Growth in the human population and relentless consumption is closely related to an increase in energy demands. However, the destabilized temperature equilibrium of the Earth forces the search for clean energy sources in order to reduce greenhouse gas emissions. Directive (EU) 2018/2001 [1
] on the promotion of the use of energy from renewable sources obliges the EU Member States to achieve a 32% share of renewable energy in final energy consumption by 2030. According to Eurostat [2
], in 2018, wind and hydropower each contributed around one-third of the total electricity from renewable sources, with wind power (36%) edging hydropower (33%) as the most important sources.
Hydropower (or hydroelectric energy) is the most important and world’s largest source of renewable energy. It may harness the power of moving water (kinetic energy) or the water stored in dams (potential energy) and is often considered a very clean form of electricity generation. However, at present, most hydroelectric energy comes from hydropower plants that generate energy by damming of water. According to the International Energy Agency, hydropower accounts for about 17% of global electricity production. China is the largest producer of hydroelectricity, followed by Brazil, the United States, and Canada [3
], while Africa has the highest percentage of untapped technical hydropower potential in the world [4
Hydropower is a source of energy that changes over time because it is directly dependent on water resources and the hydrological cycle. In countries where the amount of electricity generated by hydropower plants is substantial, projections of hydropower potential are considered as a task of national importance. For example, in the United States, where hydropower is a key contributor to the renewable energy portfolio, the Department of Energy, directed by Congress, has conducted a second five-year assessment [5
] examining the potential effects of, and risks from, global climate change associated with water supplies for federal hydroelectric power generation. This report indicated that the most critical climate change effect on hydrology is likely to be the trend toward earlier snowmelt and change in runoff seasonality. Under the projections of increasing winter–spring runoff and decreasing summer–fall runoff, water resource managers may need to consider different water use allocations. Rich in water resources, Canada, where national energy security is mostly dependent on the hydropower, is getting warmer and wetter with more contribution from rainfall than snow [6
]. Consequently, under the continuation of climatic trends, production potential is expected to increase. However, the net gain/loss is subject to significant variations across different regions. In Norway, a country almost perfectly made for hydropower, a current concern about the future potential of this energy resource resulted in a report [7
] that indicated the river flow increase due to the projected higher amounts of precipitation. The most significant increase in river flow occurs in winter, and at the same time, slightly smaller snowmelt floods are expected. By the end of the century (in 2071–2100), according to the RCP8.5 scenario, the total hydropower production in Norway is going to grow by 8% (compared to 1961–1990). In contrast, hydropower plants in Germany are affected negatively due to declining river discharge [8
]. A special report from the German Advisory Council on the Environment [9
] assumed that the hydropower potential for electricity generation in Germany is limited to about 28 TWh a−1
for orographic reasons. Thus, the additional development potential of hydropower is rated to be minimal. For the Alpine region, the average annual electricity generation of run-of-river plants in 2031–2050 (relative to 1961–1990) is estimated to decrease slightly for all climate scenarios considered (up to −8%) [10
]. Whereas for Austria, two scenarios project a slight increase (not more than +5%), and according to the other two scenarios a slight decrease (not more than −5%) is expected. Most of the studies assess only the potential energy of water because a vast amount of hydropower is generated in large power plants having water storage reservoirs. However, the research of hydrokinetic energy resources remains limited. The potential of these resources on a national level has been previously assessed in Canada and the continental United States [11
Hydroelectric energy potential varies significantly in time and space. Hydropower systems are dependent on water availability and can either increase competition or mitigate water scarcity [13
]. In the long run, the success of hydropower generation and development will largely depend not only on the geographical region, economic opportunities, political approach, and environmental constrains, but also on future climate-induced river runoff changes. The National Energy Independence Strategy of Lithuania [14
] stipulates to increase the share of electricity consumption from renewable energy sources up to 45% in 2030 and 80% in 2050 compared to the final electricity consumption. In this context, an assessment of the current and future potential of hydroelectric energy in Lithuania is very important. Jablonskis and Lasinskas [15
] were the first to evaluate theoretical potential of hydropower in Lithuania. According to them, over 22,000 rivers (watercourses) with a total length about 77,000 km together with the slopes of the earth’s surface produce a total of 688.8 MW of hydropower and about 6.0 billion kWh a−1
. However, the exploitation of hydropower resources is severely limited by environmental requirements; after assessing all nature protection restrictions, it is estimated that efficient technical energy will be 159.1 million kWh a−1
, which is only 2.6% of the total annual potential [16
]. Currently, the total installed capacity of the hydropower plants is 128 MW (15.3% of all RES). This figure indicates the conventional hydropower resources: 101 MW of installed capacity at Kaunas Hydropower Plant (on the Nemunas—the largest Lithuanian river) and the rest at 97 small hydropower plants (on smaller rivers). River hydrokinetic resources, at the state level, have never been assessed. Gailiušis et al. [17
] estimated that resources of hydrokinetic energy of Lithuanian small and medium-size rivers amount to 82.1 MW, but due to the specified exclusionary criteria (environmental constrains and weather conditions), the capacity of riverine hydrokinetic energy decreases to 13.6 MW; thus, only 0.7% of the total electric energy demand for the national economy can be met.
Up to now, no projections of future hydroelectric energy resources have been made in Lithuania. This study is designed to project the changes of river runoff and the total hydropower resources in both gauged and ungauged Lithuanian river catchments. The main objective is to assess the impact of climate change on the future of this renewable energy resource potential in Lithuania.
