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Isotopic composition of precipitation in Greece
Journal of Hydrology (2006) 327, 486– 495
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/jhydrol
Isotopic composition of precipitation in Greece Athanassios A. Argiriou
a,*
, Spyros Lykoudis
a,b
a
University of Patras, Department of Physics, Section of Applied Physics, GR-265 00 Patras, Greece National Observatory of Athens, Institute for Environmental Research and Sustainable Development, GR-152 36 Palaia Pendeli, Greece
b
Received 9 February 2005; received in revised form 24 November 2005; accepted 28 November 2005
KEYWORDS Athens; Stable isotopes; Tritium; Precipitation; Rainfall
Summary The contribution of stable isotopes in meteorological, climatological and hydrological research is well known. Until this date and despite the fact that several hydrological studies of water sources in Greece have been published, no systematic isotopic study of precipitation has been performed in the country. This paper presents all the available isotopic data collected since 1960 in several Hellenic measurement stations. This data is divided in two periods: the first covers data that was collected in the past, in the frame of a preliminary survey of the isotope composition of precipitation in the Eastern Mediterranean Sea and specific hydrological studies; the second is the result of a three-year coordinated research project of the International Atomic Energy Agency, in which the authors participated, aiming at the systematic study of stable isotopes (2H and 18O) and 3H in precipitation around the Mediterranean basin. No statistically significant behavior between the two periods of data was found. The isotopic content of precipitation presents characteristics intermediate of those of the Eastern and Western Mediterranean. The tritium concentration in precipitation declines as expected towards the pre-bomb levels, however there is an indication that tritium concentrations are higher in Northern Greece. 2005 Elsevier B.V. All rights reserved.
Introduction The importance of deuterium (2H), tritium (3H) and 18O in hydrological, meteorological and climatological applications has been demonstrated in many publications (e.g., IAEA, 1981; Rozanski et al., 1993; Bar-Matthews et al., 1999). Spatial and temporal variations in the isotopic composition of precipitation are due to isotopic fractionation
* Corresponding author. Tel.: +30 2610 996078. E-mail address: [email protected] (A.A. Argiriou).
occurring during the evaporation of seawater and condensation during the advection of water vapor (Dansgaard, 1964). Also the isotopic composition of local precipitation is primarily controlled by regional scale processes, like the trajectories of the water vapor transport over the continents and the average rainout history of the air masses giving precipitation at a particular place (e.g., Rozanski et al., 1982). Stable isotopes can therefore be used to investigate the precipitation formation conditions, and to monitor their changes as well as the impact on water resources, in relation to observed and to expected climatic changes.
0022-1694/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.11.053
Isotopic composition of precipitation in Greece The isotopic composition of hydrogen and oxygen in water is expressed in units of d2H and d18O, respectively. In precipitation these quantities follow in general the socalled meteoric line equation: d2H = 8d18O + d. The parameter d, termed ‘‘deuterium excess’’ (D-excess) (Dansgaard, 1964), depends among others, on the location: the worldwide mean of D-excess was first reported by Craig (1961) to be 10&. Using long-term arithmetic mean values from 206 stations Rozanski et al. (1993) reported a value very close to that of Craig (10.35 ± 0.65)&, while Bowen and Revenaugh (2003) using global maps derived by interpolation from more than 340 stations, report a global value of D-excess practically equal to that obtained by Craig. Large variability is observed across the globe but on local scales as well. Values higher than 15& are found in the Eastern Mediterranean (Gat and Dansgaard, 1972; Bowen and Revenaugh, 2003), with a gradient of 6& from the Aegean to the coast of Israel (IAEA, 2001). The meteoric line for the Eastern Mediterranean is estimated to be on average d2H = 8d18O + 20& (IAEA, 2001; Bowen and Revenaugh, 2003; Aouad et al., 2004) while for the Western Mediterranean it is d2H = 8d18O + 13.7& (Celle-Jeanton et al., 2001a). This reflects the difference of origin and the vapor supply and removal history of the air masses over the two areas, which is also observed in recent isotopic data of atmospheric vapor collected across the Mediterranean Sea (Gat et al., 2003). The area of the Mediterranean basin is sensitive to climatic change (IPCC, 2001). It is therefore important to be able to predict the availability of water resources both under the current and the future climatic conditions. General circulation models are able to simulate synoptic circulation patterns and through them predict temperature and precipitation fields. Yet these require extensive verification. This can be achieved using the stable isotope composition of precipitation, which is a very useful tracer of the atmospheric moisture transport systems (Hoffmann et al., 2000; Yoshimura et al., 2004). Due to the multitude of the factors involved, detailed knowledge of the local relationships between the stable isotopic composition of water vapor and precipitation, and atmospheric circulation patterns, is needed. The isotopic composition of precipitation over many specific areas has been analyzed in relation to synoptic circulation patterns and air mass trajectories (e.g., Rindsberger et al., 1990; Cruz-San Julian et al., 1991; Celle-Jeanton et al., 2001b; Celle-Jeanton et al., 2004; Aouad et al., 2004; Asaf et al., 2005) with meteorological or hydrological aims. Also local spatial and temporal variations have been investigated and new data were introduced to the global stable isotope database (El-Asrag et al., 2003; Longinelli and Selmo, 2003; Boronina et al., 2005). Precipitation in Greece is produced from air masses originating either from the Atlantic or from the Mediterranean (Flocas and Giles, 1991). It is therefore expected that the isotopic signature of precipitation will vary significantly depending on the origin of the water vapor. However and despite the fact that several isotopic studies of spring waters of Greece have been published in the past (e.g., Payne et al., 1978; Leontiadis et al., 1988; Leontiadis et al., 1996), only precipitation isotopic data from the sixties have been reported for this area (Gat and Carmi, 1970). Precipitation isotopic data have been collected in
487 the past in several locations, but within a time span suitable only for surface water studies. Consequently, the available data have important gaps. The International Atomic Energy Agency sponsored a three-year program aiming among others to establish a common database of precipitation isotopic data around the Mediterranean basin. During this period the isotopic composition of precipitation in terms of 2H, 3H and 18 O has been determined in four Greek locations. This paper reviews the data collected during this systematic three-year campaign, along with the data obtained in the past. The spatial and temporal variations of oxygen and hydrogen isotopes in the precipitation of this northeastern part of the Mediterranean basin are investigated in order to provide basic information and identify the locally significant parameters that affect stable isotopic distributions. The data used in this paper cover two distinct periods termed A and B (see Table 1). During period A only monthly samples were collected. During period B – the systematic measurement campaign period – monthly samples were collected in the locations of Athens (city center – Thission), Patras (Western Greece) and Thessaloniki (Northern Greece), while event-based samples were collected in Athens (northern suburbs – Pendeli) (Fig. 1). Information regarding the stations and the number of samples available is provided in Table 1.
Stable isotopes The spatial and temporal distribution of stable isotope abundances depends on a number of weather parameters that characterize the origin and the vapor supply and removal history of the air moisture generating the precipitation, and the particular sampling site. In the mid-latitude regions of the northern hemisphere, where Greece is located, the isotopes are correlated with the ambient temperature, the seasonal variation of which, affects the amount of precipitable water. This is due to the variation of rainout as the air masses move from the vapor source to the sampling station. This leads to more depleted values during winter, compared to those during summer. This phenomenon is further enhanced as the moisture moves from the coast to the mainland. This is the so-called continentality effect. Inland stations show more depleted values compared to the coastal ones. The continentality effect is less marked during summer because of recycling of rainwater due to evaporitranspiration. The complexity of the terrain is another factor that influences the isotopic signature in a region: sites located at the lee side of the moist air path show depleted values since precipitation is induced orographically at the other side of the mountain. The altitude changes the isotopic content of precipitation also, since moist air masses as uplifted, precipitate the heavier isotopes first. Stable isotope data are examined in order to establish the local relationships between d2H and d18O as well as the effects of temperature and seasonality, precipitation amount and altitude on the isotopic composition of precipitation. Furthermore the data of the two periods are examined for possible trends. Monthly data are the most commonly available and most of the aforementioned isotopic effects have been documented on this time basis.
9.0 ± 1.6 12.1 ± 1.7 38.9 ± 3.6 32.9 ± 2.6 9.9 ± 1.3 15.3 ± 1.3 45.0 ± 1.1 36.7 ± 1.0 6.86 ± 0.12 6.50 ± 0.11
5.99 ± 0.49 5.63 ± 0.38
11.3 ± 1.5 12.9 ± 2.0 35.2 ± 3.7 35.0 ± 4.4 12.9 ± 1.3 16.7 ± 1.3 40.2 ± 1.1 45.2 ± 1.2
40.61 37.97 Thessaloniki Thission
22.96 23.72
32 107
10/2000–9/2003 00/2000–03/2003 04/2003–05/2003 10/2000–08/2003 10/2000–12/2003 38.28 38.05 Period B Patras Pendeli
21.79 23.86
100 495
Monthly Event Monthly Monthly Monthly
29/28 163/163 2/2 33/33 33/33
6.62 ± 0.11 7.73 ± 0.12
5.80 ± 0.48 6.0 ± 0.60
13.8 ± 1.5 17.3 ± 1.5 12.4 ± 2.0 15.2 ± 1.9 33.4 ± 2.3 32.8 ± 2.9 29.0 ± 2.9 25.3 ± 2.4 6.10 ± 0.26 6.29 ± 0.36 5.17 ± 0.42 5.11 ± 0.32 14.1 ± 1.3 17.1 ± 1.3 12.8 ± 1.3 15.7 ± 1.3 34.8 ± 1.0 36.0 ± 1.0 29.2 ± 1.0 23.5 ± 1.0 11/1960–11/1991 12/1960–01/1987 01/1963–01/1968 01/1963–04/1987 37.90 35.33 36.83 36.46
23.73 25.18 21.72 28.10
27 47 33 42
Monthly Monthly Monthly Monthly
61/50 22/23 10/10 23/18
6.36 ± 0.10 6.67 ± 0.11 5.24 ± 0.11 4.96 ± 0.10
d18O (&) D-excess (&) d2H (&) d18O (&)
Period A Hellinikon Heraklion Methoni Rhodes
Station
Latitude ()
Longitude ()
Alt (m)
Period
Sample type
Sample # d18O/d2H
xw uðxw Þ
x uðxÞ
d2H (&)
D-excess (&)
A.A. Argiriou, S. Lykoudis Table 1 Precipitation sampling stations across Greece and corresponding, long-term, precipitation amount weighted as well as simple arithmetic mean values xw ; x and combined standard uncertainties uðxw Þ; uðxÞ, for d18O, d2H and D-excess
488
Figure 1
Location of Greek sampling stations.
