Atmospheric Moisture (vapor collection) at Rehovot, 2000–2004

The isotopic composition of the vapor samples undergo the marked seasonal cycle noted before [6], with ¹⁸δ values between -13‰ and -9‰ and the d-excess of 5‰ ≤ d ≤ 22‰ in summer. Winter values are more scattered ranging between 10‰ ≤ ¹⁸δ ≤ -18‰ and with d-excess values up to +40‰. The data from spring and autumn follow a trend line between these two seasonal values. Samples from rain days often show the most extreme values.

The results are summarized on a plot of d-excess vs. ¹⁸δ in Figs. 3 to 7 (values are shown in ‰).

FIG. 3: d-excess vs. δ¹⁸O in water vapour collected at Rehovot during spring periods.

Summer

FIG. 4: d-excess vs. δ¹⁸O in water vapour collected at Rehovot during summer periods.

Autumn

Winter

FIG. 6: d-excess vs. δ¹⁸O in water vapour collected at Rehovot during winter periods.

0

FIG. 7: d-excess vs. δ¹⁸O in all water vapour samples collected at Rehovot during period 2000-2003.

The overall trend line for all samples is given by: ²δ = 5.¹⁸δ – 18.5(‰) [r² = 0.60], which translates into the correlation: d = -3. ¹⁸δ – 18.5(‰).

A weak temperature dependence of the data [r² = 0.34] is obviously a reflection of the seasonal pattern rather than pertaining to the individual samples.

On 21 occasions where vapor data on rainy days were available (Table 3), the difference in ¹⁸δ between the vapor and rain was usually close to the equilibrium value for the prevailing temperature.

With the exception of two cases in late autumn of 2001, the difference in the value of the d-excess was within the large analytical error for such a comparison, which is estimated to be ± 5 ‰. One needs to consider, however, that the time span for the vapor and precipitation sampling is not the same in most instances.

Table 3. Comparison of isotope data of vapour and precipitation at Rehovot (all given in ‰) It is of interest to note that the isotope composition of vapor samples collected all over the Mediterranean during the cruise of the Research Vessel “Meteor” in January 1995 [17] fall inside the field of the Rehovot data set, which indicates that some of the air masses that penetrate to our shores maintain their marine characteristics.

The general validity of these observations, using the data from previous years within their meteorological and synoptic context is now being examined. One salient feature already detected is exemplified by the case of the period of May 17 to May 23, 2001 where a drop of the d-excess value from 23,15 to 5.4 is on record with a commensurate change of the δ values, It appears that in this case the vertical mixing of the air column above the sea was an important factor; during these days the

involved in determining the isotopic composition. Thus during May 2001 (Fig. 8) the prolonged sojourn of the air-masses over the Mediterranean prior to May 22 enabled the establishment of marine characteristics with a low d-excess value at that time. In contrast, as shown in Fig.9 during the period from March 31 to May 5, 2003, the lower d-excess values ranging from d=8‰ to d=10.25‰ were associated with air-masses with a continental trajectory over the north African continent (April 3 and 7) or over the eastern Levant (May 4 and 5), whereas the classical routes over the eastern Mediterranean resulted in higher d-excess values, e.g. for March 31, April 10 and April 24 under relatively high stability of the air-masses. Evidently the role of stability all along the trajectory needs to be carefully investigated for these varying synoptic situations.

It is to be further noticed that on a few occasions when the vapor sampling was extended to 24 hours, specifically on the Nov. 20 and Nov. 27, 2001 and Febr. 3, 2002, the ¹⁸δ values were more enriched by 2.5‰ on the average when compared to the daytime samples, presumably due to the more stable air column at night.

Table 4. Selected vapour data from Rehovot

Date ¹⁸δ(‰) ²δ(‰) d-excess(‰) Stab-1 Traj#

* Stab-1 is the averaged value of the vertical the difference between (lon,lat,p+50hPa) and (lon,lat,p-50hPa) calculated for every trajectory at the closest position to Rehovot where ECMWF data were available, subject to the limitation that these are available only every six

# Classification of trajectory backtracks:

#1a - classical from Europe through Aegean and north-eastern Mediterranean

#1b – from Europe through Aegean and south-eastern Mediterranean

#2 – over eastern Med. from Eastern Europe via Turkey

#3 - from Western Europe along axis of central and eastern Mediterranean

#4 – from the west over North Africa

#5 – from the north over lands of the Levant (no contact with Mediterranean)

#6 – local, from eastern Mediterranean

#7 – from south, Red-Sea area

FIG. 8. Backtracking trajectory analysis in May 2001.

