Impact of Climate Variability and Change on the Kupa River Dušan Trninić, Tomislava Bošnjak Meteorological and Hydrological Service Zagreb, CROATIA Abstract The latest studies on climate variability and changes in South and East Europe, including Croatia, the continued air temperature increase trend is expected to be accompanied by further decrease in precipitation and, consequently, number of days with snow and snow cover, decrease in runoff, land humidity and availability of water resource. The results of studies on climate variability and changes (WMO, UNEP, 2008) indicate 23-36 % decrease in annual runoff in South and East Europe for time thresholds set at 2020, 2050 and 2070. The Kupa River annual runoff analysis for the locations near Hrvatsko and Kamanje (period: 1957-2008) shows a decreasing trend. An outcome of such, less than optimistic, forecast is a need for demanding water resources management. The observations and forecasts of climate variability and changes should be continued, focusing in particular on the near future. Also, it is necessary to continue with observation of meteorological and hydrological values for the region and take into consideration the anthropological impact on climate variability and changes. Keyword: climate variability and changes, runoff, Kupa River Introduction Significant meteorological, hydrological, morphological, sediment and other changes have recently been observed in the Kupa River catchment area. Some results of the annual precipitation amount and annual runoff analyses are presented in this paper. The increasing presence of scientifically verified climate variability and changes should be highlighted as well as intensified anthropological impact on hydrological regime. The increasing climate variability and changes, and presence of anthropological impacts, complicate the water-related problems at the entire Croatian territory, including the Kupa River catchment area. These issues have been frequently addressed by the Croatian Hydrometeorological Service (2009), The Sava River Directorate (1975), WMO, and UNEP (2008). The background documents used in preparation of this paper were reports (1996, 2009) and books by Srebrenović (1986), Šiklomanov (1979) and Žugaj (2000). Annual precipitation amount Annual precipitation amount analysis results are an important input in analysis of the Kupa annual runoff. Fig. 1. shows defined linear trends of annual precipitation amounts recorded at the weather stations in: Parg (1951-2009), Rijeka (1961-2009), Ogulin (1949-2008) and Karlovac (1949-2009). These results indicate that the calculations carried out using data from the Parg Weather Station show decrease in annual precipitation of 1.75 mm/year, those from Rijeka of 1.82 mm/year, Ogulin only 0.69 mm/year and Karlovac 1.06 mm/year. Hydrological characteristics of the Kupa River The Kupa River catchment area is 10,236 km2, of which 8,412 km2 is on the Croatian and the rest on the Slovenian territory. The Kupa River is 294 km long, and the border course reach is 100 km long. The Kupa River springs from a turquoise karst lake the Risnjak National Park, Gorski Kotar region. Its major tributaries are: Čabranka, Kupica, Utinja, Kupčina, Trepča, Glina and Odra. BALWOIS 2010 – Ohrid, Republic of Macedonia – 25, 29 May 2010 1 Figure 1. Annual precipitation amount trends for Parg (1951-2009), Rijeka (1961-2009), Ogulin (1949- 2008) and Karlovac (1949-2009) Detailed information on basic and hydrogeological characteristics of two water gauging stations in the upper course of the Kupa River is shown in Table 1. which presents the basic characteristics of the gauging stations: catchment area F (km2), distance from the river mouth L (km), and elevation ˝0˝ (m a.s.l.), and characteristic flows (m3 s-1): Qmin, Qmin mean, Qmean, Qmax mean, Qmax at the Hrvatsko and Kamanje gauging stations in the upper reach of the Kupa catchment. Table 1. Basic characteristics of gauging stations and characteristic Kupa flows at Hrvatsko and Kamanje CHARACTERISTIC FLOWS '' 0 '' F L STREAM STATION 2 (m Q Q (km ) (km) Q min Q max Q a.s.l.) min mean max (m3s-1) mean (m3s-1) mean (m3s-1) (m3s-1) (m3s-1) HRVATSKO 370 286.75 285.212 0.659 1.99 20.2 260 419 KUPA KAMANJE 2047 171.15 123.827 4.070 8.90 72.6 806 1145 Changes in annual runoff The results of annual runoff analysis for the Kupa at Hrvatsko and Kamanje show a trend of decrease in annual runoff trend, as shown in Fig 2. A negligible decrease trends of 0.0027 109 m3 (0.55% of annual mean) and 0.0125 109 m3 (0.42% of annual mean) was recorded at the Hrvatsko gauging station. BALWOIS 2010 – Ohrid, Republic of Macedonia – 25, 29 May 2010 2 Figure 2. Annual runoff trend V (109 m3) for the Kupa at Hrvatsko and Kamanje (1957-2008) Contemporary hydrological studies point to unacceptable orientation encountered in water management and hydrological calculations towards using steady mean annual runoff. V.G. Andrijanov was the first author to stress that such calculations should focus more on specific transfer of previous hydrological data to the