Position-in-a-topographic-sequence"an index of whether you are shedding or receiving water But terrain is 3D, therefore position-in- catchment is a better description Area above point in catchment is critical ie the up-slope area or up-slope contributing area At drainage line, USCA is large At crest, USCA is small or o USCA therefore potential discharge( )of water through that point/location• “Position-in-a-topographic-sequence” an index of whether you are shedding or receiving water But terrain is 3D, therefore position-in-catchment is a better description Area above point in catchment is critical ie. the up-slope area or up-slope contributing area At drainage line, USCA is large At crest, USCA is small or  > USCA therefore > potential discharge () of water through that point/location