What Happens to Precipitation? Hydrologic cycle P = RO + ET + S, Where, P ≡ precipitation RO ≡ runoff ET ≡ evapotranspiration S ≡ storage

Focus on Evapotranspiration Evaporation from Water surface Bare soil 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 D = 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 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 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 0.1233 inches per day