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Sediment delivery to the three gorges:
Geomorphology 41 Ž2001. 143–156 www.elsevier.comrlocatergeomorph
Sediment delivery to the three gorges: 1. Catchment controls D.L. Higgitt a,) , X.X. Lu b a
b
Department of Geography, UniÕersity of Durham, Science Laboratories, South Road, Durham, DH1 3LE, UK 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 Project ŽTGP.. 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 The potential impact of sedimentation on the operation and life span of the Three Gorges Reservoir )
Corresponding author. Fax: q44-191-374-2456. E-mail address: [email protected] ŽD.L. Higgitt..
is one of the key environmental issues that has focused attention on the dynamics of soil erosion and fluvial sediment transport in the catchment of the Upper Yangtze ŽGu and Douglas, 1989; Edmonds, 1992; Qian et al., 1993; Luk and Whitney, 1993.. There are two major issues of concern—whether proposed reservoir regulation procedures are effi-
0169-555Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 0 1 . 0 0 1 1 2 - X
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cient enough to control the conveyance of most of the sediment-laden waters, and whether the long-term observed sediment delivery to the Three Gorges is representative of future trends. Investigation of the temporal and spatial variability of sediment transport within the Upper Yangtze catchment leads to the following paradox. There is widespread evidence that the extent and magnitude of soil erosion across southern China has increased dramatically during the last 30–40 years ŽSmil, 1993; Wen, 1993; Edmonds, 1994.. There is no evidence from the sediment load measurements at the Yichang gauging station Ždownstream of the TGP dam site. of a trend in sediment yield delivered from the Upper Yangtze catchment. The average annual load Žfor a catchment area of just over 1 million km2 , is reported as 520 Mt ŽMason, 1999.. A question for catchment managers Žand geomorphologists. is whether the observed spatial and temporal variability of both sediment production and conveyance can be explained adequately by a model of sediment delivery for the Upper Yangtze. In attempting to address this issue, the appropriateness of methodologies and available data sets to analyse sediment delivery within large river catchments can be considered. The paper first provides a discussion on approaches to modelling regional sediment yield. Second, background information about the Upper Yangtze catchment and the construction of data sources for examining sediment yields is described. Third, results focusing on estimates of sediment delivery to the Three Gorges reservoir are presented, along with description of sediment responses to catchment disturbance and environmental variables. Evidence for recent changes in soil erosion and sediment delivery to the reservoir from the area surrounding the Three Gorges is addressed in the second paper ŽLu and Higgitt, this volume..
2. Regional sediment yields The geomorphological and hydrological literature contains many attempts to relate global or regional sediment yields to controlling factors. Langbein and Schumm Ž1958. produced a model of sediment yield in relation to effective precipitation, reaching a maximum in semi-arid environments and declining as vegetation cover protects the land surface. Xu Ž1994.
has proposed that the Langbein–Schumm model is broadly applicable to explain national variations within China. As the availability of gauging station data has increased a more complex relationship with climate and vegetation has emerged ŽDouglas, 1967; Wilson, 1973; Jansen and Painter, 1974; Walling and Webb, 1983; Jansson, 1988., mainly because of the impact of human activity on natural vegetation cover. At a global scale, the importance of topography and the significance of relatively small mountainous catchments as major contributors to global continental sediment export has been noted ŽMilliman and Syvitski, 1992; Summerfield and Hulton, 1994.. The difficulty of obtaining sufficiently detailed, spatially distributed data on catchment characteristics has hampered attempts to disentangle the various controls on sediment yields within large catchments. However, the availability of global environmental data sets comprising description of hydroclimatic, biological and geomorphological characteristics of the Earth offers a means of extracting catchment variables for integration with sediment yield data. This approach has been used to examine global variations in sediment yield ŽSummerfield and Hulton, 1994; Ludwig and Probst, 1998. where individual catchments are represented by a single sediment yield value. In the present study it is extended to the investigation of sediment yields within the Upper Yangtze. The prediction of sediment yields is complicated by the interaction of controlling variables, human impact on the hydrological system, and by scale effects associated with different catchment sizes. The proportion of sediment eroded from catchment slopes that is exported, decreases with catchment size in most environments as opportunities for storage increase downstream ŽWalling, 1983.. This proportion can be quantified by the sediment delivery ratio. The scale-dependency inhibits the direct comparison of specific sediment yields Žt kmy2 ay1 . from catchments of contrasting sizes. In addition, sediment delivery ratios may change over time, damping the response of suspended sediment loads to the magnitude and extent of soil erosion. Time lags between initial sediment mobilisation and export from the catchment outlet may be considerable ŽTrimble, 1998., and have significant effects on the management of sediment laden waters.
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
3. The Upper Yangtze: characteristics and data sources 3.1. Catchment characteristics The Upper Yangtze catchment, with the exception of the extreme west, experiences a subtropical monsoon climate. Precipitation varies from - 250 mm on the northern edge of the Qinghai–Xizang Plateau to ) 1000 mm in the east of the catchment. Population densities range from - 10 persons kmy2 in the mountainous west to ) 500 persons kmy2 in the Sichuan Basin. The Upper Jinsha, Yalong, Dadu and Min principally drain the mountainous areas to the west of the catchment. To the east, the Tuo, Fu, Jialing and Qu flow through areas of high population density and agricultural activity. The basin of the Wu, the only significant right bank tributary of the Yangtze, is largely agricultural but also drains the karst uplands of Guizhou Province ŽFig. 1.. The large-scale changes associated with land use practices and resource exploitation in rural China over the last 40 years are particularly significant for the five AeasternB tributaries noted above. Widespread deforestation and the extension of agricultural land have resulted in a large increase in the area reported as suffering from soil erosion. Land use inventories carried out in the 1950s and 1980s estimate that forest cover reduced form 19% to 12% in Sichuan and from 23% to 13% in Guizhou ŽYu et al., 1991.. Land affected by erosion increased from 16% to 67% in Sichuan and from 11% to 31% in Guizhou. Notwithstanding the qualitative nature of the inventories, an increase in the extent of land degradation is expected to show in an increased sediment load of the Upper Yangtze. However, this period also witnessed rapid development of local water conservancy projects comprising ponds, check dams, ditches and small headwater reservoirs, which have trapped and temporarily stored a proportion of the eroded sediment ŽLuk and Whitney, 1993.. Together with more recent large hydro electric power schemes it has been estimated that total reservoir capacity in the Upper Yangtze catchment upstream of the TGP exceeds 16 billion m3. 3.2. Data sources The sediment yield and runoff data were extracted from the China Hydrological Yearbooks, which sum-
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marise measurements from a network of hydrographic stations throughout the Upper Yangtze. The original records for each station provide information on the station co-ordinates Žlatitude and longitude., catchment area, mean monthly and annual water discharge and sediment load, and the magnitude and date of occurrence of the maximum daily discharge. The use of historical gauging station data raises the question of data quality. The monthly sediment loads are based on discrete rather than continuous measurements of suspended sediment at daily to weekly intervals. As such, the frequency of sampling does not ensure that all ranges of flow were sampled. The sampling intervals and the exclusion of the bedload component may result in underestimation of sediment discharge during peak flows. The annual measured sediment loads were recorded in units of 10 6 or 10 4 t, depending on the gauging station contributing area or a particular year. In transcribing the yearbook records from paper to a series of linked spreadsheets a number of errors were recognised and corrected by recalculating annual load from the monthly data ŽHiggitt and Lu, 1996.. Sediment load data were extracted from the records of 250 stations in the Upper Yangtze for the period from 1956 to 1987. Post-1987 records are not in the public domain. There are large variations in the spatial distribution, length and period of operation, and the catchment area of gauging stations, which have implications for statistical analysis. Approximately 25% of the gauging stations serve catchment areas - 1000 km2 , while 9% cover areas ) 100 000 km2 . About one quarter of stations have records lasting less than 5 years. Given the marked year to year variations in climatic conditions experienced within the catchment, care is needed in extracting data for comparison. There are 56 stations with records of 25 years or more. Together with six stations from the Wu tributary Žwhich is otherwise unrepresented., these 62 sub-catchments are used in more detailed analysis of sediment yield variability. Watershed boundaries were generated from the Asian 30 arcsecond Digital Elevation Model, supplemented by digitising 1:1,500,000 maps ŽLu and Higgitt, 1999.. The resolution of the DEM is approximately 1 = 1 km and catchment variables can be determined for each pixel and then calculated for the defined sub-catchment boundaries. From the elevation data,
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Fig. 1. The Upper Yangtze catchment: ŽA. Principal tributaries with location of major dams and gauging stations mentioned in text. The location of stations showing significant trends in sediment yield Žbased on Lu and Higgitt, 1998. is also indicated; ŽB. Standardised residuals of sediment yield contribution, identifying relative sediment contribution within the catchment Žbased on Higgitt and Lu, 1996..
