Determining the Surface Drainage Network Causing Flash Floods in Central Erbil Sub-Basins in Northern Iraq Using GIS and Geospatial Technique

Abstract


Introduction
The study of the characteristics of the surface drainage networks of dry valley basins, especially in dry and semi-dry environments, is of importance in reflecting the geomorphology and hydrology of the valleys and thus on the pattern and drainage of water resources in general (Aziz et al., 2020).The quality of the surface drainage network is a true reflection of the quality of climate, the land's topography, the quality of soil and rocks, and the presence of tree cover in particular (Jirjees et al., 2022).Quantitative measurements and analysis of surface drainage networks through morphometric studies of valleys are among the priorities of applied hydrological studies in many human activities and land uses, often concentrated near rivers and running water sources.This human interaction may have a significant impact on them.The surface drainage network is also one of the topographical phenomena on which the runoff water is based, which transports the running water from its sources to its outlets, and this represents a true reflection of river discharge patterns and thus clarifying the surface drainage network of basins (Fatah et al., 2020).Basins of rivers, which are affected by catchment discharges and comprise the entire area between a river's source and mouth, are crucial geographic units for managing water resources (Sakthivel et al., 2019).Rapid and unsustainable growth has disrupted hydrological cycles in the river basins.This has frequently led to increased frequency and intensity of flooding, droughts, and pollution (Al-Dabbas et al., 2020).
The people residing in these river basins suffer significant economic and social losses due to the decline and loss of biodiversity.There is an increasing need for water for drinking and other uses because of urbanization and population increase, resulting in a problem affecting surface and groundwater (Singh and Thakur, 2011;Thakur et al. 2011;Diwakar and Thakur, 2012).Therefore, appraisal of water resources is vitally needed for the economy and sustainability of livelihoods (Singh et al. 2013).Development and management plans are essential to the ecosystem's survival and guarantee that it continues to provide the community's inhabitants with essential goods and services (Al-Shammari and Al-Dabbas, 2015).Utilizing freshwater resources most effectively and sustainably is also necessary for innovative basin and water management methods.Watershed management strategies require understanding the hydrological character of rocks within the basin, which can be obtained using a basin's quantitative morphometric analysis (Singh et al., 2014).
A watershed's primary structural, geological, and hydrological components are revealed through the stream network.It is necessary to be aware of the geological setting, topography, stream network and its pattern, and geomorphological structure for the watershed to be managed and a conservation plan implemented (Sreedevi et al., 2013).Several regional hydrological models constructed using the watershed's geomorphological properties resolve various hydrological issues of a watershed.Esper (2008) asserts that morphometric descriptions for watersheds are essential for assessing the hydrological system in conjunction with geology and geomorphology.The morphometric study is essential for hydrological research, drainage basin development, and management in a drainage basin (Rekha et al., 2011).The main factors of basin-scale running water ecosystems in operation are morphometric parameters and climatic circumstances (Frissel, 1986;Lotspeich and Platts, 1982), according to Malik et al. (2011), river basin appraisal, watershed prioritizing for water and soil preservation, and natural resources.
The quantitative examination of morphometric factors can significantly improve management at the watershed level because the watershed's morphometric features control all surface runoff.The watershed is regarded as the optimal type of regional unit (Lima et al., 2011).Different drainage parameters can be analyzed, including the arrangement of basin area and different streams, the perimeter of drainage channels and the length, frequency of streams, drainage density, texture ratio, bifurcation ratio, ruggedness number, basin relief, and time of concentration to evaluate morphometric factors (Kumar et al., 2000;Nag and Chakraborthy 2003).
Numerous researchers have examined the different catchments' basin morphometric characteristics using traditional methods (Strahler 1957;Smith 1950;Horton, 1945), Geographic Information System (GIS), and remote sensing (RS) methods (Narendra et al., 2006;BiSBas et al., 1999;Agarwal 1998;Srivastava and Mitra, 1995;Krishnamurthy and Srinivas 1995), and other techniques.GIS, Geospatial Technology, GPS, and RS have been employed as useful tools for planning and managing land and water resources that address most issues (Banerjee et al., 2015;Tripathi et al., 2013;Soni et al., 2013).GIS techniques provide a powerful tool for modifying and analyzing spatial information, especially for information extraction and identification for improved understanding in the future.They are used to analyze the various topography and morphometric aspects of drainage basins (Vijith and Satheesh, 2006).
The main aims of the study are to determine basin geometry and describe the morphometric characteristics of the Central Erbil sub-basin.Using morphometric analysis to understand water resource management and conservation methods for sustainable livelihood through GIS and RS.This research aims to determine the drainage network that causes floods in the Central Erbil sub-basin by identifying the morphometric characteristics of the basin as well as the causes that lead to the occurrence of floods in Erbil city, especially recent flash floods and flood have occurred in Erbil city and led to significant material and human losses.

