The gully erosion rates in the black soil region of northeastern China: Induced by different processes and indicated by different indexes

Over 295,000 gullies have developed in the black soil region of Northeast China, and approximately 80% were observed in cropland, which caused intensive land degradation that threatens crop production in this region. There are two important issues for predicting gully erosion which have often been ignored in previous studies: 1) the same gully erosion rates may induce by completely different development processes and dynamics and 2) how to indicate the gully erosion rate. In this study, the topographic changes of five active gullies (two valley floor gullies and three hillslope gullies) that formed between 1997 and 2002 in Heshan farm were continuously monitored for three to six years. Although all of the gullies showed significant growth in volume and the average erosion rates ranged from 47.71 to 213.07 m(3).a(-1), the development processes were quite different for the valley floor and hillslope gullies. The valley floor gullies showed intensive headcut retreat and were prone to show deposition in the years with moderate rainfall. The widening and deepening in the middle parts were the key locations of soil loss in the hillslope gullies, and the volumes increased steadily during the periods observed. Four different indexes were adopted to indicate the gully erosion rates in terms of volume: the volume change per year (GER(V)), volume change per length/area changes (Delta V/Delta L, Delta V/Delta A) and the increasing ratio (IRv). All the indexes of the valley floor gullies showed significant parabolic relationships with the maximum rainfall (R-max), except for Delta V/Delta L (0.93 < R-2 < 0.99, P < 0.01, n = 10), while for the hillslope gullies, IRv, GER(v) and Delta V/Delta A showed exponential growth relationships with a compound index Rg being equal to the product of the logarithmic values of the snowfall (Sf), erosivity of the runoff generation rainfall (EI30runoff) and the R-max of each year (R-2 = 0.58, 0.50 and 0.60 respectively, P < 0.01, n = 14). The IR, showed a better ability than the other indexes to indicate gully erosion rates, which reflected the gully erosion rates directly, and reduced the influence of the different initial sizes of the gullies.