1 Introduction

Morphometric analysis consists in the quantitative evaluation of the characteristics of the shape of the earth surface and of any landform unit. This is the most common technique in basin analysis, since the morphometric shape is an ideal area unit for the interpretation and analysis of fluvially originated landforms where they show an example of open systems of operation.

The composition of the stream system of a drainage basin is quantitatively expressed with stream order, drainage density, bifurcation ratio and stream length ratio (Horton 1945). A major emphasis in geomorphology has been given aiming the development of quantitative physiographic methods and to describe the evolution and behaviour of surface drainage networks (Horton 1945; Leopold and Maddock 1953; Abrahams 1984).

Morphometric analysis is very useful in drainage basin evaluation, silt erosion control, flood frequency analysis, watershed prioritization, natural resources management and conservation. Analysis of morphometric parameters of a drainage basin and its associated stream network show its hydrological behaviour (Pophare and Balpande 2014). An integrated set of dependent and independent variable shapes controls the watershed characteristics over time (Aisuebeogun and Ezekwe 2013). Quantitative parameters of drainage network of the basin have a very helpful function in the definition of the hydrological model, watershed prioritization, natural resource management and rehabilitation (Choudhari et al. 2018). In many regions, most of the basins are either ungauged or difficult to access, so study on geomorphology of basins becomes much more important (Khare et al. 2014).

Morphometric analysis and prioritization of river drainage basin using Remote Sensing (RS) and Geographic Information System (GIS) techniques have been attempted by a number of researchers (Agarwal 1998; Akram et al. 2009; Prafull et al. 2012; Gajbhiye et al. 2014). These studies allowed to conclude that the remote sensing and the geographic information system are the most effective tools for analysing land use, morphometric analysis and controlling soil erosion and to take measures to preserve the soil from further erosion. In this context, the current study, intends to analyse the morphometric parameters of Welmal Watershed, from Ganale-Dawa River Basin (Ethiopia), aiming at the conservation of soil erosion, avoiding its erosion and defining the prioritization of sub-watersheds. This information will be of great importance in the definition of a hydrographic basin sustainable development plan for this region and to prevent soil erosion.

2 Description of the study watershed

2.1 Location

Welmal watershed found in Southeastern part of Ethiopia, in Oromia regional state, Bale Zone at about 423 km of Addis Ababa. The catchment situated in Genale-Dawa basin at the uppermost portions of the Web Sub-watershed, which is one of the Sub-watersheds of Genale-Dawa Basin. It is located between 5° 30′ 00″ N–7° 70′ N latitudes and 39° 00′ 00″ E–41° 00′ E longitude as shown in Fig. 1. It covers a total drainage area of 506.32 km2. Almost all of the rivers are flowing from NW to SE. Most parts of it have an arid or semi-arid climate that is prone to drought and erratic rainfall (Hurni et al. 2016).

Fig. 1
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Location map of the study area

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2.2 Climate

The northern part of the watershed has a bimodal rainfall distribution, as in most parts of southeastern Ethiopia: from February to May (the “short rains”), with a peak in April, and; from June to September (the “long rains”), with a peak in August (Degefu et al. 2017). The mean annual 80% dependable rainfall is 918.7 mm and annual Potential evapotranspiration (PET) is 8540 mm. Mean maximum, minimum and average annual temperature is 29.10 °C, 15.6 °C and 22.3 °C respectively.

2.3 Geology and geomorphology

Geomorphology of the area is represented by denuding hills, which are structurally controlled with or without complex folding and faulting, residual hills, plateaus, valley fill, pediments, and buried pediments, pedi-plains, and alluvial plains. Pediments are gently sloping smooth surface of erosional bedrock with thin detritus. Alluvial plains land surface produced by extensive deposition of alluvium, usually adjacent to a river that periodically overflows its banks. The major part of the area is characterized by an undulating topography with innumerable local depressions (Welmal Irrigation project 2006). The Welmal watershed (Fig. 2) represented by older group of rocks, granitoid gneisses and basement complex rocks. The older rocks consist of hornblende schist, biotite schist, sandstone and syenite, while granitoid gneisses are represented by hornblende gneiss, hornblende-biotite gneiss, and migmatitic gneiss. Basement complex rock includes hornblende and biotite granites (Welmal Irrigation project 2006).

