Application of GIS and remote sensing in morphometric analysis of river basin at the south-western part of great Ganges delta, Bangladesh

Watercourses in nature have a significant impact on and shape the physical environment where the river basins efficiently define its spatial boundaries. The term ‘morphometry’ refers to the science of determining the structure of the earth's surface, including the shape and scale of its landforms, by measuring and studying them (Bárdossy & Schmidt 2002; Kulkarni 2013; Shrivastava et al. 2017). It depicts the measurement of shapes and is used to quantitatively describe landforms. Drainage basin analysis using morphometric parameters is critical for watershed planning because it gives details on the basin's slope, topography, soil quality, drainage characteristics, and surface runoff generation. Those characteristics help to determine some major essential aspects like ground infiltration capacity, surface water potential, groundwater potential, hydrological behaviour, and environmental assessment (Chandrashekar et al. 2015; Albaroot et al. 2018). A comprehensive morphometric analysis of a drainage basin is required to fully comprehend the effects of drainage morphometry on landforms and their features. This is because the physiographic characteristics of a drainage basin, such as its scale, form, texture, pattern, gradient, and drainage density, can be linked to a variety of hydrological phenomena. Developing and using techniques to measure the land exteriors, morphometric analysis helps to show the drainage basin by exposing the related hydrological and geomorphic processes that take place in the basin (Albaroot et al. 2018). It also provides the framework for a thorough assessment of the flood risk, vulnerability, and hazard in the basin. Horton (1932) first initiated the morphometric analysis in the field of stream basins, and since then the area of interest was chosen by many contemporary researchers (Horton 1945; Strahler 1952, 1964; Schumm 1956, 1963). According to Strahler (1964), measuring a drainage basin's linear, areal, and relief (gradient) properties is necessary for a thorough analysis of its morphometry. The linear characteristics of a drainage basin are closely related to the drainage system's stream patterns, where the topological characteristics of the streams in relation to open connectivity are analysed. It addresses variables including stream order, stream number, stream length, bifurcation ratios, and Rho coefficients (Chandrashekar et al. 2015; Venkatesh & Anshumali 2019). In order to produce a numerical representation of the landscape division and runoff potential, drainage density is often determined with the help of research on aerial features. In this aspect, parameters such as the basin's extent factors, length of surface runoff, stream frequency, drainage density and texture, elongation ratio (ER), circularity ratio (CR), form factor, compactness coefficient, constant of channel maintenance, time of concentration, and recession are all interpreted (Bárdossy & Schmidt 2002; Oyedotun 2020). The analysis of three-dimensional features involving area, volume, and altitude of vertical dimension of landforms, is intrinsically linked to the gradient (relief) aspects of drainage basins in order to investigate various geohydrological characteristics. For the investigation of relief aspects, common criteria include total basin relief, relief ratio, relative relief ratio, ruggedness number, Melton ruggedness number, ruggedness index, slope analysis, and dissection index (Oyedotun 2020).

Through the use of morphometric analysis, Mahala (2020) demonstrated disparities in flood potentiality, geological and geomorphological control, and the volume of drainage water between the Kangsabati River and Kosi River in India. The geomorphological parameters (stream length, slope, and degradation) are the most important criteria, according to Bárdossy & Schmidt (2002) and they also set the limit of parameters for morphometric-based analysis for evaluating stream water quality.