indices of slope and relief can be generated using a GIS utility. Population density, precipitation and land cover classification classes are derived from global databases that are available in the public domain. With the exception of the precipitation data Žfrom the Global Ecosystem Database on CD-ROM., all data sets have been extracted freely from the Internet. Again, the time-bound nature of historical sedi-
ment load data has some implications for analysis. For example, population density and land cover is derived from 1992 to three databases which are non-synchronous with the sediment load data. Demographic and land use change will have been marked within the time period used for averaging sediment loads. The resolution of the environmental data on a 1 = 1 km grid also poses some difficulty. Topo-
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Table 1 Definition, derivation and source of catchment variables used in multivariate analysis Variable
Definition
Source
Specific Sediment Yield ŽSY. Mean Elevation ŽME.
Annual sediment load per unit area wt kmy2 ay1 x Mean of elevation of all cells within delineated catchment boundary wmx Catchment above gauging station wkm2 x
China Hydrological Yearbooks 30 arcsecond DEM Žpixel size approx. 1 = 1 km, height accuracy "30 m. China Hydrological Yearbooks—boundaries derived from DEM or digitised from various map sources ArcrInfo utility
ŽSub-. Catchment Area ŽDA. Basin Length ŽBL. Basin Relief ŽBR. Relative Relief ŽRR. Mean Slope ŽMS. Agricultural Slope Index ŽSI. Population density ŽPD. Precipitation ŽPP. Runoff ŽRO. Land Cover Class V1-8
Maximum straight line distance between watershed and gauging station wkmx Difference between maximum and minimum cell elevation within catchment boundary wmx Basin ReliefrBasin Length wm kmy1 x Mean of slope of all cells within delineated boundary w8x Function of mean slope and presence of agricultural land Number of persons per km2 Mean annual precipitation from 0.5 by 0.58 resolution wmmx Mean annual runoff wmmx Regrouped land cover classifications Žsee Table 3.
graphic indices such as mean slope may have limited application in terms of predicting soil erosion potential and land cover classification is problematic as Chinese landscapes are frequently mosaics of varied land uses that are aggregated into one unit at the resolution of the classification system. Definitions and units for the extracted and derived variables are provided in Table 1.
4. Sediment delivery and its response in the Upper Yangtze 4.1. Sediment deliÕery to the Three Gorges Sediment delivered to the Three Gorges reservoir from upstream of the reservoir can be approximated by using data from three major stations: Beipei, Zhutou and Wulong ŽFig. 1A.. The mean annual sediment load delivered to the reservoir will be 497.4 Mt. The area above Zhutou station accounts for 64% of this total, but in terms of specific sediment yield, as noted in earlier studies ŽGu et al., 1987; Chen and Gao, 1988; Gu and Douglas, 1989., that the Jialing tributary is a major source of sediment. The mean
From statistical file after clipping DEM using delineated catchment boundary Derived index ArcrInfo slope grid generation Žmaximum elevation changes within nine neighbouring cells. ArcrInfo grid utility to match land cover and slope data Asian Population database, derived from 1992 census Global Ecosystems Database ŽVersion 1.0, 1992. China Hydrological Yearbooks China Land Cover Database, EROS Data Center Žbased on AVHRR data.
specific sediment yield of the Jialing at Wusheng is 928 t kmy2 ay1 . The relationship between catchment area and specific sediment yield makes it difficult to compare contributions of unit area sediment yield directly. As a first attempt to isolate scale effects from the data, Higgitt and Lu Ž1996. produced a regression equation, SY s 1126.2DAy0 .0826 , for the relationship between specific sediment yield ŽSY, t kmy2 ay1 . and sub-catchment area ŽDA, km2 . for the 187 stations with measurement records covering at least 5 years. The equation is used to estimate the relative contribution of sub-catchments. Spatial variability in relative contribution can then be defined by plotting the standardised residuals from the SY–DA regression ŽFig. 1B.. Locations with significantly higher sediment contributions occur in the headwaters of the Jialing and the western margins of the Chengdu Plain, and to a lesser extent across the eastern side of the Sichuan Basin into the Three Gorges area itself. Locations with significantly lower sediment contributions occur in the west and along the southern margins of the catchment in Yunnan and Guizhou. Table 2 lists the mean suspended sediment load ŽMt ay1 . for stations on the major tributaries. The
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
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Table 2 Relative contributions of the principal tributaries to the Upper Yangtze sediment load Žappended. River
Station
DA Žkm2 .
U Jinsha Yalong Min Dadub Jialing Fu Qu Wu Ýtributaries Main
Shigu Xiaodeshi Pengshan Shaping Wusheng Guodukou Xiaoheba Wulong
232,651 118,294 30,661 75,016 79,714 31,626 29,420 83,035 680,417 1,005,501
Yichang
"S.D. ŽMt ay1 .
Mean contribution Ž%.
Mean relative contributiona
Maximum relative contributiona
Minimum relative contributiona
21.2 29.4 10.3 32.2 74.0 19.9 19.1 32.4
9.8 13.5 4.8 10.5 44.8 10.9 16.6 12.3
8.89 12.11 4.58 13.62 31.96 8.40 8.34 14.42 100.0
0.28 0.68 0.91 1.10 2.54 1.63 1.75 1.20
0.41 0.93 1.11 1.37 3.45 2.29 2.32 1.78
0.20 0.41 0.66 0.90 1.90 0.54 1.07 0.67
527.2
98.8
Mean load ŽMt ay1 .
a
Mean relative contribution is the proportion of actual load divided by predicted load Žfrom SY–DA regression., averaged across gauging station measurement period. Maximum and minimum relative contribution are for individual years within this measurement period. b Gauging station relocated from Tunjianzhi ŽDA s 77,202 km2 . in 1967. Mean loads and contribution corrected for change in DA.
