CHARACTERIZATION AND MAPPING OF GULLY EROSION FEATURES IN TWO GEOLOGICAL FORMATIONS OF EASTERN NIGERIA USING GLOBAL POSITIONING SYSTEM (GPS) AND SATELLITE AERIAL PHOTO

ABSTRACT

With the aid of Global Positioning System (GPS) and Satellite Aerial Photo, a comparative characterization and mapping of soil gully erosion features on two geological formations were carried out in Nsukka area of eastern Nigeria. The two geological formations were Ajali  and  Mamu  formations.  The study involved  the use of base map  created  using a Geographic Information System (GIS) (GPS Track Marker) and Satellite Photo downloaded from the internet using the same GIS (GPS Track Marker). This aided the field work for erosion site study and data collection. A total of seventy (70) erosion sites with an average length of about 1606.5 meters, average width of about 64.2 meters and average depth of 8.6 meters were visited in Ajali formation. On the other hand, only nine with an average length of about 484.2 meters, average width of about 6.5 meters and an average depth of about 3.7 meters  were  visited  in  Mamu.  In  Ajali  formation,  forty  three  new  erosion  sites  were identified to add to the twenty seven old sites while in Manu formation only five new sites were identified to add to the four old sites. Three profile pits were dug in each formation to represent the soils. They are sites of Ada (Mpt1), Agu-Orba (Mpt2) and Agu-Ekwegbe (Mpt3) on Mamu Formation while Iheaka (Apt1), Ede-Oballa (Apt2) and Aku (Apt3) were sites on Ajali Formation. The soils from the pits were sampled and analyzed for some physical and chemical properties. The properties were, colour, texture, soil reaction, organic matter, exchangeable bases, total nitrogen and available phosphorus, exchangeable acidity, cat ion exchange capacity, and aggregate stability. There was a significant difference in the value of gully length and width while the depth was statistically the same. There was a positive significant correlation between length and width (r = 0.409), depth and width (r = 0.862), but non significant correlation between length and depth (r = 0.188) in Mamu Formation, while a positive and significant correlation was found between length and depth (r = 0.635), length and width (r = 0.578), depth and width (r = 0.689) in Ajali formation. The results of the soil percentage state of aggregation (PSA) and percentage aggregate stability (PAS) was low at both soils. There was no significant difference between their mean weight diameters (MWD). Their low MWD values (1.1mm) in the soil of Mamu formation and 1.2mm in the soil of Ajali formation were indication that the soil were highly susceptible to erosion. Both soils of the studied area have low silt content (8% in the soil of Ajali and 5% in the soil of Mamu), but moderate to high fine sand values (18% in Ajali and 49% in Mamu). These could be one of the factors promoting the soil erodibility. At micro level,  the  water/calgon  dispassion  ratio  (DR)  was  very high.  There  was  no  significant difference in the chemical properties of the soils of the two underlying geological formations using t-test analysis. Gully erosion affected both Mamu and Ajali formations and led to loses to all the soil nutrients. Low soil pH due to heavy rainfall and the acidic nature of the underlying geology (false bedded sand stones and coal measures) and possible acidic precipitation affected the soil structure and promoted erodibility. The organic matter content of the soils      was  generally  low  (5.8g/kg  in  Ajali  Formation  and  4.3g/kg  in  Mamu Formation). The total nitrogen values were low (average of 0.1g/kg in the soil of Ajali and 0.07g/kg on the soil of Mamu formation) .The effective cat ion exchange capacity (ECEC) (cmol kg-1) values were also very low. The available phosphorus (mg/kg) was very low (6.6m/kg in Mamu formation and 6.7m/kg in Ajali formation) compared to the critical value 8-15m/kg. All these signify low soil fertility status partly due to severe land wash by soil erosion. This is getting worse due to anthropogenic effects on the soil cover (deforestation) and soil disruption due to sand and stone excavation. To this, effort is urgently needed to rescue the inhabitants of the agricultural areas such as Agu-Ukehe, Agu Ekwegbe, Agu- Orba,  Imilike-Agu,  Ezimo  and  Obollo-Eke  where  threatening  gullies  were  identified. Farmers should be encouraged to practice conservation tillage and use more organic manure as against inorganic fertilizer. These should be reforestation especially at the eastern aspect of the Ajali formation where the soil structure is becoming poorer year after year. Government should set-up a task-force to control sand and stone excavation which was identified as part of the major initiator of gullies in the studied area.

CHAPTER ONE

INTRODUCTION

Land degradation was a significant global issue during the 20th century and remains of high importance in the 21st century as it affects the environment, agronomic productivity, food security, and quality of life (Eswaran et al., 2001). No soil phenomenon is more destructive worldwide than the erosion caused by wind and water (Brady and Weil, 1999). Soil erosion remains the world’s biggest environmental problem, threatening sustainability of both plant and animal in the world and over 65 percent of the soil on earth is said to have displayed degradation phenomena as a result of soil erosion, salinity and desertification (Okin, 2002).

