WaterEnergyBudgetforVegetatedSoil

# WaterEnergyBudgetforVegetatedSoil - ESM 203:...

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1 ESM 203: Land-atmosphere interactions: Water and energy balance of a vegetated soil Jeff Dozier & Tom Dunne Fall 2007

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2 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 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 Δt (e.g day or month) on a unit area of land, the mass balance equation for water is 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 Quickflow P E net Advection of sensible heat ( H ) Ground water t SM e arg ch Re Quickflow E P = - - -

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4 Suppose …. We can measure or predict P (depth or volume/area per time) We can predict QuickFlow (e.g. as a fraction of P ) 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.
5 And then suppose …. The soil has a fixed maximum water holding capacity: 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) max fc SM D θ =

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6 Water and energy balance of a vegetated, soil-covered land surface For some Δt (e.g day or month) on a unit area of land, the mass balance equation for water is 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 Quickflow P E net Advection of sensible heat ( H ) Ground water t SM e arg ch Re Quickflow E P = - - -
7 And that …. Soil moisture content, SM (m 3 /m 2 ), varies each day as a result of the accounting: If SM rises to SM max , “excess” water draining from the soil recharges the ground water store, which has a volume per unit area (i.e. a depth), V that also changes each day SM P E QuickFlow DelayedFlow t - - - =

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8 We need three values for this accounting: The water-holding capacity (field capacity) of the soil profile, θ fc The root zone depth (D) for SM max = D x θ fc The evapotranspiration rate, E
9 Soil particles and soil pores Soils consist of particles with a range of size from clay to gravel Between the particles 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)

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10 Water-holding capacity of soil or sponge Soil contains pores of differing sizes (cm to μ m)
11 Experiment in the bathtub to understand the water-holding capacity of a soil 1. Get into the bathtub

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12 Experiment in the bathtub to understand the water-holding capacity of a soil 2. Squeeze a sponge underwater, release pressure so that sponge is saturated, and place it on belly
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## This note was uploaded on 08/06/2008 for the course ESM 203 taught by Professor Dozier,dunne during the Fall '07 term at UCSB.

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WaterEnergyBudgetforVegetatedSoil - ESM 203:...

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