Map analysis of soil erodibility rates and gully erosion sites in Anambra State, South Eastern Nigeria

Romanus Udegbunam Ayadiuno; Dominic Chukwuka Ndulue; Christopher Uche Ezeh

doi:10.1515/geo-2025-0830

Article Open Access

Map analysis of soil erodibility rates and gully erosion sites in Anambra State, South Eastern Nigeria

Romanus Udegbunam Ayadiuno , Dominic Chukwuka Ndulue and Christopher Uche Ezeh

Published/Copyright: August 18, 2025

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From the journal Open Geosciences Volume 17 Issue 1

Abstract

Nigeria faces severe food security challenges caused by insecurity, flooding, and soil erosion. In the north, insecurity has prevented many farmers from accessing their farms, while in the south, especially in Anambra State, gully erosion is causing a severe menace not only to farmers and their farmlands but also to properties, and even social infrastructures in the state are not spared. Many research studies have been carried out on soil erosion and gullies in Anambra State, and solutions have been suggested, yet the menace continues. There are also conflicting reports from researchers about the actual number of gully erosion sites in Anambra State including disparities in the estimated soil erodibility in the state. This study analysed and mapped the susceptibility of soils to rainfall erosivity (high, medium, and low) and gully erosion sites in Anambra State. It reveals that the differences in erodibility result from sources of soil data and their spatial resolution. Climatically, the state falls within Koppen’s Af region, with the mean annual temperature and rainfall of around 27°C and 1,870 mm, respectively. The data requirements for this study were from primary and secondary sources. The primary data include field visits, observations, measurements, photographs, soil specimens, experimentation, and brain sensing. The secondary data sources include the Internet, related literature, soil data from the Food and Agriculture Organization (FAO), and satellite images. Geographic information system-based analyses were done using ArcGIS 10.4.1. The results are presented in two-dimensional topographic raster and vector overlaid maps of Anambra State, showing the geological formations, areas in the state with high-, medium-, and low-susceptibility to soil erodibility, and the existing gully erosion sites in the state.

Keywords: soil erodibility; gully erosion sites; terrain and anthropogenic effects; GIS-based analyses; Anambra State; South Eastern Nigeria

1 Introduction

Soil erosion has been majorly attributed to human factors [1]. Activities of man and choice of settlements have made soil erosion endemic to man. Soil erosion is a part of the processes of landforms and soil formations. Rainfall erosivity and erodibility of soils ensure the detachment, removal, and deposition among others, which form the processes of terrain modification [2,3,4]. Soil erosion should have been a good catastrophic event and an agent of terrain correction. However, due to neglect and abuse of the environment, soil erosion has become a serious environmental disaster [5]. Many studies have linked soil erosion to anthropogenic activities like unwholesome farming practices, deforestation, mining, and overgrazing among others [6], and natural phenomena like slope, soil types, and climatic actions (rainfall and winds). Ishaya et al. [2], Xiong et al. [7], and Adepoju and Adekoya [8] attributed the problems of soil erosion to population growth, which has led to the transformation of green areas (natural vegetation) to brown areas (bare land).

There have been challenges of food security in Nigeria caused by insecurity, flooding, and soil erosion among others [9,10,11]. In the north, insecurity has prevented many farmers from accessing their farms [10,11,12], while in the south, especially in Anambra State, gully erosion is causing a severe menace not only to farmers and their farmlands but also to properties, and even social infrastructures in the state are not spared [13].

The menace of soil erosion has attracted many researchers who have investigated soil erosion and gullies in Anambra State and suggested mediation measures [14], yet the menace continues. There are conflicting reports from researchers about the actual number of gully erosion sites in Anambra State and also disparities in the rate of soil erodibility in the state [15,14].

Anambra State is the state with the most gully erosion sites, particularly in southeast and Nigeria in general – about 700 gully erosion sites [15,16,17]. Fagbohun et al. [18] reported the soil erodibility to range from 0.08 to 0.19 ton ha hr MJ⁻¹ mm⁻¹, Ajibade et al. [19] calculated the range to be from 0.05 to 0.80 ton ha hr MJ⁻¹ mm⁻¹, Egbueri et al. [13] recorded a range of 0.100–0.310 ton ha hr MJ⁻¹ mm⁻¹, Ayadiuno et al. [9] analysis result showed a range from 0.091 to 0.176 ton ha hr MJ⁻¹ mm⁻¹, and Ezeh et al. [17] found the soil erodibility range to be 0.034–0.042 ton ha hr MJ⁻¹ mm⁻¹.

Soil erodibility is the proneness of the soil to be eroded. It can be determined using the erodibility factor (K factor), which is an arm of the RUSLE (revised universal soil loss equation) [20,21]. It calculates the rate at which soils/sediments are removed from a particular space. The major variables used for calculating soil erodibility are the particle size distribution (sand, silt, and clay) and organic content of the soils [18,19].

