Drought Assessment in Nestos River Basin (N. Greece) for the Period 1955-2018

Abstract

Drought is a significant meteorological phenomenon that can seriously impact natural ecosystems. Drought assessment and monitoring should be continuous in regions of high ecological importance, for the conservation of natural vegetation distribution and dynamics. In this study, we analyzed monthly precipitation data obtained from five meteorological stations in the Nestos' river sub-basin in Greece in order to detect drought episodes during the period 1955-2018 by employing the Standardized Precipitation Index (SPI). The results indicate that the coastal and altitudinal lower areas of the region faced more frequent and more severe drought events during the recent years compared to the past, whereas the climatic conditions From the trend analysis of the SPI it appears that the lower part of the sub-basin is anticipated to face even more droughts in the future. These patterns may have significant impacts on the natural rainfed vegetation, especially at the river's Delta where habitat types of high priority are located. Thus, the need to adopt measures for the conservation of the local ecosystem is critical, considering also that the local habitat types are characterized by high demands for water availability.

1. Introduction

The Mediterranean basin includes diverse types of habitats and it is considered a biodiversity hotspot [ 1,2 ], due to the combination of climate conditions and the variety of geographical and topographical features of the region. Climate shifts and extreme events, such as droughts, in this area may lead to alterations of the ecological balance and cause pressures to human activities. Many studies have identified trends, especially in precipitation timeseries [ 3-9 ], as well as in drought patterns [ 10-13 ], indicating a tendency to drier conditions [ 14,15 ] in various areas within the Mediterranean region. The efficient management of drought risks is essential for compensating the impacts of droughts on several sectors, including the economy, the societies and the environment. To this end, proper understanding, identification and monitoring of drought characteristics is essential, especially in economically and ecologically important and vulnerable regions. Additionally, year to year drought conditions is critical

2022 Copyright for this paper by its authors. to be monitored for assessing the effects on the sustainability of the natural rainfed ecosystems [ 16 ], as decreased precipitation and droughts have a strong impact on plants’ growth dynamics [ 17,18 ].

Nestos is a transboundary river, shared between Bulgaria and Greece. The river delta, which is located in the Greek sub-basin, is considered as of high ecological significance. Many parts of the basin, including Nestos’ Delta, have been declared as Natura 2000 protected sites. Especially the river’s Delta hosts a variety of priority habitats that require high water availability in order to be sustainable. Therefore, drought monitoring is crucial in this region, considering also that the hydrological regime of Nestos river is rather sensitive to drought episodes [ 19 ]. Furthermore, drought analyses using proper tools, such as drought indices, is important for indicating appropriate management measures in transboundary rivers [20].

The present study is based on previous studies by Proutsos et al. [21,22] that analyzed the precipitation trends in the Greek part of Nestos river and assessed the changes occurred since the 1950s. In this work, the drought conditions of this region are evaluated, analyzing the annual changes of the SPI drought index and assessing the spatial variability in the basin.

2. Study Site and Climatic Data

The geophysical pattern of the study area, which includes the Greek sub-basin of Nestos river, is presented in Figure 1. Details about the characteristics and information concerning the high ecological importance of the area can be found in Proutsos et al. [ 21-23 ].

The climate of the area is humid [ 15,24 ] according to UNEP’s [25] aridity classification system based on Thornthwaite’s [26] water balance approach, whereas the recent precipitation trends [21,22] indicate significant reduction in the coastal areas of the region, but also precipitation increase in the mountainous part of the basin with high seasonal variability [21]. The changing trends are more rapid at the current climatic period [22].

In this work, we assessed the drought conditions for the Greek part of the basin. Monthly precipitation data series, obtained from five long-operating meteorological stations, were analyzed for the period 1955-2018, on a hydrological year basis. The specific characteristics of the meteorological stations are presented in Table 1. The drought assessment was performed by employing the widely used Standardized Precipitation Index (SPI) [27], using DrinC software [28, 29]. The index is based solely on precipitation data and can be applied for several timesteps. In this study, the annual timestep of the hydrological year (October to September) was adopted, an approach that is considered suitable for assessing drought impacts on water availability [30,31]. SPI values classify the adopted time period of each year to one of the following eight classes (four wet and four dry): extreme drought (SPI≤-2.0), severe drought (-2.0<SPI≤-1.5), moderate drought (-1.5<SPI≤-1.0), mild drought (-1.0<SPI≤-0.5), near normal (-0.5<SPI≤0.5), mildly wet (0.5<SPI≤1.0), moderately wet (1.0<SPI≤1.5), severely wet (1.5<SPI≤2.0), and extremely wet (SPI>2.0).

