challenges Article Operating Small Hydropower Plants in Greece under Intermittent Flow Uncertainty: The Case of Tsiknias River (Lesvos) Ourania Tzoraki Department of Marine Sciences, University of the Aegean, 81100 Mytilene, Greece; [email protected]; Tel.: +30-22510-36840 Received: 3 July 2020; Accepted: 28 July 2020; Published: 3 August 2020 Abstract: In arid and semi-arid parts of the world, river exploitation is intensive, involving water storage for irrigation or hydropower generation. In Greece, 100 small hydropower plants (SHPs) take advantage of less than 10% of the hydropower potential of low flow streams (<2 m3/s), a very small amount in relation to the 70% of the European Union. The energy policy of complete decarbonization of the country by 2023 on a national scale opens the road for new investments in SHP projects, especially in intermittent-flow streams of the Greek islands. Simulated flows by the Modello Idrologico SemiDistribuito in continuo (MISDc model) are used to construct the annual flow duration curve (FDC) to study and assess the hydropower potential of an intermittent stream (Tsiknias river, Lesvos, Greece). For Tsiknias River, but also for six other intermittent-flow rivers of Crete island, the capacity factor (CF), which represents the mean annual power of the hydropower plant, should remain >75% to exploit the river’s potential. The FDC and CF are essential in designing SHP projects in intermittent-flow streams with long no-flow periods. The development of public participatory approaches and a closer cooperation among policy makers and stakeholders should work to promote hydropower exploitation and accelerate licensing procedures. Keywords: small hydropower plants; intermittent-flow river; flow duration curve; capacity factor; hydrological model 1. Introduction Climate change (CC) is expected to significantly decrease annual rainfall in the Mediterranean region [1–3], as increase in surface temperature is expected through the end of this century. Projections by the fifth iteration of the Intergovernmental Panel on Climate Change (IPCC) point towards a “likely to very likely” decrease in precipitation over the Mediterranean by 30–45%, especially if temperature rises by 1.4 ◦C[4,5]. IPCC further projects that over the coming years, meteorological droughts and floods in the region will increase in intensity and frequency, seriously altering the ecological and hydrological patterns of the rivers and streams. The transition from flow conditions to dry river beds creates an ecosystem vulnerable to climate change and flow variability [6]. Under the pressure of the climate change, the number of intermittent-flow streams is expected to increase in the future. Nowadays, intermittent-flow streams experience dry phases following a natural, periodical pattern and cover almost 30% of the total length of rivers at global scale [7]. Only in the 19 European countries, the terrestrial area covered by intermittent streams of known hydrological regime (due to existing gauging stations) reaches 10,180 km2, almost 11% of these countries’ terrestrial territory [8]. In arid and semi-arid parts of the world, their exploitation is intensive, involving water storage for irrigation or hydropower generation, surface and groundwater pumping, channelization and urbanization of some parts. Challenges 2020, 11, 17; doi:10.3390/challe11020017 www.mdpi.com/journal/challenges Challenges 2020, 11, 17 2 of 15 Statistics show that hydropower plants produce around 20% of the electricity consumed on a global scale and 50% of that produced in the United States. More than 60 countries in the world have made important investments in hydropower plants [9]. For instance, in Greece, the installed hydropower potential exceeds 3061 MW [10]. The use of hydroelectric power is expected to increase in the future due to the possible exhaustion of conventional fossil fuels and the shift to more sustainable 3 1 renewable energy resources. Small rivers (<2 m s− ) hide high potential for run-of-the-river (RoR) hydroelectric power generation, which is a more sustainable solution towards dam construction or other renewable sources, as there is not any disturbance in the river regime and limited socioeconomic impacts [11]. In particular, the investment in RoR plants is crucial in moving to clean energy solutions with zero environmental footprint, especially in Natura 2000 areas. However, RoR plants are dynamic systems with the river flow as a crucial parameter that affects their operation. The operation of turbines follows the weather conditions and the flow response to rainfall or snowpack melting. Consequently, the plant operation and efficiency are strongly depended on