Study Area
The Tuirial River Basin covers 1419.17 km2 area of land and water, it lies between longitudes 92°42′E–92°52′E and latitudes 23°26′N–23°52′N. The highest elevation is 1690 m above MSL seen at Hmuifang hill in the south western corner of the water divide line, and about 23 m above MSL is the lowest at Saipum village north eastern corner of the watershed within the state of Mizoram. The river is about 117 kilometres long, from source to outlet. It originates from the eastern site of Chawilung village at an elevation of 1083 metres above MSL about 62 km from Aizawl city. It flows northward to join the Barak River in Assam. It also formed the district boundary between Kolasib and Aizawl districts on the eastern side. Important perennial streams like the Lungdai Lui, Tuisen Lui, Keitum Lui, Tuiritai Lui, and Hachhe Lui join the Tuirial river before it joins the confluence of the Barak river in the Cachar district of Assam. As the watershed is formed by the structural hills and valleys, it is elongated and spreads in a south-to-north direction. Clay, Loamy soil, Fine loamy, Loamy skeletal, and Coarse loamy are the soils found in the Tuirial watershed. The mean annual precipitation is about 2500 mm of which most of it is received during southwest monsoon in the months of early June to September. Dense, medium, open, and bamboo forests are the different forest types occur in this watershed.
Material Used
The Tuirial basin water divide was extracted from the survey of India’s topographical sheets of 83D/14A, 83D/14B, 83D/15, 83D/16, 83H/3, 83H/3, 84A/12, 84A/13, 84A/14, 84A/15, 84E/1, 84E/2, and 84E/3 at 1: 50,000 scale, with reference to the Watershed Atlas of Mizoram using QGIS 3.22 software. The thirteen (13) topographical sheets covering the watershed area were rectified and mosaicted to transform the coordinate system to the WGS_1984_UTM_Zone_46N projection system before digitizing settlement and linear features like streams, and roads (Fig. 1). The rainfall data from seventeen rain gauge stations was collected from NASA power data. A soil texture map was generated using the soil texture data obtained from Mizoram Remote Sensing Application Centre (MIRSAC). The LULC types of the study area were classified from the satellite data (Sentinel 2C) based on-screen visual interpretation techniques in the GIS environment. Advanced Land Observing Satellite (ALOS) Phased Array Type L-band Synthetic Aperture Radar (PALSAR) digital elevation model (DEM) data at 12.5-metre spatial resolution was acquired from the ASF database for the generation of slope in degrees, flow direction, and flow accumulation (Table 1).
Table 1
Source | Available at | Data |
COPERNICUS | https://earthexplorer.usgs.gov/ | SENTINEL 2C |
ASF | https://search.asf.alaska.edu/#/ | ALOS PALSAR 12.5m |
MIRSAC | Directorate of Science and Technology, Mizoram | Soil Texture |
NASA POWER | https;//Power/larc/data/access | Rainfall data |
Google Earth | https://www.google.com/intl/en_in/earth/versions/ | Study Area |
Survey of India | Department of Geography and Resource Management, Mizoram University | 83D/14A, 83D/14B, 83D/15, 83D/16, 83H/3, 83H/3, 84A/12, 84A/13, 84A/14, 84A/15, 84E/1, 84E/2, 84E/3 |
Methodology
The RUSLE (Renard et al., 1997) which is widely used approach was used to estimate average annual soil loss (A) and erosion risk zones in the Tuirial basin (Eq. 1). Also, the details methodological flow chart highlighted in Fig. 2.
A = R×K ×L×S×C×P, (1)
Where A means averages annual soil loss per unit area (t ha− 1yr− 1), R is the rainfall erosivity (MJ mm ha− 1 yr− 1), K is the soil erodibility factor (ha hr. MJ− 1 mm), L is the slope length factor, S is the slope steepness factor (dimensionless), C is the cover management factor (dimensionless), and P is the support and conservation practice factor (dimensionless).
Rainfall Erosivity Factor (R)
The Rainfall erosivity factor of soil loss depends on the amount and intensity of rainfall (Renard et al., 1991), and it is also the most important to determine soil loss (Thakuriah, 2023). The rainfall erosivity factor is determined by two rainstorm characteristics like kinetic energy (E) and the maximum 30-minute intensity (I) (Wischmeier, 1959). In the present study, thirty (30) years (1992–2022) of NASA power daily rainfall data of 413-metre resolution at seventeen rain gauge stations was downloaded to compute the R factor. The rainfall data was interpolated using the inverse distance weighted (IDW) tool as this method is suitable for smooth rainfall distribution due to the least error (Bakis et al., 2021). Thus, the rainfall erosivity factor (R) was estimated using (Eq. 2) the raster calculator in ArcGIS 10.4, after Chattopadhyay et al. (2014) and Vanlalchhuanga et al. (2021).
