WaterEnergyBudgetforVegetatedSoil

WaterEnergyBudgetforVegetatedSoil - 10/20/2005 Fluxes...

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10/20/2005 ESM 203 - Hydrologic cycle - land-atmosphere interactions 1 ESM 203: Land-atmosphere interactions: Water and energy balance of a vegetated soil 1 Jeff Dozier & Tom Dunne Fall 2007 Fluxes between stores (note that the ocean area is about twice the land area) Evaporation from ocean 117 cm/yr Precipitation onto ocean 107 cm/yr Precipitation onto land 74 cm/yr vaporation from land 9 cm/yr 2 Evaporation from land 49 cm/yr Runoff from land 25 cm/yr Over the ocean, E > P Over land, P > E. Suggests that the storage of water on land causes some kind of “resistance” to evaporation, so that some of the precipitated water “escapes” evaporation and survives to run off the continents as streamflow (R). Water and energy balance of a vegetated, soil-covered land surface ± For some Δ t (e.g day or month) on a unit area of land, the mass balance equation for water is Quickflow P E R net Advection of sensible heat ( H ) SM e arg ch Re Quickflow E P Δ = ± SM is the water content of the soil ± Units are [m 3 /(m 2 x t)] or depth/time (e.g., m/mo) Soil Recharge Delayed flow Ground water Suppose …. ± We can measure or predict P (depth or volume/area per time) ± We can predict QuickFlow (e.g. as a fraction of P ) 4 ± We can predict E (depth or volume/area per time) ± Rainfall that does not run away quickly over or under the surface, and is not immediately evaporated enters the soil. And then suppose …. ± The soil has a fixed maximum water holding capacity: h t h t i d t h () d max fc SM D θ = × 5 where D is the rooting zone depth (m) and θ fc is the “field capacity” of the soil (m 3 /m 3 ), and depends on soil texture ± Thus, SM max has dimensions of m (m 3 /m 2 of land surface) Water and energy balance of a vegetated, soil-covered land surface ± For some Δ t (e.g day or month) on a unit area of land, the mass balance equation for water is Quickflow P E R net Advection of sensible heat ( H ) M 6 ± SM is the water content of the soil ± Units are [m 3 /(m 2 x t)] or depth/time (e.g., m/mo) Soil Recharge Delayed flow Ground water SM e arg ch Re Quickflow E P =
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10/20/2005 ESM 203 - Hydrologic cycle - land-atmosphere interactions 2 And that …. ± Soil moisture content, SM (m 3 /m 2 ), varies each day as a result of the accounting: ± If SM rises to SM ax “excess” water draining from SM P E QuickFlow DelayedFlow t Δ −− = Δ 7 max , g the soil recharges the ground water store, which has a volume per unit area (i.e. a depth), V that also changes each day We need three values for this accounting: ± The water-holding capacity (field capacity) of the soil profile, θ c 8 f ± The root zone depth (D) for SM max = D x θ fc The evapotranspiration rate, E Soil particles and soil pores ± Soils consist of particles with a range of size from clay to gravel ± Between the particles re irregular- aped 9 are irregular shaped conduits called pores ± The diameter of the pores is roughly proportional to the sizes of the particles (pores in sand > pores in silt) Water-holding capacity of soil or sponge ± Soil contains pores of differing sizes (cm to μ
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WaterEnergyBudgetforVegetatedSoil - 10/20/2005 Fluxes...

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