Fable
The new sluice first let a small canal test the water
Daohe Town relied on a large sluice gate for irrigation. The old gate opened and closed slowly, but it had adapted to the temperament of each canal over many years. The town decided to install a new gate that would automatically adjust water volume according to field demand. The craftsmen tested it many times in the courtyard, and the new gate performed well. Someone suggested taking over the whole town's canals the next day: since it had been tested, there was no need to delay.
The old farmer shook his head. The water in the courtyard carries no silt, and it never encounters the hard soil of the north slope, the shallow ditch of the south slope, or a midnight rainstorm. Real fields will force out the problems that cannot be seen in the courtyard.
The mayor first wanted to assign more people to watch the whole town go live. The old farmer said that if hundreds of acres of fields are short of water or flooded at the same time, no amount of watching will be fast enough to save them. When the scope of failure is too large, correction turns into compensation.
In the end they chose a small canal. There was little farmland there, farmers were willing to cooperate, and the old gate could be reconnected at any time. The new gate first served only this canal, and water level, opening and closing counts, farmer complaints, and seedling conditions were all recorded separately.
On the first day, the new gate performed well in clear water; on the third day, silt made the water-level gauge read high, and the water volume was one tenth less. Because the scope was small, the craftsmen replaced the filter that same day, and the old gate could serve as a fallback at any time. The main fields were not affected.
The mayor did not listen to that proposal. He first connected the new gate to a small canal at the north end, where there were few fields and shallow water, so even if something went wrong it would be easy to block. The first night the water volume was good, but the second night upstream leaves jammed the float gauge, and the small canal's water level rose and fell erratically. If the whole town had opened at once, half the rice fields would have been damaged.
The craftsman repaired the floating ruler, watched for another three days, and only then connected the second and third channels. With each extension, someone stood by the channel recording water levels, listening to farmers' complaints, and ready to retreat to the old gate at any time. The new gate eventually took over the whole town, not because it had been tested in the yard, but because it had weathered problems in real water and real fields on a small scale.
A week later the small canal was stable, and only then did the town expand to three canals, then to half the town. Each step had a threshold for scaling up and conditions for reverting to the old gate. The mayor later remembered this: let a small scope discover problems in the real environment for the whole town first; that is what it means to be responsible to farmers.