Impact of elevation and weather patterns on the isotopic composition of precipitation in a tropical montane rainforest

Abstract. This study presents the spatial and temporal variability of δ¹⁸O and δ²H isotope signatures in precipitation of a south Ecuadorian montane cloud forest catchment (San Francisco catchment). From 2 September to 25 December 2010, event sampling of open rainfall was conducted along an altitudinal transect (1800 to 2800 m a.s.l.) to investigate possible effects of altitude and weather conditions on the isotope signature.

The spatial variability is mainly affected by the altitude effect. The event based δ¹⁸O altitude effect for the study area averages −0.22‰ × 100 m⁻¹ (δ²H: −1.12‰ × 100 m⁻¹). The temporal variability is mostly controlled by prevailing air masses. Precipitation during the times of prevailing southeasterly trade winds is significantly enriched in heavy isotopes compared to precipitation during other weather conditions. In the study area, weather during austral winter is commonly controlled by southeasterly trade winds. Since the Amazon Basin contributes large amounts of recycled moisture to these air masses, trade wind-related precipitation is enriched in heavy isotopes. We used deuterium excess to further evaluate the contribution of recycled moisture to precipitation. Analogously to the δ¹⁸O and δ²H values, deuterium excess is significantly higher in trade wind-related precipitation. Consequently, it is assumed that evaporated moisture is responsible for high concentrations of heavy isotopes during austral winter.