Morphometric Analyses of Osun Drainage Basin, Southwestern Nigeria

This study evaluated some morphometric parameters with a view to assessing the infiltration potential of Osun Drainage Basin, Southwestern Nigeria. Input data were derived from SPOT DEM using ArcGIS 10.3 platform. The basin has an area extent of 2,208.18 km2, and is drained by 1,560 streams with total length of 2,487.7 km. Drainage Texture (0.52), Stream Number (1,560), Total Stream Length (2,487.7 m) and Main Stream Length (119 m) indicate that larger percentage of annual rainwater would leave the basin as runoff. Infiltration Number increases with increasing Stream Frequency (r = 0.95) and Drainage Density (r = 0.78). Length of Overland Flow increases with decreasing Drainage Density (r = -0.83), Stream Frequency (r = -0.51) and Infiltration Number (r = -0.45). Regression analysis show that Stream Frequency accounts for 97.43% of the strength of the overall regression model. Thus, Stream Frequency is a strong variable that can solely give meaningful explanation of infiltration potential. However, Basin Perimeter, Length of Overland Flow and Drainage Density also have significant influence on infiltration potential at varying degrees. The overall relationship explains 93.4% of the regression plain. Thus, Stream Frequency, Basin Perimeter, Length of Overland Flow and Drainage Density constitute a set of strong variables that can predict Infiltration Number and consequently, give meaningful explanation to infiltration potential within a basin. The study concluded that infiltration potential is moderate within Osun Drainage Basin as suggested by the mean Infiltration Number.


Introduction
Drainage basin can be defined as a geographically delimited finite area on the earth surface that is drained by a network of streams through a single pore point (Akinwumiju, 2015).Drainage basin is an ideal unit for the interpretation and analysis of fluvial originated landforms where they exhibit an example of open system of operation.Thus, a drainage basin is a fundamental unit of virtually all catchment-based fluvial investigations.The continuous interaction between climate and geology often result to the evolution of landform pattern across a given basin, which can be qualitatively (morphology) and quantitatively (morphometry) analyzed.This topographic expression is known as terrain analysis (Jones, 1999;Obi-Reddy et al., 2002).Terrain analysis is the study of elements relating to the geometric form, the underlying materials, geomorphogenesis and the spatial pattern of landforms (Schmidt and Dikau, 1999).Early studies on terrain analyses were mostly qualitative in approach, which were devoid of numerical analysis of drainage basin (Gregory and Walling, 1973;Ajibade et al., 2010).As a result, detailed understanding of drainage evolution as well as the mechanics of surface runoff was lacking (Ajibade et al., 2010).However, notable scientific approaches to terrain analyses were evident in the literature as far back as 17th Century (Penck, 1894(Penck, , 1896;;Passarge, 1912).Since its introduction by Horton (1940), morphometric analysis has been providing elegant description of basin-scale landscape as well as quantitative parameterization of the earth surface (Easterbrook, 1993;Ajibade et al., 2010).Usually, morphometric analysis is undertaken in many hydrologic investigations such as groundwater potential assessment, pedology, water resource management, flood control, environmental impact assessment and pollution studies among others (Jayappa and Markose, 2011).Furthermore, morphometric analysis could be undertaken with the aim of assessing the impacts of tectonic activities across a drainage basin (Hurtex and Lacazeau, 1999;Sinha-Roy, 2002;Singh, 2008;Walcott and Summerfield, 2008).Thus, morphometric parameters have earlier been observed as crucial indices of surface processes within a given basin.Consequently, these parameters have been determined and analyzed in many geomorphological and surface hydrological studies such as sediment deposition, flood parameterization as well as the evolution of basin morphology (Jolly, 1982;Adejuwon et al., 1984;Anyadike and Phil-Eze, 1989;Lifton and Chase, 1992;Moglen and Bras, 1995;Chen et al., 2003;Haung and Niemann, 2006).More recently, morphometric analyses have been playing a major role in modeling of surface processes such as soil erosion and flooding (Nogami, 1995;Singh et al., 2008;Ajibade et al., 2010;Sumira et al., 2013).
Until recently, scientists usually rely on data garnered from field measurements and or extracted information from existing topographic maps as major inputs in morphometric analyses.Currently, remotely sensed data and Geographic Information System (GIS) have gained recognition as preferred data source and analytical platform for morphometric analyses respectively.For example, multi-resolution Digital Elevation Models have been extensively utilized in various morphometric analyses (Nag and Anindita, 2011;Somashekar and Ravikumar, 2011).Today, many GIS platforms are embedded with various types of morphometric-specific algorithms that enable scientists to determine many morphometric parameters automatically, thereby increasing efficiency as well as reducing rigor and time (Schmidt and Dikau, 1999).Recently, a comprehensive inter-disciplinary-based groundwater potential assessment was undertaken within Osun Drainage Basin, involving terrain analyses.In this study therefore, we present and analyze the adopted morphometric parameters with the aim of evaluating the geomorphometric characteristics; particularly in relation to infiltration potential of the basin.

