Spatial analysis of the geomorphic evolution of Tigris River basin using developed ArcGIS-Morphometric toolbox

Studied parts of Tigris river basin extended over an area of about 202702 square kilometres mostly extended within Turkish, Iranian and Iraqi territories with a narrow area in Syria and located between latitudes 32° 30′ to 38° 30′ N and longitudes 40° 00′ to 48° 00′ E. The basin is characterised by a complex topography, lithology, and structural features, which have impacts on the geomorphic development stages of the sub-basins. The study aims to evaluate the geomorphic stages of the river sub-basins based on hypsometric and volumetric analysis of each sub-basin using ArcGIS-morphometric toolbox developed by the author. The analysis was carried out using SRTM one arc-second DEM data. The basin is divided into 34 sub-basins and the hypsometric and volumetric parameters i.e., area, elevation, volume ratios, hypsometric and volumetric curves and integrals are calculated. The results show Tigris river basin has been passing in the Monadnock phase, while the sub-basins are classified into three geomorphic development stages i.e, middle - later maturity stage, early Monadnock phase and later Monadnock phase. According to the hypsometric curves, hypsometric integral, volumetric integrals; most of the sub-basins are passing in steady state of erosions and weathering processes with the prevailed fluvial system, terrain slopes and rock types.


Introduction
Estimation of the geomorphic development of a large basin represents a challenge for the researchers, due to the difficulty of measuring the required morphometric parameters. The inaccurate explanation of rivers or valley basins as "young, mature, old and poor drain or well drain" is probably due to the lack of suitable measuring and operating tools [1]. The hypsometric curve of area -altitude distribution is suitable for defining the slope of the basin [2]. Strahler [3] recognized three development stages of the basins according to the value of Hypsometric integral (HI) i.e., inequilibrium (youth stage) with HI greater than 60%, equilibrium (Mature stage) with HI between 60% and 35% and Monadnock when HI drop below 35%. Watershed conditions are well recognized using hypsometric curves and hypsometric integral, whereas the differences in their shapes and values indicate the imbalance of tectonic forces and erosive [4][5][6][7][8]. Basin conditions can be evaluated based on hypsometric integral and hypsometric curves [5,8]. Convex hypsometric curves are typical of a youthful stage; s-shaped curves are related to a maturity stage, and concave curves are indicative of a peneplain stage [3,7,9]. In past, hypsometric analysis has been limited due to intensive computation requirements and lack of effective analysis equipment; Strahler was involved planimeter as a tool for area measurements increased the efficiency of assessment of contours, and dramatic advances in GIS technique after that was rigorously increased the quantitative measurement of basin characteristics [10]. Pérez-Peña, Azañón [11] have been developed CalHypso-ArcGIS extension to calculate statistical moments of hypsometric curves using Visual basic and ArcObjects with input layers i.e.,  [12] has developed ArcGIS-morphometric toolbox including script to calculate the area, height and volume ratios, plotting the hypsometric and volumetric curves and to calculate the HI and VI using trapezoidal method. Several factors are controlling the hypsometry of the basin i.e., shape, size, relief and dominant erosion process, as well as HI is sensitive to erosion resistance, basin outcrops and uplift rate [13]. The analysis and development of landscape can be initially assessed using HI [14,15]. Hamza [16] referred to the importance of hypsometric parameters in distinguishing between the tectonically active area and the inactive area in a basin. The cycle of erosion in any basin can be indicated by hypsometric integral [3,17].
Tigris river is one of the two major water resources of Iraqi lands beside Euphrates river. The river basin extends on a large area recognised by a different geological setting of lithology and structures with terrains varied in elevations and slopes. In fact, these characteristics are impacted in various manners on the development of the river sub-basins.
The current study aims to a). Assessment of the geomorphic development phases of Tigris river sub-basins based on hypsometric and volumetric analysis of SRTM-DEM data, b) Implementing the analysis using ArcGIS-morphometric toolbox, c). Evaluating the geological setting and slopes impact the geomorphic development of each sub-basin using the spatial analysis methods.

