1、Evaporation, Transpiration,Sublimation,Processes by which water changes phase- Liquid or solid to gas vapor,New Temperature and Snow Data 2012,Learning Objectives: Evapotranspiration (ET) Learn what conditions are necessary for evaporation to occur Learn what factors control evaporation rates Learn

2、how to measure ET Learn where to find or how to compute variables needed to estimate ET Understand the difference between potential evapotranspiration (PET) and actual evapotranspiration (AET) Understand the difference between evaporation and transpiration Learn what factors control transpiration Be

3、come aware of common equations used to estimate ET Understand how ET varies in time and space,Evaporation,Phase change liquid to gas Hydrogen bonds broken vapor diffuses from higher to lower vapor pressure At an open water surface, net evaporation = 0- bonds constantly forming and breaking Most take

4、s place over open water surfaces such as lakes and oceans,/karl/Unit2_Lecture1.ppt,What controls evaporation?,Energy inputs Temperature Humidity Wind Water availability,What controls evaporation?,Evaporation is energy intensive- latent heat of vaporization is 540 cal/gram Provided mai

5、nly by Solar energy - radiation Sensible heat temperature transferred via conduction and convection kinetic energy of water internal energy, heat Energy that is absorbed during phase changes of water is not available to increase the surface temperature.,Energy Budget,Net radiation: Rnet is determine

6、d by measuring incoming & outgoing short- & long-wave rad. over a surface. Rnet can or + If Rnet 0 then can be allocated at a surface as follows: Rnet = (L)(E) + H + G + Ps,L is latent heat of vaporization, E evaporation, H energy flux that heats the air or sensible heat, G is heat of conduction to

7、ground and Ps is energy of photosynthesis. LE represents energy available for evaporating water Rnet is the primary source for ET & snow melt.,/faresa/courses/nrem600/10-02%20Lecture.ppt,In a watershed Rnet, (LE) latent heat and sensible heat (H) are of interest. Sensible h

8、eat can be substantial in a watershed, Oasis effect where a well-watered plant community can receive large amounts of sensible heat from the surrounding dry, hot desert.,Advection is movement of warm air to cooler plant-soil-water surfaces. Convection is the vertical component of sensible-heat trans

9、fer.,/faresa/courses/nrem600/10-02%20Lecture.ppt,What controls evaporation?,Energy inputs Temperature Vapor content Wind Water availability,Temperature,Measure of heat energy Affects vapor pressure- Saturation vapor pressure increases with air temperature Can compute with a

10、n equation if know temperature Saturation vapor pressure minus actual vapor pressure = saturation deficit The amount of additional water vapor that air can hold at a given temperature,What controls evaporation?,Energy inputs Temperature Vapor content Wind Water availability,Measuring the Vapor Conte

11、nt,There are a number of ways that we can measure and express the amount of water vapor content in the atmosphere: Vapor Pressure Mixing Ratio Relative Humidity Dew Point Precipitable Water Vapor Others (absolute humidity, specific humidity),Humidity can be describe in many ways, for example, Measur

12、esymbolunits Volumetric concentration cwvmol m-3 Vapor pressure ea, also pH2O kPa (the partial pressure of H2O vapor) Relative humidityRH% =(ea/es)* 100, where es is saturation vapor pressure Vapor pressure deficitVPDkPa =es ea,/bond/fs561/lectures/humidity%20and%20transpiration.ppt,

13、Vapor Pressure (e),Vapor pressure (e) is simply the amount of pressure exerted only by the water vapor in the air The pressures exerted by all the other gases are not considered The unit for vapor pressure will be in units of pressure (millibars and hectopascals are the same value with a different n

14、ame),Relative Humidity (RH),The relative humidity (RH) is calculated using the actual water vapor content in the air (mixing ratio) and the amount of water vapor that could be present in the air if it were saturated (saturation mixing ratio) RH = w/ws x 100% The relative humidity is simply what perc

15、entage the atmosphere is towards being saturated Relative humidity is not a good measure of exactly how much water vapor is present (50% relative humidity at a temperature of 80 degrees Fahrenheit will involve more water vapor than 50% relative humidity at -40 degrees) Relative humidity can change e

16、ven when the amount of water vapor has not changed (when the temperature changes and the saturation mixing ratio changes as a result),Dew Point (Td),The dew point temperature is the temperature at which the air will become saturated if the pressure and water vapor content remain the same The higher

17、the dew point, the more water vapor that is present in the atmosphere The temperature is always greater than the dew point unless the air is saturated (when the temperature and dew point are equal),Precipitable Water Vapor (PWV),Precipitable water vapor (PWV) is the amount of water vapor present in

18、a column above the surface of the Earth Measured in units of inches or millimeters It represents the maximum amount of water that could fall down to the surface as precipitation if all the water vapor converted into a liquid or a solid Can be measured easily by weather balloons or satellites,What co

19、ntrols evaporation?,Energy inputs Temperature Vapor content Wind Water Availability,Wind,Creates turbulent diffusion and maintains vapor pressure gradient Turbulence a function of wind velocity and surface roughness Evaporation can increase substantially with turbulence up to some limit that is a fu