Event-based data, on the other hand, due to their temporal resolution, allow a better investigation of specific aspects of these effects, as well as possible correlation to other environmental parameters such as synoptic weather systems. Long-term values of d18O, d2H and D-excess calculated both as simple arithmetic and as precipitation amount weighted means, are presented in Table 1. Also reported are standard uncertainties combining the statistically induced uncertainty represented by the standard deviation of the mean (Type A), and the analytical uncertainty reported by the analyzing laboratory (Type B), according to the Guide to the Expression of Uncertainty (GUM), (ISO, 1995). Analytical standard uncertainties for d18O were taken to be ±0.1& and for d2H ± 1&. (Gat and Carmi, 1970; Aragua ´s-Aragua ´s, 2005). For the calculation of the standard deviation, rðxw Þ, of the weighted mean values, xw , the following formula was used: !#)1=2 ( ," X X X 2 rðx w Þ ¼ pi ðxi xw Þ pi pi 1 ; i
i
i
where pi is the precipitation amount corresponding to each sample (SAS, 1999). Also, since the calculation of combined standard uncertainties requires that Type B uncertainties are constant for all measurements considered, the data from Pendeli were entered as events in all calculations. Whenever this was not possible, an average Type B uncertainty was calculated and used instead. The number of significant digits in reporting all values, measured or calculated, was defined in accordance to Wilrich (2005). The results of Table 1 reveal a difference in the behavior of the data collected during period A as compared to the data of the recent period (period B). For period A stations, the weighted and arithmetic mean values, of all the presented parameters, cannot be considered as different within the calculated uncertainty limits. On the other hand, the d18O and d2H weighted means during period B, are much
Isotopic composition of precipitation in Greece
489
lower than the corresponding arithmetic ones reflecting the fact that 8 out of 10 lowest monthly d18O values in our database, occurred during the cold and rainy seasons of this period. These differences suggest an increased intensity of the heavy frontal precipitation episodes, and this was indeed the case in many areas of Greece during that period. Notably the Thission station, with a climatic normal precipitation height of 375 mm, in 2002 reported a record (since 1841) precipitation amount of 985 mm. The precipitation amount recorded in the Thessaloniki station during this year also presented an annual amount of about 30% higher than the normal.
Local meteoric water lines Fig. 2 illustrates the relation d2H = f(d18O) for all sampling stations. The monthly d18O values range between +2& and 14&; the d2H values range between +20& and 100&, in accordance to what is expected in these latitudes. The three most 18O-enriched cases correspond to light summer precipitation with intense evaporation of the raindrops beneath the cloud base-surface air temperatures around 26– 27 C – and present characteristically negative D-excess values (Fro ¨hlich et al., 2002). Best-fit lines, using monthly
40 All monthly data - Greece
20
-20 LWML GMWL Hellinikon Heraklion Methoni Patras Pendeli Rhodes Thessaloniki Thission
-40
2
δ H (‰)
0
-60 -80 -100 -120 -16
-14
-12
-10
-8
-6
-4
-2
0
2
4
18
δ Ο (‰ )
Figure 2
Deuterium and d18O relationship for Greece.
data (weighted mean values for Pendeli) were calculated using three regression models. Ordinary least squares regression (OLSR) is the most commonly used approach, yet it has significant shortcomings when it is applied to real life data. The basic assumption of OLSR, that x-values, in our case d18O, are exactly known thus error-free, is clearly violated. A better approach is the so-called reduced major axis or orthogonal distance least squares regression (ODLSR) (IAEA, 1992) that allows for x-values to have a standard deviation. ODLSR can handle cases where the ratio of y and x-value standard deviations is constant. Finally a full error-in-variables generalized least squares regression model (GENLS) was used, that is suitable for cases where the errors of both x and y-values are not constant and can also incorporate analytical or other Type B uncertainties (Macdonald and Thompson, 1992). The best GENLS fit line – the Local Meteoric Water Line (LMWL) – together with the Global Meteoric Water Line (GMWL), defined by the equation d2H = 8d18O + 10&, are plotted in Fig. 2. The results presented in Table 2 show that the stations sampled during the period 1960–1990 (period A) have higher slopes than those of period B (2000–2003). Furthermore, period A slopes cannot be considered different from that of the GMWL, on a 95% confidence level. Methoni has a different behavior possibly due to the limited number of available data. On the other hand, all period B slopes are lower than 8, with more pronounced those of the stations located in the Athens area (Pendeli and Thission) possibly indicating some non-equilibrium evaporation processes during the fall of the drops below the cloud base (Dansgaard, 1964). This pattern could seem to be indicating a change towards warmer and less humid climatic conditions over Greece, that would account for this assumed evaporation increase, yet this is not the case. A more detailed examination of the data reveals that the older data consist mainly of winter time precipitation, that is normal for Greece, while the data collected during the 2000–2003 period contain a significant proportion of summer precipitation as well, thus reflecting erroneously a climatic change towards warmer and drier conditions. As a matter of fact, the precipitation amount in Greece during that period increased, especially during
Table 2 Least squares regression results for the relationship d2H = a Æ d18O + b, using ordinary least squares regression (OLSR), orthogonal distance least squares regression (ODLSR), and errors-in-variables generalized least squares regression (GENLS) Station
OLSR
ODLSR
GENLS
a ± ra
b ± rb
a ± ra
b ± rb
a ± ra
b ± rb
Period A Hellinikon Heraklion Methoni Rhodes
7.50 ± 0.43 7.86 ± 0.47 5.9 ± 1.2 6.6 ± 1.4
10.8 ± 2.7 16.4 ± 3.1 1±6 8±7
8.00 ± 0.42 8.14 ± 0.45 6.7 ± 1.0 8.5 ± 1.3
13.8 ± 2.6 18.2 ± 3.0 6±6 18 ± 7
7.88 ± 0.44 8.08 ± 0.48 6.4 ± 1.2 8.1 ± 1.6
13.1 ± 2.8 17.8 ± 3.2 4±6 16 ± 8
Period B Patras Pendeli Thessaloniki Thission
7.28 ± 0.30 6.38 ± 0.42 6.98 ± 0.31 6.09 ± 0.41
7.2 ± 1.9 3.2 ± 2.9 3.0 ± 2.1 1.4 ± 2.5
7.44 ± 0.29 6.77 ± 0.41 7.20 ± 0.30 6.51 ± 0.40
8.1 ± 1.8 5.6 ± 2.8 4.3 ± 2.0 3.7 ± 2.4
7.39 ± 0.30 6.61 ± 0.54 7.13 ± 0.31 6.36 ± 0.42
7.8 ± 1.9 3.8 ± 3.3 3.8 ± 2.1 2.8 ± 2.5
All data
6.86 ± 0.17
6.2 ± 1.1
7.31 ± 0.16
8.8 ± 1.1
7.24 ± 0.18
8.2 ± 1.1
490 the summer months, yet the precipitation regime has returned to normal during the past three years (2003–2005). The higher than 10& intercepts reported for the period A stations, except Methoni, indicate that the Mediterranean sea is a significant source of water vapor for the precipitation over Greece.