Study Area
The research area is located in the north of Iraq, bounded by latitudes 35°30'00"-36°30'00" N and longitudes 43°30'00"-44°30'00" E (Fig. 1).The area occupies 2107 km2.The elevation ranged from 206 to 1076 m a.s.l.It is bounded from the north by the Bastura Valley and Greater Zab River, from the west by Greater Zab River, from the south by the Awna mountain range, and from the east and southeast by a group of convex structures.As a result of being surrounded by a group of convex structures from north, east, and south, it has formed a hydrological sub-basin whose waters drain ends in the Greater Zab River to the west.The research area generally consists of mountainous areas in the northeastern part, followed by hilly areas, and then the area begins to be almost flat towards the southwest.About 75 percent of the research area has a slight slope in the southwestern parts; the remaining 25 percent is in the northeastern and contains low anti-mass folds with overlapping decaying valleys.
According to Erbil meteorological station, the highest temperature in the region ranges from 30 degrees in September to 35 degrees in July and August, while the lowest ranges between 8.48°C in January to 13.38°C in March.Evaporation is considered an essential climatic element in determining the balance of the water system and a crucial factor in the hydrological cycle.Stability prevails in the case of high precipitation rates in December, January, and February.Evaporation reaches about 194.56 mm and may reach 213.41 mm, and the evaporation rate decreases in December to about 27.21 mm.The research area experiences lengthy, scorching summers, moderately heavy monsoon rains, and cool winters.This type of climate is classified as the sub-continental monsoon climate category.Long hot summers, monsoon showers, and chilly winters are typical for this climatic type.The long, dry, and hot summer season starts in March, after which the temperature rises quickly and stays high until August.The average annual rainfall is 300-600 mm.According to the Erbil Meteorological Station, the climate of the study area is classified as semidry (Khoshnaw, 2022).The rainfall occurs between October and May and approximately disappears in months for June, July, August, and September; the highest average rainfall occurs in January (94 mm), and the average annual precipitation is 472.2 mm from 1992 to 2022.
Geologically, according to Jassim and Goff (2006), the exposed rocks in the area of interest belong to the Miocene, Middle Miocene, and Pliocene, which are the Injana, Bai Hassan, and Mukdadiyah formations.These rocks are under thick beds for Pleistocene and Holocene rocks.The lithology of the Injana is variable, but the unit is composed of mostly red or grey-colored silty marl or claystone and siltstones of the same color.Bai-Hassan Formation consists of alternating claystones and conglomerates with sandstones and siltstones.The Mukdadiya Formation comprises up to 2000m of fining upward cycles of gravelly sandstone, red mudstone, and sandstone.The major units of the Quaternary deposits involve the alluvial fan, depression fill, floodplain, and aeolian sediments.Quaternary sediments are unconsolidated, and their grains are finer than those in the Mukdadiya and Bai Hassan formations.The productive hydrogeological unit in the studied area is the Bai-Hassan Formation (confined aquifer, unconfined aquifer), composed of sandstone and gravel, consecutive with clay and conglomerate masses.All wells in the area partially penetrate this formation, and the recharge sources are located in the northeast area outside the study area, where its layers are exposed.The general direction of groundwater flow in the study area is from northeast to southwest toward the Greater Zab River, and the hydraulic gradient (I) average is 0.0183.

