Watershed Prioritization of the West Rapti Basin based in Morphometric Parameters Using Geospatial Analysis

It Prioritizes Lower Rapti Basin due to higher drainage density and bifurcation ratio, requiring river training works to manage discharge and sediment

 Watershed Prioritization of the West Rapti Basin based in Morphometric Parameters Using Geospatial Analysis

Authors: 

✏️ Isha Karn1, Ladli Gupta1, Rabin Karki1, Alisha Budhathoki1, Animesh Khadka2

1Department of Agricultural Engineering, Tribhuvan University, IOE Purwanchal Campus, Dharan, Nepal.  

2GPKoirala College of Science and Technology, Purwanchal University, Gothgaun 

 

Abstract   

Prioritization of watershed has picked up significance in watershed management. Morphometric analysis is being commonly applied to prioritize the watershed. The present study makes an effort to organize sub watersheds dependent on morphometric characteristics using GIS techniques in West Raptibasin, located in the Mid-Western region of Nepal. There are nine Sub watersheds under this. Various morphometric parameters namely Bifurcation ratio (Rb), Drainage density (Dd), Stream frequency (Ns), Texture ratio (T), Form factor (Rf), Circularity ratio (Rc), Elongation Ratio (Re), length of overland flow, shape factor (Bs), drainage texture, compactness ratio (Cc) has been determined for each sub-watershed and allotted position on premise of relationship as to arrive at a computed value for final ranking of watershed. The morphometric parameters range between Rb (1.682-2.240), Dd (0.8831.280), Fs (1.047-1.233), T (1.795-5.266), Rf (0.22-0.27), Rc (0.11-0.32), Re (1.061.18), C (0.781-1.132), Lof (0.391-0.566). The very important quantitative watershed aspects such as linear, relief and areal have been taken in consideration, and six morphometric parameters have been selected and used for ranking and prioritizing of subwatersheds. In this regard, lower rapti Subbasin, lundri Subbasin and jhimruk Subbasin have categorized into very high priority, upper Rapti sub-basin has categorized into high priority, siban and dundungad Subbasin into medium priority and Arun and madi Subbasin have categorized into low priority. However, depending upon the weightage of morphometric parameters and mean raster values, lower rapti Subbasin has been ranked into the first priority due to higher drainage density as well as the bifurcation ratio.   

Keywords: GIS, DEM, Morphometric analysis, Subbasin, Priority  

1. Introduction  

Watersheds play a vital role in the hydrological cycle, influencing the quality and quantity of water resources. However, they are often subjected to various geomorphological and topographical conditions that can impact their health and sustainability. In order to better understand and assess watersheds, morphometric analysis has emerged as a valuable tool in geomorphology. By quantifying the morphology of a river basin, morphometric analysis provides insights into its shape and hydrological characteristics.  

Morphometric parameters such as stream length ratio (Rl), bifurcation ratio (Rb), drainage density (Dd), drainage texture (Dt), stream frequency (Fs), elongation ratio (Re), circularity ratio (Rc), form factor (Rf), length of overland flow (Lg), and relief ratio (Rh) are commonly used to evaluate different aspects of watersheds. These parameters help in understanding the linear, areal, and relief features of a watershed and can provide valuable information for watershed management and conservation efforts. The stream length ratio (Rl) indicates the ratio of the total stream length of one order to the next lower order of stream segment. It reflects the maturity of the geomorphic status of the watershed. A higher stream length ratio suggests a more developed watershed. Bifurcation ratio (Rb) measures the ratio of the number of stream segments of a specified order to the number of streams in the next higher order. It provides insights into the control of geologic structures on the drainage pattern. Drainage density (Dd) quantifies the total length of streams of all orders per unit area. It helps in understanding the density and interconnectedness of the stream network within a watershed. Drainage texture (Dt) is the total number of stream segments of all orders in a river basin divided by the perimeter of the basin. It provides information about the risk of soil erosion within the watershed. Stream frequency (Fs) measures the total number of channel segments of all stream orders per unit area. It can indicate the relief characteristics of a watershed. Elongation ratio (Re) compares the diameter of a circle with the same area as the basin to the maximum basin length. It helps classify watersheds as elongated or circular. Circularity ratio (Rc) represents the ratio between the area of a watershed and the area of a circle with the same circumference as the watershed's perimeter. Form factor (Rf) assesses the ratio of the watershed area to the square of its length. It can provide insights into flood hydrograph characteristics. The length of overland flow (Lg) measures the average overland flow within a watershed. It helps assess the vulnerability to flash flooding. Relief ratio (Rh) quantifies the difference in elevation between the highest and lowest points in a watershed, indicating the relief characteristics.  