D-excess Table 1 summarizes the calculated D-excess values for all stations and periods.With the exception of Thessaloniki, these vary within the range defined by the respective values for the Western (13.7&) and Eastern Mediterranean (20&). Their spatial distribution follows closely the one presented by IAEA (2001). The lowest weighted mean D-excess value is observed at Thessaloniki in the north, followed by the western stations of Patras and Methoni, the southeastern station of Rhodes and finally reaching a maximum value at Heraklion in the south. The Athens stations, Hellinikon, Thissio and Pendeli also present high D-excess values. The low D-excess values of Thessaloniki can be attributed to the unusually high proportion of the warm period – May to October for the local climatic conditions (Maheras, 1988; Argiriou et al., 2004) – precipitation (of about 44%) that is also characterized by low D-excess values due to evaporation of raindrops under the cloud base. The western stations of Methoni and Patras have lower D-excess values possibly because they receive relatively larger amounts of precipitation originating either from the Atlantic or the Western Mediterranean. On the other hand, the precipitation at the remaining stations, especially Rhodes and Heraklion, is more likely due to water vapor originating from the Aegean or the Mid-eastern Mediterranean. This is in accordance with the D-excess values of vapor samples collected in several locations in the Mediterranean Sea (Gat et al., 2003). D-excess values for the Athens stations present a rather unusual pattern: Hellinikon and Thission, situated within the city, show significantly lower than that of Pendeli, that is located some 10–15 km to the North and at a higher elevation. Since a raise of Pendeli’s D-excess by addition of re-evaporation of precipitated water is not a plausible argument for such short distances, it is the evaporation of raindrops beneath the clouds that should be examined as a possible explanation of the observed D-excess pattern. From this point of view it is Thission and Hellinikon that should be regarded as presenting lower D-excess values compared to Pendeli, due to an increased evaporation of raindrops below the cloud. The fact that, throughout the year, the surface air temperature in both Hellinikon and Thission is 2–3 K higher than that of Pendeli and the relative humidity is generally low in all stations, could support this argument, yet this point needs further investigation. Processing of monthly data from the global isotopic network has shown that in case of oceanic precipitation D-excess values are scattered; in continental stations, although the range of the d18O values is wider compared to that of the marine stations, the scatter of the D-excess values is rather reduced. This characteristic is quantified by the ratio of the range of D-excess, calculated as the difference between the maximum and minimum monthly values to the range of the d18O values (Gat, 2001). This ratio
A.A. Argiriou, S. Lykoudis for European coastal stations is 2.0, 2.3, 1.8 and 2.8 (Valentia – Ireland, Reykjavik – Iceland, Faro – Portugal and Weathership E – North Atlantic). For continental stations the following values are provided: 0.4 (Berlin), 0.35 (Krakow) and 0.7 (Vienna) (Gat, 2001). For the Greek stations it takes values between 1.7 and 5.3, therefore all stations have clearly coastal characteristics, and indeed all are located from a few hundred meters to a few kilometers from the coastline. These characteristics appear to be more pronounced in Rhodes, and Methoni. This could be attributed to the fact that the precipitation over these two stations has the characteristics of a first condensate of the vapor, due to the tracks of the weather systems affecting the area. However, according to this explanation, the ratio for the Heraklion and Patras stations should have had a similar value, but the contrary occurs, with these stations having the lowest values (2.2 and 1.7, respectively). The remaining three stations have ratio values between 2.4 and 3.0, while the overall value for Greece is 3.0. In order to assess whether the mean D-excess values differ statistically between stations the t-test was applied to all possible pairs of stations, using the weighted means and the combined standard uncertainties presented in Table 1, as sample variances. The obtained results show that there are only three pairs of stations for which the hypothesis of equal mean values cannot be rejected at a 95% confidence level. For one of these pairs, the western stations of Patras and Methoni, this is directly justified because of the similarity of the climatic and meteorological conditions of these stations. Thessaloniki stands alone in the North while the Athens’ stations exhibit variable affiliations with the southern stations of Rhodes and Heraklion but not between them. The variation of D-excess is illustrated in Fig. 3. From Fig. 3(a) it can be seen that the D-excess determined from the precipitation episodes at the Pendeli station, varies with time as expected, i.e., showing maximum values during winter and minimum during summer. The weighted mean D-excess values for each calendar month for the stations of Thessaloniki, Pendeli, Thission and Hellinikon are presented in Fig. 3(b) and for Patras, Methoni, Rhodes and Heraklion in Fig. 3(c). Precipitation intensity seem to be a key parameter affecting the isotopic composition of monthly precipitation samples over Greece since the stations of the Athens area (Hellinikon, Pendeli, Thission), that is characterized by the typical dry Mediterranean precipitation regime (average intensity frontal precipitation during winters and few thermal storms giving high intensity precipitation during summer) present marked minima during summer that could be explained by the few intense rainstorms that occurred, aided by the high air temperature–low humidity conditions that enhance the precipitation amount effect. The remaining stations present a weaker annual variation with maxima during either winter or during the transitional seasons and minima during summer. The temporal profiles of D-excess variation do not show any statistically significant trend, neither on a monthly nor on an event basis. The only exception is the Hellinikon station, where a negative linear trend is detected over the period 1960–1974. This trend is statistically significant over a 95% confidence interval, however this finding could not be
Isotopic composition of precipitation in Greece
491
40
Pendeli - Events 30
4
40
2
20
0 0
-2
δ O (‰)
18
10 0
-20
-6
-40
-8
-60
-10 -80
-10
-12
----
-14
-20
-16 0.0
a -30 09/00
12/00
03/01
06/01
09/01
12/01
03/02
06/02
09/02
5.0
15.0
20.0
18
δ O = 0.192•T (˚C) - 8.84‰
-100
2
δ H = 1.08•T(˚C) - 50.8‰
25.0
30.0
-120 35.0
Air temperatuture (˚C)
Figure 4 Variation of d18O and d2H as a function of temperature.
20 15
2 Thessaloniki
10
0
Pendeli
5
-5
18
Pendeli Thission
δ O (‰)
0
b Jan
Hellinikon
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Thission
-2
Thessaloniki
-10
Hellinikon
-4 -6 -8
Dec
25
-10
20
-12 Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
15
2 Patras
10
0
Methoni
5 Patras Methoni
-5
Rhodes
-10
c Jan
Heraklion
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 3 Variation of D-excess with time: (a) episodes; (b) and (c) weighted monthly means.
Rhodes
-2
18
0
δ O (‰)
D-excess (‰)
10.0
01/03
25
D-excess (‰)
-4
δ2H (‰)
D-excess (‰)
20
Heraklion
-4 -6 -8 -10 -12 Jan
confirmed using another, non-parametric method, because of the gaps in the time series. Treating the Hellinikon and Thission series as one, since both stations are located in Athens, did not produce any trend.
Temperature and seasonal effect The relation between monthly d18O and d2H values and surface air temperature for all available data is shown in Fig. 4. The observed behavior is as expected for mid-latitude stations, i.e., the lower the temperature, the more depleted the precipitation. Thission, Hellinikon, Methoni and Rhodes have lower slopes than the aggregate, while the rest of the stations have steeper slopes. The weighted mean d18O values, for each calendar month, are presented in Fig. 5. The d2H variations follow those of d18O and are not shown. The stable isotopes variation presents the expected pattern following closely that of the surface air temperature, with the exception of Thessaloniki, Pendeli and Thission. These stations have very
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
18
Figure 5 Variation of weighted monthly mean d O values with time.
depleted d18O during February and December. A close inspection of the data revealed that these months include, as a rule, the minimum d18O samples, usually collected under rather extreme meteorological conditions, like the December 2001 sample from Thessaloniki, with d18O = 13.96& corresponding to a mean monthly temperature of 2.5 C compared to the climatic normal of 6.9 C, and the samples of February 2003 from Thission and Pendeli that include water from a heavy snowfall. The contrasting minimum of Pendeli during June 2001 represents a single very weak precipitation event (0.4 mm).
Amount effect Fig. 6 illustrates the variation of d18O as a function of the precipitation amount. On this graph, data from all stations
492
A.A. Argiriou, S. Lykoudis
have been reported distinguishing winter and summer precipitation. On an annual basis, it can be seen that up to an amount of about 100 mm of precipitation per month, the depletion increases with the precipitation amount whereas for higher monthly amounts this behavior does not appear. Considering the d18O – precipitation amount relationship on a seasonal basis though, it can be seen that during winter there is very slight isotopic depletion trend with increasing amount, while during summer the depletion is more intense and of different shape, suggesting a difference in the seasonal mechanisms generating precipitation over Greece. Winter precipitation over Greece is mainly generated by frontal depressions that approach Greece from the northwest or west, whereas summer precipitation is mainly generated by thermal instability induced either by locally developed thermal lows or depressions approaching Greece from southwest (Maheras, 1983; Flocas and Giles, 1991). The winter frontal depressions’ tracks cause southwesterly winds over Greece that force stagnating maritime air eastwards. This air is potentially unstable and its instability is released by orographic uplift and frictional convergence when it reaches mainland Greece. On the other hand thermal lows are associated with convective instability that can produce precipitation either as short-lived intense thermal storms or as showers from trailing fronts or weak depressions coming from southwest. The temporal evolution of the isotopic composition of convective precipitation events is expected to be L-shaped, meaning a sharp decrease followed by a more or less stable isotopic composition, whereas that of frontal precipitation is V and sometimes W-shaped (Rindsberger et al., 1990; Celle-Jeanton et al., 2004). The extent of overall depletion during an event may depend on the intensity and the duration of the event, but this is mainly determined by the prevailing synoptic circulation patterns. Summer precipitation over Greece, primarily of convective origin, presents the expected isotopic depletion versus amount behavior. Several light showers with high d18O can be identified, followed by a sharp d18O decrease up to a 50 mm amount and remain rather constant beyond that value. The non-linear relationship between d18O and the precipitation amount is obvious since the R2 for a linear fit is 0.11 while for the logarithmic fit presented in Fig. 6 the R2 is 0.30.
4
During winter, on the other hand, the relationship between d18O and the precipitation amount is linear even though their correlation appears to be very weak. Winter precipitation over Greece is mainly of frontal character and the associated V-shaped relationship between depletion and amount tends to smooth out the overall effect, when considered on an aggregate basis. Furthermore there are two stations, namely Rhodes and Methoni that were previously identified as presenting a first-condensate behavior. These stations show extremely weak, if any, correlation between precipitation amount and d18O throughout the year, possibly indicating a continuous supply of moist air masses as the systems move across extended marine regions before reaching these stations. Should these two stations be excluded, Fig. 6 would present a more plausible decreasing pattern for winter. Fig. 7 illustrates the evolution of d18O during the event of 24–25/03/2002. The bars illustrate the precipitation rate, while the line with the marks shows the d18O values. The pattern observed is typical of a frontal event with a sharp drop of d18O when the precipitation rate intensifies between 11:00 and 13:00 h. Then the d18O value increases again, possibly as the result of mixing of depleted rainwater from the second half of the rainstorm and water from the last part of the rainstorm that is being re-enriched as the drops leaving the base of the cloud travel through a d18O rich environment.
