1. Evapotranspiration

2. Deposition

3. Water Vapor
Liquid Water

4. Solar Radiation
Wind
Evaporation
Temperature
Relative Humidity

5. Evaporation

6. Evaporation

7. Evaporation

8. Evaporation

9. Evaporation

10. Evaporation

11. Evaporation

12. Evaporation

13. Evaporation

14. High Evaporation High Winds High Solar Radiation High Temperature
Low Relative Humidity

15. Mean Daily Solar Radiation
Mean Annual Lake Evaporation
Mean Daily Solar Radiation (langleys)
Annual Lake Evaporation (inches)
10-20
20-30
30-40
40-50
50-60
60-70
70-80
80+
Mean daily solar radiation in the United States and Puerto Rico.(Source: Data from the U.S. Department of Commerce, 1968). From Hanson 1991.
Mean annual lake evaporation in the conterminous United States, Data not available for Alaska, Hawaii, and Puerto Rico.(Source: Data from U.S. Department of Commerce, 1968). From Hanson 1991.

16. Evapotranspiration
Evaporation from the land surface, including evaporation of soil moisture and transpiration from plants
Figure by Mwtoews (Own work) [GFDL ( or CC-BY-3.0 ( via Wikimedia Commons

17. Transpiration
Evaporation of water from plant leaves Water moves from the soil into plant roots, through the plant, and finally out through the leaves
water vapor
Background image licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons -

18. Transpiration Evaporation of water from plant leaves water vapor
Microscopic structure of a leaf, and the processes of photosynthesis and transpiration. Image credit nasa.gov
Background image licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons -

19. Transpiration is an evaporative process
Same meteorological factors:
solar radiation
temperature
wind
humidity
And more factors
plant structure, age, health and other factors
Image used with permission from

20. Estimated Mean Annual Precipitation, for the Period 1971 to 2000
Estimated Mean Annual Actual Evapotranspiration (ET) for the Period
Estimated Mean Annual Precipitation, for the Period 1971 to 2000
Estimates are based on a regression equation of ET⁄P that includes land cover multiplied by the mean annual precipitation from the PRISM climate data for the same period. Calculations of ET were made first at the 800-m resolution of the PRISM climate data. The mean values for the counties (shown) were then calculated by averaging the 800-m values within each county.
Data compiled from PRISM Climate Group, Oregon State University (Daly et al., 2008), accessed July From Sanford and Selnick 2012.
Images Copyright (2012) Wiley. Used with permission from: Sanford, Ward E. and David L. Selnick, Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data. Journal of the American Water Resources Association, John Wiley & Sons.

21. Actual ET – varies with climate and plant type
Reference ET = ETo
Well-watered standard grass

22. Reference ET, or ETo Reference ET standard height and stage of growth
factors vary with location and climate
Image modified from FAO, 'Guidelines for computing crop water requirements' Chapter 1. [ This is an adaptation of an original work by FAO. Views and opinions expressed in the adaptation are the sole responsibility of the author or authors of the adaptation and are not endorsed by FAO.

23. PF Landscape ET = ETL L plant factor landscape ET reference ET
(well-watered grass)
landscape ET
landscape
area
Image modified from FAO, 'Guidelines for computing crop water requirements' Chapter 1. [ This is an adaptation of an original work by FAO. Views and opinions expressed in the adaptation are the sole responsibility of the author or authors of the adaptation and are not endorsed by FAO.

24. Measuring ET and estimating irrigation
Precipitation gauge
Atmometer – measures reference ET
Image from CoCoRahs.org
Image from etgage.com

25. Measuring ET and estimating irrigation
Water surplus
Water Deficit

26. Very Low Water Demand, PF = 0.2
Landscape water needs
Sprinkler efficiency  70%
Very Low Water Demand, PF = 0.2
Image from Detweiler et al. 2005
Image from Hattendorf 2012
High Water Demand, PF = 0.8

27. Very Low Water Demand, PF = 0.2
Landscape water needs
sprinkler efficiency
ETL = ETo x PF
Irrigation = (ETL – P) / 0.7
For example, If ETo = 1 in, and P = 0.1 in, for July
For Yucca, PF = 0.2
ETL = 1 in x 0.2 = 0.2 in
Irrigation = (0.2 in – 0.1 in) / 0.7 = 0.14 in
For Turf, PF = 0.8
ETL = 1 in x 0.8 = 0.8 in
Irrigation = (0.8 in – 0.1 in) / 0.7 = 1.0 in
Very Low Water Demand, PF = 0.2
Image from Hattendorf 2012
High Water Demand, PF = 0.8

28. Summary
Evapotranspiration = soil evaporation + plant transpiration
Meteorological factors
Solar radiation, temperature, wind, humidity
Plant factors
Age, health, season, structure and others
Estimating ET
Landscape ET, ETL = ET from different plants
Reference ET, ETo = ET from well-watered grass (varies with meteorological factors)
Plant Factor, PF = plant dependent factor used to estimate ET of a particular plant
ETL = ETo PF

29. Summary Irrigation = (ETL – P) / 0.7
Landscape Water Needs = ETL - Precipitation
If ETL > P, then irrigation is necessary
Must consider sprinkler efficiency (0.7) when estimating irrigation rates
Irrigation = (ETL – P) / 0.7

30. References
Allen, R.G., L.S. Pereira, D. Raes and M. Smith, 1998, Crop evapotranspiration - Guidelines for computing crop water requirements – FAO Irrigation and drainage paper 56, Food and Agriculture Organization of the United Nations, Rome, Italy, Detweiler, A.J. P. Griffiths, and R. Olson, An Introduction to xeriscaping in the High Desert And Pictorial Plant Guide for Central & Eastern Oregon, Oregon State University Extension Service. Hanson, R.L., 1991, Evapotranspiration and Droughts, in Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., Compilers, National Water Summary –Hydrologic Events and Floods and Droughts: U.S. Geological Survey Water-Supply Paper 2375, p Hattendorf, M.J Northern Water Irrigation Management – Xeriscape Irrigation Recommendations, ons.pdf Sanford, W. E. and D. L. Selnick, Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data. Journal of the American Water Resources Association (JAWRA) DOI: /jawr.12010