INVESTIGATING FREQUENCY AND SPATIAL CHARACTERISTICS OF DROUGHTS IN THE CENTRAL ANATOLIAN REGION, TURKEY Osman YILDIZ Kırıkkale University Engineering Faculty Civil Engineering Department 71451 Kırıkkale-TURKEY Tel: 0 318 357 3571-1083 Fax: 0 318 357 2459 E-mail:
[email protected]
ABSTRACT
In this study, the temporal and spatial characteristics of meteorological droughts in the Central Anatolian region, a semi-arid region in Turkey, are investigated in order to provide a guide for sustainable water resources management. For this purpose, a drought intensity-areal extent-frequency curve is developed using the Standardized Precipitation Index (SPI) time series between 1953 and 2003 from 28 meteorology stations across the region. Using this curve the severity of historical droughts of 1956, 1973, 1989 and 2001 occurred in the region are assessed and their intensity, areal extent and return periods are obtained. Keywords: Drought, SPI, frequency curve, return period, semi-arid region. INTRODUCTION
As natural hazards droughts affect our life in many different ways. Droughts occur as a result of precipitation deficit. Different definitions for droughts are given in the literature with respect to their effects [Dracup et al., 1980; Wilhite and Glantz, 1985]. Meteorological, agricultural, hydrological and socio-economic drought definitions are among the most common ones. Located in a semi-arid region Turkey experiences frequent drought events resulting from atmospheric, geographic, and climatic factors. Sudden decreases in precipitations in the African Sahel and the subtropics during the 1960s affected the Eastern Mediterranean Basin and Turkey as well in the 1970s, especially during winter months. Severe droughts were observed across the country in 1973, 1977, 1984, 1989-1991 and 1999-2000 periods [Türkeş, 1996; Komuscu, 2001].
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The early scientific researches on droughts in Turkey started in the 1940s. In the literature, there are various studies covering the whole country, a specific region or an area. A summary of such studies is presented in the following paragraphs. Using De Martonne and Thorntwaite formulas Tümertekin (1956), and Tümertekin and Cöntürk (1956) investigated the number of drought months across Turkey. In his study, Erinç (1957) prepared drought maps for Turkey with the use of a drought index based on the ratio of total precipitation to temperature. In addition to precipitation and temperature, Aydeniz (1988) utilized relative humidity and duration of sunshine in his drought studies in Turkey. The author determined the drought categories of each geographical regions of the country. With the use of drought index developed by Erinç (1957), Türkeş (1990) explored the drought regions in Turkey, major drought events and periods. Recently, Sırdaş (2002) prepared drought maps for Turkey using precipitation, temperature and humidity data between 1930 and 1990 from 60 meteorology stations. In her three-variable drought approach, the author investigated the relationship between precipitation deficits (i.e. droughts) and the other two variables. In their study, Sırdaş ve Şen (2003) presented a spatial and temporal drought analysis in the Trakya region of Turkey. Using the SPI method and the run analysis, the authors determined the drought characteristics (magnitude, duration and intensity) of the selected stations in the region and they also prepared drought maps for the entire region. Yeğnidemir (2005) obtained drought characteristics of 26 meteorology stations in the Central Anatolian region from the monthly precipitation data between 1953 and 2003 with the use of the SPI method and the run analysis. The author also presented regional drought maps for various drought periods in his study. The objective of this study is to assess the temporal and spatial characteristics of meteorological droughts in the Central Anatolian region for providing a guide for sustainable water resources management. For this purpose, a drought intensity-areal extent-frequency curve is developed for the region by using the method proposed by Henriques and Santos (1999). The SPI time series between 1953 and 2003 from 28 meteorology stations across the region are utilized in the study. With the use of drought intensity-areal extent-frequency curve major droughts occurred in the region are evaluated and their intensity, areal extent and return periods are obtained. STUDY AREA
The region under study is located at 30-39oE longitudes and 37-40.5oN latitudes and has a land surface area of nearly 184,165 km2 (Figure 1). As a semi-arid region it is surrounded by the Northern Anatolian mountain ranges in the north, the Taurus
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Mountains in the south, and the Eastern Anatolian mountain ranges and high plateaus in the east. The region is generally characterized by highlands in the north and east and by lowlands in the west and south directions with an average altitude of 1150 m. The region covers a major part of upstream drainage area of the Kızılırmak River basin. Agricultural crops produced in the region are mainly wheat and barley. A significant amount of the country’s cereal need is provided from the Central Anatolian region.
Figure 1. A map of Turkey and the Central Anatolian region.
HISTORICAL DROUGHTS IN THE REGION
A short analysis of historical droughts occurred in the region is given in this section. The SPI method developed by McKee et al. (1993) is utilized for the temporal analysis of droughts. The method is simple and straightforward since precipitation is the only meteorological variable to be used. Another advantage of this method is the utility of various time periods in drought assessments. Standardized precipitation series is calculated using the arithmetic average and the standard deviation of precipitation series. For a given X1, X2, Xn series standardized precipitation series, xi, is calculated from the following equation: −
xi =
Xi − X Sx
where X is the average of precipitation series and Sx is the standard deviation of precipitation series. Negative values obtained from this equation indicate precipitation deficits (drought events), while positive values stand for precipitation excesses (wet events). Four different drought categories are defined by the same authors as in Table 1.
