You're talking about a small decimal point variation in heat of vaporization aka enthalpy of vaporization. You are correct it does in fact vary by temp, but as a percentage, less than my vast rounding errors. One problem is heat of vaporization usually drops with temp. So evaporating water at 400 degrees in a boiler takes somewhat less energy than evaporating the same amount on a open to the air pond, and there's not much of a pressure vessel around the plant anymore anyway. For all intents and purposes given reasonable rounding the energy required to evaporate the water is more or less constant in that location.

This has to do with the strength of the hydrogen bond in water. The energy require to vaporize the water is the same as hundreds of degrees of merely heating the water, thats why the starting temp has little effect on water boiling rate, etc.

Still I stand by my numbers, 2 MW continuously can evaporate your 50 tons of water per day, to one sig fig.

Note that we're not arguing about much. Sunlight is like a KW per sq meter so we're arguing about the energy from 2000 or so sq meters of sunlit area, which isn't much compared to a giant nuclear plant facility.

It just doesn't strike me as much of an engineering challenge to evaporate 50 tons/day given that size of a facility and the decay heat and the solar insolation.

Those are some awesome calculations. Since we are talking Japan's food supply and the Pacific Ocean, it would be nice to have some independent measurements taken on site, in addition to your nice back-of-the-envelope calculations.

Keep in mind this water is all indoors, so unless you have some openings you are blowing air in, there will be no evaporation whatsoever.