R’ Elazar ben Azarya says you cannot make a new irrigation canal on Chol Hamoed and Shmitta. The Chachamim say you can make a new canal on Chol Hamoed and Shmitta and fix the old broken ones.

You can fix damaged cisterns on public property that are meant for the public and also clean them out. You can fix public roads, streets and mikvaous [ritual baths] for the public. You can mark graveyards and go out to check for fields that are kilayim [mixed vegetation] to have them uprooted.

Moed Katan 1:3

R’ Elazar ben Yaakov says if water is around one tree you can drag it to another as long as you don’t water the whole field as that is too much work.

Seeds that were not watered before Chol Hamoed cannot be watered on Chol Hamoed. The Chachamim permit it.