## 4 Hierarchical Drainage Basins Analysis  
Here the drainages of Greece were selected and analyzed according to their tectonic regime, mean elevation value and mean value of Topography Position Index (TPI). In terms of geotectonics, a distinction between the non-metamorphic External Hellerides of western Greece on the one hand, characterized by Triassic-Cenozoic sedimentary sequences, and the Internal Hellerides of eastern Greece on the other, with metamorphic zones of pre-Alpine formations (Figure 2) is used, in order to see the watershed distribution pattern in this two main geotectonic units that form Greece. The External Hellerides mainly consist of Meso- and Cenozoic sedimentary rocks deposited in a series of platforms (Pre-Apulian and Gavrovo zones) and deep basins (Ionian and Pindos zones) that formed the eastern rifted margin of the Apulian plate, bordering towards the east the Pindos Ocean [48; 49; 50]. These units were developed during Tertiary times following the closure of the Pindos Ocean and the consequent continent-continent collision between the Apulian and Pelagonian micro-continents to the east [51; 52]. This process induced the inversion of Mesozoic basins in the northern margin of Apulia as well as the formation of a series of thrust sheets comprising the External Hellerides thrust belt ([48] and references therein). To search the possible connection of drainages hierarchy with the geotectonic environment as indicated in a number of previous works [21; 23; 24; 46; 47] we select to search the drainage distribution in the two drainage subsets defined by the two main tectonic patterns of Greece. Moreover, having in mind that different elevation values may reflect different landscape dynamics probably affecting the drainage basins area distribution, zonal statistical analysis was performed on all the cells of GDEM that belong to each one drainage basin (i.e., zone). In this way, we obtained the mean elevation value of all the drainage basins in Greece. The latter were classified in 6 classes according to their mean elevation: 0-30 m, 30-90 m, 90-150 m, 150-300 m, 300-900 m and over 900 m (Figure 3). The range of each class was adopted from [PERSON]'s 1964 methodology for classifying and mapping landforms [53]. Furthermore, the size distribution of Greek drainages classified in several classes according to their mean value of the Topography Position Index (TPI) [54] is given. The latter is a morphometric parameter derived from DEM and therefore an objective quantitative way of landform classification and watersheds characterization. The topographic position index (TPI) was introduced by [PERSON] (2001) as a GIS application for landform classification as well as watersheds characterization [55]. The creation of an ESRI ArcView 3.x extension by [PERSON] in 2006 [54], led to a broad application of TPI in several scientific fields, such as geomorphology [56; 57]; geology [58]; hydrology [59]; geocarchaeology [60] risk management [61]. The TPI is defined as:  
\[\mathrm{TPI_{i}}=\mathrm{M_{0}}-\sum\
olimits_{\mathrm{n}^{-1}}\mathrm{M_{n }/n}\]  
where \(\mathrm{M_{0}}\) is the elevation of the model point under evaluation, \(\mathrm{M_{n}}\) the elevation of grid, and \(\mathrm{n}\) the total number of surrounding points employed in the evaluation. TPI compares the elevation of each cell in a DEM to the mean elevation of a specified neighborhood around that cell. Mean elevation is subtracted from the elevation value at center. The neighborhood size is substantial for the analysis and is related to the scale of landscape feature being analyzed. To identify large landforms, a large circular neighborhood is proposed [56]. Choosing the correct neighborhood is  
Figure 3: Drainage basins classification according to their mean elevation value. Figure 2: Map of Greece showing the division of Hellenides in External and Internal ones. an iterative process with several trials before the most appropriate size of neighborhood is decided.