soil erosion, conservation, and eco-environment changes in the loess plateau of china

land degradation & developmentLand Degrad. Develop. 24: 499–510 (2013)

Published online 10 September 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.2246

SOIL EROSION, CONSERVATION, AND ECO-ENVIRONMENT CHANGES IN THELOESS PLATEAU OF CHINA

G. ZHAO1,2, X. MU1,2*, Z. WEN1,2, F. WANG1,2, P. GAO1,2

1Institute of Soil and Water Conservation, Northwest A&F University, 26 Xinong Road, Yangling, 712100, Shaanxi Province, China2Institute of Soil and Water Conservation, Chinese Academy of Sciences & Ministry of Water Resources, 26 Xinong Road, Yangling, 712100, Shaanxi

Province, China

Received 9 July 2013; Revised 11 August 2013; Accepted 12 August 2013

ABSTRACT

As one of the best-known areas in the world, the Loess Plateau, has long been suffering from serious soil erosion. The present paper reviewed thehistorical variation of climate, vegetation cover, and environment changes in order to understand the causes of severe soil erosion. Documentaryevidence indicated that climate changes and vegetation cover were the dominant natural factors influencing the soil erosion rates during theHolocene. Intensive human activities consisting of warfare, population growth, deforestation, and soil and water conservation measures wereresponsible for the changes of soil erosion during the anthropogenic period. Spatial and temporal changes of specific sediment yields presentedsignificant decrease within the last several decades, which resulted from decreasing rainfall, large scale soil and water conservation measures,agricultural irrigation, and reservoir construction. Different phase of soil conservation measures demonstrated the development of policies andtechniques on soil erosion control. Effective strategies of soil and water conservation, consisting of terracing, afforestation, natural rehabilitation,and check-dams construction, were carried out on the Loess Plateau during the past six decades. The progress of soil conservation measuresconfirmed that the check-dams systems might be suitable for Loess hilly Plateau, and natural vegetation rehabilitation is the best way for soilerosion control and should be implemented in other regions with emphasis of improving the quality of conservation measures based on naturalrehabilitation. Copyright © 2013 John Wiley & Sons, Ltd.

keywords: Loess Plateau; soil erosion; soil and water conservation; runoff and sediment; eco-environment changes

INTRODUCTION

Soil erosion is an important driving force in the processaffecting the landscape and has become one of the mostserious environmental problem attracted much attentionthroughout the world. Accelerated soil erosion, in additionto causing loss of topsoil as a result of soil quality degradationin an irreversible direction, also lead to catastrophic floods,droughts, and famine threatening food and environmentalsecurity worldwide (Lal, 2003). The transportation ofsediment to water bodies is accompanied by loss of nutrients,which lead to farmland infertile and freshwater eutrophicated(Vanacker et al., 2003). Each year, about ten million hectaresof cropland are lost because of soil erosion, which reducedthe limited cropland for food production and led to millionsof people malnourished. It has been reported that modern soilloss rate is 10 to 40 times faster than the rate of soil renewalimperiling future human food security and environmentalquality (Pimentel, 2006). Severe soil erosion also led tolarge amount of sediment discharged into large rivers and

*Correspondence to: X. Mu, Institute of Soil and Water Conservation,Northwest A&F University, 26 Xinong Road, Yangling, 712100, ShaanxiProvince, China.E-mail: [email protected]

Copyright © 2013 John Wiley & Sons, Ltd.

possibly deposited in the reservoir, therefore harm dams andreservoirs, and eventually make them unusable for theirintended purposes. Thus, better understanding the historicalchanges of environment and current status of soil erosion andconservation are greatly helpful for watershed management,soil erosion control, and ecological restoration at regional scale.The Loess Plateau, facing the Gobi Desert and the

Mongolian Plateau to the north, is located in northwesternChina. The Loess Plateau was once a high, flat plain with themost widely distributed region of loess on Earth. Now, morethan 70% of the area is a gully-hill dominated region owingto massive soil erosion during the later Quaternary and intensehuman activities over the past thousands of years. The loess,being loose, porous and homogeneous, is easy to cultivateand has been an early and long-lasting center of cultivationbecause of fertile farmland (Liu & Ding, 2004; Montgomery,2007; Wang et al., 2010). Agriculture started about 7,000yearsago on the Loess Plateau, and hence, it was considered as thecradle of Chinese civilization (Zhu, 1989).Today, the Loess Plateau plays a critical role on the food

production and energy sources in Chinese national economicdevelopment, but severe soil erosion undermine the environ-mental sustainability of the Loess Plateau and degrade thelivelihood of 104·2 million people who live there (Tang et al.,

