Gully Erosion History,development Rates And Key Processes In A Typical Agricultural Catchment In The Northeast China

In China the Mollisol region is the largest granary that dominates food security and provide a buffer for the environment.However with agricultural intensification,the quality of Mollisols is increasingly threatened,accelerated water and wind erosion especially gully erosion are increasingly concerned in recent years.Due to the coupling controlling factors of gully erosion as well as limited data acquisition methods,the development process and mechanism of gully erosion have always been the weak link in soil erosion research.In this study,an integrated approach which involved aerial photographs interpreting,field measurement,interview techniques,detailed soil profiles analysis with radioisotope dating,and in-situ experiment with soil moisture before freezing were utilized.The objectives of this study were to 1)identify the contemporary gully development history and rates on head retreat and soil loss using UAVs and d-GPS as well as radioisotope techniques for 50 years and 100 years,2)quantify the gully distribution,development processes,rates and controlling factors by comparing satellite image combined with UAVs monitoring,and 3)to characterize the temporal and spatial variation of soil water content during the freeze-thaw processes and the critical point of cracks formation and sidewall collapse.The main results were as follows:1.The gully erosion history and sedimentation rate of agricultural catchment in Black Soil Region were investigated.The typical soil profiles along gully bank is layered from top to bottom:the mollic soil,loess like parent materials,sandy loess and sand layers as well as sand and shale.The geochemical features and magnetic susceptibility vary in different layers significantly,the characteristic of cross-section of gully bank is closely related to the texture of exposed layers.From the beginning of human settlement around 1900,soil erosion was no significant.The first phase was characterized by sheet erosion results from the main period of cropland expansion about100 years ago.We concluded that the main period of gully formation did not started before the 1960ies and continued until the present.The average sediment accumulation rate of gully erosion estimated by the ¹³⁷Cs dating was 1.8 to 2.2 m yr^-1for the period?1963-2018?.We confirmed that severe human-induced gully development in this region was only 50 years,whereas the gully erosion rates were as comparable with the higher erosion rates in permanent gullies around the world.2.The reason of still active gullies detected in restoration of forestland after agricultural terraces were analyzed.Gullies are normally initiated at the slope positions where overland flow can remove surface soil along drainage line and create a waterfall that plunges over the gully head and scours the subsoil.As these processes continue,the gully develops as the gully head retreats backwards the direction of overland flow,gully side walls are slumped and widened and gully floor is deepened.Once the watercourses are altered,gully heads will be directed along altered overland flows,resulting in twisted gully morphologies,rather than in V-or U-shapes.After the reforestation in 1990s,overland flow was concentrated and directed to the pre-existing watercourse due to lack of maintenance of the terraces,resulting in quicker gully head retreating to the terraced lands.Because the terraces were slightly inclined,the gullies were further twisted towards the terrace direction from the slope direction,or widened due to massive failure along gully banks.Relevant recommends to identify gully erosion sensitive area,prevent overland flow concentration before land abandonment and gully development after land abandonment were given as well.3.The long-term gully development features and rates in Black Soil Region were quantified.The study showed that the gully erosion started at the downslope areas,then the gully went further upslope over several decades to the upslope areas,with average retreat rates of gully head were 2.0 to 3.4 m yr^-1 during past 50 years?1968-2018?,gully density increased from 1.0 km km^-2to 2.0 km km^-2during the last 50 years.The soil loss rates of gullying were 25.7 to 44.7 t ha^-1 yr^-1 in the contributing catchment area?initial to present state?,and in 2018 the gully erosion rate was 381 t ha^-1 yr^-1 in the area contributing to the gully,equivalent to over 2.7 cm soil loss over the catchment area.Current monitoring indicated that gully area extended 552 m³ and erosion processes were dominant at active gully head-cut retreat.4.The critical point and controlling factors of crack formation and gully bank collapse were revealed in this study.The variation of liquid soil water content?LSWC?showed the trend of decreasing in freezing period and increasing in melting period,and LSWC at different distances from the sidewall plays an important role in the formation and collapse of cracks.The rapid springtime air temperature rise plays a relatively strong role in the ice thawing process in shallow soil layer,which may hold more snowmelt could not migrate downward due to the freezing layers and reduce soil stability on the surface layers.Large numbers of tension crack mainly initiated at middle April.The first critical stage of bank failure occurred in April,the snowmelt result to collapse soil mass account for 17.9%in total collapse soil mass until the critical point at 25^th April.The second critical stage was in July and August which related to the rainy season,a total of 2.9 t soil mass was happened in this period,which account for 77.4%in the whole year.Gully formation began was subject to a large-scale expansion of agriculture in the first half of the 20th century.The studied gullies were detected as small scale in 1968aerial photos,which estimated began from 1960 to 1966,corresponding with the intensive agriculture.The erosion and accumulation process were caused by heavy rainfall and started at the downslope areas.Then the gully erosion went further upslope in a self-dynamic process over several decades to the upslope areas.In our study,the quantification of sediment highlights the connectivity between gully erosion history and different phases of agricultural land use.Understanding of the initiation and development of the severe gully erosion in relation to these control factors is very critical for optimizing soil and water conservation practices,and further guide effective land management actions.