The obtained results of this study are reported under three sections. The first section deals with the basic morphometric parameters which are derived from DEM using Arc-GIS tool. The second section deals with the linear, areal, slope and landscape parameters. The third section deals with the prioritization of watershed on the basis of linear, areal, slope and landscape parameters.
3.1 Basic Morphometric Parameters
The basic parameters are very much important for evaluating linear, areal, slope and landscape indices (Hembram et al. 2018). The information about the basin area (A), basin perimeter (P), basin length (Lb), maximum height (Hmax), minimum height (Hmin) and mean height (Hmean) of the basin are derived from delineated watershed. The parameters are estimated using relations mentioned in Table 1 and obtained results are reported below in Table 2.
The watershed 4D7G7 has the biggest area while the watershed 4D7H5 is smallest one. The elevation of 4D7H7 watershed is the highest while the elevation of watershed 4D7F1 is lowest one. Basin length is the longest dimension of the basin parallel to the principal drainage line. It can be observed from Table 2 that watershed 4D7G7 have greater basin length whereas the watershed 4D7G1 have least basin length. Basin length is directly related with the erodibility. Higher the basin length, high is the erodibility, and vice versa.
3.2 Linear Morphometric Parameters
Linear parameters like stream order (N), number of streams (Ns), stream length (Ls), bifurcation ratio (Rb) and mean bifurcation ratio (Rbm) of watersheds are measured in GIS environment. The parameters are estimated as per the estimating relations given in Table 1 and obtained results of the parameters are reported below in Table 3.
3.2.1 Stream Order (N)
Stream order is the first step of morphometric analysis. Stream order is delineated in each watershed of the Bhima basin. It is delineated according to the Strahler’s stream ordering rule (Strahler et al. 1957). According to Strahler’s system of stream ordering, fingertip tributaries are taken as 1st order stream. Two 1st order streams combine to form a 2nd order stream. Two 2nd order streams combine to form a 3rd order stream and so on. It is found that stream order increases, number of streams decreases.
3.2.2 Stream Number (Ns)
The total stream segments present in a particular order of stream is called as stream number. The total stream number in a particular stream order and in each watershed of the Bhima basin is estimated by using ArcGIS 10.3. Watershed 4D7G4 have more streams while the watershed 4D7H2 have less streams.
3.2.3 Stream Length (Ls)
The total length of stream order in a particular order of stream is called as stream length. ArcGIS is used to evaluate the total stream length in a particular stream order and in each watershed of the Bhima basin. Generally, stream length decreases with increase in stream order. Longer stream length indicates flatter slope and shorter stream length indicates steep slope and finer texture. Lengthwise the watershed 4D7G7 have more stream lengths while the watershed 4D7H5 have less stream lengths. The results are tabulated below in Table 3.
3.2.4 Bifurcation Ratio (Rb)
Bifurcation ratio is derived as the ratio of the stream numbers in a particular order and the stream numbers in the next higher order. It is denoted by Rb. It shows the form of drainage basin. It is considered as the index of relief. It is a dimensionless property. Lower Rb values are the characteristics of structurally less distributed watersheds without any distortion in drainage pattern (Meshram et al. 2015). Higher bifurcation ratio implies high runoff and low permeability of the drainage basin. From Table 3 values of bifurcation ratio ranges from 0.16 to 5.46.
3.2.5 Mean Bifurcation ratio (Rbm)
Mean bifurcation ratio is calculated by the arithmetic mean of the bifurcation ratio of all the streams. It is denoted by Rbm. Higher the Rbm high is the erodibility, and vice versa. Values of mean bifurcation ratio ranges from 1.56 to 2.77. The obtained result is tabulated as per watershed of the basin in Table 4. The categorization of watersheds as per mean bifurcation ratio is shown below in Fig. 3.
In this study, stream length and stream numbers are determined using GIS tool. The drainage network pattern of Bhima basin indicates that it is a 6th order watershed consisting steams of various order. The detail of all the steam orders, stream numbers and bifurcation ratio of 19 watersheds is given in Table 3. It reveals that there are 17417 streams of 1st order, 8053 of 2nd order, 4114 of 3rd order, 3055 of 4th order, 2237 of 5th order and 304 of 6th order. It is also seen from Table 3 that highest stream lengths are found in watershed 4D7G7 (1598) and the lowest stream lengths are found in 4D7H5 (396). The total length of all streams for the watersheds is 15731 Km.