500 G. ZHAO ET AL.

1991; He et al., 2004). Food cultivation on the Loess Plateau isexacerbating soil erosion, sedimentation of the Yellow River,and groundwater recession (Miao et al., 2012; Gao et al.,2013; König et al., 2013; Wang & Shao, 2013; Wang et al.,2013; Zhao et al., 2013a).The Loess Plateau belongs to the continental monsoon

region with an average annual rainfall ranged from 250 to600mm, increasing from northwest to southeast. As reportedby NDRC et al. (2010), erosion-prone area reached up to472,000km2 in the Loess Plateau. The area with soil erosionmodulus higher than 8,000Mgkm�2 is approximate91,200km2, which is the most serious soil erosion region inChina and the world. Soil erosion not only restricted thesustainable development in regional economy and society butalso determined the evolution process of the Yellow Riverand the security of the lower reaches (Fan et al., 2012).During the past several decades, climate change and

intensive human activities have largely altered the landscape,geomorphology, and hydrology. Eco-environment changesand soil erosion related issues have extracted much attentionfrom both public, academic cycles, and governments(Mu et al., 2012; Zhao et al., 2012). Wang et al. (2006)reviewed the historical changes of the environment andaddressed that the decline in vegetative cover of the LoessPlateau was related to increased soil erosion rates as well asthe increased frequency of natural disasters such as floods,droughts, and dust storms. He et al. (2004) pointed out thatserious accelerated soil erosion has occurred during thelast 2, 500 years because of man-induced devastation ofvegetation and other anthropogenic disturbance of theenvironment according to the published archeological andpedosedimentary evidence of climatic and flood events ofthe Yellow River. Modern soil erosion rates are a combinationof both intensive natural and human-induced erosions, which

Figure 1. Location of the Loess Plateau. This figure is availab


is much higher than that of the geological past on the LoessPlateau. Although numerous studies have been conducted onthe historical changes of soil erosion and their potentialcauses, the variation in eco-environment, current status of soilerosion, and conservation policies in the Loess Plateau havenot been investigated in detail. The changes in climatevariables, streamflow, and sediment load were not completelyaddressed. Particularly, the effects of climate changes andlarge scale ecological restoration projects on eco-environmentof the Loess Plateau were still not well understood. Therefore,the objectives of this study are to conduct a comprehensiveinvestigation on the historical changes of environment andcurrent status of ecosystem for better understanding theevolution of Loess Plateau and the causes of serious soilerosion and to analyze the progress of soil and water conserva-tion on the Loess Plateau for providing good references andmore effective strategies on eco-rehabilitation and watershedmanagement in the future.

BACKGROUND

Geography Settings

The Loess Plateau is located in the middle and upper reachesof the Yellow River, covering an area of 630,000 km2

(Figure 1). The area covers most of Shaanxi and GansuProvinces, all of Shanxi Province and parts of three others(Henan, Qinghai, and Ningxia Provinces) among the south-ern Yinshan Mountain, northern Qinling Mountain, easternRiyue-Helan Mountain, and western Taihang MountainThe elevation of the plateau is between 1,000 and 1,600mabove sea level, and the surface is covered by continuousloess on hills, basins, and alluvial plains of differing heightwith a range from 100–300m in thickness. The loess has

le in color online at wileyonlinelibrary.com/journal/ldr.

LAND DEGRADATION & DEVELOPMENT, 24: 499–510 (2013)

501SOIL EROSION AND ECO-ENVIRONMENT CHANGES IN THE LOESS PLATEAU

been geologically transported from the northwestern Gobidesert by wind and has accumulated on the Loess Plateausince the beginning of the Quaternary (Cai, 2001).The Loess Plateau lies in the semi-humid and semi-arid

transitional zone and has an annual average temperature of4·3 °C in the northwest and 14·3 °C in the southeast. Limitedrainfall is highly variable both spatially and temporally andmostly concentrated within summer months. Frequently,storms, steep landscape, low vegetation cover, and highlyerodible loessial soil have led to the Loess Plateau to be oneof the most severely eroded regions throughout the world(Wang et al., 2005; Zhang & Liu, 2005). Large amount ofsediment is delivered into the Yellow River and its tributariesbecause of severe soil erosion in the Loess Plateau. Approxi-mately, 90% of the sediment in the Yellow River wascontributed from the Loess Plateau (Tang et al., 1991). It hasbeen addressed that the runoff discharge and sediment loadhave significantly decreased, particularly since the late1980s, because of numerous soil and water conservationpractices implementation (Xu, 2005; Mu et al., 2012).

Climate Changes

Time series in precipitation and temperature since 3,000 yearsBP were derived from tree ring analysis by Wang (1991). Awarmer period can be clearly observed from 2,500 to1,000 years BP according to the annual temperature differencesas shown in Figure 2a. The Loess Plateau experienced a relativecolder period since 500years BP, nearly 1.5 °C lower thanpresent. Precipitation fluctuated considerably during the past3,000 years, particularly within the period before 1,000 yearsBP. Slightly wet (1700–1300 BP) and dry periods (1300–980BP) can be respectively identified from the average annualprecipitation. Although it illustrated stable trend due to itsslighter fluctuation compared with that before 1,000years BP.The changes of precipitation and temperature are largelycorresponded to the East-Asian Summer Monsoon activitiesduring the past thousands of years.Figure 2b demonstrates the long term changes of annual

precipitation and temperature in the Loess Plateau from

Figure 2. Long term variation of precipitation and temperature in the Loess Pljournal/l


