GIS and RS data-based morphometric analysis of the Talas river watershed characteristics Open Access

Water resource assessment and management are of particular importance and relevance and also have notable socio-economic and political character. Its relevance, in turn, increases with the growing influence of anthropogenic factors and climate change on both global and regional scales. In the current context, where the effects of climate change and human activities are becoming increasingly evident, the study and analysis of river catchments have become key elements of sustainable environmental management and water resources management (Shaidyldaeva et al. 2014; Alimkulov et al. 2021). To determine and analyze the morphometric characteristics of a river basin, a large number of different techniques are currently available. The success of their application depends on the availability of baseline data and information regarding the hydrographic and physiographic characteristics of the study area (Mudassir & Murtala 2024). Until recently, they were determined by traditional labor-intensive manual measurements on topographic maps. The intensive development of modern digital technologies makes it possible to obtain the desired morphometric characteristics of a river and its catchment through Geographic Information Systems (GIS) and Remote Sensing (RS) data in modern conditions (Sharma & Sarma 2018; Bhatt et al. 2024).

Morphometric analysis is an important tool for the study of river basins, rivers and other water bodies, allowing a better understanding of their physical characteristics and hydrological behaviour.

Morphometric analysis, as a field of scientific research, encompasses the mathematical analysis of the shape and size of water bodies and catchment landscapes. In foreign studies, morphometric analysis includes the measurement of linear, aerological, and landform characteristics of catchments, such as basin length, area, perimeter, and the gradient and degree of branching of the river network (Moid et al. 2019).

One of the first significant contributions to the morphometric study of catchments was made by Strahler (1964), who proposed a classification system for tributaries based on their order, which allowed formalizing methods for calculating hydrological parameters of catchments. According to studies (Kalaivanan et al. 2014), the key parameters for morphometric analysis are not only linear characteristics (river length, number of tributaries), but also relief parameters such as maximum basin elevation, energy capacity, which determines the activation of erosion and channel processes and favorable conditions for drainage.

The morphometric characteristics of a river and its catchment are important for understanding and explaining the characteristics of river regimes, and its combination with GIS and RS offers new opportunities for better understanding and investigation of hydrological processes (Kumar & Singh 2024).

Various hydrological processes are related by the physiographic characteristics of the catchment area such as its size, shape, slope of the catchment area, drainage density, and the size and lengths of the elements affecting it (Pande et al. 2021).

A more comprehensive list of determinable morphometric characteristics of a river catchment is given in Tursunova et al. (2023) and includes the following parameters: river length with an accuracy of 0.1 km (L, km), catchment area (F, km²), mean catchment elevation (H_avg, m), mean slope (%), centroid of the river basin, drainage density (D, km/km²). In short, the morphometric parameters of a river basin describe its shape and size, including the study of geometric and morphological parameters of the basin. While the geometric parameters of a river basin are identical to the full list of morphometric characteristics given above, the following parameters can be classified as morphological parameters: limnicity, cropland, woodland, wetland, urban and built-up lands, permanent snow and ice (Surface Water Resources of the USSR 1973).

Morphometric and morphological characteristics of a river basin are widely used in the practice of hydrological calculations as basic parameters of many calculation formulas. However, there are very few sources that contain all the morphometric data on a river and its catchment necessary to assess the conditions of river flow formation, or that reflect individual river basins of different levels (Tursunova et al. 2023). As for the foreign literature, studies in the field of analyzing and determining morphometric characteristics are based on GIS and RS with writing separate scripts to automate the whole process of morphometric analysis with the introduction of some new parameters to improve the assessment of the stage of basin development (Ali 2015). In morphometric analysis of a river basin, it is important to take into consideration the spatial resolution of DEM and the threshold value of flow accumulation that can provide the best correlation with observed data, otherwise they may lead to misidentification of basin, order and length of watercourses (Kumar & Singh 2024). The applicability of GIS and RS techniques in morphometric analysis of river basins is well demonstrated in Kumar & Varija (2022).

According to the set of rules of the State Hydrological Institute (SHI) (Federal State Budgetary Institution ‘State Hydrological Institute’ 2017; Federal Service for Hydrometeorology and Environmental Monitoring 2018; STO GHI 52.08.48-2020 2020), morphometric characteristics are quantitative indicators of water bodies and their catchments, and morphological characteristics are qualitative and quantitative indicators of the structure of the catchment surface. Together they form hydrographic characteristics that give a fairly complete picture of the nature, shape, size, extent of water bodies and some physiographic features of their catchments.