Tritium Tritium in the atmosphere is a result of the interaction of cosmic radiation with various atmospheric components. In the beginning of the sixties however, the atmospheric nuclear tests resulted to an increase of the tritium concentration in the atmosphere. The activity of 3H is expressed in tritium units (TU); 1 TU is equal to 1 3H atom in 1018 1H atoms. The tritium activity in precipitation before the tests was of about 5 TU in central Europe. This number reached an average of about 5000 TU in northern Europe in 1963 (Gat, 2001). After the nuclear tests ban, the tritium activity started decreasing more or less exponentially and today it has reached the values of the pre-test era. Tritium measurements from period A extend from 1960 to 1991. Heraklion, Rhodes and Hellinikon are represented by a significant number of samples (116, 92 and 72, respectively) while the during period B Thessaloniki and Patras
2 0
18
δ O (‰)
-2 -4 -6 -8 without Rhodes and Methoni
-10
18
-12
--- δ O = -0.0031•P - 6.21‰
-14
δ O = -0.0117•P - 5.88‰ - - δ18O = -0.77•Ln(P) - 2.25‰
18
-16 0.0
50.0
100.0
150.0
200.0
250.0
300.0
Precipitation (mm)
Figure 6 Variation of d18O as a function of the monthly amount of precipitation for winter (circles) and summer (triangles).
Figure 7 Variation of d18O (line) as a function of the amount of precipitation during a single precipitation event (bars).
Isotopic composition of precipitation in Greece 10000
Hellinikon Heraklion Methoni Patras Rhodes Rhodes Thessaloniki Thission
1000
Tritium (TU)
493
100
Conclusions This paper attempts for the first time a comprehensive survey of the composition of precipitation over Greece in stable isotopes (2H, 18O) and 3H. All sparse data from past together with the results of a recent systematic measurement campaign have been presented and analyzed. The complete dataset consists of two distinct periods:
10
1 12/60 12/64 12/68
Figure 8
12/72 12/76 12/80 12/84 12/88 12/92 12/96
12/00
Tritium variation for all stations.
have only 1 full year of data. The results from all stations are shown in a graphic form in Fig. 8. The tritium variation follows the expected exponential decrease as for the other stations around the world. For each curve of Fig. 8, the slope of the line ln (TU) = a + b (Date) was calculated. The obtained results showed that these slopes are not statistically different. It is therefore concluded that the decrease rate of tritium concentration is practically the same for all the stations. The tritium concentrations of the current period (B) are shown in detail in Fig. 9. Although the number of data available for this period is still small to allow definitive conclusions, it is interesting to observe that while the tritium concentrations in Patras and Athens (Thission) are practically the same, the corresponding values in Thessaloniki are, from spring to autumn, two or three times higher. The relative increase in concentration between spring and autumn observed both in the samples of Patras and Thessaloniki, can be explained by the well documented ‘‘spring leak of the tropopause’’ (Gat, 2001). The higher tritium levels at Thessaloniki, might be the result of transport from tritium emitting installations such as the Bulgarian nuclear power plant at Kozloduy located some 350 km north of Thessaloniki. It should be noted that elevated tritium levels in the precipitation of Swiss stations, attributed to the emissions of nuclear power plants, are comparable to the levels observed in Thessaloniki, if one takes into account the distance between the plant and the city, the corresponding atmospheric dispersion and the prevailing synoptic circulation patterns (Schu ¨rch et al., 2003).
18 16
Patras Thessaloniki
14
Tritium (TU)
Thission
12 10 8 6 4 2 0 10/00
Figure 9
12/00
01/01
04/01
06/01
08/01
10/01
12/01
Tritium variation during the period 2000–2001.
• Period A, when only monthly samples were collected in several locations, but not on an identical systematic basis in all of them. • Period B, during which monthly samples were collected systematically in three locations, situated at the central, western and northern part of the country. During this period and for one of the above locations, samples were collected also during precipitation episodes. For the Period A stations the arithmetic and weighted mean values of the various parameters characterizing the signature of the stable isotopes in precipitation, cannot be considered as statistically different within the calculated uncertainty limits. This is not the case between the arithmetic and weighted mean values of d2H and d18O values for the Period B stations, reflecting thus the increased intensity of heavy frontal precipitation episodes in Greece during that period (2000–2003). Concerning the Local Meteoric Water Lines (LMWL), their slopes for the Period A stations cannot be considered to be different from the slope of the GMWL (8&) on a 95% confidence level, while the LMWLs of Period B have slopes lower than 8&. This is due to the fact that summer precipitation during the Period B (2000–2003) had increased significantly compared to Period A (1960–1990). The spatial distribution of the average D-excess is not homogeneous, reflecting the differences in the origin and history of water vapor masses generating precipitation over the Western, Central and Eastern parts of the country. Differences in average D-excess values were observed also between the stations located in the greater Athens area, i.e., those of Pendeli (northern semi-urban site) and Hellenikon and Thission located in the main urban canopy. The last two urban stations have lower D-excess values compared to the semi-urban Pendeli. This could be explained by the fact that the ambient temperature in Pendeli is 2–3 K lower compared to that of the urban stations possibly leading to less D-excess reduction due to evaporation of raindrops beneath the cloud base. The temperature and seasonal dependence of the stable isotopic content in all stations was found to be as expected in mid-latitudes. Summer precipitation versus amount presents the expected isotopic depletion. During winter, the relationship between d18O and precipitation amount, although weakly correlated, appears to be linear, reflecting the frontal character of winter precipitation in Greece. During summer a logarithmic shape represented a better fit on the data, implying a predominantly convective character of the precipitation. Finally, tritium concentrations in precipitation collected at the Greek stations during all those years, followed the same trend as in all other stations around the world and
494 are actually at the pre-test era levels. However, tritium concentrations in the Thessaloniki precipitation samples, which all have been collected during Period B, were found to be systematically higher compared to those of Athens and Patras. This could be attributed to emissions of nuclear power plants located in northern neighboring countries, dispersed over Northern Greece.
Acknowledgements This work has been partly financed by the International Atomic Energy Agency (IAEA), Department of Nuclear Sciences and Applications, Isotope Hydrology Section, through the Co-ordinated Research Project ‘‘Isotopic Composition of Precipitation in the Mediterranean Basin, in relation to Air Circulation Patterns and Climate’’ (IAEA Contract No. 11319). The authors acknowledge the assistance of Dr. Laurence Gourcy, in charge of the project. The samples have been analyzed at the Isotope Hydrology Laboratory of the IAEA. The authors are grateful to Prof. Joel R. Gat for providing the 1960–1974 datasets and for his critical review of the manuscript.
