4.2 Linear Aspects
The 4th order Kuttiyadi river is made up of 96 different lower-order streams. The highest stream order among the 6 subwatersheds is 4th shown by the 4 watersheds named SW1, SW2, SW5, and SW6 respectively, and while the remaining two watersheds have the highest stream of order 3rd named SW3 and SW4 (Table 3). The discharge carrying capacity of a stream and the flowing velocity increase with the stream order. In addition to contributing to greater sediment loads to the river, higher velocity and discharge exacerbate the erosion of the river bed and banks [47]. Additionally, it is observed that the number of stream segments decreases as the stream order increases and vice versa [39]. The stream length (Lu) of the stream segments is maximum in the case of first-order streams and stream length decreases as the order of streams increases [48]. The mean stream length (Lsm) of any given order is greater than that of the lower order streams [39]. The higher stream length and mean stream length is shown by the SW1 and SW4 (Table 3). As a result, these two watersheds SW4 are capable of conveying less surface runoff volume to the mainstream but due to high vegetal cover, uniform slope, and loamy soil, a large amount of surface runoff will infiltrate through cracks under the dry conditions of the soil. However, The lag time of the SW6 has very little as compared to others because it has less watershed area and perimeter, and also the catchment has very few stream lengths (Table 3). Stream length ratio (RL) is the ratio of the mean stream length of the individual stream segment to the next lower order stream segment [39]. A larger stream length ratio indicates a greater potential to carry a large volume of runoff [14]. This study showed that watershed uplift is negligible, with uniform lithology throughout the watershed. The Mean bifurcation ratio (Rbm) of the 6 subwatersheds of the Kuttiyadi river is 2.33 2.13, 2.00, 3.28, 2.63, and 1.75 for SW1, SW2, SW3, SW4, SW5, and SW6 respectively (Table 3). There is a possibility in the watershed that the mean Bifurcation ratio may not remain constant from one order to the next because of variations in geometry and lithology [49]. The low Rbm indicates the delay in the peak of hydrograph ie; the time of concentration is more and the high Rbm indicates the early rise in the hydrograph ie; the time of concentration is less so that the Rbm is sometimes used to identifying the watersheds which are responsible for flash flooding [14, 50]. Overall the Rbm of the subwatersheds is less showing the delayed peak in the hydrographs. The linear aspect of geo-morphometric parameters demonstrates that the watershed area depends only on the drainage characteristics for the runoff movement.
Table 3
Computed values of the Linear aspect of the geo-morphometric parameters for the sub-watershed of the Kuttiyadi river
S.No. | U | Nu | Lu (km) | Lsm(km) | RL | Rb | Rbm |
SW1 | 1 | 20 | 40 | 2 | 1 | 3.33 | 2.33 |
2 | 6 | 19.72 | 3.29 | 0.493 | 3 | |
3 | 2 | 7.61 | 3.81 | 0.39 | 2 | |
4 | 1 | 6.87 | 6.87 | 0.90 | 1 | |
SW2 | 1 | 12 | 29.68 | 2.47 | 1 | 4 | 2.13 |
2 | 3 | 21.59 | 7.20 | 0.73 | 1.5 | |
3 | 2 | 3.84 | 1.92 | 0.18 | 2 | |
4 | 1 | 4.52 | 4.52 | 1.18 | 1 | |
SW3 | 1 | 6 | 16.66 | 2.78 | 1 | 3 | 2.00 |
2 | 2 | 10.21 | 5.11 | 0.61 | 2 | |
3 | 1 | 0.27 | 0.27 | 0.02 | 1 | |
4 | - | - | - | - | - | |
SW4 | 1 | 17 | 37.95 | 2.23 | 1 | 2.83 | 3.28 |
2 | 6 | 10.5 | 1.75 | 0.27 | 6 | |
3 | 1 | 14.23 | 14.23 | 1.36 | 1 | |
4 | - | - | - | - | - | |
SW5 | 1 | 13 | 26.7 | 2.05 | 1 | 6.5 | 2.63 |
2 | 2 | 2.97 | 1.49 | 0.11 | 1 | |
3 | 2 | 16.15 | 8.08 | 5.43 | 2 | |
4 | 1 | 5.86 | 5.86 | 0.36 | 1 | |
SW6 | 1 | 6 | 11.83 | 1.97 | 1 | 3 | 1.75 |
2 | 2 | 6.7 | 3.35 | 0.57 | | |
3 | 2 | 3.21 | 1.60 | 0.48 | | |
4 | 1 | 2.39 | 2.39 | 0.75 | | |
4.3 Relief Aspects
In a watershed, relief indicates the difference between its maximum and minimum elevation. It is important for a watershed to examine its relief to investigate the behavior of surface runoff [51]. The results show the watershed relief of the SW4 is quite high as compared to the others (Table 4). It shows there is a lot of variation between maximum and minimum elevation. This may happen if the watershed is elongated enough or either the watershed having very steep slopes. An elongated watershed shows a delayed hydrograph peak with a larger time of concentration [14]. However, out of the total watershed area, 127.35 km2 area falls under less than 15⁰, 73.20 km2 area falls under 15-30⁰ slope, 29.29 km2 area falls under 30-45⁰ slope and only 3.79 km2 area falls under the greater than 