This study aimed to identify, analyse, and map the areas that are most susceptible to soil erosion based on their varying soil erodibility (high, medium, and low), and gully erosion sites in Anambra State. Based on the conflicting soil erodibility reports, there is the need for a revisit of the area and for the re-analysis, identification, documentation, and mapping of the soil erodibility, locations, and directions of gully erosion site for ease of identification and remediation through proper and suitable drainage construction.

There is also the need to consider built-up settlements and their contribution to the soil erosion menace in the state. It is evident that cemented surfaces which characterized the urban settlements do not allow for infiltration. Most of the rains that fall end up generating so much runoff. With the aid of concrete constructed drainage channels in the urban settlements, these runoffs are evacuated from their points of contact and are made to join the others as the drainage channels directed and form large volumes of runoff with high velocity capable of tearing down land surfaces and triggering gully erosion.

2 Materials and methods

2.1 Description of the study area – Anambra State

The study area is Anambra State (Figure 1). The geographical area of Anambra State is 4,844 km², with geological substrates and geomorphological features that cut across the state boundaries. The state is located at latitude 5°40′N to 6°46′N and longitude 6°35′E to 7°21′E, which climatically lies within Koppen’s Af region [22,23], with a mean annual temperature of around 27°C and a mean annual rainfall of 1,870 mm. The state has warmer temperature at the southwest and western parts, with high humidity due to the presence of water bodies – River Niger, etc., and low terrain coupled with the anthropogenic effects of built-up in the areas. The central part of the state through the north and northeast has a relatively colder temperature with low humidity (drier) as a result of elevation and limited water bodies [24]. The climatic conditions of the tropics – torrential rainfall has influence on the natural denudational processes in the region, which the state is part of. The rainy season in the region starts from March/April to October/November every year, with a break in August – little dry season which is prominent in the coast of West Africa. The region experiences dry season in October/November through March/April, with the appearance of harmattan (a dry tropical disturbance) from November/December to February/March [23,25].

Figure 1

Study area. Source: Diva-GIS, modified by the authors.

2.2 Geologic and soil substrates of the study area – Anambra State

Anambra State is found within the region of cretaceous sedimentary rocks in the north, and the shale (Imo clay) formation that underlies the northern part of the state is exposed in the Mamu/Omambala basin, through the Idemili–Niger flood plains [27,28,29]. Covering the shale (Imo clay) are the deposits of the Eocene–Miocene era, the Bende-Ameke formation, Nanka sand formation, and Ebenebe sandstones, among others. “The sands and sandstones of Nanka formation are the products of the older substrates that were later broken by the faults of the Maastrichtian and have continued to be worn down through weathering, and widened by the actions of the streams issued from these aquiferous sands and sandstones of the Eastern Upland, from where the streams in the state emanates” [30,31,32]. The actions of the surface and subsurface runoffs in the state reveal the picture of a broken-like topography, carved out by the streams (on and off streams) on the relatively impermeable Shale (Imo clay) covered by sands and sandstones (Bende–Ameke and Nanka formations) with various degrees of resistance to weathering and lost by the “water-based denudation processes” leading to the formation of scarps in some areas in the state [26,34]. The geology of Anambra State is presented in Figure 2.

Figure 2

Geology of Anambra State. Source: Okoro et al. [26] and Diva-GIS, modified by the authors.

The soils in Anambra State are arranged in relation to the various geological formations of the state [32]. The shale (Imo clay) formation also known as Ogwashi/Asaba formation covers some parts of the western side of the state and is predominantly made up of gleysols (alluvium, clay soils, and silt). This formation is the youngest of the three formations and consists of in situ decayed rock materials of Bende–Ameke in the northern transition zone and deposits of silt and fine sand in the Southwestern parts of the formation. The Bende–Ameke formation is largely made of dystric fluvisols/durisols (gravels, coarse sands, and sandstones), which covers mostly the eastern part of the state with subsections of minor formations like Nsugbe, Ebenebe, Ajali, and Mamu. The soils of Bende–Ameke formation in the transition zone of Nanka Sands formation (South of Bende–Ameke formation) in Azigbo and other parts of Nnewi South LGA and in Ozubulu and other parts of Ekwusigo LGA, majorly consist of fine sand and silt (evidence of in situ rock decay). The Nanka Sands formation cover the southern part of the state and is made up of dystric nitosols/acrisols (dark brown and red soil), which is loose and friable in nature, and very easily detachable and erodible (Figures 3 and 4) (www.fao.org, https://www.isric.org/sites/default/files/WRB). The sands in this formation are younger than that of Bende–Ameke formation and are deposits of eroded materials from the older formation – the Bende–Ameke formation.

Figure 3

DSMW soil map of Anambra State showing five classes. Source: Diva-GIS and DSMW/WRB [33], modified by the authors.

