Stable isotopic composition of fog and rainfall in a Macaronesian cloud forest

Abstract

Stable isotopes of water are commonly used to investigate the role of fog in cloud forests. This first requires the characterization of the isotopic signatures of both fog and local rainfall. The isotopic composition of rain and fog water and their variability were analysed for a Macaronesian cloud forest in Tenerife, Canary Islands, from samples collected over 5 years within the Global Network of Isotopes in Precipitation (GNIP) project. The results showed little differences in the isotopic signature of fog and rain with the former being slightly higher with respect to the latter (differences in absolute value of 1‰ in δ18O, 7‰ in δ2H and 0.7‰ in d-excess, on average) and with a similar statistical variability. Their meteoric lines, LMWL (local meteoric water line for precipitation): δ2H = 6.56 δ18O + 11.56 and FWL (fog water line): δ2H = 5.57 δ18O + 9.98, which are very close to each other and above the GMWL (Global Meteoric Water Line), reveals that rain and fog may originate from water evaporated from the same water sources and under moisture conditions typical of semi-arid regions. Different slopes and interception in LMWL and FWL point to seasonal processes affect fog and rain differently, i.e. summer evaporation of small fog droplets and autumn-winter rainfalls isotopically depleted from frontal or low-pressure systems that are decoupled from fog formation processes. Both isotopic signatures showed low correlation with temperature and total rainfall (R2 <0.3 in all cases). However, the high anticorrelation observed (R2 values >0.6) between the isotopic composition of rain and fog with rainfall totals with values between 100 and 150 mm for both fog and rain, associated with a well-defined synoptic rainfall pattern affecting the Canary Islands, supports the hypothesis that the “amount effect” is related to the moisture source region and transport patterns. Annual variability of the isotopic signatures showed a marked pattern, typical of these latitudes, with enriched values in summer and more depleted in winter for both rainfall and fog. The high amplitude observed in the seasonal pattern of d-excess for both rain and fog, point to the Mediterranean Sea as a potential source of moisture in the winter months or to a mixing process when the moist air mass trajectories pass over North Africa. Despite the similarities observed in these seasonal patterns, some anomalies were found in the summer d-excess, indicating the participation of some local processes to which d-excess is sensitive, such as sub-cloud evaporation or moisture recycling.