The Study Area
Osun Drainage Basin (ODB) lies within Latitudes 7°35' and 8° 00' north of the Equator; Longitudes 4°30' and 5°10' east of the Greenwich Meridian; in the forested undulating Yoruba Plain of Southwestern Nigeria (Figure 1).Osun Catchment extends from the upland area of Ekiti State to the low lying area of Osun State, covering 21 Local Government Areas with projected population of 6.2 million as at December, 2014 (Akinwumiju, 2015).ODB is a watershed that is drained by a sixth order river network, comprising various perennial rivers that take their courses from Ekiti-Ijesa mountainous region.The basin constitutes the upland northeastern watershed, which is a major donor sub-basin of the much larger Osun-Ogun Drainage Basin in Southwestern Nigeria.Osun-Ogun River Network is one of the few drainage systems in the Southwestern Nigeria that empties its contents directly into the Gulf of Guinea.The climate of the study area is characterized by long rainy season from March to November.The basin lies within the Humid Tropical Climatic Zone that normally experience double maximal rainfall that peaks in July and October.Precipitation is relatively high across the basin (1,500 -1,700 mm per annum) and the only dry months are January and February.Relative humidity rarely dips below 60% and fluctuates between 75% and 90% for most of the year.In the rainy season, cloud cover is nearly continuous, resulting in mean annual sunshine hours of 1,600 and an average annual temperature of approximately 28oC.The vegetation of the study area is characterized by disturbed rainforest, light forest and patches of thick forest.Experience from change detection analysis showed that the heavily disturbed vegetation has the potential to rejuvenate under sustainable natural resources utilization and management (Akinwumiju, 2015).The study area is underlain by the Precambrian Basement Complex that is characterized by both foliated and non-foliated rocks such as quartzite/quartz schist, amphibole schist, mica schist, migmatite, porphyritic granite, biotite granite, pegmatite, granite gneiss, banded gneiss and charnockite (De Swardt, 1953;Elueze, 1977;Boesse and Ocan, 1988;Oluyide, 1988;Odeyemi et al., 1999;Awoyemi et al., 2005).A unique attribute of Osun Drainage Basin is it's been located at the heart of Ilesa Schist Belt, which is a zone of regional metamorphism that is characterized by notable geological structures such as the Efon (psammite formation) Ridge and Zungeru-Ifewara Mega Fault Line (Akinwumiju, 2015).

Analytical Procedure
This study relied on the medium resolution Digital Elevation Model (SPOT DEM, 20 m resolution) of Osun Drainage Basin that was acquired from the Office of the Surveyor-General of the Federation in Abuja, Nigeria.
Digital spatial data (such as sub-basin and river network maps) were extracted from Akinwumiju (2015).
Analyses were undertaken in three stages.The first stage involved the determination of independent morphometric variables such as basin area, basin perimeter, basin relief, stream length, basin length, basin width, maximum order of streams, and number of streams in each order.Thus, automated feature attribute extraction (Add Geometry Attributes) module was adopted to derive the independent morphometric parameters on ArcGIS

3.3
Relief Ratio

Shape
The value watershed manage as course of t     The results of the stepwise regression analysis showed that Stream Frequency accounts for 97.43% of the strength of the overall regression model (eq.5).The interpretation of this is that Stream Frequency is a strong variable that can solely give meaningful explanation of infiltration potential in the study area.However, basin perimeter, Length of Overland Flow and Drainage Density also have significant influence on infiltration potential at varying degrees.The overall relationship (eq.5) explains 93.4% of the regression plain, which is quite significant.Thus, it can be affirmed that Stream Frequency, Basin Perimeter, Length of Overland Flow and Drainage Density are strong parameters that can give meaningful explanation of Infiltration Number in Osun Drainage Basin.Therefore, infiltration potential can be predicted based on these parameters.
Table 9 presents the values of some morphometric parameters for the present study area (ODB) and Calabar Drainage Basin in the South-southern Nigeria (Eze and Efiong, 2010).The values of Elongation Ratio, Circularity Ratio and Form Factor computed for the two basins revealed that they are both relatively elongated, which implies that the basin are at advanced stage of landform development.However, based on the classification of Chow (1964), these basins have the tendency of becoming more elongated in the process of time as fluvial processes proceed.Moreover, the values of Area-Perimeter Ratio showed that ODB has higher potential to expand in the process of time.