Materials and Methods
The hydrogeological behavior of the drainage basin is shown through quantitative analysis, which also characterizes the types of rock, geomorphology, and structure.The morphometric study also provides the basin's geometry, rock permeability, and the basin's storage capacity.The first step of the analysis is to define the drainage basin and catchment area, which are prepared for stream and boundary delineation throughout the basin.The DEM was obtained from http://earthexplorer.usgs.govwith a spatial resolution of 12.5 m, and updates to basin boundaries and streams were made using the data.The drainage network within the basin boundary was derived from DEM using the ArcGIS 10.4 application.Utilizing ArcGIS 10.4, these data were utilized to compute the linear aspect, areal aspect, and relief aspect (Table 2).Length of the stream Horton (1945) Mean stream length (Lm) Lm = Lu/Nu Horton (1945) Stream length ratio (Rl) Rl=Lu/L(u-1), where Lu; is stream length order u and L(u-1); is stream segment length of the next lower order Horton (1945) Bifurcation ratio (Rb) Rb=Nu/N(u-1), where Nu is the number of streams of any given order, and N(u-1) is the number in the next higher order.Horton (1945) Rho coefficient ()  = Rl/Rb Horton (1945) Areal aspects Drainage density (Dd) Dd=L/A, where L; is total stream length, A; is area of watershed Horton (1945) Stream frequency (Fs) Fs=N/A, where N is the total number of streams, and A is the area of the watershed.Horton (1945) Drainage texture (Dt) T = Dd * Fs Smith (1950) Length of overland flow (Lg) Lg = 1/2 Dd Horton (1945) Constant of channel maintenance (C) C = 1/Dd Schumm (1956) Form factor (Ff) Ff = A/Lb 2 Horton (1945) Circularity ratio (Rc) Rc = 4πA/P 2 Miller (1953) Elongation ratio (Re) Re = 2√(A/π)/Lb, where A is the area of the watershed, π is 3.14, and Lb is the basin length.Schumm (1956) Shape index (SB) SB = 1.27 * Form Factor Horton (1945) Relief aspects Basin relief (R) R = H -h, where H is maximum elevation, and h is minimum elevation within the basin Schumm (1956) Relief ratio (Rr) Rr = R/Lb Schumm (1956) Ruggedness number (Rn) Rn = R * Dd Strahler (1958) Dissection index (Di) DI = R/Ra, where Ra is an absolute relief Singh and Dubey (1994) Gradient ratio (Rg) Rg = Es -Em/Lb, where Es is the elevation at the source, and Em is the elevation at the mouth.

Sreedevi et al. (2009) Melton ruggedness number (MRn)
MRn = H -h/A 0.5 Melton (1965) A Digital Elevation Model, SRTM (Shuttle Radar Topography Mission), was used for digitizing the study watershed and producing drainage patterns.A variety of techniques were employed to create stream networks and define watersheds.Software Arc GIS 10.4 was utilized to examine the basin's morphologic features.The SRTM is used to generate watershed and stream networks for the study using the arc toolbox in ArcGIS 10.4.The following are the general methods used to define and create stream networks.Using the ArcGIS arc toolbox, re-project DEM data using the coordinate system of UTM-WGS84 zone 38N.Using the Fill tool included in Spatial Analyst tools of Hydrology Applications, DEM filled in pits.The hydrology Application's flow direction and accumulation are tracked using the same spatial analyst tools used to create the watershed's flow direction and accumulation map.In order to create the stream networks, a raster computation procedure was carried out with 1000 as the threshold, and an appropriate watershed outlet point was chosen.The watershed's area, the streams' length, and their order were calculated once the watershed boundary was plotted.Typically, stream ordering comes first when analyzing a drainage basin.The hierarchy of its tributaries is being used to define the size of the steam.The Horton (1932) law, modified by Strahler (1958), was utilized for stream ordering.Each order stream's total number and length were calculated and recorded (Fig. 2).The drainage layer is used to derive basic parameters, including area, perimeter, basin length, stream length, and number of streams.Based on formulae proposed by Horton (1945), Miller (1953), Schumn (1956), and Strahler (1964), drainage basin was analyzed by computing values of the morphometric properties, which include drainage density, form factor, stream frequency, texture ratio, elongation ratio, bifurcation ratio, and circularity ratio.