To detect possible trends of the annual SPI, the Mann-Kendall test was applied [ 32,33 ] and the trend slopes were estimated by the Sen’s slope method [34]. These methods are widely used and are considered reliable in hydrological and climatic trend analysis [ 6,21,35-37 ]. For the trend detections, the evaluation of their significance and the calculation of their magnitudes, the MAKESENS 1.0 software was applied [38]. The spatial patterns presented in this work were developed by the application of the Kriging’s interpolation technique, where the unknown SPI slope values at all points across a defined spatial domain are estimated using a weighted average of all known values around the point [39].

3. Results and Discussion

The annual values of the SPI, based on the precipitation data of the five meteorological stations, are presented in Figure 2. The annual SPIs clearly indicate that the altitudinal lower coastal areas (KAV and XAN stations) of the region, located at the lower part of the basin, experience more intense and more frequent extreme or severe drought events during the recent years (after 1990). On the contrary, at the altitudinal higher positions (MES and PRA stations) the drought events were more frequent and intense in the previous decade (1980s), whereas at the mountainous areas (SID station) no extreme droughts were detected.

More specifically, for KAV station, 1998-1999 and 2003-2004 were hydrological years of extreme drought, 1996-1997 and 2001-2002 years of severe drought and 1994-1995, 1995-1996, 1997-1998, 2000-2001 and 2002-2003 years of moderate drought. The most severe drought in KAV occurred in 1998-1999, when SPI was -2.2.

XAN faced no extreme drought events, however 1998-1999, 1999-2000 and 2001-2002 were years of severe droughts. The most severe drought year was 1999-2000 (SPI=-1.8).

In MES and PRA, 1984-1985 and 1988-1989 were extreme drought years. Severe droughts were recorded in 1973-1974 and 1992-1993 and moderate droughts in 1989-1990, 1993-1994 and 2016-2017 for PRA station. Also, moderate drought was identified during the years 1967-1968, 1986-1987, 19891990, 1991-1992, 1992-1993, 1993-1994 and 2000-2001 for MES station. The most extreme drought years were 1988-1989 for MES (SPI=-2.3) and 1984-1985 for PRA (SPI=-2.5).

For SID station, no extreme droughts were identified. However, 1977-1978, 1980-1981 and 19921993 were years of severe drought and 1975-1976, 1976-1977, 1984-1985 and 1991-1992, were years of moderate drought. With regard to the available data, 1977-1978 was the year with the most stressful conditions (SPI=-1.7).

Considering the years with the highest negative values in each station, i.e. 1998-1999 for KAV, 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 1999-2000 for XAN, 1988-1989 for MES, 1984-1985 for PRA and 1977-1978 for SID, the spatial patterns of Figure 3 were produced. These patternsconfirm that in the recent years the coastal areas of the basin faced drier conditions compared to the past, whereas the mountainous regions faced more severe drought events in the past, but not during the recent years.

12-month SPI (Oct-Sep): KAV

Extremedrought Severedrought Moderatedrought Milddrought 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 5 5 6 6 6 6 6 7 7 7 7 7 8 8 8 8 8 9 9 9 9 9 0 0 0 0 0 1 1 1 1 1 -19155 -19157 -19159 -19116 -19136 -91561 -19176 -19196 -19117 -19137 -19157 -19177 -19197 -19118 -19138 -19158 -19178 -19198 -19119 -19139 -19159 -19179 -20991 -22010 -22030 -22050 -20702 -22090 -22011 -22031 -22051 -22071 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 0 Figure2:AnnualvaluesoftheSPIindexfortheoperatingperiodofthefivemeteorologicalstationsin Nestos’riversub-basin.

The trend analysis of the SPI values produced the pattern depicted in Figure 4, which indicates that SPI presents negative and strong decreasing trends in the low altitude coastal areas, whereas at the mountainous areas of the basin the conditions are more wet, however these latest positive trends are not significant. These changes are anticipated to cause problem to the local high priority habitats that are located at the lower part of the basin, since they are highly vulnerable in the changes of precipitation. Such high water demanding ecosystems, are considered of high risk and require interventions to ensure that they have the necessary water quantities in order to be sustained.

