this particular medieval unit comes from irrigation
maybe a rice crop requires three feet of water from planting to harvest, and you normally only get three inches of rain during that time, so you need 2.75 acre-feet of irrigation water per acre of rice
maybe a rice crop requires three feet of water from planting to harvest, and you normally only get three inches of rain during that time, so you need 2.75 acre-feet of irrigation water per acre of rice