Geomorphic threshold conditions for ephemeral gully incision

Ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies. Furthermore, little is known about the factors controlling ephemeral gully erosion. In this paper the available information on the initiation and location of (ephemeral) gullies is summarised, focusing on the relationship between the upslope drainage area (A) and the critical slope gradient (S-cr) for ephemeral gully initiation. By plotting the non-linear relationship between critical slope gradient (measured immediately upstream of the incision head) and drainage area (at the incision head) for gullied sites it was possible to draw a straight line on log-log paper through the lower-most points for each of the available datasets, representing a critical slope-area relationship for incision. Consequently, below this line no incision occurs. This line or critical relation can also be written as a power function between critical slope and area. Although many factors vary between the different datasets, the exponent of the drainage area relationship (-0.4) showed very little variation. The observed critical slope-area relationship can be related to a simple model of channel initiation by overland flow. Furthermore, this relationship can be used to identify potentially unstable sites where control measures should be undertaken.