CHARACTERIZATION AND MAPPING OF GULLY EROSION FEATURES IN TWO GEOLOGICAL FORMATIONS OF EASTERN NIGERIA USING GLOBAL POSITIONING SYSTEM

 
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 suscep tible 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.