45⁰ slope (Fig. 2). The maximum area of the watershed lies under the low relief and less slope. The results also indicate that the study area has been no probable uplift due to the uniform geology and lithology. Quantitatively, the relief ratio of a watershed measures the overall steepness [51]. Also, it is a primary indicator of erosion intensity which operates on the mountainous slopes [52]. Relief ratio increases with decreasing the area and size of a watershed [53]. The computed value of the subwatersheds shows very low values of relief ratio. So, the subwatersheds are less prone to erosion (Table 4). The subwatersheds have relatively very low erosion based on the minor change in Rh. The ruggedness number (Rn) indicates the structural complexity of the watershed terrain with the relief (Wh) and drainage density (Dd). It also describes the areas which are prone to soil erosion [65]. The computed values of Rn for the subwatersheds SW1, SW2, SW3, SW4, SW5, and SW6 are 0.08, 0.16, 0.23, 0.46, 0.14 and 0.04 respectively (Table 4). The lesser values of the Rn indicate the subwatersheds are less prone to the soil erosion associated with the low drainage density, low relief, and gentle slopes (Fig. 2a and 4b).
Table 4
Computed values of the Relief aspect of the geo-morphometric parameters for the sub-watershed of the Kuttiyadi river
S.No. | Wh | Rh | Rn |
SW1 | 546 | 0.03 | 0.08 |
SW2 | 1021 | 0.07 | 0.16 |
SW3 | 1253 | 0.10 | 0.23 |
SW4 | 1509 | 0.08 | 0.46 |
SW5 | 677 | 0.04 | 0.14 |
SW6 | 192 | 0.02 | 0.04 |
4.4 Aerial Aspects
The drainage density (Dd) of the subwatersheds of the Kuttiyadi river is 0.145, 0.154, 0.182, 0.307, 0.282, and 0.194 for the watersheds SW1, SW2, SW3, SW4, SW5, and SW6 respectively (Table 5). The factors affecting the drainage density are the length of the streams, climate, weathering, and permeability, etc; [54]. The drainage density also affects the travel time of runoff in the watershed [55]. The results show that low drainage density which might be due to the impermeable surface or subsurface materials with high vegetation cover and low catchment relief (Fig. 4c). The drainage texture (Rt) of the subwatersheds of the Kuttiyadi river is 0.119, 0.079, 0.066, 0.069, 0.051, and 0.077 for the subwatersheds SW1, SW2, SW3, SW4, SW5, and SW6 respectively (Table 5). The result shows the subwatersheds have a very fine drainage texture so that all the subwatersheds show a longer duration to the peak runoff. The results also show the watershed has very low relief with a low drainage density [42]. Stream frequency is defined as the total number of streams of all orders per unit watershed area. The stream frequency (Fs) is related to the watershed infiltration rate and capacity, permeability, and relief [14, 18]. The result shows that the SW2 watershed has the minimum stream frequency among all the subwatersheds. This might be due to the fact that the watershed has a high relative relief and it contains some rocky terrain as well (Table 1 and 5). The SW4 shows high stream frequency as compared to other watersheds. This may be due to the watershed having comparatively low relief and having more vegetation as compared to the others (Table 1 and 5). Overall the stream frequency of the watershed is low because the watershed of the Kuttiyadi river is covered by adequate vegetation and loamy soil such that the catchment has a very good infiltration rate and infiltration capacity. So, the early rise in the hydrograph peak is not possible due to the low stream frequency of the watershed ie; the runoff will take more time to reach the watershed outlet [56–58]
Length of overland flow (Lg) is one of the most independent parameters which affect both hydrological and hydrographical development of the watersheds [40, 59]. The computed values of Lg for the subwatersheds SW1, SW2, SW3, SW4, SW5, and SW6 are 3.46, 3.26, 2.47, 1.63, 2.40, and 5.57 respectively (Table 5). The larger values of Lg which is shown by all the subwatersheds indicate gentle slopes with longer flow paths for the runoff. Constant channel maintenance (C) which is reciprocal of the Drainage density (Dd) indicates how much watershed drainage area is required to maintain a unit length of a channel [60]. The computed values of C for the subwatersheds SW1, SW2, SW3, SW4, SW5, and SW6 are 6.92, 6.51, 5.48, 3.26, 4.80, and 5.14 respectively (Table 5). The large values of the C which is shown by all the subwatersheds indicate the watershed has high resistive soils, with high vegetation and comparatively a plane area which also indicates the subwatersheds are less prone to erosion (Fig. 4b and 4d).
