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Energy budget method of lake evaporation

Short answer

Apply energy conservation to the lake: net radiation equals heat used for evaporation, sensible heat, storage change, and advection. Sensible heat is obtained via Bowen ratio β, then evaporation from lake EL = (Qn - Qθ - Qv) / [ρL(1+β)]. It requires detailed radiation, temperature, and flow data.

Explain the energy budget method of estimating evaporation from a lake.

The energy budget method applies the law of conservation of energy to a lake over a period (usually a day or a week) to find the energy used in evaporation.

Energy balance

Qs−Qr+Qa−Qar−Qbr=Qe+Qh+Qθ+QvQ_s - Q_r + Q_a - Q_{ar} - Q_{br} = Q_e + Q_h + Q_\theta + Q_v

In short, Qn=Qe+Qh+Qθ+QvQ_n = Q_e + Q_h + Q_\theta + Q_v, where

TermMeaning
QsQ_sincoming short-wave solar radiation
QrQ_rreflected short-wave radiation
Qa,QarQ_a, Q_{ar}incoming long-wave atmospheric radiation and its reflection
QbrQ_{br}long-wave back radiation emitted by the water surface
QnQ_nnet radiation energy received
QeQ_eenergy used for evaporation =ρLEL= \rho L E_L
QhQ_hsensible heat conducted to the air
QθQ_\thetachange in heat stored in the water body
QvQ_vnet energy advected by inflow/outflow

Evaporation

Sensible heat is found by the Bowen ratio β=Qh/Qe=γ(Tw−Ta)/(ew−ea)\beta = Q_h/Q_e = \gamma (T_w - T_a)/(e_w - e_a). Substituting:

EL=Qn−Qθ−QvρL(1+β)E_L = \frac{Q_n - Q_\theta - Q_v}{\rho L (1 + \beta)}

Data needed

  • Radiation: from a pyranometer or computed from sunshine hours.
  • Water temperatures at various depths (for QθQ_\theta), and air temperature and humidity (for β\beta).
  • Inflow, outflow and their temperatures (for QvQ_v).

Merits and demerits

  • It gives accurate evaporation, can be used for any time period, and is the basis for calibrating other methods.
  • It needs costly and detailed instruments, and each term is difficult to measure; QvQ_v and QθQ_\theta are especially hard to find. So it is used for research on large lakes rather than for routine estimation.

✓ Answer checked by Claude Sonnet 5.5

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