The study looked into how changes in a watershed's shape influence how much water runs off the land. They focused on a place in China called Qiantang Estuary, known for its big tides but lacking runoff data. Using a simple model and GIS technology, they found that as cities grew, the runoff increased significantly over 35 years. The runoff depth went up by about 36mm per year, leading to a total yearly volume increase of a huge amount. The shape of the land plays a big role in how much water flows over it, affecting flooding and the environment in a watershed.