12-VegetationProcesses

# 12-VegetationProcesses - Water balance for a landscape ESM...

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5/14/2008 ESM 266: Vegetation – processes, biomass 1 ESM 266: ESM 266: Vegetation Vegetation— processes, biomass 1 Jeff Dozier Water balance for a landscape Precipitation (P) Evapotranspiration (ET) 2 water table slope Runoff (R = Quickflow + Delayed Flow) (Quickflow leaves the landscape within a few hours of rainfall. Delayed flow represents slow drainage from soil water and ground water.) Conceptual model •F o r s om e Δ t (day, … month) on a unit area of land, the Continuity Equation is Quickflow P ET R net Advection of sensible and latent heat Δ −− = Δ U P Quickflow Delayed flow ET t 3 U is the water content of the “underground store” (soil or ground water) • Units are V/(A×T) or depth/time (e.g., mm/mo, m/s) Soil Recharge Delayed flow Ground water We need to evaluate evapotranspiration ( ET ) • If we could evaluate ET we could use continuity equation as an accounting procedure to calculate runoff and soil moisture for any time period • We could account for the land-atmosphere interaction, including any factor such as vegetation type that might 4 including any factor such as vegetation type that might affect it ET is extremely difficult to measure directly for long periods, so we usually estimate it from the energy balance (rate of expenditure of latent heat) • Phase change to vapor requires an input of 2.5×10 6 J/kg Basic principle • Net radiation ( R net ) drives the sum of sensible ( H ) and latent ( L ) heat exchange with the atmosphere and heat flow into or out soil ( G ) G is normally small – Temperature vapor pressure and soil moisture 5 Temperature, vapor pressure, and soil moisture determine how R net is partitioned between H and L • i.e., the magnitude of the temperature gradient vs. the vapor pressure gradient The energy balance equation (flux per unit area, W m –2 ) S = solar radiation α = albedo: water 0.06; conifer forest 0 09; Amazon H = sensible heat transfer (+ is surface to atmosphere) L = latent heat transfer in water evaporating or condensing (+ is 4 (1 ) net radiation s net SF T H L G R αε σ −+− =++ 6 conifer forest 0.09; Amazon broadleaf forest 0.12; grassland 0.2–0.4 F ± = downward infrared radiation, depends on temperature, water vapor, and clouds T s = surface temperature evapotranspiration, – is condensation) G = heat conducted into soil

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5/14/2008 ESM 266: Vegetation – processes, biomass 2 Convert latent heat flux into mass flux of water Latent heat exchange per unit area converts a volume of water (per unit area) to vapor
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12-VegetationProcesses - Water balance for a landscape ESM...

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