Download Vaporization Process

What Happens to Precipitation?
• General hydrologic equation
PG = RO + ET + ST,
Where,
•
•
•
•
PG ≡ gross precipitation
RO ≡ runoff
ET ≡ evapotranspiration
ST ≡ storage
Slumping in Honduras
Forest Hydrology
• What happens to
precipitation falling on a
forest?
– Gross precipitation, PG, equals
throughfall, T, plus stemflow,
SF, plus evapotranspiration, ET
PG = T + SF + ET
Device for measuring stem
flow, http://gaia.fr.a.utokyo.ac.jp/~shirakik/fukuro/st
em/stem.html
Throughfall
Device for measuring throughfall,
http://water.usgs.gov/nrp/proj.bib/hawaii/throughfall.htm
Forest Hydrology
• Evapotranspiration, ET, includes
– Evaporation from surfaces of,
• Water
• Soil
• Vegetation
Massive slump deposited debris in La Libertad
Forest Interception
• Total interception, IF , is canopy
interception, IC, plus litter interception, IL
IF = IC + IL
• Precipitation reaching mineral soil, called
effective precipitation, PE, gross
precipitation, PG, minus, total interception, IF
PE = PG - IF
Focus on Evapotranspiration
• Transpiration
– Evaporation from
vegetative surfaces
• Primarily through
stomata
• It occurs chiefly while
the stomata are open
for the passage of CO2
and O2 during
photosynthesis.
Vaporization Process for
• Water Surface
• Bare Soil
• Vegetative Surfaces
The “Heat of vaporization” is the heat required to
vaporize one mass unit (one gram) of a substance at its
normal boiling point. The vaporization heat of water is
540 calories per gram.
Evaporation from Water Surface
• Evaporation – net loss of water molecules
• Condensation – net gain of water
molecules
• Saturation – equilibrium between
evaporation and condensation
Evaporation Rate
Water vapor is a gas.
Therefore it exerts pressure in the air.
e = vapor pressure of atmosphere in
mbar (millibars of mercury)
es = vapor pressure of saturated
atmosphere
(es – e) = saturation vapor deficit
Vapor Pressure of Water
Torr – unit of pressure force per unit area, millimeters
of mercury
Convert water vapor pressure to
water vapor density
for use in evaporation formula
ρ (g/m3) = 217e/T
T = degrees Kelvin
Evaporation Rate
V (g/cm2/sec) = - (ρs- ρa)(Dv)/dv
ρs= water vapor density at surface of water
ρa = water vapor density of air
(ρs – ρa) = vapor density gradient in g/cm3
Dv = diffusion coefficient of water vapor in the air
in cm2/sec
dv = thickness of wind speed dependent
boundary layer in cm
Evaporation from Bare Soil
• Stage I – wet soil surface, similar to
evaporation from water surface
• Stage II – evaporation rate falls below
potential rate and is determined primarily
by hydraulic conductivity
• Stage III – liquid flux of water ceases, rate
determined by vapor flux governed by soil
vapor diffusivity
Definitions
• Hydraulic conductivity or coefficient of
permeability – movement of water within
soil from areas of higher moisture content
to lower moisture content
• Flux - the rate of transfer of fluid, particles,
or energy across a given surface
Definitions
• Diffusion - the flow of energy or matter
from a higher concentration to a lower
concentration
Bare Soil
E = cumulative evaporation in cm
_
= 2(Θi – Θf)[Dt/π]0.5
where,
t = time in days
_
Dt = weighted mean diffusivity in cm2/day
Θi = initial profile wetness
Θf = final surface wetness
Evapotranspiration (EV)
Thornthwaite model of potential EV (PE)
PE = 16 (10Tm/I)a (K),
where,
Tm = monthly air temperature
I = sum of monthly heat index
∑(Tm/5)1.514
a = polynomial in I
K = adjustment in length of day for 12 hour period
Example of EV
calculations used for
watershed management in
Colorado
Forested Watershed
Actual evapotranspiration (AE)
= (1 – α)E + α(T + I),
where,
α = watershed forest coverage
E = evaporation from soil and water surfaces
T = forest transpiration (about 60% of AE)
I = forest canopy and litter interception (loss is
about 10% to 30% of precipitation)
Average Daily ET Rates by Species
Species
mm/day
inches/day
Location
Douglas fir
2.1
0.0821
Seattle, WA
Slash pine
3.0
0.1191
Gainesville, FL
2.0 to 3.3
0.0800 to 0.1310
White pine
3.1
0.12131
Cowetta, NC
Pinyon-juniper
1.2
0.0476
Flagstaff, AZ
Spruce-fir
1.2
0.0476
Frazier, CO
Aspen
1.5
0.0595
Bountiful, UT
Oak-hickory
2.6
0.1032
Cowetta, NC
Yellow poplar
1.7
0.0675
East Tennessee
Ponderosa pine
Annual rainfall of 45 inches is 0.1233 inches per day
Alpine, AZ
Occult/Horizontal Precipitation
• Condensation in
canopy exceeds
canopy
interception
• Net precipitation
increases
Monteverde Cloud Forest, Costa Rica
Cloud Forest in Turrialba Mountains, Costa Rica