Prospects of sediment deposition at small watershed scale in the black soil region of Northeast China:A mini review

Guanghui ZHANG ,Yang YANG ,Yingna LIU and Zhiqiang WANG

1State Key Laboratory of Earth Surface Processes and Resource Ecology,Faculty of Geographical Science,Beijing Normal University,Beijing 100875(China)

2School of Geography,Faculty of Geographical Science,Beijing Normal University,Beijing 100875(China)

ABSTRACT Sediment deposition is one of the most significant processes in small watersheds characterized by gentle long hillslopes in the black soil(Mollisol)region of Northeast China,as indicated by severe ephemeral gully and gully erosion on hillslopes and very low sediment concentrations in river systems.Few reviews have been conducted to summarize the related research in this region.The objectives of this review were to identify the potential factors influencing sediment deposition,review related studies,and propose future research needs in the black soil region of Northeast China.Sediment deposition is controlled by the deficit between sediment transport capacity of flow and sediment load.Hence,all factors affecting flow transport capacity and sediment load directly affect sediment deposition.For a specific small watershed,the change in slope gradient along the flow path is the key factor affecting sediment deposition.Shelterbelts,ridge tillage systems,terraces,grass strips,road distribution,ponds and reservoirs,and land-use patterns also influence the spatial distribution and rate of deposition.The trace method has been widely used to quantify sediment deposition in this region.The results of cesium-137(137Cs),lead-210(210Pb),and magnetic susceptibility reveal that serious deposition occurs on the back and foot slopes.Distinct deposition occurs in front of contour shelterbelts.Future studies should focus on the methodology,spatial and temporal variations,dominant influencing factors and their mechanisms,and the potential effects on land productivity within specific small watersheds and across the black soil region.This review provides insights into the sediment deposition process in small watersheds characterized by gentle,long hillslopes.

Key Words:Mollisol,sedimentation,soil and water conservation,soil erosion,soil redistribution

INTRODUCTION

Soil erosion is defined as the process of soil detachment,sediment transport,and sediment deposition by an erosive agent.Soil detachment is the sub-process of soil particle dislodgment by raindrops and overland or concentrated flow from soil mass at a particular location on soil surface,providing loose sediments for the subsequent sub-processes of transport and deposition(Zhanget al.,2003).Sediment transport is a sub-process of transporting eroded sediments by flow down a slope or downstream.It is controlled by sediment transport capacity(Tc),defined as the maximum equilibrium sediment load that a flow can transport.TheTcof a flow plays a vital role in determining soil detachment and sediment transport(Zhanget al.,2009).A widely recognized theory states that soil detachment occurs only when the sediment load is less thanTc.Sediment deposition occurs when the sediment load exceedsTc(Nearinget al.,1989).Relationships between soil detachment,sediment transport,and sediment deposition are shown in Fig.1.The sediment load is zero when the flow goes from non-erodible (e.g.,rock) to erodible surface.The soil detachment rate is at its maximum and then decreases,whereas sediment load increases.When sediment load is equal toTc,the detachment rate is zero.Sediment deposition occurs with the decrease inTcdue to coarser soil texture,smaller slope gradient,and the implementation of soil and water conservation facilities such as ridge and furrow systems,dense grass strips,and terraces.

Fig.1 Relationships between soil detachment,sediment transport,and sediment deposition.

The sediment load of a flow is controlled by either soil detachment orTc.The former is called detach-limited and the latter transport-limited(Meyer and Wischmeier,1969).In the case of detach-limited,sediments released by soil detachment are insufficient for transport by overland or concentrated flow.Conversely,in the case of transport-limited,Tcis not large enough to transport all detached sediments.The two limiting conditions are influenced by rainfall,topography,soil,vegetation,land use,and human activities(Zhanget al.,2003,2009).All these factors exhibit considerable spatiotemporal variations at small watershed scale.Consequently,detach-limited and transport-limited alter frequently over time or space.Nevertheless,for a given region,sediment load at the outlet of a small watershed is likely controlled by a specific limiting mechanism,particularly for frequently occurring erosive agents.

