Event precipitation amount is generally weak. Indeed, 50% of the rainy events are lower than 10 mm in Algiers, and 5 mm in Assekrem. The correlations between the isotopic contents and the climatic factors appear not very significant.
7.1. The
δ
18O = F (P) relationship (amount effect)The relationships are defined by the following equations:
In Algiers (Fig. 17): δ18O = -1.25 ln (P) -2.2; (R2 = 0.137, n = 113)
In Assekrem: δ18O = -2.14 ln (P) +22.6; (R2 = 0.29, n = 78)The relatively low correlation coefficient is due to the fact that the isotope content of precipitation is not completely controlled by precipitation amount. Thus, the amount effect appears slightly occulted, by the combined effects of other climatic factors, such as temperature and moisture.
FIG. 17. Oxygen-18 vs. precipitation amount relationship in Algiers station.
7.2. The
δ
18O = F (T) relationship
In Algiers (Fig. 18): δ18O = 0.137 (T) - 8.4; (R2=0.113; n=113) (T being the temperature during the rain).
In Assekrem δ18O = 0.137 (T) - 8.4; (R2=0.1; n=61)(T being the temperature measured at the time the the sample was taken, i.e. a few moments or a few hours after the rainy event.
FIG. 18. Oxygen-18 vs. temperature relationship in Algiers.
The seasonal effect clearly appears, showing highest heavy isotope contents that correspond to the warmest months. On the contrary, rainfalls of the cold season are depleted in heavy isotopes, with δ18O values as low as -11.9 ‰, and generally located above the GMWL.
The very bad adequacy obtained in Assekrem station, can be explained by the fact that the temperature varies considerably during the rainy event. The thermal effect, which appears dominant, explains the highest isotopic contents during the hottest months, and vice versa. The highest values are observed for August and July precipitation, which coincide with optimum of the monsoon. During this season, the high temperatures reigning in Ahaggar mountain, maintain the moisture at its lowest middle levels of the year, in spite of arrival of the Monsoon air masses. The rainy events, not frequent and of small amount, do not allow a durable cooling of the atmosphere which, after the rain, very quickly present a severe dryness profile.
In summer, the temperatures exceed 30°C at midday, and can fall abruptly after a storm. This temporary cooling, translates the intensity of the evaporation responsible for calorific consumption of energy. It is accompanied of an increase of moisture, which generates isotopic contents more impoverished at the end of the rain.
In winter, the cooling of atmosphere and a higher moisture level explianed lower isotopic contents of precipitation.7.3. The oxygen-18 vs relative humidity
The relationship appears not very significant. During rainy events, more especially in Assekrem, the moisture fluctuates quickly and very largely. The first drops which fall within a particularly dry atmosphere, are enriched in heavy isotopes. When the atmosphere temporaly saturated, precipitation is very impoverished.
8. Conclusion
The isotopic data obtained for Algiers and Assekrem precipitation permit, conjointly to the meteorological data, to obtain information on circulations of the air masses.
The station of Assekrem is located in a strategic position for the study of circulation of air masses as it is influenced by two different climatic regimes:
In the south, and precisely in the Ahaggar massif (to 2000 km from Algiers), the monsoon regime prevails.The limit between the two regimes may be located between Algiers and Assekrem, our two observation sites.
The influence of the polar climate reaches the Sahara, and even the Ahaggar massif. The influence of the Monsoon climate reaches only the Ahaggar massif. This flux from Guinea Gulf is observed at the tropical latitude, extending northward up to 3° to 4° more than the limit fixed up to present and that is 20° N.
The indices suggesting that the influence of the Monsoon could be extended up to the Ahaggar is the presence of summer precipitation, thermodynamically in non-equilibrium, and evaporated. Influence of Guinean air moisture must be taken in account to explain August precipitation in mountainous regions of Central Sahara. The low δ18O values of precipitation in winter indicate the influence of the Atlantic Ocean.
In summer, during the incursions of Monsoon air masses, the negatives values of deuterium excess confirm the evaporated character of rain. During the rainy season in winter, the deuterium excess values higher than 10 ‰ indicate processes of recycling of air masses under the polar climatic regime.
ACKNOWLEDGEMENTS
This present investigation was carried out within the framework of IAEA Coordinated Research Programme for “Research Project Isotopic on Composition of Precipitation in the Mediterranean Basin in Relation to Air Circulation Patterns and Climate”. It has been supported financially by the “Faculté des Sciences de la Terre”, University of Algiers (USTHB) and with partial support from IAEA-Research Contract (302 F3-ALG11327). Sincere thanks are due to “Fraternité Assekrem”, which carried out sampling and meteorological measurements in Assekrem. Thanks are also extended to Dr W. Balderer who carried out chemical analyses. I would also like to acknowledge the numerous persons that spent time and efforts in this study.
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ISOTOPIC COMPOSITION OF PRECIPITATION IN AUSTRIA IN