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Water budget and energy budget evaporation methods

Short answer

Water budget method uses continuity equation: evaporation = precipitation + net surface/groundwater inflow – outflow – storage change. Energy budget method uses energy conservation: evaporation = (net radiation – stored heat – advected energy)/[ρL(1+β)].

Explain the water budget and energy budget methods for estimation of evaporation.

Water budget method

Continuity equation applied to the lake over a period:

P+Vis+Vig−Vos−Vog−EL−TL=ΔSP + V_{is} + V_{ig} - V_{os} - V_{og} - E_L - T_L = \Delta S

so

EL=P+Vis+Vig−Vos−Vog−TL−ΔSE_L = P + V_{is} + V_{ig} - V_{os} - V_{og} - T_L - \Delta S

where PP = precipitation, Vis,VosV_{is}, V_{os} = surface inflow and outflow, Vig,VogV_{ig}, V_{og} = groundwater inflow and outflow, TLT_L = transpiration loss, ΔS\Delta S = change in storage (all in m³). It is simple, but seepage, groundwater flow and measurement errors make it inaccurate for short periods; it suits long periods.

Energy budget method

Conservation of energy for the lake: Qn=Qe+Qh+Qθ+QvQ_n = Q_e + Q_h + Q_\theta + Q_v. Using Bowen's ratio β=Qh/Qe\beta = Q_h/Q_e:

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

QnQ_n is net radiation, QθQ_\theta stored heat, QvQ_v advected energy, ρ\rho density and LL latent heat. It gives accurate results for any period but needs costly instruments and many measurements.

✓ Answer checked by Claude Sonnet 5.5

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