VEGETATION RESPONSE TO RAINFALL VARIABILITY IN THE SUDANO SAHELIAN ECOLOGICAL ZONE OF NIGERIA

Abstract

Rainfall variability is an important driver of vegetation shift or dynamics. However, the changes are symmetric and have great multiplying effects on the ecosystem and the general livelihood of man. The study examines the vegetation response to rainfall variability in the Sudano Sahelian Ecological Zone of Nigeria (SSEZ). Rainfall data from the Climate Research Unit (CRU) and the United State Geological Survey (USGS) for satellite imageries for the study areas was acquired for the period of 1981-2018 (37 years). The Interseasonal Rainfall Monitoring Index (IRMI) was used to compute the “actual “or “real” onset and cessation date of the raining season, onset of rains was taken as the pentad within which the index is greater than or equal to 1(≥1) for the first time. The Monsoon Quality Index (MQI) was also used to determine the moisture quality in the study area and the Perpendicular Vegetation Index (PVI) for the analysis of the satellite images at the IR and the NIR.  The results revealed that rainfall in the region is highly variable across the ecological zone, the lowest average rainfall is Nguru with an average rainfall of (536mm) and the highest average rainfall is Yelwa (1090.65). The Average Length of Raining Season (LRS) was between 120-140 days,   Monsoon Quality Index (MQI) was calculated to determine the quality of rainfall, rainfall in the region ranged from good with value <0.005 and to extremely poor with value >0.02. Satellite imageries analysis shows a change in vegetation dynamic over the years under review. The Normalize Difference Vegetation Index (NDVI) was used to determine the vegetation index, i.e. the vegetation vigour of the region; they were ranked from areas with poor, moderate and healthy vegetation, Maiduguri, Nguru have NDVI value of around -0.2 which signify poor vegetation with average annual rainfall of less than 600mm, Kano and Gusau with NDVI value of 0.0564 which signify moderate vegetation with average annual rainfall of around 800mm and Yelwa and Bauchi has NDVI value of around 0.826whcih shows healthy vegetation with average annual rainfall of more than 1000mm. from the research it was deduced that rainfall is a function of vegetation growth, vegetation can respond positively or negatively to increase or decrease in the quality of rainfall received in a region.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background to the Study

Rainfall and vegetation dynamics are tightly coupled; they are physically connected to each other

that cannot be easily separated from one another. Previous researches (Usman 2000, Ibrahim

2018 and Usman and Abdulkadir 2019) have shown that the amount of rainfall received by

vegetation is a major factor that determines its thickness or shallowness. Studies reported that

vegetation growth at high latitudes in some Northern hemisphere regions has increased from

1981 to the 1990s due to increase in amount of rainfall received but reverse is the case in the

2000’s (Nayak et al. 2010). Large scale changes in vegetation leaf area index are also known to

have led to shifts in temperature and precipitation patterns but these feedback mechanisms are

complex, varying greatly from location to location and over time.

Remote sensing provides a vital tool to capture the temporal dynamics of vegetation change in

response to weather (rainfall) or climate shifts, at spatial resolutions fine enough to capture the

spatial heterogeneity. Frequent satellite data products, for example, can provide the basis for

studying time-series of ecological parameters related to vegetation dynamics (Bradley et al.

2007, Gu et al. 2009, Jacquin et al. 2010 & Beck et al. 2011). Among the many available

remote-sensing data products, the Normalized Difference Vegetation Index (NDVI) has been

frequently used in vegetation dynamics studies, as this index is highly correlated with the leaf

area  index,  photosynthetic  capacity,  biomass,  dry  matter  accumulation,  and  net  primary

productivity (Wang et al. 2010a). Therefore, NDVI data are frequently used to assess spatio-

temporal changes in regional vegetation dynamics (Kang et al. 2011 & Zhang et al. 2011) in

response to changes in regional climate. Normalized Difference Vegetation Index (NDVI) is one

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of the main characteristic descriptors of vegetation cover and is widely used to monitor the

changing dynamics of vegetation cover, biomass and the ecosystem. In addition, NDVI can be

used to represent the proxy vegetation responses to climate changes since it is well correlated

with the fraction of photosynthetically active radiation absorbed by plant canopies and thus leaf

area, leaf biomass and potential photosynam thesis (Liu et al. 2012).