Figure 4

Soil map of Anambra State reclassified into three (3) based on DSMW reclassification. Source: Diva-GIS, DSMW/WRB [33], modified by the authors.

2.3 Data collection and analysis

The data for this study consist of primary and secondary sources. The primary data include field visits, experimentation (soil erodibility monitoring plot), and observation (Figure 5), measurements, photographs, and oral detailed reports; while the secondary data include the satellite images from Google Earth Pro (www.googleearth.org), SRTM-DEM (digital elevation model data of the shuttle radar topography mission) image data and study area shapefile (www.divagis.org), soil data from the Food and Agriculture Organization (FAO) digital soil map of the World (DSMW) (www.fao.org, https://www.isric.org/sites/default/files/WRB), and other related literature.

Figure 5

Experimental plot for the observation of rainfall erosivity impacts and soil erodibility.

The field visits include inspections and inventories of the gully erosion sites in Anambra State. These were conducted to identify the gully erosion sites’ locations. The coordinates of the gully heads were captured with a hand-held global positioning system (GPS) and were used in mapping the gully erosion sites in the state (Appendix A). An inference is drawn from the comparison of the geo-locations of the gully erosion and that of the erodibility map of the state. A map overlay of both will reveal where more gully sites are found in relation to the soil erodibility map.

The experimental plot was established to check and observe the differences in the erodibility rate of vegetated areas, porous and compacted soils. The plot was designed in a rectangular shape with planks at the edges to prevent runoff flows from outside, and it was divided into two (“A” and “B”). The plot was placed in the premises of the Federal Polytechnic Oko, Anambra State, on a land with a slope angle degrees of about 7° (Figures 5 and 6). Plot “A” (2.5 m × 10 m), an area of 25 m² was for porous soils, which was manually tilled to create a surface with high porosity level. Plot’B’ (5 m × 20 m), an area of 100 m² was continually cleared of vegetations during the experimentation process to depict bare soil. The sides of the plot were left with vegetations and observed for any change occasioned by rainfall erosivity. Runoff pipes were attached at the slope ends of both sections of the plot, fastened with bags to trap eroded sediments. These plots were set up in July 2022 and were used by Ezeh et al. [17] in their research. Further experimental observations were carried out for a period of 1 year – from January 2023 to December 2023 – for the purpose of this study. The experimental plot was being visited every 2 weeks for rainfall data collection and then aggregated into monthly data, while the sediment collection was also done on a monthly basis. The rainfall data were collected and recorded manually with the use of rain gauge placed in the plot. At the end of every month, the trapped sediments were extracted, allowed to dry at room temperature, then measured, and the data were also manually recorded. The data generated are shown in Table 1 and was used to determine the effect of rainfall on soil particle removal at the experimental plot location.

Figure 6

Reclassified soil map of Anambra State with the location of the experimental plot at Oko. Source: Diva-GIS and DSMW/WRB [33], modified by the authors.

Table 1

Data collected from the experimental plot

Month (2023)	Rainfall data (mm)	Sediments collected from Oko Plot A (kg per 25 m²)	Sediments collected from Oko Plot B (kg per 100 m²)	Sediment loss observed from vegetated areas
Jan	36.08	—	4.47	No sediment loss observed
Feb	42.56	—	3.36	No sediment loss observed
Mar	57.36	—	25.86	No sediment loss observed
Apr	63.25	—	70.16	No sediment loss observed
May	150.43	—	110.81	No sediment loss observed
Jun	177.80	—	128.25	No sediment loss observed
Jul	138.05	—	84.68	No sediment loss observed
Aug	95.62	—	35.35	No sediment loss observed
Sept	106.70	—	43.18	No sediment loss observed
Oct	275.85	—	225.95	No sediment loss observed
Nov	—	—	—	No sediment loss observed
Dec	—	—	—	No sediment loss observed
Average	102.81	—	61.01	—
Std Dev	79.86	—	65.21	—

The soil data of Anambra State were extracted from the DSMW and named after the World reference base (WRB) soil nomenclature (www.fao.org, https://www.isric.org/sites/default/files/WRB). The relevant soil data types required for soil erodibility (percentage top soils of sand, clay, silt, and organic carbon contents) were selected modified (re-evaluated to round up the value of 100%; Table 2).

