Sediment Delivery to the Three Gorges: 1. Catchment Controls

Geomorphology 41Ž. 2001 143–156 www.elsevier.comrlocatergeomorph

Sediment delivery to the three gorges: 1. Catchment controls

D.L. Higgitt a,), X.X. Lu b a Department of Geography, UniÕersity of Durham, Science Laboratories, South Road, Durham, DH1 3LE, UK b Department of Geography, National UniÕersity of Singapore, 10 Kent Ridge Crescent, Singapore, 119260, Singapore Received 10 February 2000; received in revised form 27 July 2000; accepted 29 May 2001

Abstract

The paper examines sediment yield and its response to catchment disturbance and environmental variables in the Upper Yangtze basin, where the attention of environmentalists has been drawn to the Three Gorges ProjectŽ. TGP . Information about the source and conveyance of sediment from the catchment area to the Three Gorges Reservoir has implications for management strategies. Methodologies for establishing the relationships between land cover, climatic and topographic variables with sediment yield are introduced. The analysis uses a sediment load data set, containing 250 stations with up to 30 years of measurement, a 1=1 km resolution land cover database and variables extracted from various geodatabases. The mean sediment load delivered from the Yangtze upstream of Chongqing is 318 Mt ay1, but the contribution from the Jialing tributary is higher in terms of specific sediment yield at 928 t kmy2 ay1. Long-term sediment yield at Yichang has not exhibited an upward trend despite the evidence for increased soil erosion within the basin. Examination of sediment response to catchment disturbance and spatial variability in relation to controlling variables has been undertaken in an attempt to predict future sedimentation impacts. Time series analysis illustrates that significant increases in sediment yield have occurred over about 8% of the catchment area while about 3% have experienced decreasing sediment yields. The latter are associated with major reservoir schemes on the tributaries of the Yangtze. When the spatial pattern of sediment yields within the basin is analysed, AnaturalB climatic and topographic factors explain most of the variability in the relatively sparsely populated western part of the Upper Yangtze basin, but do not afford very good prediction in the more populated eastern part. Incorporation of land cover information does not provide additional explanation of spatial variability. Examination of the response of sediment delivery to catchment disturbance and environmental variables provides an illustration which may have some lessons for the management of the sedimentation problem in the Three Gorges Reservoir and a basis for modelling future changes in sediment delivery. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Sediment yield; Sediment delivery; Geodatabases; Yangtze

1. Introduction is one of the key environmental issues that has focused attention on the dynamics of soil erosion and The potential impact of sedimentation on the op- fluvial sediment transport in the catchment of the eration and life span of the Three Gorges Reservoir Upper YangtzeŽ Gu and Douglas, 1989; Edmonds, 1992; Qian et al., 1993; Luk and Whitney, 1993. . ) Corresponding author. Fax: q44-191-374-2456. There are two major issues of concern—whether E-mail address: [email protected]Ž. D.L. Higgitt . proposed reservoir regulation procedures are effi-