In the study area, it is seen from Fig. 3 that the watershed 4D7F5 has the highest value of mean bifurcation ratio, indicates a high structural complexity and low permeability of terrain. More the value of Rbm the more will be the soil erosion (Malik et al. 2019). Low value of Rbm means less distortion due to drainage (Hembram et al. 2018). It is observed from Fig. 3 that, eleven watersheds are of very low mean bifurcation ratio, three watersheds are of low, three watersheds are of medium, one watershed is high and one watershed is very high category.
3.3 Areal Morphometric Parameters
The parameters drainage density (Dd), stream frequency (Fs), texture ratio (Rt), length of overland flow (Lof) and constant of channel maintenance (Ccm)have direct relation with the erodibility (Kadam et al. 2019). The categorization of watersheds as per areal morphometric parameters are shown in Fig. 4.
3.3.1 Drainage Density (Dd)
Drainage density is the ratio of total channel segment lengths cumulated for all orders within a basin to the basin area. The drainage density is expressed in terms of Km/Km2. Drainage density depends on permeability of sub surface elements, type of vegetation and terrain relief. Lower Dd indicates highly permeable subsurface, good type of vegetation, low roughness, whereas opposite condition produces high Dd. (Arabameri et al. 2020). Higher drainage density represents a relatively higher number of streams per unit area and thus a rapid storm response. Higher drainage density represents conditions favorable for higher erosion from the catchment. From Table 4, values of drainage density range from 0.94 to 1.20 Km/Km2, indicating low drainage density. The low drainage density in all the watersheds suggests that the Bhima basin is highly permeable subsoil and coarse drainage texture.
3.3.2 Stream Frequency (Fs)
Stream frequency depends upon the rate of recurrence of the stream viz., frequency and area of the sub watersheds. The morphometric parameter is direct relationship with the erodibility. higher the stream frequency, higher will be erodibility and vice versa. Stream frequency values indicate positive correlation with the drainage density for all watersheds indicating increase in stream population with respect to increase in drainage density. From Table 4, it can be observed that 4D7G4 (Fs = 3.58) have maximum stream frequency whereas 4D7F4 (Fs = 1.75) have minimum stream frequency.
3.3.3 Texture Ratio (Rt)
Texture ratio is the crucial consideration in drainage analysis depending on the relief aspect of the terrain. From Table 4, it can be observed that the maximum texture ratio is with respect to 4D7G4 (Rt = 27.40) refers to high sensitivity to erosion and minimum texture ratio is with respect to 4D7H5 (Rt = 6.54) refers to low sensitivity to erosion.
3.3.4 Length of Overland Flow (Lof)
The overland flow and surface runoff are different terms; overland flow refers to that flow of precipitated water which moves over the land surface leading to the stream channel, while the channel flow reaching outlet of the watershed is termed as surface runoff. From Table 4, it can be observed that the maximum length of overland flow is with respect to 4D7F1 and 4D7F3 (Lof = 0.53 Km) refers to high sensitivity to erosion and minimum length of overland flow is with respect to 4D7G2 and 4D7H6 (Lof = 0.42 Km) refers to low sensitivity to erosion.
3.3.4 Constant of Channel Maintenance (Ccm)
Constant of channel maintenance have direct relationship to erodibility. Higher Ccm value indicates the high infiltration and low runoff. From Table 4, maximum Ccm corresponds to 4D7F3 (Ccm = 1.06 Sq.km/km) and minimum Ccm corresponds to 4D7H6 (Ccm = 0.83 Sq.km/km).
Higher values of drainage density, stream frequency, texture ratio is seen in western part of the study area (Fig. 4). It indicates that such watersheds are underlain by impermeable rocks, which are responsible for high surface run off. The study of length of overland flow shows that it is a dominant hydrological parameter which has a great effect on shape of hydrograph. In the study area, average length of overland flow is estimated as 0.47 Km (Table 4) which shows high surface runoff leading to high soil erosion. The relationship between constant of channel maintenance and soil erosion is analogous to the relationship between drainage density and stream frequency (Arabameri et al. 2020). The determination of areal parameters is found to be important for the estimation of soil erosion.
3.4 Shape Morphometric Parameters
The shape parameters form factor (Ff), elongation ration (Re), compactness coefficient (Cc) and circulatory ratio (Rc) have inverse relation with the sediment yield per unit area. The categorization of watersheds as per shape parameters are shown in Fig. 5.
3.4.1 Form Factor (Ff)
Form factor is the dimensionless ratio of basin area to the square of basin length. It indicates the shape of the basin. Lesser Ff value indicates highly elongated nature of the basin and higher Ff value indicates highly circular nature of the basin. Watershed with high Ff has high peak flows of shorter duration, whereas watershed with low Ff has lower peak flows for longer duration. From Table 4, maximum Ff occurs in 4D7G1 (Ff = 0.62) with least priority refers to circular shape and minimum Ff occurs in 4D7H5 (Ff = 0.10) with high priority refers to elongated shape of the basin.
