It is forbidden on Shabbat to water plants, grass, seeds or trees. Watering on Shabbat falls under the prohibition of "Zore’a" – planting – which includes not only actual planting, but also causing vegetation to grow, such as by adding water. Therefore, if people eat outside on Shabbat in their lawn or garden, they must ensure not wash Netilat Yadayim over the grass or plants, as this is forbidden on Shabbat. This Halacha is explicitly codified by the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in Hilchot Shabbat (8:2; listen to audio recording for precise citation).
In light of this Halacha, the question arises as to the permissibility of outdoor sinks on Shabbat. These sinks have a faucet and a pipe that brings the water to a lawn or garden where it is drained. Seemingly, it would be forbidden to use such a sink on Shabbat to wash one’s hands or to wash dishes, for example, since the water runs from the faucet to the grass, thus watering it and causing it to grow. This issue is especially relevant on the holiday of Succot, when many people use outdoor sinks for the Succa. The prohibition of "Zore’a" applies on Yom Tob just as it does on Shabbat, and the question thus arises as to whether an outdoor sink may be used on the first days of Succot.
Hacham Ovadia Yosef, in his recently published volume of Hazon Ovadia (vol. 4, p. 10; listen to audio recording for precise citation), rules that it is permissible to use such sinks on Shabbat and Yom Tob, due to the combination of several factors. When using an outdoor sink, one does not pour water directly on the lawn; the water is poured into a pipe which then carries it to the lawn. As such, this situation falls under the category of Gerama – an action that indirectly results in a forbidden action – which is generally forbidden on Shabbat on the level of Mi’de’rabbanan (Rabbinic enactment). In this case, however, one has no intention to water the grass; his purpose is to wash his hands or the dish, and has no interest in having the water run onto the lawn to water it. And even though the water will definitely end up in the lawn, a situation known as "Pesik Resheh," the faucet may nevertheless be used since, as mentioned, we deal here to begin with a Rabbinic enactment, as opposed to a Torah prohibition.
This is also the ruling of Rav Tzvi Pesach Frank (Jerusalem, 1873-1960), in his work of responsa Har Sevi (Orah Haim 207), who addresses the situation of a home sink connected to a pipe that brings the water to a garden. He writes that since the water is turned on at a distance from the garden, it is permissible for use on Shabbat. This is also the position taken by Rav Shlomo Zalman Auerbach (Jerusalem, 1910-1995), as recorded in the work Shemirat Shabbat Ke’hilchatah (chapter 12, note 51).
Summary: It is forbidden to water vegetation on Shabbat, and therefore one may not wash one’s hands directly over a lawn on Shabbat. However, one may use a sink connected to a pipe that leads the water to a lawn.
