Evapotranspiration

1. Evapotranspiration ERS 482/682 Small Watershed Hydrology

2. consumptive use by plants water becoming water vapor Definition Total evaporation from all water, soil, snow, ice, vegetation, and other surfaces plus transpiration

3. Processes • Evaporation of precipitation intercepted by plant surfaces • Evaporation of moisture from plants through transpiration • Evaporation of moisture from soil (ground) surface

4. How significant is evapotranspiration? • Can be as much as 90% of precipitation • Affected by changes in • Vegetation • Weather   ET   streamflow  air temperature  ET   streamflow 

5. Evaporation Fick’s Law: A diffusing substance moves from whereits concentration is larger to where itsconcentration is smaller at a ratethat is proportional to the spatialgradient of concentration Figure 8.2 (Chapra 1997)

6. indicates movementfrom regions of higherconcentration to regionsof lower concentration gradient (change) inconcentration will have units ofsubstance *[L T-1] Evaporation Fick’s Law: A diffusing substance moves from whereits concentration is larger to where itsconcentration is smaller at a ratethat is proportional to the spatialgradient of concentration where Fz(X) = rate of transfer of substance X in z direction DX = diffusivity of substance X C(X) = concentration of X [L2 T-1]  units depend on substance

7. Evaporation Fick’s Law: A diffusing substance moves from whereits concentration is larger to where itsconcentration is smaller at a ratethat is proportional to the spatialgradient of concentration where E = evaporation rate KE = efficiency of vertical transport of water vapor va = wind speed es = vapor pressure of evaporating surface ea = vapor pressure of overlying air [L T-1] [L T-1 M-1] [L T-1] [M L-1 T-2] [M L-1 T-2]

8. water temperature at surface relative humidity Vapor pressure, e Partial pressure of water vapor saturation vapor pressure, e*: maximum vapor pressure ea = Waea* water vapor water es = es*

9. surface water temperature (°C) Latent heat exchange, LE • Occurs whenever there is a vapor pressure difference between water and air [E L-2 T-1] where w = water density v = latent heat of vaporization 1000 kg m-3 [MJ kg-1]

10. Depends on air pressure  constant at a particular site Sensible heat exchange, H • Occurs whenever there is a temperature difference between water and air where B = Bowen ratio

11. Energy balance Equation 7-15 where Q = change in heat storage per unit area over time t K = shortwave (solar) radiation input L = longwave radiation H = turbulent exchange of sensible heat with atmosphere LE = turbulent exchange of latent heat with atmosphere Aw= heat input due to water inflows and outflows G = conductive exchange of sensible heat with ground All expressed in units of [E L-2 T-1]except Q [E L-2]

12. often assumed negligible Classification of ET processes • Surface type: • Open water • Bare soil • Leaf/canopy type • Crop type • Land region • Water availability • Unlimited vs. limited • Stored energy use, Q • Water-advected energy, Aw

13. Free-water evaporation “Potential evaporation” Evaporation that would occur from an open-water surface in the absence of advection and changes in heat storage Depends only on climate/meteorology Evaporation: net loss of water from a surface resulting from achange in the state of water from liquid to vapor and the nettransfer of this vapor to the atmosphere

14. 0 0 0 recall: Free-water evaporation “Potential evaporation” • Penman equation • Standard hydrological method

15. Free-water evaporation “Potential evaporation” • Penman equation • Standard hydrological method psychrometric constant

16. dimensionless Free-water evaporation “Potential evaporation” • Penman equation • Standard hydrological method Table 4-6 Dunne & Leopold (1978)

17. 12 in. Free-water evaporation “Potential evaporation” • Pan-evaporation • Direct measurement method where W = precipitation during time t V1= storage at beginning of period t V2= storage at end of period t Class-A evaporation pan Diameter = 1.22 m Height = .254 m

18. 0.7 average for US Free-water evaporation “Potential evaporation” • Pan-evaporation • Direct measurement method Efw = (PC)Epan See Morel-Seytoux (1990) for pan coefficients No adjustments necessary for annual values

19. Bare-soil evaporation • Stages • Atmosphere-controlled stage (wet soil surface) • Evaporation rate  free-water evaporation rate • Soil-controlled stage (dry soil surface) • Evaporation rate << free-water evaporation rate

20. Transpiration Transpiration: evaporation of water from the vascular system of plants into the atmosphere Figure 6.1 (Manning 1987)

21. Transpiration • Dry soils soil capillary pressure > osmotic pressure • Saline soils water concentrationsoil < water concentrationplant Figure 6.2 (Manning 1987)

22. Cleaf LAI: fraction of area covered with leaves shelter factor Transpiration • Leaf/canopy conductance • Depends on • Number of stomata/unit area • Size of stomatal openings • Density of vegetation Penman-Monteith model (Equation 7-56)

23. Transpiration Figure 3.4 (Brooks et al. 1991)

24. Potential evapotranspiration (PET) Rate at which evapotranspiration would occur from a large area completely and uniformly covered with growing vegetation with unlimited access to soil water and without advection or heat-storage effects

25. Potential evapotranspiration (PET) • Thornthwaite method where Et = potential evapotranspiration Ta = mean monthly air temperature I = annual heat index a = 0.49 + 0.0179I – 0.000077I2 + 0.000000675I3 [cm mo-1] [°C]

26. Potential evapotranspiration (PET) • Thornthwaite method Figures 5-4 and 5-5 (Dunne & Leopold 1978) Index must be adjusted for # days/mo and length of day

27. Potential evapotranspiration (PET) • Blaney-Criddle formula where Et = potential evapotranspiration Ta = average air temperature k = empirical crop factor d = monthly fraction of annual hours of daylight [cm mo-1] [°C]

28. Potential evapotranspiration (PET) • Notes • Wind speed has little or no effect • Local transport of heat can be significant • Taller and widely spaced vegetation tend to have greater heat transfer

29. Measuring evapotranspiration • Cannot be measured directly • Transpiration • Lysimeters Figure 6.3 (Manning 1987)

30. Measuring evapotranspiration • Cannot be measured directly • Transpiration • Lysimeters • Tent method • Evaporation • Evaporation pans • Water budget: ET + G = P – Q • Paired watershed studies Figure 3.5 (Brooks et al. 1991)