CIRCULATION — CASE STUDY: APPLICATION IN A PILOT BASIN IN CENTRAL ANATOLIA
Y. INCI TEKELI
2. PRECIPITATION DATA AND METHODS
In this study, δ18O and δD values derived from 30 precipitation samples collected from the Guvenç Basin are used. Samples are part of precipitation isotope record of suffi cient length to study the impact of individual storm types on precipitation isotopic variability. All samples were fi lled to near the capacity of the container to avoid air entrapment, stoppered tightly and then labeled. The samples were sent to the Isotope laboratory for analysis at the Technical Research and Quality Control Division of the State Hydraulic Works (DSI) Ankara. Stable isotopes are measured relative to VSMOW using internal standards calibration with international standards. Sample precision is ±0.1‰ for δ18O and ±1.0‰ for δD.The atmospheric circulation during each precipitation event was evaluated using a 48 — hour back trajectory approach originating from Guvenc Basin. Back trajectories were calculated using the HYSPLIT model provided by National Oceanographic and Atmospheric Administrations (NOAA), Air Resource Laboratory. Each trajectory was initiated from at 500 meters above ground level to 3000 m using archived data from the Middle East Technical University Environmental Engineering Department. Trajectory data were coupled with weather conditions for each precipitation event using synoptic-scale surface maps.
3. RESULTS AND DISCUSSION
3.1. Relationships between isotopic composition and depth-intensity of precipitation
The characteristics of precipitation (mm) collected from 7 individual storms are given in Table 1 with date, amount (mm) and intensity (mm/h).
Three of seven samples (5–7) resulted from rainfall amount greater than 5 mm with intensity varying from 5 to 21.5 mm/h. The other four (1–4) produced the amounts ranging between 2.3 and 4.8 mm with intensities varying from 2.5 to 4.8 mm/h.
A scatter plot showing the δ18O and δD values for each sample is presented in Fig. 1. Also shown in Fig. 1 is the global meteoric water (GMWL) line as reported by [3]. Deviations from the GMWL are found on a seasonal basis in Guvenc Basin, mainly as a result of partial evaporation, temperatures changes and small seasonal fl uctuations of the deuterium excess of precipitation.
The samples may also differ substantially from the GMWL in cases where the atmospheric circulation regime of the given basin area varies seasonally bringing moisture to the area from air mass sources.
In general there is a close relationship between amount of precipitation and δ18O values. When precipitation amount increases, δ18O values of precipitation decreases. Three storm types were observed during the period of record. From the start of sampling (23 April 2003) until (21 April 2004), spring convective storms occurred frequently in study area. These convective storms were typically
TABLE 1. PRECIPITATION AMOUNT, INTENSITY AND DURATION.
Dates Precipitation
amount (mm)
Precipitation intensity
(mm/h) Duration
1 – 8.04.2003 2.3 3.0 18:00–18:10
2 – 18.04.2003 4.7 2.5 15;00–15:10
3 – 23.04.2003 4.8 4.8 14:30–15:00
4 – 25.04.2003 2.7 3.0 14:50–15:00
5 – 25.10.2003 9.2 5.0 22:00–22:15
6 – 16.12.2003 5.3 11.6 14:15–14:20
7 – 21.04.2004 5.6 21.5 16:20–16;30
small in area and of short duration. Storms in 25 April 2003 provided the most isotopically enriched precipitation, with an average δ18O of –5.06 ‰ and 2.7 mm amount originating in the Siberia- Balkan. In October and December, a maritime tropical brought high amounts and intense rains to the entire basin, amounting to 9.2 mm with an average δ18O of –9.16 ‰. A plot of rain δ18O versus amount (Fig. 2) showed no systematic pattern except that the most enriched precipitation samples were associated with low precipitation amounts.
FIG. 1. δ18O and δD values for each precipitation event.
FIG. 2. Precipitation depth and δ18O ‰ relationships.