the flow transition following climatic conditions and the long-term CC impact on flow variation. By 2023, Greece is planning to replace existing lignite power plants with renewable energy solutions. Starting in 1954 with the construction of Louros hydropower reservoir, the Hellenic public power corporation (PPC) produces today more than 3173 MW energy from the large hydropower plants and ~300 MW from the 78 small hydropower plants (SHP). A SHP can be characterized as producing power less than 15 MW. The majority of SHPs are located at mountainous terrains facing particular operation conditions (e.g., environmental flow, turbine max and min discharge, shortage of operational days, intermittent energy source). Unfortunately, Greece, with thousands of intermittent streams, operated 2940 water mills in the past and nowadays has fewer than 100 SHPs in operation. Until now, there is no study examining the potential of intermittent rivers to serve small hydropower plants. The objective of the current study is (a) to investigate the current status of the small hydropower plants in Greece, (b) to explore the hydropower potential of intermittent-flow streams, and (c) to make recommendations for the hydrological models and tools used in the design of the SHP in river basins of intermittent flow. Tsiknias River (Lesvos, Greece) is used as a case study to understand the effect of intermittency impact into the river flow regime and hydropower production. In order to assess the flow regime of Tsiknias River and to put forward management recommendations related to hydropower production, hydrological model simulations up to the year 2020 were conducted for the site. The flow duration curve (FDC) is used as a tool to correlate the hydrological regime to the hydropower potential by calculating the capacity factor index (CF), which represents the mean annual power of the hydropower plant. 2. Methodology The main methodological steps include (a) an extended literature review related to explore the SHP status and perspectives in Greece; (b) spatial, flow and climate data collection of the Tsiknias River basin as a representative case study of a small river with a mountainous and intermittent flow; (c) the use of Modello Idrologico SemiDistribuito in continuo (MISDc) (Matlab code) [12] to conduct hydrologic analysis of the historical conditions and climate change scenario inputs. (d); the use of the flow duration curve (FDC) in designing SHPs. 2.1. Study Area The Tsiknias River catchment, located in the north-central part of Lesvos, Greece (Figure1), flows south into the island’s Gulf of Kalloni and through the settlement of Kalloni. The catchment is approximately 91.3 km2 in size and encompasses a lowland and low mountainous environment with its highest point of elevation being 968 m (Bormann et al., 2018). Tsiknias River is a major river discharging into the closed gulf of Kalloni in terms of water amount and nutrient load contribution. The area is of a Mediterranean climate type and is primarily occupied by agriculture (olive groves), grassland, and brushland habitats, with some small pine and oak forests at the northern tip of the Challenges 2020, 11, 17 3 of 15 catchment [13]. The soil in the largest part of the area originates from various lithological formations such as volcanic lava, pyroclastics, and ignimbrite. Only in small segments and especially at the mouth of the river, the soil is composed of torrential concentration/fluvial deposits (clays, sands, fluvial deposits). The soils of the study site are of shallow depth (from 5 to 30 cm) to moderately deep depth (from 30 to 65 cm), are well drained, [14] and the maintenance of terraced fields by the farmers prevents soil erosion [15]. The river experiences flash flooding during the rainy season (November to April) and reachesChallenges no-flow 2020 during, 11, x summer months, with major flood events causing extensive economic4 of damage 16 in Agia Paraskevi village and in the main road close to the river estuaries [16]. The mean outflow of the river is estimated to be 21.4 hm3 and the annual loads of nitrogen and phosphorus flowing into the Gulf of Kalloni are approximately 23 t N and 8.5 t P [17]. Figure 1. Tsiknias River hydrographic network and basin at Lesvos island in north-east Greece. Figure 1. Tsiknias River hydrographic network and basin at Lesvos island in north-east Greece. The river experiences intermittent flow, flash flooding and long dry periods annually and the effects2.2. of Continuous climate change Rainfall-Runoff on these Mode flowl—MISDc variations (Matlab should Code) be Model considered Setup in the management of water bodies. 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