4. Conclusions

In this work, a drought assessment was performed for the Nestos’ river sub-basin in N. Greece. The region hosts a variety of significant habitats that require high water availability in order to be preserved. The precipitation changes can highly affect the vegetation patterns in the region, thus the continuous monitoring of drought is necessary. From the results of this study, it is indicated that the altitudinal lower coastal part of the basin is at high risk and faces, in the recent years, more severe and frequent drought events compared to the past. The western coastal areas are even more vulnerable. On the other hand, the climatic conditions appear to be more favorable nowadays, compared to the past, in terms of drought frequency and severity. This is expected to favor the existing mountainous ecosystems. However, specific measures should be undertaken to conserve the highly valuable coastal ecosystem and conserve the priority habits of the coastal zone. Such interventions should be assisted by the operation of the existing uphill dams and impose the continuous co-operation of the stakeholders from Greece and Bulgaria and the implementation of common strategies in order to achieve the ecological goals.

5. Acknowledgements

This work is financially supported by the project “Bioclima and natural vegetation in Greece” which is financed by the Hellenic Agricultural Organization “DEMETER”. The climate data for the KAV and XAN stations were kindly provided by the Hellenic National Meteorological Service (HNMS). The authors also express their gratitude to the Public Power Corporation S.A. (DEH) and especially to Mrs. Kreouza Mangina, Acting Head of Hydrology Sector of the Hydro-Electric Projects' EngineeringConstruction Department of DEH, for supporting this research by providing the climate data for MES, PRA and SID stations.

6. References

volume 26, Advances in Natural and Technological Hazards Research, Springer, Dordrecht, 2009, pp. 243–256, doi: 10.1007/978-1-4020-9045-5_16. [20] F. A. Al-Faraj, M. Scholz, D. Tigkas, M. Boni, Drought indices supporting drought management in transboundary watersheds subject to climate alterations, Water Policy 17 (2015) 865–886, doi: 10.2166/wp.2014.237. [21] N. Proutsos, E. Korakaki, A. Bourletsikas, G. Karetsos, K. Tsagari, C. Georgıadis, 50-year precipitation trends in Nestos Delta-Natura 2000 site, in: Proccedings of the 16th International Conference on Environmental Science and Technology, CEST2019, University of the Aegean, GlobalNest, Rhodes-Greece, 2019 (in press). [22] N. Proutsos, A. D. Solomou, P. Koulelis, A. Bourletsikas, N. E. Chatzipavlis, Detecting changes in annual precipitation trends during the last two climatic periods (1955-1984 and 1985-2018) in Nestos River basin, N. Greece, in: Proceedings of the 10th International Conference on Information and Communication Technologies in Agriculture, Food and Environment, HAICTA 2022, Athens-Greece, CEUR Workshop Proceedings, 2022, (in press). [ 23 ] N. Proutsos, G. Karetsos, K. Tsagari, Determining the phenological stages of Nestos-Delta (Alnus glutinosa) riparian forest (Natura 2000 site) by using radiation transmittance data, in: Proceedings of the 17th International Conference on Environmental Science and Technology, CEST2021, University of the Aegean, GlobalNest, Athens-Greece, 2021, (in press). [24] N. D. Proutsos, I. X. Tsiros, P. Nastos, A. Tsaousidis, A note on some uncertainties associated with Thornthwaite's aridity index introduced by using different potential evapotranspiration methods, Atmospheric Research 260 (2021) 105727, doi: 10.1016/j.atmosres.2021.105727. [25] UNEP, World Atlas of Desertification, Edward Arnold, London, 1992. [26] C. W. Thornthwaite, An approach toward a rational classification of climate, Geographical review 38(1) (1948) 55–94, doi: 10.2307/210739. [27] T. B. McKee, N. J. Doesken, J. Kleist, The relationship of drought frequency and duration to time scales, in: Proceedings of the Proceedings of the 8th Conference on Applied Climatology, pp. 179– 183, 1993. [28] D. Tigkas, H. Vangelis, G. Tsakiris, DrinC: a software for drought analysis based on drought indices, Earth Science Informatics 8 (2015) 697–709, doi: 10.1007/s12145-014-0178-y. [29] D. Tigkas, H. Vangelis, N. Proutsos, G. Tsakiris, Incorporating aSPI and eRDI in Drought Indices Calculator (DrinC) Software for Agricultural Drought Characterisation and Monitoring, Hydrology 9(6) (2022) 100, doi: 10.3390/hydrology9060100 [30] D. Tigkas, H. Vangelis, G. Tsakiris, Drought and climatic change impact on streamflow in small watersheds, Science of the Total Environment 440 (2012) 33–41, doi: 10.1016/j.scitotenv.2012.08.035. [31] D. Tigkas, H. Vangelis, G. Tsakiris, Drought characterisation based on an agriculture-oriented standardised precipitation index, Theoretical and applied climatology 135 (2019) 1435–1447, doi: 10.1007/s00704-018-2451-3. [ 32 ] M. G. Kendall, Rank Correlation Methods. London: Charles Griffin & Co. 1975. [33] H. B. Mann, Nonparametric tests against trend, Econometrica, Journal of the econometric society 13(3) (1945) 245–259, doi: 10.2307/1907187. [34] P. K. Sen, Estimates of the regression coefficient based on Kendall's tau, Journal of the American

