The study watershed was delineated to seven sub-watersheds with coded as SW1 to SW7. Predominantly it is dendritic to sub-dendritic drainage pattern which intersecting branches at acute angles. This indicates the presence of homogenous rocks and uniform soil type in mountainous and hilly area. Map of each sub-watersheds and drainage networks is located in Fig. 3.
Watershed morphometric analysis
Depending on the formulae and methods described in Table 1, the computed results of morphometric parameters are summarized in Table 2, and their implications are also discussed.
Table 2 Computed results of watershed morphometric parameters Basic parameters
Watershed area (A) is the total area projected upon horizontal plane of a watershed. It is the most important watershed characteristics since it directly reflects volume of water in a watershed. Larger size intercepts greater volume of rainfall, higher runoff and peak discharge. Sometimes, the smaller size has also recorded maximum flooding and sedimentation. This is due to other watershed morphometric characteristics like stream networks, shape and length and relief parameters. In present study, the area ranges 81.90–151.16 km2, the smallest at SW6 and the largest at SW7.
Watershed perimeter (P) is length of delineated watershed boundary. It also represents watershed size. The perimeter ranges 46.34–97.21 km; the shortest at SW6 and the longest at SW7.
Watershed length (Lb) it is the longest dimension of basin parallel to the principal drainage channel (Schumm 1956). It shows the main channel of watershed in which the greatest volume of water travels. In this regard, the length of sub-watersheds is the longest at SW7 (22.69 km) and the shortest at SW6 (16.02 km).
Watershed relief (Bh) is the elevation between outlet and the highest point on the perimeter of a watershed. Here, at the study area, elevation ranges 1200–3020 m.a.s.l.
Stream order (U) is a measure of the position of a stream in the hierarchy of tributaries. Stream ordering or stream categorization is based on the number and the type of tributary junctions, which is the initial step in morphometric analysis. The streams are given order designation based on Strahler (1964) method. The smallest finger type and not branched tributaries are nominated as first stream orders; where two first stream orders join, these are second stream orders; where the second stream orders join, these are third stream orders, and so on. It increases from upstream to downstream due to geomorphology of a watershed, and the highest order of stream segment is the main channel in which all discharges, runoff and sediment pass (Kumar and Lal 2017). The main and highest order (5th order) is shown at SW1 and SW7; and others with the 4th order.
Stream number (Nu) is the number of stream segments present in each order. The relationship between the number of streams of a given order and the stream order form an inverse geometric series (Horton 1945). It informs about surface runoff features. For example, a watershed with maximum numbers of first-order streams indicates highest permeability and infiltration rate, and erodible topography. A watershed with higher in number and order of streams is characterized by soft shale and slate rocks (Biswas 2016). In the present study, number of first stream order is highest and lowest at SW7 (68) and SW6 (33), respectively.
Stream length (Lu) is the average lengths of streams of each of the different orders in a drainage basin that tends closely to approximate a direct geometric series in which the first term is the average length of streams of the 1st order. Stream length is the highest in first-order consequently it increases as stream order increases (Horton 1945). Relatively, shorter the stream length is at area of steep slopes and finer texture, whereas longer the stream lengths are at area of lower slopes (Strahler 1964). Besides, it measures hydrological characteristics and bedrock formation of the area. Relatively, the permeable bedrock and well-drained watershed is formed a smaller in stream number and longer in stream length and the vice versa (Sethupathi et al. 2011). For the current study, the longest and the shortest stream lengths of all orders are at SW7 (158.75 km) and SW6 (85.761 km), respectively (see Table 3).
Table 3 Stream orders and numbers Derived parameters
Linear aspects
Bifurcation ratio (Rb) is the ratio between total numbers of streams in a given order (Nu) to the number of next higher order (Nu + 1) (Schumm 1956). Rb is related to the branching pattern of a drainage network which shows degree of integration between streams of various orders (Horton 1945). Lower Rb values show structurally less disturbed watersheds without any distortion in drainage pattern (Suji et al. 2015). In the present study, Rb value is the highest and lowest at SW1 (14.33) and SW6 (10.21), respectively. This implied that SW1, SW7 and SW3 are relatively more disturbed sub-watersheds than others, whereas SW6, SW4 and SW5 are relatively sustainable sub-watersheds.
