The standard diagram of δ18
H (Figure 4) shows the position of all samples relative to the Global Meteoric Water Line (GMWL: δ2
] and the Local Meteoric Water Line of the Tunis-Carthage (LMWL: δ2
] closest to our study area belonging to the GNIP network. This diagram shows that all meteoric water samples lay between the GMWL and the LMWL. This arrangement signifies that the precipitation ensuring the recharge of the El Kef area originates from a mixture of Oceanic and Mediterranean vapor masses. This suggests that the precipitation as the predominant source from the Atlantic (with the intercept +40%) and from the Mediterranean (with the intercept up to +60%) during this period of rain (January to May 2001). However, quantifying the contribution of each of these origins to the total recharge of the basin is difficult, and no simple and reliable methods are currently available. These findings and estimations remain to be developed in the future but with a lot of isotopic data. However, this contribution varies from one period to another and to a geographical area to another; depending on several other parameters (climate, geographical position, storm trajectory, atmospheric moisture, temperature, latitude, altitude, distance from the coast, and the amount of precipitation, also the influence of the isotopic signature of local precipitation “local vapor sources”). The same phenomenon was also observed in the central and southern Tunisia [21
]. A mixed isotopic signature of Atlantic and Mediterranean origin was also observed in the karst aquifers of southeastern and south Spain [46
] and northeastern of Algeria-El Eulma basin [48
The air passing over El Kef area reflect the combined effects of oceanic-sea vapor sources, fractionation due to local precipitation, and slower equilibration of the larger raindrops nucleated by a maritime aerosol (Mediterranean Sea from the North and from the East). Both larger raindrops as well as the potential for more night-time precipitation [49
] may explain the variation isotopic signature of precipitation at El Kef area [23
]. Remember, that the observations at this site are based on only half of one year of precipitation data, and additional monitoring may be necessary to confirm the results presented here.
Additionally, the samples that are characterized by a relatively depleted oxygen-18 and deuterium contents, indicate that they are not significantly affected by evaporated. Therefore, the most 18
O-enriched value corresponds to rain collected during summer (evaporative influence). This value corresponds to summer precipitation with intense evaporation of the raindrops beneath the cloud base with surface air temperatures around 40°C. The deuterium values are linearly and positively correlated to the δ18
O values (Figure 4). The linear correlation was found by many scientists [17
]. The physical basis for this correlation lies in the fractionation of isotopes during evaporation-condensation processes [51
]. This is attributed to the air temperature gradient and the massifs of the country (Tunisian Atlas). One of the factors that can be identified as influential is the distance of the stations from the sea (˜100 km).
Tunisia is located in the Western Mediterranean, which represents a climatic transition zone open to the influence of the cool North Atlantic air masses and the warm Mediterranean air masses [13
]. Moreover, specific geomorphologic characteristics of Tunisia i.e. the absence of high mountains with elevations exceed 1,500 m and the relatively limited geographic extensions allow the integration of Saharan air masses into the atmospheric circulation [34
]. However, hydro-meteorological studies [35
] suggest the existence of two major trajectories for dominant air masses. These are (i) Atlantic air masses that circulate from the west over Northern Africa and (ii) Mediterranean air masses that come from the north (Figure 1). Quantitatively, Mediterranean precipitation represents ˜66% of the total rainfall. The main part of the regional aquifer recharge is supplied by Mediterranean rain events. The question about how this affects the local precipitation and precipitation δ18
O needs future detailed work, for this influence may vary in time and in space.
Deuterium-excess (D-excess) (Table 1), defined as (D-excess=δ2
O), is generally associated with the moisture sources of the precipitation and Sea Surface Temperature (SST; positive correlation) at the moisture source [6
]. It is generally negatively correlated with the relative humidity of the air masses formed above the ocean [53
], and appears to be used to indicate climatic changes in the moisture source regions [56
]. Using this relationship, D-excess has been used in many studies to determine the temporal changes in moisture supply for a given location [58
]. The average D-excess in El Kef basin during 2001 (from January to May) is 11.94‰. The d values, varying from 7.24‰ (April) to 17.3‰ (February). The high D-excess for the winter samples indicating evaporation in low humidity conditions and low D-excess in summer (coupled with the higher δ18
O) indicating the possible re-evaporation below the cloud.
These results indicate a larger proportion of local moisture for the precipitation in 2001 in northwestern Tunisia. To improve the quality of this moderate interpretation, probably we should adopt a model analysis on trajectory of air-mass with combination with several isotopic analyses. Moreover, like several scientific authors in the word, I hope that application of these isotopic and modeling methods (GIS) would become of common use in order to advance in our knowledge of the oxygen, deuterium isotopes and D-excess composition at different areas in the word and to improve and facilitate the interpretations of paleoclimate depending on it and with the inter-discipline inter-collaboration.