Various researchers from all across the world have conducted morphometric analyses of several drainage basins using different approaches based on their ideology. Faisal (2021) used the analytical hierarchy process (AHP) through GIS for building up the geographic model using 14 morphometric parameters for the sub-watershed in the Bastora river basin. Geographical information systems (GIS) have been used as the most viable one to determine different basin parameters where digital elevation model data available from the Shuttle Radar Topography Mission (SRTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is used for effective determination, interpretation, and study of spatial knowledge relevant to drainage basins. According to certain research, ASTER and SRTM are both excellent alternatives to local 1:50,000 hypsographic data and are both valuable in absolute and relative terms (Albaroot et al. 2018; Oyedotun 2020). These global elevation data sets can be used to analyse several surface processes precisely, described by the relative assessment that focuses mostly on hydrological systems. This is immensely helpful for geomorphologists. Studies on water and land resource management are greatly needed, particularly in Bangladesh's Khulna division where uncontrolled expansion is putting the resources in jeopardy (Rahaman et al. 2017). The majority of studies calculated the morphometric values to do the river basin analysis, while a small number attempted to identify the morphometric values from various morpho-climatic conditions. Again, Bangladesh is a river-prone country, although this type of morphometric analysis is done a little from that perspective. The majority of research investigations are conducted in either the north-western (Rahaman et al. 2017) or the central regions of the nation, mostly ignoring the significant south-western region (which is also a part of the huge Ganges delta). Additionally, the research area's soil quality and agricultural land production have also been impacted by the degrading water quality. Ecosystems are deteriorating to the point where without quick intervention, they will sustain irreparable harm. Understanding the physiographic status of the land in this context requires morphometric study. Therefore, the current study aims to quantify and evaluate the morphometric parameters in order to better understand the physiographic characteristics of the south-western part of Ganges delta watershed (primarily covering the Khulna division) as well as its hydrological and geomorphic processes. This will provide a strong foundation for the management of the watershed for the optimum development of the resources within it and will also serve as a solid guideline for future studies.

Parameters of the basin such as its area, perimeter, length, and width, as well as the number and length of streams in various stream orders, are automatically extracted using ArcGIS tools. Again, stream length ratio, bifurcation ratio, rho coefficient, stream frequency, drainage density, drainage texture, circulatory ratio, elongation ratio, form factor, etc., are derived from those parameters using several pre-established formulae given in Equations (1)–(20). The Digital Terrain Model (DTM)-based approach also produces flow direction and flow accumulation.

Perimeter (P) (km), basin length (L_b) (km), and basin area (A) (km²) are drawn from digital elevation modelling (DEM) (Schumm 1956). The basin length is the longest length measured from the headwaters to the confluence point of the basin. A basin with a small area represents greater rapidity of rainwater flow to the channel than a larger basin (Shrivastava et al. 2017).

The height of the basin mouth (z) and the maximum height of the basin (Z) are vital components of geomorphological investigations for watershed development and morphometric analysis (Shrivastava et al. 2017) extracted from DEM.

The slope of the basin surface is its steepness, and an aspect is often the direction that a slope faces. Microclimates are directly impacted by factors, especially in dry and semi-arid regions. To depict the spatial variation of the aspect in this study, the slope and aspect map is produced from the SRTM DEM.

The parameters considered for assessing the linear aspects are computed according to the standard mathematical formulae shown in Equations (1)–(6). Values of the parameters of the linear aspect are presented in Table 1 and are discussed below:

The variation of mean stream length values for the drainage basin (0.06–0.16 km) in Table 1 and the values of the stream length ratio (L_ur) (0.25–1.48) indicate that the area contains fairly resistant rocks, moderate slope, and topography (Radwan et al. 2017). The drainage basin shows stream length ratio values of less than 2 indicating the mature stage of geomorphic development (Shrivastava et al. 2017).

The values of the bifurcation ratio (R_b) in this study denote a mean value of 2.80. Such discrepancies in R_b values among the stream orders demonstrate variations in the geological and lithological characteristics of the basin (Venkatesh & Anshumali 2019). The first-order stream's high R_b value (9.02) shows that the upper basin area got a significant amount of water. Once more, the low R_b value (0.47–3.09) in the second to ninth stream orders indicates an increase in water pressure at lower reaches. A low mean R_b also indicates that there is water stress in the drainage basin (Mahala 2020). As the ratios are less than 5 (except first order), it expresses the uniformity of underlined materials of all other order basin layers (Chandrashekar et al. 2015).

The rho coefficient (ρ) is represented in Table 1 as the proportion of stream length to bifurcation ratio. The drainage basin showed a high rho variation, indicating high hydrological storage during the flood period thus attenuating the erosion effect during the elevated discharge (Venkatesh & Anshumali 2019).