Result and Discussion
Morphometric analysis was used to measure the landforms' dimensions and the arrangement of Earth's surface.The purpose of this analysis is to plan and manage the water resources as well as to analyze the drainage basin quantitatively.The three primary characteristics identified for investigation are Linear, Areal, and Relief.In morphometric parameters, basin length, bifurcation ratio, stream length, Stream order, and stream number are characteristics of linear aspects.The features of the catchment area are reflected in the areal aspect, which demonstrates how the catchment region controls hydrological behavior.The relief aspect determines the terrain arrangement of the catchment and terrain characteristics.Following is an analysis and description of morphometric parameters of Central Erbil sub-basins and its subdivision:

Stream Network Generation
Using ArcGIS Hydrology and Spatial Analyst tools, stream networks were drawn from DEM in Fig. 2. The watershed's slope is contrasted with the streams' defined boundaries.The slope in the downstream part of the Central Erbil sub-basin is smooth and mild, whereas, in the upper part of the Central Erbil sub-basin, Fig. 3 is steep.Fewer long streams are delimited in the gentler portion of the Central Erbil sub-basin (Fig. 4), whereas a more significant number of short streams are generated at the steeper slope.This suggests that the topography with more undulations is more vulnerable to soil erosion and flooding.

Drainage Pattern
A drainage pattern is a river system layout incorporating geological data and the drainage basin's main slope.How the streams are arranged in a drainage system determines the drainage pattern.This reflects the lithological or structural controls of subsurface rocks.Three kinds of drainage patterns exist: parallel, radial, and dendritic.The parallel type is located in the southwestern part of the study area, the radial type is located in the northwestern part of the study area, and the dendritic type is located in the northeastern part of the study area (Fig. 5).Rocks, and soil are homogeneous and uniform, as shown by the dendritic drainage pattern.Parallel drainage patterns suggest a consistent, mild slope with weaker bedrock.The radial drainage pattern is created by streams that, from a central higher point, diverge dramatically in all directions.Perfect radial patterns seem to be encouraged by dome formations, residual ridges, small plains, and isolated mountains, as shown in Fig. 5.

DEM Collection and Processing
Sink and fill pits in depression

Linear Aspects
Linear aspects of basin morphometric analysis, including bifurcation ratio, stream length ratio, stream order, number of streams, length of the stream, perimeter, length of the basin, and Rho coefficient, are discussed below.

Perimeter
The overall perimeter of the Central Erbil sub-basin (ES) is 252.64 km, whereas Table 3 expresses data for six sub-basins (SB).All sub-basins are elongated because the perimeter increases as the area increases (r = 0.359).The Daratwo sub-basin has the biggest perimeter (125.23 km), encompassing a greater basin area of 349.99 km2.Darband sub-basin, on the other hand, has the smallest perimeter, measuring 93.38 km, covering an area of 121.13 km2.

Length of basin (Lb)
Length of basin of ES is 87.68 km and the remaining six sub-basins (Kasnazan sub-basin (SB1), Daratwo sub-basin (SB2), Qushtapa sub-basin (SB3), Grd-Jotyar sub-basin (SB4), Qoritan sub-basin (SB5) and Darband sub-basin (SB6) table 3 discusses this.Each sub-basin is longer than the others.It exhibits a positive connection (r = 0.98) with erosion tending toward the head of the basin.The distance of a straight line between a basin's mouth and its outlet point is known as basin length.(Horton, 1932).

Stream order (Nu)
A positive integer describes the order of a river system's branching in geomorphology and hydrology.Different stream ordering techniques exist (Leopold et al., 1964;Strahler, 1957;Horton, 1945).This study uses Strahler's technique, a slightly modified version of Horton's method, to evaluate the stream order.Streams that are first order are the smallest and lack branches.The smallest (not

Parallel Dendritic Type
Radial Type Drainage branching) streams are called first-order streams.When two first-order streams converge, they form second order, and when two second-order streams converge, they become third order, and so on.However, when two distinct levels of streams converge, the highest-level stream will always remain.The Central Erbil sub-basin is a sixth-order stream.A total of 884 streams were found, of which 688 were first orders, 149 were second orders, 35 were third orders, 8 were fourth orders, 3 were fifth orders, and 1 were sixth orders (Table 3).Understanding the hydrological process in a basin requires understanding the stream network's properties.The six sub-basins are arranged in Table 3.There are 884 streams totaling 2107.28 km 2 in the analyzed area, making it a 6 th -order drainage basin (Fig. 2).