Table 2

FAO soil data and the modified version

DOMSOI	FAOSOI	Sand	Silt	Clay	Orgc	Total (%)
Nd15-1a	Dystric nitosols	38.9	17.6	43.6	1.57	101.67
G2-2/3a	Gleysols	32.9	23.7	43.4	2.02	102.02
Jd3-2a	Dystric fluvisols	35.9	39.4	24.8	2.16	102.26
Nd21-1a	Dystric nitosols	38.9	17.6	43.6	1.57	101.67
Nd5-1a	Dystric nitosols	38.9	17.6	43.6	1.57	101.67

DOMSOI	FAOSOI/WRB, 2014	Sand (modified)	Silt (modified)	Clay (modified)	Orgc (modified)	Total (%)
Nd15-1a	Dystric nitosols/acrisols	38.3	17.3	42.9	1.5	100.0
G2-2/3a	Gleysols	32.2	23.2	42.5	2.0	100.0
Jd3-2a	Dystric fluvisols/durisols	35.1	38.5	24.3	2.1	100.0
Nd21-1a	Dystric nitosols/acrisols	38.3	17.3	42.9	1.5	100.0
Nd5-1a	Dystric nitosols/acrisols	38.3	17.3	42.9	1.5	100.0

There are several methods for erodibility estimation such as the original formula by Wischmeier and Smith [35], but the Williams [36,37] formula was applied and processed in ArcGIS 10.4.1 (Figures 7 and 8). The variables required are easily assessed, and the output is much comparable to the original nomograph [38,39]. Additionally, the decision and justification of applying the EPIC model in the analysis are based on the argument of Anache et al. [40] that “the equations for indirect K factor estimation fit better when the soils subjected to indirect erodibility estimation experience similar conditions to those used for model validation,” and also on the reports of many authors such as Zhang et al. [41], Wawer et al. [42], Hussein et al. [43], Zhang et al. [44], Wang et al. [45], and Ezeh et al. [17] among others who adopted the model in their various studies.

Figure 7

Diagrammatical representation of RUSLE analysis with emphasis on soil erodibility.

Figure 8

Diagrammatical representation of the soil erodibility (K factor) analysis of the RUSLE model.

The formulae for soil erodibility analyses are thus stated as follows:

(1) K factor = f sand × f clay × f orgc × f silt × 0.1317 ,

(2) f sand = 0.2 + 0.3 × esp − 0.256 × f sand 1 − f silt 100 ,

(3) f clay = f silt f clay + f silt 0.3 ,

(4) f orgc = 1 − 0.0256 × f orgc f orgc + esp [ 3.72 − 2.95 × f orgc ] ,

(5) f silt = 1 − 0.7 1 − f sand 100 1 − f sand 100 + exp − 5.51 + 22.9 1 − f sand 100 .

0.1317 is used to multiply f _sand, f _clay, f _orgc, and f _silt to convert the erodibility value (k factor) to SI units of metric ton hectare hour hectare⁻¹ megajoule⁻¹ millimetre⁻¹ (ton ha hr MJ⁻¹ mm⁻¹) [46,47].

Here, the K factor is the erodibility factor measured in ton ha hr MJ⁻¹ mm⁻¹, f _sand is the % sand (0.05–2.0 mm), f _silt is the % silt (0.002–0.05), f _clay is the % clay (<0.002), and f _orgc is the % organic matter.

Also, the locations and directions of some of the gully sites in the area were captured and overlaid on the soil erodibility (K factor) map for ease of comparison and identification.

2.4 Gully erosion site assessment

The gully erosion sites were assessed via fieldwork, which involved taking inventories of their presence and recording their locations using a handheld Garmin GPS, as shown in Table 3. By visual inspection, the gullies were assessed to determine if they are inactive, they are stabilized by intervention efforts of the state and the people, or if they are active, which imply that the intervention action was ineffective, or no attempt has ever been made to stabilize them. Additionally, inference was drawn from the soil erodibility (k factor) map when juxtaposed with the gully site maps, and by that it can be inferred if gully erosion sites are higher in locations with high erodibility rate or not.