When analyzing morphometric characteristics, it is important to distinguish between stream morphometry and catchment morphometry. Stream morphometry studies the geometric characteristics of the stream (river) itself, such as length, average river slope, drainage density and tortuosity coefficient. Catchment morphometry focuses on the characteristics of the catchment area, including its area, mean catchment elevation and slope, polygon centroid which is necessary for the assessment of water resources and hydrological regime. Catchment morphology, which includes its elongation, compactness, symmetry and shape, also needs to be considered, which affects the distribution of river flow and flooding processes. Together, the morphometry and morphology of a river basin provide a complete picture of its hydrography. In assessing the water resources of a river basin, along with morphometric and morphological characteristics, the landscape features of the basin (Federal State Budgetary Institution ‘State Hydrological Institute’ 2015) play an important role. These include indicators such as forest cover, lake cover, ploughing, wetlands, urbanization, and glaciation (SHI), which are an integral component of the landscape and exert a significant influence on hydrological regime, particularly affecting descriptors such as surface runoff and evapotranspiration. Changes in land use and land cover (LULC) affect precipitation regime and temperature, fundamental drivers of the hydrological cycle (Bati et al. 2023). Subsequently, this alters the water balance of catchments, showing the difference between evapotranspiration, groundwater recharge and runoff. Moreover, land use conversion, combined with land cover patterns, can significantly transform the morphometric characteristics of a basin, affecting its hydrological response (DeFries & Eshleman 2004). Thus, taking into account LULC characteristics as landscape parameters of a river catchment is key to an integrated analysis of hydrological behavior, allowing a more accurate assessment of changes in water balance, hydrological regime descriptors and morphometric characteristics of basins under the influence of natural and anthropogenic factors (Nyamathi & Kavitha 2013; Mane & Shelar 2019; Thakur et al. 2019).

This study seeks to analyze the morphometric and morphological characteristics of the Talas River basin using GIS and RS, contributing to the understanding of hydrological processes and water resources management in the river basin. It should be noted that the morphometric analysis of the Talas River basin requires clarification, as much of the data published in the hydrological literature (Surface Water Resources of the USSR 1973) is now objectively outdated, as significant changes have occurred since its publication, for example, in the patterns of land use and river system. Water resource assessment based on morphometric aspects is becoming increasingly important in effective water resources planning and basin management. The novelty of the study lies in the use of modern GIS and D33 technologies to update the morphometric characteristics of the Talas River catchment. For the first time, RS data were applied to improve the accuracy of hydrological calculations. The study includes a detailed analysis of morphometric, morphological and landscape characteristics, which allows a more accurate assessment of water resources status and identification of risk zones. This approach contributes to effective water resources management under changing climate and anthropogenic pressures.

Thus, determining the morphometric characteristics of river basins is essential for planning and effective water resources management, designing engineering structures, developing flood forecasting models in the context of sustainable development and amid changing climate conditions and anthropogenic impacts. The use of GIS and RS complements this analysis by providing spatial data and the ability to monitor change on a multi-year basis.

The river channel united the mountain basin with a series of tectonic depressions and in the lower reaches formed a number of deltaic areas of different ages. This is an area of intensive manifestation of neotectonic movements, which strongly influenced the formation of complex deltaic-alluvial plains of the Talas River and its tributaries. As a result, the deltaic-alluvial plain was taken out of the sphere of plunging areas and turned out to be elevated above the base of depression erosion by 150–300 m (Dzhanaliyeva 1998).

The Talas River basin is divided into two parts – mountainous, extending to Taraz city (Kazakhstan) and plain. The entire basin area is 52,700 km², which includes the mountainous area of 9,240 km² (Dostay et al. 2012; Shaidyldaeva et al. 2014).

From the confluence of the rivers to Kirovskoye village (Kyrgyzstan), the Talas River flows through a wide (up to 15 km) intermountain valley, receiving almost all of its tributaries, with the left tributaries predominating: Kolba, Beshtash, Uchmaral, Kumyshtag, Karabura. On the right side, the Talas River receives its two largest tributaries – Kenkol and Neldy. In addition to these rivers, numerous karasu (dry valleys) flow into the Talas River, the most significant of which are the Kirov rivers. All tributaries are utilized for irrigation to a greater or lesser extent.

After cutting through the Ichkeletau ridge near Kirovskoye village via a narrow gorge, the Talas River flows again through a wide valley along the foot of the southern slope of the Kyrgyz ridge and having passed its western end, the river emerges near the Taraz city to the plains, with which the river valley merges. Below the city, the river is extensively diverted for irrigation, forming a dense irrigation network. As a result of this diversion for irrigation and water loss due to infiltration and evaporation, the Talas River reduces its water content and dissipates in the plains, failing to reach the Shu River (Surface Water Resources of the USSR 1973).

The climate of the basin is continental. During winter, it is under the influence of high pressure areas, which contributes to the establishment of cloudless, frosty weather with sharply pronounced temperature inversions and strong cooling. The average January temperature ranges from −11 to −14 °C, while the absolute minimum can reach −40 °C. Summers in the Talas River basin are characterized by high heat, while winters, despite the geographical latitude, are quite severe. Precipitation in the basin is very low, less than 200 mm per year. In the foothill areas this figure is slightly higher and is about 300 mm Scientific and Applied Reference Book on the Climate of the USSR (1989). The elevation of the area plays a decisive role in shaping the climate.

