Combining Heat and Mass Flux Methods for Estimating Real-Time Evaporation from a Water Surface

Abstract:

Quantifying the heat and mass fluxes associated with evaporation from lakes and reservoirs is a
challenge for hydrologists and water managers. This is in large part due to a lack of comprehensive
measurement data for most systems, which is itself related to the inherent difficulties associated with
measuring turbulent quantities. An alternative to direct measurement is to develop better models for the
evaporative flux, based on the mean terms (as opposed to the turbulent terms) that drive evaporation.
Algorithms for the evaporative heat and mass flux must reflect changes in heat storage in the system as
well as the other components of a mass balance (inflow, outflow, and precipitation). The energy budget based
approach requires records of all the other energy fluxes across the air-water interface to separate
out the latent heat component. Other approaches utilize the similarity between atmospheric velocity,
temperature and humidity profiles. This study seeks to combine these approaches to build and calibrate
heat flux models that can be used to accurately recreate a long-term record of mass storage change
from a sub-set of meteorological data, lake surface temperature data, and hydrologic observations. High
frequency lake level data are used to check that the mass balance is in fact achieved. Good agreement is
shown between the heat flux methods and the mass balance results through comparison with a three-year
record of lake level. The results demonstrate that a combination of mass and heat flux approaches can
be used to generate accurate values of evaporation on daily or even sub-daily time-scales.