As soon as Kublai Khan, the Mongol ruler who founded the Yuan Dynasty, chose a site for his capital city, he realized that he needed a hydraulic engineer. He found one in Guo Shoujing. What did Kublai need Guo to do for the city of Beijing? He needed Guo to design a way to route water to Beijing, which had no natural waterways nearby.
He needed Guo to design a palace that would be grander than all of the Chinese emperors before him.
He needed Guo to build a dam that would hold back the large lake near Beijing, in case it flooded.
He needed Guo to build windmills that could be used to ground large amounts of grain into flower.