3.4.2 Elongation Ratio (Re)
Elongation ratio is defined as the ratio of diameter of a circle having the same area as that of the basin to the maximum length of the basin. Elongation ratio varies from 0 to 1. Higher value of Re indicates the circular shape of the watershed and lower value indicates the elongated shape of the watershed. From Table 4 maximum Re value occurs in 4D7G1 (Re = 0.89) with least priority and minimum Re value occurs in 4D7H5 (Re = 0.36) with high priority.
3.4.3 Compactness Coefficient (Cc)
It is calculated as ratio of basin perimeter to the circle perimeter of same area of watershed. It develops the relationship between actual hydrologic basin and circular basin having same area. It is directly proportional to infiltration. From Table 4, maximum Cc value occurs in 4D7H5 (Cc = 1.98) and minimum Cc value occurs in 4D7G1 (Cc = 1.23). The values of Cc vary from 1.23 to 1.98.
3.4.4 Circulatory Ratio (Rc)
Circulatory ratio is calculated as ratio of area of basin to the area of circle having equivalent circumference to the perimeter of the basin. Generally, the value of Rc varies from 0 to 1. Lower value indicates elongated nature whereas higher value indicates circular nature of the basin. (Pawar et.al. 2014). From Table 4, the value of Rc ranges from 0.26 to 0.66 indicates that all the watersheds are elongated in shape. Lower Rc value have high priority whereas higher Rc value have low priority. Maximum Rc value occurs in 4D7G1 (Rc = 0.66) and minimum Rc value occurs in 4D7H5 (Rc = 0.26).
In the study area values of form factor varies from 0.10 to 0.62. Values greater than 0.78 indicated the circular basin (Javarayigowda et al. 2018), while smaller values suggest the elongated basin. The average form factor of Bhima basin is estimated as 0.24 (Table 4), indicating the basin is more elongated and susceptible to more soil erosion. The estimation of elongation ratio for watersheds in Bhima basin are observed in the range of 0.36 to 0.89 (Table 4). The watershed 4D7G1 (Re = 0.89) has circular shape with sharp peak flood discharge, whereas the remaining watersheds are marked as elongated shape with peak flows for longer duration. More the value of compactness coefficient, less is the soil erosion and vice versa. Cc value > 1 represents that the shape of watershed is deviated from circular shape. From Table 4 it is observed that Watersheds 4D7H5 (Cc = 1.98), 4D7F1 (Cc = 1.93) and 4D7H1 (Cc = 1.83) indicating higher infiltration capacity and hence less susceptible to soil erosion as compared with the remaining watersheds in study area. The average circulatory ratio of Bhima basin is 0.42. This shows that the basin is elongated and permeable to geologic material. Watershed 4D7H5 (Rc = 0.26) (Table 4) shows the lowest Rc value which in turn reflect rapid discharge and more soil erosion. Watershed 4D7G1 (Rc = 0.66) (Table 4) shows the highest Rc value reflects low discharge from watershed and less soil erosion.
3.5 Landscape Morphometric Parameters
Landscape parameters of the basin are relating to the elevation features to analyze terrain characteristics. These parameters have direct relation with the soil erosion. The parameters basin relief (R), ruggedness number (RN), relief ratio (Rr), relative relief (Rrf) relief peakedness (Rpk), hypsometric integral (HI), slope (G) and ruggedness index (RG) are grouped under landscape parameters. The categorization of watersheds as per landscape parameters are shown below in Fig. 6.
3.5.1 Basin Relief (R)
Basin relief is the difference between the elevation of highest point and lowest point of the watershed. Higher basin relief indicates low infiltration and high runoff. From Table 4, 4D7F1 (R = 129 m) has the least basin relief and 4D7H7 (R = 547 m) has the highest basin relief. Higher basin relief indicates low infiltration and high run off, hence more erosion.
3.5.2 Ruggedness Number (RN)
Ruggedness number is a dimensionless number, which can be obtained by product of basin relief and drainage density of same unit. From Table 4, maximum value of ruggedness number (RN = 0.61) occurs in 4D7H7 watershed and minimum value of ruggedness number (RN = 0.12) occurs in 4D7F1 watershed. The value of RN varies from 0.12 to 0.61. RN values indicate the structural complexity of the watersheds in association with relief and drainage density. Higher values implies that the area is more susceptible to erosion.
