Morphometric analysis of the Middle Tuirial watershed, Mizoram, India and its significance for soil loss risk

This study aims to highlight the quantitative analysis of Middle Tuirial watershed in Mizoram, India and its significance for soil loss risk. In addition to understanding landscape evolution and hydrological characteristics of the river basin, it is crucial to implement appropriate soil and water conservation practices, to reduce further soil erosion risk in the basin. Linear, areal and relief aspects of morphometric parameters were analyzed using a survey of India topographical maps, advanced land observing satellite (ALOS) phased array type L - band synthetic aperture radar (PALSAR) digital elevation model (DEM), and geographic information system (GIS). The study reveals a high drainage density of 5.22 km/km 2 and stream frequency of 10.58 km 2 , which denotes the basin exhibits high surface run - off with low ground - water recharge. In addition, it has 0.6 form factor, 0.3 elongated ratio and 0.43 circularity ratio indicating a highly elongated shape of the basin. Furthermore, the hypsometric integral values of 0.48 with an s - curve show the basin has attained a mature stage of landscape evolution.


Introduction
The analysis of landforms, shape and surface configuration measuring quantitatively and mathematically is known as morphometry.The morphometry of the river basin reveals the characteristics of the river catchment of hydrological and geomorphic processes including the drainage system.Geology, soil texture, terrain, topographic features and climate are the natural factors that govern river flow, direction, and pattern of drainage system in the watershed.Horton 1,2 introduced the quantitative measurement of the drainage basin to analyze the drainage system.Later, adjustment and further analysis were performed by Strahler 3,4 .
Pioneering works of Horton 1,2 are concentrated in stream number, stream length, drainage density spatial approach of morphometric studies of the river basin is successfully conducted by various researchers. 10,11They found out that, morphometric analysis by remote sensing (RS) and geographic information system (GIS) technique provides the principle information of landforms and hydrogeological features of the watershed.The advantage of geo-spatial approach is low in capital investment, expenditure and time saved.In the present study the linear aspects, areal aspects, and relief aspects of the morphometric parameters of middle Tuirial river basin were computed by applying formulae derived from pioneering works with geospatial data and GIS techniques.No significant study was conducted in this area with an attempt to establish relationship of relief aspects with hydrological parameters.This study aims to highlight the quantitative analysis of middle Tuirial watershed along with the morphometric parameters and its significant contribution for the soil loss risk.In addition to understand landscape evolution and drainage characteristics of the basin, to recommend the appropriate techniques for soil and water conservation .

Study Site
The Middle Tuirial river basin is geographically located in between 23° 48' N -24° 28' N latitudes 92° 45' E -93° 58' E longitudes (Fig. 1).It forms the boundary between the two districts of Aizawl and Kolasib.Tuirial hydro-power station is constructed along the river with having 60 MW installed capacity to meet the requirement of power demand in Mizoram.The middle Tuirial river basin spreads an area about 683.10 km 2 with having 54 perennial tributaries river on both sides.The basin receives about 2000 mm of annual rainfall, mostly influenced by the southwest monsoon.There are 40 settlements, mostly found along the water divide.This river provides the water demand for public and irrigational facilities.Tuirial River basin is divided into three watersheds such as upper Tuirial D2CBAR26, middle Tuirial D2CBAR27 and lower Tuirial D2CBAR28, of which middle Tuirial watershed D2CBAR27 is selected for this study.Built-up land agricultural land, bamboo forests, open forests, and dense forests are the prominent land use and land cover types in the basin, while loamy, clay, and silt are the common soils found in the basin.In addition, shale, siltstone and sandstone are the major lithounits of Bokabil and Bhuban formations under the Surma group believed to be formed in Miocene to upper Oligocene epoch.

Data used and methodological approaches
Remotely sensed ALOS PALSAR digital elevation model (DEM) at 12.5 metres spatial resolution is acquired from Alaska university website 12 for analysis of Stream length ratio (L ur ) Bifurcation ratio (R bm ) R bm = average Horton 2 Basin length (km) ArcGIS Relief features Maximum basin height (m) (H) ArcGIS

