how to prevent evaporation of liquid for soup

Is it dried seaweed? It absorbs a LOT of water and doubles or triples in size. Perhaps your water is not evaporating, but it’s just getting slurped up by your thirsty seaweed. In one video, Maangchi put a little square of kelp in the boiling water. Soon, it looked like a huge black skirt. So maybe use less seaweed. But also, you can keep the soup boiling very gently, and put a lid on it, you will lose less water too. Hope that helps!