1960 to 2010. It can be clearly seen that inter-annual averagerainfall and temperature had large fluctuations within the last50 years. However, decreasing precipitation and increasingtemperature have been evidently observed (Figure 2b).Simple linear fit for annual average rainfall indicates adownward trend of �1·27mmy�1. An average decrease of�0·97mmy�1 has been examined by Xin et al. (2011).In the Loess Plateau, the annual mean temperature varied

between 7·5 and 10·5 °C during the past 50 years. Increasingannual average temperature of 0·03 °Cy�1 has been exam-ined. As shown in Figure 2b, temperature changes before themid-1980s were not very evident, whereas the results fromlinear regression showed significant increasing trends at 95%confidence level after 1985. This is consistent with the resultsreported by Yang et al. (2004) and Xin et al. (2011).Owing to the large stretch of the Loess Plateau being

relative dry-cold northwest and humid-warm southeast, wesimply applied least square fit to the climate variables seriesin the entire study area to quantify the spatial pattern andtemporal changes of climate variables. Figure 3a displaystrends of annual precipitation from 1960 to 2010. Only fourstations in the west and north part indicate increasing trend(0–1·7mmy�1), and all the other stations present downwardtrend ranging from �4·85 to �0·02mm, particularly nearLanzhou and Yinchuan cities as well as the northeast ofthe plateau.Temperature in the whole Loess Plateau illustrated

homogenously upward trends (Figure 3b). The increasingrates lie between 0·02 and 0·70 °C per decade and nearly50% of the stations have relative high increasing rates above0·03 °C y�1. Consistent with our results, Lü et al. (2012)found an average increase of 0·02 °C y�1 in annual tempera-ture, only a little higher than the global average increasingrate of 0·013 °C y�1 temperature. Yang et al. (2004) exam-ined the changes in both precipitation and temperature byusing 50 years of meteorological data from 108 stations inthe Yellow River. It was found that the annual precipitationgenerally decreased (�45·3mm 50 y�1), whereas the tem-perature generally increased (+1·28 °C 50 y�1).

ateau. This figure is available in color online at wileyonlinelibrary.com/dr.


Figure 3. Spatiotemporal trends analysis of precipitation and temperature inthe Loess Plateau. This figure is available in color online at

wileyonlinelibrary.com/journal/ldr.

502 G. ZHAO ET AL.

Population Growth

The loess soil was easy to farm in ancient times, which contrib-uted to the development of early Chinese civilization around theLoess Plateau. Historically, the Loess Plateau was the political,

Figure 4. Changes of population, soil erosion rates, and vegetation coverage in thecom/journa


economic, and cultural center of China, and the capital of uni-fied dynasty from theWestern Zhou (1046–771BC) to the TangDynasty (618–907 AD) seated in the region. As reported by Liu(1985), there were about ten million people living on the LoessPlateau during the late Western Han dynasty (2 AD). The popu-lation decreased to 3·5 million by 280 AD because of long pe-riod of war and social instability. Population in the LoessPlateau accounted for approximately 25% of the country’s totalfrom the 6th to the 15th century. During this period, the popu-lation evidently fluctuated with dynasties change. The changesin the population of the Loess Plateau are mainly resulted fromgovernment policies, the outbreak of wars, and the occurrenceof natural disasters such as floods, droughts, and earthquakes.During the Qing dynasty (1644–1911 AD), the population

in the Loess Plateau increased more quickly because of thetax policies of the government (Wang et al., 2006). In1749, the population was estimated to be 23·5 million, andit reached up to more than 40 million by 1860 (Figure 4).A relative slight decrease in population from 1920s to thelate 1940 was caused by the extreme drought during 1928and 1931, as well as extreme floods in 1933 and long termSino-Japan War since 1937. The population has continuedto grow much more rapidly in the last 60 years, especiallyfrom 1950s to 1970s. Since the founding of China in 1949,the population of the Loess Plateau has tripled, reaching104 million people by 2000. Large number of populationput severe pressure on the environment and resulted inseriously depletion of water and land resources, high levelof soil erosions, massive environment degradation, andwidespread poverty on the Loess Plateau.

Variation in Sediment Load

Sediment loads of the Yellow River were largely contributedby the soil erosion from the Loess Plateau. Thus, the pristinelevel of the sediment load of the Yellow River was relativelylow during the Holocene period. The Loess Plateau experi-enced a cold and dry period in the early Holocene, a warm

Loess Plateau. This figure is available in color online at wileyonlinelibrary.l/ldr.



and wet period in the Middle Holocene Climatic Optimum,and a cool and dry period in the later Holocene (He et al.,2006). Soil erosion rates were very low because of limitedprecipitation during early Holocene and extreme highvegetation cover within the middle Holocene (Figure 2).Sediment load discharged to the sea through the YellowRiver was estimated to be about 0·1–0·2Gt y�1 from thedeltaic sediments and marine sediments during the Holocene(Figure 5), when there was negligible human intervention(Milliman et al., 1987; Saito et al., 2001). Sediment loadwas an order of magnitude lower than that discharged tothe sea during the last 1,000 year (Ren & Zhu, 1994; Saitoet al., 2001; Liu et al., 2004). Corresponding to the sharplyincreasing sediment load is the increasing soil erosion ratesand decreasing vegetative coverage (Figure 2), whichresulted from anthropogenic activities since 3,000 years BP.During the early anthropogenic period, the sediment load