0169-555Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0169-555XŽ. 01 00112-X 144 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 cient enough to control the conveyance of most of has proposed that the Langbein–Schumm model is the sediment-laden waters, and whether the long-term broadly applicable to explain national variations observed sediment delivery to the Three Gorges is within China. As the availability of gauging station representative of future trends. Investigation of the data has increased a more complex relationship with temporal and spatial variability of sediment transport climate and vegetation has emergedŽ Douglas, 1967; within the Upper Yangtze catchment leads to the Wilson, 1973; Jansen and Painter, 1974; Walling and following paradox. There is widespread evidence Webb, 1983; Jansson, 1988. , mainly because of the that the extent and magnitude of soil erosion across impact of human activity on natural vegetation cover. southern China has increased dramatically during the At a global scale, the importance of topography and last 30–40 yearsŽ Smil, 1993; Wen, 1993; Edmonds, the significance of relatively small mountainous 1994. . There is no evidence from the sediment load catchments as major contributors to global continen- measurements at the Yichang gauging stationŽ down- tal sediment export has been notedŽ Milliman and stream of the TGP dam site. of a trend in sediment Syvitski, 1992; Summerfield and Hulton, 1994. . The yield delivered from the Upper Yangtze catchment. difficulty of obtaining sufficiently detailed, spatially The average annual loadŽ for a catchment area of just distributed data on catchment characteristics has over 1 million km2 , is reported as 520 MtŽ Mason, hampered attempts to disentangle the various con- 1999.Ž . A question for catchment managers and geo- trols on sediment yields within large catchments. morphologists. is whether the observed spatial and However, the availability of global environmental temporal variability of both sediment production and data sets comprising description of hydroclimatic, conveyance can be explained adequately by a model biological and geomorphological characteristics of of sediment delivery for the Upper Yangtze. In the Earth offers a means of extracting catchment attempting to address this issue, the appropriateness variables for integration with sediment yield data. of methodologies and available data sets to analyse This approach has been used to examine global sediment delivery within large river catchments can variations in sediment yieldŽ Summerfield and Hul- be considered. The paper first provides a discussion ton, 1994; Ludwig and Probst, 1998. where individ- on approaches to modelling regional sediment yield. ual catchments are represented by a single sediment Second, background information about the Upper yield value. In the present study it is extended to the Yangtze catchment and the construction of data investigation of sediment yields within the Upper sources for examining sediment yields is described. Yangtze. Third, results focusing on estimates of sediment The prediction of sediment yields is complicated delivery to the Three Gorges reservoir are presented, by the interaction of controlling variables, human along with description of sediment responses to impact on the hydrological system, and by scale catchment disturbance and environmental variables. effects associated with different catchment sizes. The Evidence for recent changes in soil erosion and proportion of sediment eroded from catchment slopes sediment delivery to the reservoir from the area that is exported, decreases with catchment size in surrounding the Three Gorges is addressed in the most environments as opportunities for storage in- second paperŽ. Lu and Higgitt, this volume . crease downstreamŽ. Walling, 1983 . This proportion can be quantified by the sediment delivery ratio. The scale-dependency inhibits the direct comparison of 2. Regional sediment yields specific sediment yieldsŽ t kmy2 ay1. from catchments of contrasting sizes. In addition, sediment The geomorphological and hydrological literature delivery ratios may change over time, damping the contains many attempts to relate global or regional response of suspended sediment loads to the magni- sediment yields to controlling factors. Langbein and tude and extent of soil erosion. Time lags between SchummŽ. 1958 produced a model of sediment yield initial sediment mobilisation and export from the in relation to effective precipitation, reaching a maxi- catchment outlet may be considerableŽ Trimble, mum in semi-arid environments and declining as 1998. , and have significant effects on the manage- vegetation cover protects the land surface. XuŽ. 1994 ment of sediment laden waters. D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 145