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Table 1. SPI drought categories.
SPI Value (0.0) – (-0.99) (-1.0) – (-1.49) (-1.5) – (-1.99) ≤ -2
Category Mild drought Moderate drought Severe drought Extreme drought
Figure 2 shows SPI time series for the region calculated from monthly precipitation data between 1953 and 2003. The arithmetic averages of data from 28 stations (Table 2) were used for this purpose. Previously mentioned severe drought events for the whole country can be detected for the region from the figure. The droughts that occurred in 1956, 1973, 1984, 1989, 1993, 1994 and 2001 have SPI values are as low as -2.5 as a result of precipitation deficits, especially, during summer and fall seasons (see Figure 3). Total precipitation deficits during these drought years reach up to 30%. Table 2. List of meteorology stations used in the study.
No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Station Name Aksaray Ankara Beypazarı Boğazlıyan Bolvadin Cihanbeyli Çankırı Develi Eskişehir Gemerek Ilgın Kaman Kangal Karaman Karapınar Kayseri Kırıkkale Kırşehir Kızılcahamam Konya Kulu Nevşehir Niğde Pınarbaşı Polatlı Sivas Yozgat Zara
Altitude (m) 965 891 682 1066 1018 969 751 1180 787 1173 1034 1075 1545 1025 1004 1093 747 1007 1033 1031 1010 1260 1211 1500 885 1285 1298 1348
Latitude (o) 38.23 39.57 40.10 39.12 38.43 38.39 40.36 38.23 30.57 39.11 38.17 39.22 39.14 37.11 37.43 38.44 39.51 39.09 40.28 37.52 39.06 38.35 37.58 38.43 39.35 39.45 39.49 39.54
Longitude (o) 34.05 32.53 31.55 35.15 31.03 32.56 33.37 35.30 39.78 36.04 31.55 33.43 37.23 33.13 33.33 35.29 33.31 34.10 32.39 32.29 33.00 34.40 34.41 36.24 32.09 37.01 34.48 37.45
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Figure 2. SPI time series for the region (1953-2003).
Figure 3. Monthly precipitation distributions of the selected drought years in the region.
In order to find out the influence of precipitation deficits on streamflows the annual average flow rates at the Yamula station (near the city of Kayseri) on the Kızılırmak River between 1956 and 1994 were compared against the long term average value at this station (Figure 4). As the figure indicates there is a significant amount of decreases in flow rates during the drought years mentioned above. For example, the average flow rate in 1973 is only half the long term average value.
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140 Long Term Averag e
Annual Averag e
120
Flow (m^3/s)
100 80 60 40 20 0 1953
1958
1963
1968
1973
1978
1983
1988
1993
Year Figure 4. Long term average and annual average flow rates at Yamula station between 1953 and 1994.
REGIONAL DROUGHT ANALYSIS
The method proposed by Henriques and Santos (1999) is among the recent methods used for regional drought analysis. In this method, both temporal and spatial characteristics of regional droughts are analyzed by a drought intensity-areal extentfrequency curve. In order to develop this curve, the authors employed the Thiessen polygons to determine the area of influence of each individual station and utilized synthetic precipitation series in their study area. However, the Thiessen polygons method does not take into account the stochastic characteristics of precipitation data. As an alternative, Kim et al. (2002) proposed the application of geostatistical techniques to obtain regional distribution of precipitation data. They used the kriging technique to obtain regional precipitation distributions in the Conchos River basin, a semi-arid region under severe drought effects in Mexico. With the use of drought intensity-areal extent-frequency curve for the region the authors determined intensity, areal extent and return periods of severe drought events in the 1990s. In this study, a regional drought analysis was performed for the Central Anatolian region using the drought intensity-areal extent-frequency curve with the method proposed by Kim et al. (2002). According to the method the following steps were followed in this study. Annual drought intensity values at each station were first
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obtained from the monthly SPI time series. For this purpose, in a given year, SPI values lower than -1.0 were added together. Then the annual totals were multiplied by the number of months with SPI values lower than -1.0 and divided by 12. Later, the regional distributions of annual drought intensities from 28 stations were obtained via the kriging method. Here, as an example, the regional distributions of 1956 and 1973 are shown in Figure 5. Next, percent areal distributions of drought intensities for selected threshold levels (e.g., -1.0, -1.5 etc.) were calculated and then frequency analyses of percent areal distributions were performed to determine return periods of drought intensities. In this study, the Extreme Value Type I probability distribution was selected to fit drought intensities as suggested by Henriques and Santos (1999), and Kim et al. (2002). The negative values of drought intensities were converted to positive values in order to represent the extreme conditions and to analyze the risks of droughts using the exceedance probability.