Statistical Association 63:324 (1968) 1379–1389, doi: 10.1080/01621459.1968.10480934. [35] N. Proutsos, E. Korakaki, A. Bourletsikas, K. Kaoukis, C. Georgiadis, Analyzing temperature attributes for the last half century in Heraklion–Crete, Greece, in: L. Garrote, G. Tsakiris, V. A. Tsihrintzis, H. Vangelis, D. Tigkas (editors), Proceedings of the 11th World Congress on Water Resources nd Environment: Managing Water Resources for a Sustainable Future, EWRA 2019, European Water Resources Association, Madrid, Spain, 2019, pp. 477–478. [36] N. Proutsos, E. Korakaki, A. Bourletsikas, A. Solomou, E. Avramidou, C. Georgiadis, A.

Kontogianni, K. Tsagari, Urban temperature trends in east Mediterranean: The case of HeraklionCrete, European Water 69/70 (2020) 3–14. URL: http://ewra.net/pages/EWRA2019_Proceedings.pdf [37] N. Proutsos, C. Tsagari, G. Karetsos, Precipitation variations over the last half of the century in Greece, in: Proceedings of the VI European Water Resources Association International symposium, Water Engineering and Management in a Changing Environment, EWRA 2011, European Water Resources Association, Catania, Sicily-Italy, 2011. [38] T. Salmi, A. Määttä, P. Anttila, T. Ruoho-Airola, T. Amnell, Detecting trends of annual values of atmospheric pollutants by the Mann-Kendall test and Sen’s slope estimates MAKESENS–The excel template application. Finish Meteorological Institute, Helsinki, 2002. [39] L. Salvati, M. Zitti, R. Di Bartolomei, L. Perini, Climate aridity under changing conditions and implications for the agricultural sector: Italy as a case study, Geography Journal 2013 (2013), 923173, doi: 10.1155/2013/923173.

References

32..00 -5195659 -7591958 -9591960 -1691962 -3691964 -5691966 -7691968 -9691970 -1971972 -9731974 1 - 97519762 -- 7971987m -7991908on - 9811928th - 8931948SP -8951968I (- 8971988Oc - 8991909t -S- 9911929ep-9931949):-9951969PR - 997199A -999200 -001200 -003200 -005200 -007200 -009210 -011210 -013210 -015210 -017210 8 0 2 4 6 8 0 2 4 6 8

23..00 -565915 -758591 -950691 -162691 -364691 -566691 -768691 -960791 -172791 -137749 -15776912-177789-m-197089on - 118289th - 138489SP -158689I (- 178889Oc - 198099t -S- 119299ep-139499):-159699SI - 17999D -29900 -21000 -23000 -52000 -70002 -90102 -11102 -31102 -51102 -71102 8 0 2 4 6 8 0 2 4 6 8

[1] N. Myers , R. A. Mittermeier , C.G. Mittermeier , G. A. Da Fonseca , J. Kent , Biodiversity hotspots for conservation priorities , Nature 403 ( 2000 ) 853 - 858 , doi: 10.1038/35002501.

[2] A. D. Solomou , N. D. Proutsos , G. Karetsos, K. Tsagari , Effects of Climate Change on Vegetation in Mediterranean Forests: A review , IJEAB 2 ( 2017 ) 240 - 249 , doi: 10.22161/ijeab/2.1.31.

[3] M. Boccolari , S. Malmusi , Changes in temperature and precipitation extremes observed in Modena, Italy . Atmospheric Research 122 ( 2013 ) 16 - 31 , doi: 10.1016/j.atmosres. 2012 . 10 .022.

