Moed Katan, Chapter 1, Mishnah 3. This Mishnah deals with the laws of watering on Chol HaMoed: which activities were permitted and which were forbidden, and where the line is drawn between an activity that merely brings benefit and one whose omission causes an actual loss.
"Moshchin es hamayim me'ilan le'ilan":
Rabbi Eliezer ben Yaakov says that one may draw water that has collected around one tree over to another tree. According to the Bartenura, the case is a rain-fed field, a field that survives on rainwater alone and does not require irrigation. Even so the activity was permitted, since moving water from one tree to another involves no great exertion, and it brings gain and benefit, even though no loss would result if it were not done.
The Mishnah, however, adds a limitation: "uvilvad shelo yashkeh es kol hasadeh" - the permission applies only to drawing the water from tree to tree, and not to flooding the entire field with water.
"Zera'im shelo shasu lifnei hamo'ed - lo yashkem bamo'ed":
Vegetables and seeds that were not watered before Yom Tov may not be watered on Chol HaMoed. The principle behind the ruling: even in a beis hashelachin, a field that requires irrigation, the need to water it immediately on Chol HaMoed exists only when it was watered shortly beforehand. But if it was not watered before the festival, failing to water it on Chol HaMoed will cause no loss. This is therefore a case of gain and benefit alone and not of preventing a loss, and for that reason the watering was forbidden.
"Vachachamim matirin bazeh uvazeh":
The Sages disagree with Rabbi Eliezer ben Yaakov and permit both cases:
Drawing the water: the permission is not limited to moving water from one tree to another; one may even water the entire field, since an activity that brings benefit is permitted even when no loss results from omitting it.
Seeds that were not watered before the festival: one may water them on Chol HaMoed, for although no loss results from leaving them unwatered, there is benefit in watering them.
In summary: in this Mishnah, Rabbi Eliezer ben Yaakov and the Sages disagree as to whether a watering activity that brings benefit and gain, but whose omission causes no loss, is permitted on Chol HaMoed. According to Rabbi Eliezer ben Yaakov, only drawing water from tree to tree was permitted, and only provided that one does not water the entire field, while seeds that were not watered before the festival may not be watered during it. The Sages permit both cases, because the benefit involved in the activity is itself sufficient grounds to permit it.