of the Yellow River increased gradually to 0·5Gt y�1, and itabruptly increased to 1·0–1·2Gt y�1 from 2,000 to1,000 years BP according to the land growth rate of theYellow River delta. This is mainly contributed by enhancedcultivation and deforestation on the Loess Plateau (Millimanet al., 1987; Ren & Zhu, 1994). Within the past 1,000 years,sediment load in the Yellow River remained relatively stableand was at a relative high level of 1·2–1·3Gt y�1 (Figure 5).Therefore, human accelerated erosion plays a dominant rolein the historical variation of sediment load from the YellowRiver. Soil erosion contributed by human activities in theloess hilly area accounted for only 2% about 6,000 yearsago, was 8% 4,000 years ago, 18% 2,000 years ago, andreached 30% by the mid-1980s (Lu, 2000).However, annual sediment load measured at Lijin station

(40 km from the river mouth) demonstrated significant lineardecreasing trend (p< 0.01) since 1950s. Particularly, abruptreduction in 1960, 1968, 1986, and 1999 can be clearly seenin Figure 5, which is correspondingly because of the trappingeffect of large reservoirs along the Yellow River. Wang et al.(2007) estimated that 30% of the decrease in sediment loadhas resulted from climate change (decrease of precipitation)

Figure 5. Historical changes of sediment load of the Yellow River. This fig


and 70% from human activities including soil andwater conser-vation (terracing, afforestation, and check-dams construction),agricultural irrigation, and hydraulic projects.

Deforestation and Afforestation

The original vegetation is steppe on the plateau surfaces anddeciduous broad leaved forests or mixed conifer andbroadleaved forests in the valleys of the plateau as well as inthe rocky mountain areas. In the Loess Plateau, vegetationhas played a critical role on soil and water conservation.Research evidence indicated that soil erosion rates had strongresponse to the changes in vegetation coverage (Cerdà, 1998;Nearing et al., 2005), and vegetation is very vulnerable to theeffects of human activities. Numerous researches addressedthat soil erosion on the Loess Plateau was mainly induced byirrational land use and low vegetation coverage due to waterresource constraint (Kimura et al., 2006; Jiao et al., 2007; Fuet al., 2011; Zhao et al., 2013a).In the early-middle Holocene, the vegetation cover was

dominated by the densest temperate forest due to well-developed wetland-swamp environmental condition. After-wards, the steppe and desert-steppe dominated landscapealternated within the late Holocene under relatively drierclimatic variation (Feng et al., 2006). During theWestern Zhoudynasty, more than 50% of the Loess Plateau was covered byforests. The forested area mainly lay in the western ShanxiProvince, northern Shaanxi Province, and central GansuProvince. A steadily decline of the forest coverage of 30%occurred within the following several centuries (420–1911 AD).Forest coverage reduced to about 40% by the time of thenorth–south dynasties (420–589AD) and was 33% during theTang and Song dynasties (618–1275 AD). From the mid of14th century to the beginning of 20th century, the forest cover-age is approximately 15% in the Loess Plateau. By 1949, therewere only 3·7million ha of forestland covering nearly 6% of theLoess Plateau, mostly remained in the mountainous region.The changes in the vegetation are possibly attributed to

the climate changes, outbreak of wars between differentminorities and dynasties, and natural disasters. Guo et al.

ure is available in color online at wileyonlinelibrary.com/journal/ldr.


Figure 6. Topography feature and soil erosion pattern over the LoessPlateau. This figure is available in color online at wileyonlinelibrary.com/

journal/ldr.

504 G. ZHAO ET AL.

(2000) reported that the Loess Plateau experienced a relativewarm and humid period during the mid Holocene (about7·6–5·9 ka BP), with high vegetation coverage. Afterwards,a cool and dry period in the late Holocene made the LoessPlateau to be steppe. The summer monsoon circulationshowed an overall weakening trend due to the reduction ofsolar radiation in the low latitudes of the NorthernHemisphere during the period. From the late 15th centuryto 19th century, dry-cold climate together with frequentoccurred wars and unsustainable agricultural practices wasresponsible for the decline of vegetation cover. During thelast century, human activities may be the dominant factorinfluencing the vegetation on Loess Plateau. Abrupt increaseof population needs more farmland and houses for foodsupply and living condition. Deforestation also occurred inthe 1950s because of the Great Leap Forward campaignlaunched by the government leaders, which led to rapidand serious deforestation problems (Rozelle et al., 1997).Since 1960s, a series of soil and water conservationmeasures were implemented including terracing, forestation,and grassing. Particularly, the Green for Grain projectedlaunched in 1999, returned numerous slope cropland toforest and grassland and had increased grassland andwoodland coverage at a rate of 4·5% with a total restoredarea of 4·83 million ha. It was also confirmed by otherresearch that vegetation cover in the whole Loess Plateauincreased at an approximate rate of 6–8% during2000–2006 (Xin et al., 2008) or 12·5% at local level in thecentral Loess Plateau during 1998–2005 (Cao et al., 2009).

