The method adopted to filter the elevation map and rectify it is based on the paper TITLEd "Software Tools to Extract Structure from Digital Elevation Data for Geographic Information System Analysis" by S.K. Jenson and J.O. Domingue (1988).
The procedure takes an elevation layer as input and initially fills all the depressions with one pass across the layer. Next the flow direction algorithm tries to find a unique direction for each cell. If the watershed program detects areas with pothholes, it delineates this area from the rest of the area and once again the depressions are filled using the neighborhood technique used by the flow direction routine. The final output will be a depressionless elevation layer and a unique flow direction layer.
The flow direction map can be encoded in either ANSWERS (Beasley et.al, 1982) or AGNPS (Young et.al, 1985) form, so that it can be readily used as input to these hydrologic models. The resulting depressionless elevation layer can further be manipulated for deriving slopes and other attributes required by the hydrologic models.
Jenson, S.K., and J.O. Domingue. 1988. Extracting topographic structure from digital elevation model data for geographic information system analysis. Photogram. Engr. and Remote Sens. 54: 1593-1600.
Beasley, D.B. and L.F. Huggins. 1982. ANSWERS (areal nonpoint source watershed environmental response simulation): User's manual. U.S. EPA-905/9-82-001, Chicago, IL, 54 p.
Young, R.A., C.A. Onstad, D.D. Bosch and W.P. Anderson. 1985. Agricultural nonpoint surface pollution models (AGNPS) I and II model documentation. St. Paul: Minn. Pollution control Agency and Washington D.C., USDA-Agricultural Research Service.