The black soil(Mollisol)region of Northeast China has an area of 1.09×106km2.The soil in the region is rich in organic matter and fertile,and the region is the dominant food supply base in China.However,this region suffers from severe soil erosion induced by irrational land use structure and intensive disturbances by farming activities.The consequence of long-term severe erosion is land degradation,characterized by a decrease in the thickness of the top black soil layer and soil fertility,and an increase in soil hardness(Zhanget al.,2022).A gentle(1°-5°)long(500-2 000 m)slope is a distinct feature of the topography.The flow shear stress and energy are low for transporting all eroded sediments in a small watershed because both are functions of slope gradient (Nearinget al.,1989;Zhanget al.,2009).Hence,the sediment yield of a small watershed is controlled by the mechanism of transport-limited(Meyer and Wischmeier,1969).Most of the eroded sediments from the upslope,particularly from the shoulder slope,deposit at the back slope,foot slope,channel,and stream,which is directly confirmed by the well-developed ephemeral gully and gully erosion on the hillslope (Zhanget al.,2007;Donget al.,2019)and very low sediment concentrations in river systems(Gaoet al.,2016).It is critical to understand the mechanisms of sediment deposition at small watershed scale to mitigate soil degradation.The objectives of this review were to identify the potential influencing factors of sediment deposition,review relevant studies,and prioritize future research needs at small watershed scale in the black soil region of Northeast China.

POTENTIAL FACTORS INFLUENCING SEDIMENT DEPOSITION

Sediment deposition is controlled by the deficit betweenTcand sediment load.The excessive sediment not transported by flow will deposit on land surface due to gravity.Sediment deposition rate is a function of the deficit betweenTcand sediment load,flow velocity,flow depth,and sediment size(Nearinget al.,1989;Morganet al.,1998;Zhanget al.,2011).It decreases with flow velocity,depth,and viscosity but increases with fall velocity,which is defined as the speed of sediment with a given size and density falling through a column of water (Morgan and Duzant,2008).The fall velocity of coarse sediment is much larger than that of fine sediment,which explains the selectivity during the process of sediment deposition.

For a specific small watershed,the potential influencing factors on sediment deposition can be systematically summarized as climate,topography,soil,vegetation,land use,and human activities.The intensity and cycle of freezing and thawing can alter soil properties,which further affects soil hydrological processes and increases soil erodibility (Zhang and Liu,2018).These influences disappear after tillage in croplands,but likely exist for a relatively long time under other land use types.Concentrated rainfall from June to August and frequent heavy storms directly influence flow discharge and promote soil detachment by raindrops,flowing water,andTc(Zhanget al.,2009,2022).The spatial change in slope gradient along hillslopes,gullies,and streams,particularly the obvious decrease in slope gradient from shoulder to back and foot slopes,leads to most eroded sediments being deposited in low areas within small watersheds.The long slope length accumulates surface runoffalong the flow path,stimulating detachment rate on the one hand and increasingTcon the other.Soil organic matter content is high,and aggregates are well-developed in Northeast China.Nevertheless,soil infiltration rate is low because of the high clay content and strong dispersion of aggregates(Zhanget al.,2022).Hence,soils are prone to runoffgeneration and detachment.Vegetation greatly affects hydrological and erosion processesviaits canopy cover,plant stems,litter cover,and root systems.TheTcof overland flow decreases exponentially with plant stem and litter coverage(Muet al.,2019;Dinget al.,2020).Seasonal variations in the near-surface characteristics of vegetation and crops also affect sediment deposition.Land use,particularly the area and pattern of a slope cropland,is one of the most significant factors influencing soil erosion processes in the black soil region.Slope cropland can produce a large amount of sediment,especially under heavy storms,which is transported to and deposited in low areas of small watersheds due to the low sediment connectivity(Zhanget al.,2022).

