Why did Su Hu and the world appear in the Song Dynasty?

"If Suhu is ripe, the world is full" This proverb has been circulating since the Southern Song Dynasty. The source can be found in Gao Si's "Manuscripts of the Shame Hall·Ning Guofu's Encouraging Farmers" and other books. Su refers to Suzhou, the area around present-day Wuxian County, Jiangsu Province, east of Taihu Lake. Lake refers to Huzhou, which is in the present-day Wuxing area of ??Zhejiang Province, south of Taihu Lake. The Taihu Lake Basin has flat terrain, fertile land, and small rivers all over. "Hundreds of streams and numerous waterways (dú, waterways) twist and turn", so it is called "a water town and a country". From the period of Emperor Gaozong of the Song Dynasty to Emperor Xiaozong of the Song Dynasty (1127-1189), water conservancy projects were built in the Taihu Lake area. To the north of Kunshan and Changshu, estuaries were opened to guide lake water into the rivers and seas. During the reign of Emperor Xiaozong, a gate was set up at the exit of Taihu Lake to regulate the water volume. The fields are low beside the lake, and the polder bank is built high to protect against the wind and waves. Due to the dredging of water conservancy, serious floods have been eliminated, and both low and high fields can be irrigated. Coupled with the advanced agricultural production technology here, farmers cultivated intensively and carefully, "ploughing without waste, mowing without leaving ridges", and using the latest agricultural tools "flails" to thresh grains. As a result, the Suhu area became a high-yield agricultural area in the Southern Song Dynasty. It is harvested twice a year, and the yield of the upper field reaches five or six stones per mu, so it is known as "Suhu is ripe and the world is full".