mean contribution of each tributary is given as a percentage of the sum of tributary loads. The relative contribution of each tributary is calculated by dividing the measured mean sediment load by that predicted from the SY–DA relationship assuming loads were spatially uniform across the catchment. The regression is based on data averaged across the measurement period, but relative contributions for individual years can also be calculated. Maximum and minimum contributions are also indicated in Table 2. The Jialing is the dominant contributor over the record period at 2–3 times the expected level, except during the late 1960s to late 1970s. Similar, but less marked, trends are observed in the behaviour of the Fu and Qu, tributaries of the Greater Jialing, the latter generally increasing contribution over time. The rivers draining the Sichuan Basin appear to carry about twice the sediment load that would be expected if sediment delivery was spatially uniform. The Upper Jinsha and Yalong are relatively less important and the Wu, Dadu and Min are close to the predicted output. 4.2. Sediment deliÕery response to catchment disturbance Yichang station has not measured any significant trend in sediment load despite the evidence for increased erosion in parts of the catchment ŽWen,
1993.. The long-term mean annual sediment load consequently has been used as the base for analysing the potential sedimentation problems of the Three Gorges Project ŽTGP.. The implication is that changes in upstream headwater sediment transport resulting from erosion are damped or lagged by the time that they reach the main channel. Two hypotheses can be posed: Ž1. That sediment supply has increased through soil erosion but that conveyance downstream has been reduced through reservoir sedimentation; Ž2. That the spatial and temporal dynamics of land use change have led to an increase in the extent of degraded land without increasing overall sediment delivery rates. The first hypothesis is tested by conducting a time series analysis on 187 stations. A statistically significant linear regression model of sediment yield against year of measurement Ž a s 0.05. could be fitted to 16 of the stations, of which 10 Ž7.9% of catchment area. displayed increasing trends and six Ž2.8%. displayed decreasing ones. The majority of sub-catchments are relatively small Ž4000 km2 . and the changing sediment yields may reflect land use alterations, but six gauging stations are located on major tributaries ŽFig. 2.. Of the three large stations with decreasing sediment trends, Bikou and Sanleiba are located on the Jialing River downstream of the Bikou Reservoir, while Denyenyan is on the Tuo River which now has the highest proportion of reservoir capacity to catchment area of any of
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
the Yangtze tributaries. Comparison of time series plots of stations above and below major reservoirs clearly demonstrates their impact on reducing sediment yield measurements immediately downstream ŽFig. 2.. The Dadu, Qu and Wu show increasing sediment yield from their basins. The Dadu and Wu have relatively low provision of reservoir capacity. For most of the Upper Yangtze, no evidence of higher sediment yields arising from widespread erosion is apparent. However, subtle variations within the catchment and the association of sites of decreasing yields with reservoir construction, indicate the need to consider the longer term implications of sediment delivery as storage sites are progressively filled. In addition to the analysis of annual variation in sediment loads, evidence exists that the tributaries of the Yalong, Dadu, Min, Qu and Wu, have witnessed significant increases in seasonal and daily sediment load over the last 40 years. The evidence for trends in sediment yield at individual gauging stations within the Upper Yangtze may provide some insight into the changing nature of sediment delivery. The second hypothesis concerns the way in which an expansion of erosion-inducing land use change impacts upon long-term sediment yield. The impact of catchment disturbance on sediment delivery over time can be simulated using a simple model ŽHiggitt and Lu, 2000.. In many earth surface systems, it has been observed that initial disturbance Žsuch as deforestation or extension of agricultural land. will generate a pulse of increased sediment yield before the system returns towards a pre-disturbance sediment supply if the disturbance is not sustained ŽSchumm and Rea, 1995.. The impact of a disturbance on any cell within a grid representing the catchment can be expressed by the exponential decay function: SY s meynT q k where m, n, k are constants and T s time Žyears.. A sediment delivery term can be included such that the sediment exported from each cell that reaches the basin outlet is a function of the travel distance and represented by, SYiX s SD p SYiX s sediment
where yield from cell i that is exported to basin outlet; SD s sediment delivery ratio per cell length; p s travel distance in cell lengths.
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Values for the constants were chosen to represent a fivefold increase in soil erosion following disturbance which declined to 50% after 3 years and 10% after 10 years Ž m s 4, n s 0.25, k s 1.. The catchment is represented by 100 grid cells. Sediment yields are in arbitrary units where the pre-disturbance conditions would produce an output of 100 units if the sediment delivery ratio was 1. Values for sediment delivery per cell length, rate and direction of disturbance were varied in the simulation. For most simulations, sediment yield increases over the time period Ž20 years. but the rate of increase began to slow after 5–10 years ŽFig. 3.. Thus, the extent of erosion within the catchment has increased but the effect on the magnitude of sediment redistribution is less marked. The direction of disturbance is important because a larger proportion of sediment contributes to basin yield when cells close to the basin outlet are affected. When disturbance progresses from the divide towards the catchment outlet, new sediment sources are created close to the outlet and sediment loads continue to increase. However, in the case of upstream propagation of disturbance Ži.e., from the outlet towards the divide. sediment yield increases slow down quickly. As the direction of land use change in China has tended to spread up-basin both in terms of disturbance by extension of agricultural land and by deforestation, the results of the AupstreamB simulation require field validation. If the disturbance has acted as a pulse that has migrated across the Upper Yangtze basin in a general westward and upstream direction, the impact of the visible increase in land degradation on sediment load may be less dramatic. The simulation model does not allow for any variation in erosion sensitivity in any of the cells, nor distinguish between grades of disturbance. Similarly, the parameter values describing recovery rate are chosen arbitrarily for demonstration purpose. Work is now in progress to reconstruct the spatio-temporal pattern of agricultural extensification and deforestation across central and southern China to test the model with field investigations in the zones of most recent disturbance. 4.3. Sediment deliÕery response to enÕironmental Õariables Having examined sediment delivery response to catchment disturbance, multivariate techniques can
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D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
be employed to examine the extent to which spatial variation in sediment yields can be explained by catchment controls. The 62 sub-catchments with longer-term records were analysed. The interrelationships between catchment variables were examined using Spearman’s rank correlation. At sub-catchment level, mean elevation is significantly correlated with many variables. Mean slope increases with mean elevation whereas population density and precipitation decrease. Specific sediment yields are significantly correlated with only two variables—mean elevation and runoff. Elevation appears to act as a surrogate for aridity and human impact, constrained by the geography of the catchment. There is wide-
spread interest in attempting to quantify human impact on sediment yields, but it is difficult to disentangle the effects from the natural background, particularly where anthropogenic variables such as population density and land use are likely to be strongly correlated with elevation and precipitation. The high degree of spatial variability in sediment yields and catchment characteristics causes difficulty when attempting to model the controlling relationships across the whole data set. Previous studies of global sediment yield variation have used the strategy of grouping data into suitable categories to reduce scatter ŽJansson, 1988; Summerfield and Hulton, 1994; Ludwig and Probst, 1996.. Based on
Fig. 2. ŽA. Relatively large gauging stations showing a trend in sediment yield across the measurement period. ŽB. Impact of large dams on downstream sediment conveyance indicated by measurements from nearest station above and below impoundments Žbased on Lu and Higgitt, 1998..
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
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Fig. 2 Ž continued ..
tributary groupings clear patterns in sediment yields become evident in the groupings for the Jinsha–Yalong and Dadu–Min catchments. Multiple regression
indicates that 87% and 95%, respectively, of the scatter in sediment yields in these tributaries is explained 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 year Župper panel. and 4% per year Žlower panel. for sediment delivery ratios Žper cell length. of 0.99, 0.98 and 0.95 Žbased on Higgitt and Lu, in press.. The direction of disturbance is upstream Žaway from outlet., downstream Žaway from watershed..
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D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
tation, mean elevation, basin relief, mean slope and population density ŽLu and Higgitt, 1999.. In contrast only 33% of the variance is explained in the Jialing and it is here that soil erosion rates are highest and the control of sediment delivery most critical. Interestingly, there are notable significant correlations between sediment yield and individual
catchment variables within the Jialing sub-catchment data set including an apparent inverse relationship with population density. This probably reflects the fact that the highest population densities are encountered within the flatter Chengdu Plain. The relationships between sediment yields and mean annual precipitation or mean annual runoff are noteworthy ŽFig.
Fig. 4. Sediment yield–precipitation and sediment yield–runoff relationships for data grouped by catchment size, elevation and tributary Žbased on Lu and Higgitt, 1999..
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
153
Fig. 4 Ž continued ..
