Water resources management requires the knowledge of the environmental conditions and hydrologic processes. The development of Geographic Information Systems (GIS) has allowed the use of spatially and physically based hydrologic models in order to simulate the hydrology in a complicated natural system. It is very important to use such tools especially in regions with limited available data. In this context, the objective of this study is to model the hydrologic conditions in the island of Thasos by using the Soil and Water Assessment Tool (SWAT) in a GIS environment. In order to understand the hydrologic processes, it was required to predict the water balance of the island and simulate the discharges of its ungauged torrents. The outputs of the model revealed the simulated values of the hydrologic phases, the torrent discharges and a map that identifies the watersheds with the highest discharges. The results could be used by experts and by every interested authority in order to manage the surface stream water for various purposes, such as suppression of forest fires.
Surface water and especially stream and torrent flow is important for environmental
scopes including the sustainability of living organisms and habitats in and adjacent to
the stream and torrent but also for human purposes
It is essential to determine the stream/torrent discharge in order to meet the above
objectives. A quick, widely and frequently used technique, but also efficient
application is to use hydrologic models
The study area is the island of Thasos, located in northern Greece that belongs to
Kavala Prefecture. It is the northernmost Greek island and the 12th largest by area.
The surface of the island occupies about 378 km² while the shape of the island is
almost rounded and the perimeter is approximately 102 km
The SWAT model is a physically based, semi-distributed model Neitsch et al.,
2011). The model predicts the impact of land management practices on water,
sediment and agricultural chemical yields. It can accomplish these takes in large
complex basins with varying soils, land-use and management conditions over long
periods of time
In the second phase the channel hydrology is simulated, more specifically the
loadings that are calculated earlier are routed through the stream network of the basin
The major benefits of the model are that it is a free product, it is applicable with
the available data, it is documented well, it has good technical support through many
databases and assistance programs
The data used in this study was the topographic map of Thasos that developed
based on the Military Geographic Service that had a scale of 1:50,000. This map was
used to create the Digital Elevation Model (DEM) in ArcGIS 10.1 by digitizing the
contour lines and the stream network (Figure 1.A.). The SWAT model requires the
DEM for the creation and delineation of watersheds. There is the possibility of
burning onto the hydrographic network for better results (Figure 1.B.). The next
process is to fill the sinks of the DEM and calculate the flow direction and flow
accumulation grids
Most watersheds are typically not homogenous as they consist of different soils
and land-uses. As it was mentioned, SWAT can divide the watershed into smaller
hydrologic response units (HRU) that are unique combinations of soil type, land
cover and slope category
The soil map was downloaded from the European Soil Portal and the Harmonized World Soil Database Viewer 2012 that had a scale of 1:1,000,000. In addition, the geological map was digitized and the geological parent material was considered and combined in order to create the final soil map (Table 1).
The CORINE 2000 vector files by European Environment Agency were acquired in order to display the land-cover of the island and had a scale of 1:100,000. The land cover of CORINE was reclassified to the categories of land cover that already exist in the SWAT database (Table 2). All of these maps were geo-referenced in the Greek Geodetic System (D_GGRS_1987). Finally, the weather data were obtained by a meteorological station located in the city of Limenas. The data concerned monthly precipitation and temperature that covered the period from 1975-1997 (Figure 2).
The simulated outputs were printed in a monthly step. The average precipitation was 844.7 mm. The results showed that most water becomes lateral flow (238.9 mm) through the soil profile because of the highly permeable soils and parent material such as marbles that allow the infiltration. The average surface runoff was about 70 mm and the groundwater flow 63 mm. These three phases consist the total water yield of the area (371.26 mm). The average percolation in aquifers was about 72 mm. There was also a high percentage of evapotranspiration (449 mm) due to the high vegetation of the island.
In addition, a map was produced (Figure 3) that identifies the average simulated stream discharges based on the outputs of SWAT. The depiction is in the view of the sub-basins/watersheds and shows which watersheds discharge had the highest amount of water. Five categories were selected based on the statistics of the flows and specifically they were divided in: i) 0-0.05, ii) 0.05-0.10, iii) 0.10-0.25, iv) 0.250.50 and v) 0.50-1.0 (the discharge values are in m3/sec).
The average monthly water discharge of the major torrent is depicted in figure 4. The upper part of this torrent is situated in central Thasos while its outlet is in southeast region of the island. The specific outlet is also depicted in figure 3 as the left red colored watershed. The simulated results showed that the discharge of the specific torrent ranged between 0.04 to 1.2 m3/sec. The almost no flow period was noted in the summer period, specifically in July and August, while the highest during November and December.
Main challenges of the study included the lack of important data. Specifically, daily weather inputs were not available, only monthly statistics were used. Observed field discharge data were not available for the calibration of the model. Also, an extensive soil map would be very helpful for the parameterization. Finally, the land cover concerns the 2000 period as Greece has not update the database. Any current satellite images could be helpful for the classification of land cover because Thasos has suffered by many fire disaster during the last 15 years.
The results could be used for many purposes. The produced map could be a practical guide for water managers and other scientists for water resources management plans. Furthermore, future study could include the implementation of Multi-Criteria Analysis to combine other information such as wildfire maps and road network to identify best locations for water reservoirs to save water resources. The properly managed and stored surface stream water could be utilize for agricultural irrigation, the production hydro-electric power, water supply for municipalities during the summer where problems arise due to tourism and for fire suppression.
This study was an attempt to predict the water balance and the stream discharges of Thasos Island. For the specific scope, the hydrologic model SWAT was implemented to simulate the hydrologic conditions. The data used were the topographic map through the DEM, the soil map, the geological map, the land cover map and monthly weather data of the meteorological station located in Limenas. The produced results, maps and hydrographs have some limitation but could be used for water resources management plans and especially to sustainably utilize the surface torrent waters.
Acknowledgments. This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) – Research Funding Program: ARCHIMEDES III. Investing in knowledge society through the European Social Fund.