SOIL EROSION AND CONSERVATION

Spatial Pattern of Soil Erosion

Loess is a highly erosion-prone soil that is susceptible towater erosion. Predominant types of soil erosion in theLoess Plateau are sheet and rill erosion on the cultivatedslopes, gully, and gravital erosions in the gully area.Because of different geomorphological and soil componentsfeatures, soil erosion demonstrates distinct types within theLoess Plateau. Shown in Figure 6, the Loess Plateau aredivided into six regions, 140,000 km2 of hilly plateau,107,200 km2 of rocky mountains, 200,000 km2 of highplainplateau, 63,600 km2 of Fen-Wei River valley, 792,000 km2

of deserts, and 58,700 km2 of Hetao alluvial plains.Sediment delivery ratio in the plateau was considered to

be near 1, which means all the sediment yields on the slopewill be transported to the channel because of the silty com-position of the loess soil and steep landforms of the plateau(Cai, 2001). Relative higher soil erosion rates are detected inthe Loess Plateau except for the rocky mountain areas, theflat Fen-Wei River valley, and the desert area with annualprecipitation less than 300mm. The hilly plateau regions


suffered the most severe soil erosion in the Loess Plateau(emphasized with red color in Figure 6), and the erosionrates are mostly higher than 5,000Mg km�2 y�1, evenreached to 25,000Mg km�2 y�1 in the northern part of theLoess Plateau (such as Huangfuchuan and Kuyehe Riverbasin) during the period from 1950s to 1970s. TheFen-Wei River valley, two major tributaries of the YellowRiver, was elevated by large amount of sediment depositionon the alluvial plain (Xu, 2009).Severe soil erosion in the Loess Plateau is largely associated

with natural factors. Frequent heavy storms, easily erodedloess soil, and low vegetation cover are the main causes forhigh erosion rates, characterizing by steep slopes and highlydistributed gullies of landform. On the other hand, long-termhistorical exploitation of land for agriculture production isthe dominant anthropogenic factor accelerating soil erosionin the plateau. Historically, a large number of forest areas inthe flat valley and gentle slopes were reclaimed for cultivationbecause of increasing population. Wars between minorities,alternation of dynasties and refugee mitigation may influencethe eco-environment and aggregate soil erosion in an indirectway (Kiernan, 2013). Zhang et al. (2009) investigatedhistorical sediment yields in a small watershed of theHuangtuwa watershed in Zizhou County, Shaanxi Province,and found that the inter-gully areas were intensively cultivatedfor producing grains to supply the army in the Ming Dynastyand the sediment yield rates at that time were close tothe present.

Temporal Variation of Specific Sediment Yield

Specific sediment yield (SSY) was obtained by using annualsediment load and catchment area controlled by 68



hydrological stations. There are two periods selected forinvestigating the spatial and temporal changes of SSY in theLoess Plateau, a referenced period from 1955 to 1969 withoutmuch soil erosion control measures and a changing period from2000 to 2009 with intensive soil and water conservationpractices implemented.Figure 7 shows the spatial distribution of average annual

SSY on the Loess Plateau within the two periods. Duringthe period from 1955 to 1969, the most severe soil erosionregions with SSY higher than 8,000Mg km�2 y�1 lie in thesection between Toudaoguai and Longmen stations(Figure 1), which is also called ‘the Coarse Sandy HillyCatchments’ (Zhang et al., 2008). This region covers an areaof 7·86 × 104 km2, accounting for only 14·8% of the wholeYellow River basin but producing nearly 80% of the coarsesediment to the Yellow River (Xu et al., 1998).

Figure 7. Spatial and temporal variation of specific sediment yield in theLoess Plateau. This figure is available in color online at

wileyonlinelibrary.com/journal/ldr.


After three decades of soil and water conservation, theaverage annual SSY decreased evidently (Figure 7b). Duringthis period, the region with SSY above 11,000Mgkm�2 y�1

disappeared, and only a very small catchment (with an areaof 923 km2) in the upstream of Qingjian River has the SSYbetween 8,000–11,000Mgkm�2 y�1. Clearly seen inFigure 7b, a majority area of 52·5 × 104 km2 have SSY lessthan 1,000Mgkm�2 y�1, accounting for more than 80% ofthe entire Loess Plateau.Comparison of annual SSY between two periods presents

remarkable spatial and temporal variation in different water-sheds. Annual SSY generally displayed a significant lineardecreasing trend (p< 0.01) when comparing the SSY from2000 to 2009 with that between 1955 and 1969 on the LoessPlateau. The region with a significant decreasing trend wassituated primarily in the middle reaches of the Yellow River(Zhao et al., 2012). Particularly in the section fromToudaoguai and Longmen stations, average annual SSY was975·5Mg km�2 y�1 much lower than that of the earlier period(7,576·8Mgkm�2 y�1) and represented approximately 87%reduction. The sediment in the Yellow River is mostlyproduced by the Toudaoguai-Longmen region. The evidentdecline in SSY is largely attributed to both intensive humanactivities and obvious decline in erosive rainfall. A series oflarge scale soil conservation measures, agricultural irrigation,and hydraulic projects are the dominant factors reducing ortrapping abundant sediment to the Yellow River (Zhaoet al., 2012). Additionally, significant decrease of annualerosive rainfall (p< 0·001) reduces sediment yield in the landsurface, as well as weakens the sediment transport capacity byreducing runoff (Xin et al., 2011).