Table 3

Gully erosion sites and locations in Anambra State

S/N	Location	Latitude	Longitude	Active	Remediation work
1	Oba Rojenny villa	6.071	6.817	Yes	Yes
2	Oba	6.067	6.833	Yes	No
3	Oba Urueze	6.088	6.858	Yes	No
4	Ojoto Mgbowo	6.077	6.87	Yes	No
5	Ojoto	6.067	6.871	Yes	Yes
6	Ojoto near CGC company	6.067	6.871	Yes	No
7	Ojoto near LG HQ	6.1	6.9	Yes	Yes
8	Amatutu Agulu	6.114	7.041	No	Yes
9	Agulu	6.109	7.039	No	Yes
10	Amako Nanka	6.044	7.08	Yes	Yes
11	Umuabiala Agulu	6.121	7.027	No	Yes
12	Ekwulobia	6.029	7.086	No	Yes
13	Eziabor (I) Oko	6.032	7.093	Yes	Yes
14	Umudim Nnewi	6.018	6.912	Partially	Yes
15	Nyaba Umudim Nnewi	6.000	6.917	Yes	Yes
16	Nnewichi	6.046	6.92	Yes	Yes
17	Ndam Nnobi	6.071	6.943	Yes	Yes
18	Umuolum Rd Alor	6.086	6.953	Yes	Yes
19	Ezioko Oko	6.04	7.074	Yes	Yes
20	Umudiana Adazi-Nnukwu	6.115	7.015	No	Yes
21	Eke Oraukwu	6.099	6.979	Yes	No
22	Abatete-Oraukwu Rd Abatete	6.107	6.957	No	Yes
23	Umunakwa Abatete	6.125	6.962	Yes	No
24	Ideani Junction	6.088	6.931	Yes	No
25	Uru_Amada Oraukwu	6.106	6.982	Yes	No
26	Abagana	6.176	6.973	Yes	No
27	Ogbunike	6.18	6.901	Yes	No
28	Isiji Oraifite	6.026	6.812	Yes	Yes
29	Ekulo Oraifite	6.011	6.828	Yes	Yes
30	Ndikpa (I) Umunze	5.95	7.235	Yes	Yes
31	Ndiabor Umunze	5.962	7.236	Yes	Yes
32	Ndikpa (II) Umunze	5.957	7.231	Yes	Yes
33	Ndikpa (III) Umunze	5.961	7.234	Yes	No
34	Ndikpa (IV) Umunze	5.963	7.229	Yes	Yes
35	Akpu Orumba north	6.042	7.217	Yes	Yes
36	Obuagu Ufuma	6.071	7.18	Yes	No
37	Ufuma Gully Head	6.072	7.185	Yes	No
38	Umugamu Abba	6.199	6.961	Yes	Yes
39	Nkwo Abba	6.217	6.974	Yes	No
40	Nkpo flyover	6.152	6.828	Yes	Yes
41	Awka high court	6.239	7.08	Yes	Yes
42	100 ft road Awka	6.238	7.07	Yes	Yes
43	Amaenyi Awka	6.225	7.855	Yes	Yes
44	Nkpor	6.153	6.828	Yes	No
45	Abagana	6.166	6.964	Yes	No
46	Amaugo Awgbu	6.083	7.087	Yes	No
47	Amawbia	6.199	7.061	Yes	Yes
48	Uruokpala Abagana	6.171	6.969	Yes	Yes
49	Akpu Abagana	6.174	6.968	Yes	No
50	Ugwunano Umunze	5.959	7.223	Yes	No
51	Amabor Abba	6.227	6.967	Yes	No
52	Amawbia	6.199	7.0614	Yes	Yes
53	Awgbu	6.083	7.087	Yes	NO
54	Amadunu Nnobi	6.043	6.943	Yes	Yes
55	Oye Udide Nnobi	6.043	6.941	Yes	No
56	Ire Ojoto	6.099	6.895	Yes	Yes
57	Ideoma Umuoji	6.092	6.877	Yes	No
58	Ideoma Umuoji Diversion	6.092	6.877	Yes	No
59	Umuoma Umuoji	6.098	6.878	Yes	No
60	Isiama-Igbo Agulu	6.083	7.065	No	Yes
61	Umuakwali Akwaukwu	6.036	6.808	Yes	No
62	Eziabor (II) Oko	6.038	7.093	Yes	Yes
63	Otta Oraukwu	6.099	6.992	Yes	No
64	Oko Polytechnic	6.060	7.102	Yes	Yes
65	Nanka (I)	6.080	7.059	Yes	No
66	Nanka (II)	6.043	7.076	Yes	No
67	Ekwulobia (II)	6.031	7.093	Yes	No

3 Results and discussion

The map representation of the soil and soil erodibility rates (K factor) are presented in Figures 9 and 10. The analyses show the soil erodibility rate of 0.02509 ton ha hr MJ⁻¹ mm⁻¹ for Gleysols, 0.02515 ton ha hr MJ⁻¹ mm⁻¹ for Dystric fluvisols/Durisols, and 0.02525 ton ha hr MJ⁻¹ mm⁻¹ for Dystric nitosols/Acrisols (Figures 9 and 10). The values differ from those of previous studies [9,10,11,12,13,14,15,16,17,18,19]. However, the values are closer to those of Ezeh et al. [17] and Fagbohun et al. [18] in terms of spatial distribution. This is partly because this study and that of Fagbohun et al. [18] used soil data from the same source.

Figure 9

Point locations of main cities and soil erodibility rates (K factor values in ton ha hr MJ⁻¹ mm⁻¹) of Anambra State. Source: Diva-GIS and DSMW/WRB [33], modified by the authors.

Figure 10

Map overlay of varied soil erodibility (K factor in ton ha hr MJ⁻¹ mm⁻¹) rate, soil types, and gully erosion sites in Anambra State. Source: Diva-GIS, DSMW/WRB [33], and field work data, modified by the authors.

