Integrative Assessment of Geomorphic Indices and Land Cover Dynamics for Sustainable Development: A Case Study of the Asan Watershed, Doon Valley, Uttarakhand, India

Understanding watershed dynamics and the application of geomorphic indices and GIS in watershed management play crucial roles in ensuring sustainable water resources and environmental conservation. By comprehending the intricate interactions between landforms, hydrology, and human activities within a watershed, researchers can make informed decisions for effective water management, erosion control, and flood mitigation, assess the health of watersheds, identify vulnerable areas, and implement targeted interventions, fostering a more resilient and ecologically balanced watershed management approach.

The present paper describes the drainage and terrain characteristic of the Asan watershed obtained through Remote Sensing and a GIS-based approach. Entire geomorphic and hydrologic processes that occur within the watershed contain significant information about their formation and development [1]. Geomorphic indices are responsible for providing a numerical measure of the stream network, which is an important aspect in determining a watershed’s characterization [2, 3] was the first to use quantitative techniques (mathematical equations for indices) in morphometric analysis of drainage basins from a topographic map using manual methods. Later, certain modifications in related equations were adopted by the studies [4-7].

GIS techniques are currently used for the extraction of stream networks and characterization of the watershed by several researchers [8-10]. Also, this technique helps assess various terrain attributes through the generation and study of Slope, Aspect and LULC analysis [10-12].

Several researchers have studied morphometric properties for different terrains as an indicator of structural influences on stream development [13-17]. The morphometric and morphotectonic properties have proven to be an important tool in the generation of up-to-date and statistical information for drainage basin characterization [10]. The geomorphological setup is strongly affected by the deformational processes that occur within the watershed. It is strongly dependent upon the tectonic activities occurring in it, which influences the drainage system, terrain topography and landscape development.

Many researchers studied the interplay between the structure of the drainage system and other factors such as basin relief, geology and climatic conditions present in the watershed [4, 18, 19]. In structurally active regions, the drainage network is susceptible to tectonic features such as faults, thrusts and folds, as well as tectonic movements caused by incision, diversion, tilting and asymmetry [11]. Watersheds behave as the basic unit of the riverine formations and act as an ideal entity for understanding the tectonic activity in the area [2].

Numerous studies have examined the influence of Land Use and Land Cover (LULC) on land use dynamics [20] analysed LULC changes and drivers in the Gubalafto district, North-eastern Ethiopia, over 30 years using RS, GIS, and community surveys. Cultivated and settlement areas increased by 9% while grazing land and forest cover were drastically reduced due to population growth, land insecurity, poverty, and climate change. Ensuring the sustainable utilization of resources is crucial for safeguarding the ecosystem services of the region. The study [21] employs a research approach that integrates map interpretation, RS and GIS. The study identifies the expansion of urban residential areas and evolving economic dynamics as primary drivers of changes in land use/cover, ultimately resulting in landscape degradation. Furthermore, these alterations in and around Conakry city and its adjacent urban areas over 59 years have prompted concern within both the Guinean government and the scientific community. Socio-economic development, climatic patterns, topography manipulation, and policy implementation are the key factors affecting land use/cover. [5], highlight the significance of digital change detection techniques for understanding expansion in the area. The study used RS and GIS to assess land use/land cover dynamics in Ramnagar town, Uttarakhand, over two decades (1990-2010). The built-up area and sand bar increased by 8.88% and 3.98%, while vegetation, agricultural land, and water bodies decreased by 9.41%, 0.69%, and 2.76%, respectively. The expansion was mainly towards the southern direction along National Highway-121.

All of these parameters are further validated using field visits, toposheets, aerial photographs and satellite images.

Here, this multidisciplinary study demonstrates the interplay between human activities and natural processes. By adopting an integrated approach, the study advocates for sustainable practices that encompass biodiversity & environment conservation. This involves striking a balance between the demands of the growing population, fostering economic development and implementing enhanced environmental management practices. This understanding can be extrapolated to other watersheds, providing valuable insights for studying and managing watersheds worldwide.

This research aims to examine a detailed study of different geo-environmental aspects which influenced the study area in the long run. Further, these aspects have been categorized into three specific objectives: (a) Conducting a detailed examination of geomorphic indices, which includes evaluating morphometric and morphotectonic parameters. This analysis is based on an in-depth study of the drainage network. (b) Focus on the structural impact of the watershed (c) Generation of decadal LULC maps through different satellite datasets. These maps will serve as a crucial tool for studying changes over time.