References Aouad, A., Travi, Y., Blavoux, B., Job, J-O., Najem, W., 2004. Isotope study of snow and rain on Mount Lebanon: preliminary results. Hydrolog. Sci. J. 49 (3), 429–441. Aragua ´s-Aragua ´s, L., 2005. Personal communication. Argiriou, A., Kassomenos, P., Lykoudis, S., 2004. On the methods for the delimitation of seasons. Water Air Soil Poll: Focus 4 (4), 65–74. Asaf, L., Nativ, R., Hassan, M.A., Shain, D., Geyer, S., Ziv, B., 2005. Influence of small- and large-scale variables on the chemical and isotopic compositions of urban rainwater, as illustrated by a case study in Ashdod, Israel. J. Geophys. Res. 110 (D11), D11307. Bar-Matthews, M., Ayalon, A., Kaufman, A., Wasserburg, G.J., 1999. The Eastern Mediterranean paleoclimate as a reflection of regional events: Sorec cave, Israel. Earth Planet. Sci. Lett. 166, 85–95. Boronina, A., Balderer, W., Renard, P., Stichler, W., 2005. Study of stable isotopes in the Kouris catchment (Cyprus) for the description of the regional groundwater flow. J. Hydrol. 308, 214–226. Bowen, G.J., Revenaugh, J., 2003. Interpolating the isotopic composition of modern meteoric precipitation. Water Resour. Res. 39, 10, art. no 1299. Celle-Jeanton, H., Travi, Y., Blavoux, B., 2001a. Isotopic typology of the precipitation in the Western Mediterranean region at three different time scales. Geophys. Res. Lett. 28 (7), 1215– 1218. Celle-Jeanton, H., Zouari, K., Travi, Y., Daoud, A., 2001b. Isotopic characterisation of the precipitation in Tunisia. Variations of the stable isotope compositions of precipitation events related to the origin of air masses. CR. Acad. Sci. A Math. 333 (10), 625– 631. Celle-Jeanton, H., Gonfiantini, R., Travi, Y., Sol, B., 2004. Oxygen18 variations of rainwater during precipitation: application of the Rayleigh model to selected rainfalls in Southern France. J. Hydrol. 289 (1–4), 165–177. Craig, H., 1961. Isotopic variations in meteoric waters. Science 133, 1702–1703. Cruz-San Julian, J., Araguas, L., Rozanski, K., Benavente, J., Cardenal, J., Hidalgo, M.C., Garcia-Lopez, S., Martinez-Garrido,
A.A. Argiriou, S. Lykoudis J.C., Moral, F., Olias, M., 1991. Sources of precipitation over South-Eastern Spain and groundwater recharge. An isotopic study. Tellus 44B, 226–236. Dansgaard, W., 1964. Stable isotopes in precipitation. Tellus 16 (4), 437–468. El-Asrag, A.M., Al-Gamal, S.A., Youssef, A., Ahmed, D.M., 2003. Spatial and temporal variation of rainwater stable isotopes in Egypt and the east Mediterranean. Theor. Appl. Climatol. 74, 191–202. Flocas, A.A., Giles, B.D., 1991. Distribution and intensity of frontal rainfall over Greece. Int. J. Climatol. 14, 429–442. Fro ¨hlich, K., Gibson, J.J., Aggarwal, P.K., 2002. Deuterium excess in precipitation and its climatological significance. In: Proceedings of Study of Environmental Change Using Isotope Techniques, IAEA, Vienna, 2002, IAEA-CSP-13/P, 54–66. Gat, J.R., 2001. Volume II – atmospheric water. In: Mook, W.G., (Ed.), Environmental Isotopes in the Hydrological Cycle. IHP-V Technical Documents in Hydrology No. 39 – UNESCO, Paris. Gat, J.R., Carmi, I., 1970. Evolution of the isotopic composition of the atmospheric water in the Mediterranean Sea area. J. Geophys. Res. 75, 3039–3048. Gat, J.R., Dansgaard, W., 1972. Stable isotope survey of the freshwater occurrences in Israel and the Jordan Rift Valley. J. Hydrol. 16, 177–211. Gat, J.R., Klein, B., Kushnir, Y., Roether, W., Wernli, H., Yam, R., Shemesh, A., 2003. Isotope composition of air moisture over the Mediterranean Sea: an index of the air-sea interaction. Tellus 55B, 959–965. Hoffmann, G., Jouzel, J., Masson, V., 2000. Stable water isotopes in atmospheric general circulation models. Hydrol. Process. 14 (8), 1385–1406. IAEA, 1981. Deuterium and oxygen-18 in the water cycle. In: Gat, J.R., Gonfiantini, R. (Eds.), Stable Isotope Hydrology, IAEA Technical Report Series No. 210, International Atomic Energy Agency, Vienna. IAEA, 1992. Statistical Treatment of Data on Environmental Isotopes in Precipitation, Technical Reports Series No. 331, IAEA, Vienna, p. 781. IAEA, 2001. GNIP Maps and Animations, International Atomic Energy Agency, Vienna. Available from:
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/jhydrol
Isotopic composition of precipitation in Greece Athanassios A. Argiriou
a,*
, Spyros Lykoudis
a,b
a
University of Patras, Department of Physics, Section of Applied Physics, GR-265 00 Patras, Greece National Observatory of Athens, Institute for Environmental Research and Sustainable Development, GR-152 36 Palaia Pendeli, Greece
b
Received 9 February 2005; received in revised form 24 November 2005; accepted 28 November 2005
KEYWORDS Athens; Stable isotopes; Tritium; Precipitation; Rainfall
Summary The contribution of stable isotopes in meteorological, climatological and hydrological research is well known. Until this date and despite the fact that several hydrological studies of water sources in Greece have been published, no systematic isotopic study of precipitation has been performed in the country. This paper presents all the available isotopic data collected since 1960 in several Hellenic measurement stations. This data is divided in two periods: the first covers data that was collected in the past, in the frame of a preliminary survey of the isotope composition of precipitation in the Eastern Mediterranean Sea and specific hydrological studies; the second is the result of a three-year coordinated research project of the International Atomic Energy Agency, in which the authors participated, aiming at the systematic study of stable isotopes (2H and 18O) and 3H in precipitation around the Mediterranean basin. No statistically significant behavior between the two periods of data was found. The isotopic content of precipitation presents characteristics intermediate of those of the Eastern and Western Mediterranean. The tritium concentration in precipitation declines as expected towards the pre-bomb levels, however there is an indication that tritium concentrations are higher in Northern Greece. 2005 Elsevier B.V. All rights reserved.
Introduction The importance of deuterium (2H), tritium (3H) and 18O in hydrological, meteorological and climatological applications has been demonstrated in many publications (e.g., IAEA, 1981; Rozanski et al., 1993; Bar-Matthews et al., 1999). Spatial and temporal variations in the isotopic composition of precipitation are due to isotopic fractionation
* Corresponding author. Tel.: +30 2610 996078. E-mail address: [email protected] (A.A. Argiriou).
occurring during the evaporation of seawater and condensation during the advection of water vapor (Dansgaard, 1964). Also the isotopic composition of local precipitation is primarily controlled by regional scale processes, like the trajectories of the water vapor transport over the continents and the average rainout history of the air masses giving precipitation at a particular place (e.g., Rozanski et al., 1982). Stable isotopes can therefore be used to investigate the precipitation formation conditions, and to monitor their changes as well as the impact on water resources, in relation to observed and to expected climatic changes.
0022-1694/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2005.11.053
Isotopic composition of precipitation in Greece The isotopic composition of hydrogen and oxygen in water is expressed in units of d2H and d18O, respectively. In precipitation these quantities follow in general the socalled meteoric line equation: d2H = 8d18O + d. The parameter d, termed ‘‘deuterium excess’’ (D-excess) (Dansgaard, 1964), depends among others, on the location: the worldwide mean of D-excess was first reported by Craig (1961) to be 10&. Using long-term arithmetic mean values from 206 stations Rozanski et al. (1993) reported a value very close to that of Craig (10.35 ± 0.65)&, while Bowen and Revenaugh (2003) using global maps derived by interpolation from more than 340 stations, report a global value of D-excess practically equal to that obtained by Craig. Large variability is observed across the globe but on local scales as well. Values higher than 15& are found in the Eastern Mediterranean (Gat and Dansgaard, 1972; Bowen and Revenaugh, 2003), with a gradient of 6& from the Aegean to the coast of Israel (IAEA, 2001). The meteoric line for the Eastern Mediterranean is estimated to be on average d2H = 8d18O + 20& (IAEA, 2001; Bowen and Revenaugh, 2003; Aouad et al., 2004) while for the Western Mediterranean it is d2H = 8d18O + 13.7& (Celle-Jeanton et al., 2001a). This reflects the difference of origin and the vapor supply and removal history of the air masses over the two areas, which is also observed in recent isotopic data of atmospheric vapor collected across the Mediterranean Sea (Gat et al., 2003). The area of the Mediterranean basin is sensitive to climatic change (IPCC, 2001). It is therefore important to be able to predict the availability of water resources both under the current and the future climatic conditions. General circulation models are able to simulate synoptic circulation patterns and through them predict temperature and precipitation fields. Yet these require extensive verification. This can be achieved using the stable isotope composition of precipitation, which is a very useful tracer of the atmospheric moisture transport systems (Hoffmann et al., 2000; Yoshimura et al., 2004). Due to the multitude of the factors involved, detailed knowledge of the local relationships between the stable isotopic composition of water vapor and precipitation, and atmospheric circulation patterns, is needed. The isotopic composition of precipitation over many specific areas has been analyzed in relation to synoptic circulation patterns and air mass trajectories (e.g., Rindsberger et al., 1990; Cruz-San Julian et al., 1991; Celle-Jeanton et al., 2001b; Celle-Jeanton et al., 2004; Aouad et al., 2004; Asaf et al., 2005) with meteorological or hydrological aims. Also local spatial and temporal variations have been investigated and new data were introduced to the global stable isotope database (El-Asrag et al., 2003; Longinelli and Selmo, 2003; Boronina et al., 2005). Precipitation in Greece is produced from air masses originating either from the Atlantic or from the Mediterranean (Flocas and Giles, 1991). It is therefore expected that the isotopic signature of precipitation will vary significantly depending on the origin of the water vapor. However and despite the fact that several isotopic studies of spring waters of Greece have been published in the past (e.g., Payne et al., 1978; Leontiadis et al., 1988; Leontiadis et al., 1996), only precipitation isotopic data from the sixties have been reported for this area (Gat and Carmi, 1970). Precipitation isotopic data have been collected in
487 the past in several locations, but within a time span suitable only for surface water studies. Consequently, the available data have important gaps. The International Atomic Energy Agency sponsored a three-year program aiming among others to establish a common database of precipitation isotopic data around the Mediterranean basin. During this period the isotopic composition of precipitation in terms of 2H, 3H and 18 O has been determined in four Greek locations. This paper reviews the data collected during this systematic three-year campaign, along with the data obtained in the past. The spatial and temporal variations of oxygen and hydrogen isotopes in the precipitation of this northeastern part of the Mediterranean basin are investigated in order to provide basic information and identify the locally significant parameters that affect stable isotopic distributions. The data used in this paper cover two distinct periods termed A and B (see Table 1). During period A only monthly samples were collected. During period B – the systematic measurement campaign period – monthly samples were collected in the locations of Athens (city center – Thission), Patras (Western Greece) and Thessaloniki (Northern Greece), while event-based samples were collected in Athens (northern suburbs – Pendeli) (Fig. 1). Information regarding the stations and the number of samples available is provided in Table 1.
