Video: Understanding Evaporation and Transpiration

About US Army Corps of Engineers Savannah District

The U.S. Army Corps of Engineers Savannah District oversees a multi-million dollar military construction program at 11 Army and Air Force installations in Georgia and North Carolina. We also manage water resources across the Coastal Georgia region, including maintenance dredging of the Savannah and Brunswick harbors; operation of three hydroelectric dams and reservoirs along the upper Savannah River; and administration of an extensive stream and wetland permitting and mitigation program within the state of Georgia. Follow us on Twitter @SavannahCorps and on
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  • Ferris

    Great demonstration of the amount of water transpired by just a few leaves! Transpiration is invisible, as the video says, because evaporated water is an invisible gas. Overnight cooling and increased humidity in the bag changed the gas to a liquid, showing just how much water was transpired. The increased humidity in the bag might have reduced the transpiration rate. The temperature is lower under a large tree because of the shade and because the transpired water removes heat from the surrounding air in order to evaporate, the same reason a damp cloth hanging in the shade gets cool. Because it is heavier than the surrounding air, the cooler air gradually migrates toward the ground for us to enjoy. Thanks also for the brief tour! ~Ferris

  • George C. Selfridge, Jr.

    I think it would have been much more informative if the evaporation number was stated in number of inches per day verses the cubic feet.

    • Ferris

      Hi George, I like your idea of presenting level drop due to evaporation over a specific time period and decided to perform the calculations.

      A rule of thumb for lake level drop due to evaporation is 1″ a week from mid-April to mid-August. To be more specific, the level drop increases from 1″ a week in mid-April to 1-1/4″ a week during June. Beginning in July, the level drop decreases from 1-1/4″ to 1″ a week by mid-August. Around Jan 1 the level drop reaches a low of 1/4″ a week. The transitions are fairly linear. These estimates are subject to the caveats in the following two paragraphs. These caveats may be part of the reason for no response to your post.

      Evaporation rates can vary dramatically depending upon weather conditions. They will be much lower during cool/humid/rainy periods and higher during hot/dry/sunny/windy periods. There is less surface area when lakes are below the summer full pool level, resulting in relatively lower evaporation rates.

      A USACE graph shows lake evaporation for the 3 lakes at 900 CFS in mid-April, rising to over 1,100 CFS for all of June, and falling to 900 CFS by mid-August. These rates are consistent with the peak of 1,200 CFS in the video. Consider these numbers as typical with fluctuations occurring from year to year, much as precipition amounts are not consistent from year to year.

      Basis and Calculations:
      – There are a combined 151,400 acres of water when the lakes are full. “Corps Lakes on the Savannah River”. (55,950+26,650+68,800 = 151,400).
      – From USACE Conversion Factors: 1 CFS-Day = 1.9832 AF
      – Assume 1,000 CFS-Day for 7 Days or 13,882 AF/Week of Evaporation
      – 13,882 AF-Week/151,400 A = 0.0917 Ft/Week or 1.10″/Week.

      – For 200 CFS, the drop is 0.22″/Week.
      – For 900 CFS, the drop is 0.99″/Week.
      – For 1,100 CFS, the drop is 1.21″/Week.
      – For 1,200 CFS, the drop is 1.32″/Week.

      I hope this helps. ~Ferris

      • This is a good answer Ferris. You are correct. Measuring evaporation by inches over a period of time is very complex – not only because the rates change with the conditions, (season, weather, humidity etc,) but also because the volume of an inch of water at a higher elevation is greater than an inch of water at a lower elevation, due to the sloping shape of the basin. ~Russell Wicke

        • Ferris

          Hi Russell. Your comment about less volume per inch brings to mind another layer of complexity that was not included. The USACE graph did not indicate lake levels, and without thinking it through I used the rates as if the lakes were full. Reconsidering, the evaporation rates are presumably based on average levels which are about 3 Ft below full by the end of August for Hartwell and Thurmond. This helps explain why evaporation rates begin dropping so rapidly at the beginning of Jul. Calculations become more complex because the average level and surface area change daily, and I lack the hard data and desire for this much detail. Just putting my thumb on the scale for a small tweak, I now estimate that in July the typical level probably drops in the range of 1-1/4″/Week because of evaporation, with 1″/Week reached around the end of Aug. ~Ferris

  • Fish1

    You have a good presentation on ‘invisible’ losses of water from our system. I still don’t understand why, with below average rainfall in July and Aug, releases could have been lowered toward 4,000 to keep more water in our lakes. We left the rule curve as we entered July and continued downward thru the month. Aug saw no recovery with continuing releases of about 6,000. Realizing that transpiration and evaporation will take place and reduce available water, reduced rainfall should require prudent reduced releases by the Corps to keep us closer to the rule curve while harming no one.

    • scottl

      Why is because of hydro. They will release whatever is required to meet their commitment until trigger level 1 is reached. “Balancing” Hartwell and thuond is even more important than reducing release rates. Just look at today. The lakes are still at barely above the trigger yet releases this week will be 5600cfs. And the lakes may even enter trigger 1 next week as a result even with rainfall above normal for the first half of the month.