The soul of the paper lies in the integrated parametric evaluation to understand the physiography and geometry of the watershed, which is associated with the study of its formation, development, structural imbalances and geomorphic processes. Further, the generation of LULC maps for different years helps us to study the historical and present condition of the watershed which leads to the rapid expansion of built-up areas over time compared to the forest and agricultural land.

Asan River flows along the axis of the asymmetrical synclinal valley. It arises in the clayey dipping in the west of the Asarori-Dehra Road and afterwards the northwesterly course of about 42 Km, it merges into the Yamuna. The physiographic units of the watershed are striking in NW-SE to ENE-WSW. The research area is located in the Doon valley. The watershed lies between Lesser/Lower Himalaya to the north and the Outer Himalaya (Siwalik) to the south. The northern and southern part of the watershed is characterized by rough mountainous terrain having rocks predominantly of sedimentary and metamorphic origin. Small seasonal channels from the Siwaliks and Mussoorie that meet the mainstream are almost dry for the majority of the year. The drainage pattern is influenced by slope, lithology and structure to determine the severity of erosion. It is also susceptible to tectonic features viz., faults, folding and tilting. The major portion of the study area is covered by a piedmont zone containing pebbles, gravels, boulders etc., with scanty exposures of Siwalik in the southern part of the watershed. These have been transverse by several structural dislocations, namely the Main Boundary Fault (MBF), exposed in the north along with other associated lineaments, some of which are tectonically active. The entire watershed is divided into four major hydrogeological/lithostratigraphic units viz., Lesser Himalaya, Siwaliks, Doon gravels and recent alluvium. Stratigraphically, each succession is the representation of age, thickness and environment of deposition [30, 31]. Their stratigraphic description is mentioned in the subsequent sections (Figure 5, Table 3).

The directional alignment of structural features such as faults and thrusts from NE to SW through the north-western part of the Doon Valley. The study area is bounded in the north by the Main Boundary Thrust (MBT) with the Lesser Himalaya and in the south by the Mohand Thrust with the Lower Siwalik, which falls outside the watershed. The MBT situated in the region brings Lesser Himalaya (Precambrian) strata over Doon fan gravel (Quaternary). The activities of various faults and thrusts provide maximum relief into the watershed [32]. The Santaugarh thrust [33, 34], the Bhauwala thrust [35], the Asan and Tons Faults [36], are the other structural features present in the area. The MBT and rocks of Middle Siwalik ride over the older fan deposits whereas the younger fan deposits are eroded into the Santaugarh Fault hanging blocks. This indicates that the fan gravel found in the area is derived from the erosion of MBT and Santaugarh hanging blocks.

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Figure 5. Geological and structural map of the Asan watershed [30, 34]

The foothills of the previously mapped Mussoorie hills descend in a number of large fans made up of pebbles, cobbles, and boulders with a sandy matrix [37]. An alluvial fan is formed by the accumulation of sand, gravel, silt, etc. (collectively known as alluvium) over a long period due to the interaction of the river of the stream with mountains or hills. Because of their resemblance with a fan or section of a shallow cone, it is called an alluvial fan. It is one of the major geomorphological units of the watershed identified by the studies [32, 36]. The maximum and minimum elevation of the Alluvial fan ranges from 1778m to 345m above MSL and covers an area of 133.51Km²~18.74% of the entire watershed and the slope ranges from <5° i.e., gentle. The three alluvial fans Donga, Dehradun, and Bhogpur flow from west to east through the Lesser Himalaya's front range, depositing terraces made of a loose mixture of gravel, sand, silt, and clay along the Asan, Song, and Tons rivers. In addition, a system of springs covers the entire area [31]. Both the northern and southern portions have rugged mountainous terrain, with the majority of the rocks being sedimentary and metamorphic in origin.