The areal characteristics of a basin are its two-dimensional qualities. Table 2 gives the detailed results of the areal aspect parameters. The equations for the calculation of areal aspect elements are listed in Equations (7)–(9) and discussed below:

In Table 2, the total area (A) and its perimeter (P) indicate that the area has a vast coverage where rainwater needs to flow more paths to reach the channel. In addition, basin length expresses basin shape; therefore, in this case, a high basin length indicates a more extended basin. Additionally, the length of the study area's basin displays the basin's elongated shape, which stands for its hydrological characteristics.

The lemniscate (k) determines the gradient of the basin. Its value for the drainage basin is 6.59, which shows that the maximum inception area is comprised of large higher-order stream numbers (Kumar et al. 2018).

The form factor (F_f) expresses a basin with a high value has a high peak flow for a short period, whereas elongated basins with a low form factor have a low peak flow over a longer period (Shrivastava et al. 2017). As a result, flood flows in elongated basins are simpler to manage than those in circular basins. The form factor value of the basin shows that it is elongated and devoid of any abrupt peak discharges close to the outflow.

The relief aspect is critical for analysing the erosive feature of the basin. The parameters considered for assessing the relief aspects are calculated according to the standard mathematical equation shown in Equations (10)–(14). The results are presented in Table 3 and discussed below:

Total basin relief (H) is critical to comprehending the basin's denudation features, as well as stream gradient, flood pattern, and sediment volume that can be transported. The total relief of the basin is 40 m indicating high infiltration and low surface runoff condition and consequently decreased sediment load and erosion rate in the study area (Venkatesh & Anshumali 2019).

The overall steepness of a basin is typically indicated by the relief ratio (R_h), which may also be impacted by a single, isolated peak in the basin region. While a moderate slope and low relief are associated with a low relief ratio, a steep slope and high relief are associated with a high relief ratio (Albaroot et al. 2018). Khulna basin's R_h value indicates that it has an overall gentle slope and low relief.

Relative relief (R_hp) and relief values in terms of structural control are good indications of lithology changes. Low values imply clastic or less resistant rock in contrast to high values (Al-Saady et al. 2016). R_hp value of the Khulna drainage basin represents a relatively low value (Table 3).

The research area is less prone to erosion since the ruggedness number (R_n) (0.15) is low, implying a mature basin with good drainage density and a moderate slope (Venkatesh & Anshumali 2019). The Khulna basin's Melton ruggedness number (MR_n) value indicates that the basin is vulnerable to debris floods (Wilford et al. 2004; Table 3).

The aspect map (Figure 6(b)) of the Khulna drainage basin constructed using SRTM DEM shows the spatial variation of the aspect. As they are exposed to different micrometeorological conditions, sun-facing and shaded slopes in dry and semi-arid regions have different vegetation patterns. Additionally, because the moisture content is retained longer on the shaded side, the density of vegetation cover on shaded slopes is typically higher than on sun-facing slopes (Al-Saady et al. 2016). The basin showed 34.57% of the area has an NE–E–SE aspect of slope that is oriented towards sunlight during the warmest hours of the day and hence hotter than all the other sides (Table 5). Many of the parameters will experience differential exposures to sun and shadow because of the microclimate variance, which might result in differences in drainage density, vegetation cover, and erosion rate.

Drainage texture analysis entails investigating the features of drainage networks and the layout of streams carved into the land surface by drainage systems. Table 6 gives the detailed results of the drainage texture parameters. The equations for the calculation of drainage texture elements are listed in Equations (15)–(19) and discussed below:

The stream frequency (F_s) largely depends on the basin's lithology including the drainage network's texture (Albaroot et al. 2018). The extent of division and runoff is directly related to stream frequency, whereas mean annual rainfall is inversely related. Therefore, the basin is predicted to have low mean annual rainfall while having a significant degree of division and runoff, as shown by the higher F_s value (52.25 streams/km²).