Stable isotopes The spatial and temporal distribution of stable isotope abundances depends on a number of weather parameters that characterize the origin and the vapor supply and removal history of the air moisture generating the precipitation, and the particular sampling site. In the mid-latitude regions of the northern hemisphere, where Greece is located, the isotopes are correlated with the ambient temperature, the seasonal variation of which, affects the amount of precipitable water. This is due to the variation of rainout as the air masses move from the vapor source to the sampling station. This leads to more depleted values during winter, compared to those during summer. This phenomenon is further enhanced as the moisture moves from the coast to the mainland. This is the so-called continentality effect. Inland stations show more depleted values compared to the coastal ones. The continentality effect is less marked during summer because of recycling of rainwater due to evaporitranspiration. The complexity of the terrain is another factor that influences the isotopic signature in a region: sites located at the lee side of the moist air path show depleted values since precipitation is induced orographically at the other side of the mountain. The altitude changes the isotopic content of precipitation also, since moist air masses as uplifted, precipitate the heavier isotopes first. Stable isotope data are examined in order to establish the local relationships between d2H and d18O as well as the effects of temperature and seasonality, precipitation amount and altitude on the isotopic composition of precipitation. Furthermore the data of the two periods are examined for possible trends. Monthly data are the most commonly available and most of the aforementioned isotopic effects have been documented on this time basis.
9.0 ± 1.6 12.1 ± 1.7 38.9 ± 3.6 32.9 ± 2.6 9.9 ± 1.3 15.3 ± 1.3 45.0 ± 1.1 36.7 ± 1.0 6.86 ± 0.12 6.50 ± 0.11
5.99 ± 0.49 5.63 ± 0.38
11.3 ± 1.5 12.9 ± 2.0 35.2 ± 3.7 35.0 ± 4.4 12.9 ± 1.3 16.7 ± 1.3 40.2 ± 1.1 45.2 ± 1.2
40.61 37.97 Thessaloniki Thission
22.96 23.72
32 107
10/2000–9/2003 00/2000–03/2003 04/2003–05/2003 10/2000–08/2003 10/2000–12/2003 38.28 38.05 Period B Patras Pendeli
21.79 23.86
100 495
Monthly Event Monthly Monthly Monthly
29/28 163/163 2/2 33/33 33/33
6.62 ± 0.11 7.73 ± 0.12
5.80 ± 0.48 6.0 ± 0.60
13.8 ± 1.5 17.3 ± 1.5 12.4 ± 2.0 15.2 ± 1.9 33.4 ± 2.3 32.8 ± 2.9 29.0 ± 2.9 25.3 ± 2.4 6.10 ± 0.26 6.29 ± 0.36 5.17 ± 0.42 5.11 ± 0.32 14.1 ± 1.3 17.1 ± 1.3 12.8 ± 1.3 15.7 ± 1.3 34.8 ± 1.0 36.0 ± 1.0 29.2 ± 1.0 23.5 ± 1.0 11/1960–11/1991 12/1960–01/1987 01/1963–01/1968 01/1963–04/1987 37.90 35.33 36.83 36.46
23.73 25.18 21.72 28.10
27 47 33 42
Monthly Monthly Monthly Monthly
61/50 22/23 10/10 23/18
6.36 ± 0.10 6.67 ± 0.11 5.24 ± 0.11 4.96 ± 0.10
d18O (&) D-excess (&) d2H (&) d18O (&)
Period A Hellinikon Heraklion Methoni Rhodes
Station
Latitude ()
Longitude ()
Alt (m)
Period
Sample type
Sample # d18O/d2H
xw uðxw Þ
x uðxÞ
d2H (&)
D-excess (&)
A.A. Argiriou, S. Lykoudis Table 1 Precipitation sampling stations across Greece and corresponding, long-term, precipitation amount weighted as well as simple arithmetic mean values xw ; x and combined standard uncertainties uðxw Þ; uðxÞ, for d18O, d2H and D-excess
488
Figure 1
Location of Greek sampling stations.
Event-based data, on the other hand, due to their temporal resolution, allow a better investigation of specific aspects of these effects, as well as possible correlation to other environmental parameters such as synoptic weather systems. Long-term values of d18O, d2H and D-excess calculated both as simple arithmetic and as precipitation amount weighted means, are presented in Table 1. Also reported are standard uncertainties combining the statistically induced uncertainty represented by the standard deviation of the mean (Type A), and the analytical uncertainty reported by the analyzing laboratory (Type B), according to the Guide to the Expression of Uncertainty (GUM), (ISO, 1995). Analytical standard uncertainties for d18O were taken to be ±0.1& and for d2H ± 1&. (Gat and Carmi, 1970; Aragua ´s-Aragua ´s, 2005). For the calculation of the standard deviation, rðxw Þ, of the weighted mean values, xw , the following formula was used: !#)1=2 ( ," X X X 2 rðx w Þ ¼ pi ðxi xw Þ pi pi 1 ; i
i
i
where pi is the precipitation amount corresponding to each sample (SAS, 1999). Also, since the calculation of combined standard uncertainties requires that Type B uncertainties are constant for all measurements considered, the data from Pendeli were entered as events in all calculations. Whenever this was not possible, an average Type B uncertainty was calculated and used instead. The number of significant digits in reporting all values, measured or calculated, was defined in accordance to Wilrich (2005). The results of Table 1 reveal a difference in the behavior of the data collected during period A as compared to the data of the recent period (period B). For period A stations, the weighted and arithmetic mean values, of all the presented parameters, cannot be considered as different within the calculated uncertainty limits. On the other hand, the d18O and d2H weighted means during period B, are much
Isotopic composition of precipitation in Greece
489
lower than the corresponding arithmetic ones reflecting the fact that 8 out of 10 lowest monthly d18O values in our database, occurred during the cold and rainy seasons of this period. These differences suggest an increased intensity of the heavy frontal precipitation episodes, and this was indeed the case in many areas of Greece during that period. Notably the Thission station, with a climatic normal precipitation height of 375 mm, in 2002 reported a record (since 1841) precipitation amount of 985 mm. The precipitation amount recorded in the Thessaloniki station during this year also presented an annual amount of about 30% higher than the normal.
Local meteoric water lines Fig. 2 illustrates the relation d2H = f(d18O) for all sampling stations. The monthly d18O values range between +2& and 14&; the d2H values range between +20& and 100&, in accordance to what is expected in these latitudes. The three most 18O-enriched cases correspond to light summer precipitation with intense evaporation of the raindrops beneath the cloud base-surface air temperatures around 26– 27 C – and present characteristically negative D-excess values (Fro ¨hlich et al., 2002). Best-fit lines, using monthly
40 All monthly data - Greece
20
-20 LWML GMWL Hellinikon Heraklion Methoni Patras Pendeli Rhodes Thessaloniki Thission
-40
2
δ H (‰)
0
-60 -80 -100 -120 -16
-14
-12
-10
-8
-6
-4
-2
0
2
4
18
δ Ο (‰ )
Figure 2
Deuterium and d18O relationship for Greece.
data (weighted mean values for Pendeli) were calculated using three regression models. Ordinary least squares regression (OLSR) is the most commonly used approach, yet it has significant shortcomings when it is applied to real life data. The basic assumption of OLSR, that x-values, in our case d18O, are exactly known thus error-free, is clearly violated. A better approach is the so-called reduced major axis or orthogonal distance least squares regression (ODLSR) (IAEA, 1992) that allows for x-values to have a standard deviation. ODLSR can handle cases where the ratio of y and x-value standard deviations is constant. Finally a full error-in-variables generalized least squares regression model (GENLS) was used, that is suitable for cases where the errors of both x and y-values are not constant and can also incorporate analytical or other Type B uncertainties (Macdonald and Thompson, 1992). The best GENLS fit line – the Local Meteoric Water Line (LMWL) – together with the Global Meteoric Water Line (GMWL), defined by the equation d2H = 8d18O + 10&, are plotted in Fig. 2. The results presented in Table 2 show that the stations sampled during the period 1960–1990 (period A) have higher slopes than those of period B (2000–2003). Furthermore, period A slopes cannot be considered different from that of the GMWL, on a 95% confidence level. Methoni has a different behavior possibly due to the limited number of available data. On the other hand, all period B slopes are lower than 8, with more pronounced those of the stations located in the Athens area (Pendeli and Thission) possibly indicating some non-equilibrium evaporation processes during the fall of the drops below the cloud base (Dansgaard, 1964). This pattern could seem to be indicating a change towards warmer and less humid climatic conditions over Greece, that would account for this assumed evaporation increase, yet this is not the case. A more detailed examination of the data reveals that the older data consist mainly of winter time precipitation, that is normal for Greece, while the data collected during the 2000–2003 period contain a significant proportion of summer precipitation as well, thus reflecting erroneously a climatic change towards warmer and drier conditions. As a matter of fact, the precipitation amount in Greece during that period increased, especially during
Table 2 Least squares regression results for the relationship d2H = a Æ d18O + b, using ordinary least squares regression (OLSR), orthogonal distance least squares regression (ODLSR), and errors-in-variables generalized least squares regression (GENLS) Station
OLSR
ODLSR
GENLS
a ± ra
b ± rb
a ± ra
b ± rb
a ± ra
b ± rb
Period A Hellinikon Heraklion Methoni Rhodes
7.50 ± 0.43 7.86 ± 0.47 5.9 ± 1.2 6.6 ± 1.4
10.8 ± 2.7 16.4 ± 3.1 1±6 8±7
8.00 ± 0.42 8.14 ± 0.45 6.7 ± 1.0 8.5 ± 1.3
13.8 ± 2.6 18.2 ± 3.0 6±6 18 ± 7
7.88 ± 0.44 8.08 ± 0.48 6.4 ± 1.2 8.1 ± 1.6
13.1 ± 2.8 17.8 ± 3.2 4±6 16 ± 8
Period B Patras Pendeli Thessaloniki Thission
7.28 ± 0.30 6.38 ± 0.42 6.98 ± 0.31 6.09 ± 0.41
7.2 ± 1.9 3.2 ± 2.9 3.0 ± 2.1 1.4 ± 2.5
7.44 ± 0.29 6.77 ± 0.41 7.20 ± 0.30 6.51 ± 0.40
8.1 ± 1.8 5.6 ± 2.8 4.3 ± 2.0 3.7 ± 2.4
7.39 ± 0.30 6.61 ± 0.54 7.13 ± 0.31 6.36 ± 0.42
7.8 ± 1.9 3.8 ± 3.3 3.8 ± 2.1 2.8 ± 2.5
All data
6.86 ± 0.17
6.2 ± 1.1
7.31 ± 0.16
8.8 ± 1.1
7.24 ± 0.18
8.2 ± 1.1
490 the summer months, yet the precipitation regime has returned to normal during the past three years (2003–2005). The higher than 10& intercepts reported for the period A stations, except Methoni, indicate that the Mediterranean sea is a significant source of water vapor for the precipitation over Greece.