Minimum basin height (m) (h) ArcGIS
Basin relief (m) (R) R = (H -h) Schumm 5 Relief ratio (R r ) Rr = R/L b Schumm 5 Relative relief (R hp ) R hp = R/A Smith 27 Dissection index (D i ) Di = R hp /R a Pike and Wilson 7 Gradient ratio (G r ) Gr = (a -b)/L s Sreedevi et al. 10 Ruggedness number (R n ) Rn = R × D d Schumm 5 Hypsometric integration (HI) Elmean-Elmin/Elmax-Elmin Pike and Wilson Form factor (F f ) Where, N u = number of stream segment, L u = total stream length of order (u), L u -1 = the total stream length of its next lower order, N u + 1 = number of segments of the next higher order, R l = stream length ratio, R b =bifurcation ratio, H = highest elevation, h = lowest elevation, B h = basin relief, L b = basin length, R = basin relief, A = basin area, R hp = relative relief, R a = absolute relief, a = the elevation at the source of the river, b = is the elevation at mouth of the river, L= is the length of main stream, D d = Drainage density, P = perimeter of basin.
Table 1: Details of morphometric parameters and equation slope, elevation, areal and relief aspects of morphometric parameters.The Tuirial river shape file is acquired from Mizoram remote sensing application center (MIRSAC) for the analysis of linear and areal morphometric parameters.The river catchment is digitized from survey of India toposheets after rectification and projection to WGS 1984 UTM 46N coordinate system.The daily rainfall data at 9 rain gauge stations is acquired from National Aeronautics and Space Administration 13 (NASA) power to generate rainfall thematic layer.
Since the river basin covers a large area that falls in two tiles of DEM were merged using ArcGIS 10.4 software.The study area shape file is used as a mask layer for clipping the DEM by clip vector tool and re-projected to WGS 1984 UTM 46N.Using hydrological tools, fill sink command was executed by Wang and Liu method, and other hydrological methods were performed by various pioneering works to accomplish the morphometric analysis (Table 1).

Results and discussion
Linear morphometric parameters of middle-Tuirial sub-watersheds According to Horton stream ordering, the  3 and Fig. 2), with the variation in stream order and size of the watershed influenced by structural and physiographic conditions. 14The sub-watersheds exhibit varying drainage patterns like, trellis, dendritic, sub-dendritic and parallel, of which the dendritic pattern is predominant as the region is controlled by physiographic conditions and active tectonic activity. 11The middle-Tuirial watershed was counted for 7273 streams, of which 882 streams fall in D2CBAR27l and 277 streams found in D2CBAR27b sub-watersheds to contribute the highest and lowest stream frequency, respectively.The total stream length of the watershed decreases with the increase in the stream order as per the law of Strahler.But, in origin and outlet, whereas in Strahler stream order it shortens the main stream length because more than one stream confluences form the bigger stream order, then it forms the main river.The middle Tuirial watershed stream length constitutes 1 st stream order of 47.26 %, 2 nd order 22.85 %, 3 rd order 13.36, 4th order 6.34 % 5th order 4.93 %, 6 th order 2.02 % and 7 th order of 3.24 % of the total stream length.The 7 th order stream length (115.63 km) is higher than that of the 6 th (72.01 km) as the similar result highlighted by Iqbal and Sajjad 14 due to variations of the slope, topographic and lithological 15,16 .In addition, the steep slope and length of the slope with gentle slope gradient indicate short and longer stream lengths respectively. 17The bifurcation ratio of the middle Tuirial River basin is 4.08, 2.77, 4.08, 3.60, 1.30, and 0.82 for the stream order of 1, 2, 3, 4, 5, and 6, respectively.The mean bifurcation ratio ranges from 2.89 to 4.36.The highest bifurcation ratio was found in D2CBAR27e with 4.36 while 2.89 is the lowest in D2CBAR27i.The bifurcation ratio between 3 and 5 indicates the drainage system of the basin is not controlled by geological structure, whereas less than 3 indicates that the basin is plain. 4In this study, the middle Tuirial watershed, the mean bifurcation ratio ranges between 2.89 and 4.36 which implies the basin of the geological structure has no significant impact on the hydrological pattern, but it is highly affected by topographic structure (Table 3).

Areal features of middle Tuirial watershed
The analysis revealed that these subwatersheds such as D2CBAR27a, D2CBAR27c, D2CBAR27f, D2CBAR27h, D2CBAR27j, D2CBAR27l, and D2CBAR27n are exhibiting high drainage show moderate drainage density with 4-6km/km 2 (Table 2), whereas low drainage density with < 4 km/km 2 was found only in D2CBAR27g sub-watershed.The average drainage density of the basin is 5.22km/ km 2 .This implies that the basin is highly susceptible to erosion 2,4,18 with high run-off for triggering landslides, 11 low infiltration rate, high relief with low groundwater recharge.Whereas, low drainage density implies high groundwater recharge, coarse drainage texture and dense vegetation with low relief of the basin 19 but, the middle Tuirial river basin has high relief with dense vegetation cover (Fig. 5).The average elongated ratio of the middle Tuirial basin is 0.37.It is against Strahler 4 stated values of 0.6 to 1, it continues further, if the value close to 1 denotes homogenous low relief with a circular shape, inverse in the values close to zero is characterized by high relief and high infiltration. 20In addition to the above, it is highly elongated with a low elongated ratio.This less elongated ratio of the middle Tuirial River basin implies high surface runoff with huge sediment transport towards low gradient.The average form factor of the middle Tuirial basin is 0.62, as Chopra et al. 21stated if the value is less than 0.45 indicates the basin is more elongated, and a value greater than 0.78 exhibits more circular shape of a basin. 4The sub-watershed D2CBAR27c, D2CBAR27f and D2CBAR27g are close to circular shape with more than 0.8 value, it indicates prone to high peak flow for short-term which led to flash floods in the lower course of river. 22In addition to elongated ratio and form factor, the circularity ratio is also inferring the shape of the basin.The length of stream and frequency are influenced circularity of the basin.The value ranges from 0 to 1, if the values close to one more circular and vice-versa.20 The subwatershed D2CBAR27a and D2CBAR27b are characterized by more than 0.6 circularity value.