3.5.3 Relief Ratio (Rr)
Relief ratio is obtained by dividing the basin relief and basin length. Relative relief is directly related with the slope, also affects the hydrological process (Arabameri et al. 2020). The relief ratio is an indicator of erodibility in watershed which measures the steepness of watershed. High value of Rr represents the hilly region and vice versa. From Table 4, maximum value occurs in 4D7H7 (Rr = 10.94) with high priority and minimum value occurs in 4D7F1 (Rr = 1.64) with least priority. Thus it can be said that Rr has a direct relation with the erodibility. The values of Rr varies from 1.64 to 10.94.
3.5.4 Relative Relief (Rrf)
It is an important morphometric parameter used for overall assessment of morphometric characteristic of any topography (Javarayigowda et al. 2018). Relative relief is given by the ratio of basin relief to the perimeter of the watershed. From the Table 4, watershed 4D7H7 has the maximum Rrf (Rrf = 4.53) with high priority and 4D7F1 has the minimum Rrf (Rrf = 0.71) with least priority.
3.5.5 Relief Peakedness (Rpk)
Relief peakedness is given by the ratio of mean elevation to maximum elevation of the watershed (Khawaja et al. 2008). From the Table 4, 4D7F1 has the maximum Rpk (Rpk = 0.88) and 4D7G4 has the minimum Rpk (Rpk = 0.62). The values of Rpk varies from 0.62 to 0.88. Relative peakedness has direct relation with erosion.
3.5.6 Hypsometric Integral (HI)
It is used to determine the geomorphic stages of development of watersheds and express how the mass is distributed within a watershed from bottom to top (Meshram et al. 2015). It is given by (Hmean - Hmin) / (Hmax - Hmin) (Yang et al. 2014). From the Table 4, 4D7F1 has the maximum HI (HI = 0.43) and 4D7G2 has the minimum HI (HI = 0.15). Hypsometric integral has direct relation with erosion.
3.5.7 Slope (G)
Slope is the ratio of mean elevation of the watershed to the square root of the watershed area. Steep slopes are observed at the periphery of the watershed. Slope plays key role in runoff and stream discharge (Kadam et al. 2019). Slope are directly proportional to land and water degradation where higher value of slope gives greater sediment yield per unit area. From the Table 4, 4D7H5 has the maximum slope (G = 37.43) with high priority and 4D7G7 has the minimum slope (G = 17.38) with least priority. The slope value varies from 17.38 to 37.43.
3.5.8 Ruggedness Index (RG)
Ruggedness index is given by R*A− 0.5 (Yang et al. 2014). From the Table 4, 4D7H5 has the maximum RG (RG = 25.53) and 4D7F1 has the minimum RG (RG = 4.85). From Table 4, 4D7H5 has the maximum ruggedness index with high priority and 4D7F1 has the minimum ruggedness index with least priority. The ruggedness index has direct relation with erosion. The ruggedness index value varies from 4.85 to 25.53.
Figure 6 Landscape parameters of the study area: a. ruggedness number, b. relief ratio, c. relative relief, d. relief peakedness, e. hypsometric integral, f. slope, g. ruggedness index
Landscape parameters are the topographic parameters and derived from DEM. Higher values relate with more erosion and lower values less erosion except relief peakdness, which is inversely proportional to erosion. From Fig. 6 it is stated that more soil erosion is seen in uppermost part of the study area, where more values of elevation are estimated from DEM.
3.6 Priority Ranking
To determine the importance and contribution of each morphometric parameter to determine soil erosion potential priority ranking of watersheds are required. In prioritization process, hierarchy of different watersheds according to priority ranks are then taken up for soil conservation and treatment (Kadam et al. 2019).
3.7 Initial Priority Ranking (IPR)
Initial priority ranking is done on the basis of morphometric characteristics which are estimated according to linear, areal, shape and landscape parameters. The parameters which are directly relate with the erosion are, for them largest value given as highest rank. Similarly, the parameters which are inversely related with the soil erosion, for these parameters lowest value given as highest value. If two watersheds have same value, then equal rank is assigned to such watersheds. The sum of all the parameters for each watershed is calculated. The compound factor of every watershed is calculated as dividing the sum by number of parameters. Finally, initial priority ranking according to the calculated compound factor value. Table 5 shows the initial priority ranking of 19 watersheds estimated for 17 morphometric parameters.
