Investigating the hydrology of tropical cloud forest and coastal fog ecosystems using water stable isotopes

Background/Question/Methods

The stable isotope ratios of water, ¹⁸O/¹⁶O and ²H/¹H, are good tracers for fog and cloud water climate patterns. Isotopes delineate sources and pathways of water in an ecosystem, and are used to determine inputs to soils, streamflow and groundwater, and to identify water sources for plants.  However, quantifying fog or cloud water in water budgets remains difficult, and physical measurements and isotope tracer methods often do not agree.  We are using water stable isotopes to investigate the importance of fog and cloud water to the hydrology of different ecosystems, including the northern coast of California where fog is the only precipitation source during part of the year, and tropical mountain cloud forests in Hawaii and Puerto Rico, where cloud water is abundant but may be changing as regional warming increases cloud base altitude. 

Results/Conclusions

Isotopic composition indicates that cloud water (fog and drizzle) is an important component of headwater streamflow year-round (62% in Puerto Rico, 37% in Maui); and groundwater isotopic composition on Bodega Peninsula in California indicates fog contributes to recharge. Oceanic fog sustains ecosystems in Northern California summers through fog drip and suppression of transpiration; stable isotope tracers can be used to determine whether it also contributes to recharge and streamflow.  On Maui, Hawaii, xylem water isotopic composition was used to trace root-zone water back to its source as rain or cloud water, and indicated different water use strategies by trees at windward (wet) and leeward (dry) mountain slopes. Samples recently taken from the same trees (12 years later) will show whether water sources have changed in the interim.  In Puerto Rico and California, measurement of stable isotope composition at 30-minute intervals during overnight cloud events showed that cloud water isotopic composition is surprisingly variable. Sample volume, changes in source vapor, and temperature all appeared to influence the isotopic composition of the fog.  In Puerto Rico, cloud base altitude and frequency of cloud immersion is monitored with trail cameras, temperature/relative humidity (T/RH) sensors, and a ceilometer, to quantify cloud water deposition and cloud base height, and to develop measurement strategies that can be used at other remote mountain sites.  Initial results show that while T/RH measurements indicate cloud immersion, statistical processing of time-lapse images may resolve liquid water content at sub-daily timescales. Further development of isotope tracer methods, combined with other measurements, has great potential for understanding the importance of fog and cloud water in ecohydrology.