Tosfos is bothered by the Gemara earlier, which says that if the river came and irrigated the field it is the loss of the Ba'al ha'Bayis, and the Gemara does not stipulate that it has to be a regular event (and that if it is unexpected then it is the loss of the Po'el).

Tosfos answers that if the river irrigated the field there must have been irrigation ditches, which proves that it is a regular event.

Also, Tosfos asks why the Gemara here differentiates between whether the Po'alim are local residents or not, while earlier it does not make this distinction.

Tosfos answers that in the case of this Gemara, whether the river dries up or not is common knowledge to all the local residents, and therefore if the Po'alim are locals they could have expected it. In the earlier Gemara, however, there is no reason for even local Po'alim to know the effect the river will have on the field of the Ba'al ha'Bayis. Therefore it is never Peseida d'Po'alim.

D. Zupnik