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GEO3020/4020 Evapotranspiration

Meteorological Elements Energy Balance Evapotranspiration. GEO3020/4020 Evapotranspiration. Definition and Controlling factors Measurements Physics of evaporation Estimation of free water evaporation, potential and actual evapotransp.

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GEO3020/4020 Evapotranspiration

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  1. Meteorological Elements • Energy Balance • Evapotranspiration GEO3020/4020Evapotranspiration • Definition and Controlling factors • Measurements • Physics of evaporation • Estimation of free water evaporation, potential and actual evapotransp. • Processes and estimation methods for bare soil, transpiration, interception

  2. Weather is determined by the energy and mass transport at the surface: Energy transport LE: 15% H: 60% Oceans: 25% Meteorological variables are used to describe the weather and to calculate the components of the energy and water balance equation.

  3. Meteorological variables • Precipitation • Radiation • Air temperature • Air humidity • Wind • Air pressure

  4. Radiation Why do we want to calculate the radiation budget at the land surface?

  5. 70% 30%

  6. Summary = Extraterrestrial Radiation on a horizontal plane =Extraterrestrial Radiation on a sloping plane = Total daily clear sky incident radiation on a horizontal plane at the earth surface = global short wave radiation at the earth surface = backscattered radiation (= ) and

  7. Structure of the atmosphere Composition Vertical structure Pressure-temperature relation (Ideal gas law) Adiabatic lapse rate (dry & wet) Vapour Vapour pressure, ea Sat. vapour pressure, ea* Absolute humidity, ρv Specific humidity, q = ρa/ρv Relative humidity, Wa = ea/ea* Dew point temperature, Td

  8. GEO3020/4020Lecture 2: I. Energy balance II. Evapotranspiration

  9. Energy balance equation where: K net shortwave radiation L net longwave radiation LE latent heat transfer H sensible heat transfer G soil flux Aw advective energy ΔQ/Δt change in stored energy Units: [EL-2T-1] Bowen ratio = H/LE replace H = B∙LE

  10. Controlling factors of evaporation I. Meteorological situation • Energy availability • How much water vapour can be received • Temperature • Vapour pressure deficit • Wind speed and turbulence

  11. Controlling factors of evaporation II. Physiographic and plant characteristics • Characteristics that influence available energy • albedo • heat capacity • How easily can water be evaporated • size of the evaporating surface • surroundings • roughness (aerodynamic resistance) • salt content • stomata • Water supply • free water surface (lake, ponds or intercepted water) • soil evaporation • transpiration The wind speed immediately above the surface. • The humidity gradient away from the surface. • The rate and quantity of water vapor entering into the atmosphere both become higher in drier air. • Water availability. • Evapotranspiration cannot occur if water is not available.

  12. EvapotranspirationMeasurements Free water evaporation • Pans and tanks • Evaporimeters Evapotranspiration (includes vegetation) • Lysimeters • Remote sensing

  13. GEO3020/4020Lecture 3: Free water Evaporation

  14. Flux of water molecules over a surface

  15. Zveg Z0 Zd velocity

  16. Momentum, sensible heat and water vapour (latent heat) transfer by turbulence (z-direction)

  17. Steps in the derivation of LE • Fick’s law of diffusion for matter (transport due to differences in the concentration of water vapour); • Combined with the equation for vertical transport of water vapour due to turbulence (Fick’s law of diffusion for momentum), gives: DWV/DM (and DH/DM) = 1 under neutral atmospheric conditions

  18. Latent heat, LE • Latent heat exchange by turbulent transfer, LE • where • where • ra = density of air; • λv = latent heat of vaporization; • P = atmospheric pressure • k = 0.4; • zd = zero plane displacement • height z0 = surface-roughness height; za = height above ground surface at which va & ea are measured; va = windspeed, ea = air vapor pressure es = surface vapor pressure (measured at z0 + zd)

  19. Sensible heat, H • Sensible-heat exchange by turbulent transfer, H (derived based on the diffusion equation for energy and momentum): • where • where • ra = density of air; • Ca = heat capacity of air; • k = 0.4; • zd = zero plane displacement • height z0 = surface-roughness height; za = height above ground surface at which va & Ta are measured; va = windspeed, Ta = air temperatures and Ts = surface temperatures.

