shoyu evaporating while fermenting

[Jwest]

I am making shoyu for the first time and am noticing that the liquid is evaporating out and as it does so, the frequency of white mold on the top in increasing. I set it up last February and was planning on letting it go for a year, but by then, there will be no liquid! Can I add liquid (brine?) and if so, what ratio salt/water?
I started with about 2 gallons in a 4 gallon pail, covered with a cloth.