Identification of Hydrological Droughts in Lithuanian Rivers

Globally, hydrological droughts are most commonly identified based on various indices calculated from water flow values. However, the water flow rate is calculated from a flow rate curve that needs to be updated constantly, so it takes a long time to resolve its true value. For this reason, the possibility of identifying a hydrological drought on the basis of hourly and prompt treated water levels seems much more attractive. 8 water gauging stations (WGS) operating along 7 important rivers and covering the hydrological areas of visas in the Lithuanian region were selected for the study. In this study, a modified SPI function of the R programming language SPEI package (traditionally used to calculate the standardized precipitation index, SPI) was applied for the streamflow drought index (SDI) calculations. Given how it was applied to the SDI calculation, just like the baseline data, this was the ten-day mean water flow and then the water level. The suitability of water level data for SDI calculations was assessed by analyzing the relationships between SWLI (Standartized Water Level Index) calculated from water level data and SDI calculated from water flow information. SWLI and SDI in all 8 WGS are closely interconnected. It was found that the possibility of recurrence of droughts of different severity identified by both methods is significantly influenced by the profile of the river bed in a specific section. In areas where riverbanks have steeper slopes, the SWLI and SDI similarly describes the water situation and the recurrence of droughts. It is believed that a modified SDI methodology (SWLI), which is based on water level data, may become a good alternative in our country for identifying hydrological droughts. Keywords: Lithuanian rivers, hydrological drought, identification of droughts, water level, SDI, SWLI.

Uncertainty of annual runoff projections in Lithuanian rivers under a future climate

Uncertainties of runoff projections arise from different sources of origin, such as climate scenarios (RCPs), global climate models (GCMs) and statistical downscaling (SD) methods. Assessment of uncertainties related to the mentioned sources was carried out for selected rivers of Lithuania (Minija, Nevėžis and Šventoji). These rivers reflect conditions of different hydrological regions (western, central and southeastern). Using HBV software, hydrological models were created for river runoff projections in the near (2021–2040) and far (2081–2100) future. The runoff projections according to three RCP scenarios, three GCMs and three SD methods were created. In the Western hydrological region represented by the Minija River, the GCMs were the most dominant uncertainty source (41.0–44.5%) in the runoff projections. Meanwhile, uncertainties of runoff projections from central (Nevėžis River) and southeastern (Šventoji River) regions of Lithuania were related to SD methods and the range of uncertainties fluctuates from 39.4% to 60.9%. In western Lithuania, the main source of rivers' supply is precipitation, where projections highly depend on selected GCMs. The rivers from central and southeastern regions are more sensitive to the SD methods, which not always precisely adjust the meteorological variables from a large grid cell of GCM into catchment scale.

Analysis of Requirements for Effluent Quality and their Impact on Status of Rivers

The article analyses Lithuanian rivers, 32 of which have been selected as non-complying with the require-ments of good condition. Furthermore, 15 discharge facilities that discharge wastewater to the aforementioned rivers from agglomerations with population equivalent (PE) greater than 2,000 have been selected. The assessment of the quality of treated wastewater according to the allowable impact on the reception facility has been carried out and has been compared with the currently regulated minimum allowable values of concentrations of pollutants. The analysis of the impact of quality of treated wastewater on the ecological status of rivers has been presented. It has been established that the pollution of rivers would be decreased by an average 8% (organic pollutants and total nitrogen) and by 22% (total phosphorus), if the allowable concentrations of pollutants according to the impact on the reception facility were ensured in the analysed treatment plants with PE greater than 10,000.

Phytobenthos Cyanobacteria Species New To Lithuanian Rivers

A total of 23 new to Lithuania cyanobacteria species were recorded in phytobenthos of 70 rivers in 2009–2011. New species belong to the Chroococcales and Oscillatoriales orders, 10 families and 12 genera. Descriptions of species morphology and original photos are presented; cyanobacteria’s ecological groups and distributions are analysed. The paper contains information about the possibility of employment of new species as indicators for the assessment of ecological status of Lithuanian rivers.