(atMospHere, surface Waters and
Water QualIty)
ON THE IsOTOPIC ALTITuDE EFFECT OF PRECIPITATION IN THE NORTHERN ADRIATIC (CROATIA)
Z. roller-lutZ a, d. Mance a, t. HunJaK a, H.o. lutZ a,b
a stable Isotope laboratory,
Medical faculty, university of rijeka, rijeka, croatia,
roller@medri.hr
b physics faculty, Bielefeld university, Bielefeld, germany
Abstract
the upper (northern) adriatic is very rich in precipitation. this input into the water sys-tem and its stable isotope composition is a basic factor, knowledge of which is required for proper use and management of water resources. the geomorphology of the region (e.g., moun-tains of 1400 m next to the sea) can cause specific local conditions. The isotopic composition of precipitation has been measured in various locations at different altitudes. For δ18o this
‘altitude effect’ is found to lie around −0.2‰ /100 m; its exact value depends on the specific location and the season. The δ2H values and the d-excess vary correspondingly.
1. IntroductIon
the use and protection of the karst aquifers in the border area of slovenia and Croatia presents a problem since it is strongly influenced by the transboundary hy-drological situation. three out of seven catchment areas between the Kvarner and the trieste bay belong to transboundary aquifers [1], while the geomorphology (e.g., mountains of up to 1400 m next to the sea) causes specific local conditions. Rije -ka, the third largest city in croatia, is located in this region, and her water supply needs special attention. evidently, the management of the region’s water resources requires a broad knowledge of all related aspects including the precipitation as input into the water system. Besides meteorological data, isotopic tracers are an important tool to understand and quantify such complex problems. the isotopic composition of precipitation is primarily determined by the passage of the water into and through the atmosphere. as the air mass passes over the continent preferentially the heavy
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isotopes rain out due to fractionation; as a consequence, the remaining vapour and thus also the further precipitation experience a progressive depletion of the heavy isotopes along the way, and, as is well known, such a mostly temperature related depletion occurs also with increasing altitude. since only scattered information ex-ists about the isotopic composition of precipitation in our region of interest [2, 3], the stable Isotope laboratory (sIlab, rijeka university) has initiated a network of precipitation collection stations on the upper (northern) adriatic, i.e. in the rijeka region (fig. 1). In this paper we will report on the altitude effect of the isotopic com-position of precipitation in this area. the isotopic comcom-position of a water sample is usually expressed by abundance ratios r (2H/1H and 18o/16o, respectively) in terms of the δ-values, δ (‰) = (Rsample/rstandard – 1) × 103, with rstandard the abundance ratio of an internationally accepted standard (e.g., the Vienna standard Mean ocean Water, VSMOW). Ref. [3] gave a mean value of −0.3‰ /100 m for δ18o of precipitation in the area of interest here; however, as we shall see, the situation is more complicated and depends on the location and the season.
FIG. 1. The upper Adriatic and the precipitation collection stations (■ — recently installed stations;
●
, ▲ — stations with isotope record of two full years).IAEA-CN-186/12
2. sIte descrIptIon
croatia is located in southeast europe, bordering on the adriatic sea. gen-erally, the islands and the coast have a Mediterranean climate. from autumn until spring, Icelandic cyclones can cause weather instability, frequently accompanied by secondary Mediterranean cyclogenesis with strong southerly winds along the coast and heavy precipitation, the sirocco. In summer, azorean and siberian anticyclones can result in rather stable conditions, often with strong, dry northerly winds — bora
— at the seaside. In the northern adriatic the amount of precipitation during the year is quite considerable. Thus, by the Thornthwaite classification, the climate is mostly humid, while in the mountainous hinterland of the Kvarner gulf and the wider region of rijeka orographic effects can intensify the precipitation to over 1500 mm/a and the climate is perhumid [4]. This orographic diversity has an important influence and can cause strong variations within short distances of a few kilometers. for example, between Istria and the Rijeka region the Učka Mountain massive rises up directly from the sea to about 1400 m a.s.l. and is responsible for occasional very heavy lo-cal precipitation. Other highlands and mountain chains between the flat Pannonian plain in the north and the sea provide a rather efficient divide and somewhat miti-gate weather exchange. average temperatures show quite pronounced regional varia-tions: in winter they range from –2ºc inland with snow in the mountains to 5 ºc along the coast and in summer they range from 15 ºc to 25 ºc, respectively.
