Methods of estimating lake evaporation

Evaporation from a lake or reservoir is estimated by the following methods.

1. Evaporation pans: Standard pans (US Class A land pan, ISI pan, sunken Colorado pan) are placed near the lake and the fall of water level is measured. Lake evaporation $= C_p \times$ pan evaporation, where the pan coefficient $C_p$ is 0.6-0.8 (0.7 for Class A). Simple and widely used, but the pan coefficient is only an approximation.

2. Empirical formulae: Evaporation is related to vapour pressure deficit and wind speed.

   • Meyer's formula: $E_L = K_M (e_w - e_a)\left(1 + \dfrac{u_9}{16}\right)$ (mm/day).
   • Rohwer's formula: $E_L = 0.771(1.465 - 0.000732 P_a)(0.44 + 0.0733 u_0)(e_w - e_a)$. They are easy to use but need calibration for the local area.

3. Water budget method: Applies continuity to the lake: $E_L = P + V_{is} + V_{ig} - V_{os} - V_{og} - \Delta S - T_L$. It is simple in principle, but seepage and the other terms are hard to measure, so errors are large for short periods.

4. Energy budget method: Applies conservation of energy with Bowen's ratio: $E_L = \dfrac{Q_n - Q_\theta - Q_v}{\rho L(1+\beta)}$. Accurate but needs detailed instruments.

5. Mass transfer (aerodynamic) method: Uses theory of turbulent transfer of vapour: $E_L = B(e_w - e_a)$ with $B$ a wind-speed function.

6. Penman (combination) method: Combines energy balance and aerodynamic terms: $E = \dfrac{A H_n + E_a \gamma}{A+\gamma}$ with $r = 0.05$ for water. Gives good results from routine weather data.

7. Analytical/remote sensing methods: Used for large lakes and where ground data are scarce.