Fig. 2
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Geological map of the study area

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2.4 Soil

The Welmal watershed area has eight soil types, i.e., eutric cambisols, chromic cambisols, leptosols (formerly lithosols), rendzic leptosols, developed from limestone (formerly rendzinas), chromic luvisols, pellic vertisols and chromic vertisols (Yared et al. 2020). The largest area is occupied by chromic vertisols and rendzic leptosols (altogether, 49% of the studied area) (Yared et al. 2020).

2.5 Land cover

The land cover of the watershed encompasses dense forest in the northern part and open forest in the middle of the watershed; however, the lower altitude southern part of the study area is characterized by shrub lands, grasslands and degraded bare grounds (Hurni et al. 2016).

3 Materials and methods

Remote Sensing (RS) data and Geographic Information System (GIS) applications are integrated for the morphometric analysis of watershed characteristics of the Welmal River watershed. Drainage map of the study area, considered as the base-map, has been prepared in 1:50,000 scale using a topographical map and a Digital Elevation Model (DEM), with 12.5 m of resolution were obtained from the Ministry of Water Irrigation and Energy (MWIE) of Ethiopia. Watershed boundary was delineated and drainage network map was derived as a line coverage giving unique ID for each order of stream from DEM data. ArcGIS (version 10.4.1) software was used for integrating maps and other relevant data where the attributes were allocated to generate the digital data base for drainage layer of the river basin. Prioritization of sub-watershed was done to identify the critical zone with high erosion so that appropriate conservation measures can be taken for minimizing soil erosion in the area. The methodological flow chart used for watershed morphometric analysis is shown in Fig. 3. Morphometric analysis parameter of Welmal river basin addresses the linear, aerial and relief aspects.

Fig. 3
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Flowchart of research in the study area

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The study dealt mainly with erosion risk assessment parameters and has been used for prioritizing sub-watersheds. Parameters, such as bifurcation ratio (Rb), drainage density (Dd), stream frequency (Fs), and drainage texture (Rt), have direct relationship, i.e., higher the value, more is the erodibility and ranked accordingly. Whereas the other parameters, i.e. circulatory ratio (Rc), elongation ratio (Re), form factor (Rf), basin shape (Bs) and constant channel maintenance (C), have inverse relationship, i.e. it means lower the value, more is the erodibility and ranked accordingly (Biswas et al. 1999).

Prioritization of the sub-watersheds of Welmal watershed was performed by assigning the ranks based on the highest or lowest values for linear, aerial and relief parameters as the case may be. After assigning the ranks to each parameter from nine sub-watersheds, rank values were added and averaged out to get a compound value (i.e. Cp). On the basis of the range of Cp values, the sub-watersheds were then categorized into three classes: high (2.6–3.6), medium (4.0–4.5) and low (> 4.5) priority. The sub-watershed with the highest Cp value was assigned as having the least priority.

4 Results and discussion

The morphometric analysis of Welmal river basin is performed with the obtainment of linear parameters and areal parameters.

4.1 Linear aspects

Computation of the linear aspects, such as stream order, stream number for various orders, bifurcation ratio, stream lengths for various stream orders and length ratio, is described below.

Stream order (U) the first step in morphometric analysis of drainage basin analysis is a measure of the position of a stream in the hierarchy of tributaries (Strahler 1964). According to Strahler (1964), the smallest fingertip tributaries are designated as order 1. Where two first-order channels join, a channel segment of order 2 is formed; where two of order 2 joins, a segment of order 3 is formed, and so forth. The trunk stream through which all discharge of water and sediment passes is therefore the stream segment of highest order. It was observed that, as the stream order increases, number of streams decreases. This shows that the Horton’s law (1945) is applied to the basin. Application of this ordering procedure through GIS shows that the drainage network of the study area is of a fourth-order basin. One sub-watershed (6) was in the second order, seven sub-watersheds (1–5, 7 and 8) were identified in third order, and one sub-watershed (9) was in fourth order (Table 1).

Table 1 Linear morphometric parameters of the Welmal sub-watersheds
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Stream length (Lu) is the distance measured along the stream channel from the source to a given point or to the outlet. This distance was estimated for all stream orders within a watershed and they were added together to find out the total stream length of the watershed. The maximum total length of the watershed is 571 km and that of the nine sub-watersheds are 70, 59, 51, 66, 38, 43, 83, 91 and 70 km, respectively (Table 1).

Stream length ratio (RL) is defined as the ratio of mean stream length (Lu) of segment of order u, to mean stream segment length (Lu − 1) of the next lower order u − 1 (Horton 1945). The values of the RL vary from 0.37 to 5.04 for the sub-watersheds, while it ranges from 1.65 to 2.92 for the whole basin. This parameter reveals that the sub-watershed 1 and 3 have higher surface runoff and also high stream erosion as compared to other sub-watersheds (Table 1).