Number of streams (Nt)
Stream number refers to how many streams are in a certain watershed for each order.According to Horton's law of stream order (1945), the number of streams in each order yields an inverse geometric sequence when compared to stream order.The last sub-basin is characterized by lithological and topographic variation.A straight line with a variation represents the relationship between the logarithm of the number of streams and their order, indicating that the number of streams decreases as the order of streams rises and describes homogeneous sub-surface material subject to weathering.(Nag and Lahiri, 2011).

Length of stream (Lt)
One of the factors with the greatest potential for comprehension of the hydrological properties of the basin is stream length.Streams with longer lengths typically have flatter catchments.A stream segment's cumulative length is usually the longest in the first order and grows shorter as the stream order increases (Table 3).ArcGIS 10.4 software was used to calculate the total number and length of streams of the Central Erbil sub-basins.Watershed's stream qualities were consistent with Horton's law of stream length, which stipulates that cumulative stream length tends to rise by a constant length ratio starting at the mean segment length of first order (Horton, 1932 and1945).Strahler (1964) describes the typical network component parts size by the mean stream length.Table 3 displays the relationship between stream order and the cumulative length of the streams that are being studied.The change of streams from one order to another indicates that flow is to flatter ground from high altitude, lithological variation, and steep slope.

Bifurcation ratio (Rb)
A bifurcation ratio is the ratio of one-order streams to the following higher-order, according to Schumm (1956).It affects the morphometry of the landscape and has some control over the "peakedness" of runoff (Chorley, 1969).The bifurcation ratio measures the stream network's degree of spread (Mesa, 2006).When geologic characteristics have little effect on stream network, the bifurcation ratio value in networks produced on homogenous rocks ranges from 3.0 to 5.0 (Vittala et al., 2004;Strahler, 1964;Nag, 1998;Verstappen, 1995), and values greater than 10 when elongate basins dominate structural controls (Chow et al., 1988;Mekel, 1970).Verstappen (1983) and Ghosh and Chhibber (1984) show that the bifurcation ratio data reflects the basin's shape.Circular basins have large values for the bifurcation ratio, but elongated basins have low bifurcation ratio values (Morisawa, 1985).The bifurcation ratio values for the Central Erbil sub-basin range from 0.33 to 4.62, with the mean bifurcation ratio value being 3.316.The rest of the sub-basins have greater bifurcation ratio values ranging from 3.16 to 4.40 (Table 3).This shows that the mountainous topography contributes significantly to overland flow and discharge.SB4 has the lowest mean bifurcation ratio value, which indicates a high infiltration rate.

Stream length ratio (Rl)
A ratio of one order's mean length to subsequent lower order in stream networks is known as stream length ratio.An overall impression of the relative permeability of rock formation can be obtained from the stream length ratio.According to Horton's law (1945), Stream length increases with the higher order of streams, and the average stream length segments of every order that follows in a basin typically approximate a straight.according to the mean stream length ratio of the Central Erbil sub-basin and the variations in stream length ratios across streams of different orders, the streams in the studied area are still in the late youth stage of their geomorphic development.(Vittala et al., 2004;Singh and Singh, 1997) 4.3.8. Rho coefficient (Rho) According to Horton (1945), the rho coefficient is the relationship between the bifurcation ratio and stream length.The Rho coefficient indicates the drainage network's storage capacity.ES's Rho coefficient value is 0.62, and the rest of the 6 SB's values range from 0.21 to 0.49, indicating larger hydrologic storage during floods.

Area
The area of Central Erbil sub-basin is 2107.28km 2 .SB6 is the smallest, 121.13 km 2 , whereas SB2 349.99 km 2 is the largest among six sub-basins Table 4.