In addition to natural factors,sediment deposition is also significantly affected by human activities.The shelterbelt of this region is the dominant component of the Three North Shelter Forest Program of China,which was originally developed to mitigate wind erosion.The shelterbelt,particularly the contour belt,can effectively reduceTcdue to its great roughness.Obvious sediment deposition can occur in front of a shelterbelt (Fang and Wu,2018).The frequent deep tillage considerably decreases soil bulk density and increases soil erodibility.The influence declines exponentially over time,which is attributed to the process of soil consolidation due to raindrop impacts and gravity.Compaction by large machines increases soil bulk density,reduces soil infiltration rate,and enhances flow discharge andTc.The influence of a ridge system on sediment deposition depends on its orientation and size.The contour ridge system is effective in depositing sediment in furrows under frequent slight or moderate erosive events.Nevertheless,it fails under heavy and extreme storms as the ridge breaks when runoffstored in furrows exceed their capacities(Xuet al.,2018).The widely applied up and down ridge system,which partially contributes to the influence of the shelterbelt,is available for sediment transport.Terraces are among the most significant measures used in the black soil region.It can shorten slope length,enhance rainfall infiltration,decrease flow discharge and the local slope gradient,and thereby promote sediment deposition.Ponds and small reservoirs can trap sediments transported by the flowing runoffwater(Fang,2015).Their benefits to sediment deposition depend on their storage capacity and spatial distribution within small watersheds.

PROGRESSES OFSEDIMENT DEPOSITION IN SMALL WATERSHEDS

Few studies have been carried out to directly investigate the process of sediment deposition at small watershed scale in the black soil region.To provide a more comprehensive review of the related studies,this section is organized based on the methods used.The methodologies applied to investigate sediment deposition at small watershed scale can be categorized into field surveys,trace methods,sediment budgets,and modeling.Few studies have been performed to quantify sediment deposition in the black soil region using field surveys and modeling methods.Sediment budget quantifies the sediment behavior based on the difference in sediment load between two sub-catchments.A nesting system is needed to observe the sediment balance within a small watershed.However,no related studies have been conducted in this region.The spatial variations in sediment load or concentration along hillslope and gully system or between different sediment cascades remain unknown.The trace method is widely used to identify the spatial distribution of sediment deposition in small watersheds.Fanget al.(2012)collected 61 soil samples to measure cesium-137 (137Cs)activity and determine the magnitude and spatial pattern of soil redistribution rates within a 28.5-ha agricultural catchment in Heshan,Heilongjiang Province.The calculated soil redistribution rates ranged from-56.8 to 171.4 t ha-1year-1.Sediment deposition mainly occurred along the shelterbelts and at the catchment outlets at average rates of 78 and 33 t year-1,respectively(Fanget al.,2012).Fanget al.(2013)utilized lead-210(210Pb)to trace sediment deposition in the same small watershed and found that soil redistribution rates ranged from-106.5 to 166.9 t ha-1year-1with a mean of-3.9 t ha-1year-1.The results confirmed that severe sediment deposition occurred in small watersheds.Nevertheless,the estimated deposition rates from the two trace elements differed because a long period was traced by210Pb.By combining137Cs with210Pb,Fang and Wu(2018)quantified the effect of shelterbelt on sediment deposition in the Hebei small watershed in China.They found that shelterbelts significantly contributed to sediment trapping,particularly in the front of shelterbelts.The deposition rate depended on the slope gradient and the angle between shelterbelt and contour.The mean deposited sediment over the 50-and 100-year periods by the two shelterbelts accounted for 18.8%and 7.0%,respectively,of the total eroded sediment in the studied small watershed(Fang and Wu,2018).The advantages of the trace method are that it directly displays the spatial distribution of sediment deposition and provides an estimated deposition rate.However,the computed deposition rate is closely related to the location and number of references as well as the selection of mass conversion model(Zhanget al.,2015).