In  the last  30  years, there have been  many research  studies  conducted  on  the relationship

between vegetation activity and its driving factors. The global vegetation presents an obvious

greening trend, especially in middle and high latitudes of the Northern Hemisphere. Despite

great  spatial  heterogeneity,  vegetation  greenness  in  arid  and  semi-arid  regions  on  average

experienced an increase in both global and regional scale, for example, in Central Asia, Inner

Asia, Eurasia, Sahel, and Australia (Yang et al. 2014). The greening trend also appears in the

Tibet Plateau and the arid regions in northwest China.

Potential drivers of vegetation change can be divided into two categories: natural and human

factors. Natural factors include temperature, precipitation, photosynthetically active radiation,

atmospheric concentrations of CO2, etc. Human factors mainly include cultivation, afforestation,

deforestation, urbanization as well as improved agricultural management practices. Generally,

large scale variations are affected by climatic factors that represent the dominant limitation to

plant growth. Elevated air temperature and increased water availability are the dominant climatic

factors. Shule River Basin is a typical arid region in northwest China and is strongly disturbed by

human activities. Compared with related research about Shiyang River Basin and Heihe river

basin in the Hexi region, research about Shule River Basin is very limited. On the basis of

MODIS NDVI data with 250-m spatial resolution and daily meteorological data, their research

employed panel data models to analyze the vegetation dynamic and its response to climate

change from 2000 to 2015. The purpose of their research was to reveal the characteristics, trend

and spatio-temporal difference of vegetation change, and to quantify impacts of climatic factors

on NDVI, which might provide some scientific basis for the comprehensive basin management

(Yang et al. 2014).

Geographic information systems (GIS) and Remote Sensing (RS) have become critical tools in

agricultural research and Natural Resource Management (NRM), yet their utilization in the study

area  is  minimal  and  inadequate.  Utilization  of  GIS  spatial-interpolation  techniques  such  as

Inverse Distance Weighted (IDW) and Spline and Kriging interpolation techniques are some of

the  ArcGIS  application  tools  essential  for  data  reconstruction.  To  aid  in  understanding

spatiotemporal occurrence and patterns of agro-climatic variables (e.g., rainfall), accurate and

inexpensive quantitative approaches such as GIS modeling and availability of long-term data are

essential (Wang et al. 2010b).

Vegetation, the main component of the terrestrial biosphere, is a crucial element in the climate

system. Its variation is an important indicator of regional changes in ecology and environment.

Understanding the relationship between the greenness of vegetation and climate is an important

topic in global change research. It is understood that the vegetation in the Guinea Savanah Zone

of Nigeria is sensitive and fragile to the global climate change (IPCC 2007, Maiangwa et al.

2007). Therefore, this region is a key area worthy of study to provide information in relation to

global environmental changes. This explains why this study is utilizing spatial interpolation

techniques to aid in the understanding of the influence of rainfall variability on  vegetation

dynamics.

Vegetation  dynamics  is  affected  by  lot  of  factors,  rainfall  inclusive,    this  research  assess

vegetation response to early or late rainfall, quality of rainfall or the amount of rainfall received

in a location over some selected years.

Sudanano-Sahelian Ecological Zone of Nigeria is the largest ecological zone in the country

because this zone occupies almost one-third of the total land area of the country. It stretches from

the Sokoto plains through the northern section of the high plains down to the Chad Basins

(Odekunle et al. 2008). The whole zone is covered by Savanna vegetation consisting of Sudan

and  Sahel  vegetation  with  the  density  of  trees  and  other  plants  decreasing  as  one  move

northwards. These two zones are together referred to as the Sudano-Sahelian Ecological Zone

(SSEZ) (Abaje et al. 2012).

1.2       Statement of the Research Problem

In a typical tropical country like Nigeria, rainfall varies spatially as the rain-belt follows the

relative northward and southward movements of the sun. In this tropical situation of a marked

seasonal rainfall regime, variability of the onset and cessation of rain is highly significant, and its

estimation and prediction are necessary. A delay of 1 or 2 weeks in the onset is sufficient to

destroy the hopes of a normal harvest (Odekunle, 2004a) and in return have adverse effects on

vegetation dynamics. A false start of planting, encouraged by a false start of rainfall, may be

followed by prolonged dry spells whose duration of 2 weeks or more may be critical to plant

germination  and/or  growth.  For  instance,  in  1973,  the  onset  was  earlier  in  Nigeria,  which

encouraged early planting and animal migration. However, this was a false onset, resulting in

both crop and animal loss (Odekunle, et al. 2008). Although, some studies exist (Amekudzi et

al., 2015; Hachigonta, et al., 2008; Ibrahim, et al., 2017; Jiang,    et al., 2011; Oladipo and Kyari,