Soil and Water Conservation since the 1950s

Because of severe soil erosion and its induced eco-environmental problems, considerable attention has been paidto effectively reduce sediment yield, improve agriculturalproduction, and maintain the Yellow River healthy. Since the1950s, a large amount of soil and water conservation practiceshave been planted, which are consisted of terracing, plantingtrees, natural vegetation rehabilitation, and check-damsconstruction (Tang, 2004). Terracing is an efficient way tocontrol soil erosion on slope cropland by leveling groundsurfaces and reducing the slope lengths. Planting trees andnatural vegetation rehabilitation reduce soil loss throughincreasing vegetation cover on the plateau surface. Both ofthese methods focus on controlling soil erosion on the slope,whereas the check-dams system built up in the gullies orchannels could trap most of the incoming sediment from theupstream. Soil and water conservation measures alteredduring the past several decades. It can be divided into fivestages according to their characteristics, technical progressesand dominant policies of soil and water conservation measures


506 G. ZHAO ET AL.

in the Loess Plateau (Wang et al., 2009; Zhao et al., 2013b;Zhou et al., 2013).Stage I (1950s–1963): Initial preparation period. Limited

soil conservation projects were scattered on the LoessPlateau, and the policy making focused on improving theagricultural production. Soil erosion control measures wereimplemented on the slope land, mainly including treeplanting in the fish scale pits and check dams. During thistime, measurement data from plot and catchment scalesindicated that extreme high erosion rates occurred becauseof severe rill erosion and gully erosion.Stage II (1964–1978): Period for soil erosion control in

highly erodible area. Soil conservation measures aimed toefficiently reduce slope and gully erosion and to improvecultivation. Large amount of soil erosion control measuresincluding terracing, afforestation, and conservation tillagepractices were implemented. The rapid increase in thenumber of check dams is benefited from the developmentof sluicing-siltation technique. Abundant check dams inthe gullies and channel completely altered surface hydrolog-ical processes and made the sediment transport capacitydecline. The check dam was thought to be the most effectiveway for soil erosion control in the Loess Plateau, as well asproduced a large flat land area for agricultural use with eightto ten times higher productivity than that on the slopefarmland (Xu et al., 2004). Because of its advantages, checkdams were encouraged in the Loess Plateau by the Chinesegovernment from 1950s to the mid-1970 (Chen et al.,2007). It was confirmed that the trapping effects of checkdams on sediment control contributed a large proportion tothe sediment decline during this period (Tang, 2004).Stage III (1979–1989): Comprehensive management of

small watersheds. Previous studies have convinced that only asmall part of sediment resulted from slope land erosion,whereas 60–70% was contributed by rill and gully erosion. Insome areas, more than 80% of sediments were from gullyerosion (Cerdà, 1999 ; Valentin et al., 2005) Thus, a govern-ment launched policy named as ‘Comprehensive managementof small watersheds’ aimed to control soil erosion in gulliesand inter-gullies regions and improve the eco-environment onthe Loess Plateau. Mechanization was gradually applied in soilconservation implementation. Furthermore, higher agriculturalproduction is achieved through rational land use practices thatmake full use of natural resources (including self-sufficiencyin crop production, forest protection, orchard production, andanimal husbandry) (Zhao et al., 2013b).Stage IV (1990–1999): Period of natural restoration. In this

period, both biological and engineering measures wereencouraged for soil erosion control and developing ecologicalagriculture regime. Natural and artificial rehabilitationswere widely applied for environment restoration. Comprehen-sive strategies were developed to optimize the land usestructure and to control soil erosion in the sandy and Pisha


sandstone regions in the northern part of the Loess Plateau.Check-dams system was applied to trap sediment andmaintain dams a relative stable state. The integrated treatmentframework was proposed to effectively utilize water resourcesand to improve agricultural production.Stage V (1999–Present): Period of eco-rehabilitation. In

1999, the Chinese government initiated the ‘Grain-for-Green’program (GFGP) for ecological restoration and soil erosioncontrol. The GFGP is the largest land retirement projectthroughout the world and uses a public payment scheme thatdirectly engages millions of rural households as core agentsof project implementation (Lü et al., 2012; Zhao et al.,2013a). The project promoted the conversion of cropland toforest and grassland, as previously mentioned, the vegetationcover has increased greatly because of large scale of foresta-tion and rehabilitation since 1999 (Xin et al., 2008).