FIG. 9. Backtracking trajectory analysis from March to May 2003.

REFERENCES

[1] GAT, J.R., KARFUNKEL, U. NIR, A., Tritium content of rainwater from the eastern Mediterranean area, In: Tritium in the Physical and Biological Sciences, IAEA 1 (1962) 41–

54.

[2] GAT, J.R., DANSGAARD, W., Stable isotope survey of freshwater occurrences in Israel and the Jordan Rift Valley, Journal of Hydrology 16 (1972) 177–211.

[3] GAT, J.R., CARMI, I., BAUMAN, N., The isotopic composition of precipitation at Bet-Dagan, Israel: the climatic record (1961-1990). Israel Meteorological Research Papers 5 (1994) 10–19.

[4] International Atomic Energy Agency, Statistical Treatment of Data on Environmental Isotopes in Precipitation, Technical Reports Series No. 331, IAEA, Vienna (1992).

[5] GAT, J.R., The isotopic composition of atmospheric waters in the Mediterranean Sea area and their interpretation in terms of air-sea interactions, In: Rapp. Comm. Int, Mer Medit. 19 (1969) 923-926.

[6] TZUR, Y., Isotope effects in the evaporation of water into air, Ph.D. thesis, The Weizmann Institute of Science, Rehovot (1971).

[7] RINDSBERGER, M., YAFFE, S., RAHAMIM, S., GAT, J.R. , Pattern of the isotopic composition of precipitation in time and space: data from the Israeli storm water collection program, Tellus 42B (1990) 263–271.

[8] WINDERBAUM, S.. MSc. Thesis, Dpt. of Meteorology, Hebrew Univ., Jerusalem (1992).

[9] LEVIN, M., GAT, J.R., ISSAR, A., Precipitation, flood and groundwaters of the Negev Highlands: an isotopic study of desert hydrology, In: Arid Zone hydrology; Investigations with Isotope Techniques, IAEA-AG-158 (1980) 3–23.

[10] LEGUI, C., RINDSBERG, M., ZANGWIL, M., ISSAR, A., GAT, J.R., The relation between the oxygen-18 and deuterium contents of rainwater in the Negev Highlands and air mass trajectories, Isotope Geosciences 1 (1983) 205–218.

[11] GAT, J.R., Variability (in time) of the isotopic composition of precipitation: consequences regarding the isotopic composition of hydrologic systems (Proceedings of a symposium on Use of Isotope Techniques in Water Resources Development), STI/PUB/757, IAEA, Vienna (1987) 551-563.

[12] GOODFRIEND, G.A., MAGARITZ, M., GAT, J.R., Stable isotope composition of land snail body water and its relation to environmental waters and shell carbonate, Geochim.

Cosmochim. Acta 53 (1989) 3215–3223.

[13] AYALON, A., BAR-MATTHEWS, M., SASS, E., Rainfall-recharge relationships within a karstic terrain in the eastern Mediterranean semi-arid region, Israel. Journal of Hydrology 207 (1998)18-31.

[14] GAT, J.R., RINDSBERGER, M., The isotopic signature of precipitation originating in the Mediterranean sea area: a possible monitor of climate modification, Israel J. of Earth Sciences 34 (1985) 80–85.

[15] GAT, J.R., CARMI, I., Effect of climate changes on the precipitation pattern and isotopic composition of water in a climate transition zone; -case of the Eastern Mediterranean area, In: The Influence of Climate Change and Climate Variability on the Hydrologic regime and Water resources, IASH 168 (1987) 513–523.

[16] GAT, J.R., ADAR, E., ALBERT, P., Inter- and intra storm variability of the isotope composition of precipitation in southern Israel: -are local or large scale factors responsible?

(Proceedings of an International Conference on the study of environmental change using Isotope Techniques), C&S Papers Series 13/P, IAEA, Vienna, (2000) CD-ROM.

[17] GAT, J.R., KLEIN, B., KUSHNIR, Y., ROETHER, W., WERNLI, H., YAM, R., SHEMESH, A., Isotope composition of air moisture over the Mediterranean Sea: an index

STABLE ISOTOPES (

δ

δ

RESULTS AND COMPARISON WITH GROUNDWATER PERCHED