Figure 5. The regional distributions of drought intensities for 1956 and 1973.
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DISCUSSION OF RESULTS
Figure 6 shows the drought intensity-areal extent-frequency curve obtained for the Central Anatolian region with selected return periods. The drought intensity, areal extent and return periods of selected historical droughts in the region are indicated on the same figure. From the figure, the 1956 drought has a return period ranging from 2 to 50 years. The drought intensity is lower than -2.0 at nearly 70% of the region meaning that a major part of the region was under extreme drought conditions during this year. The drought of 1973 has a return period ranging from 2 to 10 years. The areal extent of extreme drought conditions is about 20% during this drought period. Similarly, the drought event occurred in 1989 has a return period of 2-10 years but extreme droughts covered more than half of the region. In 2001, the drought event seems to have a relatively low return period (2-5 years). During the same year, extreme drought conditions prevailed at nearly 10% of the region. 0,0
2 Year 5 Year
Drought Inyensity (SPI)
-1,0
10 Year 30 Year
-2,0
50 Year -3,0
100 Year 1956
-4,0
1973 1989
-5,0
2001 -6,0 0
10
20
30
40
50
60
70
80
90
100
Areal Extent (%) Figure 6. Drought intensity-areal extent-frequency curve for the Central Anatolian Region with the selected historical droughts.
CONCLUSIONS
A regional drought analysis for the Central Anatolian region of Turkey, where semi-arid climatic conditions are predominant, was performed in this study. For this aim, a drought intensity-areal extent-frequency curve was developed for the region using the regional drought analysis approaches proposed by Henriques and Santos (1999), and Kim et al. (2002). The curve enabled us to obtain and evaluate spatial and
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temporal characteristics of historical droughts occurred in the region. Overall, the study results suggest that this method provides a useful tool for the analysis and evaluation of regional droughts both spatially and temporally. REFERENCES
Aydeniz, A. 1988. Drought Assesment in Turkey with the Aydeniz Method (in Turkish), The Turkish State Meteorological Service, Agricultural Meteorology and Climate Observations Department, Ankara, Turkey. Dracup, J.A., Lee, K.S. and Paulson Jr., E.G. 1980. “On the Definition of Droughts.” Water Resources Research, 16(2), 297-302. Erinç, S. 1957. Applied Climatology and the Climate of Turkey (in Turkish), İstanbul Technical University, Hydrogeology Institute, İstanbul, Turkey. Henriques, A.G. and M.J.J. Santos. 1999. “Regional Drought Distribution Model.” Phys. Chem. Earth (B), No.1-2, 19-22. Kim, T-W., Valdes, J.B., and Aparicio, J. 2002. “Frequency and Spatial Characteristics of Droughts in the Conchos River Basin, Mexico.” IWRA, Water International, 27(3), 420-430. Komuscu, A.Ü., 2001, “An Analysis of Recent Drought Conditions in Turkey in Relation to Circulation Patterns”, Drought Network News, 13(2-3), 5-6. McKee, T. B., Doesken, N. J. and Kleist. J. 1993. “The Relationship of Droght Frequency and Duration to Time Scales, Reprints, 8th Conference on Applied Climatalogy, Anaheim, CA, USA, pp.179-184. Sırdaş, S. 2002. A Meteorological Drought Modelling and its Application in Turkey (in Turkish), Ph.D. Thesis, İstanbul Technical Univeristy, Science Institute, İstanbul, Turkey. Sırdaş, S. and Şen, Z. 2003. “Spatio-temporal Drought Analysis in the Trakya Region, Turkey” Hydrological Sciences, 48(5), 809-820. Tümertekin, E. 1956. “Drought Seasons in Turkey (in Turkish)” T.C.D (Journal of Turkish Geography) No. 15-16: 193-197, İstanbul, Turkey. Tümertekin, E. ve Cöntürk, H. 1956 “Drought Investigation in Turkey Using Statistical Methods (in Turkish)”, İstanbul Üniversitesi Coğr. Ens. Derg. (İstanbul University, Journal of Geography Institute) Vol. 4, No 7: 107-123, İstanbul, Turkey. Türkeş, M. 1990. Drought Regions and Severe Drought Years (in Turkish), Ph.D. Thesis, İstanbul University, Sea Sciences and Geography Institute, İstanbul, Turkey. Türkeş, M. 1996. “Spatial and Temporal Analysis of Annual Rainfall Variations in Turkey”, International Journal of Climatology, 16, 1057-1076. Wilhite, D.A., and H.Glantz, M.H.1985. "Understanding the Drought Phenomenon: The Role of Definitions." IWRA, Water International 10(33), 110–120. Yeğnidemir, M. K. 2005. Drought Analysis in the Central Anatolian Region Using the Standardized Precipitation Index (SPI) (in Turkish), M.Sc. Thesis, Kırıkkale University, Science Institute, Kırıkkale, Turkey.