Table 3. Lithostratigraphic succession of Doon Valley [30, 35]

The study [40] defines Bifurcation ratio ($R_b$) as “the ratio between the total number of streams ($N_u$) and the number of streams of the next higher-order ($N_u$+1)”. The value of the ‘$R_b$’ for the watershed ranges from 3-5 for all orders. The drainage network with high '$R_b$' indicates that the area is tectonically active with prominent soil erosion [2]. Higher ‘$R_b$’ represent the younger stage while the lower value indicates the mature stage of development [41]. The mainstream (Asan River) passes through the Asan Fault (AF) which corresponds to higher ‘$R_b$’ (Figure 5). This supports that mainstream formation is younger than the geologic structures. Mean Bifurcation Ratio ($R_{bm}$) for the watershed is 3.9.

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Figure 7. Stream network belongs to different stream order of the study area

Based on the classification given by the study [42], ‘$R_{b m}$’ is classified into three categories <3: flat region; 3-5: least effect of geologic structure on drainage network; >5: drainage network is structurally controlled. The calculated value of ‘$R_{b m}$’ indicates low structural impact and the presence of moderate slopes on the drainage development of the watershed.

5.1.2 Aerial aspect of the watershed

The aerial aspects of the watershed share a significant connection with the study of two-dimensional features of the watershed. The mathematical equations associated with the parameters are described in Table 5.

The texture and proximity of channels within a watershed are predicted by drainage density $\left(D_d\right)$. Based on the classification adopted after [43], the value of ‘$D_d$’ ranges from 0 to 5 based on lower to higher degree of density; 0-1: Low, 1-2: Moderate, 2-3: Moderately high, 3-4: High, 4-5: Very high. In the present study, it is calculated as 2.1 Km^-1 which indicates that the area comes within the range of medium drainage density and rock surfaces drained by the watershed having moderately permeable strata with medium run-off and infiltration [41]. Drainage texture ($D_t$) is a degree of the relative spacing of drainage lines in a fluvial-dissected terrain [6]. The calculated ‘$D_t$’ for the area is 6.32 Km. According to Smith’s classification, the different categories of ‘$D_t$’ are: <2: Very Coarse; 2-4: Coarse; 4-6: Moderate; 6-8: Fine; >8: Very fine. Based on the classification, the watershed represents moderate drainage texture. It also indicates that the area is developed on a bedrock of moderate permeability (e.g., thin-bedded sandstone). Stream frequency ($F_s$) is “the total number of stream segments of all orders per unit area” [43]. Based on the classification which categorises ‘$F_s$’ into five categories: 0-1: Low; 1-2: Moderate; 2-3: Moderately high; 3-4: High; 4-5: Very high. Using the methodology (Table 5), the calculated ‘$F_s$’ for the study area is 1.64 Km² which indicates that the watershed has moderate ground slopes, rock permeability, run-off and infiltration capacity of the watershed [43, 44]. The length of overland flow ($L_g$) is “the length of water over the ground before it gets concentrated into certain stream channels approximately equal to half of the reciprocal of drainage density” [4]. It affects the geographical and hydrological development of the watershed. Based on the length of the flow path, ‘$L_g$’ is classified into different categories: <0.2: Low flow path; 0.2-0.3: Moderate flow path; >0.3: High flow path. The calculated value of ‘$L_g$’ is 0.24 shows a well-developed stream network with moderate slopes showing surface runoff travels at a relatively shorter distance before flowing through the channels. The horizontal projection of the watershed shape can be represented using the Elongation Ratio ($R_e$) [45]. The calculated value of ‘ $R_e$ ’is 0.47. According to the classification adopted after [40], ‘$R_e$’ is divided into different categories based on the shape of the watershed: <0.7: Elongated; 0.7-0.8: Less elongated; 0.8-0.9: Oval; 0.9-1.0: Circular. This indicates that the shape of the watershed is elongated with low to medium relief and moderate infiltration.

5.1.3 Relief aspect of the watershed

The relief aspects of the watershed share a significant connection with the “study of the three-dimensional feature includes area, volume and altitude of the vertical dimension of landform to analyses of varying geohydrological characteristics” [6]. This aspect helps in the study of various geomorphological features which includes analysis of terrain relief, computing surface, permeability, sub-surface flow, and landform development. The mathematical equations associated with the parameters are described in Table 5.

