Lecture4 - WaterBudgetII: Evapotranspiration P =Q+ET +G+S...

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Water Budget II: Evapotranspiration P = Q + ET + G + Δ S
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Evaporation •Trans fer of H 2 O from liquid to vapor phase – Diffusive process driven by • Saturation (vapor density) gradient ~ ( s a ) •Aer ia l resistance ~ f(wind speed, temperature) •Energy to provide latent heat of vaporization (radiation) • Transpiration is plant mediated evaporation –Same result (water movement to atmosphere) •Summa t ive process =
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Evapo Transpiration •ET is the sum of – Evaporation: physical process from free water •So i l •P lan t intercepted water •Lake
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Four Requirements for ET Vapor Pressure Gradient Energy Water Wind NP TP
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NASA 3850 zettajoules per year
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Energy Inputs •Rad ia t ion Budget –R total = Total Solar Radiation Inputs on a horizontal plane at the Earth’s Surface –R net = R total – reflected radiation = R total * (1–albedo) –A lbedo ( α ) values •Snow 0.9 • Hardwoods 0.2 •Wa te
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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*T mean +8) – Where p is the mean daytime hours –T mean 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 •D i s tan ce between actual conditions and saturation line –G rea te r distances = larger evaporative potential •S lope of this line (
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Water Availability: PET vs. AET •PET (potential ET) is the expected ET if water is not limiting –G iven conditions of: w ind, Temperature, Humidity •AET (actual ET) is the amount that is actually abstracted (realizing that water may be limiting) –AET = * PET –Whe re is a function of soil moisture, species, climate –In Florida, ~ is unity for the summer, 0.75 otherwise •ET :PET is low in arid areas due to water 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 –D ry conditions: when E o :P → ∞ , ET:P 1 and R:P 0 –We t conditions: when E o :P 0 ET
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Budyko Curve
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Wind •W i th evaporation, boundary layer gets saturated –Th i s
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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 - WaterBudgetII: Evapotranspiration P =Q+ET +G+S...

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