Estimation of Evaporation From Reservoirs

1. Estimation of Evaporation From Reservoirs Kevin Stamm, Kellie Bergman, Tim Temeyer Water Management, Omaha District Dr. Steven F. Daly ERDC/CRREL Second Annual Missouri Basin River Forecaster’s Meeting15-17 November 2010

2. Overview • Results from a long term, relatively low cost study to estimate evaporation from reservoirs in the Omaha District and NWD-RCC • Requirements: • Physically based • No new instrumentation installed in field • Remove existing evaporation pans • Work within the CWMS framework

4. Evaporation Estimation Two Processes: • Bulk flux evaporation estimation • 1-D temperature heat budget to estimate vertical temperature profile in reservoir Using: • Met data available through Internet from NWS or estimated • Reservoir geometry data

5. Bulk-Flux Algorithm • Estimate reservoir surface roughness based on wind speed • Assume neutral stratification in air • Estimate transfer coefficients • Calculate fluxes • Estimate air stratification

8. Surface Temperature is Key Surface temperature has a strong influence on the evaporation flux because the saturation humidity immediately above the water is a strong function of the water temperature

9. 1-D temperature heat budget to estimate surface temperature • Heat transfer through surface with atmosphere • Distribution of heat throughout depth of reservoir: 1-D

10. Flow Through times

11. Heat transfer through surface with atmosphere • Sensible heat flux • Latent heat flux • Solar radiation • Long wave radiation in and out • Both solar and long wave require information on cloud cover – NWS METAR

12. Distribution of heat throughout depth of reservoir: 1-D • Solar Radiation penetration • Thermal diffusion • Mixing due to Wind • Mixing due to Potential Energy

13. Solar Radiation penetration • Majority of solar radiation is absorbed at surface • But the remaining solar energy can penetrate to depth and be absorbed below the surface • The clearer the water the deeper the penetration • Measured using Secchi disk

14. Wind and Density Mixing • Density mixing cause denser water to mix with less dense water below. (Primarily in the fall) • Wind mixing uses the work of the wind blowing over the surface to mix denser water from below with less dense water above (Anytime the wind blows.) • Algorithms are available for both

16. TriLakes Chatfield

17. TriLakes Chatfield

19. Pipestem

20. Pipestem

21. Pipestem

22. Oahe

23. Oahe

24. Application • Operating in Omaha District CWMS • Each reservoir is assigned a NWS station • Decode NWS METAR messages • CWMS runs model once each day for each reservoir • Stores evap. and water temperature profile for next run

26. Further Work • Resolve CWMS problems • Ice cover model • Convective heat transfer (inflow and outflow)