GEO3020/4020 Evapotranspiration 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 I.Meteorological Elements II.Energy Balance III.Evapotranspiration

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

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

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

5 30% 70%

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, e a –Sat. vapour pressure, e a * –Absolute humidity, ρ v –Specific humidity, q = ρ a /ρ v –Relative humidity, W a = e a /e a * –Dew point temperature, T d

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

Energy balance equation 9 where: Knet shortwave radiation Lnet longwave radiation LElatent heat transfer Hsensible heat transfer Gsoil flux Awadvective energy ΔQ/Δtchange in stored energy Units: [EL -2 T -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.

Evapotranspiration Measurements Free water evaporation -Pans and tanks -Evaporimeters Evapotranspiration (includes vegetation) -Lysimeters -Remote sensing 12

GEO3020/4020 Lecture 3: Free water Evaporation

Flux of water molecules over a surface 14

15 Z veg ZdZd Z0Z0 velocity

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

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: D WV /D M (and D H /D M ) = 1 under neutral atmospheric conditions 17

18 Latent heat, LE Latent heat exchange by turbulent transfer, LE where where  a = density of air; λ v = latent heat of vaporization; P = atmospheric pressure k = 0.4; z d = zero plane displacement height z 0 = surface-roughness height; z a = height above ground surface at which v a & e a are measured; v a = windspeed, e a = air vapor pressure e s = surface vapor pressure (measured at z 0 + z d )

19 Sensible heat, H Sensible-heat exchange by turbulent transfer, H (derived based on the diffusion equation for energy and momentum): where  a = density of air; C a = heat capacity of air; k = 0.4; z d = zero plane displacement height z 0 = surface-roughness height; z a = height above ground surface at which v a & T a are measured; v a = windspeed, T a = air temperatures and T s = surface temperatures.

Selection of estimation method Type of surface Availability of water Stored-energy Water-advected energy Additional elements to consider: 1)Purpose of study 2)Available data 3)Time period of interest 20

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

22 Mass-transfer method Physical based equation: or Empirical equation: -Different versions and expressions exist for K E and the empirical constants b 0 and b 1 ; mainly depending on wind, v a and actual vapour pressure, e a

23 Calculation of evaporation using energy balance method Substitute the different terms into the following equation, the evaporation can be calculated where Latent Heat of Vaporization : v = (2.36 × ) T a [MJkg -1 ] or 2495 J/g at 0 o C LE has units [EL -2 T -1 ] E [LT -1 ] = LE/ρ w λ v

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 A w, and change in energy storage  Q/  t. II. The sensible-heat transfer flux, H, is given by: I. + II. gives the Penman equation:

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

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

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

Modelling transpiration 28 Rearrange to give:

Atmospheric conductance, C at 29

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

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

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

Estimation of potential evapotranspiration 33 Definition: function of vegetation – reference crop Operational definitions (PET) 1.Temperature based methods (daily, monthly) Empirical 2.Radiation based methods (daily) Homogeneous, well watered surfaces, e.g. P-T 3.Combination method (daily) Penman or Penman-Monteith (C leaf : no soil moisture deficit) 4.Pan methods

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 34

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

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) 36 Streamflow response to precipitation (rain or snow) input

Definition of terms 37 Refer Table Time instants, t - Time durations, T

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

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

Mechanisms producing event response III. Subsurface flow I.Channel precipitation II.Overland flow (surface runoff) A.Hortonian B.Saturation excess III.Subsurface flow A.Saturated zone 1.Local groundwater mounds 2.Perched saturated zones B.Unsaturated zone 1.Matrix (Darcian) flow 2.Macropore flow 40

Questions? 41