ABSTRACT
This study examined the role of extreme rainfall events in gully erosion formation at Umuhute/Umuaroko-Ahiaeke, Umuahia North Local Government Area, Abia State. The specific objectives were; to identify major causes of soil erosion in the study area, to identify extreme rainfall events in the study area over a period of 5 years and to evaluate the effects of extreme rainfall events on soil erosion. The study is developed via a descriptive research design. Data on daily rainfall for a period of 5 years were collected from the archive of the Nigerian Meteorological Agency (NIMET) and extreme rainfall events were extracted. Furthermore, a questionnaire survey was also carried out to generate first-hand information from residents whom had lived more than 5years in the area and not less than 23 years of age at time of distribution. The student T-test was used to compare extreme rainfall events and to test the following hypothesis: There is no significant difference in rainfall events over the past 5years (2013 – 2017) in the study area. Results showed that, rainfall, topography and type of soil were the major causes of gully erosion in the study area. Results also indicated that annual extreme rainfall totals amounted to 873.7 mm, 681.2 mm, 783.3 mm, 1123.7 mm and 818 mm for from 2013, 2014, 2015, 2016 and 2017 respectively. The wettest year is the year 2016 with an annual extreme rainfall amount of 1123.7 mm and with 16 extreme rainfall events. The two observed gullies have a 9% and 17% gradient with a sandy loamy textural class. Student T-test also revealed that, there is statistically significant difference in extreme rainfall events over the past 5years (2013-2017) at P<0.05 (F=4.5). It could therefore be stated that as a result of extreme rainfall events, the rate of gully formation increases. This situation can be exacerbated by climate change which has been reported to be increasing the frequency of exteme rainfall events. The study recommends that vegetation should be encouraged to curtail the impact of raindrops, especially the extreme rainfall type on soil.
TABLE OF CONTENTS
Cover page page
Title
Page i
Declaration
ii
Certification iii
Dedication iv
Acknowledgement v
Table of Contents vi
List of Tables ix
List of Figures x
List of Plates xi
Abstract xii
CHAPTER
1:
INTRODUCTION 1
1.1
Objectives of the Study 5
CHAPTER
2: LITERATURE REVIEW 6
2.1 Theoretical Framework 6
2.2 Erosivity 7
2.3 Erodibility 7
2.4 Gully Erosion 8
2.4.1 Factors of gully erosion 9
2.4.2 Deforestation 9
2.4.3 Geology 9
2.4.4 Topography 10
2.4.5 Anthropogenic influence 11
2.5 Climate
in Gully Erosion Formation 11
2.5.1 Influence
of climate change on gully development 11
2.5.2 Influence
of rainfall regime on gully development 12
2.6
Impact of Gully Erosion 12
2.7 Mitigation of Gully Erosion 13
2.7.1 Afforestation, deforestation and
agro-forestation 13
2.7.2
Role of organic matter in erosion
control 13
CHAPTER 3: MATERIALS AND METHODS
14
3.1
Study Area 14
3.2. Research
Design 15
3.3 Population of the Study 16
3.4 Sources of Data 16
3.4.1 Primary data 16
3.4.2
Secondary data 16
3.5
Sample Size 16
3.6 Sampling Techniques 17
3.7 Methods
of Data Collection 17
3.7.1
Personal observation 17
3.7.2
Use of digital camera 18
3.7.3 Questionnaire 18
3.7.4 Collection of rainfall data 18
3.7.5 Field measurement 19
3.7.6 Collection of soil samples 19
3.8 Laboratory Analysis 21
3.9 Methods
of Data Analysis 21
CHAPTER 4: RESULTS
AND DISCUSSION 22
4.1 Characteristics of Respondents 22
4.2 Causes
of Gully Erosion 22
4.3 Soil Morphology 23
4.4 Physio-Chemical Properties of the Soils 26
4.4.1 Particle size distribution 26
4.4.2
Textural classification of soil 27
4.4.3
Erodibility and erosivity 27
4.4.4
Soil pH of the study area 28
4.4.5 Hydraulic conductivity 29
4.4.6. Aggregate stability 29
4.4.7 Soil organic material/matter (OM) 30
4.5
Rainfall characteristics over the
years 32
4.5.1 Hypotheses 32
CHAPTER
5 :
CONCLUSION AND RECOMMENDATIONS 35
