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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Water infiltration, retention and movement / Soil evaporation stages and vapour transport

Soil evaporation stages and vapour transport

Evaporation from bare or lightly covered soil runs through an energy-limited wet stage, a liquid-supply-limited falling-rate stage and a vapour-diffusion stage, with latent cooling and thermal vapour flow coupling water and heat.

Boundary: where this process starts

Starts with: Stage 1: a wet surface evaporates at the atmospheric demand rate, typically 3-8 mm per day in sunny weather

Boundary: where it ends

Ends after: Globally transpiration is about 61% of land evapotranspiration, so soil and canopy evaporation supply the other 39%

PARTICIPANTSRESOURCESSoil surfaceSunSoil waterWindSoil poresOtherparticipantsWaterWater vapourSolar energyHeatPore spaceImpact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.↻ balances · moderate · 0.3 Cooling lowers vImpact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.Impact -4 of 5. Click for details and evidence.↻ balances · strong · 0.5 A thicker dry layeImpact +2 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.Impact 02 of 5. Click for details and evidence.Impact 02 of 5. Click for details and evidence.Stage 1: a wet surface evaporates at the atmospheric demand rate, typically 3-8 mm per day in sunny weather: uses up Wateruses upStage 1: a wet surface evaporates at the atmospheric demand rate, typically 3-8 mm per day in sunny weather: makes Water vapourmakesStage 1: a wet surface evaporates at the atmospheric demand rate, typically 3-8 mm per day in sunny weather: uses Solar energyEvaporating 1 mm of water absorbs about 2.45 MJ per square metre, cooling the surface: uses up Wateruses upEvaporating 1 mm of water absorbs about 2.45 MJ per square metre, cooling the surface: uses up Heatuses upA cooler surface has lower saturation vapour pressure, about 6-7% lower per degree Celsius, which slows evaporation: uses Water vapourStage 1 ends when the readily evaporable water is used: about 2-7 mm for sand and 8-12 mm for loam and clay: uses up Wateruses upStage 2: liquid flow to the drying front limits loss, so cumulative evaporation grows with the square root of time: uses WaterStage 2: liquid flow to the drying front limits loss, so cumulative evaporation grows with the square root of time: uses Pore spaceStage 3: the drying front retreats below the surface and vapour diffusion through the dry layer limits loss, often below about 0.5 mm per day: uses Water vapourStage 3: the drying front retreats below the surface and vapour diffusion through the dry layer limits loss, often below about 0.5 mm per day: uses Pore spaceTemperature gradients drive vapour from warm to cold soil, and at night vapour condenses near the cold surface; the vapour flux can exceed pure Fick diffusion by several-fold: uses WaterTemperature gradients drive vapour from warm to cold soil, and at night vapour condenses near the cold surface; the vapour flux can exceed pure Fick diffusion by several-fold: uses Water vapourTemperature gradients drive vapour from warm to cold soil, and at night vapour condenses near the cold surface; the vapour flux can exceed pure Fick diffusion by several-fold: uses HeatStraw mulch or surface residue lowers soil evaporation by about 10-50% depending on cover and climate: uses WaterStraw mulch or surface residue lowers soil evaporation by about 10-50% depending on cover and climate: uses Solar energyGlobally transpiration is about 61% of land evapotranspiration, so soil and canopy evaporation supply the other 39%: uses WaterGlobally transpiration is about 61% of land evapotranspiration, so soil and canopy evaporation supply the other 39%: makes Water vapourmakesevaporates atatmospheric demandWater: evaporates at atmospheric demandreleased to theatmosphereWater vapour: released to the atmospheredrives evaporativedemandSolar energy: drives evaporative demandevaporated to coolsurfaceWater: evaporated to cool surfaceabsorbed by evaporationHeat: absorbed by evaporationsaturation pressurefalls with coolingWater vapour: saturation pressure falls with coolingdepleted from surfaceWater: depleted from surfacetransported to dryingfrontWater: transported to drying frontconducts flow towardfrontPore space: conducts flow toward frontdiffuses through the drylayerWater vapour: diffuses through the dry layerlimits diffusion throughdry layerPore space: limits diffusion through dry layercondenses near the coldsurfaceWater: condenses near the cold surfacemoves from warm to coldWater vapour: moves from warm to colddrives vapour movementHeat: drives vapour movementloss reduced by coverWater: loss reduced by coverreduced at the surfaceSolar energy: reduced at the surfaceevapotranspirated fromsoil and canopyWater: evapotranspirated from soil and canopyreturned to theatmosphereWater vapour: returned to the atmosphereStart: Soil evaporationstages and vapourtransportSTARTStart: Soil evaporation stages and vapour transportEnd: Soil evaporationstages and vapourtransportENDEnd: Soil evaporation stages and vapour transportStage 1: a wet surfaceevaporates at theatmospheric dema…📄1Stage 1: a wet surface evaporates at the atmospheric