[4] N. Katerji , G. Rana, R. M. Ferrara , Actual evapotranspiration for a reference crop within measured and future changing climate periods in the Mediterranean region , Theoretical and Applied Climatology 129 ( 2017 ) 923 - 938 , doi: 10.1007/s00704-016-1826-6.

[5] S. Pasiardis , M. Theofilou , Climatic changes in Cyprus-Statistical data and reports for the last 100 years (in greek) , Meteorological Service of Cyprus, CY , 2011 . URL: https://slideplayer.gr/slide/2695653/.

[6] N. Proutsos , K. Tsagari , G. Karetsos , A. Liakatas , T. Kritikos, Recent temperature trends over mountainous Greece , European Water 32 ( 2010 ) 15 - 23 , URL: http://www.ewra.net/ew/issue_32.htm

[7] A. Sofroniou , S. Bishop , Water scarcity in Cyprus: a review and call for integrated policy , Water 6 ( 2014 ) 2898 - 2928 , doi: 10.3390/w6102898.

[8] S. Stefanidis , V. Alexandridis , Precipitation and potential evapotranspiration temporal variability and their Relationship in two forest ecosystems in Greece , Hydrology 8 ( 4 ) ( 2021 ) 160, doi: 10.3390/hydrology8040160.

[9] A. Zeroual , A. A. Assani , M. Meddi , Combined analysis of temperature and rainfall variability as they relate to climate indices in Northern Algeria over the 1972-2013 period . Hydrology Research 48 ( 2017 ) 584 - 595 , doi: 10.2166/nh. 2016 . 244 .

[10] T. Caloiero , S. Veltri , P. Caloiero , F. Frustaci , Drought analysis in Europe and in the Mediterranean basin using the standardized precipitation index , Water 10 ( 8 ) ( 2018 ) 1043, doi: 10.3390/w10081043

[11] M. Hoerling , J. Eischeid , J. Perlwitz , X. Quan , T. Zhang , P. Pegion, On the increased frequency of Mediterranean drought , Journal of Climate 25 ( 2012 ) 2146 - 2161 , doi: 10.1175/ JCLI-D- 11- 00296 .1.

[12] J. Ruffault , N. K. Martin-StPaul , S. Rambal , F. Mouillot, Differential regional responses in drought length, intensity and timing to recent climate changes in a Mediterranean forested ecosystem , Climatic Change 117 ( 2013 ) 103 - 117 , doi: 10.1007/s10584-012-0559-5.

[13] D. Tigkas , Drought characterisation and monitoring in regions of Greece , European Water 23/24 ( 2008 ) 29 - 39 , URL: http://www.ewra.net/ew/issue_ 23 - 24 .htm

[14] P. M. Sousa , R. M. Trigo , P. Aizpurua , R. Nieto , L. Gimeno , R. Garcia-Herrera , Trends and extremes of drought indices throughout the 20th century in the Mediterranean , Natural Hazards and Earth System Sciences 11 ( 2011 ) 33 - 51 , doi: 10.5194/nhess-11- 33 - 2011 .

[15] I. X. Tsiros , P. Nastos , N. D. Proutsos , A. Tsaousidis , Variability of the aridity index and related drought parameters in Greece using climatological data over the last century ( 1900 -1997), Atmospheric Research 240 ( 2020 ) 104914, doi: 10.1016/j.atmosres. 2020 . 104914 .

[16] D. Tigkas , G. Tsakiris, Early estimation of drought impacts on rainfed wheat yield in Mediterranean climate , Environmental Processes 2 ( 2015 ) 97 - 114 , doi: 10.1007/s40710-014- 0052-4.

[17] N. Proutsos , Α. Liakatas, S. Alexandris , I. Tsiros , Carbon fluxes above a deciduous forest in Greece , Atmósfera 30 ( 2017 ) 311 - 322 , doi: 10.20937/atm. 2017 . 30 .04.03.

[18] N. Proutsos , D. Tigkas , Growth response of endemic black pine trees to meteorological variations and drought episodes in a Mediterranean region , Atmosphere 11 ( 6 ) ( 2020 ) 554, doi: 10.3390/atmos11060554.

[19] D. Pangalou , D. Tigkas , H. Vangelis , G. Tsakiris , A. Nanou-Giannarou , Drought severity thresholds and drought management in Greece ,in:A. Iglesias , A. Cancelliere , D. A. Wilhite , L. Garrote , F. Cubillo (eds) Coping with Drought Risk in Agriculture and Water Supply Systems,