The soil erodibility (K factor) analyses show that Gleysols has the lowest rate of erodibility (0.02509 ton ha hr MJ⁻¹ mm⁻¹). These soils are alluvial in nature and are found in the northern and southwestern parts of the state. They are very malleable and impermeable. The other variants of the soils are the Imo clay, fine sand, and silt of the Idemili-Niger plains. They comprise the Ogwashi/Asaba formation. The Dystric fluvisols/Durisols have a moderate erodibility rate (0.02515 ton ha hr MJ⁻¹ mm⁻¹). These types of soils are found in the northeastern through the southeastern parts of the state. The soils are mainly gravel and sandstone. According to Ezeh et al. [17], the soils of this part are denser than other parts of the state. They are generated as a result of disintegration of parent rock formation (Bende–Ameke) with other subformations such as Nsugbe, Ajali, and Ebenebe. The Dystric nitosols/Acrisols have the highest erodibility rate (0.02525 ton ha hr MJ⁻¹ mm⁻¹). These types of soils are found in the central through the southern parts of the state. The soils consist of deep weathered materials of the parent rock and the newly deposited weathered rock materials of the Bende–Ameke formation. The soils are dark brown and red in colour, loose, friable, easily detachable, and mainly consist of coarse and fine sands (Figures 9 and 10).

These findings are relatively different from those of other researchers who had carried out studies on soil loss rate, rainfall erosivity, and soil erodibility among others. There are variations in the findings of Fagbohun et al. [18], with an erodibility rate of 0.08–0.19 ton ha hr MJ⁻¹ mm⁻¹; Ajibade et al. [19], 0.05–0.80 ton ha hr MJ⁻¹ mm⁻¹; Egbueri et al. [13], 0.100–0.310 ton ha hr MJ⁻¹ mm⁻¹; Ayadiuno et al. [9], 0.091–0.176 ton ha hr MJ⁻¹ mm⁻¹; and Ezeh et al. [17], 034–0.042 ton ha hr MJ⁻¹ mm⁻¹. These variations may be caused by different soil data sources, estimation algorithms, and models adopted by the researchers. Fagbohun et al. [18] used soil data obtained from a soil map produced in 1990 by the Soil Division of Federal Department of Agricultural Land Resources and applied Williams [37,38] equations, while Ajibade et al. [19] adopted the Wischmeier and Smith [34] equation. Egbueri et al. [13] accessed the soil data from the Nigerian soil map table, while Ayadiuno et al. [9] and Ezeh et al. [17] used empirical and primary methods of soil data collection analysed in a soil laboratory and also applied Williams [37,38] equations.

The observation from the experimental erodibility plot suggests high erodibility (Table 1) that has rendered the area susceptible to erosion menace, especially gullies. Another reason for the escalating large number of gully erosion sites in Anambra State is attributed to artificial spaces (built-up areas). These gully erosion sites coincidentally are located in upland areas of the state overlain by Nanka Sands and with partial geological substrate of the Bende–Ameke formations. These locations are also densely populated which add to the pressure on the soil structure and the cohesive binding forces (Figures 11–13 and Appendix B). It was observed from the erodibility plot (Figure 5) that areas with vegetation cover having flat and porous surfaces impedes abrupt runoff generation as highly porous soils and vegetative areas are highly permeable. However, areas with little vegetation, low organic content, and very close to artificial spaces, especially built-up cities and high slope, mostly have a high erodibility rate and are very susceptible to gully erosion (Figures 11–13 and Table 3).

Figure 11

Map overlay of land use land cover change, soil types, and gully erosion sites in Anambra State. Source: Diva-GIS, DSMW/WRB [33], and field work data, modified by the authors.

Figure 12

Map overlay of the slope angle and gully erosion sites in Anambra State. Source: Diva-GIS and field work data, modified by the authors.

Figure 13

Map overlay of elevation, soil types, and gully erosion sites in Anambra State. Source: Diva-GIS, DSMW/WRB [33], and field work data, modified by the authors.

In Table 1, the data collected from the experimental plot show that in the month of January, about 36.08 mm of rain fell, which led to the dislodgement of about 4.47 kg of sediments from the 100 m² plot (Plot B). In February also, about 42.56 mm of rain fell, which led to the dislodgement of about 3.36 kg of sediments from the 100 m² plot (Plot B). The low level of sediment removal is attributed to the season. January and February fall within the dry season in Southern Nigeria. During this season, rainfall is usually low, while infiltration and potential evaporation are very high due to the absence of moisture occasioned by high solar radiation and temperature. At this time, the soils are unsaturated, and as such runoff generation is very low. In the months of March, April, May, June, July, August, September, and November, about 57.36, 63.25, 150.43, 177.80, 138.05, 95.62, 106.70, and 275.85 mm of rain fell, respectively, which led to the dislodgement of about 25.86, 70.16, 110.81, 128.25, 84.68, 35.35, 43.18, and 225.95 kg of sediments from the 100 m² plot (Plot B), respectively. Excessive sedimentation loss in March and April as recorded in the table is because of the potential and kinetic energies of the rains at the onset of rainy season, which are able to disintegrate soils that are not compacted and so lacks cohesive binding forces.