4.. Many of the earlier studies to examine global sediment yields have attempted to reveal the nature of the relationship. Grouping strategies reveal contrasting fits to the data. Linear or near linear relationships can be fitted to data from the Jinsha–Yalong and Dadu–Min tributaries, but a polynomial fit describes the relationship in the Jialing tributary. Similarly, linear or near-linear relationships can be fitted to data for sub-catchments with maximum elevation ) 5000 m and those with catchment area ) 100,000
km2 . In the catchment size category 10,000–100,000 km2 , the relationship between sediment yield and precipitation is inverse, while a polynomial fits the sediment yield–runoff data. In this case and the same relationship for the Jialing data set, a minimum turning point occurs at annual runoff of 500 mm. Higher sediment yields at lower values of runoff is consistent with the Langbein and Schumm Ž1958. model, while the increase beyond 700 mm runoff is a feature that has been described previously ŽWilson,
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
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1973; Walling and Webb, 1983.. Using data from 700 rivers in China, Xu Ž1994. reported a peak sediment yield at 400 mm runoff. The analysis here suggests that any relationship is not consistent and in many cases can be explained by the geography of the Upper Yangtze. For example, the headwaters of the Jialing contain some loess areas which have high sediment yields but also have low runoff. Multiple regression results suggest that much of the variability in sediment yields in the west of the Upper Yangtze catchment can be explained in terms of AnaturalB catchment characteristics, but these provide a relatively poor explanation of sediment yields in the more populated east of the catchment. Land cover information has been obtained from the China land cover database, a part of the global land cover project of Earth Resources Observation System ŽEROS. Data Center. The database is derived from 1 = 1 km Advanced Very High Resolution Radiometer ŽAVHRR. data. The 157 different land cover types have been regrouped into 8 land cover classes ŽTable 3.. Agricultural land is represented in three classes—paddy field, high-density cropland Ž) 50%. and low density cropland Ž- 50%.. Broadly, the high density cropland and the paddy field land cover dominate in the Sichuan Basin and its margins, while the lower density of cropland is situated in valleys further west. The relationship between specific sediment yield and any measure of agricultural land for the 62 sub-catchments is not significant. As the highest density of agricultural land tends toward the less steep land, an agriculture
slope index was devised to account for the coincidence of agricultural land and slopes at pixel level within each of the sub-catchment boundaries ŽTable 1.. Although this provides a statistically significant relationship, the degree of explanation remains poor. Incorporating land cover into the multiple regression equations also fails to improve the degree of explanation. Employing factor analysis as an alternative to multiple regression reveals some clustering of subcatchments of similar sediment yields within factor space ŽHiggitt and Lu, 1999.. It has been suggested that catchments with specific sediment yields of 500–1000 t kmy2 ay1 plot within factor space controlled by anthropogenic variables Žpopulation density and cropland land cover. but the areas supplying the highest sediment yields are well dispersed. One interpretation is that the incidence of the highest specific sediment yields Ž) 1000 t kmy2 ay1 . is attributed to particular local conditions. For example, conditions promoting very high sediment yields are the presence of loess sediments in the Upper Jialing, areas of frequent landslide activity in the incised valleys to the west of the Chengdu Plain, or specific land use changes in relatively small catchments. Clearly, the influence of land use on sediment yields within a large catchment is subtle and confounded by the interaction of other variables and scale effects. Obtaining reliable and sufficiently high resolution data to tease out the anthropogenic influences from the natural factors remains a challenge. The precise influence of land use remains somewhat inconclusive 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
v1
Unvegetated Žwater, desert, ice and snow. Alpine meadow or steppe Grassrshrub Shrubrwoodland Woodland Cropland Žmosaic- 50%. Cropland Žmosaic ) 50%. Paddy field
v2 v3 v4 v5 v6 v7 v8
% of ground cover in the Upper Yangtze
Number of categories grouped
Dominant location
3
17
High elevation
25 9 13 11 8 23 8
34 13 13 28 13 27 12
Qinghai–Xizang Plateau upper parts of western tributaries upper parts of western tributaries mid elevations of western tributaries valleys in western tributaries eastern tributaries Sichuan Basin
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
ronmental data sets will enable more sophisticated land use representation and associated modelling to be attempted.
5. Conclusion The paper has described some of the aspects of the temporal and spatial variation of sediment yields in the Upper Yangtze. The apparent paradox of increasing catchment erosion without increasing sediment export has been investigated. Certain subcatchments experience marked increases in sediment yield over time, but large tributary reservoirs succeed in restricting downstream conveyance. Thus, the sediment load delivered to the Three Gorges may reflect a balance between increased sediment production counteracted by increased storage in tributary impoundments. However, a hypothetical model of sediment delivery from catchments undergoing a wave of disturbance Žsimilar to the recent land use history of southern China. indicates that in some circumstances a near constant yield may be produced. This model needs to be tested against field evidence. Work is in progress to identify key points in the basin where sediment yields have been affected by recent land use changes for more detailed sediment budget investigation, using the spatial and temporal analysis of sediment loads in the Upper Yangtze. The study has demonstrated that novel approaches to using GIS techniques to extract spatially distributed data can be applied within large catchments. The high degree of scatter has been overcome to some extent by grouping strategies that allow variations in the nature of the relationship in different parts of the Upper Yangtze or at different scales to be exhibited. Ultimately, explanation of the variation of sediment yield in the eastern portion of the catchment remains relatively poor though individual outliers can often be attributed to special conditions. Future improvement in the resolution and availability of global environmental data sets may permit within catchment modelling to be advanced. The examination of spatial and temporal variability of sediment yields indicates the main source areas of sediment within the Upper Yangtze, the areas where sediment loads are apparently increasing and the extent to which the pattern can be explained by a
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combination of topographic, hydroclimatic, population and land use variables. The detailed analysis of sediment loads within a large basin has implications for the management of potential sedimentation and for policy development at the catchment scale. Critical areas for sediment control can be identified. One of these is the immediate land area surrounding the Three Gorges, where soils are susceptible to erosion and the resettlement of the displaced population may generate further land degradation. Consideration of the local component to sediment delivery to the Three Gorges is addressed in the second paper ŽLu and Higgitt, this volume..
Acknowledgements We gratefully acknowledge the contribution of Professor Chen Zhongyuan and Professor Avijit Gupta in organising the IAG Large Rivers Conference on the Yangtze River and editing the papers. Comments from Gordon E. Grant helped to improve the manuscript. Cartographers from the University of Durham and the National University of Singapore produced the diagrams.
