MISHNAH. IF [THE AREA OF] A COURTYARD WAS LESS THAN FOUR CUBITS NO WATER MAY BE POURED OUT INTO IT ON THE SABBATH UNLESS IT WAS PROVIDED WITH A TROUGH HOLDING TWO SE'AH FROM ITS EDGE DOWNWARDS, IRRESPECTIVE OF WHETHER IT WAS (flowing) WITHOUT (the Courtyard) OR WITHIN, EXCEPT THAT IF IT WAS WITHOUT IT IS NECESSARY TO COVER IT AND IF IT WAS WITHIN IT IS NOT NECESSARY TO COVER IT.
That is, if the trough flows from or to a public domain from the private one, it must be covered with boards so that it is clearly a “free” domain.
Why does there have to be a two se’ah space in the trough? According to the rabbis that is supposed to be the amount of water a person uses in a day.
Of course, the usual thing in those days, particularly for those in the upper stories, is to pour waste water into the roof gutters and let it flow into the drain. In a small courtyard (less than 4 cubits) that would be restricted as well. Or maybe not:
One taught: This applies only to the hot season, but during the rainy season a person may pour his water again and again without any limit.
Waste water added to a gutter in the rainy season would not add noticeably. And, equally important, it would not be noticed and cause others to think that Sabbath restrictions were not in place. In the dry season, it would stand out!