The elongation ratio (Re) depends on a wide variety of climate, geology, and relief of a watershed. If Re is greater than 0.9 then the watershed lies in the category of the circular watershed, if Re lies in between 0.9-0.7 then it is categorized under an oval watershed and if Re lies less than 0.7 then the watershed is called elongated in shape [14]. The computed values of Re show the SW4 has larger elongation in shape as compared to other watersheds (Table 5), and it shows a larger concentration-time for the hydrograph peak for a particular storm event. The form factor (Ff) of a watershed indicates the duration of runoff to reach the outlet. The high values of form factors indicate high peak flows with shorter duration whereas the low form factor indicates the lower peak flow with longer duration [61]. The form factor of the subwatersheds of the Kuttiyadi river is 0.446, 0.439, 0.292, 0.212, 0.171, and 0.363 for the watersheds SW1, SW2, SW3, SW4, SW5, and SW6 respectively (Table 5). Results indicate the subwatersheds have low values of form factors which show that the lower peak flows so the runoff will take a large time to reach the outlet of the watershed. Result also shows the watersheds are elongated in shape and they are quite capable to manage flood flows as compare to circular watersheds.
The compactness coefficient (Cc) expresses the relationship of a watershed with that of a circular watershed having the same area. A drainage channel in the circular watershed drains the water in the shortest period as compared to the other shapes to the outlet i.e. the peak of a hydrograph will occur in the shortest period after a storm event [62, 63]. If Cc approaches unity, the watershed behaves like a completely circular watershed and if Cc is greater than 1 watershed shows more deviation from its circular shape [64]. The computed values of Cc are greater than one suggesting that all the watersheds deviate from the circular shape and show some elongation. The Cc value of 3.49 for the SW4 shows higher elongation in its shape and consequently, it will have the longest concentration time to reach the water at the watershed outlet (Table 5).
The circulatory Ratio (Rc) of the watershed influences the frequency and length of the streams, geological structures, LULC, relief, climate, and slope [53]. The computed values of Rc for the subwatersheds SW1, SW2, SW3, SW4, SW5, and SW6 ranges from 0.064 to 0.436 (Table 5). The SW1, SW2, SW3, and SW6 show a large value of Rc as compare to SW4 and SW5. The low values of Rc might be due to the low relief and gentle slope shown by the SW4 and SW5 shows the larger time of concentration for the peak runoff and the rest of the watershed shows less time of concentration [14]. Also, low values of Rc indicate the watershed having permeable surface and sub-surface materials which allows infiltration and contributes to the base flow. Generally for any watershed, if the value of the Rotundity factor (Rf) approaches unity it indicates the perfectly circular watershed [46]. The computed value of the Rf ranges from 0.56 to 1.17 which indicates all the subwatersheds are elongated in shape (Table 5). The SW4 shows a very large elongation in its shape as compared to the other subwatersheds. The results show the watersheds having week stratum and permeable surface and sub-surface material which allows infiltration of the water and approaches to a flatter hydrograph with a larger time of concentration. The rate of flow and the erosion rate of a watershed along with the main river is largely affected by the shape index (Si) [40, 54]. It depends on the relief and length of the watershed. If a computed value of the shape index approaches to unity, the watersheds show maximum erosion rate. The computed values of Si are 1.72, 2.272, 2.480, 3.457, 3.563, and 2.698 for the subwatersheds SW1, SW2, SW3, SW4, SW5, and SW6 respectively (Table 5). The SW1 and SW2 shows a higher erosion rate with very little time of concentration and SW4 and SW5 shows less erosion rate and larger concentration-time as compared to other subwatersheds [46].
