文本讲稿水文学原理chapter4evaporation.pptxVIP

Chapter 4 Evaporation;4.1 introduction definitions;The source water for evaporation Open water: lakes, reservoirs, rivers, oceans, etc Bare soil: no vegetation; the space between Vegetation: interception;Transpiration: Special case of evaporation Taken up by plant’s roots from soil Evaporate through leaves May continue for long after the drying up of foliage or upper soil layer;Evapotranspiration: Total evaporation Combined process of evaporation from soil and water surfaces plus transpiration from plants Consumptive use Water used by vegetation in the production of plant materials Rather than a mere “loss”;The function of evaporation Driving force of hydrological cycle Controlling the earth’s energy budget Sensible heat: Warms the ground by conduction convection Its effect can be measured or sensed by a change in T Latent heat: A change in state without a change in T;4.2 The process of evaporation;Description of the evaporation process Constant motion of water molecules(分子) Adding heat to water → molecules more energized →move more rapidly → distance increased → force weakened. Higher T, more water escapes from the surface to lower layer of overlying air.;All atmosphere contains some water vapor Water molecules in the lower air layers also moving Some penetrates into the underlying mass of water Water vapor pressure is the driving force for water molecules back into the water body. Water vapor pressure ranges from 0.1 to 4Kpa, the total atmosphere pressure is 100Kpa;The rate of E depends on the balance between Vaporization (汽化): molecules into atmosphere Condensation(凝结): molecules into water from atmos. If more molecules into the air from water, E occurs; If more molecules returns to water than leaving it, condensation occurs.;Saturation vapor pressure (SVP) Calm conditions: Water vapor into overlying air → increase the water content → vapor pressure increases → condensation equalized with vaporization → evaporation ceases The air is called to be saturated. Th