Water evaporating from a pond at \(298 \mathrm{~K}\) does so by molecular diffusion across an air film \(1.5 \mathrm{~mm}\) thick. If the relative humidity of the air at the outer edge of the film is \(20 \%\), and the total pressure is 1 bar, estimate the drop in the water level per day if conditions in the film remain constant. The vapor pressure of water as a function of temperature can be accurately estimated from the Wagner equation (Reid et al., 1987)

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Pw In P C -7.7645T+1.458471.5 -2.775873-1.23306 T=1-T T - T " T = Pw water vapor pressure T r