Basin Relief (H) described as “the basin relief refers to differences in elevation between the drainage summits and the watershed outlet” [6]. It is helpful to understand the processes of geomorphology and several landform characteristics. Basin Relief (H) of the watershed is 1833m, with a minimum value (z) is 349m in the vicinity of the mouth (outlet) and a maximum value (Z) of 2182m in the Lesser Himalayan region. It indicates runoff and transportation of sediments from the upper to the lower reaches of the watershed. Relief ratio ($R_h$) is “the indicative of the stream’s gradient flow and intensity of erosional process operating on the slope of the watershed” [40]. According to the classification adopted after [3], the ‘$R_h$’ is classified based on the degree of steepness of the slope present in the watershed and is categorised into several categories: >0.29: Steep; 0.28-0.17: Moderate; <0.17: Gentle. The calculated value of ‘$R_h$’ is 0.23, which indicates the dominance of moderate to gentle slopes and less resistant rocks in the watershed. Ruggedness number ($R_n$) measures the extent to which basin topography is dissected [7]. According to classification of basin ruggedness [46], the different categories include: <1: Low; 1-2: Moderately low; 2-3: Moderate; 3-4: Moderately high; >4: High. The calculated ‘$R_n$’ of the study area is 3.8 which suggests that the watershed has rugged topography and is susceptible to erosion.

5.2 Morphotectonic parameter

The purpose of the qualitative and quantitative analysis is to describe the part of tectonism that controls river behaviour and the development of drainage networks [14, 47, 48]. The entire Doon valley has experienced strong denudational forces and most of the streams in the valley transported a huge number of debris. Failure of sediments, deformed rocks, jointed and shattered rocks, and prominent landslides are all of the results of tectonism prevalent in the study area. The Doon is bordered on the east by Ganga and on the west by the Yamuna, both of the rivers are flowing along the tear fault mark. The Lesser Himalayan zone, which rises abruptly to approximately 2000m in the area, is represented by the northern portion of MBT, whereas the Siwalik hills, which rise to approximately 640m and are longitudinally transverse by the Santaugarh Fault, are represented by the southern portion of MBT [34]. A broad, active syncline runs through the heart of the Doon Valley. The MBT runs parallel to the Santaugarh and Bhauwala Thrust. Several parameters are used in the study with their mathematical equations, described in Table 6.

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Figure 8. Drainage Basin symmetry

5.2.1 Basin Asymmetry Factor ($A_f$)

The Basin Asymmetry Factor ($A_f$) is “the ratio of the right side of the stream facing downstream to the total area of the watershed” [15]. This parameter determines the tilting of the watershed due to lateral tectonic shift concerning the trunk-ordered stream at smaller as well as larger scales [15, 49].

From the methodology adopted by the study [50], the calculated value of ‘$A_f$’ is 57, which suggests tilting toward the left side of the area (Table 6). In Figure 8, it is also found that the mainstream of the watershed flows towards the NE direction and tilts toward the south. The tributaries on the right side of the trunk stream cover a larger area than the tributaries on the left, which cover a smaller area. When the underlying rocks are of the same composition, this parameter expresses the best result. The difference between an observed and neutral value is used to calculate the absolute value of ‘$A_f$’ is (50). Based on the classification adopted after [51], the absolute value of ‘$A_f$’ is categorised into different categories: $A_f$<4 (Symmetrical basin); 4<$A_f$<8 (Gently asymmetrical basin); 8<$A_f$<12 (Moderately asymmetrical basin); 12<$A_f$<16 (Asymmetrical basin); $A_f$>16 (Strongly asymmetric basin) the observed absolute value of ‘$A_f$’ is categorized the watershed into the gently asymmetrical watershed.

5.2.2 Hypsometric Integral ($H_i$) & hypsometric curve

Hypsometric Integral ($H_i$) is a general but significant geomorphic index for erosional development in the watershed. ‘$H_i$’ explains the distribution of elevation in the watershed [14]. The Hypsometric Index (value) and the Hypsometric curve are the result of the hypsometric analysis. Both of them provide details about the tectonic, erosional, lithological and climatic factors controlling it [52]. For the study area, the calculated value of ‘$H_i$’ is 0.50. Based on the shape of the hypsometric curve mentioned by the study [53], the ‘$H_i$’ values are grouped into three categories based on: $H_i$≥ 0.5: convex curves; 0.4≤$H_i$<0.5: concave-convex/s-shaped curves; $H_i$<0.4: concave curves (Figure 9). The classification is associated with young (weakly eroded), mature (moderately eroded) and old (highly eroded) respectively.

