Imagine you have a big swimming pool in your backyard, and it just rained really hard. The rain water is pouring into your pool, and it's starting to overflow.
That's kind of like how a reservoir works. A reservoir is basically a big man-made pool that collects water from rain, snowmelt, or other sources. But instead of just letting the water overflow, we control how much water flows out of the reservoir.
We do this by using a runoff model, which is like a fancy calculator that tells us how much water is coming into the reservoir, and how much we need to let out to make sure the water doesn't overflow or cause any problems downstream.
The runoff model takes into account a lot of factors, like how much rain fell in the area, how much water is already in the reservoir, how much water is being used by people or animals, and how much water is flowing out of the reservoir.
By using the runoff model, we can make sure that we have enough water for things like drinking, farming, and generating electricity, while also making sure that we don't have too much water that could cause flooding or other problems.
So just like how you control the water in your swimming pool, we use a runoff model to control the water in a reservoir, and make sure that everything stays safe and under control.