References Chen, G.J., Gao, F.H., 1988. The ecology and environment of the Yangtze and the Three Gorges, and the Three Gorges project. The Leading Group of the Research Project of Chinese Academy of Sciences. The Effect of the Yangtze Three Gorges Project on Ecology and Environment and Countermeasures, Science Press, Beijing, pp. 1–15 Žin Chinese.. Douglas, I., 1967. Man, vegetation and the sediment yield of rivers. Nature 215, 925–928. Edmonds, R.L., 1992. The Sanxia ŽThree Gorges. Project: the environmental argument surrounding China’s super dam. Global Ecology and Biogeography Letters 2, 105–125. Edmonds, R.L., 1994. Patterns of China’s Lost Harmony: A Survey of the Country’s Environmental Degradation and Protection. Routledge, London. Gu, H.Y., Douglas, I., 1989. Spatial and temporal dynamics of land degradation and fluvial erosion in the middle and upper Yangtze River basin, China. Land Degradation and Rehabilitation 1, 217–235. Gu, H.Y., Ai, N.S., Ma, H.L., 1987. Sediment sources and trend of sedimentation in the Three Gorges reservoir area. Leading Group of the Three Gorges Project Ecology and Environment Research Project, Chinese Academy of Sciences. Collected
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Papers on Ecological and Environmental Impact of the Three Gorges Project and Countermeasures, Science Press, Beijing, pp. 522–541 Žin Chinese.. Higgitt, D.L., Lu, X.X., 1996. Patterns of sediment yield in the Upper Yangtze Basin, China. In: Walling, D.E., Webb, B.W. ŽEds.., Erosion and Sediment Yield: Global and Regional Perspectives ŽProceedings of the Exeter Symposium.. IAHS Publ., vol. 236. International Association of Hydrological Sciences, Wallingford, pp. 205–214. Higgitt, D.L., Lu, X.X., 1999. Challenges in relating land use to sediment yield in the Upper Yangtze. Hydrobiologia 410, 269–277. Higgitt, D.L., Lu, X.X., 2000. Determining variations in sediment yield in large river basins: an example of the Upper Yangtze. In: Stone, M. ŽEd.., The Role of Erosion and Sediment Transport in Nutrient and Contaminant Transfer ŽProceedings of the Waterloo Symposium, July 2000.. IAHS Publ., vol. 263. International Association of Hydrological Sciences, Wallingford, pp. 19–28. Jansen, J.M.L., Painter, R.B., 1974. Predicting sediment yield from climate and topography. Journal of Hydrology 21, 371– 380. Jansson, M.B., 1988. A global survey of sediment yield. Geografiska Annaler 70A, 81–98. Langbein, W.B., Schumm, S.A., 1958. Yield of sediment in relation to mean annual precipitation. Transactions of the American Geophysical Union 39, 1076–1084. Lu, X.X., Higgitt, D.L., 1998. Recent changes of sediment yield in the Upper Yangtze, China. Environmental Management 22, 697–709. Lu, X.X., Higgitt, D.L., 1999. Sediment yield variability in the Upper Yangtze, China. Earth Surface Processes and Landforms 24, 1077–1093. Lu, X.X., Higgitt, D.L., 2001. Sediment delivery to the Three Gorges: 2. Local response. Geomorphology 41 Ž2–3., 157– 169. Ludwig, W., Probst, J.L., 1998. River sediment discharge to the oceans: present-day controls and global budgets. American Journal of Science 298, 265–295. Luk, S.H., Whitney, J., 1993. Unresolved issues: perspectives
from China. In: Barber, M., Ryder, G. ŽEds.., Damming the Three Gorges. Earthscan, London, pp. 89–99. Mason, R., 1999. The three gorges dam of the Yangtze river, China: engineering geology in China. Geology Today 15, 30–33. Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphicrtectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. Journal of Geology 100, 525–544. Qian, N., Zhang, R., Chen, Z.C., 1993. Some aspects of sedimentation at the Three Gorges Project. In: Luk, S.-H., Whitney, J.B. ŽEds.., Megaproject A case Study of China’s Three Gorges Project. M.E. Sharpe, Armonk, pp. 121–160. Schumm, S.A., Rea, D.K., 1995. Sediment yield from disturbed earth systems. Geology 23, 391–394. Smil, V., 1993. China’s Environmental Crisis: An Inquiry Into the Limits of National Development. M.E. Sharpe, New York. Summerfield, M.A., Hulton, N.J., 1994. Natural controls of fluvial denudation rates in major world drainage basins. Journal of Geophysical Research 99 ŽB7., 13871–13883. Trimble, S.W., 1998. Decreased rates of alluvial sediment storage in the Coon Creek Basin, Wisconsin, 1975–93. Science 285, 1244–1246. Walling, D.E., 1983. The sediment delivery problem. Journal of Hydrology 65, 209–237. Walling, D.E., Webb, B.W., 1983. Patterns of sediment yield. In: Gregory, K.J. ŽEd.., Background to Palaeohydrology. Wiley, Chichester, pp. 61–100. Wen, D.Z., 1993. Soil erosion and conservation in China. In: Pimental, D. ŽEd.., World Soil Erosion and Conservation. Cambridge Univ. Press, Cambridge, pp. 63–85. Wilson, L., 1973. Variation in mean annual sediment yield as a function of mean annual precipitation. American Journal of Science 273, 335–349. Xu, J.X., 1994. Zonal distribution of river basin erosion and sediment yield in China. China Science Bulletin 39, 1356– 1361. Yu, J.R., Shi, L.R., Feng, M.H., Li, R.H., 1991. The surface erosion and fluvial silt in the upper reaches of Changjiang River. Bulletin of Soil and Water Conservation 11 Ž1., 9–17 Žin Chinese..
Sediment delivery to the three gorges: 1. Catchment controls D.L. Higgitt a,) , X.X. Lu b a
b
Department of Geography, UniÕersity of Durham, Science Laboratories, South Road, Durham, DH1 3LE, UK 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 Project ŽTGP.. 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 The potential impact of sedimentation on the operation and life span of the Three Gorges Reservoir )
Corresponding author. Fax: q44-191-374-2456. E-mail address: [email protected] ŽD.L. Higgitt..
is one of the key environmental issues that has focused attention on the dynamics of soil erosion and fluvial sediment transport in the catchment of the Upper Yangtze ŽGu and Douglas, 1989; Edmonds, 1992; Qian et al., 1993; Luk and Whitney, 1993.. There are two major issues of concern—whether proposed reservoir regulation procedures are effi-
0169-555Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 0 1 . 0 0 1 1 2 - X
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cient enough to control the conveyance of most of the sediment-laden waters, and whether the long-term observed sediment delivery to the Three Gorges is representative of future trends. Investigation of the temporal and spatial variability of sediment transport within the Upper Yangtze catchment leads to the following paradox. There is widespread evidence that the extent and magnitude of soil erosion across southern China has increased dramatically during the last 30–40 years ŽSmil, 1993; Wen, 1993; Edmonds, 1994.. There is no evidence from the sediment load measurements at the Yichang gauging station Ždownstream of the TGP dam site. of a trend in sediment yield delivered from the Upper Yangtze catchment. The average annual load Žfor a catchment area of just over 1 million km2 , is reported as 520 Mt ŽMason, 1999.. A question for catchment managers Žand geomorphologists. is whether the observed spatial and temporal variability of both sediment production and conveyance can be explained adequately by a model of sediment delivery for the Upper Yangtze. In attempting to address this issue, the appropriateness of methodologies and available data sets to analyse sediment delivery within large river catchments can be considered. The paper first provides a discussion on approaches to modelling regional sediment yield. Second, background information about the Upper Yangtze catchment and the construction of data sources for examining sediment yields is described. Third, results focusing on estimates of sediment delivery to the Three Gorges reservoir are presented, along with description of sediment responses to catchment disturbance and environmental variables. Evidence for recent changes in soil erosion and sediment delivery to the reservoir from the area surrounding the Three Gorges is addressed in the second paper ŽLu and Higgitt, this volume..
2. Regional sediment yields The geomorphological and hydrological literature contains many attempts to relate global or regional sediment yields to controlling factors. Langbein and Schumm Ž1958. produced a model of sediment yield in relation to effective precipitation, reaching a maximum in semi-arid environments and declining as vegetation cover protects the land surface. Xu Ž1994.
has proposed that the Langbein–Schumm model is broadly applicable to explain national variations within China. As the availability of gauging station data has increased a more complex relationship with climate and vegetation has emerged ŽDouglas, 1967; Wilson, 1973; Jansen and Painter, 1974; Walling and Webb, 1983; Jansson, 1988., mainly because of the impact of human activity on natural vegetation cover. At a global scale, the importance of topography and the significance of relatively small mountainous catchments as major contributors to global continental sediment export has been noted ŽMilliman and Syvitski, 1992; Summerfield and Hulton, 1994.. The difficulty of obtaining sufficiently detailed, spatially distributed data on catchment characteristics has hampered attempts to disentangle the various controls on sediment yields within large catchments. However, the availability of global environmental data sets comprising description of hydroclimatic, biological and geomorphological characteristics of the Earth offers a means of extracting catchment variables for integration with sediment yield data. This approach has been used to examine global variations in sediment yield ŽSummerfield and Hulton, 1994; Ludwig and Probst, 1998. where individual catchments are represented by a single sediment yield value. In the present study it is extended to the investigation of sediment yields within the Upper Yangtze. The prediction of sediment yields is complicated by the interaction of controlling variables, human impact on the hydrological system, and by scale effects associated with different catchment sizes. The proportion of sediment eroded from catchment slopes that is exported, decreases with catchment size in most environments as opportunities for storage increase downstream ŽWalling, 1983.. This proportion can be quantified by the sediment delivery ratio. The scale-dependency inhibits the direct comparison of specific sediment yields Žt kmy2 ay1 . from catchments of contrasting sizes. In addition, sediment delivery ratios may change over time, damping the response of suspended sediment loads to the magnitude and extent of soil erosion. Time lags between initial sediment mobilisation and export from the catchment outlet may be considerable ŽTrimble, 1998., and have significant effects on the management of sediment laden waters.