D-excess Table 1 summarizes the calculated D-excess values for all stations and periods.With the exception of Thessaloniki, these vary within the range defined by the respective values for the Western (13.7&) and Eastern Mediterranean (20&). Their spatial distribution follows closely the one presented by IAEA (2001). The lowest weighted mean D-excess value is observed at Thessaloniki in the north, followed by the western stations of Patras and Methoni, the southeastern station of Rhodes and finally reaching a maximum value at Heraklion in the south. The Athens stations, Hellinikon, Thissio and Pendeli also present high D-excess values. The low D-excess values of Thessaloniki can be attributed to the unusually high proportion of the warm period – May to October for the local climatic conditions (Maheras, 1988; Argiriou et al., 2004) – precipitation (of about 44%) that is also characterized by low D-excess values due to evaporation of raindrops under the cloud base. The western stations of Methoni and Patras have lower D-excess values possibly because they receive relatively larger amounts of precipitation originating either from the Atlantic or the Western Mediterranean. On the other hand, the precipitation at the remaining stations, especially Rhodes and Heraklion, is more likely due to water vapor originating from the Aegean or the Mid-eastern Mediterranean. This is in accordance with the D-excess values of vapor samples collected in several locations in the Mediterranean Sea (Gat et al., 2003). D-excess values for the Athens stations present a rather unusual pattern: Hellinikon and Thission, situated within the city, show significantly lower than that of Pendeli, that is located some 10–15 km to the North and at a higher elevation. Since a raise of Pendeli’s D-excess by addition of re-evaporation of precipitated water is not a plausible argument for such short distances, it is the evaporation of raindrops beneath the clouds that should be examined as a possible explanation of the observed D-excess pattern. From this point of view it is Thission and Hellinikon that should be regarded as presenting lower D-excess values compared to Pendeli, due to an increased evaporation of raindrops below the cloud. The fact that, throughout the year, the surface air temperature in both Hellinikon and Thission is 2–3 K higher than that of Pendeli and the relative humidity is generally low in all stations, could support this argument, yet this point needs further investigation. Processing of monthly data from the global isotopic network has shown that in case of oceanic precipitation D-excess values are scattered; in continental stations, although the range of the d18O values is wider compared to that of the marine stations, the scatter of the D-excess values is rather reduced. This characteristic is quantified by the ratio of the range of D-excess, calculated as the difference between the maximum and minimum monthly values to the range of the d18O values (Gat, 2001). This ratio
A.A. Argiriou, S. Lykoudis for European coastal stations is 2.0, 2.3, 1.8 and 2.8 (Valentia – Ireland, Reykjavik – Iceland, Faro – Portugal and Weathership E – North Atlantic). For continental stations the following values are provided: 0.4 (Berlin), 0.35 (Krakow) and 0.7 (Vienna) (Gat, 2001). For the Greek stations it takes values between 1.7 and 5.3, therefore all stations have clearly coastal characteristics, and indeed all are located from a few hundred meters to a few kilometers from the coastline. These characteristics appear to be more pronounced in Rhodes, and Methoni. This could be attributed to the fact that the precipitation over these two stations has the characteristics of a first condensate of the vapor, due to the tracks of the weather systems affecting the area. However, according to this explanation, the ratio for the Heraklion and Patras stations should have had a similar value, but the contrary occurs, with these stations having the lowest values (2.2 and 1.7, respectively). The remaining three stations have ratio values between 2.4 and 3.0, while the overall value for Greece is 3.0. In order to assess whether the mean D-excess values differ statistically between stations the t-test was applied to all possible pairs of stations, using the weighted means and the combined standard uncertainties presented in Table 1, as sample variances. The obtained results show that there are only three pairs of stations for which the hypothesis of equal mean values cannot be rejected at a 95% confidence level. For one of these pairs, the western stations of Patras and Methoni, this is directly justified because of the similarity of the climatic and meteorological conditions of these stations. Thessaloniki stands alone in the North while the Athens’ stations exhibit variable affiliations with the southern stations of Rhodes and Heraklion but not between them. The variation of D-excess is illustrated in Fig. 3. From Fig. 3(a) it can be seen that the D-excess determined from the precipitation episodes at the Pendeli station, varies with time as expected, i.e., showing maximum values during winter and minimum during summer. The weighted mean D-excess values for each calendar month for the stations of Thessaloniki, Pendeli, Thission and Hellinikon are presented in Fig. 3(b) and for Patras, Methoni, Rhodes and Heraklion in Fig. 3(c). Precipitation intensity seem to be a key parameter affecting the isotopic composition of monthly precipitation samples over Greece since the stations of the Athens area (Hellinikon, Pendeli, Thission), that is characterized by the typical dry Mediterranean precipitation regime (average intensity frontal precipitation during winters and few thermal storms giving high intensity precipitation during summer) present marked minima during summer that could be explained by the few intense rainstorms that occurred, aided by the high air temperature–low humidity conditions that enhance the precipitation amount effect. The remaining stations present a weaker annual variation with maxima during either winter or during the transitional seasons and minima during summer. The temporal profiles of D-excess variation do not show any statistically significant trend, neither on a monthly nor on an event basis. The only exception is the Hellinikon station, where a negative linear trend is detected over the period 1960–1974. This trend is statistically significant over a 95% confidence interval, however this finding could not be
Isotopic composition of precipitation in Greece
491
40
Pendeli - Events 30
4
40
2
20
0 0
-2
δ O (‰)
18
10 0
-20
-6
-40
-8
-60
-10 -80
-10
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----
-14
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-16 0.0
a -30 09/00
12/00
03/01
06/01
09/01
12/01
03/02
06/02
09/02
5.0
15.0
20.0
18
δ O = 0.192•T (˚C) - 8.84‰
-100
2
δ H = 1.08•T(˚C) - 50.8‰
25.0
30.0
-120 35.0
Air temperatuture (˚C)
Figure 4 Variation of d18O and d2H as a function of temperature.
20 15
2 Thessaloniki
10
0
Pendeli
5
-5
18
Pendeli Thission
δ O (‰)
0
b Jan
Hellinikon
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Thission
-2
Thessaloniki
-10
Hellinikon
-4 -6 -8
Dec
25
-10
20
-12 Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
15
2 Patras
10
0
Methoni
5 Patras Methoni
-5
Rhodes
-10
c Jan
Heraklion
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 3 Variation of D-excess with time: (a) episodes; (b) and (c) weighted monthly means.
Rhodes
-2
18
0
δ O (‰)
D-excess (‰)
10.0
01/03
25
D-excess (‰)
-4
δ2H (‰)
D-excess (‰)
20
Heraklion
-4 -6 -8 -10 -12 Jan
confirmed using another, non-parametric method, because of the gaps in the time series. Treating the Hellinikon and Thission series as one, since both stations are located in Athens, did not produce any trend.
Temperature and seasonal effect The relation between monthly d18O and d2H values and surface air temperature for all available data is shown in Fig. 4. The observed behavior is as expected for mid-latitude stations, i.e., the lower the temperature, the more depleted the precipitation. Thission, Hellinikon, Methoni and Rhodes have lower slopes than the aggregate, while the rest of the stations have steeper slopes. The weighted mean d18O values, for each calendar month, are presented in Fig. 5. The d2H variations follow those of d18O and are not shown. The stable isotopes variation presents the expected pattern following closely that of the surface air temperature, with the exception of Thessaloniki, Pendeli and Thission. These stations have very
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
18
Figure 5 Variation of weighted monthly mean d O values with time.
depleted d18O during February and December. A close inspection of the data revealed that these months include, as a rule, the minimum d18O samples, usually collected under rather extreme meteorological conditions, like the December 2001 sample from Thessaloniki, with d18O = 13.96& corresponding to a mean monthly temperature of 2.5 C compared to the climatic normal of 6.9 C, and the samples of February 2003 from Thission and Pendeli that include water from a heavy snowfall. The contrasting minimum of Pendeli during June 2001 represents a single very weak precipitation event (0.4 mm).