The dislodgement that occur in May, June, July, August, September, and October is attributed to heavy rainfall, and the influence of antecedent soil moisture as soil becomes increasingly wet and nears saturation cum super-saturation. Subsequently, runoffs are readily generated in those months, especially towards the end of the rainy season. It was observed that an average of 102.81 mm and a standard deviation of 79.86 mm of rain fell in the area of the experimental plot (Federal Polytechnic Oko) and removed an average of 61.01 kg per 100 m² and a standard deviation of 65.21 kg per 100 m² of sediments, respectively.

A closer examination of Table 1 shows that sediment loss is considered in terms of rate per m². No soil loss was recorded in the 25 m² plot (Plot A) due to the high porous nature of the soil which encourages high infiltration and no runoff generation. The 100 m² plot (Plot B) recorded infinitesimal values in January and February (about 4.47 and 3.36 kg, respectively). Values above 25 kg per 100 m² were recorded from March through October, which coincide with the months of heavy rainfall in Anambra State, and erosion becomes severer (noticed from the amount of sediments trapped) in the 100 m² plot (Plot B). This indicates that land areas with high porosity discourage erosion due to limited runoff generation, especially at the onset of rainy seasons. But, over time, as the land gets compacted due to the impact of rain drops, runoff generation increases and with it soil removal. Hence, bare soils that have been compacted get saturated and super-saturated quickly and become highly vulnerable to high runoff generation because of the absence of pore spaces and vegetation cover. Vegetation cover and re-growth reduce runoff generation, soil erosion, and at the same time stabilize the soil. This observation is in line with other studies’ findings that vegetation cover limits erosion occurrence [17,18].

It is worth noting that the data collected from a single erodibility experimental plot such as the one carried out in Oko by the authors cannot be used to make generalization of sediments or soil loss or rate of erodibility dynamics in Anambra State. It is merely intended to show variations in the rate of soil erodibility of an area under the same climatic conditions with the same rainfall amount but with different levels of vegetative cover, state of soil porosity, and compaction of the bare lands.

The areas on the periphery of the built-up cities with high population density receive devastating runoffs generated already in the cities’ impermeable artificial surfaces and are propelled with force by high gravity occasioned by high slope (Figures 11–13). Irrespective of the soil properties, the surface attributes of high slopes, built-ups and the associated bare grounds, runoffs with high volume and velocity will eventually overpower the soil strength, and as a result areas with low soil erodibility features but very close to covered spaces like cities also often experience severe erosion (Figures 10–13). These are the phenomena mostly responsible for and explain the concentration of large number of gully erosion sites in the central and southeastern parts of the state. It also explains the presence of gully erosion sites at the peripheries of cities like Onitsha, Nkpor, Awka, Ekwulobia, and Nnewi. Vegetated surfaces and porous soils tend to experience less erosion than compacted or bare surfaces (Figures 5 and 6). The central and eastern parts of the state with higher elevation, larger concentration of settlements (built-ups and high population density), bare surfaces, and little vegetation cover experience higher erosion than the western and northern parts with low and gentle slopes, lesser concentration of settlements (low population density), and much vegetation cover (Figures 11–13 and Appendix B). Additionally, the western part of the state with denser vegetative cover, lesser population, and artificial spaces has the lowest soil erodibility rate and therefore is less susceptible to rainfall erosivity and soil erosion in the state and the absence of gully erosion sites in the area. Gullies are a critical part of sediment yield in a catchment [48]. The first step in studying their contribution to sediment yield is identifying their locations.