1993; Tadross et al., 2005) on the onset and cessation of rains, substantial number of them were

with respect to rainfed  agriculture. Similarly, while some recent studies (Alli, et al., 2012;

Sivakumar et al., 2014; Sultan and Gaetani, 2016 and   Ibrahim, et al., 2018) have shown a

general rise in the rainfall of the savanna region of Nigeria, there is rarely a study that links such

rise with vegetation response. In addition, while literature abounds documenting the rate of

desertification in the Sudano-Sahelian regions of West Africa (e.g. Kusserow, 2017, Reshma and

Roy, 2018) and the links between rainfall and crop dynamics, how vegetation responds to rainfall

changes  at  the  beginning,  at  the  end,  and  throughout  a  given  season,  is  yet  to  be  fully

investigated  in  the  Sudano-Sahelian  region  of  Nigeria.  Consequently,  there  still  exists  a

knowledge gap with respect to vegetation dynamics in relation to onset and cessation of rainfall.

There is also the need to establish whether, like in the case of crop production, vegetation

response is influenced any differently by the quality of seasonal rainfall as it is by annual rainfall

receipt.

1.3       Justification for the Study

Rainfall gives life to vegetation and other species that exist in the ecosystem. The Sudanano-

Sahelian Ecological Zone of Nigeria over the years has been threatened with the effects of

climate change and global warming as seen in the loss of vegetation and surface water resources

through desertification. It is important to note that the change in vegetation dynamics has a great

effect on climate change and global warming through land-atmosphere coupling mechanisms

that  influence  convection  processes  on  a  large  scale  (Omotosho  et  al.  2000).  This  present

research try to examine how rainfall variability, onset and cessation impacts the vegetation

response. The study is important in such a way that it will allow researchers, scientists, farmers

and pastoralists to know how the changes in vegetation affect their decision making. It is also

important to note that the study was carried out with the use of remote sensing techniques,

allowing  the  researcher  to  collect  adequate  data,  simulate  and  model  future  scenarios  and

contribute to the efforts for identifying climate change adaptation options to guide sustainable

livelihoods decisions. The research could also contribute important insights into the causative

factors of security challenges as they affect the North East and help reduce the farmers-herdsmen

clashes in the country.

1.4       Scope of the Study

The study covers the entire Sudanano-Sahelian Ecological Zone of Nigeria, with rainfall data

collected from the following meteorological stations; Sokoto, Gusau, Yelwa, Katsina, Kano,

Nguru, Bauchi and Maiduguri. The years under study is from 1981 to 2018 (37 years). Satellite

images covering the entire Sudano Sahelian Ecological Zone was acquired for the entire years

under study. The research was carried out using integrated Remote Sensing (RS) and Geographic

Information  System  (GIS)  Techniques.  The  factors  considered  in  the  study  include,  land-

use/land-cover changes, rainfall intensity, rainfall spread throughout the year, onset and cessation

of rainfall, temperature, and other factors as it affects vegetation response to rainfall variability.

1.5       Aim and Objectives of the Study

The aim of the study was to examine vegetation response to rainfall variability in the Sudano-

Sahelian Ecological Zone of Nigeria. The specific objectives for the study are to:

i.      Examine vegetation response to early or late onset of rainfall in the study area

ii.      Capture the spatiotemporal patterns of vegetation response to quality of rainfall across the

study area.

iii.     Examine the patterns in lag relationships between vegetation response and interannual

rainfall variability.

1.6       Research Questions

In order to achieve the objective of the study, the following guiding questions or lines of inquiry

were proposed:

1.   How does the vegetation in the Sudano-Sahelian Ecological zone respond to early and

late onset of rainfall?

2.   What is the spatiotemporal pattern of vegetation response to quality of rainfall across the

study area?

3.   What are the lag relationships between vegetation and inter-annual rainfall variability?

1.7       The Study Area

1.7.1    Location of the study area

The Sudano-Sahelian Ecological Zone (SSEZ) is located in northern Nigeria between latitude

100N and 140N and longitude 40E and 140E. This is the largest ecological zone in the country

because this zone occupies almost one-third of the total land area of the country. It stretches from

the Sokoto plains through the northern section of the high plains down to the Chad Basins

(Odekunle et al. 2008). The average annual rainfall in this zone varies from less than 500mm in

the extreme northeastern part to 1000mm in the southern sub-region in only about five months in

the year, especially between May and September (Abaje, et al. 2012). The rainfall intensity is

very high between the months of July and August. The pattern of rainfall in the zone is highly

variable in spatial and temporal dimensions with inter-annual variability of between 15 and 20%

(Oladipo, 1993).