4. Discussion and Conclusions
This study set out to assess the future potential of hydropower resources in the Lithuanian river catchments. Two challenges accompanied the investigation: projection of ungauged river runoff and assessing the total potential of hydroelectric energy according to the new climate scenarios.
The projection of hydropower resources is a complex process. When managing water resources, water specialists often face the problem of insufficient data or a complete lack of data from hydrological observations. For the present assessment of the total potential of hydroelectric energy, it was necessary to include the runoff data of ungauged rivers. Information transfer from gauged to ungauged Lithuanian river catchments was performed using isoline maps created by interpolating specific runoff derived from the hydrological modeling. Such spatial proximity approach is one of the earliest and most widely used regionalization methods [38
]. This method enabled us to get the data of ungauged catchments necessary for the projection described in the paper. The average annual specific runoff differed only by −7.7%–+3.8% from the calculated one, based on observational data using the isoline method. That confirms that the selected regionalization scheme was successful.
The effects of climate change on the hydrological regime of rivers are well studied. Due to changing precipitation and air temperature patterns in the Baltic Sea basin, alterations in the annual and seasonal runoff distribution are observed and they are projected to continue in the future [41
]. The aforementioned major drivers result in higher winter runoff and lower runoff during other seasons. However, in different regions of the planet, runoff projections may vary due to dissimilarities in natural (primarily climatic) conditions [46
]. The present study results demonstrate that even among different hydrological regions of Lithuania, river runoff projections may differ. The most significant and intimidating changes are likely to occur under the most pessimistic scenario (RCP8.5) at the end of the 21st century. In the rivers of the hydrological region of western Lithuania, the runoff will decrease from 4.5 (RCP2.6) to 10.4% (RCP8.5), in central Lithuania from 4.0 (RCP2.6) to 31.2% (RCP8.5), and in south-eastern Lithuania from 1.3 (RCP2.6) to 24.6% (RCP8.5) relative to the reference period. The central Lithuanian hydrological region will experience the greatest decline of runoff according to all projections and in both the near and far future periods. This finding corresponds with a previous study [51
] that highlighted vulnerability and a high risk of an increasing number of rivers drying up in response to climate change in this particular hydrological region.
Changing climate and runoff patterns provide additional uncertainty for hydropower generation. IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation [13
] states that the impacts of climate change on hydropower generation are likely to be small on a global basis. However, still significant regional changes in river flow volumes and timing may pose challenges for planning. Hamududu and Killingtveit [52
] assessed that, globally, hydroelectric energy generation is predicted to change very little (by 0.46 TWh, i.e., less than 1% of the 2005 generation level) by the year 2050. According to Turner et al. [53
], by the end of the century, depending on the used general circulation model, a change in net global hydropower production of between −8% and +5% under RCP8.5 is projected. Lehner et al. [54
] estimated, for the whole of Europe, a decline of the gross hydropower potential by about 6% by the 2070s according to the moderate climate and global change scenario. The present results demonstrate a clear shrink of hydropower potential in Lithuania due to the already discussed decline of river runoff. In 2081–2100, small hydropower plants’ energy production is expected to lower from 7.1% (under RCP2.6) to 13.7% (under RCP8.5), whereas the reduced river runoff in spring will result in a decrease of almost 50%. In the only large Kaunas HPP, a decline from 7.4% to 30.6%, depending on the scenario in the far future, is projected. Such a decrease in Kaunas HPP electricity production is 1.6 times higher than the annual electricity production of all small hydropower plants. Thus, the loss of such energy would be significant for Lithuanian hydropower production. This estimation is also crucial concerning the possibilities of future development of small hydropower. In neighboring Latvia, it has been assumed that a projected decrease of the mean annual river runoff [41
] will bring negative changes to hydropower supply as well, whereas in north Estonia, the climate change impact on hydropower potential is likely to be positive [55
], likewise in Norway [7
An object of the vast majority of studies on hydropower resources are conventional power plants that harness the potential energy of dammed water. Hydrokinetic energy is one among promising renewable energy resources; however, it has yet to be proven commercially viable [56
]. Although some hydrokinetic power potential assessments have been accomplished and are available publicly, the authors of the present paper could not find any study dedicated to projections of this riverine hydroelectric energy resource. The current findings revealed that future climate alterations would bring runoff modifications and, consequently, changes of hydrokinetic power potential. In the near future, the total kinetic resources of two major Lithuanian rivers are projected to decrease from 5.0 to 7.5%, and in the far future period, from 6.6% to 15.1% depending on the chosen RCP scenario. The hydrokinetic resources of the main Lithuanian rivers (excluding the Nemunas and Neris) will decrease from 4.3 (RCP2.6) to 7.8% (RCP8.5) in the near future, while it will decrease twice as much compared to the reference period in the distant future.
As hydroelectric power plants’ development using river potential (head) energy is limited in Lithuania due to environmental constraints, installing kinetic (non-head) hydroelectric power plants could be a great alternative in the future. In general, the Nemunas and Neris might have considerable potential as they have favorable hydrokinetic energy generation conditions due to a high flow rate and depth. Small conventional hydropower generation in smaller rivers should not be developed. In the projected conditions of limited water availability, finding the balance between human (energy production) and environment (suitable habitat for aquatic species) needs might be very complicated and hardly achievable. The provided projections could help understand what kind of benefits and challenges water resource managers may face in the future and how to transform the country’s electricity sector development into something more sustainable.