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
3. The Upper Yangtze: characteristics and data sources 3.1. Catchment characteristics The Upper Yangtze catchment, with the exception of the extreme west, experiences a subtropical monsoon climate. Precipitation varies from - 250 mm on the northern edge of the Qinghai–Xizang Plateau to ) 1000 mm in the east of the catchment. Population densities range from - 10 persons kmy2 in the mountainous west to ) 500 persons kmy2 in the Sichuan Basin. The Upper Jinsha, Yalong, Dadu and Min principally drain the mountainous areas to the west of the catchment. To the east, the Tuo, Fu, Jialing and Qu flow through areas of high population density and agricultural activity. The basin of the Wu, the only significant right bank tributary of the Yangtze, is largely agricultural but also drains the karst uplands of Guizhou Province ŽFig. 1.. The large-scale changes associated with land use practices and resource exploitation in rural China over the last 40 years are particularly significant for the five AeasternB tributaries noted above. Widespread deforestation and the extension of agricultural land have resulted in a large increase in the area reported as suffering from soil erosion. Land use inventories carried out in the 1950s and 1980s estimate that forest cover reduced form 19% to 12% in Sichuan and from 23% to 13% in Guizhou ŽYu et al., 1991.. Land affected by erosion increased from 16% to 67% in Sichuan and from 11% to 31% in Guizhou. Notwithstanding the qualitative nature of the inventories, an increase in the extent of land degradation is expected to show in an increased sediment load of the Upper Yangtze. However, this period also witnessed rapid development of local water conservancy projects comprising ponds, check dams, ditches and small headwater reservoirs, which have trapped and temporarily stored a proportion of the eroded sediment ŽLuk and Whitney, 1993.. Together with more recent large hydro electric power schemes it has been estimated that total reservoir capacity in the Upper Yangtze catchment upstream of the TGP exceeds 16 billion m3. 3.2. Data sources The sediment yield and runoff data were extracted from the China Hydrological Yearbooks, which sum-
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marise measurements from a network of hydrographic stations throughout the Upper Yangtze. The original records for each station provide information on the station co-ordinates Žlatitude and longitude., catchment area, mean monthly and annual water discharge and sediment load, and the magnitude and date of occurrence of the maximum daily discharge. The use of historical gauging station data raises the question of data quality. The monthly sediment loads are based on discrete rather than continuous measurements of suspended sediment at daily to weekly intervals. As such, the frequency of sampling does not ensure that all ranges of flow were sampled. The sampling intervals and the exclusion of the bedload component may result in underestimation of sediment discharge during peak flows. The annual measured sediment loads were recorded in units of 10 6 or 10 4 t, depending on the gauging station contributing area or a particular year. In transcribing the yearbook records from paper to a series of linked spreadsheets a number of errors were recognised and corrected by recalculating annual load from the monthly data ŽHiggitt and Lu, 1996.. Sediment load data were extracted from the records of 250 stations in the Upper Yangtze for the period from 1956 to 1987. Post-1987 records are not in the public domain. There are large variations in the spatial distribution, length and period of operation, and the catchment area of gauging stations, which have implications for statistical analysis. Approximately 25% of the gauging stations serve catchment areas - 1000 km2 , while 9% cover areas ) 100 000 km2 . About one quarter of stations have records lasting less than 5 years. Given the marked year to year variations in climatic conditions experienced within the catchment, care is needed in extracting data for comparison. There are 56 stations with records of 25 years or more. Together with six stations from the Wu tributary Žwhich is otherwise unrepresented., these 62 sub-catchments are used in more detailed analysis of sediment yield variability. Watershed boundaries were generated from the Asian 30 arcsecond Digital Elevation Model, supplemented by digitising 1:1,500,000 maps ŽLu and Higgitt, 1999.. The resolution of the DEM is approximately 1 = 1 km and catchment variables can be determined for each pixel and then calculated for the defined sub-catchment boundaries. From the elevation data,
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Fig. 1. The Upper Yangtze catchment: ŽA. Principal tributaries with location of major dams and gauging stations mentioned in text. The location of stations showing significant trends in sediment yield Žbased on Lu and Higgitt, 1998. is also indicated; ŽB. Standardised residuals of sediment yield contribution, identifying relative sediment contribution within the catchment Žbased on Higgitt and Lu, 1996..
indices of slope and relief can be generated using a GIS utility. Population density, precipitation and land cover classification classes are derived from global databases that are available in the public domain. With the exception of the precipitation data Žfrom the Global Ecosystem Database on CD-ROM., all data sets have been extracted freely from the Internet. Again, the time-bound nature of historical sedi-
ment load data has some implications for analysis. For example, population density and land cover is derived from 1992 to three databases which are non-synchronous with the sediment load data. Demographic and land use change will have been marked within the time period used for averaging sediment loads. The resolution of the environmental data on a 1 = 1 km grid also poses some difficulty. Topo-
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147
Table 1 Definition, derivation and source of catchment variables used in multivariate analysis Variable
Definition
Source
Specific Sediment Yield ŽSY. Mean Elevation ŽME.
Annual sediment load per unit area wt kmy2 ay1 x Mean of elevation of all cells within delineated catchment boundary wmx Catchment above gauging station wkm2 x
China Hydrological Yearbooks 30 arcsecond DEM Žpixel size approx. 1 = 1 km, height accuracy "30 m. China Hydrological Yearbooks—boundaries derived from DEM or digitised from various map sources ArcrInfo utility
ŽSub-. Catchment Area ŽDA. Basin Length ŽBL. Basin Relief ŽBR. Relative Relief ŽRR. Mean Slope ŽMS. Agricultural Slope Index ŽSI. Population density ŽPD. Precipitation ŽPP. Runoff ŽRO. Land Cover Class V1-8
Maximum straight line distance between watershed and gauging station wkmx Difference between maximum and minimum cell elevation within catchment boundary wmx Basin ReliefrBasin Length wm kmy1 x Mean of slope of all cells within delineated boundary w8x Function of mean slope and presence of agricultural land Number of persons per km2 Mean annual precipitation from 0.5 by 0.58 resolution wmmx Mean annual runoff wmmx Regrouped land cover classifications Žsee Table 3.
graphic indices such as mean slope may have limited application in terms of predicting soil erosion potential and land cover classification is problematic as Chinese landscapes are frequently mosaics of varied land uses that are aggregated into one unit at the resolution of the classification system. Definitions and units for the extracted and derived variables are provided in Table 1.