Stream length ratio (Rl): is the ratio of mean stream length of a given order (Lu) to the mean stream length of the next smaller order (Lu − 1) (Horton 1945). In fact, the mean length of a stream of any given order is always greater than the mean length of a stream of the next lower order. It gives an idea about chronological developments of the stream segments and relative permeability of the rock formation in a watershed.
According to Horton (1945), two fundamental laws are related to the stream numbers and lengths of different orders in a drainage basin.
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1.
Law of stream numbers: the relation between streams number of a given order, and stream order in terms of an inverse geometric series, in which bifurcation ratio is the base. This law was checked for all sub-watersheds and the results are in line with that of Horton’s law of stream numbers (Fig. 4), where the number of streams and stream order exhibited well inverse and strong relationship with coefficient of determination (R2) ranges from 0.78 (at SW1) to 0.99 (at SW3).
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2.
Law of stream lengths: the average length of streams of a given order in terms of stream order, average length of streams of 1st order, and the stream length ratio, this law takes the form of a direct geometric series. This law was also checked for all sub-watersheds and the result deviated from that of Horton’s law of stream length (Fig. 5), where the stream length and stream order show weak relationship with coefficient of determination (R2) ranges from 0.36 (at SW1) to 0.83 (at SW6). Here, the deviation and the differences among sub-watersheds may indicate the sub-watersheds are varying in lithology (bedrocks) and presence of geological control in addition to other environmental factors, for example water erosion processes.
Stream frequency (Fs): is total number of streams per unit area (Horton 1945). Values of drainage density and stream frequency for small and large drainage basins are not directly comparable because they usually vary with the size of the drainage area. But stream frequency exhibits positive correlation with drainage density to indicate the increase stream population drainage density at the same (Suji et al. 2015). In this analysis, stream frequency is higher at SW7, SW5 and SW6; and lower at SW1, SW2 & SW3.
Drainage density (Dd) is the total length of streams per unit area (Horton 1945). It represents channels development and their spacing closeness in watershed. Here, drainage density is higher at SW7, SW6 and SW2; and lower at SW5, SW3 and SW1.
Drainage texture (Dt) is the total number of streams per perimeter of a watershed (Horton 1945). It shows the relative spacing of drainage lines. In the present study SW7, SW6 & SW4 are higher; and SW1, SW2 and SW3 are lower in drainage texture.
Length of overland flow (Lo) is roughly equal to half of the reciprocal of drainage density (Horton 1945). It is used to describe the length of flow of water over the ground before it becomes concentrated in definite stream channels. In the study area, SW5, SW3 and SW1 are higher and SW7, SW6 and SW2 are lower in length of overland flow, respectively.
Drainage intensity (Di) is the ratio of stream frequency to the drainage density (Faniran 1968). A watershed with low values in drainage density, drainage texture and drainage intensity is sensitive to flooding, erosion and landslide. In the present analysis, SW5, SW7 and SW3 are higher, whereas SW1, SW2 and SW4 are lower in drainage intensity.
Rho coefficient (ρ): is the ratio between the stream length ratio and the bifurcation ratio. It is an important parameter to relate drainage density to physiographic development which determines the storage capacity of drainage network and ultimate degree of drainage development (Horton 1945). Here, Rho coefficient is higher at SW1, SW2 and SW5, while lower at SW6, SW7 and SW3. This suggests SW1 has the highest storage capacity during floods and attenuation effects of erosion during elevated discharge.
Infiltration number (If) is the product of drainage density and stream frequency. At higher infiltration number, lower the infiltration rate and higher in surface runoff (Faniran 1968). In the present study, SW7, SW6 and SW5 are higher, and SW1, SW2 and SW3 are lower values in infiltration number. This indicates that SW7, SW6 and SW5 are relatively dominating higher values in linear aspects while SW1, SW2 and SW3 are lower characteristics.
Areal aspects
Circulatory ratio (Rc) is ratio of watershed area to area of a circle having the same circumference as perimeter of the watershed (Miller 1953). A circular watershed is the most susceptible to peak discharge because it will yield the shortest time of concentration. Lower, medium and higher values of Rc indicate young, mature and old stages of watershed development. SW1, SW2 and SW3 are lower, while SW6, SW4 and SW7 show higher values in circulatory ratio at the study area.
Elongation ratio (Re) is the ratio between the diameter of a circle of the same area as the watershed and the maximum length of the watershed (Schumm 1956). It usually ranges from 0.6 to 1.0. If the value is equal to one, a watershed is equal from all sides. Re is lower values at SW7, SW3 and SW1, whereas SW6, SW4 and SW5 have higher values of elongation ratio at the study area.
