1. Interception
Interception is the amount of water retained in vegetation
It never reaches soil and evaporates back to atmosphere
In heavily forested regions it may account up to 25% of annual precipitation
Conifers can intercept more water than hardwoods
Interception is usually a function of storm characteristics, vegetative species, density and season
Crown interception: leaf, twig, stem
Forest floor interception
Forest floor interception averages about 5% of annual “P” under mature pine and 3% under mature hardwoods
Any benefit?

2. Litter Interception
Reduces quantity of precipitation reaching the mineral soil
Affects the velocity of overland flow, which allows more time for soils to absorb runoff water
Protects ground surface from direct impact of raindrop energy and wind energy as well as shades the soil surface, which in turn can reduce soil evaporation
Thus, conservation of soil moisture due to evaporation reduction for a period of time can outweigh litter interception of the forest floor
Runoff and sediment production in areas covered by litter are much lower than in areas with no cover of litter
1-cm thick floor of crop litter can reduce potential evaporation to 46% and can further reduce it to about 17 and 6% if the litter floor is increased to 3 and 5 cm in depth, respectively.
A litter floor of 3 cm is common in forest stands. The soil moisture conservation of a 3-cm litter floor in a 5-day period can reach 20 mm of water, much greater than the 1-5% of rainfall that can be lost due to litter interception in one or two storms.

3. Net Precip
I = Ican + Ilit Pnet = Ptot – (Ican + Ilit) Ptot = Pstem + Pthrough + Ican Pnet = Pstem + Pthrough - Ilit
T, E E

4. Throughfall & Stemflow as a Function of Gross Precipitation
Species
Throughfall (mm)
Stemflow (mm)
Red pine
0.87P-1.02
0.02P
Loblolly pine
0.80P-0.25
0.08P-0.51
Shortleaf pine
0.88P-1.27
0.03P
Ponderosa pine
0.89P-1.27
0.04P-0.25
Eastern white pine
0.85P-1.02
0.06P-0.25
Spruce-fir-hemlock
0.77P-1.27

6. Interception
Studies have shown that total interception loss in dark fir and spruce forests in Russia can reach 40 to 60%.
Interception increases with maturity of trees
Total interception loss (mm) in a season for eastern white pine stands in western NC (Helvey 1967)
10-yr stand: 0.08P n
35-yr stand: 0.12P n
60-yr stand: 0.18P n
n: # of storms larger than a trace amount

8. Interception Ic : Total interception for the projected canopy area [L]
Sv : Storage capacity of vegetation for projected area of canopy usually inches ( mm)
LAI : Leaf area index; ratio of vegetal surface area to its projected area. Forest LAI: generally 5-15
E : Evaporation rate during storm from plant surfaces
td : Duration of rainfall
P : Total precipitation depth (same unit with Sv)

9. Interception
For:
LAI =5
E = 6 mm/day
td = 2 hr
2nd term = 0.1 in

10. Interception
In general interception within a storm can be expressed as
I = a + b ∙ Pn (inch)
Some typical values for a, b, and n (Gray, 1973) a b n
Orchards
0.04
0.18
1.0
Oak
0.05
Maple
Willow, Shrubs
0.02
0.40
Pine
0.20
0.5
Corn
0.005h
Meadow, grass
0.08h
h = plant height
(in feet)

11. Canopy Condensation
Along coastal areas, on top of mountains, or at sites prone to the occurrence of fog, the contact of moist, warm air mass, fog, and cloud with dry, cold forest canopies causes condensation of water vapor on the foliage.
Condensed water then drips from the foliage or runs down the stems to the ground as an additional water supply. This process, reversing the effect of canopy interception, is often referred to as horizontal precipitation, occult precipitation, cloud drip or fog drip.
In San Francisco summer is dry and most rainfall occurs in winter. Fog drip under tan oak over a period of about 1 month (July 20 to August 28) in Santa Cruz Mountains totaled mm of water, higher than annual precipitation.