4. Sediment delivery and its response in the Upper Yangtze 4.1. Sediment deliÕery to the Three Gorges Sediment delivered to the Three Gorges reservoir from upstream of the reservoir can be approximated by using data from three major stations: Beipei, Zhutou and Wulong ŽFig. 1A.. The mean annual sediment load delivered to the reservoir will be 497.4 Mt. The area above Zhutou station accounts for 64% of this total, but in terms of specific sediment yield, as noted in earlier studies ŽGu et al., 1987; Chen and Gao, 1988; Gu and Douglas, 1989., that the Jialing tributary is a major source of sediment. The mean
From statistical file after clipping DEM using delineated catchment boundary Derived index ArcrInfo slope grid generation Žmaximum elevation changes within nine neighbouring cells. ArcrInfo grid utility to match land cover and slope data Asian Population database, derived from 1992 census Global Ecosystems Database ŽVersion 1.0, 1992. China Hydrological Yearbooks China Land Cover Database, EROS Data Center Žbased on AVHRR data.
specific sediment yield of the Jialing at Wusheng is 928 t kmy2 ay1 . The relationship between catchment area and specific sediment yield makes it difficult to compare contributions of unit area sediment yield directly. As a first attempt to isolate scale effects from the data, Higgitt and Lu Ž1996. produced a regression equation, SY s 1126.2DAy0 .0826 , for the relationship between specific sediment yield ŽSY, t kmy2 ay1 . and sub-catchment area ŽDA, km2 . for the 187 stations with measurement records covering at least 5 years. The equation is used to estimate the relative contribution of sub-catchments. Spatial variability in relative contribution can then be defined by plotting the standardised residuals from the SY–DA regression ŽFig. 1B.. Locations with significantly higher sediment contributions occur in the headwaters of the Jialing and the western margins of the Chengdu Plain, and to a lesser extent across the eastern side of the Sichuan Basin into the Three Gorges area itself. Locations with significantly lower sediment contributions occur in the west and along the southern margins of the catchment in Yunnan and Guizhou. Table 2 lists the mean suspended sediment load ŽMt ay1 . for stations on the major tributaries. The
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Table 2 Relative contributions of the principal tributaries to the Upper Yangtze sediment load Žappended. River
Station
DA Žkm2 .
U Jinsha Yalong Min Dadub Jialing Fu Qu Wu Ýtributaries Main
Shigu Xiaodeshi Pengshan Shaping Wusheng Guodukou Xiaoheba Wulong
232,651 118,294 30,661 75,016 79,714 31,626 29,420 83,035 680,417 1,005,501
Yichang
"S.D. ŽMt ay1 .
Mean contribution Ž%.
Mean relative contributiona
Maximum relative contributiona
Minimum relative contributiona
21.2 29.4 10.3 32.2 74.0 19.9 19.1 32.4
9.8 13.5 4.8 10.5 44.8 10.9 16.6 12.3
8.89 12.11 4.58 13.62 31.96 8.40 8.34 14.42 100.0
0.28 0.68 0.91 1.10 2.54 1.63 1.75 1.20
0.41 0.93 1.11 1.37 3.45 2.29 2.32 1.78
0.20 0.41 0.66 0.90 1.90 0.54 1.07 0.67
527.2
98.8
Mean load ŽMt ay1 .
a
Mean relative contribution is the proportion of actual load divided by predicted load Žfrom SY–DA regression., averaged across gauging station measurement period. Maximum and minimum relative contribution are for individual years within this measurement period. b Gauging station relocated from Tunjianzhi ŽDA s 77,202 km2 . in 1967. Mean loads and contribution corrected for change in DA.
mean contribution of each tributary is given as a percentage of the sum of tributary loads. The relative contribution of each tributary is calculated by dividing the measured mean sediment load by that predicted from the SY–DA relationship assuming loads were spatially uniform across the catchment. The regression is based on data averaged across the measurement period, but relative contributions for individual years can also be calculated. Maximum and minimum contributions are also indicated in Table 2. The Jialing is the dominant contributor over the record period at 2–3 times the expected level, except during the late 1960s to late 1970s. Similar, but less marked, trends are observed in the behaviour of the Fu and Qu, tributaries of the Greater Jialing, the latter generally increasing contribution over time. The rivers draining the Sichuan Basin appear to carry about twice the sediment load that would be expected if sediment delivery was spatially uniform. The Upper Jinsha and Yalong are relatively less important and the Wu, Dadu and Min are close to the predicted output. 4.2. Sediment deliÕery response to catchment disturbance Yichang station has not measured any significant trend in sediment load despite the evidence for increased erosion in parts of the catchment ŽWen,
1993.. The long-term mean annual sediment load consequently has been used as the base for analysing the potential sedimentation problems of the Three Gorges Project ŽTGP.. The implication is that changes in upstream headwater sediment transport resulting from erosion are damped or lagged by the time that they reach the main channel. Two hypotheses can be posed: Ž1. That sediment supply has increased through soil erosion but that conveyance downstream has been reduced through reservoir sedimentation; Ž2. That the spatial and temporal dynamics of land use change have led to an increase in the extent of degraded land without increasing overall sediment delivery rates. The first hypothesis is tested by conducting a time series analysis on 187 stations. A statistically significant linear regression model of sediment yield against year of measurement Ž a s 0.05. could be fitted to 16 of the stations, of which 10 Ž7.9% of catchment area. displayed increasing trends and six Ž2.8%. displayed decreasing ones. The majority of sub-catchments are relatively small Ž4000 km2 . and the changing sediment yields may reflect land use alterations, but six gauging stations are located on major tributaries ŽFig. 2.. Of the three large stations with decreasing sediment trends, Bikou and Sanleiba are located on the Jialing River downstream of the Bikou Reservoir, while Denyenyan is on the Tuo River which now has the highest proportion of reservoir capacity to catchment area of any of
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
the Yangtze tributaries. Comparison of time series plots of stations above and below major reservoirs clearly demonstrates their impact on reducing sediment yield measurements immediately downstream ŽFig. 2.. The Dadu, Qu and Wu show increasing sediment yield from their basins. The Dadu and Wu have relatively low provision of reservoir capacity. For most of the Upper Yangtze, no evidence of higher sediment yields arising from widespread erosion is apparent. However, subtle variations within the catchment and the association of sites of decreasing yields with reservoir construction, indicate the need to consider the longer term implications of sediment delivery as storage sites are progressively filled. In addition to the analysis of annual variation in sediment loads, evidence exists that the tributaries of the Yalong, Dadu, Min, Qu and Wu, have witnessed significant increases in seasonal and daily sediment load over the last 40 years. The evidence for trends in sediment yield at individual gauging stations within the Upper Yangtze may provide some insight into the changing nature of sediment delivery. The second hypothesis concerns the way in which an expansion of erosion-inducing land use change impacts upon long-term sediment yield. The impact of catchment disturbance on sediment delivery over time can be simulated using a simple model ŽHiggitt and Lu, 2000.. In many earth surface systems, it has been observed that initial disturbance Žsuch as deforestation or extension of agricultural land. will generate a pulse of increased sediment yield before the system returns towards a pre-disturbance sediment supply if the disturbance is not sustained ŽSchumm and Rea, 1995.. The impact of a disturbance on any cell within a grid representing the catchment can be expressed by the exponential decay function: SY s meynT q k where m, n, k are constants and T s time Žyears.. A sediment delivery term can be included such that the sediment exported from each cell that reaches the basin outlet is a function of the travel distance and represented by, SYiX s SD p SYiX s sediment
where yield from cell i that is exported to basin outlet; SD s sediment delivery ratio per cell length; p s travel distance in cell lengths.
