Water in the Erhfeng underground dam coming from groundwater. We all know that underwater and surface water are connected. They are from the same waterhead so any forms of developments will eventually affect these two water supply. Then if the water above the riverbed is plentiful, the water under the riverbed would be plentiful as well. Because these two water supply are connected to each other. The geology in Pingtung mainly consists of gravel with large openings. When it comes to rain, upper and middle reaches here would sink a large amount of water into the gravel alluvial fans. These groundwater will not be influenced by the droughts on the ground. If we properly use subsurface flow in the upper terrain, land subsidence will not occur and we also get natural reservoirs. Even we don’t use these water resources they will flow into sea eventually and become a waste. And if it comes a heavy rain, the groundwater will be too much and may cause some damage.