3. saMplIng and analysIs
In the northern adriatic, our network comprises at present 16 stations. since several stations have been installed only recently (square symbols in fig. 1), we will show here only the data of those eight stations which cover two complete years (circles and triangles). they are located at different altitudes in the center of rijeka (fakultet, 28 m) and the hinterland of rijeka (Kukuljanovo, 281 m; Kastav, 350 m;
Benkovac, 873 m), what is called below ‘Kvarner’, as well as on Mount Učka (Ičići, 2 m; Veprinac, 484 m; Poklon, 927 m; and on the top of Učka, 1381 m), ‘Učka’ in the following. We will display the data for the two regions separately since the Učka stations are very close together; therefore, they are particularly well suited to deter-mine altitude effects without local influences. A new type of collector is used, which allows the storing of the precipitation for weeks and even months without notice-able evaporation and associated fractionation. the performance of the collector has been studied in detail and will be published in a separate paper. Monthly samples are brought to the stable Isotope laboratory sIlab at rijeka university where they are analysed for their isotopic composition. δ2H and δ18o are measured in a thermo-finnigan deltaplusXp isotope ratio mass spectrometer using a combination of dual inlet and equilibration unit as periphery. the samples are equilibrated with H2and
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co2, whereby their isotopic composition is transferred to these gases and then ana-lysed in the mass spectrometer. The precision of the δ-values thus determined is 1‰
for δ2H and 0.1‰ for δ18o. the required working standards were produced by col-lecting waters of different δ-values and storing them under nitrogen gas in stainless steel barrels. they were calibrated through comparison with Iaea standards.
4. results and dIscussIon
the rather large precipitation amounts in rijeka and some stations in the gen-eral area are displayed in fig. 2. the gengen-eral behaviour shows strong winter maxima and weak precipitation in summer; secondary maxima e.g. in May and september 2010 are characteristic for the variable character of the local climate. occasionally, some local orographic events emphasize this general behaviour (e.g. Kastav in winter 2009/2010).
At all eight stations, the δ18o values (fig. 3) vary between maximum values (around −4‰) in the warm season and minimum values (around −12‰) in the cold season; the exact values depend on the location. δ2H (not shown) follows δ18o and varies between around −20‰ and −70‰.
from the data we have extracted the altitude dependence (fig. 4) of the pre-cipitation weighted mean δ18o values, separately for the warm (april-september) and the cold season (october-March). In both sets of stations, as expected, the winter data are shifted to more negative values, while the slopes are practically identical
FIG. 2. Precipitation amount at four stations. Left: Kvarner region; right: Učka Mountain.
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(Učka) or slightly different (Kvarner). The yearly averaged δ18o altitude effects (ca. −0.2‰ /100 m for Učka and −0.22‰ /100 m for the Kvarner) are considerably weaker if compared to the −0.3‰ /100 m as given earlier in Ref. 3. For the station on the top of Učka a few winter data had to be eliminated due to ice formation. How-ever, using the data from a lower-lying station (e.g. Veprinac) where no ice formation occurred, and applying the respective altitude relation as shown in Fig. 4, the Učka
FIG. 3. Precipitation δ18O in the Kvarner region (left) and on Učka (right).
FIG. 4. δ18O altitude effect of the weighted means for the warm (solid lines) and the cold season (dashed) lines.
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data including the missing months can be reconstructed (broken curves in fig. 3).
the agreement with the actually measured values is rather good.
The linear relationship between δ2H and δ18o (the local Meteoric Water line, LMWL) has been determined as δ2H = 7.11 δ18O + 6.75 and δ2H = 7.05 δ18o + 5.16 for Učka and Kvarner, respectively. Both LMWLs are close to the Global Meteoric Wa-ter Line (GMWL, δ2H = 8 δ18o + 10 [5]). slopes and intercept are somewhat smaller if compared to the 2001–2003 data for Portorož and Kozina on the western side of Istria [3]. as suggested in ref. [3], such a temporal change might indicate a change to warmer (and possibly less humid) climate conditions. Since local influences (evapo -rative enrichment of falling rain drops beneath the cloud base, snow formation, etc.) may follow different fractionation rules, the lMWl is more of a descriptive char-acteristic [6]. The deuterium excess d = δ2H – 8 δ18o appears to be a better indica-tor [7]. for example, it is generally thought to be useful in identifying the origin of the precipitating air mass: atlantic air masses have a d-excess typically of around 10‰, while Mediterranean air masses are characterized by a larger d-excess [8]. In the cold season d-excess is generally higher and shows a significant altitude effect, too, at least partly related to the altitude dependence of the temperature. this may require further study.
5. conclusIon
the paper presents the isotopic composition of precipitation collected in a sta-tion network on the Northern Adriatic. The δ18o values decrease with increasing alti-tude. the slope of this relation depends on the season and on the location; generally it FIG. 5. Altitude effect of the precipitation-weighted mean d-excess for the warm (solid lines) and the cold season (dashed lines).
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is, however, somewhat smaller than the one reported in ref. [3] for the Western side of Istria. Also the d-excess shows a significant altitude effect in that it increases with increasing altitude. To our knowledge, this is the first time that a systematic measure-ment of an altitude dependent d-excess has been reported.
ACkNOWLEDGEmENTs
The authors want to thank M. Majetić for her continuing help in the labora-tory. this work has been supported by the Ministry for science, education and sport (MZos) of croatia (project 062–098 2709–0510), and the International atomic en-ergy agency (Iaea, crp project 14432).
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