Imagine you have a big bathtub filled with water. Sometimes you want to fill it up really fast, but other times you might only want to fill it halfway. Hydrological optimization is a way to figure out how to make the water in the bathtub work best for you.
But instead of a bathtub, we're talking about a river. You might want to figure out the best way to use the water in the river for things like generating electricity, providing water for drinking, or helping plants grow.
People who study hydrological optimization use science and math to figure out when to fill up the river and when to let some water out. They try to find the best balance so that the water is used in the most efficient way possible. This helps people and animals who rely on the river to use the water in the best way possible, without wasting any.
So basically, hydrological optimization is like trying to be smart about how we use the water in a river so that everyone who needs it can use it in the best way possible.