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Figure 9. Graph showing the hypsometric curve

Table 8. Classification of R² with alternate categories [54]

Another classification of ‘$H_i$’, is based on the watershed’s susceptibility to erosion (Table 6). The calculated value of ‘$H_i$’ reveals that the watershed is susceptible to fluvial erosion dominantly and corresponds to the mature stage of development (the susceptibility of erosion is also an indicator of the presence of the tectonic condition in the watershed and is associated with a dissected drainage network). The trend line represents a polynomial equation of the curve for the Coefficient of determination (R²). The calculated value for ‘R²’ is 0.87 which suggests a very strong relationship between the ratio of relative area and the relative elevation equation. To estimate the area under the curve, ‘R²’ values are incorporated within the range of 0 to 1 (based on the non-dimensional nature of the graph) for estimating the area under the curve. According to the ‘R²’ classification (Table 8), the watershed is sufficiently adjusted to the prevailing conditions [54]. These conditions can be climatic, topographic and tectonic.

5.2.3 Channel Sinuosity Index ($C_{S I}$)

Various researchers have briefly described and formulated Mueller’s sinuosity index, including [17, 55-57] has redefined this parameter to integrate hydraulics sinuosity (which is developed by a channel not influence any alignment related to valley-wall) and topographic sinuosity (which is disseminated by valley geometry). The hydro-geomorphic condition of the river, as well as other factors such as terrain characteristics, cause it to deviate from its previous path, which is well measured by ‘$C_{S I}$’. Figure 10, depicts the location of the valley and the length of the channel in the watershed.

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Figure 10. Channel Sinuosity Index showing Channel length ($C_l$) and Valley length ($V_l$)

The value of ‘$C_{S I}$’ calculated for the mainstream of the Asan River using the equation proposed by the study [17] is 1.3. This conventional classification suggests that the stream has a braided course (Table 6). The sinuosity of the river is dependent upon underlying topography which includes rock type and structure, climate and vegetation that dominate the area, the impact of different hydrological factors viz., evapotranspiration, precipitation, rainfall, temperature etc., sediment deposition in the course of the river channel.

5.2.4 Basin Shape Index ($B_s$)

The Basin Shape Index ($B_s$) was proposed by the study [14]. In tectonically active areas, it is one of the most important parameters for determining the relatively younger drainage basin. Basin shape influences the fluxes, runoff and sediment from headwater reaches. In the tectonically active area, the relatively younger watershed is elongated in a shape corresponding to the topographic slope or young active fault [14]. In the present study, the classification proposed by the study [58] is adopted. The calculated value of the ‘$B_s$’ for Asan watershed is 1.4 suggesting that the shape of the watershed is elongated and tectonically active (Table 6). This conversion in the shapes of the watershed is because the width of the watershed is narrower near the mountain front in the actively tectonic area.

5.2.5 Stream Length to Height Ratio ($S L_{index}$)

Stream Length to Height Ratio ($S L_{index}$) was given by the study [16]. The variation in channel gradient affects the $S L_{index}$ and this effect helps to evaluate the tectonism in an area. River gradient also reflects the variation of lithology and is an indication of the tectonically disturbed river [55, 59]. The structural features such as fault and structural lines are consistent with the steep slope. It also helps to evaluate relative tectonic activity. Various tectonic, lithological and/or climatic factors are responsible for the deviation of river profile [16]. In the present study, $S L_{index}$ is computed using DEM. For every 200m elevation difference, the distance from the source is determined in a GIS environment [59]. The main tributary stream of the 6^th order, five streams of the 5^th order and three streams of the 4^th order are taken for the calculation of $S L_{index}$ (Figure 11).

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Figure 11. Location of the 4^th, 5^th and 6^th ordered stream taken for the analysis of stream length ratio along with major faults and thrust

The longitudinal profile for corresponding streams and numerous pronounced topographic break or knick point distributions along the profile in the middle and upper reaches of all the prominent streams of the study area are shown in Figure 12. These breaks or knick points help in examining the polycyclic nature of landform development and are influenced by major tectonic structures (fault and thrust) present in the area.

