Watershed prioritization for soil conservation in a drought prone watershed of Eastern India: Tel River Basin, Odisha

ABSTRACT In this research paper a detailed study of the morphometry of Tel River watershed to understand its hydrological behaviour and characteristics. The morphometric parameters derived are further used for prioritization of the sub-watersheds based on weighted sum analysis for planning and management. A total of 26 parameters covering linear, areal and relief aspects have been considered. These parameters are computed using standard formulae available from literature. Streams, watershed and sub-watersheds have been delineated from freely available 30 metres resolution SRTM Dem using automated processes in Arc GIS. The results show that Tel River is a seventh order stream having five sixth-order and one seventh-order sub-watershed, having a total drainage area of 22,815 km2. It is a normal watershed, slightly elongated, characterized by low relief, gentle slope and permeable sub-surface. Prioritization results show that sub-watersheds SW-6 and SW4 covering 58.41% watershed area are the most susceptible sub-watersheds to erosion, sub-watersheds SW-1, SW-2 and SW-3 covering 32.38% watershed area are moderately susceptible and sub-watershed SW-5 covering 9.21% watershed area is least susceptible to erosion. The weighted sum analysis is a robust method which can be useful for evolving strategies to manage and conserve resources at micro level.


Introduction
Water resource management is a grave challenge today particularly in areas deficient in water resources.Optimum utilization of surface and groundwater, their storage and conservation are the core issues concerning water resource management.Watersheds have long been the considered hydrological units for management of land and water resources however; diverse soil conditions, changing land use pattern and differing topographic characteristics have necessitated the need to have micro level hydrological units for their better planning, management and optimum utilization.Therefore, sub-watersheds are now the most preferred micro level hydrological units for prevention of soil erosion, management of floods, drought and land degradation.
Watershed Prioritization involves identification and ranking of environmentally degraded micro watersheds for treating them for the conservation of soil and degraded land on priority basis.Erosion-prone areas meriting prioritization can be easily recognized through morphometric parameters.They provide valuable quantitative information on the drainage, slope and relief characteristics that are vital for prioritization of watershed.The development of space technology and statistical tools has given a significant impetus to morphometric and watershed prioritization studies in the last four decades.Availability of high resolution digital elevation models (DEM), satellite images and their byproducts have made extraction of drainage basin parameters fast, precise, updated and cost effective for carrying out watershed analysis.Likewise, statistical software like SPSS, "R", Stata etc. have eased the use of various useful mathematical models like PCA, factor analysis, correlation analysis etc. in deriving precise and meaningful results of watershed prioritization that can provide useful information for various water resource managers, planners and administrators.
Several methods and techniques viz.compound factor value of morphometric parameters (Abdeta et al., 2020;Arulbalaji & Padmalal, 2020;Thakkar & Dhiman, 2007) principal component analysis (Arefin et al., 2020;Meshram & Sharma, 2017), weighted sum Analysis (Aher et.al. 2014;Malik et al., 2019), multi criteria decision of sub-watershed prioritization are in vogue today.In this study, weighted sum analysis approach propounded by Aher et al. (2014) is used for prioritization of sub-watersheds of Tel River watershed that flows through the drought prone regions of Odisha.The nature of monsoon is very erratic in the watershed area.At times it can result in more than 1100 mm of rainfall in a month at a given station and on the other hand there is no rainfall for seven to eight consecutive months.The region, therefore, often suffers from both droughts and flash floods causing extensive soil erosion.A substantial part of the study watershed suffers from moderate to severe soil erosion which is a major and continuing problem (Biswal, 2015).In light of this the main objectives of this research are (i) to derive the various morphometic parameters of the sub-watersheds of Tel River basin using SRTM DEM to understand their hydrological characteristics.(ii) To delineate critical subwatersheds in terms of soil erosion through priority ranking using weighted sum approach.1).Its total drainage area is 22,818 km 2 .Udanti, Indra, Lant, Sutkel, Hatti, Ret, Uttei, Raul and Khadago are its principal tributaries.
The various morphometric parameters viz.linear, areal and relief were calculated using the standard formulae(Table 1).The entire watershed was divided into 1 km by 1 km grid to compute and obtain grid-wise values of drainage density, ruggedness index, dissection index, relative relief etc.These were interpolated and mapped using Inverse Distance Weighing Interpolation Method.

