1. Evaporation

2. Evaporation
the process in which water changes phase from liquid to vapor and is transported from the Earth’s surface to the atmosphere
We will use the following distinctions
transpiration (T): through plants
evaporation (E): from soil, the exterior surfaces of plants, or surface water bodies
evapotranspiration (ET): ET = E + T

3. Necessary conditions for evaporation
A supply of heat
latent heat of vaporization: the energy input required to overcome the molecular forces of attraction between water molecules in liquid form
2.5 x 106 J kg-1 at 15C
the joule (J) is the SI unit of energy (kg m2 s-2)
4.18 J = 1 calorie
heat can come from external sources or can be withdrawn from the body undergoing evaporation

4. Necessary conditions for evaporation
A vapor pressure gradient
vapor pressure of the atmosphere < vapor pressure of the evaporating surface
this gradient drives transport of water by diffusion
transport by convection (bulk air flow) is also important

5. Necessary conditions for evaporation
A supply of water
sufficient water transport from or through the interior of the body to the site of evaporation
Thus evaporation can be limited by either
the evaporative demand: the supply of heat and the transport of vapor away from the surface
the soil: its ability to transport water to the surface

6. Evaporation from a water table
the greater the capillary rise the greater the potential for evaporation from the water table
shallow water tables are major contributors to the problem of soil salinization

10. Steady evaporation from a water table
assume evaporation is occurring but no change in soil water content
steady-state flow
apply Buckingham-Darcy Law

11. Reading assignment
Evaporation, p

12. Evaporation in the absence of a water table
Three stages
First, constant-rate stage
Second, falling rate stage
Third, slow-rate stage

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19. Reducing evaporation During first or constant rate stage
Reduce the evaporative demand
Maintain soil cover
Reduce irrigation frequency
During second or falling rate stage
Decrease the hydraulic conductivity rapidly
“Dust mulch” was advocated in the early 1900’s
These management practices usually involve tradeoffs and should be carefully evalutated.

20. How much water is lost to evaporation?
Water lost during fallow periods as a percentage of total evapotranspiration of the crop rotation. Data cover 15 months ( ) at Lahoma, OK. (Patrignani et al., 2010, OWRRI poster).
Abbreviations:
Conventional tillage (CT)
No-till (NT)
Wheat-double crop sunflower-grain sorghum (W/DC SF-GS)
Wheat-double crop grain sorghum-soybean(W/DC GS-SB)
Wheat-double crop soybean-corn (W/DC SB-CN)

21. Reading assignment
Plant uptake of soil moisture, p