PROCEEDINGS OF THE LATVIAN ACADEMY OF SCIENCES. Section B, Vol. 70 (2016), No. 2 (701), pp. 78–87. DOI: 10.1515/prolas-2016-0013 LONG-TERM CHANGES IN THE WATER TEMPERATURE OF RIVERS IN LATVIA Inese Latkovska1,2 # and Elga Apsîte1 1 Faculty of Geography and Earth Sciences, University of Latvia, Jelgavas iela 1, Rîga LV-1004, LATVIA, e-mail: [email protected] 2 Latvian Environment, Geology, and Meteorology Centre, Maskavas iela 165, Rîga LV-1019, LATVIA # Corresponding author Communicated by Mâris Kïaviòð The study describes the trends of monthly mean water temperature (from May to October) and the annual maximum water temperature of the rivers in Latvia during the time period from 1945 to 2000. The results demonstrated that the mean water temperatures during the monitoring period from May to October were higher in the largest rivers (from 13.6 oC to 16.1 oC) compared to those in the smallest rivers (from 11.5 oC to 15.7 oC). Similar patterns were seen for the maxi- mum water temperature: in large rivers from 22.9 oC to 25.7 oC, and in small rivers from 20.8 oC to 25.8 oC. Generally, lower water temperatures occurred in rivers with a high groundwater inflow rate, for example, in rivers of the Gauja basin, in particular, in the Amata River. Mann-Kendall test results demonstrated that during the monitoring period from May to October, mean water tem- peratures had a positive trend. However, the annual maximum temperature had a negative trend. Key words: water temperature, long-term changes, river, Latvia. INTRODUCTION tors, of which the following are the most important: climate conditions, the surrounding landscape (forests, an open The water temperature is among the physical characteristics field), the inflow of groundwater, the distribution of sandy of the surface water that has a direct impact on the flora and soils and lakes in the inflow basin, the size of the river and fauna in the water ecosystems, as an increase of water tem- its geomorphology, as well as hydropower plants located on perature very often has a negative impact on water quality the river (Webb et al., 1996; Uehlinger et al., 2003; Carriv- (Webb and Nobilis, 1995). An increased water temperature, ick et al., 2012; Jurgelënaitë et al., 2012; Gebre et al., for example, promotes eutrofication (Webb and Nobilis, 2014). A number of global studies have shown that among 1995; Poole et al., 2001; Jurgelënaitë et al., 2012). Changes anthropogenic factors, hydro power plants have the largest in the river water temperatures also affect other processes impact on river water temperature (Prats et al., 2010, Jur- such as the formation of the ice (ice starts to form when the gelënaitë et al., 2012; Dickson et al., 2012). In densely water temperature falls below 0 oC), chemical and biologi- populated areas large impact is also caused by waste water cal processes (for example, volume of dissolved gases, rate emitted to the river and the water collection points (Xin and of various chemical reactions, and activity of organisms), Kinouchi, 2013). the volume of flow of dissolved substances due to minerali- sation and chemical composition. Water temperature is also The studies that have been recently performed worldwide an essential indicator in the economy, for example, in cool- on change in river water temperature are mainly related to ing systems and for household needs (Mihailov et al., the evaluation of the impact caused by climate change 2005). (Caissie, 2006; Arai, 2009; Kaushal et al., 2010; Prats et al., 2010; Van Vliet et al., 2011). The thermal regime of rivers The dynamics of river water temperature is complicated and depends on climate conditions, particularly solar radiation, can be studied at various scales: on a macroscale (the loca- and thereby water temperature directly depends on the air tion of a river in various latitude and altitude, and continen- temperature. A close correlation between air and water tem- tality), on a mezoscale (climate and hydrology of the river peratures has been documented, which indicates that the basin), and microscale (river geometry, shadowing of banks change in river water temperature is a direct result of the and soils in the river basin) (Garner et al., 2013). The distri- climate change (Kaushal et al., 2010; Prats et al., 2010; bution of the water temperature in rivers is mainly deter- Floury et al., 2012; Van Vliet et al., 2013; Gebre et al., mined by local physical geographic and anthropogenic fac- 2014). 