The main features of the basin relief of the territory are expressed by the system of mountain ranges alternating with intermountain areas. The rugged terrain is caused by the appearance of altitudinal physiographic zonality, which, in turn, determines moisture conditions and river flow regime. The main part of runoff is formed on mountain slopes. Within the plains and intermountain depressions, surface runoff decreases sharply and infiltration increases.

Both topographic and land use data were used for morphometric analysis of the Talas River basin characteristics. The original topographic maps were obtained from https://maps.vlasenko.net/, which are Soviet General Staff maps at a scale of 1:200,000 with automatic georeferencing. Morphometric characteristics of the watercourse and catchment, such as length, slope and catchment area, were extracted directly from these maps, providing an accurate understanding of the morphometric characteristics of the watercourse and the river basin itself.

ArcGIS 10.x program (ESRI Inc.) was used to prepare the DEM of the Talas River basin. At the first stage, the DEM projection system was transformed from surface to rectangular plane using the Asia North Equidistant Conic projection coordinate system. This procedure ensured spatial consistency and accuracy of the data, which is critical for subsequent river flow modeling and morphometric analysis. The transformation of the projection system was necessary to correctly represent the geographic features of the area in the model, which in turn improves the quality of the morphometric analysis and makes the results more reliable. The procedure of image extracting along the Talas River basin boundaries was carried out in ArcGIS 10.x using Data Management Tools functions, which allowed focusing on the study area. At the final stage of DEM preparation, ‘false depressions’, which are voids caused by interpolation and rounding errors in elevation values, were eliminated. These depressions were eliminated using the Fill function from the Spatial Analyst module in ArcGIS 10.x, which ensured the continuity and accuracy of the elevation model for correct morphometric analysis and a more reliable understanding of the geographic features of the basin.

Additionally, land use images from Sentinel at 10 m resolution were used covering the period from 2017 to 2023. These data were projected into a unified coordinate space and processed in ArcGIS 10.x, with classification into nine LULC categories. Based on these data, the percentages of the landscape characteristics of the river basin such as limnicity, cropland, woodland, wetland, urban and built-up lands, permanent snow and ice were calculated, allowing a detailed analysis of the morphological characteristics of the river basin.

ArcGIS 10.x software was used for data processing and cartographic visualization of the results. All calculations and analytical procedures presented in this paper were performed in accordance with the methodology and formulas outlined in the works (Horton 1932, 1945; Strahler 1964; Federal State Budgetary Institution ‘State Hydrological Institute’ 2017). The analysis was carried out using maps (in *.shp format) digitized at a scale of 1:200 000. The investigated morphometric characteristics, such as the number, orders of watercourses and their length, were directly calculated in software with the use of spatial analyst tools. The methods of statistical, mathematical, cartographic and geoinformation analysis were applied within the framework of this study.

The capabilities of the Spatial Analyst module of ArcGIS 10.x software were used to define watershed boundaries. The main tool in this process was the Hydrology toolset, which provides efficient modeling and analysis of hydrological processes based on DEMs.

Water flow direction was determined using the Flow Direction tool, which calculates the flow direction for each cell of the processed raster model based on elevation data. This step provides a basis for further analysis of the flow accumulation. The accumulated flow was calculated using the Flow Accumulation tool, which converts the results of the flow direction calculation into a raster model showing the total runoff in each cell. This layer is necessary to identify areas where water drains to. The cumulative flow calculation method continues to be the primary method for extracting hydrographic networks from DEM data due to its simplicity and computational efficiency. Additionally, DEMs can be integrated with other datasets (e.g., precipitation and land use) to refine hydrological models, making them a powerful tool for water resource management and environmental studies. In widely used GIS platforms, 1% of the maximum flow accumulation value is applied as the default threshold value of accumulated flow (Çadraku 2023). The delineation of watershed boundaries was carried out using the Watershed tool. This tool allows the delineation of watershed boundaries based on a given runoff point and estimated flow direction and accumulated flow. Boundary vectorization was performed using the Raster to Polygon function, which converts raster watershed boundary data into vector format. This step provides a more accurate and convenient representation of the boundaries for subsequent analysis.

All operations were performed using standard techniques and formulas outlined in several works (Federal State Budgetary Institution ‘State Hydrological Institute’ 2017; Federal Service for Hydrometeorology & Environmental Monitoring 2018; Bajjali 2018; Kushwaha et al. 2022), and the results were stored and visualized for further analysis.

The procedure for determining the morphometric characteristics of terrestrial water bodies and their catchments using GIS technology from digital maps and satellite images is given in several works (Federal State Budgetary Institution ‘State Hydrological Institute’ 2017; Federal Service for Hydrometeorology & Environmental Monitoring 2018).

River morphometric characteristics provide information on the size, shape, and slope of watercourses. These parameters enable the assessment of spatial and dynamic features of watercourses, which is crucial for their hydrological analysis and water resources management.

• (1) River length (L, km). The lengths of rivers and other watercourses are measured using topographic materials (maps, images). The length of a river is taken as the length of its representation on a map or image, from the source to the mouth or gauging station. The hydrographic length of a river is measured along the centerline from source to mouth, including sections with lakes and reservoirs, and requires all key points and elevations to be considered for accurate calculations.