3. The Upper Yangtze: characteristics and data marise measurements from a network of hydro- sources graphic stations throughout the Upper Yangtze. The original records for each station provide information 3.1. Catchment characteristics on the station co-ordinatesŽ. latitude and longitude , The Upper Yangtze catchment, with the exception catchment area, mean monthly and annual water of the extreme west, experiences a subtropical mon- discharge and sediment load, and the magnitude and soon climate. Precipitation varies from -250 mm date of occurrence of the maximum daily discharge. on the northern edge of the Qinghai–Xizang Plateau The use of historical gauging station data raises the to )1000 mm in the east of the catchment. Popula- question of data quality. The monthly sediment loads tion densities range from -10 persons kmy2 in the are based on discrete rather than continuous mea- mountainous west to )500 persons kmy2 in the surements of suspended sediment at daily to weekly Sichuan Basin. The Upper Jinsha, Yalong, Dadu and intervals. As such, the frequency of sampling does Min principally drain the mountainous areas to the not ensure that all ranges of flow were sampled. The west of the catchment. To the east, the Tuo, Fu, sampling intervals and the exclusion of the bedload Jialing and Qu flow through areas of high population component may result in underestimation of sedi- density and agricultural activity. The basin of the ment discharge during peak flows. The annual mea- Wu, the only significant right bank tributary of the sured sediment loads were recorded in units of 106 Yangtze, is largely agricultural but also drains the or 10 4 t, depending on the gauging station contribut- karst uplands of Guizhou ProvinceŽ. Fig. 1 . ing area or a particular year. In transcribing the The large-scale changes associated with land use yearbook records from paper to a series of linked practices and resource exploitation in rural China spreadsheets a number of errors were recognised and over the last 40 years are particularly significant for corrected by recalculating annual load from the the five AeasternB tributaries noted above. Wide- monthly dataŽ. Higgitt and Lu, 1996 . spread deforestation and the extension of agricultural Sediment load data were extracted from the land have resulted in a large increase in the area records of 250 stations in the Upper Yangtze for the reported as suffering from soil erosion. Land use period from 1956 to 1987. Post-1987 records are not inventories carried out in the 1950s and 1980s esti- in the public domain. There are large variations in mate that forest cover reduced form 19% to 12% in the spatial distribution, length and period of opera- Sichuan and from 23% to 13% in GuizhouŽ Yu et al., tion, and the catchment area of gauging stations, 1991. . Land affected by erosion increased from 16% which have implications for statistical analysis. Ap- to 67% in Sichuan and from 11% to 31% in Guizhou. proximately 25% of the gauging stations serve catch- Notwithstanding the qualitative nature of the inven- ment areas -1000 km2, while 9% cover areas tories, an increase in the extent of land degradation is )100 000 km2. About one quarter of stations have expected to show in an increased sediment load of records lasting less than 5 years. Given the marked the Upper Yangtze. However, this period also wit- year to year variations in climatic conditions experi- nessed rapid development of local water conservancy enced within the catchment, care is needed in ex- projects comprising ponds, check dams, ditches and tracting data for comparison. There are 56 stations small headwater reservoirs, which have trapped and with records of 25 years or more. Together with six temporarily stored a proportion of the eroded sedi- stations from the Wu tributaryŽ which is otherwise mentŽ. Luk and Whitney, 1993 . Together with more unrepresented. , these 62 sub-catchments are used in recent large hydro electric power schemes it has more detailed analysis of sediment yield variability. been estimated that total reservoir capacity in the Watershed boundaries were generated from the Asian Upper Yangtze catchment upstream of the TGP ex- 30 arcsecond Digital Elevation Model, supplemented ceeds 16 billion m3. by digitising 1:1,500,000 mapsŽ Lu and Higgitt, 1999. . The resolution of the DEM is approximately 3.2. Data sources 1=1 km and catchment variables can be determined The sediment yield and runoff data were extracted for each pixel and then calculated for the defined from the China Hydrological Yearbooks, which sum- sub-catchment boundaries. From the elevation data, 146 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156

Fig. 1. The Upper Yangtze catchment:Ž. A Principal tributaries with location of major dams and gauging stations mentioned in text. The location of stations showing significant trends in sediment yieldŽ.Ž. based on Lu and Higgitt, 1998 is also indicated; B Standardised residuals of sediment yield contribution, identifying relative sediment contribution within the catchmentŽ. based on Higgitt and Lu, 1996 . indices of slope and relief can be generated using a ment load data has some implications for analysis. GIS utility. Population density, precipitation and land For example, population density and land cover is cover classification classes are derived from global derived from 1992 to three databases which are databases that are available in the public domain. non-synchronous with the sediment load data. Demo- With the exception of the precipitation dataŽ from graphic and land use change will have been marked the Global Ecosystem Database on CD-ROM. , all within the time period used for averaging sediment data sets have been extracted freely from the Inter- loads. The resolution of the environmental data on a net. Again, the time-bound nature of historical sedi- 1=1 km grid also poses some difficulty. Topo- D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 147