In addition to morphometric analysis, watershed prioritization is crucial for effective conservation measures. It involves ranking different micro watersheds within a larger watershed based on their treatment and conservation needs. Prioritization takes into account factors such as soil erosion, drainage density, bifurcation ratio, land use, and land cover changes.  

2. Study area:  

The study area for this research project is the West Rapti River basin in Nepal. Situated in the mid-western region of the country, the West Rapti River basin offers an ideal setting to investigate the morphometric characteristics and prioritize conservation measures (Figure 1). The geographic coordinates of the study area span from 27˚56'50'' to 28˚02'30'' North latitudes and 81˚45'00'' to 81˚40'00'' East longitudes.   

The main channel of the West Rapti River stretches for a length of 257 kilometers, originating from the middle mountains of Nepal. As it flows downstream, it traverses through diverse landscapes, including lowlands, before eventually joining the Ghagra (Karnali) River, a significant tributary of the Ganges River. Several tributaries contribute to the West Rapti River's flow, including the Jhimruk River, Madi River, Arun River, Lundri River, Sit River, Dundungad River, Sotiya, and Gandheli rivulets. Once the Jhimruk and Madi Rivers converge, the river assumes the name West Rapti River.The average slope of the West Rapti River basin is measured at 16.8%. This slope provides important insights into the terrain and topographical characteristics of the region. Understanding the slope of the basin can help in assessing the potential for water runoff, erosion, and the overall hydrological dynamics of the watershed. By focusing on the West Rapti River basin, this study aims to apply morphometric analysis and watershed prioritization techniques to gain a comprehensive understanding of the basin's features and identify areas that require conservation measures. The unique geographical attributes and diverse landscapes of the West Rapti River basin make it an ideal case study for examining the effectiveness of these approaches in managing and preserving watersheds.

Figure 1: Map of the study area

                                   

3. Materials and Methodology  

In the present study an integrated use of satellite data SRTM (Shuttle Radar Topography Mission) DEM (Digital Elevation Model) was utilized for generation of database and extraction of various drainage parameters. DEM of spatial resolution 30 m and spectral resolution 1 was used.

4. Research Design  

Figure 2: Flow chart for Watershed Delineation and Prioritization of West Rapti Basin.

   

For the prioritization by the morphometric analysis, the raster image of morphometric parameters such as drainage density, bifurcation ratio, stream frequency, texture ratio, length of overland flow and constant channel maintenance was created. After this, the raster overlay with certain weightage was done to obtain the final image. The weightage for individual raster is mentioned in the table 2 (Rekha, 2015):  

Prioritized map = Drainage density × 0.3 + Bifurcation ratio × 0.25 + Stream frequency × 0.2  + Texture ratio × 0.1 + Length of overland flow × 0.1 + Constant channel maintenance × 0.05  

Table 2: Morphometric parameters weightage

S/N	Parameters	Weight
1	Drainage density	0.3
2	Bifurcation ratio	0.25
3	Stream frequency	0.2
4	Texture Ratio	0.1
5	Length of  Overland flow	0.1
6	Constant channel maintenance	0.05

5. Results and Discussion  

5.1 Watershed delineation of the West Rapti Basin

The watershed delineation was done to determine the information of the West Rapti Basin such as area, perimeter, highest and lowest elevation, length as well as the number of drainage lines with the stream order and the information of the sub basin. The detail of the West Rapti sub basin is shown in table 3.  

Table 3: Details of West Rapti Basin

S/N	Parameters	Value
1	Area	6343.73
2	Perimeter	820.126
3	Lowest Elevation	121
4	Highest Elevation	3637

Figure 3: Watershed Delineation of West Rapti Basin

   

The watershed delineation of the West Rapti basin shown in the figure 3 clarified that the stream order of the basin varies from 1st to 6th order. Stream ordering of the West Raptibasin was computed using ARCGIS software by applying the law proposed by Horton, 1945. It is found that the total length of streams segment is maximum in first order streams and decreases as the stream order increases. This change in stream orders may indicate flowing of streams from high altitude and lithological variations. The understanding of streams in a drainage system constitutes the drainage pattern, which in turn replicates mainly structural controls of the underlying rocks. The study area possesses dendritic drainage patterns, despite stream lengths and other hydrological properties. They are generally characterized by a treelike branching system, which indicates homogeneity and uniformity. While the dendritic drainage pattern suggests a lack of significant structural controls on the overall river network, it is important to note that other hydrological properties, such as stream lengths and morphometric parameters, contribute to the comprehensive understanding of the basin's hydrological dynamics. characteristics and identify potential areas of concern for conservation and management efforts.