Weighted sum approach
Based on the morphometric parameters derived prioritization and categorization of sub-watersheds is done through weighted sum approach.In this approach selected variables are given priority ranking.The relative importance of each variable is determined by computing correlation among them, thereafter weights are assigned to each input variable and compound value calculated for final priority ranking of sub-watersheds (Aher et.al. 2014).It involves two steps: (a) initial priority ranking of the subwatersheds.(b) Final ranking and categorization of sub-watersheds based on weighted compound value.
The initial priority ranking of sub-watersheds was based on linear, areal and shape morphometric parameters.Since soil erodibility is directly related with linear and areal parameters a high value of these parameters results in more erodibility and vice versa.On the other hand, it is inversely related with shape parameters wherein its lower value propagates more erodibility.Therefore, linear and areal parameters are ranked starting from highest to lowest values whereas shape parameters are ranked beginning with lowest and proceeding to highest value (Aher, et.al. 2014;Malik et al., 2019;Nooka Ratnam et al., 2005).After that, correlation matrix between the different parameters is computed (Table 2).
For final priority ranking and categorization a weighted compound factor value is obtained by multiplying the preliminary priority rank of different morphometric parameters with their weights using Equation 1.These weights are obtained by dividing the column total of each parameter in the correlation matrix called sum of correlation by the grand total obtained by summing up the sum of correlation in the computed correlation matrix.The ranks are then assigned starting with first priority to sub-watershed with lowest value of weighted compound factor second rank to sub-watershed with next higher value and so on for all six sub-watersheds.
Where WCF is the weighted compound factor, PPR is preliminary priority rank based on i th morphometric parameter, and W i is the weight of i th morphometric parameter These sub-watersheds are then classified into high, medium and low categories depending on their final weighted compound factor value for fixing the priority.Figure 3 summarizes the entire methodology.

Morphometric analysis
Morphometric parameters including linear, areal and relief parameters determine the hydrological characteristics of a watershed; they considerably affect its soil erodibility and hence merit an elaborate analysis and discussion which is carried out in the following paragraphs.

Linear parameters
The linear parameters include stream order, stream number, stream length, stream length ratio, length of overland flow and bifurcation ratio.These factors directly affect the volume and extent of flow in the watershed and thereby are directly proportional to soil erodibility.

Stream order (μ) and number (Nμ).
Stream order is the positioning of the streams in hierarchy starting from small finger tip streams that coalesce to form larger streams.The method of stream ordering devised by Strahler's (1964) is used for ordering streams in the study basin.According to it Tel River is a seventh order watershed with five sixth-order (SW-1 -SW-5) and one seventh-order (SW-6) sub-watershed.Table 3 shows sub-watershed wise total number of streams (Nu) of a given order u and the basin length (Lb).The first-order streams account for maximum number of streams and their number decreases with each successive higher order (Table 3 and Figure 4).Among the various sub-watersheds SW-4 and SW-6 account for the maximum 57% of the total number of streams.
Stream length(L μ).. Stream length is the aggregate length of all streams in a watershed.It manifests the surface run off characteristics and soil erodibility.Longer channels are characteristic of regions with gentle slope, and softer rock strata where water can travel to a long distance giving it ample scope and opportunity to either cut through its banks or its beds causing more erosion.On the other hand small length channels are found in regions with steep slopes, hard and impermeable rocks where water though, travels with greater velocity but is unable to cause significant erosion due to the resistance provided by the hard rock strata and short distance covered by it.

Stream length ratio (Lμr).
It is a dimensionless property that reflects the ratio between the length of streams of a given stream order and length of streams of next lower order of stream segment.It has a strong imprint on the surface flow and erosional characteristics of a watershed.There is a wide variation in the stream length ratio ranging from 0.21 to 2.16 in the study basin.Higher stream length ratio is observed for sixthorder stream segment for SW-1 to SW-5 and seventhorder stream for SW-6 (Table 5).These differences arise from varied topography and uneven slope (Magesh & Chandrasekar, 2014).Abnormal low values less than (0.44) between different stream orders viz.second and third (0.40) in SW-3, third and fourth (0.26) in SW-5 and fourth and fifth (0.21) in SW-2 are probably due to neo-tectonism resulting in rejuvenation of the streams to late youth stage of geomorphic cycle' (Lahiri, 1996).