Relief features of morphometric parameter of Middle Tuirial watershed
The middle Tuirial watershed is highly variation of relief features, it has an altitude between 59.31 metres to 1728 meters above sea level at the minimum and maximum elevation, respectively.The average basin relief of the middle Tuirial watershed is 1035.77meters which indicates steep slopes with high erosion intensity.The basin relief ratio infers the ratio of basin relief to the length of the basin5.In the present study of the average basin relief ratio and relative relief ratio are 0.11  Whereas, sub-watersheds like D2CBAR27c, D2CBAR27h, and D2CBAR27i are characterized with low slope gradient and less erosion intensity.The river basin is highly dissected (Fig. 4a).The ruggedness number indicates the level of flatness and roughness of the basin topography. 22Strahler 3 stated that the ruggedness value varies from 0 to 1.The value close to 1 indicates high in basin relief and smoother near to 0. The average ruggedness value of the middle Tuirial watershed is 5.38.An empirical study conducted in Tlawng river basin of India and Dhidhessa river basin in Ethiopia showed ruggedness value of 3.111, 23 it implies the basin is characterized by highly rugged terrain and more susceptible to soil erosion. 3,11,24psometric integral and curve analysis Hypsometric Integral (HI) is the computation of partial basin area to relative altitude.25 Hypsometric curve indicates the stages of a drainage basin, like convex curve specify youth stage, s-curve indicate mature stage and concave show old stage of the basin.3 The average hypsometric integral of middle Tuirial subwatershed ranges from 0.47 to 0.48.As Strahler 3 stated HI values in the study arae range between 0.35 and 0.60 connotes maturity stage of landform and shrinkage of a huge volume of landmass over the past. Adtionally, HI values below 0.35 and above 0.60 signify the basin has been experiencing old age and youth stage of landform evolution, respectively (Fig, 3).

Significance of morphometric analysis for soil loss risk assessment
Soil loss is a major concern of environmental degradation in the present scenario over Mizoram.The geological setting, topographic conditions of rugged nature, steep slopes, high dissection, heavy rainfall, high surface run-off, elongated shape, dense stream network and the land use pattern of shifting cultivation are incorporated for prompting to heavy erosion. 26Linear, areal and relief aspects of the basin are the significant factors for the assessment of areas vulnerable to soil loss.Based on the analysis, it is observed that the basin with high density of 5.22 km/km 2 is experiencing more surface erosion by sheet flow and silt is deposited at the down slope resulting of the siltation problem.Similarly, the middle Tuirial river basin relief ranges from 59.89 -1728 metres above sea level which signifies undulating terrain (Fig. 4b).Huge variation of relative relief exhibits the degree of slope is high with more soil risk. 27n addition, higher the ruggedness number denotes that the area is prone to soil loss.Similarly, higher the value of form factor indicates the susceptibility of flood and high risk of peak flow.The steepness of slope in study area ranges from 0⁰-71⁰ it indicates the basin has high slope gradient.Higher the steepness of slope associated with heavy rainfall expected to be highly prone to soil risk (Fig. 5a; 5b; 6a and 6b).Land use and land cover pattern such as built-up area, agricultural land, bare-land and sparse vegetation cover are the significant factors of soil loss in the basin. 28

Conclusion
The present study focused on the significant of morphometric characteristics that lead to soil and water degradation in the middle Tuirial basin.Despite the high drainage density, stream frequency, relative relief and ruggedness values, the basin has reached a mature stage of landform evolution.Steep slopes, excessive rainfall and shifting cultivation system triggering extreme soil loss with less groundwater recharge.The erosion risk can be minimized in association of local people, non-government organizations (NGO's) and government by check dams with implementation of strict forest conservation measures.Furthermore, the shifting cultivators must emphasize on sustainable agro-forestry and settled farming.

Figure 1 .
Figure 1.Study area map

Figure 2 .
Figure 2. The sub-basin wise stream order

Table 2 :
Areal aspects of middle Tuirial sub-watershed Horton stream ordering one river is assigned as one order with no segment, because each river has an

Table 3 :
Linear aspects of middle Tuirial sub-watershed

Table 4 .
Relief aspects of middle Tuirial sub-watershed