Table 1 Definition, derivation and source of catchment variables used in multivariate analysis Variable Definition Source Specific Sediment YieldŽ. SY Annual sediment load per unit areawx t kmy2 ay1 China Hydrological Yearbooks Mean ElevationŽ. ME Mean of elevation of all cells within delineated 30 arcsecond DEMŽ pixel size approx. 1=1 km, catchment boundarywx m height accuracy "30 m. Ž.Sub- Catchment Area Ž. DA Catchment above gauging stationwx km2 China Hydrological Yearbooks—boundaries derived from DEM or digitised from various map sources Basin LengthŽ. BL Maximum straight line distance between ArcrInfo utility watershed and gauging stationwx km Basin ReliefŽ. BR Difference between maximum and minimum From statistical file after clipping DEM using cell elevation within catchment boundarywx m delineated catchment boundary Relative ReliefŽ. RR Basin ReliefrBasin Lengthwx m kmy1 Derived index Mean SlopeŽ. MS Mean of slope of all cells within delineated ArcrInfo slope grid generationŽ maximum boundary wx8 elevation changes within nine neighbouring cells. Agricultural Slope IndexŽ. SI Function of mean slope and presence of ArcrInfo grid utility to match land cover agricultural land and slope data Population densityŽ. PD Number of persons per km2 Asian Population database, derived from 1992 census PrecipitationŽ. PP Mean annual precipitation from 0.5 by Global Ecosystems DatabaseŽ. Version 1.0, 1992 0.58 resolutionwx mm RunoffŽ. RO Mean annual runoffwx mm China Hydrological Yearbooks Land Cover Class V1-8 Regrouped land cover classifications China Land Cover Database, EROS Data Ž.see Table 3 Center Ž based on AVHRR data .

graphic indices such as mean slope may have limited specific sediment yield of the Jialing at Wusheng is application in terms of predicting soil erosion poten- 928 t kmy2 ay1. The relationship between catchment tial and land cover classification is problematic as area and specific sediment yield makes it difficult to Chinese landscapes are frequently mosaics of varied compare contributions of unit area sediment yield land uses that are aggregated into one unit at the directly. As a first attempt to isolate scale effects resolution of the classification system. Definitions from the data, Higgitt and LuŽ. 1996 produced a and units for the extracted and derived variables are regression equation, SYs1126.2DAy0.0826, for the provided in Table 1. relationship between specific sediment yieldŽ SY, t kmy2 ay1.Ž and sub-catchment area DA, km2 . for the 187 stations with measurement records covering 4. Sediment delivery and its response in the Up- at least 5 years. The equation is used to estimate the per Yangtze relative contribution of sub-catchments. Spatial variability in relative contribution can then be defined by 4.1. Sediment deliÕery to the Three Gorges plotting the standardised residuals from the SY–DA regressionŽ. Fig. 1B . Locations with significantly Sediment delivered to the Three Gorges reservoir higher sediment contributions occur in the headwa- from upstream of the reservoir can be approximated ters of the Jialing and the western margins of the by using data from three major stations: Beipei, Chengdu Plain, and to a lesser extent across the Zhutou and WulongŽ. Fig. 1A . The mean annual eastern side of the Sichuan Basin into the Three sediment load delivered to the reservoir will be 497.4 Gorges area itself. Locations with significantly lower Mt. The area above Zhutou station accounts for 64% sediment contributions occur in the west and along of this total, but in terms of specific sediment yield, the southern margins of the catchment in Yunnan as noted in earlier studiesŽ Gu et al., 1987; Chen and and Guizhou. Gao, 1988; Gu and Douglas, 1989. , that the Jialing Table 2 lists the mean suspended sediment load tributary is a major source of sediment. The mean ŽMt ay1 . for stations on the major tributaries. The 148 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156

Table 2 Relative contributions of the principal tributaries to the Upper Yangtze sediment loadŽ. appended River Station DAŽ. km2 Mean load "S.D. Mean Mean Maximum Minimum Ž.Ž.Mt ay1 Mt ay1 contribution relative relative relative Ž.% contributionaaa contribution contribution U Jinsha Shigu 232,651 21.2 9.8 8.89 0.28 0.41 0.20 Yalong Xiaodeshi 118,294 29.4 13.5 12.11 0.68 0.93 0.41 Min Pengshan 30,661 10.3 4.8 4.58 0.91 1.11 0.66 Dadub Shaping 75,016 32.2 10.5 13.62 1.10 1.37 0.90 Jialing Wusheng 79,714 74.0 44.8 31.96 2.54 3.45 1.90 Fu Guodukou 31,626 19.9 10.9 8.40 1.63 2.29 0.54 Qu Xiaoheba 29,420 19.1 16.6 8.34 1.75 2.32 1.07 Wu Wulong 83,035 32.4 12.3 14.42 1.20 1.78 0.67