Similar to137Cs and210Pb,magnetic susceptibility can be utilized to quantify sediment deposition in small watersheds.Liuet al.(2015) collected soil samples along two typical transects of cropland and reforested hillslopes to measure mass-specific low-frequency and frequency-dependent magnetic susceptibility in the Hebei small watershed.The results revealed that the mass-specific low-frequency magnetic susceptibility of reforested hillslope was relatively homogeneous,whereas the measured values of cropland hillslope varied significantly along the tested transect.These results indicate that distinct alternations between erosion and deposition occur on cropland hillslopes(Liuet al.,2015).Sediments can be deposited at different locations in small watersheds,even within a cropland with different slope gradients.Yuet al.(2019)applied magnetic susceptibility to investigate sediment deposition in a cropland with a 40-year cultivation history.A nearby forestland was selected as the reference.The areas of erosion and deposition were determined by the difference in magnetic susceptibility along soil profile between cropland and reference.The results demonstrated that sediment deposition occupied 36% of the area in the tested cropland.The mean deposition rate was 9 mm year-1(Yuet al.,2019).Both studies suggest that magnetic susceptibility is a powerful trace method for quantifying sediment deposition at small watershed scale.Compared to137Cs and210Pb,it is rapid and economical(Liuet al.,2015;Yuet al.,2019).

PROSPECTS OFSEDIMENT DEPOSITION AT SMALL WATERSHED SCALE

Sediment deposition is a significant erosion sub-process at small watershed scale in the black soil region.Nevertheless,only a few studies have been conducted over the past several decades.More studies are needed to quantify sediment deposition in the future by mainly focusing on the methodology,spatial and temporal variations,dominant influencing factors and their mechanisms,and on-site effects.

As mentioned above,multiple approaches can be used to quantify sediment deposition at small watershed scale.However,the dominant methods currently applied in the black soil region are the trace methods of137Cs and210Pb and magnetic susceptibility (Fanget al.,2012,2013;Liuet al.,2015;Yuet al.,2019).Field surveys,mapping,and measurements,particularly after heavy and extreme storms,can provide powerful information of sediment deposition at different locations in small watersheds,such as ridge furrows,shelterbelts,grass strips,terraces,back slopes,foot slopes,roads,channels,streams,ponds,and small reservoirs.The results are useful for identifying sediment sources and sinks and quantifying the connectivity between different landscapes.The high-resolution digital elevation model generated from remote images,for example,captured by unmanned aerial vehicles,combined with geographic information systems,is a convenient and effective approach to investigate sediment deposition at small watershed scale.Regarding the trace method,the multiple-tracer technique has a greater advantage over a single tracer and can provide reliable information of sediment deposition(Polyakovet al.,2004).Magnetic susceptibility,137Cs,210Pb,and rare earth elements can be combined to quantify the sediment deposition in small watersheds.Considerable attention should be paid to the selection of references,particularly where the soil is subjected to both wind and water erosion,such as in the southwest black soil region.More studies are needed to compare and validate the results obtained using different trace methods.The process-based erosion model,i.e.,the modified Morgan-Morgan-Finney model(Morgan and Duzant,2008),is another approach for estimating sediment deposition in small watersheds.More studies are needed to develop a new or modify an existing model for other regions to simulate sediment deposition at small watershed scale in this region.

Sediment deposition is affected by many factors,as discussed above,at small watershed scale.Most of them vary considerably across the black soil region(Zhanget al.,2022),which likely induces variations in sediment deposition over space.Considerable attention should be paid to the spatial variations in the erosive force,watershed characteristics,topography,soil properties,vegetation cover,hydrological processes,land use,tillage operations,soil conservation measures,and erosion type and intensity.Furthermore,many influencing factors of sediment deposition exhibit significant temporal variations.It is important to quantify the temporal variations in sediment deposition at different time scales.Impacted by climatic change,heavy or extreme storms frequently occur in the black soil region,providing an opportunity to investigate sediment deposition at an event scale.Seasonal variations in deposition can be caused by seasonal changes in erosive force(e.g.,freezing and thawing,wind,and water),wet and dry cycles,soil properties due to human disturbances and soil consolidation,vegetation and crop growth,and straw decomposition on land surface or within the topsoil layer.Sediment deposited within a small reservoir contains rich information for evaluation of the long-term interannual variations in deposition rate at small watershed scale(Fang,2015).Both climate and land use/cover changes need to be fully considered in corresponding studies.

