3 Evaporation Transfer of H2O from liquid to vapor phase Diffusive process driven bySaturation (vapor density) gradient ~ (rs – ra)Aerial resistance ~ f(wind speed, temperature)Energy to provide latent heat of vaporization (radiation)Transpiration is plant mediated evaporationSame result (water movement to atmosphere)Summative process = evapotranspiration (ET)Dominates the fate of rainfall~ 95% in arid areas~ 70% for all of North America
4 Evapo-Transpiration ET is the sum of Evaporation: physical process from free waterSoilPlant intercepted waterLakes, wetlands, streams, oceansTranspiration: biophysical process modulated by plants (and animals)Controlled flow through leaf stomataSpecies, temperature and moisture dependent
5 Four Requirements for ET EnergyWaterNPVapor Pressure GradientWindTP
7 Energy Inputs Radiation Budget Rtotal = Total Solar Radiation Inputs on a horizontal plane at the Earth’s SurfaceRnet = Rtotal – reflected radiation= Rtotal * (1 – albedo)Albedo (α) valuesSnow 0.9Hardwoods 0.2Water 0.05Flatwoods pine plantation 0.15Flatwoods clear cut ____Burn ____Asphalt 0.05
8 Energy and Temperature The simplest conceptualization of the ET process focuses solely on temperature.Blaney-Criddle Method:ET = p * (0.46*Tmean+ 8)Where p is the mean daytime hoursTmean is the mean daily temp (Max+Min/2)ET (mm/day) is treated as a monthly variable
9 Vapor Deficit Drives the Process Distance between actual conditions and saturation lineGreater distances = larger evaporative potentialSlope of this line (d) is an important term for ET modelsUsually measured in mbar/°CGraph shows mass water per mass air as a function of T
10 Wind Boundary layer saturates under quiescent conditions Inhibits further ET UNLESS air is replacedTurbulence at boundary layer is therefore necessary to ensure a steady supply of undersaturated air
11 Water Availability: PET vs. AET PET (potential ET) is the expected ET if water is not limitingGiven conditions of: wind, Temperature, HumidityAET (actual ET) is the amount that is actually abstracted (realizing that water may be limiting)AET = a * PETWhere a is a function of soil moisture, species, climateET:PET is low in arid areas due to water limitationET ~ PET in humid areas due to energy limitation
12 Methods of Estimating ET Since ET is the largest flux OUT of the watershed, we need good estimatesTechniques have focused mostly on predicting capacity (i.e., PET, where water is not limiting)energy balance methodsmass transfer or aerodynamic methodscombination of energy and mass transfer (Penman equation)pan evaporation data
13 Evaporation from a Pan Mass balance equation Pans measure more evaporation than natural water bodies because:1) less heat storage capacity (smaller volume)2) heat transfer3) wind effectsNational Weather Service Class A typeInstalled on a wooden platform in a grassy locationFilled with water to within 2.5 inches of the topEvaporation rate is measured by manual readings or with an analog output evaporation gauge
14 Diurnal Water Level Variation (White, 1932) Diel variation in water level yields ET (during the day) and net groundwater flux (at night)Curiously, not widely used
15 Actual Diurnal DataNighttime slope is groundwater flow (inflow is UP, outflow is DOWN)Assuming constant GW flow, daytime slope is ET + GW.Specific yield (Sy)
16 What is Specific Yield?How much water (in units of cm) drains out of a soil; also called dynamic drainable porosity
17 Energy Balance Method Assumes energy supply the limiting factor. Consider energy balance on a small lake with no water inputs (or evaporation pan)heat stored in systemsensible heat transfer to airHsRnQeGnet radiationenergy used in evaporationheat conducted to ground (typically neglected)
18 Energy Budget Energy in = Energy out (conservation law) Energy In = RtotalEnergy OutAlbedoLatent HeatSensible HeatSoil Heat FluxIf Rtotal = 800 cal/cm2/day and a = 0.25Rnet = 800 * (1 – 0.25) = 600 cal/cm2/day
19 Energy Budget Estimates of ET Rnet = lE + H + GWe want to know EE = (Rnet – (H+G))/lWhat are evaporative losses if:Rtotal = 800 cal/cm2/dayAlbedo = 0.2l = 586 cal/gH = 100 cal/cm2/d (convected heat)G = 50 cal/cm2/d (soil heating)
20 Static Computation Rnet = lE + H + G = 800 * (1 – 0.2) = 640 E = 0.84 cm/dAnnual ET = 0.84 * 365 * 1 m/100 cm= 3.07 mRtot = 800 cal/cm/dAlbedo = 0.2l =586 cal/gH = 100 cal/cm/d lostG = 50 cal/cm/d lost to ground
22 Mass Transfer (Aerodynamic) Method Assumes that rate of turbulent mass transfer of water vapor from evaporating surface to atmosphere is limiting factorMass transfer is controlled by (1) vapor gradient (es – e) and (2) wind velocity (u) which determines rate at which vapor is carried away.
