Temperature Effect

Dansgaard (1964) analyzed a large body of isotopic data gathered by the International Atomic Energy Agency and showed that temperature is the major parameter that determines the isotopic values of precipitation (Fig. 9.9). In his extensive discussion, Dansgaard summed up the knowledge gained in laboratory experiments and field observations. The composition of precipitation depends on the temperature at which the oceanic water is evaporated into the air and, even more important, the temperature of condensation at which clouds and rain or snow are formed. The net effect is expressed in the following empirical function (from Fig. 9.9):

and, in a similar way, dD = 5.6Ta — 1000%, or 5.6%/°C

A local study at Heidelberg, Germany, revealed the following empirical function (Schoch-Fischer et al., 1983):

dD = (3.1±0.2)Ta — (172 + 3)%, or (2.8+0.2)%/°C

(Ta is the local mean annual air temperature.) The meteoric line (Fig. 9.4) is thus the result of the combined dD and d18O dependencies on temperature. The temperature effect is well seen in seasonal variations in regions with rains during cold and warm seasons. An elegant example from Switzerland is given in Fig. 9.10: monthly measurements in three stations revealed that the annual temperature cycle is followed by corresponding changes of d18O.

The temperature dependence of the isotopic composition of precipitation, or temperature effect, is to a large extent responsible for the large variation in the isotopic composition of groundwaters, thus equipping the hydrologist with a powerful tool. In regions with summer and winter precipitation the isotopic differences in the composition of the precipitation are traceable as winter recharge and summer recharge fronts, important in establishing groundwater velocities and identifying piston flows.

Fig. 9.9 Temperature effect. Correlation between annual mean d18O values observed in precipitation and the annual mean temperature of local air: polar ice (circles and triangles; figures in parenthesis indicate total thickness in cm); continental precipitation (•) and island precipitation (+). (From Dansgaard, 1964.)

Fig. 9.9 Temperature effect. Correlation between annual mean d18O values observed in precipitation and the annual mean temperature of local air: polar ice (circles and triangles; figures in parenthesis indicate total thickness in cm); continental precipitation (•) and island precipitation (+). (From Dansgaard, 1964.)

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