Changes of Runoff and Sediment Concentration Associatedwith Soil and Water Conservation Measures

During the past six decades, a series of soil and waterconservation practices have been implemented on the LoessPlateau. Statistics indicates that more than 30% of the entireplateau was under control by various soil and water conserva-tion measures (Yao et al., 2011), which are responsible for thesignificant decline in runoff and sediment load (p< 0.01) ofthe Yellow River. To better understand the effects of soilconservation measures, long term measurement data onstreamflow and sediment load at Longmen and Huayuankoustations was evaluated within different soil conservationperiods mentioned previously (Figure 8).During the period from 1950s to 1963, the mean annual

runoff and sediment concentration showed not much changewith relative high values. The mean annual streamflow was31·8 billionm3, much larger than the average value for the fullperiod of 1950 to 2010 at Longmen station (Figure 8a), andthe mean annual suspended sediment concentration of33.6 kgm�3 was about 33% higher than the full-periodaverage (Figure 8c). However, both streamflow and sedimentconcentration showed an abrupt decline in 1960 due to thetrapping effect of Sanmenxia Reservoir at Huagyuankou station(Figure 8b and d). Because of the trap efficiency of SanmenxiaDam, particularly in the early years (1961–1963), thesuspended sediment concentration accounted for only 41·4%of the average from 1950 to 1999. During this period, soilerosion on the hillslope was considered to contribute themajority of the sediment in the Yellow River, and the principalsoil erosion control measures of terracing and afforestation onslope lands did not show evident effects on streamflow andsediment load (Wang et al., 2007).From 1963 to the late 1970s, the average annual streamflow

presented significant decreasing trends (p< 0·01) at Longmenand Huayuankou stations, whereas the mean annual suspendedsediment concentration, although very variable, maintained a


Figure 8. Changes in average annual discharge and sediment concentrations of the Yellow River at Longmen and Huayuankou stations. This figure is availablein color online at wileyonlinelibrary.com/journal/ldr.


relative high values at two stations. The significant reduction ofstreamflow in 1969 may largely be attributed to the trappingeffect of Liujiaxia reservoir (Figure 5). The markedly increasedsediment concentration at Huayuankou station resulted fromthe operation of Sanmenxia Dam, which controlled floodswhile simultaneously releasing a large amount of sedimenttrapped in the reservoir (Ma et al., 2012). During this stage,check dam and terracing became the most popular soilconservation measures in the Plateau; however, a number ofcheck dams were destroyed by the frequently occurred floods(Zhang et al., 1997). Thus, the average annual sedimentconcentration did not show evident decreasing trend duringthis period.From 1979 to 1989, the trend of average annual

streamflow was variable but generally decreased at bothLongmen and Huayuankou stations. An abrupt declinearound 1986 can be clearly seen because of the operationof Longyangxia reservoir. Significant reduction in sedimentconcentration was detected during this period because ofimplementation of substantial soil and water conservationpractices (Pan et al., 2006). Numerous studies confirmedthat an abrupt decline in sediment load in most middlereaches of Yellow River was examined (Pan et al., 2006;Wang et al., 2007; Yao et al., 2011; Zhao et al., 2012). Thisshould be responsible for the extreme low values of meanannual sediment concentration.The average annual streamflow generally decreased

during the period of 1990 to 1999, whereas the suspendedsediment concentration increased resulting from a rise inthe floods events (Xu, 2005). The increasing agricultural


irrigation, soil and water conservation projects, and the jointoperation of large reservoirs are responsible for the declineof streamflow at these stations (Peng et al., 2010; Zhaoet al., 2012).Since 2000, both streamflow and sediment concentration

displayed decreasing trends at Longmen and Huayuankoustations (Figure 8). The mean annual streamflow was only17·3 and 24·0 km3 at Longmen and Huayuankou stations(Figure 8a and b), accounting for about 61·2% and 58·8%of the average values from 1950 to 1999, respectively. Themean annual suspended sediment at Huayuankou stationdecreased sharply since 1999 because of sediment impound-ment by the operation of the Xiaolangdi reservoir.In summary, the obvious decline in sediment load in the

Yellow River was predominantly resulted from decreasedprecipitation, a series of soil and water conservation mea-sures, water extraction, and reservoirs construction. It wasestimated that lower precipitation gave a decrease in sedimentload of 0·12Gt y�1 during 1956–1985 and 0·3Gt y�1within1985–1999 by reducing incoming water from the upstreamof the tributaries and main stem (Wang et al., 2007). Beforethe late 1970s, soil and water conservation was not effectivefor soil erosion control due to its limited scale. It wasestimated that soil and water conservation reduced sedi-ment load of nearly 0·18Gt y�1 before 1985 and was0·25–0·30Gt y�1 afterwards (Zhao, 1996; Walling &Fang, 2003), however, it is much higher in the pastdecade because of considerable measures implementation.The Sanmenxia reservoir trapped approximately 0·13Gt y�1

of sediment load since its operation in 1960, and the


508 G. ZHAO ET AL.

Xiaolangdi reservoir has trapped about 2·83Gt from 1997 to2010. To resolve severe silation of the reservoirs andelevation of river bed, the Water-Sediment RegulationScheme was launched by using the controlled release offloodwaters to deliver the sediment retained within theXiaolangdi Reservoir and scour the lower reaches since2002. So far, nearly 0.4 Gt of sediment was scoured intothe Bohai Sea.