Drainage Density (Dd)
According to Horton (1945), the total length ratio of all streams to basin surface area is known as drainage density.Drainage density is a quantitative indicator of the potential for runoff and dissection of the landscape (Chorley and Walling, 1973).The drainage density demonstrates the plant cover and infiltration capacity of the catchment (Macka, 2001).Drainage density affects the catchment area's water and sediment discharge and its susceptibility to erosion (Anon 1988;Bates, 1981;Gregory and Walling 1973).The vegetation and climate condition of a drainage basin determines drainage density.(Moglen et al., 1998), landscape characteristics, including rock and soil (Kelson and Wells, 1989) and relief (Oguchi, 1997), all affect the drainage density of the basin.Drainage density gives information about a region's potential for groundwater because of its connections to surface runoff and permeability.The permeable sub-soil, dense vegetation, and low relief are typically found in places with poor drainage density (Nag, 1998).
Conversely, high drainage density is caused by sparse vegetation, mountainous terrain, and impermeable underlying material.The drainage texture is coarse when the drainage density is low and fine when the drainage density is high.Central Erbil sub-basin drainage density, which was 1.07 km/km 2 , placed it in the medium category, which denotes terrain with mild to steep slopes, moderately dense vegetation, and less permeable soil with moderate precipitation.In Table 4, values for six subbasins are listed.

Stream frequency (Sf)
Stream frequency is the number of streams per unit area in a basin (Horton 1945).More significant surface runoff, steeper topography, higher ground relief, impermeable subsurface conditions, and sparser vegetation are all indicated by higher stream frequencies.Low stream frequency is indicated by high permeable geology and low relief.The ES stream frequency is 0.419 numbers per km2, and the 6 SB stream frequency ranges from 0.379-0.443numbers per km2 (Table 4), the stream frequency, indicating poor runoff.All sub-watershed stream frequency values show a strong association with drainage density, showing a rise in the number of streams relative to a rise in the drainage density.

Drainage texture (Dt)
Relative channel spacing in a fluvial dissected landscape is measured by drainage texture, which is produced by multiplying stream frequency (Sf) by drainage density (Dd).It is influenced by soil type, lithology, precipitation, development stage, climatic conditions, and soil type (Smith, 1950).The density and kinds of vegetation cover also significantly impact the drainage texture (Kale and Gupta, 2001).Large, resistant rocks produce a coarse texture, while a fine texture is produced by weak, soft, and uncovered rocks.The limited vegetation in an arid environment produces finer textures than those produced on equivalent rocks in a humid climate.According to Dornkamp and King (1971), vegetation type and climate are frequently related to the texture of rocks.Compared to permeable areas, impermeable areas have more drainage lines.Considering drainage density and stream frequency, Horton (1945) determined that infiltration capacity was the most critical element impacting drainage texture.Drainage texture is separated into five classes based on drainage density values: very coarse (<2), coarse (2-4), moderate (4-6), fine (6-8), and very fine (>8).The ES has a drainage texture value of 0.447, while all sub-basin values range from 0.403-0.479,and the Central Erbil sub-basin shows a very coarse texture (Table 4).

Length of overland flow (Lg)
The length of the overland flow is half of the reciprocal drainage density.According to Horton (1945), the amount of water flowing over land before it is concentrated into a mainstream affects drainage basins' hydrologic and physiographic evolution.This is known as the length of overland flow.According to Suresh (2000), when the intensity of rainfall exceeds the soil's ability to absorb water, overland flow happens.This component is affected by the kind of rock, permeability, relief, climate, and length of erosion, among other things (Schumm, 1956).Central Erbil sub-basin has a length of overland flow value of 0.533.In contrast, all sub-basin values range from 0.519 to 0.562, as shown in Table 4, which shows the impact of steep to low slopes, substantial surface runoff, low permeability, and severe structural disturbance.The Central Erbil sub-basins and sub-divisions have a mature geomorphic stage and a well-developed stream network.