Conclusion
This study has attempted to examine the morphometric characteristics of Osun Drainage Basin, Southwestern Nigeria, with a view to assessing its infiltration potential.Several parameters were determined and analyzed in order to have in-depth knowledge of the geomorphometric features as well as the infiltration potential of the study area.The study shows that the drainage network of the study area is partially structurally controlled.ODB tilts southwestward and the meandering main channel reflects the evidence of geological disturbance along its course.Results reveal that the basin is at advanced stage of landform development with the tendency to become more elongated in the process of time.Except for Length of Overland The study concluded that the basin's infiltration potential is moderate as suggested by the value of Infiltration Number.However, there is the need to examine the characteristics of the basin's vadose zones as well as the aquifers, which are the major determinant factors of groundwater percolation and accumulation. Fig = Bifurcation ratio, Nμ = No. of stream segments of a given order and Nμ +1= No. of stream segments of next higher order.= Total stream length of order 'μ' Nμ = Total no. of stream segments of order 'μ' RL= Lsm / Lsm-1 Where, Lsm=M ean stream length of a given order and Lsm-1= M ean stream length of next lower = Channel length (Kms) and Lv = Valley length (Kms) Basin Area (A) Area from which water drains to a common stream and boundary determined by opposite ridges S trahler (1969) Dd = Lμ/A Where, Dd = Drainage density (Km/Km2), Lμ = Total stream length of all orders and A = Area of the basin (Km2).Fs = Nμ/A Where, Fs = Stream frequency.Nμ = Total no. of streams of all orders and A = Area of the basin (Km2).Dt = Nμ /P Where, Nμ = No. of streams in a given order and P = Perimeter (Kms) Rf = A/Lb 2 Where, A = Area of the basin and Lb = (M aximum) basin length Horton (1932) Re= √A /π / Lb Where, A= Area of the Basin (Km 2 ) Lb=M aximum Basin length (Km) Rc = 4πA/ P 2 2.7 Where, A = Basin Area (Km 2 ) and P= Perimeter of the basin (Km) Or Rc = A/ Ac Where, A = Basin Area (Km2) and Ac = area of a circle having the same perimeter as the basin 3 Relief M orphometric Parameters C g = C c -E pp Where, C c = Channel Crest and E pp = Elevation of Pour Point R b = E h -E bm Where, E b = Highest Elevation of Basin and E bm = Elevation of Basin M outh R r = R b /L b Where, R b = M aximum Basin Relief and L b = M aximum Length of the Basin Rn = R b D d Where, R b = Basin Relief and D d = Drainage Density 4 Tectonic M orphometric Parameters (h/H):(a/A) Where, h = Lower Interval Elevation -Basin Elevation, H = Basin Relief, a = Area above bottom of Interval and A = Basin Area T = Da/Dd Where, Da = the distance from the main stream channel to the midline of its drainage basin and Dd = the distance from the basin margin (divide) to the midline of the basin AF = 100 (A r /A t ) Where, Ar = Area of the basin part to the right of the main drainage channel and At = Area of the entire basin.LP = The Graph of D c (X axis) and E c (Y axis) Where, E c = Elevation Values along main Drainage Channel and D c = Distance (in kilometer) along main Drainage Channel Figure 4 gmatite, BG = t interlayer wit nel -Traverse b. Predictors: (constant), Stream Frequency, Perimeter c. Predictors: (constant), Stream Frequency, Perimeter, Length of Overland Flow d.Predictors: (constant), Stream Frequency, Perimeter, Length of Overland Flow, Drainage Density Y = -1.570+ 2.800X 1 ………………………………………………………………(2) (R = 0.95; R 2 = 91.0%;SE = 0.92) Y = -1.767+ 2.854X 1 + 0.009X2………………………………………...................(3) (R = 0.96; R 2 = 91.6%;SE = 0.89) Y = -2.599+ 2.964X 1 + 0.010X2 + 1.540X3……………………………………….(4)(R = 0.96; R 2 = 91.9%;SE = 0.88) Y = -7.321+ 2.456X 1 + 0.009X2 + 5.774X3 + 2.810X4………………………... .(5)(R = 0.97; R 2 = 93.4%;SE = 0.80) Where, X 1 = Stream Frequency, X 2 = Perimeter, X 3 = Length of Overland Flow, X 4 = Drainage Density Flow and Drainage Density, other parameters (Basin Area, Basin Perimeter, Stream Number, Drainage Texture, Stream Length, Stream Frequency and Infiltration Number) vary heterogeneously across the sub-basins.Basin Order, Basin Area, Basin Perimeter, Stream Number, Drainage Texture and Stream Length exhibit positive and significant associations with one another.Infiltration potential-related parameters (Length of Overland Flow, Drainage Density, Stream Frequency, and Infiltration Number) do not exhibit significant association with other basin-scale morphometric parameters in the study area.Stream Frequency exhibits weak association with Basin Perimeter and River Order.The study shows that Stream Frequency is the strongest variable that influences infiltration potential.Basin Perimeter, Length of Overland Flow and Drainage Density also have significant influence on infiltration potential at varying degrees.Thus, Stream Frequency, Basin Perimeter, Length of Overland Flow and Drainage Density constitute a set of strong variables that can give meaningful explanation of infiltration potential.Analysis reveals that larger percentage of annual rainwater would leave ODB as runoff discharge as a result of its relatively low infiltration potential.Finally, results of the correlation statistics show that Infiltration Number increases with increasing Stream Frequency and Drainage Density; and Length of Overland Flow increases with decreasing Drainage Density, Stream Frequency and Infiltration Number.