The damage done to our soil by erosion has brought damage to agricultural land which   is now becoming limiting in farming; while homes, many highways, electric and telephone lines which cost billions of naira to build, are all at the mercy of erosion in many parts of Nigeria (Asadu, 1990a). Plaster (1992) observed that over the past 40 years, a stream of technological improvements, including fertilizer and improved crops varieties, have masked the effect of erosion on productivity.

Wikipedia (2008) documented that approximately 40% of the world’s agricultural land is seriously degraded and a large area of fertile soil is lost every year because of drought, deforestation and climate change. Since the late 1960’s, nearly one-third of the world’s arable land has been lost to erosion and continues to be lost at a rate of more than 10 million hectares (25 million acres) per year.  They added that in Africa, if current trends of soil degradation continue, the continent might be able to feed just 25% of its population by 2025. According to Brady and Weil (1999), the degraded productivity of farm, forest, and range land tell part of the sad erosion story while the soil particles washed or blown from the eroding areas are subsequently deposited else-where-in nearby low-lying landscape; in streams; or in down streams reservoirs, lakes and harbours. Wikipedia (2008) maintained that such lands will end up being waste lands especially under heavy population and mismanagement.

The World Bank (1990) recognized three main environmental problems facing Nigeria: soil degradation and loss, water contamination and deforestation. In addition, six others (problem areas) were specified: gully erosion, fishery loss, coastal erosion, wildlife and biodiversity losses, air pollution and the spread of the water hyacinth. According to them, gully erosion contributes to each of the three main problems and causes damage with an annual cost to the nation (Nigeria) estimated at $100 million in 1990. In Nigeria, FGN (1997) recorded an estimate of over 90% of the land mass under severe interrill, rill and gully, with the severest gully erosion occurring on 80% of Nigeria’s total land area.

In Southeastern Nigeria, Akamigbo et al., (1987) reported that the worst hit area by gully erosion in Enugu, Anambra and Ebonyi States (former Anambra state) include the former Aguata, Nnewi, Njikoka, Ihiala, Udi, Awka, Idemili, Ezeagu, Oji River, Isi-Uzo and Onitsha Local Government Councils.  As at then, the land area engulfed by gullies in these states were estimated to cover about 10% of the total land mass of the states and this is approximately 176, 750 ha (Akamigbo et al., 1987).

A lot of research interests on erosion and its control have amply been demonstrated over the years by various groups, individuals and stakeholders. The Food and Agriculture Organization of the United Nations summarized attempts to check gully erosion in eastern Nigeria, from the establishment of the Udi Forest Reserve in 1918 to the formation of the Anambra State Task Force on Soil Erosion Control in 1990 (FAO, 1990). In general, these initiatives were “top down” in design and yielded some success, especially in vegetation established, but largely unsuccessful and expensive engineering solutions (Akamigbo et al., 1987).

According  to  Ihediwa,  (1998)  soil  erosion  is  influenced  by  many  pedogenic processes and their interactions with climate and management systems. He added that processes governing soil erodibility in the eastern part of Nigeria are not well understood and so more research is required to understand the principles influencing it. Ihediwa (1998) recorded that with continuous intensive cultivation and ever increasing emphasis on urban development; soil vulnerability to erosion is likely to increase. He pointed out that soil erosion is now becoming a national problem; the first stage in solving the problem includes the identification of potential risk areas, which requires detailed studies and evaluation of the  soil  properties,  land  use  and  strategies  as  they  influence  soil  erosion  in  various geological formations.

Asadu (1990a) emphasized that the cost of the survey is often far less than the benefit accruing from the results. He insisted that soil survey is a capital intensive activity which is often considered too expensive by government and individuals to embark upon. This ugly situation according to him has led many nations and individuals to focus their attention and effort towards ameliorating degradation problems instead of eradicating it.

The use of grid technique of soil survey as employed by most Nigerian soil scientists is a major factor discouraging the involvement of many soil scientists in the crusade against soil erosion (Asadu, 1990a). Reacting to this, Brady and Weil (1999) proposed that Global Positioning System (GPS) is an obvious prerequisite for delineating the location of a soil body in  the field.  According to  them,  the soil surveyors  will  be aware of where they themselves are located as they traverse a landscape and thus can take advantage of satellite technology to identify precise locations anywhere in the world.

This  is  done following  the fact  that  when a  location  is  required,  the  GPS  unit displays the coordinates and stores the coordinates for geo-referencing (Turenne, 1996).

In this study, emphasis was laid on erodibility of the soil in relation to its geological formations. The general objective of this work was to identify, characterize and map the erosion gullies in two geological formations in Eastern Nigeria with the help of the Global Positioning System (GPS) for proper land use planning, erosion control and prevention and academic research development. The specific objectives were to:

a.   compare  the  contributions  of  two  geologic  formations  to  the  erodibility  of  the overlying soils.

b.   provide a composite soil erosion map of the study area.

c.   proffer preventive and control measures for the identified erosion types d.   relate erosion phenomenon in the two geological formations to the soil physical and chemical properties.

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