Constant of channel maintenance (Cc)
This parameter indicates the number of units of the watershed surface required to support one unit of the channel length.The inverse drainage density with a length dimension has been employed by (Schumm, 1956) as a characteristic known as the channel maintenance constant.Higher values of this parameter in drainage basins will result in lower drainage densities.In Table 4, the computed value is displayed.The channel maintenance value of Central Erbil sub-basins is 0.937, and the channel maintenance value of 6 sub-basins varies from 0.888-0.961(Table 4).Higher values of constant channel maintenance suggest moderate surface runoff, faster infiltration rates, less dissection, and a basin unaffected by structural influences.It also shows reasonable control of lithology with a high permeability surface.

Basin configuration
Basin geometry affects how floods originate and flow.It is well known that floods form and move more quickly in circular basins than they do in elongated ones.In addition, floods in the former type of basin are stronger, move faster, and possess higher transport and erosion capacities.Longer shapes promote less flooding since tributaries enter the mainstream at longer time and space intervals.4.4.7. Form factor (Ff) According to Horton (1932), the dimensionless ratio of a drainage basin's area (A) to the square of its longest length (Lb) is known as its form factor. Simple dimensionless ratios of fundamental metrics area, perimeter, and length can be used to determine the geometry of a basin (Singh, 1998).The form factor predicts the formation and movement of floods, the severity of erosion, and the capacity of a watershed to transfer sediment loads.ES has a form factor of 0.453, but the form factors of six subbasins (Table 4) range from 0.115-0.234.The form factor differs from zero (extremely elongated shape) to unity 1 (perfect circular shape).Because streams enter mainstream at wider intervals and spaces, which encourage groundwater percolation, the main basin and sub-basins have lower values of form factor, indicating a longer basin that is flatter and has low flow peaks for longer periods, less capacity for erosion and sediment movement, and a favorable decrease in floods.

Circularity ratio (Rc)
The circularity ratio is the ratio of the area of a basin (A) to the area of a circle with the same perimeter as the basin (Miller, 1953).When the basin is shaped like a complete circle, the value of the ratio is unity.However, when the basin is substantially elongated and made of homogeneous geologic materials, it is in the range of 0.4 to 0.5.The slope, basin's relief geologic structure, land use, and land cover impact the circularity ratio.The ES has a circularity ratio of 0.414, whereas, in 6 sub-basins, the value ranges between 0.174 and 0.326 Table 4. High circularity ratio values indicate a near-circular form, whereas low values suggest an elongated basin shape.The association between the circularity ratio value and the form factor is positive (r = 0.76).

Elongation ratio (Re)
According to Schumm (1956), the elongation ratio is the diameter of a circle having the same area as a basin divided by the basin's greatest length.In order to analyze basin shape, the elongation ratio is a crucial measure.An elongation ratio analysis revealed that higher-value places had reduced runoff and higher infiltration rates.According to Singh and Singh (1997), an extended basin is less effective in draining runoff than a circular basin.Elongated (0.5 to 0.7), less elongated (0.7 to 0.8), more elongated (< 0.5), circular (0.9 to 1.0), oval (0.8 to 0.9), and more elongated were categories used by Strahler (1964) to categorize elongation ratio.The elongation ratio of ES is 0.274, while the elongation ratio values of six sub-basins range from 0.077-0.179(Table 4).Elongation ratio values indicate an elongated basin shape, gentle to steep slope, and high relief.

Shape index (SB)
The shape index has no dimensions and is the opposite of the form factor.The shape index of ES is 2.208, and the remaining six sub-basins have values between 4.268 and 8.669 Table 4.A higher form index indicates a more extended basin and a weaker flood period.

Basin relief (R)
Basin relief calculations are used to demonstrate that spatial variance predominates (Rao et al., 2011).Basin relief is the largest vertical distance that separates a basin's lowest and highest points.Stream gradient is caused by basin relief, which also affects flood patterns and the amount of transportable silt (Hadley and Schumm, 1961).According to Sreedevi et al. (2009), Basin relief is essential to understanding the denudation features of the basin.ES's basin relief value is 0.80 km, while Table 5 describes the remaining six sub-basins.According to Schumm (1956), the relief ratio is a valuable measure of the features of the gradient in a watershed and the ratio of basin relief to basin length.The relief ratio indicates the drainage basin's overall steepness and strength of the erosion processes occurring on its slopes (Javed et al., 2009).The relief ratio value of the Central Erbil sub-basin is 0.0117, while values of 6 SB are shown in Table 5.Values are relatively low (< 0.1), indicating a gentle slope.