Form factor (Ff) is the ratio of watershed area (A) to the square of watershed length (Lb). Smaller the value of form factor, more the elongated watershed (Strahler 1964). A watershed with higher form factor has high peak discharge in a short period of time (Horton 1945). Here, value of Ff is lower at SW7, SW3 and SW2 while SW6, SW4 and SW5 are higher in form factor in the study area.
Lemniscate’s ratio (K) is used to determine gradient of the watershed (Chorley et al. 1957). In the present study, SW6, SW4 and SW5 have lower K values, whereas SW7, SW3 and SW2 have the higher values.
Compactness coefficient (Cc) is the ratio of perimeter of watershed to circumference of equivalent circular area of the watershed (Horton 1945). It is an independent of watershed size, but it depends on the slope. In the present study, value of Cc is lower at SW6, SW4 and SW7 whereas SW1, SW2 and SW3 are relatively higher in compactness coefficient. In general, regarding to areal and shape aspects of the sub-watersheds, SW7, SW6 and SW3 have dominated with lower values whereas SW4, SW2 and SW3 have relatively higher values.
Relief aspects
Relief ratio (Rh) is the ratio of maximum watershed relief to the maximum watershed length, which is parallel to the principle drainage line. It measures overall steepness of a watershed, and it is an indicator of erosion process and intensity on watershed slopes (Schumm 1956). Rh value is higher value at SW6, SW4 and SW2, whereas SW1, SW5 and SW7 have lower value of relief ratio at study area.
Relative relief (Rhp) has calculated by using perimeter and watershed relief (Melton 1957). In the present study, SW6, SW4 and SW7 have higher while SW1, SW2 and SW3 have lower values in relative relief.
Ruggedness number (Rn) is the product of maximum basin relief and drainage density (Strahler 1964). It combines slope steepness and length. Its higher values occur when slopes are not only steep but long as well. It has higher values at SW7, SW3 and SW6, and it has lower values at SW1, SW5 and SW4 in the study area. In general, regarding relief aspects of sub-watersheds, SW6 and SW4 are dominating the higher values whereas SW1 and SW5 are relatively lower values.
Ranking and prioritization of sub-watersheds
Linear and relief parameters have direct relationship with soil erodibility (Nookaratnam et al. 2005; Singh and Singh 2014; Sujatha et al. 2015), the highest their value shows the most erodible soil in a watershed. Therefore, a sub-watershed showed the highest value in linear and relief parameters has rated at first rank, second higher value has rated as second rank and so on; and the least value has rated at last the rank. In the contrary, areal/shape parameters have inverse relationship with soil erodibility (Javed et al. 2009; Raja et al. 2017); the lowest their value the most erodible soil in a watershed. Thus, a sub-watershed showed the lowest value in areal/shape parameters has rated at first rank, the next lower value has rated at second rank and so on, then the highest value has rated at the last rank. Compound method of averaging value was used in this study, because it has expected all morphometric parameters have equal importance for final ranking (Ajay et al. 2014; Farhan 2017). After ranking of all (seven) sub-watersheds based on every single parameter, the ranking values for all parameters of each sub-watershed have added and divided by the number of all parameters, in this case it has divided by eighteen; and then to arrive at compound value. So that, the sub-watershed with the least compound value has assigned at the highest priority and denoted by number 1, the next higher value has denoted by number 2 and so on, then the sub-watershed that got the highest compound value has assigned at the last priority number (Ayele et al. 2017; Sheikh et al. 2017; Thapliyal et al. 2017; Kumar and Lal 2017). This implies that, the highest priority indicates the greatest degree of runoff, peak discharge and soil erosion risks in that sub-watershed. Thus, it is important to plan proper land and water management practices for each sub-watersheds as per their sensitivity ranks. Eighteen morphometric parameters were selected and used for ranking and prioritizing of sub-watersheds based on their values obtained from the calculation (see Table 4).
Table 4 Computed parameters used for ranking and prioritization of sub-watersheds Figure 6 shows the final priority map of sub-watersheds. SW7, SW3 and SW4 are relatively the most susceptible to land degradation being prone to soil erosion, respectively. This is due to their inherent geomorphometric characteristics. Hence, they need immediate attention for soil and water conservation measures or practices according to the final priority.