Amount effect Fig. 6 illustrates the variation of d18O as a function of the precipitation amount. On this graph, data from all stations
492
A.A. Argiriou, S. Lykoudis
have been reported distinguishing winter and summer precipitation. On an annual basis, it can be seen that up to an amount of about 100 mm of precipitation per month, the depletion increases with the precipitation amount whereas for higher monthly amounts this behavior does not appear. Considering the d18O – precipitation amount relationship on a seasonal basis though, it can be seen that during winter there is very slight isotopic depletion trend with increasing amount, while during summer the depletion is more intense and of different shape, suggesting a difference in the seasonal mechanisms generating precipitation over Greece. Winter precipitation over Greece is mainly generated by frontal depressions that approach Greece from the northwest or west, whereas summer precipitation is mainly generated by thermal instability induced either by locally developed thermal lows or depressions approaching Greece from southwest (Maheras, 1983; Flocas and Giles, 1991). The winter frontal depressions’ tracks cause southwesterly winds over Greece that force stagnating maritime air eastwards. This air is potentially unstable and its instability is released by orographic uplift and frictional convergence when it reaches mainland Greece. On the other hand thermal lows are associated with convective instability that can produce precipitation either as short-lived intense thermal storms or as showers from trailing fronts or weak depressions coming from southwest. The temporal evolution of the isotopic composition of convective precipitation events is expected to be L-shaped, meaning a sharp decrease followed by a more or less stable isotopic composition, whereas that of frontal precipitation is V and sometimes W-shaped (Rindsberger et al., 1990; Celle-Jeanton et al., 2004). The extent of overall depletion during an event may depend on the intensity and the duration of the event, but this is mainly determined by the prevailing synoptic circulation patterns. Summer precipitation over Greece, primarily of convective origin, presents the expected isotopic depletion versus amount behavior. Several light showers with high d18O can be identified, followed by a sharp d18O decrease up to a 50 mm amount and remain rather constant beyond that value. The non-linear relationship between d18O and the precipitation amount is obvious since the R2 for a linear fit is 0.11 while for the logarithmic fit presented in Fig. 6 the R2 is 0.30.
4
During winter, on the other hand, the relationship between d18O and the precipitation amount is linear even though their correlation appears to be very weak. Winter precipitation over Greece is mainly of frontal character and the associated V-shaped relationship between depletion and amount tends to smooth out the overall effect, when considered on an aggregate basis. Furthermore there are two stations, namely Rhodes and Methoni that were previously identified as presenting a first-condensate behavior. These stations show extremely weak, if any, correlation between precipitation amount and d18O throughout the year, possibly indicating a continuous supply of moist air masses as the systems move across extended marine regions before reaching these stations. Should these two stations be excluded, Fig. 6 would present a more plausible decreasing pattern for winter. Fig. 7 illustrates the evolution of d18O during the event of 24–25/03/2002. The bars illustrate the precipitation rate, while the line with the marks shows the d18O values. The pattern observed is typical of a frontal event with a sharp drop of d18O when the precipitation rate intensifies between 11:00 and 13:00 h. Then the d18O value increases again, possibly as the result of mixing of depleted rainwater from the second half of the rainstorm and water from the last part of the rainstorm that is being re-enriched as the drops leaving the base of the cloud travel through a d18O rich environment.
Tritium Tritium in the atmosphere is a result of the interaction of cosmic radiation with various atmospheric components. In the beginning of the sixties however, the atmospheric nuclear tests resulted to an increase of the tritium concentration in the atmosphere. The activity of 3H is expressed in tritium units (TU); 1 TU is equal to 1 3H atom in 1018 1H atoms. The tritium activity in precipitation before the tests was of about 5 TU in central Europe. This number reached an average of about 5000 TU in northern Europe in 1963 (Gat, 2001). After the nuclear tests ban, the tritium activity started decreasing more or less exponentially and today it has reached the values of the pre-test era. Tritium measurements from period A extend from 1960 to 1991. Heraklion, Rhodes and Hellinikon are represented by a significant number of samples (116, 92 and 72, respectively) while the during period B Thessaloniki and Patras
2 0
18
δ O (‰)
-2 -4 -6 -8 without Rhodes and Methoni
-10
18
-12
--- δ O = -0.0031•P - 6.21‰
-14
δ O = -0.0117•P - 5.88‰ - - δ18O = -0.77•Ln(P) - 2.25‰
18
-16 0.0
50.0
100.0
150.0
200.0
250.0
300.0
Precipitation (mm)
Figure 6 Variation of d18O as a function of the monthly amount of precipitation for winter (circles) and summer (triangles).
Figure 7 Variation of d18O (line) as a function of the amount of precipitation during a single precipitation event (bars).
Isotopic composition of precipitation in Greece 10000
Hellinikon Heraklion Methoni Patras Rhodes Rhodes Thessaloniki Thission
1000
Tritium (TU)
493
100
Conclusions This paper attempts for the first time a comprehensive survey of the composition of precipitation over Greece in stable isotopes (2H, 18O) and 3H. All sparse data from past together with the results of a recent systematic measurement campaign have been presented and analyzed. The complete dataset consists of two distinct periods:
10
1 12/60 12/64 12/68
Figure 8
12/72 12/76 12/80 12/84 12/88 12/92 12/96
12/00
Tritium variation for all stations.
have only 1 full year of data. The results from all stations are shown in a graphic form in Fig. 8. The tritium variation follows the expected exponential decrease as for the other stations around the world. For each curve of Fig. 8, the slope of the line ln (TU) = a + b (Date) was calculated. The obtained results showed that these slopes are not statistically different. It is therefore concluded that the decrease rate of tritium concentration is practically the same for all the stations. The tritium concentrations of the current period (B) are shown in detail in Fig. 9. Although the number of data available for this period is still small to allow definitive conclusions, it is interesting to observe that while the tritium concentrations in Patras and Athens (Thission) are practically the same, the corresponding values in Thessaloniki are, from spring to autumn, two or three times higher. The relative increase in concentration between spring and autumn observed both in the samples of Patras and Thessaloniki, can be explained by the well documented ‘‘spring leak of the tropopause’’ (Gat, 2001). The higher tritium levels at Thessaloniki, might be the result of transport from tritium emitting installations such as the Bulgarian nuclear power plant at Kozloduy located some 350 km north of Thessaloniki. It should be noted that elevated tritium levels in the precipitation of Swiss stations, attributed to the emissions of nuclear power plants, are comparable to the levels observed in Thessaloniki, if one takes into account the distance between the plant and the city, the corresponding atmospheric dispersion and the prevailing synoptic circulation patterns (Schu ¨rch et al., 2003).
18 16
Patras Thessaloniki
14
Tritium (TU)
Thission
12 10 8 6 4 2 0 10/00
Figure 9
12/00
01/01
04/01
06/01
08/01
10/01
12/01
Tritium variation during the period 2000–2001.
• Period A, when only monthly samples were collected in several locations, but not on an identical systematic basis in all of them. • Period B, during which monthly samples were collected systematically in three locations, situated at the central, western and northern part of the country. During this period and for one of the above locations, samples were collected also during precipitation episodes. For the Period A stations the arithmetic and weighted mean values of the various parameters characterizing the signature of the stable isotopes in precipitation, cannot be considered as statistically different within the calculated uncertainty limits. This is not the case between the arithmetic and weighted mean values of d2H and d18O values for the Period B stations, reflecting thus the increased intensity of heavy frontal precipitation episodes in Greece during that period (2000–2003). Concerning the Local Meteoric Water Lines (LMWL), their slopes for the Period A stations cannot be considered to be different from the slope of the GMWL (8&) on a 95% confidence level, while the LMWLs of Period B have slopes lower than 8&. This is due to the fact that summer precipitation during the Period B (2000–2003) had increased significantly compared to Period A (1960–1990). The spatial distribution of the average D-excess is not homogeneous, reflecting the differences in the origin and history of water vapor masses generating precipitation over the Western, Central and Eastern parts of the country. Differences in average D-excess values were observed also between the stations located in the greater Athens area, i.e., those of Pendeli (northern semi-urban site) and Hellenikon and Thission located in the main urban canopy. The last two urban stations have lower D-excess values compared to the semi-urban Pendeli. This could be explained by the fact that the ambient temperature in Pendeli is 2–3 K lower compared to that of the urban stations possibly leading to less D-excess reduction due to evaporation of raindrops beneath the cloud base. The temperature and seasonal dependence of the stable isotopic content in all stations was found to be as expected in mid-latitudes. Summer precipitation versus amount presents the expected isotopic depletion. During winter, the relationship between d18O and precipitation amount, although weakly correlated, appears to be linear, reflecting the frontal character of winter precipitation in Greece. During summer a logarithmic shape represented a better fit on the data, implying a predominantly convective character of the precipitation. Finally, tritium concentrations in precipitation collected at the Greek stations during all those years, followed the same trend as in all other stations around the world and
494 are actually at the pre-test era levels. However, tritium concentrations in the Thessaloniki precipitation samples, which all have been collected during Period B, were found to be systematically higher compared to those of Athens and Patras. This could be attributed to emissions of nuclear power plants located in northern neighboring countries, dispersed over Northern Greece.
Acknowledgements This work has been partly financed by the International Atomic Energy Agency (IAEA), Department of Nuclear Sciences and Applications, Isotope Hydrology Section, through the Co-ordinated Research Project ‘‘Isotopic Composition of Precipitation in the Mediterranean Basin, in relation to Air Circulation Patterns and Climate’’ (IAEA Contract No. 11319). The authors acknowledge the assistance of Dr. Laurence Gourcy, in charge of the project. The samples have been analyzed at the Isotope Hydrology Laboratory of the IAEA. The authors are grateful to Prof. Joel R. Gat for providing the 1960–1974 datasets and for his critical review of the manuscript.
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