future ones, and he introduced a concept of ˝expected mean annual runoff˝. The concept was defined as follows: ˝The expected mean annual runoff is its average multiannual characteristic, expected for a period of operation of an engineered water structure or any phase in its operation, defined by an average quantity for a previous period with a share of adequate climate and anthropological factors˝. An analytical form of the expected mean annual runoff is: EX= QCLIM QANT (1) EX – expected mean annual runoff - mean annual runoff determined for a previous multiannual period QCLIM, QANT – expected change rates for mean annual runoff affected by climate and anthropologic factors Design engineering of numerous capital water structures is based on previously recorded climate and hydrological data for structures that will last for 50 to 100 years. In case of climate changes, these data cannot be used as realistic input for the future states. Thus, in planning and design, it is necessary to account for anthropological factors and potential future climate variability and changes. Climate variability and changes Based on the results of studies of changes in climate variability and changes by WMO, UNEP (2008), the decrease in annual runoff at the south (south-east) of Europe for individual time thresholds is given in Table 2. BALWOIS 2010 – Ohrid, Republic of Macedonia – 25, 29 May 2010 3 Table 2. Decrease in annual runoff Year Decrease Region Remark Decrease in summer 2020 up to 23 % South Europe flows 2050 up to 20-30 % South-east Europe - Decrease in summer 2070 up to 36 % South Europe low waters to 80 % The Hydrometeorological Service works intensively on the downscaling method which is used to adapt the global climate model outputs to a smaller area using a regional climate model. Therefore, the climate variability and change results are adjusted from a comparatively low resolution (e.g. 200-300 km) to a fine spatial resolution (20-50 km). Downscaling enables better presentation of spatial variations in climate parameters on smaller scales, particularly extreme events whose intensity is usually reduced in global models because the smallest spatial distribution of several hundred kilometres could encompass the entire region analyzed. Quality of downscaling results depends both on quality of the regional model used for downscaling and on quality of the global model. Annual precipitation amount trend (Hydrometeorological Service, 2009) shows its decrease at the entire Croatian territory during the 20th century, which means that it has joined the drought trend at the Mediterranean. The trend is more pronounced at the Adriatic than in inland areas. According to the data series prolonged until 2008, the decade trends of annual and seasonal precipitation amounts have not changed significantly. Decrease in annual precipitation amounts is contributed by changes in low-intensity rain days frequency and considerable increase in dry days (Rd < 1.0 mm) in entire Croatia. Anthropological factors impact on runoff Man-caused impact on the water regime changes in its character and intensity depending on the degree of economic, technical and climate variability and changes. It is sometimes very difficult to assess such an impact since a catchment might be exposed to different impacts at the same time, and some impacts might compensate for others. The most important anthropological factors affecting runoff will be given below, as an information and without numerical quantification. They include: river training and erosion control structures, irrigation, hydraulic engineering works, water supply, transfer of water in time and space, etc. The Srednje Posavlje flood control system includes the Kupa-Kupa canal, Brodarci dam, Šišljević and Kupčina dams at the Kupa-Kupa canal and Kupčina retention basin, the structures of major importance in defence of the City of Karlovac against high and flood waters. The Brodarci dam distributes the Kupa water mass by directing it into the Kupa-Kupa canal while respecting the basic criterion that the discharge through the City of Karlovac does not exceed 600-700 m3s-1 (depending on the Korana River backwater effect) and that maximum water level in Karlovac is maintained at el. 110.75 m a.s.l. Fig. 3. shows a detail of the Kupa channel near Hrvatsko. Conclusions and recommendations Analysis of annual runoff shows a decreasing trend. According to the results of studies into global climate variability and changes, decrease in annual runoff at the south (south-east) of Europe, they should be expected to be 23 to 36 % lower for particular time thresholds. Such, less than optimistic predictions for the Kupa catchment indicates that water resources management is very demanding. The trend analysis results have shown that annual precipitation amounts at the analyzed weather stations has a slight decreasing trend. Also, it is recommended to continue with monitoring and forecasts of the state development in the near future in relation to the processes of climate variability and changes in the region.