IDENTIFICATION OF DRAINAGE BASIN BORDERS AT LOCAL SPATIAL SCALE

The basin concept to nature management is used in various studies on ecological monitoring of terrestrial ecosystems. The objective of this study is to identify the borders of drainage basins on the territory of Pechora-Ilych Nature Reserve and its surroundings – Kuryinsky and Yakshinsky local forestries. Digital elevation models (DEMs) ASTER GDEM Version 2, GMTED2010, and open access data of the drainage network at a scale of 1 : 1,000,000 were the inputs for the identification of drainage basin borders of the area of interest. These data were processed and analyzed with two geographic information systems, i. e. SAGA GIS and ArcGIS Desktop software. The present study involved pre-processing of the DEM data and identification of the drainage basin borders of the area of interest. Pre-processing of DEMs data included rasters’ cell size unification, filling sinks, and determining flow direction. Identification of drainage basin borders was performed in two ways using the Basin and Watershed tools of the Spatial Analyst module of the ArcGIS Desktop geographic information system. Analysis of the findings showed that the Basin tool was only able to identify basin borders of large rivers such as Pechora and Ilych. The Watershed tool using raster data on pour points is suitable to form basin borders of both large and small rivers.

pattern, resilience and self-regulation. The functional integrity of basins results from current vertical and horizontal relations and the balance between them. F.N. Milkov believes that a drainage basin is a paradynamic system with an active exchange of matter and energy, resulting from a combination of morphological, climatic, soil and hydrological conditions (Milkov, 1981). The paper (Smolyaninov et al., 2007) considers unidirectional flows of matter and energy within a drainage basin, which makes it an ideal object for natural environment monitoring.
Arguments in support of the rationality of environmental monitoring at the basin level are given in the study (Lisetsky et al., 2014). The authors describe the internal functional integrity of migration flows of surface water and groundwater runoff, dissolved substance and suspended solids, as well as objective natural borders of the basins. In the study of Kuzmenko with co-authors (Kuzmenko et al., 2012), drainage basins are seen as the most objective and natural basis for organizing rational nature management. The authors highlight the objectivity and relative simplicity of the identification of basin borders, which increases the representativeness of territorial units (Trofimov et al., 2009). Moreover, the basin concept of nature management makes it possible to use basins as a topological unit during the gradual transition from local to regional and national levels of analysis.
The objective of this study is to identify the borders of the drainage basins on the territory of Pechora-Ilych Nature Reserve and its surroundings, i.e. Kuryinsky and Yakshinski forestries. (Fig.   1).

MATERIALS AND METHODS
Within the area of interest which is located north of the 60° latitude, the set of freely available digital elevation models (DEMs) is limited, and the available ones more often than not have distortions and incomplete data. Global digital elevation models that are currently in open access provide information about the topography of the area of interest, and the degree of data accuracy should depend on the study objectives (Mineev et al., 2015). Manual creating of a topographic maps-based DEM results in more accurate DEMs compared to global models but requires considerable time. This paper does not require accurate morphometric parameters so we decided to use a global DEM, which is in open access, to achieve the study objective. Filling sinks is necessary to remove small imperfections of the ASTER GDEM V. 2 that appeared due to data resolution or rounding off elevations to the nearest integer value (Tarboton et al. , 1991). The need to fill sinks is explained by the necessity to have a continuous raster layer of flow directions which will be created later. Filling sinks was done using the Fill tool in ArcGIS Desktop. As part of the flow direction determination, the Flow Direction tool creates a continuous raster layer that contains the direction of the biggest elevation decrease for each cell. Flow direction determination is described in detail in the article of (Plotnikova et al., 2017).

RESULTS AND DISCUSSION
The Spatial Analyst module of the ArcGIS Desktop geographic information system contains two tools that are able to define contributing areas, i. e. Basin and Watershed. As is known, the borders of drainage basins pass along watersheds that distribute the runoff around slopes. Drainage basins building operation Basin defines the borders of watersheds within the analysis window. The raster layer with flow direction is analyzed to find related units belonging to the same contributing area. Drainage basins are created by placing pour points at the edges of the analysis window and sinks. Next, the area above each pour point from which a flow passes to this point is determined.
The result is a raster layer of drainage basins.
The results of the construction of drainage basin borders with the Basin tool for ASTER GDEM VERSION 2 DEM are shown in Figure 2. Overlaying of the constructed borders of drainage basins and the drainage network, shows that the Basin tool identifies only the basin borders of large rivers, i. e. Pechora and Ilych. The obtained borders of drainage basins of small rivers will be further used as part of a comprehensive study of fire regimes of forest ecosystems at the local spatial level. Minimal spatial units for fire regimes mapping will be identified on the basis of the drainage basin borders.