The dominant factors influencing sediment deposition and the underling mechanisms are critical topics that require thorough investigation under different conditions.To promote efficiency,it is necessary to systematically combine controlled tests in the laboratory with field surveys and experiments.For controlled experiments,a concave flume with different bed conditions is a good choice for exploring the hydraulic mechanisms of sediment deposition.The results of field survey andin-situexperiments are useful for calibrating and validating the results of the indoor controlled experiments.The dominant factors influencing sediment deposition at small watershed scale likely exhibit considerable spatial variations across the black soil region.More studies are needed to quantify the spatial heterogeneity of these influencing factors at regional scale.Rational representative transects of small watersheds,in the south-north and east-west directions,are likely good choices for rapid identification of the spatial variations in dominant factors impacting sediment deposition in the black soil region.

Sediment deposited in low areas of small watersheds likely has both positive and negative effects,depending on where it is transported from and where it is deposited.In the black soil region,the soil profiles typically have two distinct layers.The top layer is fertile,but the lower layer(parent material)is poor in nutrients.If the sediments are originally eroded from the topsoil layer by interrill and rill erosion,the deposition of these particles on the back and foot slopes can increase the local thickness of the topsoil layer and enhance land productivity.Otherwise,the fertile topsoil layer in low areas will be covered by or mixedviatillage with sediments poor in nutrients if they are originally eroded from the lower soil layer by ephemeral gully and gully erosion,leading to decreases in land productivity and crop yield.The deposition of sediment in channels and streams can alter their cross sections,influence the process of flow routing,and increase the risk of flooding.Sediment deposition in ponds and reservoirs can directly reduce their capacity and service life.These on-site effects vary with the amount of deposited sediment and its erosion source,which certainly changes with the conditions of small watersheds.However,the quantitative effects of sediment deposition remain unknown.It is urgent to carry out more studies to quantify these effects across the black soil region.

CONCLUSIONS

In the black soil region of Northeast China,sediment deposition at small watershed scale is affected by rainfall properties,changes in the slope gradient along flow path,wet and dry cycles,soil properties,vegetation and crop growth,and land use structure.It is also greatly impacted by shelterbelts,ridge tillage systems,soil disturbances by tillage operations,soil consolidation by gravity and raindrop impacts,terraces,grass strips,ponds,and small reservoirs.The trace method is commonly used to investigate sediment deposition.The results of137Cs and210Pb demonstrate that distinct deposition occurs in the front of the shelterbelt and on the back and foot slopes.Magnetic susceptibility is an effective and economical technique for quantifying sediment deposition at small watershed scale.Considerable attention should be paid to the selection of reference and mass conversion models.The methodologies for quantifying sediment deposition should be improved in the future,particularly those that combine field surveys and modeling with digital elevation models and geographic information systems.Temporal variations in sediment deposition must be detected at different time scales,such as event,seasonal,and interannual.The spatial variations in sediment deposition might be identified within small watersheds and across the black soil region,based on representative transects.Further studies are needed to determine the dominant factors influencing sediment deposition,the underlying mechanisms,and their spatial variations.The on-site effects of sediment deposition on land productivity,crop yield,and reservoir capacity also need to be clarified.This review contributes to our understanding of the sediment deposition process at small watershed scale and will be helpful for the related research in the black soil region of China.

ACKNOWLEDGEMENT

This study was supported by the National Key Research and Development Program of China(No.2021YFD 1500803).