R = 79 + 0.363 x (2)
Where R is the rainfall erosivity factor (MJ.mm ha− 1 hr− 1 yr− 1), x is mean annual rainfall (mm).
Soil erodibility factor (K)
The susceptibility of soil loss is determined by the soil texture, grain size, organic content in the soil, surface run-off, and the amount and intensity of rainfall (Prasannakumar et al., 2011; vanlalchhuanga et al., 2021; Thakuriah, 2023). Soil texture and the attribute information were prepared by the data obtained from the Mizoram Remote Sensing and Application Centre (MIRSAC, 2015). K factors for each soil texture in the study were identified from the soil erodibility value after Barman et al. (2020). The K value of each soil texture was added to the attribute table and converted into a raster thematic layer using ArcGIS 10.4.
Slope Length and Slope Steepness (LS) Factors
Slope length (L) is the length of distance from the origin to where the slope gradually decreases to the extent that deposition begins and finally enters the river channel (Wischmeier and Smith, 1978). Slope steepness (S) is the angle of inclination of the slope and is a dimensionless factor. The higher the slope length and the angle of inclination, the higher the possibility of a higher risk for soil erosion (Renard et al., 1977; Wischmeier and Smith, 1978). In the initial period, the USLE and RULSE developed for gently slopes areas for one dimension however, in the rugged terrain region it becomes two-dimensional, making dimensional to estimate LS factor is difficult (Van Remortel et al., 2004). There are various methods of LS factor calculation, although, in this study we adopted LS factor computation after Van Remortel et al., (2004) and Moore and Wilson (1992). The LS factor was determined using ALOS PALSAR DEM data following the following equation.
$${\lambda =\left[ \frac{Flow Accumulation}{3.1416}\right]}^{0.5}$$
3
$$\beta =\frac{\text{sin}\theta }{\left[3\bullet {\left(\text{sin}\theta \right)}^{0.8}+0.56\right]}$$
4
$$m=\frac{\beta }{\left(1+\beta \right)}$$
5
$${L=\left[ \frac{\lambda }{22.13}\right]}^{m}$$
6
$$S=10.8\bullet \text{sin}\theta +0.03 \theta <9\%$$
7
$$S=16.8\bullet \text{sin}\theta -0.5 \theta \ge 9\%$$
8
$$LS=\left[{\left(\frac{"\lambda "}{22.13}\right)}^{\wedge } "m"\right]\times "S"$$
9
where λ is flow accumulation, m is a variable length-slope exponent, β is a factor that varies with slope gradient, and θ is the slope angle, L is the slope length (m), S is the steepness of slope.
Cover Management Factor (C)
The cover management factor is defined as the ratio of soil loss to a specific land use land cover. The P factor ranges from 0 to 1, depending upon the types and patterns of land use and land cover. It also varies with the rate and amount of soil loss depending on the land use and land cover. The area where vegetation indirect contact with raindrops on the soil particles has less erosion. While the area with bare land has more soil loss due to the direct impact of raindrops on the soil surface (Prasannakumar et al., 2012; Rahaman et al., 2015), Sentinel 2-C multispectral satellite data of 10 metre spatial resolution acquired on March 3, 2023, was used to prepare a land use and land cover layer of the study area using ArcGIS 10.4 software. Settlement, current cultivation, fallow land, water bodies, bamboo, dense, medium, and open forest types were identified as land use types in the study area (Fig. 11). The C factor value corresponding to each land cover condition was assigned as per the study carried out by Barman et al (2020).
Practice Management Factor (P)
The practise management factor is the rate and amount of soil loss under the conservation practise compared with the up and down hill slope cultivation (Dabral at el., 2008; Das at el., 2018; Vanlalchhuanga at el., 2021; Thakuriah, 2023). Practise management factors such as contouring, terraces, proper drainage, and settled agriculture help to reduce the rate and amount of soil loss because they reduce surface run-off (Renard and Foster, 1983). As per the practise management factor, the numerical values from 0 to 1 are constant. The value close to zero denotes that the conservation practise is good, whereas close to 1 refers to poor practise management (Chatterjee et al., 2014; El Jazouli et al., 2017; Ozsahin et al., 2018). The P factor values were added to the attribute table and converted into a raster thematic layer using ArcGIS 10.4. The magnitude and the spatial distribution of P factor value ranges from 0.28 to 1.00 with a mean value of 0.97 (Fig. 12).
Thematic layer integration using Spatial Analyst tool
The computed values of R, K, C, P, and LS factors were converted into a thematic raster layer, which was integrated through a raster calculator using ArcGIS 10.4, to derive the soil erosion layer of the Tuirial watershed. The thematic layer was reclassified into seven erosion risk zones in terms of ton/ha− 1/yr− 1 of soil loss by spatial analyst tools. Then, the raster layer of the soil erosion map was converted into a polygon to compute each soil loss area.