Based on the categorization of highest to lowest values of $S L_{index}$ (Table 6). The average value of $S L_{index}$ for the selected streams is 531, which indicates that the watershed is tectonically active. The $S L_{index}$ is high where the channels flow over active uplifts and cross the soft rocks e.g., alluvial. Conversely, lesser $S L_{index}$ indicates low tectonic activity and also rivers flow over less-resistant harder underlying rocks [60] e.g., Doon gravels. The break distribution shown in Figure 12, is theoretically influenced by faulting or upliftment of bedrock due to tectonic activity, base-level variation due to quaternary climatic deviation and contrasting lithology behaviour of soft and hard bedrocks [61].

The first stream of the 4^th order designated as ‘4(1)’, reflects the stream-wise value varies from 0.5-397.5 m and the higher peaks of the profile of stream are flowing across the Bhauwala Thrust (BT), Majahaun Fault (MJF), Santaugarh Thrust (ST) and Main Boundary Thrust (MBT). The second stream of the 4^th order ‘4(2)’, reflects the stream-wise value varies from 1-347.5m and the higher peaks on the stream’s profile are traverse across the Main Boundary Thrust (MBT). The third stream of the 4^th order designated as ‘4(3)’, reflects the stream-wise value varies from 0.5-685m and the higher peaks on the stream’s profile traverse across the Main Boundary Thrust (MBT). The first stream of the 5^th order designated as ‘5(1)’, reflects the stream-wise value varies from 1-138m and the higher peaks on the stream’s profile are traverse across Asan Fault (AF). The second stream of the 5^th order ‘5(2)’, reflects the stream-wise value varies from 1-322m and the higher peaks on the stream’s profile are traversed across the Asan Fault (AF). The third stream of the 5^th order designated as ‘5(3)’, reflects the stream-wise value varies from 1-758m and the higher peaks on the stream’s profile are traversed across Tons Fault (TF) and Bhauwala Thrust (BT). The fourth stream of the 5^th order ‘5(4)’, reflects the stream-wise value varies from 1-680m and the higher peaks on the stream’s profile are traversed across the Bhauwala Thrust (BT), Majahaun Fault (MJF), Santaugarh Thrust (ST). The fifth stream of the 5^th order ‘5(5)’, reflects the stream-wise value varies from 1-780m and the higher peaks on the stream’s profile are traversed across the Bhauwala Thrust (BT) and Santaugarh Thrust (ST). The stream-wise mean value of $S L_{index}$ varies from 0.5 to 566m for the Asan River (mainstream) of the 6^th order of stream network. The higher peaks on the stream’s profile are traversed across the Asan Fault (AF) and Tons Fault (TF). Increasing the level of erosion, this fault and thrust zones along the river’s course may cause landslides.

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Figure 12. Longitudinal river profile and $S L_{index}$ of the fourth, fifth and sixth ordered streams (BT-Bhauwala Thrust; ST-Santaugarh Thrust; MF-Majahaun Fault; TF-Tons Fault; AF-Asan Fault; MBT-Main Boundary Thrust)

5.3 Terrain attributes

5.3.1 Slope

The slope is one of the significant features of the earth’s surface required for hydrological study in the area. Slope measures the steepness of the surface (in degrees or percent rise) at any particular location. Several researchers such as the studies [57, 62-66] proposed various methods for expressing slope. In geomorphology, the slope is an amalgamation of ‘form' and ‘process' which includes geology, climate, vegetal cover and process of weathering respectively. In the present study, slope maps are generated from DEM using the surface analyst module in ArcMap v.10.3 (Figure 13). The entire slope range has been divided into five classes ranging from 5° to 40°. The classification presents a better picture of slope distribution in the area. This classification is adopted from the report of ‘The Commission on Slope Evolution’ published in the study [67]. The steepness of the slope is expressed by its saturation-steeper slopes have brighter colours.

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Figure 13. Slope map

Table 9. Slope classes and their aerial extent of the study area

Table 9 portrays the areal distribution of each slope category, the range of slope and the frequencies of individual slope classes. Around 73.3% of the area in the watershed belongs to a moderate to gentle slope where settlement and industries are predominant (Figure 3(b)). Because of the slightly undulating topography, moderate slopes have a good infiltration zone that leads to maximum infiltration or partial runoff. The northern part of the watershed which mostly comprises rocks of Lesser Himalayan formation such as quartzite, phyllite, limestone, and dolomite of the Chandpur, Nagthat, Blaini, Krol and Tal formation has a steep slope whereas the areas of flat terrain with gentle slope are characterised by river terraces, old landslide debris, saddles, fluvial fan and so on.

