Evapotranspiration

Deposition

Water Vapor Liquid Water

Solar Radiation Wind Evaporation Temperature Relative Humidity

Evaporation

Evaporation

Evaporation

Evaporation

Evaporation

Evaporation

Evaporation

Evaporation

Evaporation

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

Mean Daily Solar Radiation Mean Annual Lake Evaporation Mean Daily Solar Radiation (langleys) 200-250 250-300 300-350 350-400 400-450 450-500 500-550 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, 1946-55. Data not available for Alaska, Hawaii, and Puerto Rico.(Source: Data from U.S. Department of Commerce, 1968). From Hanson 1991.

Evapotranspiration Evaporation from the land surface, including evaporation of soil moisture and transpiration from plants Figure by Mwtoews (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

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 - http://commons.wikimedia.org/wiki/File:Photosynthesis.gif#mediaviewer/File:Photosynthesis.gif

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 - http://commons.wikimedia.org/wiki/File:Photosynthesis.gif#mediaviewer/File:Photosynthesis.gif

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 http://mesonet.k-state.edu/about/evapotranspiration/

Estimated Mean Annual Precipitation, for the Period 1971 to 2000 Estimated Mean Annual Actual Evapotranspiration (ET) for the Period 1971-2000 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), http://www.prism.oregonstate.edu, accessed July 2009. From Sanford and Selnick 2012. Images Copyright (2012) Wiley. Used with permission from: Sanford, Ward E. and David L. Selnick, 2012. 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.

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

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. [http://www.fao.org/docrep/x0490e/x0490e04.htm]). 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.

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. [http://www.fao.org/docrep/x0490e/x0490e04.htm]). 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.

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

Measuring ET and estimating irrigation Water surplus Water Deficit

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

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

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

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

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, http://www.fao.org/docrep/x0490e/x0490e00.htm. Detweiler, A.J. P. Griffiths, and R. Olson, 2005. An Introduction to xeriscaping in the High Desert And Pictorial Plant Guide for Central & Eastern Oregon, Oregon State University Extension Service. http://extension.oregonstate.edu/yamhill/sites/default/files/an_introduction_to_xeriscaping.pdf 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 1988-89–Hydrologic Events and Floods and Droughts: U.S. Geological Survey Water-Supply Paper 2375, p. 99-104. http://geochange.er.usgs.gov/sw/changes/natural/et/ Hattendorf, M.J. 2012. Northern Water Irrigation Management – Xeriscape Irrigation Recommendations, https://www.northernwater.org/docs/WaterConservation/ConservationGardens/XeriscapeRecommendati ons.pdf Sanford, W. E. and D. L. Selnick, 2012. 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) 1-14. DOI: 10.1111/jawr.12010