Ýtributaries 680,417 100.0 Main Yichang 1,005,501 527.2 98.8

a Mean relative contribution is the proportion of actual load divided by predicted loadŽ. from SY–DA regression , averaged across gauging station measurement period. Maximum and minimum relative contribution are for individual years within this measurement period. bGauging station relocated from TunjianzhiŽ DAs77,202 km2 . in 1967. Mean loads and contribution corrected for change in DA.

mean contribution of each tributary is given as a 1993. . The long-term mean annual sediment load percentage of the sum of tributary loads. The relative consequently has been used as the base for analysing contribution of each tributary is calculated by divid- the potential sedimentation problems of the Three ing the measured mean sediment load by that pre- Gorges ProjectŽ. TGP . The implication is that dicted from the SY–DA relationship assuming loads changes in upstream headwater sediment transport were spatially uniform across the catchment. The resulting from erosion are damped or lagged by the regression is based on data averaged across the time that they reach the main channel. Two hypothe- measurement period, but relative contributions for ses can be posed:Ž. 1 That sediment supply has individual years can also be calculated. Maximum increased through soil erosion but that conveyance and minimum contributions are also indicated in downstream has been reduced through reservoir sedi- Table 2. The Jialing is the dominant contributor over mentation;Ž. 2 That the spatial and temporal dynam- the record period at 2–3 times the expected level, ics of land use change have led to an increase in the except during the late 1960s to late 1970s. Similar, extent of degraded land without increasing overall but less marked, trends are observed in the behaviour sediment delivery rates. The first hypothesis is tested of the Fu and Qu, tributaries of the Greater Jialing, by conducting a time series analysis on 187 stations. the latter generally increasing contribution over time. A statistically significant linear regression model of The rivers draining the Sichuan Basin appear to sediment yield against year of measurement Žas carry about twice the sediment load that would be 0.05. could be fitted to 16 of the stations, of which expected if sediment delivery was spatially uniform. 10Ž. 7.9% of catchment area displayed increasing The Upper Jinsha and Yalong are relatively less trends and sixŽ. 2.8% displayed decreasing ones. The important and the Wu, Dadu and Min are close to the majority of sub-catchments are relatively small Ž- predicted output. 4000 km2 . and the changing sediment yields may reflect land use alterations, but six gauging stations 4.2. Sediment deliÕery response to catchment distur- are located on major tributariesŽ. Fig. 2 . Of the three bance large stations with decreasing sediment trends, Bikou and Sanleiba are located on the Jialing River down- Yichang station has not measured any significant stream of the Bikou Reservoir, while Denyenyan is trend in sediment load despite the evidence for in- on the Tuo River which now has the highest propor- creased erosion in parts of the catchmentŽ Wen, tion of reservoir capacity to catchment area of any of D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 149 the Yangtze tributaries. Comparison of time series Values for the constants were chosen to represent plots of stations above and below major reservoirs a fivefold increase in soil erosion following distur- clearly demonstrates their impact on reducing sedi- bance which declined to 50% after 3 years and 10% ment yield measurements immediately downstream after 10 years Ž.ms4, ns0.25, ks1 . The catch- Ž.Fig. 2 . The Dadu, Qu and Wu show increasing ment is represented by 100 grid cells. Sediment sediment yield from their basins. The Dadu and Wu yields are in arbitrary units where the pre-dis- have relatively low provision of reservoir capacity. turbance conditions would produce an output of 100 For most of the Upper Yangtze, no evidence of units if the sediment delivery ratio was 1. Values for higher sediment yields arising from widespread ero- sediment delivery per cell length, rate and direction sion is apparent. However, subtle variations within of disturbance were varied in the simulation. For the catchment and the association of sites of decreas- most simulations, sediment yield increases over the ing yields with reservoir construction, indicate the time periodŽ. 