Why did the ancient Chinese organize large groups of people to build and maintain irrigation systems? based on an order from the emperor to accommodate for lack of dependable rainfall to pay tribute to their ancestors to promote the growth of more papyrus plants

"To accommodate for lack of dependable rainfall" would be the best option as to why the ancient Chinese organized large groups of people to build and maintain irrigation systems, since when rainfall would not come for long periods of time, the crops would likely die without proper irrigation.