The dynamics of gully head recession in a savanna environment.

Field measurements have been carried out to investigate the processes and rates of gully head recession in a tropical wet-and- dry location in Nigeria over three wet seasons. A combination of gully wall erosion processes and gully floor basal sapping is found to enhance headscarp recession and gully widening through a cycle of under-cutting, collapse and removal of collapsed material. The average recession rate was 2.32 m year "SUP -1" , and the fastest recession was 4.7 m year "SUP -1" . These rates amount to 8.6 m "SUP 3" year "SUP -1" and 20.4 m "SUP 3" year "SUP -1" , respectively, yielding an average loss of some 41 m "SUP 3" ha "SUP -1" . The major on-site factors accounting for these results include the amount of runoff arriving at each gully head, the texture and structure of the aeolian material into which gully heads are cut, and the absence of land management of the gullied zone. To stabilize the gullies, the recession cycle must be broken. (A)