Currently flow accumulation in Spatial Analyst can only accomodate positive values - all negative values are treated as zero. I'd recommend creating an option (or a new tool) for flow accumulation for negative values (per
http://forums.esri.com/Thread.asp?c=93&f=995&t=243378). Even better would be an option for accomodating flows of things like water that can start out positive but when they go negative they can't go less than zero. To my knowledge this hasn't been worked out by anyone in the ArcGIS community yet (see
http://gis.stackexchange.com/questions/49462/how-can-i-get-weighted-flow-accumulation-to-deal-with-n... but it would be a tremendous resource for people who model flows of water but deal with weightings having to do with precipitation and evapotranspiration.
