how to make rain smulation with accumulation on terrain model?

hello everyone,

I am geomatic engineering student. i made a terrain project on unity by using real height data of local area where i live in. i want to make a rain smulation on this terrain model but i want to accumulation of the rain water so i can smulate behavior of rain water. for expamle rain water should accumulate in relatively deep areas, rain water should flow by slope and it should overflow if continue raining. in short i need real life rain smulation which can be a guide for engineering purpose, not a visual effect. i find various rain smulation but none of them has accumulation of rain water. if you have any suggesiton i will be gratefull.

thank you already.