While the final absorption of water that produces solid stool occurs in the large intestine, a lot of fluid is normally absorbed in the small intestine. The link below says 80%, and it occurs by osmosis.http://www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_water.html
It is possible there is some adaption after surgery to increase this amount. But, I did not find any study of adaptive absorption.
But I would imagine that to compensate for a loss of 20% of water absorption, you might start with 125% of usual fluid intake. (i.e., .8*1.25=1).
However osmosis is sensitive to the context of dissolved minerals on each side of the osmotic barrier. Water will move from the side with lower concentration of minerals to the side with higher concentration in an so as to balance the concentrations on both sides. So any dietary changes post surgery - especially in salts - can trap water in the intestine and lower absorption.
While there can be too much water so as to flush too many minerals out of the system, I think it would not hurt to take in 200% instead of just 125% to insure sufficient hydration.
Does this make sense?