Table 1 .
Morphometric Parameters and Formula

Table 6
The interpretation is that runoff would have relatively moderate time-lag to infiltrate before it will be finally confided into main drainage channels.Drainage Density of the sub-basins range from 0.58 km/km2 to 3.27 km/km2 with a mean of 1.23 km/km2.The computed standard deviation (0.36) and coefficient of variation (29.65) indicate that Drainage Density is less heterogeneous across the sub-basins.Thus, infiltration potential is generally moderate in the study area.Stream Frequency of the sub-basins range from 0.01 to 9.09 with a mean of 1.19.The values of standard deviation (1.04) and coefficient of variation (87.49) show that Stream Frequency varies heterogeneously across the sub-basins.However, the computed mean value revealed that Stream Frequency is generally low across the study area, which is an indicator of enhanced infiltration potential.Infiltration Number of the sub-basins range from 0.01 to 29.72 with a mean of 1.77.Values of standard deviation (3.06) and coefficient of variation(173.22)show that Infiltration Number varies significantly across the sub-basins.Analysis indicates that infiltration potential is high in 44% of the sub-basins (with IN < 1) while 32% of the sub-basins was adjudged to be of moderate infiltration potential (with IN ranging from 1 to 2).Analysis showed that infiltration potential was heterogeneously low in 24% of the sub-basins with Infiltration Number ranging from 2 to 30.However, the computed mean indicates that infiltration potential is generally moderate in the study area.The correlation matrix of the morphometric parameters is presented in Table7.Results reveal that Basin Order exhibit positive and strong relationship with basin area, basin perimeter, stream number, Drainage Texture and stream length with correlation values of 0.72, 0.81, 0.70, 0.77 and 0.74 respectively at α = 0.01.Results showed that Length of Overland Flow exhibit inverse but significant relationship with Drainage Density, Stream Frequency and Infiltration Number with correlation values of -0.83, -0.51 and -0.45 respectively at α = 0.01.In this case, when the Length of Overland Flow increases, Drainage Density, Stream Frequency and Infiltration Number will decrease.The interpretation of this is that high Length of Overland Flow is an indicator of high infiltration potential.Results show that Infiltration Number exhibits positive and significant relationship with Drainage Density and Stream Frequency with correlation values of 0.78 and 0.95 respectively at α = 0.01.Thus, Infiltration Number increases with increasing Drainage Density and Stream Frequency and decreasing Length of Overland Flow in the study area.This is expected since Infiltration Number is function of Drainage Density and Stream Frequency.Results also showed that Stream Frequency exhibits an inverse but weak relationship with Basin Perimeter and Basin Order with correlation values of -0.23 and -0.20 at α = 0.05.Thus, Stream Frequency decreases with increasing Basin Perimeter and Basin Order.However, these associations are weak and might not hold.Results reveal that Length of Overland Flow, Drainage Density, Stream Frequency and Infiltration Number do not have any relationship with Basin Area.The above facts imply that Infiltration Number is controlled by Stream Frequency, Drainage Density and Length of Overland Flow in the study area.And that it (Infiltration Number) does not depend on basin area and basin order.The relationship between morphometric parameters and Infiltration Number is presented in Table8and explained by the equations that follow.

Table 8 .
Relationship between Morphometric Parameters and Infiltration Number

Table 9 .
The values of some Morphometric Parameters of Osun Drainage Basin and Calabar Drainage BasinThe values of Drainage Density, Stream frequency and Length of Overland Flow showed that infiltration potential is higher in Calabar Drainage Basin compared to Osun Drainage Basin.This is expected as the former is located within the sedimentary environment while the latter is located within the Basement environment.The values of Relief Ratio suggest that the basins are located in environments of contrasting topographic characteristics.While the relief of Calabar Drainage Basin is observed to be relatively gentle, the relief of ODB is characterized by extreme topographic high and topographic low.Consequently, infiltration potential would be higher in Calabar Drainage Basin as surface runoff would have more time to infiltrate compared to ODB where surface runoff is relatively rapid.In the same vein, the values of Drainage Texture, Stream Number, Total Stream Length and Main Stream Length recorded for the basin indicate that larger percentage of annual rainfall would infiltrate within Calabar Drainage Basin while contrastingly, larger percentage of annual rainwater would leave ODB as river discharge as a result of the basin's relatively low infiltration potential.