HR: 17:00h
AN: H54C-05    [Abstracts]
TI: Using Stable Isotopes to Trace Orographic Precipitation in a Tropical Montane Cloud Forest, 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: snewell1013@yahoo.com
AF: Smith College, Department of Geology, Northampton, MA 01063 United States
AB: The ecology of the tropical montane cloud forests (TMCFs) in Monteverde, Costa Rica is influenced by sharp differences in precipitation patterns that vary with season, which is controlled by the migration of the Intertropical Convergence Zone (ITCZ) over Central America. The majority of precipitation to the region occurs during the wet season when the ITCZ brings convective rainfall. During the transitional and dry seasons, mist and fog are generated by orographic uplift of moisture carried by trade winds from the Atlantic. This hydrologic flux may be most important in sustaining canopy species during the dry season, yet it may also be most sensitive to changes in climate. To assess the viability of using stable isotopes to trace orographic precipitation through the environment, we sampled open precipitation and throughfall from June 2003-June 2004 in secondary and primary forests located on the leeward slope of the Cordillera Tilaran. The wet season yielded a wide range of isotopic values, from -13.9 to -3.1\permil (\delta$^{18}$O) and -103 to -13\permil (\delta$^{2}$H), and the lightest samples were collected when the ITCZ was positioned over Costa Rica. Rain and mist collected during the transitional and dry seasons are much heavier, ranging from -3.9 to -1.4\permil (\delta$^{18}$O) and -14 to +7\permil (\delta$^{2}$H), and are similar to cloud water compositions reported elsewhere. This observed variation is consistent with storms having different travel histories. Heavy compositions result from orographic events and compositionally light precipitation is produced from air masses with longer storm tracks. From the wet to the transitional season, d-excess values increase from +15 to +19\permil, and the highest d-excess values occur during the transitional and dry seasons when orographic precipitation dominates, indicating a recycled water contribution to the regional hydrologic cycle. The presence of a recycled water indicates that lowland deforestation, and an associated reduction in transpiration, could reduce precipitation in Monteverde, and this impact would be most severe in the dry season.
DE: 1899 General or miscellaneous
DE: 1854 Precipitation (3354)
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting