The aim of this research is to analyse and project the effects of changing climate on Lithuanian river runoff and water temperature. Climate change is expected to affect the extremes of the major river indices that impact fundamental ecological processes in river ecosystems. The available runoff and temperature data of rivers from three different hydrological regions of Lithuania were used. HBV software was applied for modelling of hydrological processes in the selected river catchments. The expected future changes of runoff and water temperature were projected according to a new set of scenarios (called representative concentration pathways) presented in the Intergovernmental Panel on Climate Change Fifth Assessment Report. The projected extreme values of runoff (flood and low flow discharges) and water temperatures in the beginning and the end of the 21st century were compared to the ones from the past period. The results showed a decrease of spring flood discharges and summer low flows and an increase of river water temperature at the end of the 21st century. The results are going to be used for an integrated assessment of the impact of climate change on aquatic animal diversity and productivity.

Hydropower (potential and kinetic energy) is one of the most important renewable energy sources in the world. This energy is directly dependent on water resources and the hydrological cycle. Ongoing climate changes are likely to influence the availability/amount of this energy resource. The present study explores the relationship between climate changes and river runoff, projects future runoff in both gauged and ungauged river catchments, and then assesses how these alterations may affect the future hydropower resources in Lithuania. Runoff projections of the gauged rivers were evaluated applying Swedish Department of Climate hydrological model, and runoff of ungauged river catchments were estimated by created isoline maps of specific runoff. According to an ensemble of three climate models and two Representative Concentration Pathway scenarios, runoff and hydroelectric energy projections were evaluated for two future periods (2021–2040, 2081–2100). The results demonstrated a decrease in future river runoff. Especially significant changes are expected according to the most pessimistic RCP8.5 scenario at the end of the century. The projected changes are likely to bring a negative effect on hydropower production in the country. These findings could help understand what kind of benefits and challenges water resource managers may face in the future.

Hydropower remains the most important and largest source of renewable energy. However, besides many additional benefits, such as dams for water supply, irrigation, flood control, recreation, navigation, etc., hydropower generation has a negative impact on the environment. This study aimed to investigate the hydrologic changes in Lithuanian lowland rivers caused by small hydropower plants (HPPs). Thirty-two indicators of hydrologic alteration (IHA) were studied in 11 rivers downstream of hydropower plants in the post-impact and pre-impact periods. The findings showed that HPPs and reservoirs considerably disturbed the primary flow of river ecosystems downstream. The largest changes in mean IHA values were found for low and high pulse characteristics (up to 57%) and the number of reversals (up to 44%). Only small or no deviations of the timing of annual extreme flows were found. The number of reversals, a low pulse count, and a fall rate were the flow characteristics that fell outside their historical ranges of variability most often. Six (out of 11) hydropower plants were identified that provoked hydrologic alterations of a moderate degree.

Recently, the number and intensity of hydrological droughts have been increasing; thus, it is necessary to identify and respond to them quickly. Since the primary hydrological data in Lithuania are water levels, and converting these data into discharge takes additional time, there is a need to develop a methodology or adapt these data to analyze and detect hydrological droughts. This paper examines the concept of the standardized water level index (SWLI) calculation, which is based on the standardized precipitation index (SPI) and streamflow drought index (SDI) methods. SDI and SWLI data were compared; SWLI was used to analyze the situation in the past and future. A total of 15 main sub-basins were considered, and the future discharge of three rivers was estimated; SWLI showed good compatibility with SDI. To better analyze droughts, the use of severe drought threshold values (SDTV) was suggested as some river data (especially those for small rivers) needed to be corrected due to dense riverine flora. The dry years and trends identified by SWLI are consistent with previous studies.