Lecture 12 Precipitation Interception (1)

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Lecture 12 Precipitation Interception (1)
Interception Processes

• General Comments
• Controls on Interception
• Interception in Woodlands
• Interception in Grasslands
• Interception by Crops
• Measurement of Relative Humidity

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Interception 
Water abstracted from gross precipitation by
leaves and stems of a vegetation canopy and
temporarily stored in its surfaces.
Interception Loss
Intercepted water lost by evaporation to the
atmosphere before reaching the soil surface
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General Comments

• Accounts for much of the variability in
  evaporation and transpiration between plant
  species or associations
•  
• Precipitation is usually intercepted by
• Tree canopy
• Grass
• Shrubs
• Litter
• Moss
• Built structures
•  
• Interception capacity is usually considered to
  be a fixed amount for a given site 
• Canopy
• Shrubs
• Urban
•  
• During filling and once storage is full, water
  passes through the canopy and reaches the soil
  as
• Throughfall (TF)
• Stemflow (SF)

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Net Rainfall
1.
2.
3.
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Terms to Remember

1. Interception loss Part of the rainfall
   intercepted by a plant canopy is evaporated back
   into the atmosphere and takes no part in the
   land-bound portion of the hydrological cycle
2. Throughfall raindrops and snowflakes that fall
   through gaps in the plant canopy and water which
   drips from leaves, twigs and stems
3. Stemflow Water run down the main stem or trunk
   from twigs and branches to the ground
4. Gross rainfall rainfall on top of plant canopies
5. Net rainfall The sum of throughfall and stemflow
6. Negative interception Water intercepted from
   fogs and mists that contributes to stemflow

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Controls on the amount of interception
1. Vegetation form/structure   Shape Branch/leaf
orientation Broad vs. needle leaves Number of
leaves/stems Surface texture Flexibility/turgidity
/stability   2. Vegetation growth
pattern/physiology   Seasonal growth Deciduous
habit Total biomass Form/structure Age Growth
rate Density of stand Leaf Area Index (LAI)  
3. Meteorological Conditions   Precipitation
intensity and duration --Heavy and long
duration precipitation will quickly exceed crown
capacity leading to greater TF and SF
--Conifers intercept more because they coincide
with gentler rain or snow --Often possible to
relate/predict losses from total P? Phase of
precipitation Snow/sleet/rain/hail Wind speed and
turbulence Energy balance Albedo related to
vegetation type
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Additional Points to Note

• These botanical and meteorological factors
  generally apply to non-botanical surfaces as well
  (e.g., urban surfaces)
•  
• Strong dependence on meteorological factors
  allows interception, TF, or SF to be estimated
  from empirical relations
•  
• Originally believed that interception losses
  were balanced by reduced transpiration losses.
  This is now believed to be incorrect
•  
• Interception is not an alternate loss, rather an
  additional one

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Interception loss during precipitation event

• Interception losses are greatest early during a
  precipitation event
•  
• Losses decrease when interception storage is
  filled
• Interception ratio (Interception loss) / (total
  precipitation)

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Interception Loss from Woodlands

• Generally deciduous crown closure gt conifer
  crown closure
• However, conifer stands tend to exhibit higher
  interception losses because of higher leaf area
  density 
• Conifer interception losses 25-35
• Decid. Interception losses 10-30
•  
• Potential reasoning needle shapes and
  distributions relative to broadleaves
• Spatially variable density of trees (spacing)

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Interception losses from grasses/shrubs

• LAI of mature, homogeneous grass cover is
  generally much smaller than that of forests
•  
• Higher aerodynamic resistance than tall
  vegetation thus, less interception loss
•  
• Grazed or cut grasslands exhibit greatly reduced
  storage
•  
• Interception losses vary 13-26
•  

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Interception losses from agricultural crops 

• Usually evenly spaced plants
•  
• Highly dependent on stage of development
• Depending on LAI
•  

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Interception of snow  

• Difficult to measure and highly variable
  spatially due to wind redistribution
•  
• Idea snow accumulation on canopy decreases
  aerodynamic resistance (smooth)
•  
• Thus, evaporation rates should be lower than for
  wet canopy
•  
• Snow often melts, slides, slips, or is blown off
  of vegetation
•  
• Studies indicate only 15 of intercepted snow
  sublimates or evaporates
•  
• Snow-stored water can be much greater than water
  storage potential for more evaporation is there
  but energy requirements are not always met
•  

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Fog and clouds  

• Deposition of fine water droplets to vegetated
  surfaces (e.g., mist, fog, clouds)
• Too fine to precipitate and would not be
  collected by rain gauges
• Negative interception Kittredge (1948)
• More common in mountainous regions and coastal
  areas
• Can be a significant addition of moisture to
  local vegetation
• Different process than dew, which is temperature
  controlled condensation of water vapor
•  

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Instrument for measuring air humidity 
http//www.mtc.com.my/publication/library/drying/f
ig5.gif
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Relative humidity 

• Ratio of the actual amount of moisture in the
  atmosphere to the amount of moisture the
  atmosphere can hold
• Therefore, a relative humidity of 100 means the
  air can hold no more water (rain or dew is
  likely)
• Relative humidity of 0 indicates there is no
  moisture in the atmosphere.

eswb Saturation vapor pressure at Twb (kPa)
esdb Saturation vapor pressure at Tdb (kPa)
ed Vapor pressure (kPa) Elv Elevation
above sea level (m) P Air pressure (kPa) Twb
Wet bulb temperature (C) Tdb Dry bulb
temperature (C)  
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Procedure for Calculating Relative Humidity
1. Approximate the air pressure, P in kPa
(kiloPascals). If you don't know your elevation,
use P 101.325 kPa. P 101.325exp(-0.0001184
? Elv) 2. Calculate a conversion factor, A. A
0.00066(1.0 0.00115 ? Twb) 3. Calculate the
saturated vapor pressure at Twb. eswb
exp(16.78 ? Twb 116.9) / (Twb 237.3)   4.
Calculate the vapor pressure, or the partial
pressure of water vapor, ed in kPa. ed eswb
AP(Tdb Twb)   5. Calculate the saturated vapor
pressure at Tdb. esdb exp (16.78 ? Tdb
116.9) / (Tdb 237.3)   6. Finally, calculate
the relative humidity, RH, in percent. RH 100
? (ed / esdb)