IMPACTS OF CLIMATE VARIABILITY ON THE RUNOFF IN THE SOUTH EASTERN PART OF BULGARIA

1. IMPACTS OF CLIMATE VARIABILITY ON THE RUNOFF IN THE SOUTH EASTERN PART OF BULGARIA Bernardo Lizama Rivas National Institute of Meteorology and Hydrology, Sofia Ivanka Koleva-Lizama University of Forestry, Sofia

2. The aim of the paper is to study the runoff variability in respect to climate change in the south eastern Bulgarian watersheds. To investigate possible impacts of climate variability on the runoff were used long term data (1952-2002) from several hydrological (Razhitsa, Kameno, Prohod, Svetlina, Fakiya, Zidarevo, Veselie, Zvezdets and Gramatikovo) and meteorological (Aytos, Burgas, Grudovo, Tsarevo and Malko Turnovo) gauging stations.

3. The most important rivers in this region are Dvoynitsa, Hadzhiyska, Aheloy, Aytoska, Rusokastrenska, Sredetska, Fakiyska, Ropotamo, Diavolska, Veleka and Rezovska rivers. The streams which debouch to the Black sea have rather diverse forestal communities. The relief is characterised by lowland which cover the eastern and middle part of the region – sea coast, low plains and low mountainous area. Location of the Study Region

4. The climate in the South Eastern region is mainly transitional Mediterranean climate. The topography in the south part of the region in concern (Strandja Mountain), plays an important role in the distribution of the precipitation. The study on long time series of hydrological data in the context of climate variability is very difficult task because of by disturbance man’s activity. During the process of time series selection was established that the streamflow in the Veleka River at Zvezdets and Gramatikovo is not disturbed, but in the catchments of the North part of the studied region exists relatively small disturbances, in view of that, these catchments were not included in this study.

5. Table 1. The basic statistical parameters for the catchments Area (A) The runoff regime in the studied catchments is characterized by considerably variability. In Table 1 are shown the statistical parameters of annual runoff (mean, standard deviation, skewness and the extremes values) for studied period for the main gauging stations. ,

6. In order to study the river runoff, variability in respect to climate change, the warm and humid conditions during the 50 last years were analyzed. To examine the temperature and precipitation variability, the time series of their anomalies are obtained (Fig. 1, 2).

7. In the catchments the temperature increase is more expressive in the mountain areas (Strandja Mountain) Fig. 1. Time series of temperature anomaly at Grudovo and Malko Turnovo stations

8. The precipitation decrease is more noticeable in the Strandja region compared to the rest of studied area. Fig. 2. Time series of precipitation anomaly at Grudovo and Malko Turnovo stations

9. In order to quantify the precipitation deficit for different time scales the Standardized Precipitation Index (SPI) was used. • In Fig. 3 is given the SPI for 3, 6 and 12 month time scales and its trends, from where is noted that the tendency to drought is evident in the studied region. Fig. 3. Standardized Precipitation Index (SPI) for 3, 6 and 12 month timescales and its trends at Malko Turnovo station

10. In Fig. 4 are shown the anomalies of annual runoff, the linear trend and the 5 years moving average for the Veleka River at Gramatikovo. • It was found that up to 1975 prevail positive runoff anomalies. From 1976 to 1994 predominate negative runoff anomalies which characterize exceptionally a low waters period, the most low values are observed in 1993 (-6.4 m3/s). From 1995 to 2002 the streamflow fluctuations are rather sudden with exceptional maximum in 1998 (12.7 m3/s ). • For the whole period 1952 – 2002 is observed a negative trend for the annual runoff. Fig. 4. Anomaly of annual runoff in the Veleka River at Gramatikovo

11. CONCLUSION The analysis of long time series of river runoff, air temperature and precipitation allows us to draw some conclusions. • The temperature increase is more expressive in the mountain areas (Strandja Mountain) and weak in the Black Sea coastal zone. • The precipitation decrease is more noticeable in the Strandja region compared to the rest of studied area. • The Standardized Precipitation Index (SPI) for 3, 6 and 12 month timescales shows that the tendency to drought is evident in the South Eastern Bulgaria. • For the whole period 1952 – 2002 is observed a negative trend for the annual runoff. • There is the tendency to weak temperature increase and good noticeable rainfall and runoff decrease, which bring to drought in the region.