Forced Evaporation

The Forced Evaporation (FE) Analysis capability has been developed to quantify increased evaporation induced by increased water temperatures due to releases from thermoelectric power plants. These power plants withdraw cooling waters, which once run through the plant and are returned/discharged to rivers or lakes at a higher temperature than the ambient water temperature. This higher temperature water causes additional evaporation (forced evaporation) from the river or lake. This additional evaporation is counted as water consumption by regulators as it is no longer available to downstream users.

Evaporation is dependent on wind speed, atmospheric humidity, and water temperature. There are a number of methods to compute FE using different wind functions as listed in Table 1. The wind functions are computed using data contained in the ASER file (ASER.INP) in EFDC.

Table 1 List of Evaporation Calculation Methods

Using these various evaporation methods, the model is first run with the power plant, and then run again without the power plant. EE then subtracts the output from two models and displays the difference which is the consumption of water from the power plant. The user may select "with power plant" option to calculate evaporation in the Temperature | General & Data tab in the frame as shown in Figure 1. The evaporation options are only available if temperature is being simulated and the ASER file is used.

Once temperature is activated and the correct Surface Heat Exchange Sub-Model option has been selected the user can choose which evaporation approach is desired. Even if evaporative losses are not a major concern, the evaporative mass fluxes should normally be activated for most models.

Heat flux due to evaporation is always included for the Full Heat and the Equilibrium Temperature (W2) options.

EFDC_DSI/EFDC_Explorer Forced Evaporation (FE) toolset results have been compared to the Electric Power Research Institute's (EPRI) FE estimates. EPRI's once through cooling FE analysis for river discharges is based on a USGS report on water consumption by thermoelectric power plants (USGS, 2013; EPRI, 2013).

Figure 1 Forced Evaporation: Evaporation Options for Water Balance.

Once the models runs have been completed, the user should go to the Model Analysis | Forced Evaporation tab where the options for model comparison are provided as shown in Figure 2.

Figure 2 Forced Evaporation Options GUI.

To display the general instructions on how to conduct a forced evaporation (FE) analysis using EE, click on the blue text box shown in Figure 2. EE will display the instructions shown in Figure 3.

Figure 3 Forced Evaporation Analysis Setup Instructions.

As outlined Figure 3, the user should then load the With Plant model as the primary (1) model. Next load model (2) using the Load Without Plant Model button shown in Figure 2. This will display the form shown in Figure 4. After selecting Enable Model Comparisons, two models will have been loaded that can be used for the FE calculations and reporting by EE.

Figure 4 Forced Evaporation: Compare Models Options.

The Model Labeling frame in Figure 2 tells the user the type of FE option that has been selected. In Figure 4, "RH with Plant" refers to model run that used Ryan-Harleman approach. Time series plots may be automatically generated for either evaporation (no plant) and forced evaporation (with plant) using the buttons in the respective frames. A plot of forced evaporation (mm/day) and cumulative volume of forced evaporation is shown in Figure 5.

The user can also produce summaries of evaporation and forced evaporation using the Tabular Summary button as shown in Figure 2.

Figure 5 Forced Evaporation: Time series using Anderson evaporation approach.

Another way to display the impact of FE between the two models is in ViewPlan. Here the user should select the Volumes | Evaporation viewing option. Selecting Alt M will toggle on the Model Comparison tool which allows the user to visualize evaporation/rainfall as "With Plant" minus "Without Plant" models as shown in Figure 6.

Figure 6 Forced Evaporation: Model Comparison using Anderson approach.

Another new and related feature in EE7.3, is the user can now generate time series of the differences in water column results as shown in Figure 7. DT means "delta temperature".

Figure 7 Forced Evaporation: Time series of model comparison for temperature.