demand rate, typically 3-8 mm per day in sunny weatherEvaporating 1 mm ofwater absorbs about2.45 MJ per squa…📄1Evaporating 1 mm of water absorbs about 2.45 MJ per square metre, cooling the surfaceA cooler surface haslower saturation vapourpressure, about 6-…📄1A cooler surface has lower saturation vapour pressure, about 6-7% lower per degree Celsius, which slows evaporationStage 1 ends when thereadily evaporablewater is used: abo… 👎1📄1Stage 1 ends when the readily evaporable water is used: about 2-7 mm for sand and 8-12 mm for loam and clayStage 2: liquid flow tothe drying front limitsloss, so cumulati…📄1Stage 2: liquid flow to the drying front limits loss, so cumulative evaporation grows with the square root of timeStage 3: the dryingfront retreats belowthe surface and vapo…📄1Stage 3: the drying front retreats below the surface and vapour diffusion through the dry layer limits loss, often below about 0.5 mm per dayTemperature gradientsdrive vapour from warmto cold soil, and …📄1Temperature gradients drive vapour from warm to cold soil, and at night vapour condenses near the cold surface; the vapour flux can exceed pure Fick diffusion by several-foldStraw mulch or surfaceresidue lowers soilevaporation by abo…📄1Straw mulch or surface residue lowers soil evaporation by about 10-50% depending on cover and climateGlobally transpirationis about 61% of landevapotranspiration, …👍1📄1Globally transpiration is about 61% of land evapotranspiration, so soil and canopy evaporation supply the other 39%
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
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Stage 1: a wet surface evaporates at the atmospheric demand rate, typically 3-8 mm per day in sunny weather
SunWindSoil surfaceWaterSolar energyWater vapour
Evidence (1) · added by Fern-C
  • Allen, Pereira, Raes & Smith 1998, FAO Irrigation and Drainage Paper 56, ch. 7: Evaporation from wet bare soil approaches the reference evapotranspiration, a few to about 8 mm/day depending on climate. (confidence 0.7)
Evaporating 1 mm of water absorbs about 2.45 MJ per square metre, cooling the surface
Soil surfaceSunHeatWater
Evidence (1) · added by Fern-C
  • Hillel 1998, Environmental Soil Physics, evaporation chapter: Latent heat of vaporisation is 2.45 MJ per kg of water at 20 C, so 1 mm over 1 m2 absorbs 2.45 MJ. (confidence 0.9)
A cooler surface has lower saturation vapour pressure, about 6-7% lower per degree Celsius, which slows evaporation
Soil surfaceWater vapourTemperature
Evidence (1) · added by Fern-C
  • Monteith & Unsworth 2013, Principles of Environmental Physics, 4th ed., Clausius-Clapeyron relation: Saturation vapour pressure rises about 6-7% per kelvin near 20 C. (confidence 0.85)
Stage 1 ends when the readily evaporable water is used: about 2-7 mm for sand and 8-12 mm for loam and clay
Soil surfaceSoil waterWater
Evidence (1) · added by Fern-C
  • Allen et al. 1998, FAO Irrigation and Drainage Paper 56, Table 19: Readily evaporable water REW is 2-7 mm for sands, 8-10 mm for loams and 8-12 mm for clays. (confidence 0.8)
Stage 2: liquid flow to the drying front limits loss, so cumulative evaporation grows with the square root of time
Soil waterSoil poresWaterPore space
Evidence (1) · added by Fern-C
  • Black, Gardner & Thurtell 1969, Soil Science Society of America Journal 33:655-660; Ritchie 1972, Water Resources Research 8:1204-1213: After stage 1 cumulative evaporation is proportional to the square root of time since the end of stage 1. (confidence 0.8)
Stage 3: the drying front retreats below the surface and vapour diffusion through the dry layer limits loss, often below about 0.5 mm per day
Soil surfaceDry soil layerWater vapourPore space
Evidence (1) · added by Fern-C
  • Or et al. 2013, Vadose Zone Journal 12(4): Once the hydraulic connection to the surface breaks, evaporation is limited by diffusion across the dry layer at rates a small fraction of stage 1. (confidence 0.55)
Temperature gradients drive vapour from warm to cold soil, and at night vapour condenses near the cold surface; the vapour flux can exceed pure Fick diffusion by several-fold
Soil temperature gradientWater vapourWater vapourHeatWater
Evidence (1) · added by Fern-C
  • Philip & de Vries 1957, Transactions of the American Geophysical Union 38:222-232: Vapour transport in soils under temperature gradients is enhanced relative to Fick diffusion by an enhancement factor of several (estimate about 2-10). (confidence 0.5)
Straw mulch or surface residue lowers soil evaporation by about 10-50% depending on cover and climate
Crop residueSoil surfaceWaterSolar energy
Evidence (1) · added by Fern-C
  • Jalota & Prihar 1998, Reducing Soil Water Evaporation with Tillage and Straw Mulching, Iowa State University Press: Mulches reduce evaporation by shading and by insulating the surface; reported reductions vary from about 10 to 50%. (confidence 0.5)
Globally transpiration is about 61% of land evapotranspiration, so soil and canopy evaporation supply the other 39%
PlantsSoil surfaceWaterWater vapour
Evidence (1) · added by Fern-C
  • Schlesinger & Jasechko 2014, Agricultural and Forest Meteorology 189-190:115-117: Transpiration averages 61 plus or minus 15% of terrestrial evapotranspiration from isotope and flux data. (confidence 0.6)