Bifurcation ratio (Rb) is the ratio between the number of streams in one order and in the next. It is calculated by dividing the number of streams in the lower by the number in the higher of the two orders. Low Rb means that the drainage pattern is not affected by the geologic structures, whereas the high class signifies that the drainage pattern is controlled by the geologic structures (Rai et al. 2017). Rb characteristically ranges between 3.0 and 5.0 for watershed where the influence of geological structure on the drainage network is negligible (Verstappen 1983). Rb of the sub-basin of the Welmal watershed varies from 3.36 to 5.81 (Table 1), which indicates that the geological structure affects the drainage networks and systematic branching pattern of the streams.

4.2 Areal aspect

The areal aspects of a drainage basin reflect the influence of lithology, geological structure, climatic conditions, and denudation history of the basin. The aerial aspects consist of morphometric parameters, such as drainage density (Dd), stream frequency (Fs), elongation ratio (Re), and circulatory ratio (Rc).

Drainage density (Dd) It indicates the closeness of spacing between channels and is a measure of the total length of the stream segment of all orders per unit area (Dikpal et al. 2017). The high Dd value indicates greater runoff and as a result low infiltration rate. While, low drainage density of an area implies low runoff and high infiltration (Resmi et al. 2019). Drainage density in the study area varies between 0.68 and 2.21 (Table 2) indicating low drainage density. The low drainage density of the study area suggests that is highly permeable subsoil and coarse drainage texture (Horton 1932). The entire drainage density of Welmal River Basin is 1.13 km/km2 (Table 2).

Table 2 Areal morphometric parameters of the Welmal sub-watershed
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Stream frequency/drainage frequency (Fs) is the total number of stream segments of all orders per unit area (Horton 1932). The stream frequency is related to the permeability, infiltration capability and relief of watershed. Fs values of the sub-watersheds vary from 0.32 (SW6) to 2.81 (SW1), indicating that the sub-watersheds have lower Fs values bearing low relief and permeable sub-surface material whereas, sub-watersheds with higher Fs values show resistant or low conducting subsurface material, sparse vegetation and high relief (Said et al. 2018).

Form factor (Rf) Rf is the ratio of basin area A, to the square of maximum length of the basin boundary (Lb). It is a dimensionless property and is used as a quantitative expression of the shape of basin form (Meshram and Sharma 2017). The sub-watershed-9 has maximum Rf value of 0.45 while sub-watershed-1 has minimum value of 0.20 (Table 2). The lower the value of the form factor is, the more elongated the basin will be. The basin with a high form factor has high peak flows of shorter duration, whereas the basin with a low form factor has lower peak flows of longer duration. Therefore, sub-watershed-1 will have lower peak flows of longer duration. However, sub-watershed-9 will have high peak flows of shorter duration. For the whole watershed, Rf value is 0.037 (Table 2).

Circulatory ratio (Rc) Rc is the ratio of the watershed area and the area of circle of watershed perimeter (P). Circulatory ratio (Rc) is influenced by the length and frequency of streams, geological structures, land use/land cover, climate, relief and slope of the basin (Miller 1953). Values of circulatory ratio of all sub-watersheds are presented in Table 2. The sub-watershed-1 has minimum Rc value (Rc = 1.22), while sub-watershed-2 has maximum Rc value (Rc = 2.15). According to the Miller range, sub-watersheds are elongated in shape, with low discharge of runoff and high permeability of subsoil condition. The Rc of the study area is 0.18 (Table 2).

Elongation ratio (Re) Re is defined as the ratio of the diameter of a circle having the same area as the basin and the maximum basin length (Schumn 1956). Elongation ratio also determines the shape of the watershed and can be classified based on these values as circular (0.9–1), oval (0.8–0.9), less elongated (0.7–0.8), elongated (0.5–0.7), and more elongated (< 0.5) (Mudashiru et al. 2017). Regions with low elongated ratio values are susceptible to more erosion whereas regions with high values correspond to high infiltration capacity and low runoff. The elongation ratio of Welmal Watershed value is 0.22 as shown in Table 2. This value (< 0.5) indicates that Welmal watershed is more elongated and more susceptible to erosion.