RECOMMENDATIONS FOR A SUSTAINABLEMANAGEMENT

Ecological Restoration through Natural Rehabilitation

Evidence indicated that re-vegetation has become one of themajor measures for soil erosion control on the Loess Plateauat present (Nearing et al., 2005; Chen et al., 2007; Wen et al.,2010). Afforestation, grassing, and grazing prohibition becamevery popular measures for re-vegetation in the Loess Plateau.Unfortunately, a majority of afforestation projects were notsuccessful. The planted trees and grasses usually grew wellfor the first few years and then became ruining because theartificial plants excluded soil moisture and the dry layerappeared in the loess profile. Decreased soil moisture in theafforestation plots, combined with reduced sunlight under thetree canopies, has decreased vegetation cover by 30.5% inafforestation plots in arid and semiarid regions of northernShaanxi Province (Cao et al., 2009).Several programs from GFGP (such as Wuqi County in the

northern Shaanxi Province) demonstrated successful resultsand perspectives for natural re-vegetation, which has beenconsidered as the most suitable way for soil erosion controland ecological restoration (König et al., 2013). Although aseries of projects have been implemented for returning slopearable land to forest or grass land, many ecological restorationprojects based on artificial tree planting failed. Afforestationmay initially increase the vegetation cover, but it has negativeeffect on biodiversity, and its ability to restore eco-environmentdepends on the type of re-vegetation carried out and its localenvironment (Lamb et al., 2005; Wang et al., 2013). The wateravailability and other ecological conditions in the local regionshould be previously considered when performing vegetationrestoration. Therefore, it might be more efficient to implementsome successful policies, such as prohibition of grazing andlogging, converting unsuitable cultivated slope land to forestand maintain the natural species for ecological restoration inthe Loess Plateau.

Sustainable Soil and Water Conservation Practices

On the Loess Plateau, soil and water conservation measuresare mainly consisted of biological measures (e.g., afforestationand grassing) and engineering structures. The biologicalmeasures have delayed effects on soil erosion control because


the trees and grass need a long period to adapt the regionalclimate, soil and topographical condition (Zhao et al.,2013b). The engineering measures affect the streamflow andsediment load by reducing flood peaks, storing the water,and trapping the sediment within check dams and reservoirs.Consequently, both the magnitude and variability of the runoffand sediment discharge will decrease.The engineering measures such as check dams can trap

sediment and rise gully channel beds to strengthen the gullyslopes, as well as generate fertile cropland. However, a lifetime of about 10 years for a small check dams to trapsediment efficiently was reported by (Xu et al., 2004). Thefailure of check dams may result in severe disasters duringcatastrophic flood events. Recent studies indicated that aseries of check dams in a gully can form a sustainable damssystem, which is much safer and more efficient to controlsediment load discharged into rivers (Cao et al., 2007).Thus, we strongly recommend that stable check-dam systemshould be widely constructed in the Hilly Plateau region,although it may not be suitable in other regions because ofspecial soil properties, landscape, topography, and geology.

CONCLUSION

The Loess Plateau experienced unstable climate from theHolocene until now. Soil erosion rates varied greatlybecause of evident changes in both climate and vegetationcover. In the pristine level, soil erosion and sediment yieldfrom the Loess Plateau were relatively low and were mainlydominated by the natural factors including climate andvegetation coverage. An abrupt increase in soil erosion ratesoccurred because of anthropogenic activities. Populationgrowth and decline of vegetation cover should be responsiblefor the increasing sediment yield since 3,000 years ago.In modern period, significant reduction in both streamflow

and sediment load were detected by the observations on theLoess Plateau, particularly since the late 1980s. The decreasingannual rainfall, large scale soil and water conservation mea-sures, water abstraction and reservoirs construction are thedominant driving forces, which led to the great decline in waterand sediment load in the Yellow River.Different phases of soil conservation measures represented

the development of policies and techniques on soil erosioncontrol. Several effective strategies of soil and water conserva-tion were carried out on the Loess Plateau, consisting ofterracing, afforestation, natural rehabilitation, and check-damsconstruction. The progress of soil conservation measuresconfirmed that the natural rehabilitation might be the bestway for soil erosion control and ecological restoration. Spatialpattern of soil erosion indicated that check-dams systemsmight be suitable for loess hilly plateau, and natural vegetationrehabilitation should be implemented in other regions with



emphasis of improving the quality of soil and water conserva-tion measures.With rapid increasing population and excessive exploitation

of resources, economic benefits are primarily considered toresolve the poverty of local farmers in the Loess Plateau.Therefore, a relative practical, scientific framework shouldbe recommended to solve the contradiction betweenenvironmental management and economic development onthe Loess Plateau.

ACKNOWLEDGEMENT

This work was supported by the Key Research Program of theChinese Academy of Sciences (No. KZZD-EW-04), the KeyProject of the National Science and Technology Ministry(No. 2012BAB02B05), the National Natural SciencesFoundation of China (Nos.: 41201266, 41271295), and WestLight Foundation of the Chinese Academy of Science. Specialappreciation was given to Prof. Zhang Xinbao in the Instituteof Mountain Hazards and Environment, Chinese Academy ofSciences, and Ministry of Water Resources for his very usefulsuggestions. The authors would also like to thank thereviewers for the very valuable comments, which greatlyimproved the quality of the paper.

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