Gully Erosion Evolvement And Effect In Typical Black Soil Region

Research on farmland gully evolvement and its effect is important for gully prevention in Northeast China. Due to limited gully erosion research as well as data acquisition methods, farmland gully evolement research in this region is weak in this region. This study is aimed to investegate the eveolment processes as well as evolvement effects of farmland rill, ephemeral gully and permental gully. Methods such as stereo photogrammetry, gully model AnnAGNPS as well as remote sensing interpretation were used to analysis gully erosion evolvement process. Thanks to stereo photogrammetry software development, we developed a new method to extract rill 3D information from runoff plot. Based on the relationship between crop yield and topsoil layer depth, the ephemeral gully erosion effect on farmer’s income was evaluated. Besides, the factors affecting farmland permental gully development were investigated. The main results were as follows:(1) Based on newly developed method using stereo photogrammetry technology, the rill evolvement process on runoff plot was investigated. This method could obtain the rill development 3D information caused by rainfall runoff erosion and snowmelt erosion. Compared with previous methods, its advantages were low-cost, fast, high-precision, undirectly-contact, easy-go. And it was also suitable to different kinds of field conditions. It took about 6 hours to carry out rill’s DSM of 0.5 mm resolution within 100 m2. The synthetical error of this method was around 50%, which was also almost satisfied. We obtained rill erosion evolvement process casued by rainfall runoff erosion in 2015. The micro relief of hillslope became smoothly, and the hillslope height variety decreased from 57% to 53%. While the erosion depth and the deposion depth of hillslope increased 7 mm and 11 mm, respectively. The runoff lines density decreased, and main runoff lines appeared, which captured surrounding small runoff lines. The main runoff lines created rills. The rills appeared at slope foot firstly, and then the rill headcuted towards slope back. Besides, amoung the early stage the main rill development process was rill headcut and incision. Amoung the late stage it shifted to rill width increasing.(2) Ephemeral gully erosion process and economics effect were evaluated by AnnAGNPS. Sensitive analysis showed that headcut detachment coefficient(HDC) was the main error source. The average topsoil depth reduction in the adjacent region along the ephemeral gully was 1.0 cm y-1, based on the assumed adjacent borrow region width(5 m) and length. The calculated cumulative economic loss associated with ephemeral gulley erosion and corn yield reduction within 15 years was $166 hm-2 with an average annual economic loss of $11 hm-2 y-1. The total economic loss(economic loss from lost production plus construction cost) due to grass waterway implementation was $1868 hm-2 y-1, which is 170 times of the economic loss associated with ephemeral gully erosion($11 hm-2 y-1) alone. The significant income differences with and without grass waterways shows that farmers have no obvious financial incentive to construct and manage grass waterways in the short term without additional financial support.(3) Gully distributions in a village district of 22.4 km2, in the center Mollisols area of Northeast China were compared by Aerial photography(1968), Quickbird image(2009) and field survey, and factors affecting gully development including land-use and topography were analyzed. Within gully initiation stage gully length increased rapidly and by 1968 the fundamental gully distribution was formed. After that the main gully development processes were gully merge and width expansion, and most of gullies in 1968 developed into gullies present in 2009. The mean catchment slope gradient for simple and complex regions types were similar while the mean catchment area for complex gully is larger than that for simple gully, which showed that the catchment area is more important than slope gradient for gully development. Furthermore, catchment area and mean slope gradient had a coupled effect on gully erosion and both of them needed to be considered for gully development. In this region the threshold catchment area between simple and complex gully was 15 to 25 hm-2. Use of this threshold value permits prediction which development process- linear or lateral expansion process, might occur in a specific catchment area with similar environmental settings. What’s more, gully control measurement was urgent because half of present gullies in number only occupied less than 7% of total gully area. It implied that potential gully erosion could be doubled if appropriate gully control implements were not applied.