Lake Evaporation by the Energy Budget Method
1) What this calculator estimates
The energy budget approach estimates lake evaporation from heat flows at the surface.
Evaporation is treated as the energy required to convert liquid water to vapor.
This method is widely used for lakes and reservoirs when radiation and heat exchange data exist.
2) The surface energy balance
A practical balance is: LE = Rn − H − G − S − A.
Here LE is latent heat flux, Rn is net radiation,
H is sensible heat flux to air, G is conduction into water,
S is heat storage change in the lake, and A represents net advected energy.
When LE is positive, evaporation occurs.
3) Net radiation and typical magnitudes
Net radiation combines absorbed shortwave and emitted longwave energy.
Over water, daily mean Rn often ranges from about 50–150 W/m²,
while sunny midday values can exceed 200 W/m².
Cloud cover, humidity, and surface temperature strongly influence longwave losses.
4) Sensible heat and atmospheric controls
Sensible heat H depends on air–water temperature difference, wind, and stability.
Warm, dry, windy conditions usually increase evaporation by enhancing turbulent transfer.
When air is warmer than water, H may become negative and add energy to the surface.
5) Heat storage matters for lakes
Lakes store heat during warm periods and release it later.
Daily storage change S can be comparable to Rn in deep or stratified lakes.
Ignoring storage may overestimate evaporation during heating phases and underestimate it during cooling phases.
6) Advection and site effects
Advection A includes heat brought by inflows, outflows, rainfall, and horizontal air movement.
Large cold inflows can reduce available energy, while warm inflows can raise it.
Coastal breezes and surrounding land can also alter the near‑surface microclimate.
7) Converting energy to evaporation depth
The latent heat of vaporization is approximately Lv ≈ 2.45×106 J/kg
near 20 °C. Evaporation rate is E = LE / Lv in kg/m²/s.
Since 1 kg/m² = 1 mm of water depth, the calculator converts flux to mm/day.
For example, LE = 120 W/m² corresponds to roughly 4.2 mm/day.
8) Data checks and uncertainty
Use consistent averaging periods and sign conventions for each flux.
A good sanity check is that warm‑season daily evaporation often falls near 2–8 mm/day,
though local climate and lake depth can push values outside that range.
Radiation and storage terms commonly dominate uncertainty, especially with limited measurements.
FAQs
1) What is the difference between evaporation rate and latent heat flux?
Latent heat flux is energy per area (W/m²) used for phase change.
Evaporation rate is the equivalent water depth lost (mm/day).
The conversion uses the latent heat of vaporization.
2) Why can heat storage change be important for lakes?
Lakes absorb heat during warming and release heat during cooling.
That storage change can shift energy away from, or toward, evaporation.
Deep or stratified lakes often show larger storage effects than shallow ponds.
3) What sign convention should I use for the inputs?
Enter Rn as positive when it adds energy to the surface.
Enter H, G, S, and A as positive when they remove energy.
The calculator also supports a guided “sign mode” to help.
4) What if my computed LE is negative?
A negative LE means the surface energy balance does not support evaporation for that period.
It may indicate condensation or missing terms, such as underestimated radiation or overestimated storage loss.
5) Do I need water temperature for this method?
Water temperature helps estimate longwave radiation and storage change, and it supports sensible heat estimates.
If you lack temperature profiles, use best available surface temperature data and treat storage as uncertain.
6) How accurate is the energy budget estimate?
Accuracy depends on measurement quality of radiation, turbulent fluxes, and storage.
With good instrumentation, energy budget estimates can be reliable.
With sparse data, uncertainty can be large, so interpret results as informed estimates.
7) Can I use this for hourly as well as daily data?
Yes. Use consistent units and averaging windows for every term.
Short windows are more sensitive to measurement noise.
Daily or multi‑day averages often provide more stable evaporation estimates.