Length of overland flow (Lg).
During and after any rainfall event the precipitated rain water after saturating the surface flows for a certain distance along the surface before it joins a stream channel.This distance covered by the precipitated water along the surface is called the length of overland flow (Lg).It is more   pronounced in small watersheds as compared to big watersheds because in small watershed due to smaller areal extent even a small amount of rainfall is sufficient enough to generate large overland flow as compared to large watershed where a comparatively large volume of rainfall is required to generate the overland flow.The extent of soil erodibility depends not only on the volume of water flowing as overland flow but also upon the exposure of the surface to the flowing water, nature of the underlying rock, slope gradient and the length of the distance travelled by flowing water prior to joining the stream channel.Longer is the distance of overland flow greater is the susceptibility of the soil to get eroded and vice-versa.Bifurcation ratio(Rb)."Bifurcation ratio is the ratio of number of streams of any giver order (Nu) to the number of streams of the next higher order (Nu+1)" (Schumm, 1956).It is an indicator of relief and dissection of the watershed (Horton, 1945).High bifurcation ratio occurs where strong geological control dominates and vice versa.The bifurcation ratio of Tel River watershed ranges between 4.60 and 5.12.For various sub-watersheds, it ranges between 4and 6 (Table 6) signifying that these are geologically stable and less disturbed.Bifurcation ratio has direct relation with erodibility of the soil.Its higher value signifies more erodibility and vice versa.

Areal parameters
Areal parameters include stream frequency, drainage density, drainage texture, compactness coefficient and various shape factors.All these factors excluding shape factors are directly proportional to soil erodibility.

Stream frequency(SF). "
The total number of streams found in a unit area is defined as stream frequency" (Horton, 1932).It directly affects soil erodibility; larger number of streams in a given area is likely to cause more erosion as compared to areas with less number of streams.Table 7 shows the overall stream frequency of Tel River watershed and its sub-watersheds.It varies between 0.57 and 0.77/km 2. Figure 5 exhibits the grid-wise stream frequency in the watershed.Bulk of the watershed is characterized by low frequency.The  presence of permeable rocks and moderate to relief is responsible for low stream frequency in the watershed.

Drainage density(Dd)
. "Drainage density is the length of streams of all orders in unit basin area" (Strahler, 1964).Table 7 and Figure 6 show the drainage density of Tel River basin and its sub-watersheds.It ranges from 1.02 km/km 2 to 1.14 km/km 2 for all the sub-watersheds whereas grid wise it lies between 0.8 km/km 2 and 4 km per km 2 .Greater part of the basin is characterised by low drainage density as it is covered with pediments and pediplains characterized by gentle slope, low relief, and erosional surfaces which allows more water to percolate down rather than flow as surface water.
Drainage texture (Dt)."Drainage texture is the number of stream segments present in per perimeter of basin area" (Horton, 1945).7).

Compactness coefficient (Cc). "Compact coefficient is
the ratio of perimeter of basin to circumference of circular area, which equals the basin area" (Horton, 1945).Its value is always >1, it does not depend on the size of the watershed but is highly slope dependent.It is directly proportional to the erosion risk assessment factors.The Cc value of Tel River watershed and its sub-watersheds ranges from 2.75 to 3.80 (Table 7).

Shape factors.
Shape is an important basin characteristic.It is influenced by slope, weathering and erosion process, geological structure, lithology, drainage pattern etc. Different scholars have developed different indices to define the shape of the watershed viz.form factor (Ff) "defined as ratio of basin area to square of basin length" (Horton, 1932).Its value ranges from 1 perfectly circular to 0 meaning highly elongated.Schumm ( 1956) developed elongation ratio (Er) "defined as ratio of diameter of a of the same area as the basin to the maximum basin length.""Er between (0.9-1) is circular, between (0.8-0.9) oval, between (0.7-0.8), less elongated, between (0.5-0.7) elongated and (< 0.5).More elongated basin shape" (Rai et al., 2018).Miller (1953) developed Circularity Ratio(Cr) "defined as the ratio between area of a basin to the ring that has the same perimeter as the comparing basin."Its high (>0.5)value indicates more circularity and more homogeneity in geological material whereas its low (<0.5)value indicates an elongated basin.Low Ff (0.23), Er (0.55), and Cr (0.13) values of Tel River watershed and its sub-watersheds (Table 7) indicates that the watershed has an elongated shape.

Relief parameters
The relief parameters determine the surface characteristics of the watershed that controls the rate and direction of flow of water.It includes relative relief, relief ratio, dissection index, and ruggedness index.