20 years but the rate of increase began need to consider the longer term implications of to slow after 5–10 yearsŽ. Fig. 3 . Thus, the extent of sediment delivery as storage sites are progressively erosion within the catchment has increased but the filled. In addition to the analysis of annual variation effect on the magnitude of sediment redistribution is in sediment loads, evidence exists that the tributaries less marked. The direction of disturbance is impor- of the Yalong, Dadu, Min, Qu and Wu, have wit- tant because a larger proportion of sediment con- nessed significant increases in seasonal and daily tributes to basin yield when cells close to the basin sediment load over the last 40 years. The evidence outlet are affected. When disturbance progresses from for trends in sediment yield at individual gauging the divide towards the catchment outlet, new sedi- stations within the Upper Yangtze may provide some ment sources are created close to the outlet and insight into the changing nature of sediment delivery. sediment loads continue to increase. However, in the The second hypothesis concerns the way in which an case of upstream propagation of disturbanceŽ i.e., expansion of erosion-inducing land use change im- from the outlet towards the divide. sediment yield pacts upon long-term sediment yield. The impact of increases slow down quickly. As the direction of catchment disturbance on sediment delivery over land use change in China has tended to spread time can be simulated using a simple modelŽ Higgitt up-basin both in terms of disturbance by extension of and Lu, 2000. . In many earth surface systems, it has agricultural land and by deforestation, the results of been observed that initial disturbanceŽ such as defor- the AupstreamB simulation require field validation. If estation or extension of agricultural land. will gener- the disturbance has acted as a pulse that has migrated ate a pulse of increased sediment yield before the across the Upper Yangtze basin in a general west- system returns towards a pre-disturbance sediment ward and upstream direction, the impact of the visi- supply if the disturbance is not sustainedŽ Schumm ble increase in land degradation on sediment load and Rea, 1995. . The impact of a disturbance on any may be less dramatic. The simulation model does not cell within a grid representing the catchment can be allow for any variation in erosion sensitivity in any expressed by the exponential decay function: of the cells, nor distinguish between grades of disturbance. Similarly, the parameter values describing s ynTq SY me k recovery rate are chosen arbitrarily for demonstration where m, n, k are constants and TstimeŽ. years . A purpose. Work is now in progress to reconstruct the sediment delivery term can be included such that the spatio-temporal pattern of agricultural extensification sediment exported from each cell that reaches the and deforestation across central and southern China basin outlet is a function of the travel distance and to test the model with field investigations in the represented by, zones of most recent disturbance. X s p SYi SD 4.3. Sediment deliÕery response to enÕironmental X s Õariables where SYi sediment yield from cell i that is exported to basin outlet; SDssediment delivery ratio Having examined sediment delivery response to per cell length; pstravel distance in cell lengths. catchment disturbance, multivariate techniques can 150 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 be employed to examine the extent to which spatial spread interest in attempting to quantify human variation in sediment yields can be explained by impact on sediment yields, but it is difficult to catchment controls. The 62 sub-catchments with disentangle the effects from the natural background, longer-term records were analysed. The interrelation- particularly where anthropogenic variables such as ships between catchment variables were examined population density and land use are likely to be using Spearman’s rank correlation. At sub-catchment strongly correlated with elevation and precipitation. level, mean elevation is significantly correlated with The high degree of spatial variability in sediment many variables. Mean slope increases with mean yields and catchment characteristics causes difficulty elevation whereas population density and precipita- when attempting to model the controlling relation- tion decrease. Specific sediment yields are signifi- ships across the whole data set. Previous studies of cantly correlated with only two variables—mean global sediment yield variation have used the strat- elevation and runoff. Elevation appears to act as a egy of grouping data into suitable categories to surrogate for aridity and human impact, constrained reduce scatterŽ Jansson, 1988; Summerfield and Hul- by the geography of the catchment. There is wide- ton, 1994; Ludwig and Probst, 1996. . Based on

