Rainfall Trends And Variability

Project and Seminar Material for Geography

Chapter One

Introduction

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 The Statement 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).

1.2 The Statement Problem

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.

1.5 Hypothesis

There is a significant difference in monthly rainfall trends from 1982 to 2011.
There is a significant variation in annual rainfall trends from 1982 to 2011.
There is a significant difference in biannual rainfall trends from 1982 to 2011.
There is an increase in the annual rainfall for the next 30 years.

1.6 Study Area

1.6.1 Location

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.

1.6.2 Climate

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.

Chapter Five

Summary, Recommendation and Conclusion

5.1 Summary

This study was carried out to investigate rainfall trends and variability in Ibadan, Oyo States from 1982-2011 with the following objectives: (1) to determine the monthly variation in rainfall from 1982 to 2011, (2) to determine the annual variation in rainfall variation from 1982 to 2011, (3) to determine the biannual variation in rainfall from 1982 to 2011, (4) to predict the rainfall in Ibadan for the next 30 years. Hypotheses were also formulated and tested based on the objectives of the study. Secondary rainfall data (1982-2011) gotten from the Nigerian Meteorological Agency (NIMET) was used for the analysis. The main statistical analysis used to test the hypothesis included charts, scatter plot, linear regression, analysis of variance(ANOVA) and the paired simple T-test.

The statistical analysis and findings can be summarized as follows:

The monthly rainfall pattern indicates that the highest average rainfall of 124.83mm was recorded in the month of June. This means that June was the wettest month in study area. This was followed by September, July and August with average rainfall of 114.33mm, 107.67mm and 101.37mm respectively. The lowest average rainfall of 17.52mm was recorded in January and this shows that January was the driest month. The total mean monthly rainfall for the study area was 78.3825mm, which indicates that January, February, March, April and November fall below the total mean monthly rainfall while May, October and December are slightly above total mean monthly rainfall of the study area.
Rainfall in Ibadan has been highly varied. The mean annual rainfall for the area was 1348.563 mm; with a mean minimum of 0.1 mm in 1996 with a mean maximum of 411.3mm in 1987. Total rainfall of the area was characterized by two distinct peaks: one in 2007 (1745.8mm) and the other (highest) peak in 1999 (1814.9mm). This shows that these years were the wettest years within the period under study (1982-2011). In general, total rainfall of the area increased in the period 1984-1997 and declined in 1998. There was a sharp increase after 1999, with a sharp decline in 2000and rose again between 2003-2004, which continued a rise and fall till about 2011, with another sharp decline in 2007. Within this period there was an interesting variation of rainfall as it experienced a rise and fall. The total rainfall continued to increase and decline in the period 1982-2011 with an overwhelming sharp rise in 1999. Rainfall amount in the period of study has shown a slight rise since 1982.
The total rainfall with the study area shows a distinct variation both for the first half and second half of the yea. The graph shows that the second half of the (in red lines) have greater amount of rainfall annually with two distinct peaks of 1109.8mm (highest) in 1999 and 1095.6mm (lowest) in 2010. The first half of the annual rainfall also has an interesting variation with the highest peak experienced in 2003 (801.2mm)
The result of the thirty year prediction shows a constant increase in the annual rainfall from 2012-2014. This result is not significant. This is because the P value is greater than 0.05. The table also shows that b1 which is 13.990 has a positive value, indicating that the predicted rainfall is rising with time though not significant. We can therefore conclude that the hypothesis which states that there is an upward trend in the rainfall variability for the next 30 years is accepted though this upward trend is not significant
The Anova was used to test for the first hypothesis which states that there is a significant difference in the monthly variation of rainfall in Ibadan from 1982-2011. Within the various months the sum of squares of all the rainfall amount is 1194026.657 (mean square of 3927.19.180) which is less than the sum of squares between groups (1346120.243) with a mean square of 122374.568. This implies that there is a larger difference in the variation of rainfall between the amount of rainfall in the various months in Ibadan than within the amounts in the different months. In essence it says that the amount of monthly rainfall shows large differences in the study area. This is supported by the significant level (P) of 0.001 (P<0.05), showing that there is a significant difference in the monthly variation of rainfall in Ibadan from 1982-2011.
The Hypothesis which states that there is a significant difference in the monthly variation of rainfall in Ibadan from 1982-2011 is accepted. Therefore H1 is accepted and H0 is rejected. It can then be concluded that there is a significant difference in the monthly variation of rainfall in Ibadan from 1982-2011.
In the table showing the descriptive statistics of the paired sample t-test for the fourth hypothesis which states that there is a significant difference in the biannual variation of rainfall in Ibadan from 1982-2011, the Annual rainfall (first half) represents the mean of all rains recorded between January and June whereas Annual Rainfall (2nd half) represents the mean of rainfall recorded between July and December. The table shows that the mean of the second half (7.4848E) of the year is higher the 1st half (6.0008E2) of the year. This result agrees with the biannual variation. This result showed whether these differences are significant. The t value is showed a difference of -3.884 at a significant of 0.001.this result is significant because P is < 0.05, indicating that it is statically significant. Therefore, we can conclude that there is a significant difference in the biannual variation of rainfall in Ibadan from 1982-2011.we then accept H1 and reject H0.

5.2 Recommendation

We can conclude that the hypothesis which states that there is an upward trend in the rainfall variability for the next 30 years is accepted though this upward trend is not significant. Hence, there would be an increase in the amount of rainfall variability in the next few years and this is going to affect agricultural practices across Ibadan and could eventually lead to increase in cost of food. I recommend that the government helps with agricultural solutions (like irrigation, production of drought resistant seeds, etc) that would help farmers cope within impending rainfall variability.

5.3 Conclusion

The purpose of this study is to observe the variations in rainfall in Ibadan from 1982 to 2011. After this study, I conclude that the rainfall variability trend is going to occur with an upward trend and it’s better to begin to prepare for it as it looks inevitable now.

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