23 Combination Method (Penman) Evaporation can be estimated by aerodynamic method (Ea) when energy supply not limiting and energy method (Er) when vapor transport not limiting Typically both factors limiting so use combination of above methodsWeighting factors sum to 1. = vapor pressure deficitg = psychrometric constant
24 Combination Method (Penman) Penman is most accurate and commonly used method if meteorological information is available.Need: net radiation, air temperature, humidity, wind speedIf not available use Priestley-Taylor approximation:Based on observations that second term (advection) in Penman equation typically small in low water stress areas.The α term is crop coefficient that assumes no “advection limitation”. Usually >1 (1.2 to 1.7), suggesting that actual ET is greater than what is predicted from radiation alone.
25 Time Scales of Variability Controls on ET create variability at scales from seconds to centuriesEddies change ET at the time scale of secondsDiel cycles affect water fluxes over 24 hoursWeather patterns affect fluxes at days to weeksWater availabilityVapor deficitWind and energyClimate variability at decadal and beyond
31 Interception Interception Loss (% of rainfall) Surface tension holds water falling on forest vegetation.Leaf StorageFir 0.25”Pines 0.10”Hardwoods 0.05”Litter 0.20”SP Plantations 0.40”.Interception Loss (% of rainfall)Hardwoods 10-20% (less LAI)Conifers 20-40%Mixed slash and Cypress Florida Flatwoods 20%
32 TranspirationPlant mediated diffusion of soil water to atmosphereSoil-Plant-Atmosphere Continuum (SPAC)Transpiration and productivity are tightly coupledTranspiration is the primary leaf cooling mechanism under high radiationProvides a pathway for nutrient uptake and matrix for chemical reactionsWorldwide, water limitations are more important than any other limitation to plant productivityCO2H2O1 : 300
33 Transpiration Dominates the Evaporation Process Trees have:Large surface areaMore turbulent air flowConduits to deeper moisture sourcesT/ETHardwood ~80%White Pine~60%Flatwoods ~75%
36 The driving force of transpiration is the difference in water vapor concentration, or vapor pressure difference, between the internal spaces in the leaf and the atmosphere around the leaf
37 TranspirationThe physics of evaporation from stomata are the same as for open water. The only difference is the conductance term.Conductance is a two step processstomata to leaf surfaceleaf surface to atmosphere
39 How Does Water Get to the Leaf? Water is PULLED, not pumped. Water within the whole plant forms a continuous network of liquid columns from the film of water around soil particles to absorbing surfaces of roots to the evaporating surfaces of leaves.It is hydraulically connected.
40 Even a perfect vacuum can only pump water to a maximum of a little over 30 feet. At this point the weight of the water inside a tube exerts a pressure equal to the weight of the atmosphere pushing down> 100 metersSo why doesn’t the continuous column of water in trees taller than 34 feet collapse under its own weight? And how does water move UP a tall tree against the forces of gravity?
41 cell wall microfibrils of carrot Water is held “up” by the surface tension of tiny menisci (“menisci” is the plural of meniscus) that form in the microfibrils of cell walls, and the adhesion of the water molecules to the cellulose in the microfibrilscell wall microfibrils of carrot
42 Cohesion-Tension Theory: (Böhm, 1893; Dixon and Joly, 1894)The cohesive forces between water molecules keep the water column intact unless a threshold of tension is exceeded (embolism). When a water molecule evaporates from the leaf, it creates tension that “pulls” on the entire column of water, down to the soil.To understand WHY the pipeline is vulnerable:LOOK AT MECHANISM: COHESION – TENSION – THEORYabout 100 yrs ago – proposed independently by Boehm and by Dixon and JolyTranspirationCapillary forces – Tension – NEGATIVE PRESUREWater is pulled up – cohesive forces between water molecules keep the water column togetherConduits are deadClass: Tug-of-war – the air pulls on one end, the soil particles on the other endVery simple mechanism and very cheap - driven by the sunOnce the pipeline is in placeplants don’t have to use energy for water transportWhat makes the pipeline “vulnerable”?It’s the MAGNITUDE of the NEGATIVE Xylem Pressure
43 ?ET = Rain * 0.80ET = Rain * 0.951,000 mm * 0.95 = 950 mm1,000 mm * 0.80 = 800 mmAssume Q & ΔS = 0G = P - ETG = 50 mmG = 200 mm4x more groundwater recharge from open stands than from highly stocked plantations.NRCS is currently paying for growing more open stands, mainly for wildlife.
45 Controls on Stand Water Use More leaves per area = more water useForesters don’t measure LAIProxies for LAI
46 A Fair Comparison Pine stands are clear-cut every 20-25 years (low ET) Compare water yield (Rain – ET) over entire rotation
47 Ecosystem Service – Water Yield Forest management may yield “new” waterWin-win for other forest servicesWho pays and how much?
48 Trading Environmental Priorities? Water for CarbonWater for EnergyJackson et al (Science)
49 Surface Water Evaporation Air TempAir relative humidityWater tempWindRadiationWater QualityActual surface water evaporation ~ pan evaporation * 0.7
50 Soil Water Evaporation Stage 1. For soils saturated to the surface, the evaporation rate is similar to surface water evaporation.Stage 2. As the surface dries out, evaporation slows to a rate dependent on the capillary conductivity of the soil.Stage 3. Once pore spaces dry, water loss occurs in the form of vapor diffusion. Vapor diffusion requires more energy input than capillary conduction and is much, much, slower.Note that for soils under a forest canopy, Rnet, vapor pressure deficit, and turbulent transport (wind) are lower than for exposed soils.
51 Soil water loss with different cover Forest Soil