5.1
Conclusion 35
5.2
Recommendations 36
REFERENCES
APPENDICES
LIST OF TABLES
Page
3.1:
Gully description and sample
collection 20
4.1:
Morphological properties of the
study area 23
4.2:
Physio-chemical properties of the
soils (A & B) 26
4.3:
Daily rainfall over the study
area (2013-2017) 32
4.4: Extreme rainfall descriptive statistics
using t-test 32
LIST OF FIGURES
Figure Page
2.1: SOM decomposition increases as the
combination of temperature and
soil moisture increases
within conditions favorable for most microbial growth 13
3.1:
Study Area: Umuaroko and Umuhute in
Umuahia North, Abia State, Nigeria 14
4.1 Respondents’
number of years lived in the study area 22
4.2
Response of respondents on causes
of gully erosion 25
4.3:
Response of respondents on
rainfall characteristics over the years 25
4.4:
Annual extreme rainfall distribution (2013-2017) 34
LIST OF PLATES
Plate
page
1: Gully erosion formation on the steep sloping
Knoxfield College,
Agbani
Road, Enugu, Enugu State 10
2: Vegetation
against raindrop impact at gully B site 22
3:
Gully A erosion site showing bare
soil exposed to raindrop impact 28
4.: Gully
B erosion site showing litters against raindrop impact
31
5: Cross
sectional area of gullies A and B showing differences
in soil organic materials
34
CHAPTER 1
INTRODUCTION
Rain is liquid water in the form of droplets that have condensed from atmospheric
water vapor and then precipitated, that is, become heavy enough to fall under gravity. Rainfall
sizes range from 0.1 to 9mm and is a major component of the water cycle
and is responsible for depositing most of
the fresh water on the Earth (Glossary of
Meteorology, 2000; Mark, 2005; Glossary of Meteorology, 2009).
The
biggest raindrops on earth were recorded over Brazil and
the Marshall Islands in 2004 and some of them were as large as 10 mm (0.39 in)
(Paul, 2004). The increased size is explained by condensation on large smoke particles
or by collisions between drops in small regions with particularly high content
of liquid water (Paul, 2004).
Rainfall
is a climate parameter that affects human activities. It affects dynamically the
ecological system, soil nutrient mobility, flora and fauna inclusive (Obot and
Onyeukwu, 2010). Rainfall, a form of precipitation, has three types namely frontal
rainfall (collision of two air masses possessing varying characteristics),
orographic/relief rainfall (forcing of air to rise by a mountain or highland)
and convectional rainfall (high surface temperature resulting in ascending of
air). The convectional type is usually associated with heavy/extreme rainfall
events and thunderstorms (Enete, 2007). Rainfall in south-south and
south-eastern Nigeria falls into bimodal rainfall regime (Enete, 2007).
Rainfall intensity is classified based on the
rate of precipitation: Light rain; when the precipitation rate is < 2.5 mm (0.098 in) per hour,
Moderate rain; when the precipitation rate is between 2.5 mm
(0.098 in) - 7.6 mm (0.30 in) or 10 mm (0.39 in) per
hour, Heavy rain; when the precipitation rate is > 7.6 mm
(0.30 in) per hour, or between 10 mm (0.39 in) and
50 mm (2.0 in) per hour and Extreme/Violent rain; when the
precipitation rate is > 50 mm (2.0 in) per hour (Eric, 2005 and Wikipedia, 2012). Boulder Area Sustainability Information
Network (BIN) (2005), reported that rainfall intensity is investigated by
grouping it into four, consisting of light rainfall (R < 5 mm/h);
moderate
( R > 5 <
10 mm/h), heavy (R >10< 50 mm/h), and extreme (R > 50
mm/h).
By definition, an extreme
rainfall event is said to have occurred when the amount of water droplets that
falls on a single occasion in the greatest or highest degree, is excessive or
far beyond the normal i.e. R > 50 mm/h. Extreme rainfall is likely
when a storm passes through a warmer atmosphere holding more water. And the
frequency and intensity of severe storms has increased in respect to climate
change (Boulder Area
Sustainability Information Network, 2005; Eric, 2005).
