HR: 1340h
AN: H43E-0533    [Abstracts]
TI: Seasonal Variation in the Stable Isotopic Composition of Precipitation in the Tropical Montane Forests of Monteverde, Costa Rica
AU: * Rhodes, A L
EM: arhodes@smith.edu
AF: Smith College, Department of Geology, Northampton, MA 01063 United States
AU: Guswa, A J
EM: aguswa@smith.edu
AF: Smith College, Picker Engineering Program, Northampton, MA 01063 United States
AU: Newell, S E
EM: snewell@Princeton.EDU
AF: Princeton University, Department of Geosciences Guyot Hall, Washington Road, Princeton, NJ 08544-1003 United States
AB: The hydrology and ecology of tropical montane cloud forests depend on frequent immersion in clouds and mist. At Monteverde, Costa Rica, climate and land-use change may be diminishing orographic clouds over tropical montane forests, stressing biota and water resources during dry seasons. From 2003-2005, we measured the stable isotopic composition of precipitation and throughfall in Monteverde to distinguish convective, wet-season rainfall associated with the Intertropical Convergence Zone (ITCZ) from dry-season, orographic rain produced by northeasterly trade winds. While event-to-event fluctuations of δ¹⁸O and δ²H values are high, monthly samples reveal a seasonal signal that may be used to trace water through the hydrologic cycle. The seasonal fluctuation in δ¹⁸O and δ²H values is consistent with different degrees of rainout between convective and orographic rainfall. The lightest compositions occur during the wet season when the ITCZ is over Costa Rica. The isotopic composition of rain collected during the drier seasons is distinctly heavier, similar to fog and cloud water reported elsewhere. An attenuated version of this seasonal signal propagates through to stream samples and provides a means of determining the importance of dry-season precipitation to water resources for the region. Calculated deuterium-excess values also show strong seasonal variation. Deuterium-excess is <10 per mil during the first three months of the wet season and rises to ~15 per mil as the wet season progresses, culminating at ~20 per mil during the transitional and dry seasons. These results suggest that recycled water evaporated from the Caribbean slope is an important flux to orographic moisture during the drier seasons. Following a shift to westerly winds at the start of the wet season, however, several months of rain are needed following an acute dry season on the northern Pacific slope before a terrestrial evaporative signal is detected in precipitation. The evaporative flux may result from a wet-season expansion of surface water bodies and flooding of seasonal wetlands.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1836 Hydrological cycles and budgets (1218, 1655)
DE: 1854 Precipitation (3354)
SC: Hydrology [H]
MN: Fall Meeting 2005