ATS2545 Lecture Notes - Lecture 3: Fog Drip, Throughfall, Terminal Velocity
25 May 2018
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Lecture 3 – Canopy Interception and Stemflow
Interception
• The temporary storage of water from precipitation on surfaces that are not part
of the mineral regolith and from which it therefore cannot infiltrate
• One of components of partitioning of rainfall
• Rain strikes organic things
• Plants have certain capacity for water
• Before the rain reaches the ground – intercepting (evaporates)
• One of the components of the partitioning of rainfall
• Some leaves are hydrophobic – loosely held water on waxy leaves: droplets of
water
• Hydrophilic leaves hold much more water
• Interception can occur on the ground – bark or canopies (leaves) or foliage
• First stage of hydrologic cycle
The Partitioning of Rainfall
• The division of arriving rain into various pathways, including canopy
interception, Stemflow, through fall, surface ponding, infiltration into the soil
and surface runoff
• The pathway of rainfall follows when it begins to interact
Canopy Storage Capacity
• The maximum depth (mm) or volume (cm^3) of water that can be held on the
plant canopy
Interception Loss
• The amount of water lost to evaporation from the wet plant canopy during an
interval such as a year
• Expressed as a % of annual rainfall but can be stated in mm
Precipitation
• Characteristics of raindrops
o Form around small salt particles (condensation nuclei) which are
hygroscopic
o Reach a terminal velocity that is a function of their weight
▪ Larger ones fall at a reduced speed
▪ Typical terminal velocities in still are 10 m/s or about 30km/h
o Raindrops only splash if there is already water present on the floor
o Raindrop sizes seen at the ground depend on two factors:
▪ Updraft speed
▪ Calm air it can drizzle and droplets can be very small
▪ Largest drop size is determined by the strength of surface
tension that holds drops into a roughly spherical shape
o Very large drops wobble and break apart into smaller pieces
o Most common size is 2-2.5 mm diameter
o Kinetic energy carried by raindrops arriving at all the soil surface at
terminal velocity is transferred to small soil particles that are splashed
into the air
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Document Summary
Interception can occur on the ground bark or canopies (leaves) or foliage. The partitioning of rainfall: the division of arriving rain into various pathways, including canopy interception, stemflow, through fall, surface ponding, infiltration into the soil and surface runoff, the pathway of rainfall follows when it begins to interact. Canopy storage capacity: the maximum depth (mm) or volume (cm^3) of water that can be held on the plant canopy. Interception loss: the amount of water lost to evaporation from the wet plant canopy during an interval such as a year, expressed as a % of annual rainfall but can be stated in mm. Stemflow: water that lands on the vegetation, water that is intercepted by stems and branches may travel down these as a film of flowing water, volume of stemflow is affected by the nature of the leaves. Methods of measurement: slit tree and wrap with hose.
