What Happens to Precipitation?

• General hydrologic equationPG = 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 + ETDevice for measuring stem flow, http://gaia.fr.a.u-tokyo.ac.jp/~shirakik/fukuro/stem/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 WaterTorr – 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/dayΘi = initial profile wetnessΘf = 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

Ponderosa pine 2.0 to 3.3 0.0800 to 0.1310 Alpine, AZ

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

Annual rainfall of 45 inches is 0.1233 inches per day

Occult/Horizontal Precipitation

• Condensation in canopy exceeds canopy interception

• Net precipitation increases

Monteverde Cloud Forest, Costa Rica

Cloud Forest in Turrialba Mountains, Costa Rica