The climate is dominated by the influence of three major meteorological features, namely: the

tropical  maritime (mT)  air  mass;  the tropical  continental  (cT)  air  mass,  and  the  equatorial

easterlies. The first two air masses (mT & cT) meet along a slanting surface called the Inter-

tropical Discontinuity (ITD). The equatorial easterlies are rather erratic and relatively cool air

masses from the east in the upper troposphere along the ITD (Odekunle, 2006; Odekunle et al.

2008  and  Abaje  et  al.  2012).  The  position  of  the  ITD  is  a  function  of  the  season  with

considerable short-period fluctuations. Generally, however, it is situated well to the north of

SSEZ in July and August, thereby allowing the area to be totally under the influence of mT air

mass. It is located south of the zone from October to May, with the effect that the whole of SSEZ

is covered by the cT air mass during this period (Odekunle et al. 2008). The whole zone is

covered by Savanna vegetation consisting of Sudan and Sahel vegetation with the density of

trees and other plants decreasing as one move northwards. These two zones are together referred

to as the Sudano-Sahelian Ecological Zone (SSEZ) (Abaje et al. 2012).

1.7.2    Climate of the study area

The climate is of the dry tropical type. Rains may occur from mid-June to mid-September with

virtually no rain from mid-September to mid-June. The mean rainfall varies from 100 mm at the

border of the desert to 600 mm at the southern limit of the Sahel, in contact with the Sudanian

ecological zone (600- 1,500 mm). The peak of the rainy season is August; duration of the rainy

season varies from 1% months in the north to 3% months in the south. The number of rainy days

(>O. 1 mm) varies from 20 to the north to 60 to the south. Rainfall variability’ goes from 40% to

the north to 25% to the south.  Temperatures are high: average maximum rises to 40-42°C with

maximums of 45°C occurring rather regularly in April-May. Average minimum drops to 15°C in

December- January with absolute minimum rarely below 10°C. Potential evapotranspiration is

extremely high: 1,800-2,300 mm/yr; class A pan evaporation is 3,000 to 3,500 mm/yr. Air

humidity is extremely low during the dry season when it is almost constantly below 40% for 6 to

9  months,  dropping  to  less  than  10%  every  afternoon  from  March  to  May.  From  July to

September, average air humidity is above 70%. (Omotosho et al. 2000).

1.7.3    Geomorphology of the study area

Altitude is low, usually 200-500m above sea level, with a few exceptions such as the Jebel Marra

rising to 3,000 m at the border of Sudan and Chad. To the northern fringe of the Sahel, there are

a series of mountainous massifs. The whole area is a gently rolling country with a flattened dune

morphology. The extension of sand on the area dates back to the late Pleistocene, where a dry

period occurred between 30,000 and 12,000 BP (Ogolian) extending the Sahara some 450 km to

the south of its present limit; this period with followed by a humid phase: 10,000 to 3,000 BP

(Chadian, Nouakchottian). (Omotosho et al. 2000).

1.7.4    Soils of the study area

Soils are predominantly sandy, yellowish-red in colour and slightly acidic (5<ph<6); they are

luvic arenosols according to the FAO classification. Some black clay soils (vertisols) may occur

in depressions. Shallow soils on fossil iron pans occur on sizeable areas in the southern half of

the zone (ferric luvisols). Soils are deficient in phosphorus and nitrogen; organic matter content

in the top layers is equal to or lower than 1%; potassium is usually in sufficient supply and trace

element have rarely been reported as a problem for plant nutrition. Fertilization provokes high

responses to phosphorus and nitrogen when these two elements are provided in conjunction.

However the cost/benefit ratio of chemical fertilization is too low to make range fertilization an

economically feasible proposition. (Omotosho et al. 2000).

1.7.5    Hydrology of the study Area

Runoff is very limited and occurs on short distances to fill up ponds and small lakes which

generally last only a few weeks after the end of the rainy season. There are neither practically no

endogenous  rivers  nor  even  wadis.  Some  exogenous  rivers  play  an  important  role:  The

Kuyanbena in the Sokoto Plane, Also the Rima River Nigeria: the Logone-Chari system in Chad;

and the Nile and its tributaries in the Sudan. These permanent exogenous rivers are of paramount

importance  in  the  livestock  industry  and  agriculture.  Deep  ground  waters  are  scarce  and

boreholes yield only small quantities of discharge, with few exceptions. This is due to the

geological structure of the region, i.e., thin sedimentary layers on the metamorphic (granitoid)

basement complex of the African shield; water is at the contact and in a thin layer of weathered

metamorphic rock. (Odekunle, et al. 2008).