4 Conclusions

Anambra State unarguably has the largest number of gully sites in Nigeria. This is as a result of the type of terrain, most of which are highly erodible. Many researchers who conducted research on erodibility in Anambra State arrived at different erodibility values. This study found out that the variations in the soil erodibility rate in Anambra State are as a result of different data sources, algorithms, and models applied. The state’s soils have low erodibility, but there is a spatial variation in its distribution across the state. The lowest is found on the western and northwestern parts. Hence, this area is the least susceptible to soil erosion, and it also has very sparse gully erosion sites. This is because most of the area is covered with dense riparian vegetation, has very low slopes, and malleable alluvial soils with high cohesive forces. The study shows that areas with high soil erodibility also correspond to high gully erosion sites. About 70% of soils in Anambra State comprises Dystric fluvisols/Durisols and Dystric nitosols/Acrisols, which have erodibility rates of 0.02515 ton ha hr MJ⁻¹ mm⁻¹ and 0.02525 ton ha hr MJ⁻¹ mm⁻¹, respectively. These areas are coincidentally on high terrains with heights ranging from about 50 to 380 m above the mean sea level and a slope angle between 25° and above 45°. The terrain characteristics of these zones and soil types (coarse soils) unfortunately attract the concentration of settlements and urbanization, leading to covered spaces such as roof tops, tarred roads, cemented compounds, and compacted open spaces among others. The artificial and anthropogenic covered spaces in these urban centers, such as Onitsha, Awka, Ekwulobia, and Nnewi, among others generate runoffs that initiate gully erosion at the periphery of the towns. These urban centres and their periphery are also the areas with the highest concentration of gully erosion in the state. This study also discovered that soil data required to conduct soil erodibility rate analysis in Anambra State are not sufficient, hence the grouping of all different types of soil variabilities in Anambra State into three groups – Gleysols, Dystric Fluvisols, and Dystric Nitosols. This reclassification may distort or alter some of the information required during analysis and thereby present results that may not be very accurate. These disparities in the soil data call for a further detailed analysis of the soils in Anambra State, Nigeria, and Africa with the aim of achieving distinct soil maps of high resolution.

The limitations observed during the study which may form the basis of further research in the direction of soil erodibility and soil gully erosion site locations in Anambra State are thus:

The experiment carried out is not sufficient to observe and establish soil erodibility rate based on land characteristics and rainfall erosivity. A single experimental plot in a location cannot be used to make a general consensus of soil erodibility rate in Anambra State. The data collected from the experimental plot for this study are not balanced and cannot also be used to adequately compare, collaborate, and contradict other findings of soil erodibility analysis in Anambra State.
The gully erosion site locations captured do not represent all the gully erosion sites in Anambra State, although the captured gully site locations were integrated into the geographic information system (GIS) to map, show, and depict the concentration of the gully erosion site locations in the state.
The soil information used for the study was obtained from DSMW, which classified the soil group in Anambra State into five. The soil group was later aggregated into three based on the merging of the “Nd” soil categories during the GIS analysis. Field observations during the study show that the DSMW soil classification does not reflect the soil reality on the ground in Anambra State.

Therefore, more studies are required with more experimental plots that will cover the whole of the state based on at least different geological formations and various soil types in the state. More data on gully erosion site locations are required to at least cover other areas being threatened by gully erosion. Finally, soil information obtained based on locally and/or manually collected soil data and analysed in a standard laboratory will provide a clearer composition of the soil types in Anambra State and consequently provide a more suitable and acceptable soil information for soil erodibility analysis.

Acknowledgments

First and foremost, we wish to express our gratitude to the Almighty God who made it possible for us to have access to those who generously supported, gave their time, energy, finance, and share ideas during the course of this study. Our appreciation goes also to members of our families for their love, members of staff of U D RUA Projects Limited who shared their field work experiences that made this study easy. We will not fail to appreciate A. T. Mozie (late) who laid the foundation for geomorphological studies that formed the basis upon which this research was carried out. We are also grateful to the editors and reviewers whose efforts and contributions have improved immensely the quality of the manuscript.

Funding information: The authors state no funding involved.
Author contributions: Conceptualization: R.U.A., D.C.N., and C.U.E.; methodology: R.U.A., D.C.N., and C.U.E.; software: D.C.N.; validation: R.U.A., D.C.N., and C.U.E.; formal analysis: R.U.A., D.C.N., and C.U.E.; resources: R.U.A.; data curation: D.C.N. and C.U.E.; writing – original draft preparation: D.C.N. and C.U.E.; writing – review and editing: R.U.A., D.C.N., and C.U.E.; visualization: R.U.A., D.C.N., and C.U.E.; supervision: R.U.A.; project administration: R.U.A. All authors have read and agreed to the published version of the manuscript.
Conflict of interest: The authors state no conflict of interest.
Informed consent statement: Informed consent was obtained from all subjects involved in the study.
Data availability statement: Data for this research will be made available upon request.

Appendix A

Gully erosion site at Ogbo, Nanka in Orumba North LGA of Anambra State. Source: Authors’ fieldwork data collection.

Gully erosion site data collection at Ogbo, Nanka gully head in Nanka, Orumba North LGA of Anambra State. Source: Authors’ fieldwork data collection.

Remediation work at the Federal Polytechnic Oko gully erosion site. Source: Authors’ fieldwork data collection.

Appendix B

Population density map of Anambra State based on LGAs and the locations of gully erosion heads. Source: Diva-GIS, Ndulue, Ayadiuno [23], and field work data modified by the authors.

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Received: 2025-03-11

Revised: 2025-06-12

Accepted: 2025-06-14

Published Online: 2025-08-18

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/geo-2025-0830

Keywords for this article

soil erodibility; gully erosion sites; terrain and anthropogenic effects; GIS-based analyses; Anambra State; South Eastern Nigeria

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