• (2) Average river slope (J, ‰). The average river slope is calculated using the data obtained by determining the elevations of the river mouth and the most distant source of the river system, which, together with the main river, represent the greatest length of the channel of this river system, as well as from measurements of the hydrographic length of the river. The average river slope is calculated using the formula (Federal State Budgetary Institution ‘State Hydrological Institute’ 2017):

  

  (1)

  where H₁ is the elevation of the river source, H₂ is the elevation of the river mouth, L is the river length.

River slope significantly influences the character of velocity distribution in the river flow, which, in turn, leads to changes in the hydrographic appearance of the channel network (Çadraku 2023; Duskayev et al. 2023). A more pronounced slope leads to increased water velocity, resulting in higher water discharge, which contributes to erosion processes and changes in channel shape (Schumm 1956). The river slope is one of the key characteristics that determine not only hydraulic conditions but also morphological changes occurring in the river landscape, which is important for studying river ecosystems and water resources management (Naumov 2019).

• (3) Drainage density (D, km/km²). Drainage density characterizes the distribution density of rivers and watercourses in a specific area and is defined as the ratio of the total length of all rivers and watercourses to the area of that territory (Horton 1945):

  

  (2)

  where ΣL is the length of rivers in the catchment area, F is the catchment area.

Drainage density is a key indicator that characterizes the degree of hydrographic network development in a catchment (Gautam 2023). This indicator is crucial for analyzing the hydrographic structure of a region, as it influences the water regime, erosion processes and hydrological conditions. A high drainage density may indicate a complex hydrographic network and a significant water resource, while a low density may indicate a less developed network of watercourses and limited water resources (Shevchenko & Demchenko 2018).

• (4) Tortuosity coefficient (K). The tortuosity coefficient is used to characterize a river's degree of tortuosity (curvilinearity) of the planned outlines of its channel. The tortuosity coefficient characterizes the degree of tortuosity of a river or watercourse and is defined as the ratio of the length of the actual river channel to the length of its direct projection between the source and the mouth (Duskayev et al. 2023). It is determined by the ratio of the river length to the length of its valley (Chupikova & Andronache 2019):

  

  (3)

  where L_river is the river length, L_o.l. is the length along the overhead line.

Depending on the values of the tortuosity coefficient, rivers are divided into straight (1.00–1.02), curved (1.03–1.08), slightly tortuous (1.09–1.20), moderately tortuous (1.21–1.35), tortuous (1.36–1.60), and highly tortuous with a tortuosity coefficient more than 1.61. The high value of the tortuosity coefficient indicates significant channel tortuosity and complexity, which may influence erosion and sedimentation processes, as well as the hydrological characteristics of the river system.

It is desirable to determine the morphometric characteristics of water body catchments using the same topographic materials from which the morphometric characteristics of watercourses are derived.

• (1) Catchment area (F, km²) is a key characteristic that significantly affects runoff volume. Catchment areas often serve as inputs to various other hydrographic characteristics, emphasizing the need to measure them as accurately as possible (Dubrova et al. 2021). A common receptor of precipitation falling on a given area in liquid or solid form is the catchment area. During snowmelt, the volume of runoff water will depend on the catchment area – the larger it is, the larger the runoff. However, this pattern can be disrupted in certain cases. For example, in some places, runoff may decrease or even cease entirely in specific areas. In the arid zone, a river may simply gradually disappear in the sands. In mountainous areas, when rivers reach the plains in the zone of insufficient moisture, runoff may also decrease. One of the main parameters in flood calculations is the catchment area (a constant parameter). The size of the catchment area, where surface runoff is formed, appears in most formulas for calculating not only the maximum flood discharge but also other characteristics of water flow in rivers (Vladimirov 2024).

• (2) Mean catchment elevation (H, m) is the average of the elevations within a catchment, calculated as the arithmetic mean of the elevations of all points within that catchment (Federal State Budgetary Institution ‘State Hydrological Institute’ 2017). This parameter has a significant impact on hydrological processes such as runoff volume and erosion rates. Mean catchment elevation is important for estimating runoff potential, analyzing water balance, and understanding the impact of topography on climatic and ecosystem conditions of a region.

• (3) Mean catchment slope (i, °). Slope is an important characteristic that significantly affects the velocity, runoff inertia and erosion potential of a catchment. It also plays a key role in groundwater recharge and influences the rate of change in topography along the main flow direction. The mean catchment slope is determined by the formula:

  

  (4)

  where h is the relief section on the working map (the difference in elevation between neighboring contours) in meters; l is the length of contours within the catchment area in kilometers; F is the catchment area in km².

• (4) Catchment polygon centroid is calculated based on the spatial distribution of points within the catchment and provide an accurate indication of the center of mass of the area. Knowledge of polygon centroid is essential for hydrologic studies, runoff modeling and water resource planning, as well as for geospatial analysis and monitoring of environmental change in the region.

