment, and quality of life. This is because so many system are tied to climate, a change in climate can affect many related aspects of where and how people, plants and animals live, such as food production, availability and use of water, and health risks. In addition, a change in the usual timing of rains or temperatures can affect when plants bloom and set fruit, when insects hatch or when streams are their fullest. This can affect historically synchronized pollination of crops, food for migrating birds, spawning of fish, water supplies for drinking and irrigation, forest health, and more (Todd et al 2001; Dominic et al 2004; Adams & Faure 1997).
1.3 SIGNIFICANCE OF STUDY
Climate change and global warming has increased at an alarming rate as a result of anthropogenic factors such as urbanization. Therefore study is relevant because it will assist researchers to understand the future consequences of rainfall variation due to climate change. This is because climate change has been linked to the climate, which in turn would affect where and how people make a living, how the flora and fauna species would thrive, how food would be produced which would be influenced by availability of water. Studies has shown that both plant and animal require a specific amount of water for their existence, excess or inadequate supply of water could lead to their demise. Carbon dioxide is emitted into the atmosphere at an accelerated rate and also the depletion of the ozone layer which has caused the earth’s surface to be heated up.
1.4 AIM AND OBJECTIVES OF THE STUDY
The aim of this study is to investigate the changes in rainfall over Ibadan from 1982 to 2011. The specific objectives include:
To determine the monthly variation in rainfall from 1982 to 2011
To determine the annual variation in rainfall variation from 1982 to 2011.
To determine the biannual variation in rainfall from 1982 to 2011.
To predict the rainfall in Ibadan for the next 30 years.
i. There is a significant difference in monthly rainfall trends from 1982 to 2011.
ii. There is a significant variation in annual rainfall trends from 1982 to 2011.
iii. There is a significant difference in biannual rainfall trends from 1982 to 2011.
iv. There is an increase in the annual rainfall for the next 30 years.
1.6 STUDY AREA
The city of Ibadan is located approximately on longitude 30 541 East of the Greenwich meridian and latitude 70 231 North of the Equator at a distance some 145kilometres North East of Lagos (Figure 1). Ibadan is directly connected to many towns in Nigeria by a system of roads, railways and air routes. The physical setting of the city consists of ridges of hills that run approximately in northwest - southeast direction. The largest of these ridges lies in the central part of the city and contains such peaks as Mapo, Mokola and Aremo. These hills range in elevation from 160 to 275 meters above sea level and thus affords the vistor a panoramic view of the city.
Figure 1: Study Area: Ibadan, Oyo State.
Ibadan has a tropical wet and dry climate, with a lengthy wet season and relatively constant temperatures throughout the course of the year. Ibadan’s wet season runs from March through October, though August sees somewhat of a lull in precipitation. This lull nearly divides the wet season into two different wet seasons. The remaining months form the city’s dry season. Like a good portion of West Africa, Ibadan experiences the Harmattan between the months of November and February. The tropical rainfall regime experienced in Ibadan is bi-modal, convectional and follows the apparent movement of the sun while both length of rainy season and the yearly total rainfall, decreases with increasing distance from the equator. August is the coldest month and coinciding with the August break while in December-January, the months are noted for the dusty and cold harmattan winds conveyed from the Sahara by the north- east trade winds.
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