1.7.6    Vegetation of the study area

Vegetation is a savanna dominated by annual grasses: Ari.\tidu mrrttrbiiis, A. cdscensionis, A.

junicdutu, Schoenejeiditr gruciiis. Cer~chms ix’jior-U.SC. . prieurii, Ductyioctenium twg~ptilrm,

Erugrostis trrmuiu, Diheteropogon hugerupii, Lolrdetici togoerisis, etc., are dominant over huge

areas. Shrubs and trees are 100 to 400 per hectare with three layers: 1-3 m, 3-5 m, and 5-10 m.

The main species are Butlunites ueGqyptitrctr. H~~~hue/~c thebuicu. Commiphoru ujricunu,

Acuciu .w~~c11A. . SCIIY~II~A, . ehren hergiunu . A . tortiiis, Moeruu crussijoiiu. Guieru

.se~lt~s~(i4~1.si.As.d unsoniu digitutu, Combreturn ~ligricwls, C. ucuietrtum. C. girrtinowm, C.

ghuzuiense, C. I,iic,i.tiritliirrii. G’re\~ki tentix. G. bicoior, Scierocuqu hirreu, Pteroc*clrpll.s

iuce~~s. Buuhiniu rxjescens, Piliosrigmu reticuiutu. The Sudano-Sahelian subzone is an area of

farming and cattle raising. The main crops are millet (Pennisetum o.phoide.s), and cow pea

(Vignu sinensis), with some sorghum in retreat flooding cultivation and some cassava. Perennial

grasses are vestigial (Aristidu puliidu, A. longij- Ioru. A . sti1wide.s. Andropo,qon guyunus.

Cenchrus ciliuris, c’~~~~~..sol)c)slo)l~r~m ido.sos ( = C. cucheri) except in the most arid parts

of   the   Northern   Sahel   where   large   areas   of   steppe   vegetation   are   dominated   by

Punicam~urgidum and Lusiunrs hirsutirs. Since perennials are only found in the driest and the

wettest  places,  it  is  believed  that  the  present  annual  grass  vegetation  is  a  fire  disclimax.

(Odekunle, et al. 2008).

The Sudano Sahelian  receives 400 to 600 mm of annual precipitation and the rainy season lasts

3 to 4 months (rainy season is understood as the period where rain is equal to or greater than 0.5

PET or 50 mm/month). Vegetation is characterized by a Combretuceue savanna where trees and

shrubs from this family are dominant: Combretum glutinosum, C. nigricuns, C. uculeutum, C.

micrunthum,  C.  ghuzulense,  Girieru  senegulensis,  etc.  ;  other  characteristic  species  are:

klerocuryu birrea, Bombux costutum, Sterculiu setigeru, Grewiu bidor. Characteristic dominant

grasses   are   the   annuals:   Diheteropogon   hugerupii,   Loudetia   togoensis,   Andropogon

yseudupricus. Ctenium eleguns, Pennisetum pedicellutum, Schizuchyrium exile; some perennials

are found in wetter places: Alldropogon guyunus, Punicum unubuptistum. (Omotosho et al.

2000).

1.7.7    People and land use

The Sudano-Sahelian subzone is an area of conflict between the nomads and settled farmers. As

in many arid zones in the world, the competition between rangelands and cropland results in

more and more rangeland being cleared in order to meet the food requirements of a fast growing

population, since population growth is of 2.5 -3.0% per annum in the settled communities and

1.5-2.0% among the pastoralists; i.e., the population of settled farmers doubles every 23-28 years

whereas the nomadic population doubles in 35-46 years, in the wrong assumption that nomads

do  not  become  settled  farmers.  An  unknown  number  of  them  obviously  do,  in  particular consecutive to the 1970-73 drought. The people are predominantly Fulani, Kanuri and Hausa

Speaking population (Omotosho, et al. 2000). This ecological zone house 25% of the Nigerian

population and supports three-quarter of cattle population, about 75% of the goats and sheep, and

almost all the donkeys, camels and horses found in the country. Major cereals such as cow peas,groundnut and cotton are the main crops grown in the region (Odekunle et al. 2008).

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