Evapotranspiration

1. Evapotranspiration Plan ET in Water Resources Evaporation physics Transpiration Process Methods to estimate E and ET

2. ET

3. Evapotranspiration

4. Evaporation Transition from liquid to vapor Sublimation from solid to vapor

6. Evapotranspiration in Water Resources Recall Homework 1, how much of the annual precipitatation was lost to ET in Dry Creek?

7. ET in Water Resources Agrimet (http://www.usbr.gov/pn/agrimet/) (http://www.kimberly.uidaho.edu/ETIdaho/)

8. ET and Streamflow

10. ET in Water Resources Water Yield � Vegetation is sometimes cleared to improve water yield http://ag.arizona.edu/oals/watershed/highlands/ponderosapine/ppmanagement.html http://books.google.com/books?id=dHo4nO4ssJYC&pg=PA124&lpg=PA124&dq=watershed+ET+yield&source=bl&ots=7WfWdoMS6n&sig=oZFAVIR8LP1oVM-0PSm0DYflaYo&hl=en&ei=Nr6sSayIDcTAnQejl8C2Bg&sa=X&oi=book_result&resnum=3&ct=result http://www.srnr.arizona.edu/nemo/newsitems/NEMO_RipETBrochure.pdf

11. Evaporation Physics Basic physical principles Conservation of Mass Conservation of Energy Ideal Gas Law related to water vapor Latent heat of phase change Turbulent transfer near the ground - (diffusion of momentum�) Conservation of Mass Diffusive process driven by a vapor pressure gradient

12. Evaporation Physics

13. Evaporation Physics Dalton�s Law E a (esat(Ts) � ea) esat(Ts) is the vapor pressure of the liquid which is related to the temperature of the liquid Evaporation occurs when the above relationship is positive. Condensation occurs when the above relationship is negative. What happens if relative humidity is 100%, but esat(Ts) > ea?

14. Evaporation Physics

15. Evaporation Physics Dalton�s law is a proportionality. We make it an equality by adding a conductance term E = KeVa (esat(Ts) � ea)) UNITS: E = (LT2M-1)(LT-1)(ML-1T-2) = L/T

16. Evaporation Physics Note the similarity between Dalton�s Law and our equation for LE: E = KeVa (esat(Ts) � ea) = L/T LE = KleVa (esat(Ts) � ea) = Energy/(L2T) The relationship between E and LE is embodied in the K terms. Ke = Kle/?w?v

17. Evaporation Physics

18. Evaporation Physics So, there are 4 conditions necessary for evaporation to occur. Energy available for phase change Water available at the surface or in root zone Vapor pressure gradient, or dry air Wind � Capacity of the atmosphere to transport away moisture Factors that influence evaporation include anything that influence the above

19. Evaporation from different surfaces are represented by different K values

20. Open Water Evaporation

21. Bare Soil Evaporation (pp 291-293

22. Bare Soil Evaporation

23. Transpiration

24. Transpiration

25. Transpiration

26. Transpiration

27. Transpiration The physics of evaporation from stomata are the same as for open water. The only difference is the conductance term. Conductance is a two step process stomata to leaf surface leaf surface to atmosphere

28. Transpiration Evaporation from the leaf surface is simply open water evaporation E = KeVa(es-ea) For transpiration we simply need to add the first step T = KeVa(es-ea) + Cleaf(est-es) Cleaf is leaf conductance and est is the vapor pressure in the stomatal cavity

29. Transpiration Leaf Conductance See equation 7-52 Related to number of stomata per unit area Species particular size of the stomatal opening (Table 7-6) Controlled by light intensity, CO2, vapor pressure difference, leaf temperature, leaf water content See figure 7-13

30. Transpiration Canapy Conductance Ccan = fs(LAI)Cleaf fs = shelter factor LAI = Leaf Area Index

31. Estimating Evaporation and ET

33. EstimatingEvaporation, POT, and actual Evapotranspiration No good direct measurement method Methods Pan Water balance Mass transfer approach Energy balance approach Combination � Penman Monteith Eddy correlation Temperate index � Thornthwaite Radiation index � Priestley Taylor

34. Pan method

35. Pan Evaporation

36. Water Balance Evaporation or Actual Evapotranspiration can be calculated as the residual in-out = ds Can be written for any body of open water, watershed, soil, pan.. difficult to apply for large water bodies accuracy increases as dt increases

38. Soil Water Balance

39. Mass Transfer Approach apply Dalton�s law � see equations 7-18a,b requires measurement of wind speed, surface vapor pressure, and air vapor pressure. None of these are commonly measured. Only good over short periods of time Gives instantaneous rate Ke is highly variable Many problems make this technique difficult to apply.

40. Energy Balance Approach Just like snowmelt calculations See equation 7-22. Solve the energy budget equation for everything except LE. LE is the residual, then convert to E Notice the addition of Aw. What is advected water energy? Again, the technique is difficult to apply � Data intensive Only good over small time intervals� Need water surface temperature Bowen ratio is often used to eliminate the need to calculate sensible heat.

42. Combination Approach A key difficulty to applying the mass-transfer or energy balance approach is the need for surface temperature. If the two approaches are combined, the need to measure Ts disappears. See equation 7-33. Evaporation is the weighted sum of the rate due to net radiation and the rate due to mass transfer. Data needed K, L, Va, Ta, RH Question: how do we obtain L? Penman is a commonly used approach Open water � eqn 7-33, page 286 ET � eqn 7-56, page 299 What�s the difference?

43. Combination Approach Let�s look at the terms in the Penman-Monteith equation D= ? (K+L) = ? Gamma = ? Cat Ccan = ? Wa=? Is this an actual or potential ET approach?

44. Combination Approach

45. Other Concepts to Know Priestly Taylor Radiation Index (not in book) Thornthwaite Temperature Index (not in book) Turbulent transfer Eddy correlation, Bowen ratio Soil Moisture Balance Isotopes