Ruggedness number (Rn)
Basin relief and drainage density are combined to provide a dimensionless ruggedness number, which combines characteristics of slope steepness and length (Strahler, 1958).Selvan et al. (2011) state that the ruggedness number measures surface unevenness.The ruggedness number for ES, which is 0.853 with the remaining six sub-basins, is listed in Table 5.The comparatively low ruggedness number value indicates a reduced susceptibility to soiling erosion and an inherent structural complexity concerning drainage density and relief.(Paretha and Paretha, 2011).

Dissection index (Di)
In order to comprehend morphometry, physiographic characteristics, and the degree of terrain dissection, the dissection index is calculated (Singh, 2000;Singh and Dubey, 1994;Schumm, 1956).The ratio of actual river dissection to potential dissection up to base levels is known as the dissection index (Pal et al., 2012).ES's dissection index is 0.72, whereas values for six sub-basins range from 0.772-0.838.Lower dissection index values indicate an older stage of the basin and less dissection (Deen, 1982).

Gradient ratio (Rg)
According to Sreedevi et al. (2009), the Gradient ratio suggests a channel slope from which runoff volume could be evaluated.The gradient ratio value of ES, which is 0.011, with the remaining six subbasins, is listed in Table 5.Low gradient ratio values indicate moderate relief terrain and plateau-based mainstream flow.

Melton ruggedness number (MRn)
The Melton Roughness Number, a slope indicator, thoroughly represents relief roughness inside watersheds (Melton, 1965).The ES has a Melton Ruggedness Number value of 0.017 and 6 sub-basins range of 0.018-0.049.This low value shows no additional debris flow and that the mainstream is flowing normally.

Conclusions
River basins are crucial geomorphological units that demonstrate topographical and hydrological unity.The Central Erbil sub-basin and its sub-divisions watershed characterization demonstrated the significance of morphometric analysis in basin dynamics and landscape depiction.The GIS technique provides high mapping and measurement accuracy in morphometric analysis.A stream's order was determined for 884 streams totaling 2348.2 kilometers.The results show that changes in stream length ratio correspond to variations in the topography and slope of the basin.
Additionally, the basin's readings for the elongation ratio is 0.274, the circulation ratio is 0.414, and the shape factor is 0.453, suggesting that the basin was semi-elongated to elongated in character.Analysis reveals a developed geomorphic stage and a well-established drainage system in the watershed.The drainage patterns in the study area are dendritic, parallel, and radial.In the sixth stream order, the drainage density value shows terrain with a moderate slope, higher infiltration rate, moderate surface runoff, and sparse to dense vegetation.Central Erbil sub-basin and its subdivisions are elongated in shape and have lower erosion and sediment transport capacity, making them less prone to flooding.Thus, morphometric parameters offer pertinent data on topography properties and the Central Erbil subbasins hydrological behavior.It is concluded that a watershed management plan would benefit from incorporating morphometric analysis with methodologies for watershed assessment.

Recommendations
-Since the Central Erbil sub-basin is located in areas far from the Greater Zab River, it must be utilized in agriculture in the best way, mainly since the Central Erbil sub-basin receives large quantities of sudden rains, which must be seized collected even by constructing pond or small dams in the area.
-Dams and barriers in some areas of the valleys should be constructed.Work to develop laws that prevent housing in the outskirts of the valleys in order to prevent damage from floods and flash floods that affect the population annually.
-Organizing the method of different uses in the basin so as not to deplete the environmental resources and waste the capabilities of the valley that can be making use of them by setting laws to regulate agriculture in particular.
-Study the appropriate pumping method for the basin, including the transmissivity amount in the basin sections and the storage coefficient.

Fig. 1 .
Fig.1.location map of the study area

Fig. 3 .
Fig.3.Contour line and stream networks of the study area

Table 2 .
Using laws to extract morphometric parameters

Table 4 .
Areal aspects of the ES and sub-watersheds

Table 5 .
Relief aspects of the ES and sub-basins