The spatial distribution of slope in degrees concerning the areal coverage and cumulative percentage is shown in Figure 14. The y-axis of a frequency curve corresponds to the cumulative percent area of slope classes while the x-axis corresponds to the cumulative slope in degrees. The area under the curve, as calculated from the graph was 91.2% accurate. In the Siwalik zone, because of the presence of a dominantly steep slope, the fans create piedmont zones along the southern periphery and moderately dipping beds are visible where water quickly runoff.

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Figure 14. Cumulative frequency distribution for Slope

The northern part of the watershed mostly comprises rocks of Lesser Himalayan formation such as quartzite, phyllite, limestone, and dolomite of the Chandpur, Nagthat, Blaini, Krol and Tal formations have a steep slope, whereas the relatively flat and gently sloping areas are characterized by deep weathering, bulges of old landslide debris, river terraces and fluvial fan. In the Siwalik zone, because of the presence of a dominantly steep slope, the fans forming the piedmont zone along the southern fringes of the watershed and moderate to a steep dip of the bed are seen where water quickly runoff.

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Figure 15. Slope-aspect map

5.3.2 Slope-aspect analysis

The steepest slope direction for a location on the surface is measured by an aspect map. The aspect map is a critical parameter for determining the impact of the sun on the local climate. In most cases, the west-facing slope will be warmer than the east-facing slope during the hottest part of the day. As a result, the aspect has a significant impact on the spread of vegetation types in the area. Aspect is the angle between the vertical directions of the steepest slope, it is always measured in a clockwise direction. Like Slope, the Aspect is also calculated from DEM (ALOS-PALSAR) and usually measured in degrees. As shown in Figure 15, the aspect categories are represented by hues (e.g., red, orange, yellow, etc.) and the degree of slope classes are mapped with saturation (or colour brilliance) so that the steeper slopes are brighter.

The aerial and frequency distribution of the aspect categories is shown in Table 10. The aspect reclassify values are designated the corresponding distribution of aspect classes.

There is a close relationship between aspects, landform types and natural vegetation in the study area. It is noted from Table 10, that the northern aspect of the watershed is steeper than the southern aspect. The north-western aspect is good for biodiversity. The curve represents the relationship between the cumulative area and aspect reclassify value increasing toward the right showing an accuracy of 98% (Figure 16).

The direction of the vegetation and forest growth are good indicators of the aspect. In the higher reaches of the watershed, the density of oak and associated species like Cupressus are well developed in the north-western aspect while the pine is well dominated in the south-eastern one.

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Figure 16. Cumulative frequency distribution for Slope-aspect

5.4 Land use/land cover mapping

The most important factors in determining the topographical features and status of classes in the watershed are changes in land use and land cover patterns. Estimation and change in land use patterns suggest resource utilization through human interferences especially utilizing urbanization and agriculture [68]. LULC becomes a significant aspect of the monitoring of different hydrological processes and natural resources management [5, 20, 21].

In the present paper, the LULC mapping for the years: 1980, 1995, 2008 and 2016 is prepared through visual interpretation of geo-referenced satellite images of Landsat and LISS-IV (data description mentioned in Table 4) using ArcMap v.10.3 and ERDAS IMAGINE. The on-screen interpretation is carried out on a scale of 1:10,000. The map thus generated was taken to the field for verification and the final vector polygon map depicting the various land use/land cover classes in the study area was prepared [22]. The digitization of the different classes of LULC is based on the classification given by the studies [69, 70]. It is classified into eight different classes: built-up (urban and rural), cropland, fallow land, plantation, evergreen needle leaf forest, deciduous broadleaf forest, mixed forest, and water bodies (Figure 6).

Remote sensing is also used in hydrologic analysis to investigate the impact of changing land-use patterns (e.g., agricultural pattern, forest coverage, urbanization and so on) on various hydrologic responses of the watershed. Table 11, shows the different classes of LULC used in the study.

In 1980, the urban area of the watershed occupied 9.13Km² (10.38%) which increased by 2.40% in 1995 to reach 11.71 Km² as revealed in Figure 17 and Table 12. The continuous urban areal increment is visible in 1995-2008 with a 12.39 Km² increase in area to reach 24.43 Km² (30.11%) in 2008 and 11.07 Km² of areal increment of 35.50 Km² (45.79%) in 2016.