-90
-80
-70
-60
-50
-40
-30 -12 -10
-8 -6
-4 -2
0
18O %o
D %o
local precipitation samples samples
♦ 08.04.2003
♦ 18.04.2003
♦ 23.04.2003
• 25.04.2003 Ÿ 25.10.2003
♦ 16.12.2003
♦ 21.04.2004
įD = 8į18O + 10 GMWL δ180 (‰)
δ2H (‰)
-12 -10 -8 -6 -4
0 0,5 1 1,5 2 2,5 3 3,5
Precipitation (mm)
O18 (%o)δ180 (‰)
3.2. Relationships between isotopic composition of precipitation and back trajectories
The circulation analysis produced one, two or three centers originating from different sources. Trajectories of individual storms are shown in Table 2.
The climate of Turkey, which is characterized mainly by the Mediterranean macroclimate, results from seasonal alternation of the mid-latitude frontal depressions, with the polar air masses, and the subtropical high pressures with the subsiding maritime tropical and continental tropical air masses. The results of recent studies [4] revealed that precipitation in Turkey and the North Atlantic Oscillation (NAO) are stronger in winter and autumn and weaker in spring and almost non-existant in summer. The NAO is defi ned as a large-scale swaying of atmospheric pressure between the dynamic subtropical anticyclone centred over the Azores region and the mid-latitude cyclone dominated over the Iceland and Greenland region in the North Atlantic. It seems that relatively wet conditions in Turkey during the negative phase of the NAO spring index are related with the cyclonic anomaly circulation over the Central Europe, although infl uences of the anomalous 500-hPa circulation patterns for the extreme NAO phases are getting weaker in spring compared with those in winter and autumn, due to the effects of the sub-regional and/or local physical geographic and meteorological factors [5]. Spatially and statistically, changes in precipitation amounts during NAO are more apparent in west and mid Turkey.
This study provides an understanding of how different seasonal circulation types in the Guvenc Basin infl uence precipitation δ18O and importance of atmospheric circulation.
TABLE 2. TRAJECTORIES OF AIR MASSES.
Dates Trajectories
8.04.2003 Atlantic coastal- N. Europe (Fig. 3a) 18.04.2003 Siberia–Middle Anatolia–South East Anatolia
23.04.2003 Siberia (Fig. 3b)
25.04.2003 Siberia–Balkan
25.10.2003 Africa–Mediterranean–Europe
16.12.2003 Africa-Mediterranean- South East Anatolia (Fig. 3c)
21.04.2004 Mediterranean (Fig. 3d)
Atmospheric circulations of air masses bringing precipitation over the basin originated from Atlantic coastal, Mediterranean- Africa and Siberia centers (Table 2). Four of the seven events can be seen in the trajectory summary in Fig. 3 Atlantic coastal trajectories (Figure 3a) bring air mass across the basin with average –7.26‰ δ18O and –58.29‰ δD isotopic values. On the other hand Siberia trajectories (Fig.3b) exhibit relatively low δ8O and δD values with an average –9.44 and –71.96‰ respectively. These events are largely associated with easterly trajectories, although many other trajectories begin over the west before entering Turkey. Africa-Mediterranean systems (Fig. 3c) produce more negative precipitation events than that of the coastal system, with an average of –9.17‰ δ18O and –58.10 ‰ δD values. Mediterranean effect precipitation events (Fig. 3d) are more negative, with an average of –10.16‰ δ18O and –67.65‰ δD, than that of the Atlantic coastal systems. These deviations can be attributed to varying temperature conditions and source of the trajectories originating from either coastal or continental areas.
3.3. Deuterium excess
The deuterium excess is defi ned as d = δD – 8δ18O [6]. It is a measure of the deviation of the given data from a line with slope 8 going through VSMOW.
Deuterium excess for the precipitation collected from 7 individual storms in the basin varies between –2.58‰ and –19.46‰. The deuterium excess offers a possibility for characterizing the interaction of different air masses and their temporal evolution.
In this study, the large range of d excess is a result of seasonal variations, with lower values in the winter season (> –15‰) and higher values (–2.58‰) in the spring season. These variations are found on a seasonal basis mainly because of mixing of dry-cold continental air masses and hot-humidity maritime tropic air masses over the study area.