Fig. 2.Ž. A Relatively large gauging stations showing a trend in sediment yield across the measurement period.Ž. B Impact of large dams on downstream sediment conveyance indicated by measurements from nearest station above and below impoundmentsŽ based on Lu and Higgitt, 1998. . D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 151

Fig. 2 Ž.continued . tributary groupings clear patterns in sediment yields indicates that 87% and 95%, respectively, of the become evident in the groupings for the Jinsha–Ya- scatter in sediment yields in these tributaries is ex- long and Dadu–Min catchments. Multiple regression plained by six catchment variables of runoff, precipi-

Fig. 3. Simulation model of changes in predicted sediment yield following a wave of disturbance through a catchment. The disturbance rate is 2% of catchment area per yearŽ. upper panel and 4% per year Ž. lower panel for sediment delivery ratios Ž per cell length . of 0.99, 0.98 and 0.95Ž. based on Higgitt and Lu, in press . The direction of disturbance is upstream Ž.Ž. away from outlet , downstream away from watershed . 152 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 tation, mean elevation, basin relief, mean slope and catchment variables within the Jialing sub-catchment population densityŽ. Lu and Higgitt, 1999 . In con- data set including an apparent inverse relationship trast only 33% of the variance is explained in the with population density. This probably reflects the Jialing and it is here that soil erosion rates are fact that the highest population densities are encoun- highest and the control of sediment delivery most tered within the flatter Chengdu Plain. The relation- critical. Interestingly, there are notable significant ships between sediment yields and mean annual pre- correlations between sediment yield and individual cipitation or mean annual runoff are noteworthyŽ Fig.

Fig. 4. Sediment yield–precipitation and sediment yield–runoff relationships for data grouped by catchment size, elevation and tributary Ž.based on Lu and Higgitt, 1999 . D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 153

Fig. 4 Ž.continued .

4. . Many of the earlier studies to examine global km2. In the catchment size category 10,000–100,000 sediment yields have attempted to reveal the nature km2, the relationship between sediment yield and of the relationship. Grouping strategies reveal con- precipitation is inverse, while a polynomial fits the trasting fits to the data. Linear or near linear relation- sediment yield–runoff data. In this case and the ships can be fitted to data from the Jinsha–Yalong same relationship for the Jialing data set, a minimum and Dadu–Min tributaries, but a polynomial fit de- turning point occurs at annual runoff of 500 mm. scribes the relationship in the Jialing tributary. Simi- Higher sediment yields at lower values of runoff is larly, linear or near-linear relationships can be fitted consistent with the Langbein and SchummŽ. 1958 to data for sub-catchments with maximum elevation model, while the increase beyond 700 mm runoff is a )5000 m and those with catchment area )100,000 feature that has been described previouslyŽ Wilson, 154 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156

1973; Walling and Webb, 1983. . Using data from slope index was devised to account for the coinci- 700 rivers in China, XuŽ. 1994 reported a peak dence of agricultural land and slopes at pixel level sediment yield at 400 mm runoff. The analysis here within each of the sub-catchment boundariesŽ Table suggests that any relationship is not consistent and in 1. . Although this provides a statistically significant many cases can be explained by the geography of the relationship, the degree of explanation remains poor. Upper Yangtze. For example, the headwaters of the Incorporating land cover into the multiple regression Jialing contain some loess areas which have high equations also fails to improve the degree of expla- sediment yields but also have low runoff. Multiple nation. Employing factor analysis as an alternative to regression results suggest that much of the variability multiple regression reveals some clustering of sub- in sediment yields in the west of the Upper Yangtze catchments of similar sediment yields within factor catchment can be explained in terms of AnaturalB spaceŽ. Higgitt and Lu, 1999 . It has been suggested catchment characteristics, but these provide a rela- that catchments with specific sediment yields of tively poor explanation of sediment yields in the 500–1000 t kmy2 ay1 plot within factor space more populated east of the catchment. controlled by anthropogenic variablesŽ population Land cover information has been obtained from density and cropland land cover. but the areas sup- the China land cover database, a part of the global plying the highest sediment yields are well dispersed. land cover project of Earth Resources Observation One interpretation is that the incidence of the highest SystemŽ. EROS Data Center. The database is derived specific sediment yields Ž)1000 t kmy2 ay1. is from 1=1 km Advanced Very High Resolution attributed to particular local conditions. For example, RadiometerŽ. AVHRR data. The 157 different land conditions promoting very high sediment yields are cover types have been regrouped into 8 land cover the presence of loess sediments in the Upper Jialing, classesŽ. Table 3 . Agricultural land is represented in areas of frequent landslide activity in the incised three classes—paddy field, high-density cropland valleys to the west of the Chengdu Plain, or specific Ž.) 50% and low density cropland Ž.- 50% . land use changes in relatively small catchments. Broadly, the high density cropland and the paddy Clearly, the influence of land use on sediment yields field land cover dominate in the Sichuan Basin and within a large catchment is subtle and confounded by its margins, while the lower density of cropland is the interaction of other variables and scale effects. situated in valleys further west. The relationship Obtaining reliable and sufficiently high resolution between specific sediment yield and any measure of data to tease out the anthropogenic influences from agricultural land for the 62 sub-catchments is not the natural factors remains a challenge. The precise significant. As the highest density of agricultural influence of land use remains somewhat inconclusive land tends toward the less steep land, an agriculture but the increasing availability and resolution of envi-

