Moed Katan, Chapter One, Mishnah Three
This mishnah returns to the subject of irrigating during the festival. As an aside, the fact that the first three mishnayot of this tractate are dedicated to this subject testifies to how crucial irrigation was in Israel, especially during Sukkot and Pesah.
1) Rabbi Eliezer ben Yaakov says: they may draw water from [one] tree to [another] tree, as long as they dont water the whole field.
2) Seeds that have not had [any] drink before the festival, he may not water them during the festival.
a) The sages however allow it in both cases.
Section one: Rabbi Eliezer ben Yaakov holds that if a lot of water had collected around one tree, they were allowed to draw the water from that tree to another tree, because this is not a lot of work. However, one cannot use this way of watering to water the entire field, because that would be too much work.
Section two: If he planted seeds before the festival but had not yet watered them, then he cannot water them during the festival, because they will do fine without being watered. The seeds dont start to open until the first time they are watered. However, if he has watered them already, then he may continue to water them during the festival, because if he does not, they will die. This illustrates an important principleif something will be lost, it is usually permitted to do that given work on the festival.
The sages allow the seeds to be watered even if they had not yet been watered before the festival. They allow this for one of two reasons: 1) they think the seeds will be lost; 2) they think that it is not a significant amount of work.