Figure 4: Regression analysis between number of streams and the length of drainage lines of West Rapti basin

                                                           

From the graph shown in figure 4, it can be concluded that there are six stream orders with different lengths and numbers. The regression analysis of the stream and length showed the positive linear behavior with the value of R2 = 0.944. Here, the number of streams are considered as independent variables and length of drainage lines are considered as dependent variables.

5.2 Morphometric Analysis  

Designation of Stream order is the first step in morphometric analysis of a drainage basin, based on the hierarchic making of streams proposed by Strahler (1964). In the study area sb1, sb2, sb4, sb5, sb6 sb8 and sb9 are of fifth order whereas sb3, and sb7 are of sixth order streams. The morphometric parameters were calculated, its shows that Bifurcation ratio (Rb) ranges from  

1.628 to 2.240, sb5 have low Rb whereas sb9 have high Rb. Stream frequency (Fs) varies from 1.047 to 1.233 with sb9 having low and sb5 has high value. Texture ratio (T) ranges from 1.795 to 5.266 with low in sb9 and high in sb7. Form factor (Rf) is low in sb6 and sb8 and high in sb5, it ranges from 0.22 to 0.27. Length of overland flow varies from 0.391 to 0.566, sb8 have low value whereas sb5 have high LoF. Constant channel maintenance (C) shows wide variation across the sub watershed; it is more in sb5 and less in sb8, it varies from 0.781 to 1.132. Elongation ratio (Re) varies from 1.06 to 1.18 with sb6 and sb7 has low and sb5 has high value. Circularity ratio (Rc) of Subbasin ranges from 0.11 to 0.32 with low in sb9 and high in sb3. The details of these values are shown in table 4.  

Table 4: Morphometric parameters of West Rapti Sub-basin

Subbasin	Name	Rb	Dd	Fs	T	Lof	C
1	Siban Subbasin	2.134	0.909	1.198	3.243	0.550	1.100
2	Dundungad Subbasin	1.688	0.927	1.159	4.165	0.539	1.078
3	Lundri Subbasin	2.137	0.899	1.163	4.693	0.556	1.113
4	Madi Subbasin	1.698	0.965	1.164	2.640	0.518	1.036
5	Arun Subbasin	1.682	0.883	1.233	2.782	0.566	1.132
6	Upper Rapti Subbasin	1.774	1.089	1.114	4.668	0.459	0.918
7	Jhimruk Subbasin	1.797	0.893	1.101	5.266	0.560	1.120
8	Lower Rapti Subbasin	2.020	1.280	1.108	4.688	0.391	0.781
9	Jhijhari Sub Basin	2.240	1.175	1.047	1.795	0.426	0.851

                         

Figure 5: Raster of Morphometric Parameters of West Rapti Basin.

                 

Raster of the different morphometric parameters are shown in the figure 5. The map shows that the Sub Basin which lies in the red zone has the highest value and the Sub Basin which lies in the dark green zone has the least value. On this basis it clarifies that, Drainage density (Dd) has its highest value in sb8 which is Lower Rapti Subbasin. similarly, bifurcation ratio (Rb) was found to be highest in Jhijhari sub basin, Stream frequency (Fs), length of overland flow (Lof) and  

Constant channel maintenance was found to be highest in the Arun sub-basin and lastly, the texture was found to be highest in the Jhimruk sub-basin. The maximum and minimum values of these morphometric parameters are shown in table 4.3  

Table 5: Maximum and minimum values of morphometric parameters of sub basins of West

Morphometric Parameters	Values
Bifurcation Ratio	Maximum	2.24
	Minimum	1.682
Drainage density (Dd)	Maximum	1.28
	Minimum	0.883
Stream frequency (Fs)	Maximum	1.233
	Minimum	1.047
Texture ratio (T)	Maximum	5.266
	Minimum	1.795
Length of overland flow(Lof)	Maximum	0.566
	Minimum	0.391
Constant channel maintenance (C)	Maximum	1.132
	Minimum	0.781

