Initial Abstractions

The initial abstractions are the first losses that must be "satisfied" in a hydrologic simulation before any runoff or infiltration occurs.  They consist of surface or depression storage as well as the water that fixes itself to vegetation such as leaves and grass.  It is impossible to calculate exact volumes for initial abstractions so estimates within a hydrologic model must be made and adjusted in order to account for them.  For a rational method simulation there is no separation of losses between initial abstractions and infiltration, whereas for the curve number method (to be discussed in the next few pages) traditional practice has been to estimate initial abstractions as 20% of the maximum potential losses (or watershed storage).