174
to 8. The cumulative evaporation increased as the value of the
transfer coefficient multiplier increased (Figure 5-25). The
cumulative evaporation for the lowest values of transfer coefficients
matched the initial phase of the cumulative evaporation fairly well,
but then around sunset, the experimental data indicated a continued
slight water loss from the soil, while the simulation showed no water
loss overnight. Evaporation during the period following the first
night showed the model significantly under-predicted the cumulative
evaporation. The simulations for the multipliers of 2.0 and 5.0 over
predicted evaporation during the first day of the experiment, while at
the end of the second day, the cumulative evaporation simulated using
the multiplier of 2.0 matched experimental data fairly well. Since the
cumulative loss had been over-predicted on the first day but was
agreement by the end of the second day, indicated that the water loss
for the second day had been under-estimated. The error between
experimental and simulated cumulative evaporation remained
approximately the same from the first to second day using a multiplier
of 5.0. Examination of the hourly evaporation rates indicated that as
the multiplier for the surface transfer coefficients increase, the
hourly evaporation rates looked more like the experimental data (Figure
5-25) showing that early in the day, the boundary coefficients or the
method in which the boundary conditions for the transfer of vapor was
modeled was limiting the evaporation of water when water was available
at the surface. Increasing the heat and mass transfer coefficients
caused the evaporation rate to exhibit the type of behavior
demonstrated by the experimental rates. A high rate of evaporation