MISHNAH: THE THREE CATEGORIES OF ACTIVITY DURING CHOL HA'MO'ED
Watering crops that would become ruined without irrigation (Beis ha'Shalchin), i.e. preventing monetary loss (Davar ha'Aved):
It may be done from a new or established spring, and may be done on Shemitah also.
It may not be done from rainwater gathered in wells, cisterns, etc. (because this is too much exertion).
One may not dig circles in the ground around the bases of vines (to hold water) (also too much exertion).
R. Elazar ben Azaryah forbids making a new water channel (on Shemitah also). The Chachamim permit making a new one on Shemitah and repairing an existing one on Chol ha'Mo'ed
Attending to public needs is permitted, for example:
Fixing problems in the public water supply
Marking graves to prevent Tum'ah.
Chol ha'Mo'ed Pesach is the time when special inspectors went to look for Kil'ayim growing in fields and destroy it.
NEW OR ESTABLISHED SPRINGS
New springs are more problematic than established ones, because they might collapse and require fixing (which is prohibited because of exertion).
Question: If the Mishnah permits even new ones, why mention established ones?
Answer: If the Mishnah had remained silent about established ones, one might have thought that it is permitted even for crops that do not depend on irrigation, but do improve with some irrigation (Beis ha'Ba'al). Therefore, it is mentioned in order to teach that it is permitted only for Beis ha'Shalchin.
Shalchin is from the Aramaic Meshalhei ("thirsty").
Ba'al is from Yeshayah 62:5, meaning "settled".
WHO IS THE AUTHOR OF THE MISHNAH?
The Mishnah clearly implies that watering is permitted (1) only to prevent loss (Beis ha'Shalchin), but not for monetary gain (Beis ha'Ba'al). And even this is (2) only permitted when there is not too much exertion. Whose opinion is this?
Answer #1: R. Eliezer ben Yakov, in the Mishnah on 6b.
Refutation: But this shows only point (2), not point (1)!
Answer #2: R. Yehudah of a Beraisa, which states three opinions about using a new spring:
R. Meir permits watering even a Beis ha'Ba'al, which certainly is against the Mishnah.
R. Yehudah permits only a Beis ha'Shalchin that has become disconnected from its usual spring.
R. Elazar ben Azaryah forbids even this (only established springs may be used).
R. Yehudah adds: It is prohibited to dredge out a channel and use this water for one's garden, because of exertion.
So we see from R. Yehudah both point (1) [from 4:c:2] and point (2) [from 4:c:4].
Question: This Beraisa discusses only a new spring. We don't know what R. Yehudah would say about an established spring - perhaps he would permit even Beis ha'Ba'al, against the Mishnah.
Answer: R. Yehudah is the only one who can possibly be identified with the Mishnah, so he probably means exactly what the Mishnah says.
WHY ARE WEEDING AND WATERING PROHIBITED ON SHABBOS?
(Rabah): They come under the category of "Plowing", because they loosen the soil.
(Rav Yosef): They come under "Sowing", because they generate growth of surrounding plants.
(Abaye): Why can't both prohibitions apply? It is, after all, possible to transgress two prohibitions with one act, as Rav Kahana said about one who prunes a tree and also intends to use the pruned branches ("Detaching" and "Sowing"). The question remains unanswered.
Question on Rav Yosef: A Beraisa says that one who weeds [or covers seeds with dirt] in a field of Kil'ayim violates the prohibition of Kil'ayim, which is "In your field you shall not sow Kil'ayim." Obviously, then, weeding comes under "Sowing."
Answer: That Beraisa holds like R. Akiva, who says that even retaining Kil'ayim is a prohibition.
Question: From where does R. Akiva derive this prohibition of "retaining Kil'ayim"?
Answer: He rearranges the punctuation of the verse to ["You shall not intermingle your animals as] Kil'ayim in your field you shall not [sow Kil'ayim]" - to get "Kil'ayim in your field you shall not", meaning even to retain what is already there.
WHY DOES THE MISHNAH PERMIT WATERING ON SHEMITAH?
Question: Whether watering comes under "Plowing" or "Sowing" it should be prohibited on Shemitah!
Answer #1 (Abaye): Our Mishnah holds like Rebbi, who says that Shemitah (ever since Sancheriv) has been only mid'Rabanan, and the Rabanan were lenient in cases of monetary loss.
How do we know that Rebbi holds Shemitah is mid'Rabanan nowadays? Because he says "At those times when Shemitah of the land applies, so does Shemitah of loans apply," implying that there are times when Shemitah does not apply (mid'Oraisa).