Table 5
Computed values of the aerial aspect of the geo-morphometric parameters for the sub-watershed of Kuttiyadi river
S.No | Dd | Fs | T | Lg | C | Si | Ff | Cc | Re | Rc | Rf |
SW1 | 0.145 | 0.056 | 0.119 | 3.46 | 6.92 | 1.762 | 0.446 | 1.513 | 0.754 | 0.436 | 0.560 |
SW2 | 0.154 | 0.046 | 0.079 | 3.26 | 6.51 | 2.272 | 0.439 | 1.626 | 0.747 | 0.378 | 0.569 |
SW3 | 0.182 | 0.06 | 0.066 | 2.74 | 5.48 | 2.480 | 0.292 | 1.818 | 0.609 | 0.302 | 0.857 |
SW4 | 0.307 | 0.117 | 0.069 | 1.63 | 3.26 | 3.457 | 0.212 | 3.942 | 0.520 | 0.064 | 1.178 |
SW5 | 0.208 | 0.073 | 0.051 | 2.40 | 4.80 | 3.563 | 0.171 | 3.175 | 0.467 | 0.099 | 0.984 |
SW6 | 0.194 | 0.089 | 0.077 | 2.57 | 5.14 | 2.698 | 0.363 | 1.798 | 0.679 | 0.309 | 0.689 |
4.5 Surface Runoff and Sediment Production Rates
Runoff from the surface of a watershed is dependent on several hydrological parameters, including precipitation, evaporation, infiltration, and transpiration, which are themselves related to climatic conditions, lithology, structure, relief, and slopes [3]. From the analysis of the aerial aspect of geo-morphometric parameters, the volume of surface runoff per unit area of the subwatersheds are found as 32.88, 26.02, 16.15, 2.13, 7.42, and 16.57 km2-cm/km2 for SW1, SW2, SW3, SW4, SW5, and SW6 respectively (Fig. 5). The subwatersheds were categorized as a zone of low runoff to high runoff based on the estimated surface runoff values (Fig. 6a). The SW4, SW5, and SW6, SW3 will be categorized under the zone of low and medium surface runoff respectively which may be due to the elongation of watersheds, low drainage density, and reasonable geological control over the surface runoff pattern while the SW1 and SW2 were categorized under the zone of high surface runoff which is maybe due to the less elongation, high drainage density, and the subwatersheds doesn’t have reasonable geological control over the pattern of surface runoff. The accumulated estimated surface runoff at the watershed outlet is 1019.7 mm which is nearly equal to the measured surface runoff as 944.03 mm for the Kuttiyadi watershed [66]. The Sediment production rate of the subwatersheds of the Kuttiyadi river are estimated as 0.017, 0.010, 0.0036, 0.0004, 0.0006 and, 0.0038 (Ha-m/100.km2/year) for SW1, SW2, SW3, SW4, SW5, and SW6 respectively (Fig. 5). The categorization of the SPR shows SW1 is more prone to soil erosion followed by SW2 whereas, SW3, SW4, SW5, and SW6 are less prone to soil erosion (Fig. 6b). The high erosion rate of the sub-watershed was due to the combination of steep slopes, the presence of sandy loam and clay loam soils, and less forest cover over the subwatersheds. The subwatersheds which are less prone to soil erosion having adequate high forest cover and gentle slope over the watersheds. The findings demonstrate a strong relationship between the geo-morphometric parameters and terrain slope in the watershed which is sensitive for the generation of surface runoff and sediment production. As a result, it is vital to emphasize these parameters in developing regional systems intended to manage, protect, and develop land resources. According to the numerical model used in this study, the drainage characteristics of basins have a direct influence on erosion rates and over sediment yield. Despite the fact that the effect of catchment land use on the overall sediment yields of rivers around the world is less clear, there have been some instances of both increasing and decreasing sediment yields. Studies of the impacts of both land-use change and climate change on river sediment loads must often separate out their impacts, though they may be closely linked. Understanding the role of sediment production rate within a drainage basin inevitably means understanding the linkages between land use, erosion, and sediment yield within a river basin.