CRITICAL SLOPE GRADIENT FOR COMPULSORY ABANDONMENT OF FARMLAND ON THEHILLY LOESS PLATEAU

Scientific evidence for the upper limit of farmland is proposed. Shall ow gully erosion is one of erosion types distributed extensively on sl oping farmland in the hilly regions of the Loess Plateau. Field observ ation and aerial photos interpretation, as well as laboratory experime nts show that the shallow gully erosion occurring on the steeper farml and in the hilly regions of the Loess Plateau is an important factor l eading to intensive erosion on slope, because of its extensive distrib ution and intensive runoff collection. The data on the formation, deve lopment and distribution of shallow gullies on sloping farmland indica te that critical slope gradient for shallow gully initiation ranges fr om 15 to 20 degrees with an average of 18.2 degrees. Therefore, it is suggested that critical slope gradient for compulsory abandonment of f armland on the hilly Loess Plateau should be kept within the critical slope gradient for shallow gully initiation to prevent shallow gully f ormation in order to control soil loss more effectively. But as the fi rst step, the cultivation on the slopes with slope larger than 25 degr ees where the maximum erosion occurs should be strictly forbidden.