Geomorphic Threshold Determination for Ephemeral Gully and Gully Erosion Areas in the Loess Hilly Gully Region

By means of GPS measurement data in research areas and combined with GIS spatial analysis and statistical regression method,this paper validated the foreign critical relations for occurring ephemeral and gully erosion in the hilly-gully region of Loess Plateau and established the critical relations for determining ephemeral gully erosion and gully erosion areas,respectively.The research results indicated that in the Loess Plateau area,with an increase of slope gradient,the threshold values(a) for occurring ephemeral gully and gully erosion increased;and meanwhile,the high rainfall intensity caused the exponential value(b) of the critical upslope area decreased,which reduced the effect of the critical upslope area.Under the same condition,occurring ephemeral gully erosion needed less critical upslope area than that of occurring gully erosion;and with the same critical upslope area,the occurrence of gully erosion needed higher slope gradient than that of occurring ephemeral gully erosion.By using the foreign critical relations of ephemeral gully and gully erosion to draw occurring areas of ephemeral gully and gully area in loess hilly-gully region,the predicted ephemeral and gully erosion area was over-estimated.By using our establishing critical relations of ephemeral gully and gully to delineate occurring areas of ephemeral gully and gully area in the loess hilly-gully region,the predicted ephemeral and gully erosion area very matched to the actual distribution of ephemeral and gully erosion in the field.That is,ephemeral gully erosion mainly happened on hillslopes with 15°~35°,accounted for 60% of the hillslope area;and the gully erosion mainly occurred on gully slopes with more than 35°,occupied 93% of the whole gully slope area.