Table 3 Regrouping of categories from the China Land Cover database into eight land cover classes No. Land cover class % of ground Number of Dominant location cover in the categories Upper Yangtze grouped v1 Unvegetated 3 17 High elevation Ž.water, desert, ice and snow v2 Alpine meadow or steppe 25 34 Qinghai–Xizang Plateau v3 Grassrshrub 9 13 upper parts of western tributaries v4 Shrubrwoodland 13 13 upper parts of western tributaries v5 Woodland 11 28 mid elevations of western tributaries v6 CroplandŽ. mosaic-50% 8 13 valleys in western tributaries v7 CroplandŽ. mosaic )50% 23 27 eastern tributaries v8 Paddy field 8 12 Sichuan Basin D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156 155 ronmental data sets will enable more sophisticated combination of topographic, hydroclimatic, popula- land use representation and associated modelling to tion and land use variables. The detailed analysis of be attempted. sediment loads within a large basin has implications for the management of potential sedimentation and for policy development at the catchment scale. Criti- 5. Conclusion cal areas for sediment control can be identified. One of these is the immediate land area surrounding the The paper has described some of the aspects of Three Gorges, where soils are susceptible to erosion the temporal and spatial variation of sediment yields and the resettlement of the displaced population may in the Upper Yangtze. The apparent paradox of generate further land degradation. Consideration of increasing catchment erosion without increasing sed- the local component to sediment delivery to the iment export has been investigated. Certain sub- Three Gorges is addressed in the second paperŽ Lu catchments experience marked increases in sediment and Higgitt, this volume. . yield over time, but large tributary reservoirs succeed in restricting downstream conveyance. Thus, the sediment load delivered to the Three Gorges may reflect Acknowledgements a balance between increased sediment production counteracted by increased storage in tributary im- We gratefully acknowledge the contribution of poundments. However, a hypothetical model of sedi- Professor Chen Zhongyuan and Professor Avijit ment delivery from catchments undergoing a wave Gupta in organising the IAG Large Rivers Confer- Ž of disturbance similar to the recent land use history ence on the Yangtze River and editing the papers. . of southern China indicates that in some circum- Comments from Gordon E. Grant helped to improve stances a near constant yield may be produced. This the manuscript. Cartographers from the University of model needs to be tested against field evidence. Durham and the National University of Singapore Work is in progress to identify key points in the produced the diagrams. basin where sediment yields have been affected by recent land use changes for more detailed sediment budget investigation, using the spatial and temporal References analysis of sediment loads in the Upper Yangtze. The study has demonstrated that novel approaches Chen, G.J., Gao, F.H., 1988. The ecology and environment of the to using GIS techniques to extract spatially dis- Yangtze and the Three Gorges, and the Three Gorges project. tributed data can be applied within large catchments. The Leading Group of the Research Project of Chinese The high degree of scatter has been overcome to Academy of Sciences. The Effect of the Yangtze Three Gorges some extent by grouping strategies that allow varia- Project on Ecology and Environment and Countermeasures, tions in the nature of the relationship in different Science Press, Beijing, pp. 1–15Ž. in Chinese . 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Spatial and temporal dynamics of catchment modelling to be advanced. land degradation and fluvial erosion in the middle and upper The examination of spatial and temporal variabil- Yangtze River basin, China. Land Degradation and Rehabilita- ity of sediment yields indicates the main source areas tion 1, 217–235. Gu, H.Y., Ai, N.S., Ma, H.L., 1987. Sediment sources and trend of sediment within the Upper Yangtze, the areas of sedimentation in the Three Gorges reservoir area. Leading where sediment loads are apparently increasing and Group of the Three Gorges Project Ecology and Environment the extent to which the pattern can be explained by a Research Project, Chinese Academy of Sciences. Collected 156 D.L. Higgitt, X.X. LurGeomorphology 41() 2001 143–156

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