• (5) Mean catchment width (B, m). Mean catchment width is defined as the ratio of the total catchment area to its length in the direction of the main flow. This parameter characterizes the average width of the watershed area and allows estimating its geometric features (Chupikova et al. 2020). Mean catchment width is important for understanding the spatial distribution of water resources, analyzing hydrological processes, and assessing the influence of catchment shape on runoff dynamics and erosion processes.

• (6) Area of endorheic basins (F_e.b., km²). The area of endorheic basins is the size of areas within a catchment that have no external flow and are characterized by enclosed water bodies or lowlands where water does not escape to other water systems. These areas include lakes, confined depressions and other locations where water flow is limited and does not connect directly to the main river network. Determining the extent of drainage-free areas is important for a comprehensive analysis of the water balance, hydrological and ecosystem processes within a catchment, as well as for assessing the influence of such areas on overall flow dynamics.

• (1) Form factor (Ff) (Horton 1932). It indicates how closely the width of the basin approximates the length of the main river, hence the closer the index value is to 1, the closer the basin configuration is to a rounded shape:

  

  (5)

  where F is the catchment area, L_b is the basin length.

The form factor is a dimensionless property that is used to quantify the shape of a basin. This factor helps to determine the degree of basin elongation, the lower the value of the form factor, the more elongated the basin under study is (Sharma & Sarma 2018). The form factor is a function of the basin shape and indicates the velocity at which water enters the stream.

• (2) Compactness index (Ic) (Gravelius 1914). The compactness index is the ratio of the basin perimeter (P) to the circumference of a circle whose area is equal to that of the basin:

  

  (6)

  where P is the basin perimeter, F is the catchment area.

The closer the compactness index is to 1, the more the shape of the basin approaches that of a circle and is considered more compact; when this parameter increases, the shape of the basin is more irregular, i.e. more elongated (Chupikova et al. 2023). The compactness index allows us to determine the degree of basin compactness, approaching a better understanding of its geometric characteristics (Shekar & Mathew 2024).

• (3) Elongation ratio (Re) (Schumm 1956). The elongation ratio is an important index for analyzing the shape of a catchment, providing valuable information about its hydrological character. The value of this index helps in understanding the flow and infiltration patterns in the basin (Schumm 1956). The elongation ratio is defined as the ratio of the diameter of a circle having the same area (F) as the basin to the basin length:

  

  (7)

  where F is the catchment area, L_b is the basin length.

Studies have shown that areas with high values of elongation ratio are characterized by high infiltration capacity and low runoff. Elongated basins with high elongation ratio produce flatter peak flows over extended periods, making them more efficient at discharging runoff compared to circular basins (Withanage et al. 2015). According to the findings of Strahler's research (Strahler 1964) the elongation ratio ranges from 0.6 to 1.0 depending on climatic and geologic conditions. He classified basins according to this ratio as follows – 0.9–1.0: circular; 0.8–0.9: oval; 0.7–0.8: less elongated; 0.5–0.7: elongated; <0.5: more elongated.

• (4) Asymmetry coefficient (α). The basin asymmetry coefficient indicates the degree of symmetry in the position of the thalwegs – whether they are lateral or centered relative to the main river, i.e. the alignment of the river network to one of the watersheds (Lisetskii et al. 2018). It allows us to assess how evenly or unevenly tributaries and the main channel are located within the catchment. A value of the asymmetric integral close to 50 suggests that the basin area is highly dissected, significantly eroded and strongly influenced by recent active tectonic activity (Sangma & Balamurugan 2017). It is defined as the ratio of the difference between the left and right parts of the basin to the total basin area:

  

  (8)

  where F_l is the area of left tributaries, F_r is the area of right tributaries.

Basin asymmetry is associated with unfavorable geological, geomorphological and climatic conditions for the formation of erosion network (Bezgodova 2021). Additionally, under the influence of historically formed geological, topographical, and climatic conditions, heterogeneity, expressed in the asymmetry of river basin slopes, can emerge. The older the river network, the more pronounced the asymmetry of the basins.

The presence of various lands in the catchments, which in one way or another affect the hydrological regime, requires the determination of such characteristics as woodland, cropland, wetland and others. These characteristics are expressed as a percentage of the total catchment area. To achieve this, it is necessary to find out the areas occupied by the respective land types, which requires precise delineation of their boundaries.

Difficulties may be encountered in delineating the boundaries of lands, especially when the map is overloaded with other elements. Problems are especially noticeable in conditions of significant fragmentation of land into small areas, complex terrain and high density of settlements. In such cases, it is recommended to carry out all preparatory work to establish the boundaries and delineation of lands on a separate layer of the digital map. To simplify the process, unnecessary elements should be deactivated and only the necessary layers of information should be used (Federal State Budgetary Institution ‘State Hydrological Institute’ 2017; Ibitoye 2021).