  20. Selection of estimation method • Type of surface • Availability of water • Stored-energy • Water-advected energy Additional elements to consider: • Purpose of study • Available data • Time period of interest

  21. Water balance method Mass-transfer methods Energy balance method Combination (energy + mass balance) method Pan evaporation method Estimation of free water evaporation Defined by not accounting for stored energy

  22. Mass-transfer method Physical based equation: or Empirical equation: • Different versions and expressions exist for KE and the empirical constants b0 and b1; mainly depending on wind, va and actual vapour pressure, ea

  23. Calculation of evaporation using energy balance method Substitute the different terms into the following equation, the evaporation can be calculated where LE has units [EL-2T-1] E [LT-1] = LE/ρwλv Latent Heat of Vaporization : lv= 2.495 - (2.36 × 10-3) Ta [MJkg-1] or 2495 J/g at 0oC

  24. Penman combination method Penman (1948) combined the mass-transfer and energy balance approaches to get an equation that did not require surface temp.: I. Simplifies the original energy balance equation: thus neglecting ground-heat conduction G, water-advected energy Aw, and change in energy storage DQ/Dt. II. The sensible-heat transfer flux, H, is given by: I. + II. gives the Penman equation:

  25. Penman equation – input data • Net radiation (K+L) (measured or alternative cloudiness, C or sunshine hours, n/N can be used); • Temperature, Ta(gives ea*) • Humidity, e.g. relative humidity, Wa = ea/ea* (gives ea and thus the saturation deficit, (ea* - ea) • Wind velocity, va Measurements are only taken at one height interval and data are available at standard weather stations

  26. GEO3020/4020Lecture 4: Evapotranspiration- bare soil - transpiration - interception • Lena M. Tallaksen • Chapter 7.4 – 7.8; Dingman

  27. Influence of Vegetation • Albedo • Roughness • Stomata • Root system • LAI • GAI Aerodynamic and surface resistance

  28. Modelling transpiration Rearrange to give:

  29. Atmospheric conductance, Cat

  30. Penman equation – 3 versions Orignal Penman Penman (physical based wind function) Penman (atmospheric conductance)

  31. Penman-Monteith Penman Penman-Monteith ”Big leaf” concept

  32. Interception: Measuring and Modelling • Function of: • Vegetation type and age (LAI) • Precipitation intensity, frequency, duration and type • Replacement or addition to transpiration?

  33. Estimation of potential evapotranspiration • Definition: function of vegetation – reference crop • Operational definitions (PET) • Temperature based methods (daily, monthly) • Empirical • Radiation based methods (daily) • Homogeneous, well watered surfaces, e.g. P-T • 3. Combination method (daily) • Penman or Penman-Monteith (Cleaf: no soil moisture deficit) • 4. Pan methods

  34. Estimation of actual evapotranspiration (ET) • Potential-evapotranspiration approaches • Empirical relationships between P-PET • Monthly water balance • Soil moisture functions • Complementary approach • Water balance approaches • Lysimeter • Water balance for the soil moisture zone, atmosphere, land • Turbulent-Transfer/Energy balance approaches • Penman-Monteith • Bowen ratio • Eddy correlation • Water quality approaches

  35. GEO3020/4020Lecture 10: Rainfall-runoff processes Lena M. Tallaksen Chapter 9.1-9.2; Dingman

  36. Streamflow response to precipitation (rain or snow) input • Basic aspect of catchment response • hillslope (and stream network) • Hydrograph separation • The Base Flow Index (BFI) • Linear reservoir model • Mechanisms producing event response • (Rainfall-runoff modeling)

  37. Definition of terms • Refer Table 9-1 • Time instants, t • Time durations, T

  38. Rapid response Base flow 6,50E+04 6,00E+04 5,50E+04 5,00E+04 4,50E+04 4,00E+04 Hydrograph separation Flow components Methods for continuous separation similarly divide the total streamflow into one rapid, qef (event flow) and one delayed component, qbf (base flow). The delayed flow component represents the proportion of flow that originates from stored sources (e.g. groundwater). The Base Flow Index BFI = Vbase flow /Vtotal flow Isotopic and chemical methods (Box 9.1)

  39. Linear reservoir model of catchment response • Box 9-2 • Catchment response time, T* • Influence of storm size and timing • Influence of drainage basin characteristics • Summary of their influence is given in Table 9.2

  40. Mechanisms producing event responseIII. Subsurface flow • Channel precipitation • Overland flow (surface runoff) • Hortonian • Saturation excess • Subsurface flow • Saturated zone • Local groundwater mounds • Perched saturated zones • Unsaturated zone • Matrix (Darcian) flow • Macropore flow

  41. Questions?

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