The circularity ratio (R_c) depends on drainage density, climate, geological structure, slope, relief, etc., of any basin. The high value of R_c in the drainage basin suggests that peak discharge occurs during the period of high rainfall. The R_c value is also a good indicator of the geomorphological evolution of a basin. The high, medium, and low values of R_c indicate the old, mature, and young levels of geomorphic changes in each basin (Mahala 2020). The R_c value of the basin suggests elongated properties. It also makes reference to the basin's modest peak discharge.

With the use of the elongation ratio (R_e), Kulkarni (2013) classified the varying slopes of the basin as round (0.9–0.10), oval (0.8–0.9), less elongated (0.7–0.8), elongated (0.5–0.7), and more elongated (<0.5). The basin's R_e value indicates that it is more elongated and has low peak discharge (as the elongated value range is 0.5–0.7, and the more elongated is <0.5).

In order to support Horton (1945) infiltration hypothesis of erosion, Melton (1957) found that drainage density is related to the percentage of bare land and the intensity-frequency of runoff, but inversely proportional to the precipitation-effectiveness index and infiltration capacity. The value of D_d (Table 6) for the basin in the current study implies that the basin has a coarse texture (Table 7).

According to Strahler (1952), young (disequilibrium phase) landforms have convex-up curves with HI > 0.6; mature (equilibrium phase) landforms have S-shaped curves with HI in the range of 0.3–0.6; and old and severely dissected landforms have concave up curves with HI < 0.3. The value of HI for the basin is 0.49 which indicates that it is in the mature stage.

The study draws the conclusion that the parameters have a wide range of opportunities to reveal the morphological and physiographic characteristics of any stream basin after evaluating the results and considering a variety of literary works. The river basins in the south-western section (Baleshwar, Bhairab, Pasur, Rupsha, Kholpetua, and Mayur) have an impact on the economic, ecological, and social elements of the area, where morphometric values show the best representation of basin features (Akter et al. 2016).

The majority of the streams are in first order among the linear parameters, where the order is inversely proportionate to the number of streams, indicating alluvial deposition and water abundance. It is also found that the basins have formation maturity and uniformity of the base materials in the basin. Additionally, the rho coefficient values in the top part of the basin reveal erosion repulsion ability. The areal morphology of the basin areas indicates that it has a huge catchment area and relatively low peak discharge that ensures more flood resilience (through form factor). The value of the total basin relief is a representation of lower sedimentation load with erosion susceptibility of the study area due to the gentle slope. Moreover, all the basins are moderate terrain nature (avg. slope <3.97°), and the sun-facing aspect make it a relatively warmer region. The basin areas are coarse in texture according to density characteristics, and the whole network has a high extent of annual rainfall runoff. Although the entire basin has an elongated shape, the young geomorphological adjustment is subject to more flood potential in the condition of high peak discharge.

Although some of the metrics can change depending on the various morpho-climatic and hydrological circumstances in other regions, it is the most effective technique to distinguish between distinct basin areas. Despite the study's foundation on a comprehensive scientific analysis of many morphometric characteristics, some restrictions must be acknowledged. This study uses remote sensing and GIS and gives a broad assessment of the physiographic parameters but it does not use any reconnaissance surveys. The major rivers of the south-western part of the Ganges delta pass through the Sundarbans area, controlling its ecological and economical values. Application of GIS and remote sensing are the fruitful assessment of morphometric attributes of such areas. The parameters can be further developed in future studies, additional high-resolution data can be used to reveal more features, and the results can be used by the relevant authorities for hydromorphological and environmental planning in this area.

The tables’ results and map data are available at the following link: https://drive.google.com/drive/folders/1ga6Q-2T6VH8HZWS_JY1kam98US3HA87K?usp=sharing.

M.A.R.J. contributed to analysis tools selection, validation, and performed the analysis; S.U. designed the study, reviewed the literature, and wrote the manuscript; K.F. helped in writing the draft manuscript and interpretation of the analysis; F.M.R.A. collected the data and helped in factor selections. The final manuscript was revised by all authors.

All relevant data are available from the online repositories linked in https://opentopography.org/ and https://cutt.ly/265lmsZ.

The authors declare there is no conflict.