To better understand the impact of different types of land use on basins and to make informed decisions in water management, the following is necessary:

1. Woodland coefficient (C_w). The relative forest cover of a catchment is understood as a value that characterizes the extent to which the catchment surface is covered by woody vegetation, expressed as a percentage of the total catchment area (Savichev et al. 2011).

2. Limnicity coefficient (C_l). The relative lakeiness of a catchment is understood as the ratio of the total area of all flowing and drainless water bodies (excluding inter-bog lakes) within the catchment to the total catchment area of a given watercourse. Relative limnicity is expressed as a percentage (Izmailovaa & Yu 2020).

3. Cropland coefficient (C_c). Relative cropland area of a catchment is defined as the ratio of the total area of cropland within the catchment to the catchment area, expressed as a percentage, characterizing the degree of disturbance of the catchment surface by cropland, which significantly affects the water regime (Seitkazy et al. 2024).

4. Wetland coefficient (C_w). Wetland coverage characterizes the extent to which the catchment surface is covered by wetlands of various types. It is quantitatively expressed as the ratio of the total area occupied by wetlands to the catchment area (Federal State Budgetary Institution ‘State Hydrological Institute’ 2017).

5. Urban and built-up lands coefficient (C_UBL). Catchment urbanization is the ratio of the area occupied by settlements and industrial, agricultural and road construction facilities to the total catchment area, expressed as a percentage.

These coefficients are key parameters for hydrologists and ecologists, as they help to assess and model hydrological processes, manage water resources, develop flood and drought protection measures, and maintain water quality and biodiversity (Federal Service for Hydrometeorology & Environmental Monitoring 2018).

River basins vary in size and shape, which has a significant impact on their hydrographic characteristics. Morphometric studies involving measurement and mathematical methods enable a detailed analysis of these differences. In the context of the Talas River basin, which is located in diverse landscape and geological-geomorphological conditions and covers the territories of two countries, a structural-morphometric analysis using GIS and RS has not been conducted so far. In this regard, a comprehensive assessment of the morphometric and morphological characteristics of this river basin, including major watercourses, catchment areas, relief and landscape features, is a key step to improve the cartographic understanding of the geomorphological, structural and hydrological parameters of the study area. This section presents the results of this work using analytical functions of GIS and RS technologies and discusses their significance and impact on understanding the structure and functioning of the Talas River basin.

The application of ArcGIS 10.x spatial toolset has significantly improved the efficiency and speed of extraction of spatial information analysis, which is a key factor for the identification of morphometric and morphological characteristics of the catchment.

In flat river sections, water flow is generally characterized by steady and uniform movement due to minimal elevation differences and a low slope. In contrast, in mountainous areas, water rushes forcefully, which is because of significant slopes and sharp drops in altitude. The slopes of the Talas and Assy rivers were determined separately for upstream, midstream and downstream (Table 2). The average slopes ranged from 2.4 to 4.3‰, indicating the diversity of hydrological conditions depending on the terrain. For example, the slopes of the Talas River in the upstream reach 11.6‰, indicating significant flow dynamics.

The average drainage density, including the smallest watercourses, is 0.03, which characterizes the degree of underdevelopment of the hydrographic network within the river basin. This indicator, which accounts for even the smallest watercourses, shows that hydrographic elements in the Talas River basin are not sufficiently connected, potentially hindering the effective distribution of water resources and limiting their availability under climate change conditions. This, in turn, increases the territory's vulnerability to droughts and other extreme climatic events.

The calculated values of the tortuosity coefficients of the Talas and Assy rivers amounted to more than 1.61, which shows the strong tortuosity of the studied rivers. The pronounced tortuosity can lead to increased erosion impacts, as well as changes in the velocity and direction of water flow. These changes, in turn, can have a negative impact on ecosystems that depend on the stability of the river channel.

In hydrology, a distinction is made between a river basin and the catchment area of a river (Mikhailov et al. 2007). Typically, the river basin and the catchment area (especially surface) coincide. In conditions of arid territories with flat relief, to which the studied river basin belongs, cases of their mismatch are common; if a portion of the territory within the river basin is drainless (a drainless area), that portion remains outside the river catchment. Therefore, within the framework of this study, the area of the entire catchment area of the Talas River, as well as the basins of the Talas and Assy Rivers were determined (Table 3). In terms of size, the Talas and Assy river basins can be categorized as medium-sized, as their area values range from 2,000 to 50,000 km². Usually, medium-sized rivers are situated within the same geographical zone and their hydrological regime is characterized by stability of flow determined by climatic conditions, type of soil and vegetation, as well as anthropogenic impact on the ecosystem.

The rivers are subdivided into plain and mountain rivers based on their flow conditions. The elevation marks of the study area vary from 279 to 4,456 m above sea level. According to the research (Meybeck et al. 2009; Li et al. 2020), the elevations of the Talas River basin were distributed into five categories (Table 4). The most significant interval of elevations from 279 to 500 m occupies 52.1% of the total area of the Talas River basin. The next elevation range is 500–1,000 m, covering 20.0% of the area. The elevation categories from 1,000 to 1,500 m account for 9.1%, while the 1,500–2,000 m zone occupies 5.2%. The elevation zone from 2,000 to 4,456 m covers 13.6% of the total area. Thus, more than 72% of the Talas River basin area is located at altitudes up to 1,000 m, which indicates a significant predominance of lowland and plain areas. At the same time, areas located at altitudes above 1,000 m occupy a smaller share (about 28%), which emphasizes the diversity of morphometric characteristics of the region.