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Figure 17. Percentage of urban growth in the area

5.4.1 Areal distribution of LULC classes

Areal statistics of different classes of LULC are given in Table 1, which corresponds to the dominance of forest class in the watershed followed by agriculture and built-up.

Table 10. Slope-aspect categories and their aerial extent

The result indicates that there is a significant change in the LULC classes of built-up and plantation. It is found that in the years 1980, 1995, 2005 and 2016 the total built-up area which includes rural and urban settlement covers 9.13 Km² in 1980 to 35.50 Km² in 2016. It covers 1.31% to 5.10% of the total area of the watershed. On the contrary, the percentage of plantations in the watershed decreases from 3.23% to 2.13% of the watershed. The area of water bodies that covers the watershed also decreases. This is because of the rapid increase in urbanization, industrialization, and population in the watershed over the past decades with an additional requirement for food and shelter. These surface areas are increasing by decreasing forest surface as well as total forest surface and water bodies. The watershed is also facing a problem of high erosion as a result of intense rainfall and LULC change.

Table 11. Different classes of LULC with description [69, 70]

Table 12. Urban growth of the Asan watershed (1980-2016)

Further, the results validated from field studies, the dislocation of jointed and weathered rocks due to the presence of slopes in the Lesser Himalayan region resulted in the activation of mass wasting in the area. The deflection and shifting of the water body and the development of flood plains, deposition of sand and landforms on both sides of the river (Figure 18). These examples interpret major pieces of evidence and the parameters which are studied in the research determine the decisive factors for the presence of tectonics in the area. The outburst of population and settlement in the entire region of the watershed particularly in the Lesser Himalayan region confirms their decadal increment. The continuous drying of water bodies over time and changes in LULC's classes have impacted the water balance of the watershed by changing the magnitude of flow, which results in increasing the extent of the water management problem.

18.png

Figure 18. Different activities responsible to make watershed fragile and controlled

The entire study is based on various geo-environmental factors that have long-term effects on the watershed. The hydrological behaviour, evolutionary background, and maturity of the terrain are revealed by the quantitative geomorphometric analysis of the study area. These are all crucial in classifying landforms into different morpho-units and determining how they relate to local geology. The overall investigation demonstrates that the watershed's drainage system is transitioning from an early to a mature stage of the fluvial geomorphic cycle. The area is tectonically active, according to the analysis of geomorphic indices. The experts are very concerned about the 36-year spatial variation in the LULC classes, particularly the four-fold expansion of the built-up area, and this study directly supports a strong need in this regard. This study directly supports the need for sustainable watershed development and management because of the 36-year spatial variation in the LULC classes, especially the four-fold expansion of the built-up area.

The overall study recommends that, to maintain sustainable development, the rate of urban growth in the watershed should be slowed and the spatial pattern of growth should be changed. Altogether, this study mainly relates to soil and water conservation in the watershed, which includes appropriate land use, runoff prevention, improvement of forests and grasslands, protection of water supply, management of local water for irrigation, and enhancement of soil fertility.

By integrating the findings from geomorphic studies, morphometric parameters, and LULC analysis, our paper provides valuable insights into the intricate relationship between human activities and natural processes in the watershed. These insights can be used to develop effective management and conservation strategies, ensuring the sustainable development of the entire area. Also, the study offers useful guidance to watershed managers in making decisions during the planning phase as well as in monitoring and assessing watershed management initiatives.

I would like to thank my mentor Dr. Dhirendra Singh Bagri for his guidance in writing the research. His guidance, advice and constructive criticism are highly appreciated. Also, I exceedingly benefitted from his critical thinking, uncertainty assessment and technical writing ability. This work uses datasets from the United States Geological Survey, USGS (https://earthexplorer.usgs.gov/); National Remote Sensing Centre, Bhuvan-NRSC (https://bhuvan.nrsc.gov.in/home/index.php); Earth Observation Research Centre-Japan Aerospace Exploration Agency and Alaska Satellite Facility, ASF (https://asf.alaska.edu/data-sets/derived-data-sets/alos-palsar-rtc/alos-palsar-radiometric-terrain-correction/), are duly acknowledged.

Conceptualization and design of the article; acquisition, methodology, interpretation and validation of the data, formal analysis, writing and original draft preparation-A.S.; Visualization, writing-review and editing-D.S.B.