Rainfall as a climatic factor is known to be
constantly changing worldwide and there has been great concern as to the
direction and effects of the changes on settlement and infrastructures (Azuwike
and Enwereuzor, 2011). With this, soils could also be affected. The potential causes of increasing extreme
precipitation events is that rising global surface temperatures have increased
evaporation and thus, addition of water vapor to the atmosphere (karl,
2003). There is enough evidence on the
rising global temperatures due to industrial increased emission of greenhouse
gases: carbon dioxide, nitrous oxide, methane, chlorofluorocarbons into the
atmosphere and when SO2, NO2 combine with H2O
molecules, they produce H2SO4 and H2NO3 thus,
increased global warming that has the capacity to trigger large-scale climatic
disturbances, which ultimately may have significant impact on the rainfall
(Somarin 2010; Ekpoh and Nsa, 2011). Aside the beneficial aspect of rainfall to
agricultural productivity, it can also be destructive in nature; natural
disasters like floods to farmlands, landslides and soil erosion (Ratnayake and
Herath, 2005).
Runoff
and erosion processes however, are affected by other multiple factors besides
land use/land cover (Sharma
et al., 1993; Dijk et al., 2002; Kinnell, 2005). Among these
factors, the most suspected one is rainfall. Extreme rainfall can lead to
severe soil erosion and runoff when it reaches the ground (Sharma et
al., 1993; Dijk et al., 2002; and
Kinnell, 2005).
Heavy rainfall that possesses large droplets has greater impact on soil
particles due to high gravitational effect when hitting the ground. In this
case, soil particles are force to disintegrate and become smaller and light
enough to be transported by water resulting to noticeable gully erosion
development (Enete, 2007). Such rainfall equally induces high runoff and
flooding, which ordinarily exacerbate soil erosion.
In
Nigeria, rainfall generally begins first in the south, spreading through the
middle belt, and eventually reaching the northern part (Enete, 2007). Bi-modal
(double maxima) rainfall regime is a determining factor to water distribution
in south eastern Nigeria (Enete, 2007). In support of Enete (2007), Akoshile et
al. (2016) have reported that long rainy season could stop soil infiltration
due to soil saturation and as a result more runoff is produced which lead to
flooding and soil erosion.
Increased
rainfall in the tropics has resultant effects due to vegetation supports and more
vegetation will result in increased plant litter on the soil surface and this
equally lead to increased soil nutrients (Nwagbara and Ibe, 2015). But if the
vegetation is destroyed by man, then the soil will be left bare. In this case,
the resultant effects are nutrients loss by leaching and loss of soil particles
and nutrients by erosion (Nwagbara and Ibe, 2015).
Soil
erosion by definition is the detachment and displacement of plant and soil
particles containing minerals from the surface to another location (Flanagan, 2002). Soil erosion is increased by intensive
rainfall, which is likely to increase under climate change. Erosion is
characterized into three main stages which are; splash, sheet, rill and gully
forms .One percent increase in precipitation is expected to lead to 1.5 – 2%
increase in erosion rates (Nearing et al.
2004). The rate of erosion depends on many factors including rainfall
intensity, soil characteristics, topography of the terrain, and land cover
type (Segura et al., 2014).
When a farmland is washed
by erosion and flooding, the top soil is mostly affected thereby exposing the
sub- soil which is not useful for farming because the nutrients needed for good
crop growth and production are contained in the top soil (Enete, 2014). It was
discovered that erosion reduces yield of crops in Osumenyi, Nnewi South Local
Government of Anambra State, Nigeria (Onyekuru and Uzuegbe, 2010).
This study focused on
gully erosion formation in Umuaroko and Umuhute of Ahiaeke in Umuahia North
Local Government Area of Abia State as it concerns the role of extreme rainfall
in it. The following questions are therefore asked: What are the major causes
of soil erosion in the study area? What is the trend of extreme rainfall events
in the study area from 2013-2017? And what are the effects of extreme rainfall
events on soil erosion?
1.1 OBJECTIVES OF THE
STUDY
This
study aimed at examining the role of extreme rainfall events in the formation
of gully erosion in Umuoroko and Umuhute in Umuahia North L. G. Area of Abia
State. The specific objectives are to:
i.
Identify major causes of soil erosion in the study area.
ii. Determine extreme rainfall events in the
study area over a period of 5 years.
iii. Evaluate the effects of extreme rainfall
events.
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