In the study area, catchment slope values are distributed over several ranges, each contributing to the overall characteristics of the river basin. The distribution of slopes in the Talas River basin emphasizes the predominance of gentle slopes (Table 5), which account for nearly 69.5% of the basin, indicating a landscape that is conducive to slow runoff and low erosion rates. At the same time, steeper slopes exceeding 15°, which comprise approximately 16% of the area, may contribute to increased erosion potential and accelerated runoff. The average slope of the catchment is 7.2°, which indicates that the Talas River basin has a moderately pronounced relief.

Catchment polygon centroid is one of the important parameters in hydrological studies and flow analysis. Catchment polygon centroid values were determined using data management tools and their determination is necessary for accurate cartographic interpretation and river flow modeling. When constructing runoff layer and runoff module maps, using catchment polygon centroid allows for greater modeling accuracy, as they help define boundaries of catchment areas and identify key factors influencing the water balance.

The total area of all endorheic basin amounted to 29% of the total study area, including 20% of the Talas River basin and 32% of the Assy River basin. From the point of view of relative stability or sustainability, the area of endorheic basin is referred to stable morphometric characteristics.

According to the study (Gregory & Walling 1973; Nisansala et al. 2018; Mudassir & Murtala 2024) flow intensity is directly related to the form factor, as low groundwater recharge is associated with higher river flow intensity. The form factor of the entire Talas River basin was 0.3 (Table 6), indicating low peak flow with a long duration of time. In addition, the low value of this index suggests that the basin is geometrically elongated and has a large area with low elevation values. One of the key advantages of elongated basins is that they are easier to manage in terms of flood control compared to circular basins (Singh & Singh 1997). Due to their shape and hydrological characteristics, elongated basins produce smoother peak flows over long periods, thus allowing for better control and management of flood waters with appropriate measures (Withanage et al. 2016).

For the Talas River basin, the compactness index was 1.9 (Table 6). This indicates that the basin is significantly elongated or has a complex shape.

According to the Strahler's classification, the Talas River basin with an elongation ratio falling within the range of 0.5–0.7 is categorized as an elongated basin (Table 6). Studies (Singh & Singh 1997) have shown that in such elongated basins the peak water flow has a gentler shape and lasts longer, which reduces the likelihood of flash and catastrophic floods.

The analysis of landscape characteristics of the Talas River basin has shown that, despite minor fluctuations, the observed general trend of the forest cover ratio indicates a low level of forest cover in the Talas River basin, reflecting limited forest resources in the region. The values of the forest cover ratio vary from 0.16 to 0.28% of the total basin area.

The limnicity coefficient value indicates a decrease in lake surface area within the basin, with only minor changes during the study period, averaging 0.3% of the total area. The average urbanization coefficient is equal to 1.40%. The highest value is observed in 2023 (1.46%) and the lowest value is observed in 2017 (1.3%). The increase in the urban and built-up lands coefficient in recent years indicates the growing impact of human activities on the landscape due to the expansion of urban areas and infrastructure. The values of the cropland coefficient indicate moderate changes in the area of cropland, with a slight decreasing trend in recent years. Changes in the wetland coefficient values are significant and indicate a decline in wetland ecosystems in the Talas River basin. The average permanent snow and ice cover coefficient is relatively stable despite minor fluctuations and on average accounts for 0.37% of the total basin area (Table 8).

Changes in landscape characteristics have a direct impact on river flow characteristics, changing both the volume and the flow regime (Gautam 2023). Thus, the results of the analysis of landscape characteristics of the Talas River basin emphasize the overall stability of natural resources and landscape changes in the basin. These data are important for further monitoring and sustainable management of the basin's natural resources.

The basic morphometric characteristics of river basins are the parameters of river catchment shape: area, length, greatest and average width, average height, average surface slope, and asymmetry coefficient (Chupikova et al. 2020). Basic and initial among these characteristics are river length and catchment area (Chupikova & Andronache 2019). These indicators play a key role because many other morphometric characteristics are directly dependent on these values: river length and catchment area. In turn, the accuracy of determining these input parameters is critical for further calculations and analysis, as even minor errors in their measurement can significantly affect the results of modeling and interpretation of hydrographic processes.

In addition, the correct determination of river length and catchment area is the basis for further studies aimed at assessing hydrological characteristics such as water flow, flood and storm regimes, and for the development of water management models. It is therefore important to use up-to-date and validated methods for measuring these characteristics to achieve high accuracy in the estimates and to ensure comparability with other regions and catchments.

