introduction to forest hydrology · introduction to forest hydrology author: hal liechty created...

Net Precipitation

Net Precipitation= This is the amount of water reaching the mineral soil

Pg-It

Pg=precipitation It=total interception

Interception & Net Precipitation

Interception= the removal of water from precipitation by

living or dead vegetation (and then evaporation) before it reaches the mineral soil

(stored on vegetation and lost as

evaporation from vegetation during

or after storm)

It=Ic+Il

It=total interception

Ic=canopy interception

Il=litter interception

Interception & Net Precipitation

Canopy Interception 10-20% lost due to interception (varies due to

stand characteristics and climate)

Forest Floor Interception 1-5% and depends on thickness of litter, water

holding capacity, frequency of wetting, and

evaporation rate.

Interception & Net Precipitation

Interception increase with time

1) Interception increases during storm event and

then levels off

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 20 40 60 80 100 120

# min. since storm start

"Inter. Storage

Capacity

Canopy Interception

Ic=Pg-Th-Sf

Pg=Total Precipitation

Th=Throughfall

Sf=Stemflow ---describe each----

Interception & Net Precipitation

Water not intercepted by canopy

1) throughfall- precipitation passing through

canopy or dripping from a canopy

2) stemflow- water passing from the canopy,

down the stem to the soil

Factors Effecting Interception

Storm Factors

-Interception loss is greater for rain than snow

-Low intensity, long duration storms have more

interception loss due to greater evaporation of

interception during the rainstorm

-Short duration storms with windy conditions

decrease interception due to movement of foliage

etc.

Factors Effecting Interception

Storm Factors

-Interception loss is greater for rain than snow

Factors Effecting Interception

Climatic Conditions -Higher humidity less evaporation & thus

interception

-Vegetation temperature, higher temperature

more evaporation & interception

Factors Effecting Interception

Climatic Conditions Interception can be an increase in net

precipitation in humid areas where foliage

intercepts fog which condenses and drips to soil

Factors Effecting Interception

Vegetation -Interception increases with increasing leaf area thus

a) interception increases with age/species

b) denser stands have more interception

c) during the dormant season

hardwoods have less interception

d) forest floor interception increases with

density, age, slow decomposition

Factors Effecting Interception

Tree Species (Density/Species)

Basal Area

(m2/ha)

%

Interception

Longleaf 20.9 12.4

Shortleaf 39.5 18.5

Loblolly 37.9 15.5

Slash 36.2 16.8

Factors Effecting Interception

Age (White pine stands)

If=1.27N +0.08 Pg (10 year old stands)

If=1.27N+0.12 Pg (35 year old stands)

If=1.52N+0.18 Pg (60 year old stands)

N=# storms

Pg=Precipitation “

Factors Effecting Interception

Age Forest Floor Int.

Factors Effecting Interception

Vegetation -Interception increases with increasing layers of vegetation (canopy structure)

Factors Effecting Interception

Vegetation -Interception increases with increasing layers of vegetation (canopy structure)

Factors Effecting Interception

Vegetation Type

Conifers =0.03-6.6mm

Hardwoods=0.3-2.0 mm

Shrubs=0.3-2.0mm

Grass=1.0-1.5mm

Factors Effecting Interception

Vegetation -Interception is altered by vegetation physiology

a) Waxy leaves have less interception

b) Smooth bark decreases interception due

an increase in water movement on bark and

an increase in stemflow

c) Needle shape and the greater leaf

area of conifers increases interception

Factors Effecting Interception

Interception Loss—10-40% of precipitation Stemflow <2-3% of precipitation Throughfall 96-99% of precipitation

Precipitation, Throughfall, & Stemflow in

Pines & Pine/Hardwoods Ouachita Mtns.

0

10

20

30

40

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60

PREC Tf PINE Sf PINETf P/H Sf P/H

Redistribution of Water By Vegetation

Types of vegetation determines stemflow , throughfall, & canopy drip -Pendulous branches: favor canopy drip -Erect of acute branches: favor stemflow