3.8 Weighted Compound Factor Method
Different watersheds behave differently according to their characteristics, It is consider that all the parameters cannot be of equal importance in the estimation of erosion and prioritization of watersheds (Aher et al. 2014). Weighted sum approach is a rigorous statistical method, which is coupled with geo-spatial technologies to identify critical parameters which should be considered in the final combination for analysis (Rahmati et al. 2019). In this method the cross-correlation matrix of these parameters have been developed as shown in Table 6. These results are used in calculating weighted compound factor for watersheds. The results obtained from initial priority ranking is multiplied with the weights obtained using cross correlation analysis to give compound factor for final prioritization of watersheds (Malik et al. 2019).
The weighted compound factor for watershed is calculated as follows:
Weighted compound factor = [(0.038 x IPR of Rbm) + (-0.014 x IPR of Dd) + (0.128 x IPR of Fs) + (0.065 x IPR of Rt) + (0.003 x IPR of Lof) + (0.003 x IPR of Ccm) + (0.061 x IPR of Ff) + (0.67 x IPR of Re) + (-0.009 x IPR of Cc) + (0.012 x IPR of Rc) + (0.119 x IPR of RN) + (0.188 x IPR of Rr) + (0.149 x IPR of Rrf) + (-0.080 x IPR of Rpk) + (0.011 x IPR of HI) + (0.113 x IPR of G) + (0.144 x IPR of RG)] (Malik et al. 2019). (2)
3.9 Final Priority Ranking (FPR)
The compound factor estimated by using weighted sum approach is used for final priority ranking of watersheds. Addition of all weighted compound factor values of parameters for 19 watersheds is taken. Thus, the soil and water conservation measures can be applied according to the priority ranking. Final priority ranking is made in such a way that the lowest value of weighted compound factor value is assigned as 1st rank, the next lower value is assigned rank 2nd and so on. Estimation of final priority ranking is tabulated below in Table 8.
It is observed from Table 6 that the combination length of overland flow (Lof) with constant of channel maintenance (Ccm), form factor (Ff) with elongation ratio (Re), relative relief (Rrf) with ruggedness number (RN) and ruggedness index (RG), has significant positive correlation, while the combination of drainage density (Dd) with length of overland flow (Lof) and constant of channel maintenance (Ccm), compactness coefficient (Cc) with circulatory ratio (Rc), ruggedness number (RN) with relief peakedness (Rpk) has significant negative correlation. Using Eq. 2 the weighted compound factor for 19 watersheds for 17 morphometric parameters are calculated and tabulated in Table 7, which is used for final priority ranking of watersheds.
As observed from Table 8 that highest priority rank 19 is assigned to watershed 4D7H7 followed by 4D7H5, 4D7H4, 4D7G3, 4D7G4, 4D7G7, 4D7G6, 4D7H6, 4D7G5, 4D7F5, 4D7G2, 4D7H3, 4D7H2, 4D7H1, 4D7G1, 4D7F2, 4D7F1, 4D7F4, 4D7F3. Figure 7 shows the final priority ranking map of Bhima basin of 19 watersheds under study. The 19 watersheds are classified in five priority categories (Aher et al. 2014; Malik et al. 2019) such as (i) very low (4.59 to 7.12), low (7.12 to 9.65), medium (9.65 to 12.19), high (12.19 to 14.72) and very high (14.72 to 17.26) as given in Table 9. It is observed from Table 9 that four watersheds (4D7G3, 4D7H4,4D7H5 and 4D7H7) are under very high category, three watersheds (4D7G4, 4D7G6 and 4D7G7) are under high category, two watersheds (4D7G5 and 4D7H6) are under medium category, five watersheds (4D7F5, 4D7G2, 4D7H1, 4D7H2 and 4D7H3) are under low category, five watersheds (4D7F1, 4D7F2, 4D7F3,4D7F4 and 4D7G1) are under very low category. Using this information final watershed priority category map is prepared shown in Fig. 7. It is revealed from Fig. 7 that watershed area under very high category is 15.94 percent, high category is 23.50 percent, medium category is 12.73 percent, low category is 23.90 percent and very low category is 23.93 percent.
Check dam structure is proposed to intercept water from catchment and store it for irrigation and other use. Storage dams can be constructed on 3rd order of streams and check dams are constructed at 2Km down stream of storage tanks on 4th or 5th order of streams (Durbude et al. 2001). Lift irrigation scheme is proposed on check dams on 5th order of streams to have irrigation of farms in command area. An earthen bund should be constructed across gentle slope in valley portion to collect water for farmland and reduce soil erosion. Where ever possible plantation of trees on barren land and forest land should be done to increase the canopy, vegetation covers to reduce the sediment yield directly coming into the reservoirs in study area, and also to enhance the life of reservoirs.