Particular attention is paid to the fact that the scale of topographic maps and the resolution of satellite images must be taken into account when determining morphometric characteristics such as river length and catchment area. These parameters have a direct impact on the accuracy of the data obtained. The geographical location of the study area also has a significant impact on the results, as topography and climatic conditions can make adjustments in the determination of these parameters. In small-scale maps, both qualitative and quantitative indicators of hydrographic characteristics are altered, leading to a decrease in the accuracy of their measurement. For example, due to generalization of small meanders of river channels and contours of plots, their length is reduced, and due to selection, the number of objects depicted on the map is reduced: lakes, rivers, separate areas of forests, wetlands and other lands. The use of large-scale maps, in turn, providing high accuracy of defined hydrographic characteristics, sharply increases the volume of cartometric works (Zhikharev & Gorbacheva 2006). Based on this, the choice of the necessary and appropriate map scale to determine the hydrographic characteristics with the required accuracy is an important and responsible preparatory stage of cartometric works.

Previously published materials (Surface Water Resources of the USSR 1973; Isabelov 2011; Dostay et al. 2012; Kireicheva 2015) cite the length of the Talas River as 661 km, but do not specify methods and technologies for obtaining these characteristics. This makes direct comparison with our results obtained during the study difficult. In our study, using 1:200,000; 1:500,000 scale topographic maps, the river length of 648 km was obtained by digitization.

Determination of the morphometric characteristics of the Talas River catchment starts with the analysis of its area, which amounted to 44,115 km². The result obtained using the GIS platform has some discrepancies with the data presented in Surface Water Resources of the USSR (1973) and Dostay et al. (2012). These discrepancies can be explained by the specific character of the relief, as well as the peculiarities of the river mouth area, which in turn is a characteristic feature of rivers located in the arid zone. In particular, the mouth of the Talas River disappears into the Moyinkum sands (Water Resources Center n.d.), which are marked in ECE (2011) as the desert runoff receiving area.

Differences between our data and previously reported values may be due to differences in approaches to measuring river length and catchment area. For example, the use of different map scales mentioned above, differences in data processing methods (e.g. manual map processing vs. digital methods), and the definition of the source and mouth of rivers may affect the results obtained. It is important to note that our study used a more accurate and detailed approach, which may explain the discrepancies compared to previous estimates.

These findings emphasise the importance of clearly specifying the methods and technologies used to obtain such data, which will allow for more accurate and comparable results in the future. In the future, harmonization of measurement methods and refinement of river boundaries should be sought to improve the reliability of the data.

The correct determination of river length and catchment area provides the basis for the assessment of various hydrological indicators such as runoff, flood and stormwater regimes, and for the development of water management models. Errors in these input data can significantly affect the results of subsequent calculations and the accuracy of models, emphasizing the need for reliable methods to measure them.

The obtained results can serve as a basis for making informed decisions aimed at preventing negative consequences associated with climate change, anthropogenic impact and natural disasters. Under conditions of changing hydrological regime, erosion processes and increasing drought, accurate and timely morphometric data become an important tool for ensuring sustainability of ecosystems and rational use of water resources in the long term. Thus, accurate and reliable baseline data on the characteristics of rivers and their catchments are essential for integrated hydrological research and effective water resources management, making these parameters important inputs for research in this area.

Updates of morphometric characteristics of water bodies and their catchments should be made whenever changes occur in the water bodies themselves or their catchments, and this begins to affect the accuracy of hydrological calculations. All morphometric characteristics of a water body should be checked and updated simultaneously. Morphometric characteristics can be updated based not only on maps but also on aerial images or field hydrographic surveys.

Morphometric analysis of the Talas River basin using GIS and RS data is an important aspect for understanding and managing water resources in the region. With the increasing influence of anthropogenic factors and climate change, such a study becomes critical for adequately assessing the status of the catchment and for planning measures for its protection and rational use.

Analysis of river morphometric characteristics such as river length, average slope, drainage density and tortuosity coefficient provide a comprehensive view of the structure and functional features of the river system. These parameters help to assess water resources, flow dynamics as well as potential risk areas associated with changes in the hydrological regime.

Using GIS and RS to determine catchment characteristics such as its area, mean elevation, catchment polygon centroid, mean catchment width and area of endorheic basin provides high accuracy and detail in the analysis. These data are the basis for more complex hydrological and ecological model buildings that are needed to develop effective water management strategies.

River catchment morphological characteristics such as form factor, compactness, elongation and asymmetry coefficients complemented the analysis by providing valuable data on catchment configuration and distribution. These indices help to understand how the shape and structure of a basin can influence hydrological processes and ecosystem conditions.

Thus, an integrated approach to morphometric analysis using modern GIS and RS technologies can not only improve understanding of hydrological processes in the Talas River basin, but also enable more effective planning and management of water resources under changing climate and increased anthropogenic pressures. The results of the study emphasize the need for continuous monitoring and updating of data, as well as integration of modern technologies for planning and effective management of water resources, designing of engineering structures, creating models for forecasting hydrological processes in the context of sustainable development.

This research was funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant Number BR18574227).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.