Cloud Water Interception

Transcription

1 Tropical Montane Cloud Forests in Hawai`i Cloud Water Interception T. Giambelluca GEOG 405 University of Hawai i at Mānoa Receive water input directly from cloud Refuge for scarce native species Important source of water Amount of cloud water input unknown Need ET: also unknown Influence of invasive species not understood Question Hawai`i Cloud Zone How much water does CWI contribute to Hawai i s ecosystems and hydrological systems? 1

2 Definitions FS: Fog Screen Catch May contain both fog and rain May be sensitive to wind direction CWF: Cloud Water Flux Wind-advected cloud water through a unit vertical area normal to wind direction Can be derived from FS or from LWC and wind CWI: Cloud Water Interception Cloud water intercepted by vegetation Can be derived from canopy water balance or isotopic balance Fog Gauge Issues Separating rain catch from fog catch Comparability among gauge types Comparability with vegetation Problems with Canopy Water Balance Herwitz and Slye (1992) EXPECTATIONS Wind-blown RF: need to correct RF for slope effects TF variability: Need adequate sample Dry cloud forest sites Ecologically significant input Possibly hydrologically significant input Wet cloud forest sites High fog input insignificant compared with much higher rainfall input 2

3 Limitations of Prior Studies Most prior studies based on fog gauge measurements Most prior studies done at exposed sites: clearings, ridge-tops, isolated trees Waikamoi: Wet Cloud Forest East Maui Field Study HaleNet Auwahi: Dry Cloud Forest Observations Analysis Rainfall Throughfall Fog screen Visibility Meteorological variables (ET) Fog gauge interpretation: (get CWF) Canopy water balance: (get CWI) 3

4 Fog gauge interpretation: Deriving CWF Estimating rainfall input Estimate quantity of water incident screen from drop-size distribution, wind velocity, and wind direction Estimate screen catch efficiency Accounting for effects of varying wind direction Estimate flux of water through surface normal to wind based on wind direction relative to the screen Fog gauge interpretation Drop Trajectory Function of drop size and wind velocity Fog gauge interpretation Canopy Water Balance: CWI Flux through normal surface Accounting for varying wind direction relative to screen Observe RF, TF Estimate ET Correct for slope effects on rain catch 4

5 Annualized Results Annualized Results AUWAHI WAIKAMOI AUWAHI WAIKAMOI No. of Rain and/or Fog Events Event Duration (hr) Mean Event Duration (h:mm) : :07 Incident Rainfall (mm) Throughfall (mm) Event Evaporation (mm) Rainfall (mm) Throughfall (mm) Fog Screen (mm) After-Event Evaporation (mm) Cloud Water Interception (mm) Total Water Input (mm) Number of CWI Events Cloud Water Flux (mm) Results Results Relationships among variables Based on event totals TF=f(R) TF=f(RI) TF=f(FS) TF=f(CWF) TF=f(CWI) CWI=f(FS) CWI=f(CWF) Auwahi r 2 (n) (464) (464) (273) (273) (464) (272) (272)* *not significant at Waikamoi r 2 (n) (438) (438) (438) (438) (438) (437) (437) 5

6 Conclusions of Maui Study Cao and Giambelluca 2007 CWI adds significantly to rainfall at both sites Wet cloud forest site: extremely high CWI (adds 46% to already high rainfall) Dry cloud forest site: CWI adds 28% to rainfall A large portion of CWI, 74% and 83%, respectively for the two sites, was estimated to become TF, adding significantly to soil water CWF estimate based on fog screen had no prediction value at dry site; slightly improved prediction over raw fog screen observations at wet site Point estimates are difficult and inadequate: Need to model process to provide spatially distributed estimates of current and future CWI Big Island Study Mami Takahashi Native and Invaded Forest Sites No fog screen measurements Both TF and SF measurements were done at the two field sites Developed a different model for this analysis Field Sites 6

7 Field Sites Single-layer Canopy Water Balance Model Invaded Forest Site ohia forest invaded by Psidium cattleianum (strawberry guava) Native Forest Site Metrosideros polymorpha ( ohia) Cibotium spp. (hapu u; tree fern) Single-layer Canopy Water Balance Model Results: Annualized Totals RF (mm) CWI (mm) TF (mm) SF (mm) TF+SF (mm) (TF+SF) RF (mm) (TF+SF) CWI (mm) E i (mm) PE (mm) Native site Invaded site

8 Conclusions of Big Island Study Annual CWI, 1188 mm and 734 mm, respectively at the native and invaded forest sites, represents an additional 37 and 20% of rainfall at the two sites. Because of changes in forest canopy structure due to strawberry guava invasion, canopy water storage is low and SF is high at the invaded site. Greater CWI at the native site, perhaps due to more efficient stripping of cloud drops by native Ōhi a 8