Relative relief (RR)
"It is the difference between the highest point on the basin perimeter to the mouth of the stream." The rate of peak run off and sediment delivery are significantly affected by relative relief (Arulbalaji & Padmalal, 2020).The basin relief of Tel River watershed is 1226 metres (Table 8).In the square sized grids, RR ranges from 8 to 564 m. 69.20% of the basin area has very low (<120 metres) relief.It extends as a wide zone from north to south in the central part of the basin covering almost entire sub-watershed SW-1 and major parts of SW-2, 3, and 4, characterized by pediments and pediplain.Low to moderate (120-360 meters) relief covers 30.25% basin area, mainly concentrated in subwatersheds SW-4, 5, and 6 characterized by high to moderately dissected hills.High to very high relief (360-600 metres) accounts for only 0.55% basin area (Figure 7).

Relief ratio(Rr)
"Relief Ratio is the ratio between maximum basin relief and basin length" (Schumm, 1956).Relief ratio depicts the aggregate steepness and intensity of erosional processes at work in a watershed.High relief ratio implies that the stream has high erosive power and can lead to high sediment loss whereas low relief ratio indicates it's opposite.Table 8 exhibits the relief ratio of Tel River and its sub-watersheds.The entire basin and its sub-watersheds are characterized by very low Rr value ranging from 0.008 to 0.011 indicating low relief, gentle slope, and less sediment loss from the basin.

Dissection index (DI)
"It is the ratio between relative relief and absolute relief."It indicates the nature and magnitude of dissection of a terrain.The grid-wise DI for the basin ranges from 0.02 to 0.69 (Figure 8).About 80% of the basin area has very low to low (<0.28)D I value, signifying mature stage of geomorphic evolution of the basin characterized by near absence of vertical erosion and predominance of lateral planation by Tel River and its tributaries.Only 4.7% area has high to very high DI value > 0.42, scattered as patches in different parts of the basin.Rest 15% basin area has moderate D I value from 0.28 to 0.42.Table 7 shows the overall DI values of Tel watershed and sub-watersheds.

Ruggedness index (RI)
It measures the unevenness or roughness in the surface created due to the interplay of various erosive forces in the watershed.It is obtained by dividing the product of drainage density and relative relief by one thousand.In the study basin, RI of the 1 km 2 grids (Figure 9) ranges from 0 to 0.96.Almost the entire basin (97%) has very low to low < 0.4 RI value.Low RI means that the watershed is characterized by highly even surface with very less fragmentation of relief.It also indicates that the watershed has reached the monadknock or the penultimate stage of geomorphic evolution.
Table 7 shows the overall RI values of Tel watershed and sub-watersheds.

Prioritization and categorization of sub-watersheds
The primary aim of watershed prioritization is to identify critical watersheds based on their diverse morphological characteristics and hydrological response to erosion.The results of the prioritization and categorization of the study watershed based on weighted sum approach are summarized from Tables 9 to 12. Table 9 shows the initial priority ranking of various morphometric parameters in terms of assessment of erosion in the different sub-watersheds.Since linear parameters are directly proportional to soil erodibility their highest value is ranked one and least value is given the lowest rank.On the other hand, shape factors behave the other way round hence, lowest value is ranked as one and highest value is given the lowest rank.Parameters having equal values are equally ranked.Table 10 shows the correlation matrix between these parameters.From it, can be observed that the combination Rb and Er (0.The weighted compound factor value computed using Equation 1shows that its value ranges between 0.73 and 6.75 for different sub-watersheds (Table 11).In terms of priority ranking SW-6 with least value of 0.73 gets the highest priority rank-I whereas SW-5 with maximum value of 6.75 has the least priority rank VI.SW-4, SW-3, and SW-2 with compound factor values of 1.84, 3.55, and 3.79 are ranked II, III, and IV respectively (Table 11 and Figure 10a).These sub-watersheds are further grouped into high, medium and low categories to distinguish the most critical sub-watersheds in terms of soil erosion that require highest priority for soil conservation measures from other sub-watersheds which are comparatively less critical.SW-4 and SW-6 with least compound factor value, accounting for 58.41% of the basin area are the most critical sub-watersheds, SW-1, SW-2, and SW-3 accounting for 32.38% basin area lie in medium category and one sub-watershed SW-5 lies low priority category (Table 12 and Figure 10b).

