Lecture4 - Water Budget II: Evapotranspiration P = Q + ET +...

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Water Budget II: Evapotranspiration P = Q + ET + G + ΔS
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Evaporation Transfer of H2O from liquid to vapor phase Diffusive process driven by Saturation (vapor density) gradient ~ ( ρ s – ρ a) Aerial resistance ~ f(wind speed, temperature) Energy to provide latent heat of vaporization (radiation) Transpiration is plant mediated evaporation Same result (water movement to atmosphere) Summative process = evapotranspiration (ET) Dominates the apportionment of rainfall ~ 95% in arid areas ~ 70% for all of North America
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Evapo-Transpiration ET is the sum of Evaporation: physical process from free water Soil Plant intercepted water Lakes, wetlands, streams, oceans Transpiration: biophysical process modulated by plants (and animals) Controlled flow through leaf stomata Species, temperature and moisture dependent
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Four Requirements for ET por Pressure Gradient Ener gy Wat er Win d NP TP
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NASA 3850 zettajoules per year
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Energy Inputs Radiation Budget Rtotal = Total Solar Radiation Inputs on a horizontal plane at the Earth’s Surface Rnet = Rtotal – reflected radiation = Rtotal * (1 – albedo) Albedo (α) values Snow 0.9 Hardwoods 0.2 Water 0.05 Flatwoods pine plantation 0.15 Flatwoods clear cut ____ Burn ____ Asphalt 0.05
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Energy and Temperature The simplest conceptualization of the ET process focuses solely on temperature . Blaney-Criddle Method: ET = p * (0.46*Tmean+8) Where p is the mean daytime hours Tmean is the mean daily temp (Max+Min/2) ET (mm/day) is treated as a monthly variable
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Vapor Deficit – Drives the Process Distance between actual conditions and saturation line Greater distances = larger evaporative potential Slope of this line ( δ ) is an important term for ET prediction equations Usually measured in mbar/°C
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Water Availability: PET vs. AET PET (potential ET) is the expected ET if water is not limiting Given conditions of: w ind, Temperature, Humidity AET (actual ET) is the amount that is actually abstracted (realizing that water may be limiting) AET = α * PET Where α is a function of soil moisture, species, climate In Florida, ~ α is unity for the summer, 0.75 otherwise ET:PET is low in arid areas due to water limitation ET ~ PET in humid areas due to energy limitation
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A Simple Catchment Water Balance Consider the net effects of the various water balance components (esp. ET) ET controlled by water availability and atmospheric demand The “Budyko” Curve Dry conditions: when Eo:P → ∞, ET:P → 1 and R:P → 0 Wet conditions: when Eo:P → 0 ET → Eo
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Budyko Curve
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Wind With evaporation, boundary layer gets saturated This inhibits further evaporation UNLESS new air with a vapor deficit replaces Turbulence at boundary layer is therefore necessary to ensure a steady supply of undersaturated air
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This note was uploaded on 12/08/2011 for the course FOR 3400 taught by Professor Staff during the Fall '11 term at University of Florida.

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Lecture4 - Water Budget II: Evapotranspiration P = Q + ET +...

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