5.3 Prioritization of sub watersheds  

Morphometric aspects such as Bifurcation ratio (Rb), Drainage density (Dd), Stream frequency (Ns), Texture ratio (T), Form factor (Rf), Circularity ratio (Rc), Elongation Ratio (Re), length of overland flow, Basin Shape (Bs), drainage texture, compactness ratio (C) is also termed as erosion risk assessment parameters and have been used for prioritizing Subbasin (Biswas et al 1999). The parameters such as Bifurcation ratio (Rb), Drainage density (Dd), Drainage Texture, Stream frequency (Ns), Texture ratio (T), have a direct relationship with erodibility, higher values of all these have been rated as rank 1 second largest is rated as rank 2 and so on with the least ranked last. Parameters such as Form factor (Rf), Circularity ratio (Rc), Elongation Ratio (Re), Basin Shape (Bs) have inverse relationship with the erodibility, higher the value less is erosion lower the value high is erodibility.  

In this manner least estimations of this are appraised as rank 1 and second least being rated as rank 2 and so on and the highest value is given last rank. After the ranking has been done based on every single parameter of each Sub Basins were added up for each of the nine Subbasins to arrive at compound value. Based on the average value of these parameters.

Figure 6: Priority Map of West Rapti Basin

                            

Subbasin with least rating was assigned highest rank next value was assigned second rank and so on and the Sub Basin with highest compound value was assigned last rank. Hence on basis of the weightage of morphometric parameters and raster values, the priority map shown in the figure 4.4 justifies that, Subbasin 8 is highly erosive which is Lower Rapti Subbasin and given first priority and the Subbasin 4 is given the last priority which is Mardi Subbasin. The priorities and the ranking of different Subbasin is shown in table 6.  

Table 6: Priorities of Subbasins and their ranks

Sub basin	Name	Rb	Dd	Fs	T	Lof	C	Area (sq km)	Value	Rank
1	Siban  Subbasin	2.134	0.909	1.198	3.243	0.550	1.100	404.014	1.48	5
2	Dundungad Subbasin	1.688	0.927	1.159	4.165	0.539	1.078	548.798	1.456	6
3	Lundri Subbasin	2.137	0.899	1.163	4.693	0.556	1.113	641.524	1.617	2
4	Madi  Subbasin	1.698	0.965	1.164	2.640	0.518	1.036	513.899	1.314	9
5	Arun  Subbasin	1.682	0.883	1.233	2.782	0.566	1.132	258.782	1.323	8
6	Upper Rapti Subbasin	1.774	1.089	1.114	4.668	0.459	0.918	1288.55	1.551	4
7	Jhimruk Subbasin	1.797	0.893	1.101	5.266	0.560	1.120	1047.35	1.575	3
8	Lower Rapti Subbasin	2.020	1.280	1.108	4.688	0.391	0.781	1309.43	1.657	1
9	Jhijhari Sub Basin	2.240	1.175	1.047	1.795	0.426	0.851	331.383	1.386	7

The graphical representation of the priority map is shown in the figure 7 which was plotted based on the values of different morphometric parameters and their corresponding raster values. Here, the thin black line shown at the top of the bar chart clarifies that Lower Rapti Sub Basin should be ranked first on the basis of priority. Similarly, Madi Subbasin should be ranked last on the basis of priority. Moreover, the graph shows that Jhmiruk Subbasin has the highest texture value in comparison to the other sub basins.  

Figure 7: Graphical representation of priority map of west Rapti basin

6. Conclusion  

The present study gives a thought regarding the basin characteristics in terms of slope, topography, soil condition, runoff characteristics, surface water potential etc. Lower Rapti Basin should be prioritized on top and least priority goes to Madi Subbasin. The higher priority in the lower Rapti basin happens due to higher drainage density as well as the bifurcation ratio. The higher drainage density is attributed to greater drainage lines in the river system that can carry massive discharge as well as the sediment in the peak period. The higher values of the bifurcation ratio mean the ability of the river to divert the river path away from the river lines. This results in the river path to meander and sometimes flow across the bank and cut the fertile land. To avoid such problems, the river training works such as construction of guide bund, spur or levee should be constructed at the vulnerable sites of the lower Rapti basin. For the basin at the middle or upper reaches, the bio engineering principles should be adopted for the watershed management.  

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This is the web copy of an article that was originally published in the print version of 'The agrineer 2023' - Annual Magazine. https://www.researchgate.net/publication/371851565