Examination of the Stable Isotope Content of Precipitation in a Pan-Tropical Context: Ecohydrological Implications

Abstract:

As part of the U.S. Department of Energy’s Next Generation Ecosystem Experiments-Tropics we are examining the stable isotope compositions of precipitation (δ²H and δ¹⁸O) at a pantropical scale. We are using the IAEA Global Network of Isotopes in Precipitation (GNIP) database supplemented with literature values from the tropics to make our assessments. These data relate to tropics ecohydrology in a number of ways including moisture sources, characteristics of tropical climate subzones, seasonality, and moisture recycling (local re-precipitation of water vapor derived from ET). We are using monthly precipitation isotope data from 219 tropical sites located within the Köppen-Geiger Tropics climate zone. We further classified the sites into the three Köppen-Geiger subzones for our initial assessments (Af-Rainforest (71 sites), Am-Monsoon (34 sites), Aw- Savannah (114 sites)). Our results show that there are statistically significant isotopic differences between Tropical Savannah (Aw) versus rainforest (Af) and monsoon (Am), but no difference was found between rainforest (Aw) and monsoon (Af). The importance of moisture recycling in the Amazon Basin is well established and d-excess values of stable isotopes clearly showed the recycling effect in previous studies. However, there has been no systematic pantropical assessment of recycling. We calculated d-excess values for the supplemented GNIP data for locations within the Köppen-Geiger Tropical subzones. Previous researchers used d-excess values over 10 permil as evidence for recycling based on monthly or seasonal data. We examined the data set based on monthly d-excess values over 10 and also 15 permil. Results show clear recycling effects for all the Köppen-Geiger Tropics subzones even when 15 permil is used as a threshold for recycling. However, stations on oceanic islands (e.g., the Caribbean) tend to show weaker recycling than other areas such as the Amazon Basin and parts of Africa. These results have strong implications for the relative partitioning of water between transpiration (minimal effect on d-excess) and evaporation (large effect on d-excess). Other aspects of the tropics stable isotope data are being investigated such as effects of the Intertropical Convergence Zone and within site seasonality.