20、nction of energy, temperature and humidity,Additional factors affecting evaporation from free water surface,Water quality More salinity means less evaporation Depth of water body Deep lakes have more evap in winter High heat capacity means lake water warmer that air temperature Shallow lakes cool fa

21、st in fall and freeze No evap in winter,Additional factors affecting evaporation from free water surface,Area of water body More evap from larger surface area but rate decreases upwind as air picks up vapor Maximum rates from small, shallow lakes in dry climates,Evaporation from soil,Same factors dr

22、ive the process as in open water 1. Soil moisture also important Evap rates decrease as surface dries 2. Soil texture: affects soil moisture content and capillary forces E.g., Fine soil- retains moisture, rates high at first but then depends on capillary forces,Evaporation from soil,Soil color affec

23、ts albedo and thus energy inputs Depth to water table If shallow such as wetlands, almost unlimited evaporation Vegetation - provides shade- limits insolation (energy and heat) - reduces windspeed at ground level - increase vapor pressure through transpiration,How do we measure/estimate evaporation?

24、,Direct measurement Pans Lake water balance Lysimeters,Pan evaporation,Class A pan 4 feet diameter, 10 inches deep- galvanized steel measure daily water loss by adding water to same level Evap = change in water level - precipitation Pan evap lake evap why? Use a pan coefficient (usually 0.6-0.8) Map

25、 of pan evap,/courses/FR3114/FieldMeas%20-%20Transpir_10_03_06.pdf,/courses/FR3114/FieldMeas%20-%20Transpir_10_03_06.pdf,/courses/FR3114/FieldMeas%20-%20Transpir_10_03_06.pdf,Soil lysimeter,Water tight box on a scale or pressure trans

26、ducer If only soil and water, loss of weight is due to evaporation of water ET = change in weight precipitation Either prevent seepage or collect and measure,Transpiration,Evaporation from plants Water vapor escapes when stomata open for photosynthesis, need carbon dioxide Related to density and siz

27、e of vegetation, soil moisture, depth to water, soil structure Of the water taken up by plants, 95% is returned to the atmosphere through their stomata (only 5% is turned into biomass!),Ratio of land ET that refalls on land,/journals/wr/wr1009/2010WR009127/2010WR009127.pdf,Water Ava

28、ilability,An open water surface provides a continuous water source Transpiration can provide water up until a certain limit based upon the plants ability to pull water up through its roots and out its stomatae (rate of transpiration),Water movement in plants,Illustration of the energy differentials

29、which drive the water movement from the soil, into the roots, up the stalk, into the leaves and out into the atmosphere. The water moves from a less negative soil moisture tension to a more negative tension in the atmosphere.,/faresa/courses/nrem600/10-02%20Lecture.ppt,The

30、driving force of transpiration is the “vapor pressure gradient.” This is the difference in vapor pressure between the internal spaces in the leaf and the atmosphere around the leaf,/bond/fs561/lectures/humidity%20and%20transpiration.ppt,Stomatal conductance balances the atmospheric d

31、emand for evaporation with the hydraulic capacity to supply water,SUPPLY Flow of liquid water = (Yleaf Ysoil) * K,DEMAND: VPD,Transpiration =,VPD * LAI * leaf conductance VPD Vapor pressure deficit LAI Leaf area index,/bond/fs561/lectures/humidity%20and%20transpiration.ppt,Leaf Condu

32、ctance,Ease of water loss affected by leaf conductance Conductance a function of light, carbon dioxide concentration, vapor pressure deficit, leaf temperature and leaf water content,Effects of Vegetative Cover,fine soils with ample soil-moisture storage, warm summers, cool winters, and little change

33、 in precipitation throughout the year,coarse soils with limited soil-moisture storage, warm, dry summers, cool, moist winters.,Effects of soil type and climate,PET,P,AET,PET,AET,P,Available Soil Water,PET Potential Evapotranspiration,Rate at which ET would occur in a situation of unlimited water sup

34、ply, uniform vegetation cover, no wind or heat storage effects First used for climate classification criteria Usually assume short grass as the uniform vegetation Compute as function of climate factors,Actual Evapotranspiration,Amount actually lost from the surface given the prevailing atmospheric a

35、nd ground conditions Provides information of soil moisture conditions and the local water balance Measured by a lysimeter (difficult to maintain, not many in existence) that weighs the grass, soil, and water above,PET equations,Penman- Monteith (based on radiation balance) Jensen-Haise (developed fo

36、r dry, intermountain west) Priestly-Taylor (based on radiation balance) Thornthwaite (based on temperature) Hamon, Malstrom (based on T and saturated vapor pressure) See table 4.3 p 95 in text,Physically-based theoretical methods- e.g. Penman Monteith,Energy budget Mass balance on energy inputs and

37、outputs Incoming solar radiation reflected solar radiation (albedo) net longwave radiation + net energy advected to vegetation = ET energy (latent heat) + sensible heat transfer from veg to air + changes in energy storage in heating soil and veg Can measure all but latent heat which equals ET,Physic

38、ally-based methods,Turbulent mass transfer Function of wind speed and vapor pressure deficit Evap = k uz ( ew ez) K is a constant, U is wind velocity, e is vapor pressure, z is some reference height, w is level at water surface Can only measure precisely over short distances Useful only for experimental si