Here's the text for China's unique advance that kicks off of Irrigation:
Get +1 Food from each Fertile space with a Settler (and no city)
that you “Collect” from (up to 2 extra Food per “Collect”).
When you “Collect” from a Fertile space outside of your city
that has a Settler on it, you gain an extra Food. You can get
at most 1 extra Food per Fertile space and at most 2 extra
Food total. The extra Food is a bonus and is not included in
the amount of resources you can “Collect” based on city size.
So, you're collecting from a fertile space with a settler on it using a city activation, so by default (without husbandry, for instance) that means the space and the settler are adjacent to the city, right?