Location of study area
Tigris river basin extends over an area of about 202702 square kilometres mostly within Turkish, Iranian and Iraqi territories with a narrow area in Syria and bounded between latitudes 32ᵒ 30′ to 38ᵒ 30′ North and longitudes 40ᵒ 00′ to 48ᵒ 00′ East (figure 1) as measured from ArcGIS software. Tigris River is one of the important water resources for Iraqi lands. The river flows through a basin of various topographic regions starting from the mountainous lands in southeaster of Turkey and northwest of Iran to the sedimentary plain in middle and south of Iraq, which merged with Euphrates river making Shatt al-Arab

Data and analysis method
In current, study the digital elevation model of SRTM-DEM data of one arc second resolution [18] are used to delineate the watersheds of sub-basins and calculation of elevation levels in hypsometric analyses (figure 2) and calculating the distribution of the slopes over the basin (figure 3). Geological map [19] was used to show the spatial distribution of rock outcrops and lithology at different subbasins (figure 4). The hypsometric analyses have been illustrated in figure 5 and implemented according to the following steps:- • Downloading the SRTM-DEM data and converting the projection into UTM-WGS84 or any suitable projected coordinates system. • Using ArcGIS-Hydrology toolbox to calculate flow direction and flow accumulation. • Execution of Con ("Flowaccum" >100, 1) from map algebra, and selecting pour point for each sub-basin to delineate the watersheds. • Extraction of the drainage network based on Strahler stream order using spatial analyst-Hydrology -Stream order tool. • Delineation of the boundaries of sub-basins watersheds using spatial analyst-Hydrology-Watershed tool. • Extraction of the DEM data for each sub-basin using an extraction tool -extract by a polygon.
• Execution the developed ArcGIS-morphometric toolbox -hypsometric script [20] to calculate the hypsometric parameters, and the input data as illustrated in figure 6.

Middle-Later Maturity stage
Many sub-basins i.e., 2, 30, 28 and 6 are identified in a group of middle-later maturity stage as shown in table 2 and figure 8 with hypsometric integrals ranging from 39.5% to 52.5%. However, according to the volumetric integrals, the remaining rock masses still waiting for erosion are ranging from 22.6% to 30.9%. The sub-basins 2 and 6 are characterized by gentle slope, but geologically the mixed sedimentary rocks are exposed with 94% and 71% of the sub basins area respectively which make the upstream area more resistant to weathering and erosion processes. While the average slope of subbasin 28 is 17.43 degrees, and covered mainly by basic volcanic rocks with 57% and the remaining area are covered by metamorphic and carbonate sedimentary rocks, despite the relative slope of the basin, however, the dominance of hard rocks reduce the denudation processes. Whereas the sub basin 30 is recognised by dominance of carbonate sedimentary rocks with 42%, 21% basic volcanic rocks and 13% metamorphic rocks of the area, which lead to active chemical weathering in the basin. All the hypsometric curves in this group are of S-shape with convex curves and the volumetric curves indicating that most of the rock masses at upstream area are more resistant to the denudation processes.  Most of the sub-basins passing in later Monadnock phase are located at the downstream area of the Tigris River basin as shown in table 2 and figure 10. The values of hypsometric integrals range from 3.7% at sub-basin 37 to 19.8% at sub-basin 21, while VI range from 10.3% at sub-basin 11 to 20.5% at sub-basin 20 with hypsometric curves of concave shape. Mixed sedimentary rocks and Quaternary deposits exposed at this group of sub-basins with average terrain slopes ranging from 0.35 to 5.26 degrees.

Conclusions
The  10 20.5%. According to the hypsometric curves and hypsometric integral values, most of the sub-basins are passing in steady state in the processes of erosions and transformation with the prevailed fluvial system, terrain slopes and rock types. The volumetric integrals indicate the remaining volumes of rock masses in the basin. Regarding the geological setting, the main lithology controlling the hypsometric parameters and consequently the geomorphic development of the sub-basins are carbonate sedimentary rocks, mixed sedimentary rocks and Quaternary deposits. The impact of knick points on the shapes of hypsometric curves is recognized in the upstream area of the sub basins due to the presence of folds and faults structures. The results of hypsometric curves show that wherever the volumetric curve comes above the hypsometric curve it means that the terrains have highly rough and complex structures.