4.3 Relief aspects

The relief aspect of a drainage basin refers to the vertical component of a drainage basin (Fig. 5). The relief aspects considered for the present study include basin relief, relief ratio and ruggedness number. The aspect-slope map generally denotes the direction to which slope faces. The value of the output raster data set represents the compass direction of the aspect (Magesh et al. 2011).The aspect map of Welmal watershed is shown in Fig. 4. On the basis of the majority of the raster cells, it is very clear that the South-facing slope mainly occur in the Welmal watershed.

Fig. 4
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Aspect grid map of Welmal Watershed

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Fig. 5
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Hilshed grid map of Welmal Watershed

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Basin relief (Bh) describes the maximum vertical distance between the highest and the lowest elevation within the basin. The basin relief plays a significant rate in landforms development, drainage development, surface and subsurface water flow, permeability and erosional properties of the terrain (Mudashiru et al. 2017). The maximum relief of Welmal River Basin is 3.67 km and minimum relief is 1.09 km (Table 3). These values indicate that the elevation of the Welmal Basin represent that the land has gentle to moderate slope.

Table 3 Relief morphometric parameters of the Welmal sub-watershed
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Relief ratio (Rh) is the dimensionless height-length ratio equal to the tangent of the angle formed by two planes intersecting at the mouth of the basin, one representing the horizontal, the other passing through the highest point of the basin (Schumm 1963). The relief ratio of the Welmal Basin is 0.035; while those of the 9 Sub-watersheds are shown in Table 3. While high values are characteristic of hill regions, low values are characteristic of pedi-plains and valley. Low relief ratios also indicate that the discharge capabilities of the Welmal River Basin in terms of groundwater potential are good (Mudashiru et al. 2017).

Ruggedness number (Rn) is the product of relief and drainage density is an index which reflects slope steepness and length (Hart 1986). Extremely high values of the ruggedness number occur when both variables are large, i.e., when slopes are not only steep but long as well (Chow 1964). The estimated ruggedness number value for the watersheds of the study area ranges from 0.93 to 7.54 (Table 3). For the whole watershed, Rn value is 4.60 (Table 3).

4.4 Prioritization of sub-watersheds

The compound values (Cp) of all nine sub-watersheds and whole morphometric parameters of Welmal watershed are calculated in Tables 4 and 5, respectively. Compound factor is computed by summing all the values of all parameters and then dividing by number of parameters. Compound parameters values are calculated and the sub-watershed with the lowest rank was given higher priority according to Vandana (2013). The prioritization was carried out by assigning ranks to the individual indicators and a compound value (Cp) was calculated. Sub-basin with highest Cp values has been low priority while those with lowest Cp values have been high priority. The sub-watersheds have been broadly classified into three priority zones according to their compound value (Cp), i.e. High (< 4.0), Medium (4.0–4.5) and Low (> 4.5) (Fig. 6).

Table 4 Priorities of sub-watersheds and their ranks
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Table 5 Morphometric parameters of the Welmal watershed
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Fig. 6
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Prioritization Map of Welmal River Basin

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High priority Highest priority indicates the greater degree of soil erosion in the particular sub-watershed and it becomes potential area for applying soil conservation measures. The two sub-watersheds, i.e. sub-watershed SW1 and SW2 with a Cp value of 3.6 and 2.6, respectively, are grouped under high-priority class should be provided with immediate soil and water conservation measures as they are likely to be subjected to maximum soil erosion.

Medium priority There are two sub-watersheds, i.e. sub-watershed SW7 and SW8 with a Cp value of 4.3 and 4.5, respectively, falling in medium priority. These sub-basins are characterized by moderate slopes, high to moderate values of drainage density, drainage texture, form factor and circulatory ratio.

Low priority The five sub-watersheds, i.e. sub-watershed SW3, SW4, SW5, SW6 and SW9 have come under the low priority with slight erosion susceptibility zone and may need agronomical measures to protect the sheet and rill erosion.

5 Conclusion

Prioritization of the watershed is one of the important aspects of planning for implementation of its development and management programs. The present study has helped to decipher information relating to the form, geomorphic and hydrologic processes of the 9 sub-watersheds of the Welmal Watershed in Bale Zone, Ethiopia. Prioritization of sub-watershed using the GIS and RS gives a clear understanding of the Soil erosion prone areas of the watershed. Results of morphometric analysis show that sub-watershed 1 and 2 are possibly having high erosion. Hence, suitable soil erosion control measures are required in these watersheds to preserve the land from further erosion. These outcomes in current research will be helpful for researchers, students, and policy makers and planners for conservation measures in the watershed management.