78 Proc. Latvian Acad. Sci., Section B, Vol. 70 (2016), No. 2. A further increase of the river water temperature has been range of 12–15 oC, medium cold rivers where the tempera- predicted (Van Vliet et al., 2013). A global increase of the ture was within the range of 18.5–20 oC, and warm rivers mean and maximum water temperature by 0.8–1.6 oC and where the water temperature was 20 oC). No research of the 1.0–2.2 oC, respectively, is forecast based on the future change in the river water temperature has been recently car- emission scenarios B1 and A2 for the time period 2071– ried out in Latvia. 2100 in comparison to the time period 1971–2000. The largest increase of the water temperature is forecast in the The goal of the study was to evaluate the long-term change United States of America, in Europe, in Eastern China, in in the monthly mean water temperature of the upper waters Southern Africa, and also in Australia, as a decrease of the of Latvian rivers (from May to October) and annual maxi- river flow is also forecast for these regions. These regions mum water temperature in the time period from 1945 to could eventually face deterioration of water quality, which 2000. would restrict the availability of water for human needs, for example, the availability of potable water, and also the ca- MATERIALS AND METHODS pacities of hydro power plants would decrease. Long-term data on water temperature of Latvian rivers from Systematic measurements of the water temperature in Lat- 19 hydrological monitoring stations (HMS) located on the via were started in 1932 (Stakle and Kanaviòð, 1941). In the largest rivers in Latvia and from 17 HMS located on me- beginning of the 1960s, Glazacheva (Glazacheva, 1964; dium and small rivers (Fig. 1) were summarised. Data on air 1965) carried out studies on the seasonal and daily change temperature were taken from 13 meteorological monitoring in water temperature of Latvian rivers, and rivers were clas- stations (MMS) — Ainaþi, Alûksne, Bauska, Daugavpils, sified based on their thermal regime. By continuing the Gulbene, Jelgava, Liepâja, Stende, Priekuïi, Rçzekne, studies carried out in 1964 and 1965, in 1967 Glazacheva Rûjiena, Zîlâni, and Rîga (Fig. 1). The data were obtained (1967) supplemented the study by classification of the riv- from the Latvian Environment, Geology, and Meteorology ers of the Latvian SSR in types depending on their thermal Centre. regime (102 rivers with an observation period from 1946 to The analysis of the long-term thermal regime change was 1962 were used in this study). The thermal regime was sin- performed for the time period 1945–2000 by analysing the gled out as one of the elements of the hydrological regime. monthly mean temperature of upper waters of the Latvian Not only were the seasonal changes of the meteorological rivers (from May to October) and the annual maximum wa- conditions evaluated, but also the impact of water tempera- ter temperature. ture on physical, chemical, and biological hydrological pro- cesses, including on ice conditions (Glazacheva, 1967). The mean temperature for decadal periods of each month Glazacheva in her study (Glazacheva, 1967) classified Lat- was calculated from measurements carried out twice daily vian rivers marking three main types (cold rivers where the (at 08:00 and at 20:00 hours). Some periods were 8, or 11 long-term mean water temperature in July was within the days). Fig. 1. Location of hydro- logical monitoring stations and meteorological moni- toring stations, Latvia. Proc. Latvian Acad. Sci., Section B, Vol. 70 (2016), No. 2. 79 Pearson correlation analysis was performed to test the rela- (17.2 oC) was recorded in year 1999. Similar patterns were tionship between water temperature and air temperature. observed in other rivers of Latvia. The Mann-Kendall test was applied to evaluate whether the water temperature changes displayed a trend. The Mann- The results of the study demonstrated that the long-term Kendall test was specifically developed for the study of mean water temperature was higher in large rivers (Table trends of change of climate parameters. The test can be used 1). The highest long-term mean water temperature was re- corded in the Lielupe River in the section Meþotne–Sloka, also when there are missing values and non-typical (very o low or very high) values. The Mann-Kendall test is based where it was 15.4–16.1 C, respectively. In the Daugava on the principle of ranges or pairs for comparing two values River, in the section from Piedruja to Jçkabpils, the long- term mean water temperature was within the range 15.3– of observations. The test can also be applied for data series o with a seasonal character, as it allows to assessing the test 15.5 C, and it was slightly lower in the Venta River in the section Vârdava–Vendzava (15.1–15.4 oC) an in the Salaca values for each individual month. The test was applied sep- o arately for each variable at each site, at a significance level river from Mazsalaca to Lagaste (15.0–14.7 C, respec- of p < 0.05.