Evaporation
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
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 15C the joule (J) is the SI unit of energy (kg m2 s-2) 4.18 J = 1 calorie heat can come from external sources or can be withdrawn from the body undergoing evaporation
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
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
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
Steady evaporation from a water table assume evaporation is occurring but no change in soil water content steady-state flow apply Buckingham-Darcy Law
Reading assignment Evaporation, p. 337-351
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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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.
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 (2009-2010) 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)
Reading assignment Plant uptake of soil moisture, p. 365-378