Conclusions
In the present work, a detailed investigation of morphometric parameters of Tel River watershed and its sub-watersheds is carried out to understand its hydrological behaviour and characteristics; these have further been used for the prioritization of watershed to delineate critical sub-watersheds from the perspective of soil erosion.In all 26 parameters covering linear, relief, and areal aspects of the watershed are considered for this purpose.From the quantitative analysis of the basin Morphometry, it is inferred that the Tel River basin is a seventh-order basin with five sixth-order and one seventh-order sub-watershed.Its bifurcation ratio indicates that it is a geologically stable watershed with very little structural control.The various shape factors viz.form factor (Ff), elongation ratio (Er), and Circularity Ratio (Cr) indicate that it is slightly elongated in shape with very little variation among different sub-watersheds.Basin relief, dissection index, and ruggedness index show that the basin is in mature stage of geomorphic evolution with moderate to low relief.Low stream frequency and low drainage density within the basin and its subwatersheds indicate the presence of gentle slope and permeable subsurface.
Erosion of soil is one of the major problems faced by this watershed and prioritization of watersheds is an important way to identify erosion prone areas to take appropriate measures of soil conservation and management.Morphometric parameters play an important role in identification of erosion prone area as they directly or indirectly propagate and control the rate and direction of movement of soil.Hence, using these parameters watershed prioritization is done to delineate critical sub-watersheds in terms of soil erosion through the weighted sum approach.Watershed prioritization and categorization result shows that substantial area of Tel River watershed is vulnerable to soil erosion.About 58.41% of its area covering sub-watershed SW-4 and SW-6 is highly susceptible to soil erosion that requires prompt soil conservation and management measures.Another 32.38% area encompassing sub-watershed SW-1, SW-2, and SW-3 is moderately susceptible whereas only 9.21% area covering sub-watershed SW-5 is least susceptible to From this study it can be inferred that geospatial technology is a robust and cost-effective technique that provides a dynamic and vibrant platform to  perform Basin Morphometry analysis and its application to watershed prioritization for soil conservation and management.The weighted sum approach used in this study for watershed prioritization is a dynamic and sustainable method over traditional watershed prioritization methods in which significance of several characterization parameters was considered.Its application in sub-watershed prioritization, for planning, management, and conservation at micro level, will be helpful to various stakeholders such as water resources managers, conservation measures planners, and decision policy makers for better decisions making.

Figure 1 .
Figure 1.Location map of Tel River Basin.
89), Rb and Dt (0.81), Ff and Dt (0.88), Er and Ff (0.87) have significantly high positive correlation whereas, Cr and Cc (-0.98) and Er and Sf (−94.1) have significantly low correlation.Further Shape Factor (Sf) and Compactness Coefficient (Cc) are negatively correlated whereas, drainage density (Dd) and length of overland flow (Lg) have positive correlation with maximum parameters.

Table 1 .
Table 4 exhibits order-wise stream length in different sub-watersheds of Tel River Basin.From the table, it can be observed that sub-watersheds SW-4 (6581.41km) and SW-6 (8055.02km) have the Formulae for calculating basin parameters, Tel River Watershed.

Table 3 .
Stream order, stream number and basin length.
Sub-watershed Area (km 2 ) Percent of total basin area Perimeter (km) Stream order (μ)

Table 5
exhibits the Lg values of Tel River and its subwatersheds.It ranges from 0.51 km for SW-5 to 0.57 km for SW-3.As large part of the watershed has flat trajectory with very gentle slope there is less surface run off and more infiltration resulting in very less generation of overland flow.

Table 4 .
Stream length and average stream length.

Table 5 .
Stream length ratio and overland flow.

Table 7 .
Areal and shape parameters.
0.75 1.12 0.17 5.79 0.47 0.07 3.80 4.85 Tel Basin 0.74 1.09 0.23 6.73 0.55 0.13 2.75 11.59 is coarse, if it lies between 4 and 10 per km it is intermediate, between 10 and 15 per km.It is fine, and >15 per km it is ultra fine.Tel River watershed has fine texture, sub-watershed wise SW-1, SW-3, and SW-4 have coarse texture whereas SW-2, SW-4, and SW-6 have intermediate texture (Table It shows how closely or far apart the streams are spaced.It depends upon climate, geology, soil characteristics, elevation and stage of geomorphic evolution of the watershed.According to Smith (1950) if the number of streams is < 4 per km, texture

Table 10 .
Correlation matrix of morphometric parameters.