​Water loss at ponds

  A. Estimating water loss

  1. How much water would ponds at installed beaver dam analogs (BDAs) lose?

  2. Would that significantly diminish flow arriving at McPhee reservoir?

Barriers | Water loss| Example

Ponding behind a beaver dam at Ryman


  B. Scientifically, it's complicated

  1. Water at ponding is lost to evaporation, transpiration, and infiltration.

  2. Losses vary due to air and water temperature, humidity, wind speed, and net radiation. [1,2].

  3. It is conventionally tested in-field with pans or tubs and meteorological information [1,3].

  4. Still, the measurements generally are of only evaporation loss.

  5. Some cite the largest loss as evapotranspiration in arid and semiarid lands, perhaps 95% [4].

  6. (Evapotranspiration is combined  evaporation and transpiration.)

  7. Others observe that infiltration can be more than half the total loss [5].

  C. Tool for evaporation loss

  1. Determined from flow per hour (cf/hour).

  2. Calculated as 0.5-1% of the flow,

  3. Depending on shaded to full sunlight conditions.

  4. Calculation is used to represent (estimate) loss per day (cf/day).

  5. Recognizes ponds as flowing systems, not static.

  6. That is, water continuously is available to be lost.

  7. Tool is applied by professional landscapers [6].

  D. Scenario approach

  1. No water loss data is known for the upper Dolores basin.

  2. Scenario or worst case assessments have been used by at other study sites to estimate possible water loss [7].

  3. For Dolores, potential pond locations in candidate streams were identified.

  4. Flows were obtained for those [8].

  5. Water loss from evaporation was calculated for those locations.

  6. Conservative assumptions were applied, maximizing estimated loss.

  7. Flows for the most vulnerable water-temperature months were used.

  8. Those flows were compared with the amount of flow entering the reservoir.

  E. Conservative assumptions and vulnerable conditions

  1. Started with the 1% factor, that is, assumed continuous full-sunlight conditions.

  2. Additional conservative assumptions

    • Assume significant infiltration loss.

    • Incorporate also transpiration loss.

  3. Upgraded to a 2-3% factor for combined evapotranspiration and infiltration loss.

  4. Used flows for July-August, the warmest months, the most habitat-sensitive.

  F. Scenario estimates​

  1. Installation, a BDA, 2 mi upstream in a candidate tributary.

  2. Assumption, main loss, evaporation, 1% factor

    • Wildcat, 90 BDAs for 1% loss at McPhee.

    • Coal, 38 BDAs for 1% loss at McPhee.

  3. Conservative assumption, combined loss, evapotranspiration and infiltration, 2% factor

    • Wildcat, 45 BDAs for 1% loss at McPhee.

    • Coal, 19 BDAs for 1% loss at McPhee.

  4. Additionally conservative assumption, 3% factor

    • Wildcat, 30 BDAs for 1% loss at McPhee.

    • Coal, 13 BDAs for 1% loss at McPhee.

  G. Simple, preliminary conclusion​

  1. Installation of several BDAs in the upper Dolores basin will not significantly reduce flow to McPhee reservoir.

  2. It is based on this assessment using a scenario approach, applying a calculation tool for estimating daily water loss at ponding, and invoking conservative assumptions.

  H. Non-ponded water

  1. Water loss by evaporation occurs all along a stream, not just at ponding.

  2. Turbulent water logically has more water surface area exposed for evaporation to occur than happens in still water.

  3. "Water movement [has been] found to increase rates of evaporation relative to non-moving water during periods of low vapor pressure deficit and no air movement" [9].

  4. That is to say, key are air and water temperature differences and wind.

  5. It appears that water-loss contributions from ponding, that is, adding to what occurs from stream flow, are seasonal.

  I. Cooling

  1. Stream water loses heat by evaporation, that is, the water cools.

  2. Evaporation is greatest when temperatures are highest, giving the most cooling.

  3. Smaller streams, having less depth and water volume, heat and cool quicker than larger streams.

  4. So cooling by evaporation is most consequential at small streams for offsetting a tendency for water temperatures to rise.

  5. Notably, the relatively small volumes of trout habitat at small streams are vulnerable to both water losses and temperature increases, and the evaporative process figures in both.


  1. Szeitz, A. J. and R. D. Moore, 2020, "Predicting Evaporation from Mountain Streams," Hydrological Processes, DOI-10-1002/hyp.18375.

  2. Friedrich, K, R. L. Grossman, J. Huntington, P. D. Blanken, J. L. Lenters, K. D. Holman, D. Gochis, B. Livneh, J. Prairie, E. Skeie, N. C. Healey, K. Dahm, C. Pearson, T. Fionnessey, S. J. Hook, and T. Kowalski, 2019, “Reservoir Evaporation in the Western United States: Current Science, Challenges, and Future Needs,” Bulletin of the American Meteorological Society, v. 99, n. 1, pp. 167-187, DOI: https://doi.org/10.1175/BAMS-D-00224.1.

  3. Taghvaeian, S. and A. Sutherland, 2016, “Evaporation Losses from Shallow Water Bodies in Oklahoma,” Oklahoma State University Extension, ID: BAE-1529.

  4. Schuster, J. L., No date, “Soil and Vegetation Management: Keys to Water Conservation on Rangeland,” Texas A&M AgriLife Extension, https://agrilifeextension.tamu.edu/library/ranching/soil-and-vegetation-management-keys-to-water-conservation-on-rangeland/.

  5. Bam, E. K. P. and A. M. Ireson, 2019, “Quantifying the Wetland Water Balance: A New Isotope-Based Approach That Includes Precipitation and Infiltration,” Journal of Hydrology, v. 570, pp. 185-200.

  6. Professional landscapers: https://willowridgegardencenter.com/how-much-water-does-a-pond-lose-to-evaporation/; https://backyardsidekick.com/how-much-water-your-pond-will-lose-to-evaporation/; https://support.aquascapeinc.com/hc/en-us/articles/205446445-Evaporation-rates.

  7. “Short Cut for Method for Wetland Drawdown Assessment, Appendix D.3” No date, https://mde.state.md.us/programs/Water/StormwaterManagementProgram/Documents/www.mde.state.md.us/assets/document/sedimentstormwater/Appnd_D3.pdf.

  8. From application of the U.S. Geological Survey Water Program StreamStats, https://streamstats.usgs.gov/ss/.

  9. Bennet, D. A., 1999, " Evaporative Heat Loss of the Upper Middle Fork of the John Day River, Northeastern Oregon," Masters Thesis, Oregon State University.