1. What Happens to Precipitation?
General hydrologic equation
PG = RO + ET + ST,
Where,
PG ≡ gross precipitation
RO ≡ runoff
ET ≡ evapotranspiration
ST ≡ storage
Slumping in Honduras

2. 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,

3. Throughfall
Device for measuring throughfall,

4. Forest Hydrology Evapotranspiration, ET, includes
Evaporation from surfaces of,
Water
Soil
Vegetation
Massive slump deposited debris in La Libertad

5. 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

6. Focus on Evapotranspiration
Evaporation from vegetative surfaces
Primarily through stomata
It occurs chiefly while the stomata are open for the passage of CO2 and O2 during photosynthesis.

7. 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.

8. Evaporation from Water Surface
Evaporation – net loss of water molecules
Condensation – net gain of water molecules
Saturation – equilibrium between evaporation and condensation

9. 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

10. Vapor Pressure of Water
Torr – unit of pressure force per unit area, millimeters of mercury

11. Convert water vapor pressure to water vapor density for use in evaporation formula
ρ (g/m3) = 217e/T
T = degrees Kelvin

12. 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

13. 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

14. 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

15. Definitions
Diffusion - the flow of energy or matter from a higher concentration to a lower concentration

16. 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

17. 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

18. Example of EV calculations used for watershed management in Colorado

19. 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)

20. Average Daily ET Rates by Species
mm/day
inches/day
Location
Douglas fir
2.1
0.0821
Seattle, WA
Slash pine
3.0
0.1191
Gainesville, FL
Ponderosa pine
2.0 to 3.3 to
Alpine, AZ
White pine
3.1
Cowetta, NC
Pinyon-juniper
1.2
0.0476
Flagstaff, AZ
Spruce-fir
Frazier, CO
Aspen
1.5
0.0595
Bountiful, UT
Oak-hickory
2.6
0.1032
Yellow poplar
1.7
0.0675
East Tennessee
Annual rainfall of 45 inches is inches per day

21. Occult/Horizontal Precipitation
Condensation in canopy exceeds canopy interception
Net precipitation increases
Monteverde Cloud Forest, Costa Rica

22. Cloud Forest in Turrialba Mountains, Costa Rica