149
Values for the constants were chosen to represent a fivefold increase in soil erosion following disturbance which declined to 50% after 3 years and 10% after 10 years Ž m s 4, n s 0.25, k s 1.. The catchment is represented by 100 grid cells. Sediment yields are in arbitrary units where the pre-disturbance conditions would produce an output of 100 units if the sediment delivery ratio was 1. Values for sediment delivery per cell length, rate and direction of disturbance were varied in the simulation. For most simulations, sediment yield increases over the time period Ž20 years. but the rate of increase began to slow after 5–10 years ŽFig. 3.. Thus, the extent of erosion within the catchment has increased but the effect on the magnitude of sediment redistribution is less marked. The direction of disturbance is important because a larger proportion of sediment contributes to basin yield when cells close to the basin outlet are affected. When disturbance progresses from the divide towards the catchment outlet, new sediment sources are created close to the outlet and sediment loads continue to increase. However, in the case of upstream propagation of disturbance Ži.e., from the outlet towards the divide. sediment yield increases slow down quickly. As the direction of land use change in China has tended to spread up-basin both in terms of disturbance by extension of agricultural land and by deforestation, the results of the AupstreamB simulation require field validation. If the disturbance has acted as a pulse that has migrated across the Upper Yangtze basin in a general westward and upstream direction, the impact of the visible increase in land degradation on sediment load may be less dramatic. The simulation model does not allow for any variation in erosion sensitivity in any of the cells, nor distinguish between grades of disturbance. Similarly, the parameter values describing recovery rate are chosen arbitrarily for demonstration purpose. Work is now in progress to reconstruct the spatio-temporal pattern of agricultural extensification and deforestation across central and southern China to test the model with field investigations in the zones of most recent disturbance. 4.3. Sediment deliÕery response to enÕironmental Õariables Having examined sediment delivery response to catchment disturbance, multivariate techniques can
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be employed to examine the extent to which spatial variation in sediment yields can be explained by catchment controls. The 62 sub-catchments with longer-term records were analysed. The interrelationships between catchment variables were examined using Spearman’s rank correlation. At sub-catchment level, mean elevation is significantly correlated with many variables. Mean slope increases with mean elevation whereas population density and precipitation decrease. Specific sediment yields are significantly correlated with only two variables—mean elevation and runoff. Elevation appears to act as a surrogate for aridity and human impact, constrained by the geography of the catchment. There is wide-
spread interest in attempting to quantify human impact on sediment yields, but it is difficult to disentangle the effects from the natural background, particularly where anthropogenic variables such as population density and land use are likely to be strongly correlated with elevation and precipitation. The high degree of spatial variability in sediment yields and catchment characteristics causes difficulty when attempting to model the controlling relationships across the whole data set. Previous studies of global sediment yield variation have used the strategy of grouping data into suitable categories to reduce scatter ŽJansson, 1988; Summerfield and Hulton, 1994; Ludwig and Probst, 1996.. Based on
Fig. 2. ŽA. Relatively large gauging stations showing a trend in sediment yield across the measurement period. ŽB. Impact of large dams on downstream sediment conveyance indicated by measurements from nearest station above and below impoundments Žbased on Lu and Higgitt, 1998..
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
151
Fig. 2 Ž continued ..
tributary groupings clear patterns in sediment yields become evident in the groupings for the Jinsha–Yalong and Dadu–Min catchments. Multiple regression
indicates that 87% and 95%, respectively, of the scatter in sediment yields in these tributaries is explained 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 year Župper panel. and 4% per year Žlower panel. for sediment delivery ratios Žper cell length. of 0.99, 0.98 and 0.95 Žbased on Higgitt and Lu, in press.. The direction of disturbance is upstream Žaway from outlet., downstream Žaway from watershed..
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D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
tation, mean elevation, basin relief, mean slope and population density ŽLu and Higgitt, 1999.. In contrast only 33% of the variance is explained in the Jialing and it is here that soil erosion rates are highest and the control of sediment delivery most critical. Interestingly, there are notable significant correlations between sediment yield and individual
catchment variables within the Jialing sub-catchment data set including an apparent inverse relationship with population density. This probably reflects the fact that the highest population densities are encountered within the flatter Chengdu Plain. The relationships between sediment yields and mean annual precipitation or mean annual runoff are noteworthy ŽFig.
Fig. 4. Sediment yield–precipitation and sediment yield–runoff relationships for data grouped by catchment size, elevation and tributary Žbased on Lu and Higgitt, 1999..
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
153
Fig. 4 Ž continued ..
4.. Many of the earlier studies to examine global sediment yields have attempted to reveal the nature of the relationship. Grouping strategies reveal contrasting fits to the data. Linear or near linear relationships can be fitted to data from the Jinsha–Yalong and Dadu–Min tributaries, but a polynomial fit describes the relationship in the Jialing tributary. Similarly, linear or near-linear relationships can be fitted to data for sub-catchments with maximum elevation ) 5000 m and those with catchment area ) 100,000
km2 . In the catchment size category 10,000–100,000 km2 , the relationship between sediment yield and precipitation is inverse, while a polynomial fits the sediment yield–runoff data. In this case and the same relationship for the Jialing data set, a minimum turning point occurs at annual runoff of 500 mm. Higher sediment yields at lower values of runoff is consistent with the Langbein and Schumm Ž1958. model, while the increase beyond 700 mm runoff is a feature that has been described previously ŽWilson,
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
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1973; Walling and Webb, 1983.. Using data from 700 rivers in China, Xu Ž1994. reported a peak sediment yield at 400 mm runoff. The analysis here suggests that any relationship is not consistent and in many cases can be explained by the geography of the Upper Yangtze. For example, the headwaters of the Jialing contain some loess areas which have high sediment yields but also have low runoff. Multiple regression results suggest that much of the variability in sediment yields in the west of the Upper Yangtze catchment can be explained in terms of AnaturalB catchment characteristics, but these provide a relatively poor explanation of sediment yields in the more populated east of the catchment. Land cover information has been obtained from the China land cover database, a part of the global land cover project of Earth Resources Observation System ŽEROS. Data Center. The database is derived from 1 = 1 km Advanced Very High Resolution Radiometer ŽAVHRR. data. The 157 different land cover types have been regrouped into 8 land cover classes ŽTable 3.. Agricultural land is represented in three classes—paddy field, high-density cropland Ž) 50%. and low density cropland Ž- 50%.. Broadly, the high density cropland and the paddy field land cover dominate in the Sichuan Basin and its margins, while the lower density of cropland is situated in valleys further west. The relationship between specific sediment yield and any measure of agricultural land for the 62 sub-catchments is not significant. As the highest density of agricultural land tends toward the less steep land, an agriculture
slope index was devised to account for the coincidence of agricultural land and slopes at pixel level within each of the sub-catchment boundaries ŽTable 1.. Although this provides a statistically significant relationship, the degree of explanation remains poor. Incorporating land cover into the multiple regression equations also fails to improve the degree of explanation. Employing factor analysis as an alternative to multiple regression reveals some clustering of subcatchments of similar sediment yields within factor space ŽHiggitt and Lu, 1999.. It has been suggested that catchments with specific sediment yields of 500–1000 t kmy2 ay1 plot within factor space controlled by anthropogenic variables Žpopulation density and cropland land cover. but the areas supplying the highest sediment yields are well dispersed. One interpretation is that the incidence of the highest specific sediment yields Ž) 1000 t kmy2 ay1 . is attributed to particular local conditions. For example, conditions promoting very high sediment yields are the presence of loess sediments in the Upper Jialing, areas of frequent landslide activity in the incised valleys to the west of the Chengdu Plain, or specific land use changes in relatively small catchments. Clearly, the influence of land use on sediment yields within a large catchment is subtle and confounded by the interaction of other variables and scale effects. Obtaining reliable and sufficiently high resolution data to tease out the anthropogenic influences from the natural factors remains a challenge. The precise influence of land use remains somewhat inconclusive 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
v1
Unvegetated Žwater, desert, ice and snow. Alpine meadow or steppe Grassrshrub Shrubrwoodland Woodland Cropland Žmosaic- 50%. Cropland Žmosaic ) 50%. Paddy field
v2 v3 v4 v5 v6 v7 v8
% of ground cover in the Upper Yangtze
Number of categories grouped
Dominant location
3
17
High elevation
25 9 13 11 8 23 8
34 13 13 28 13 27 12
Qinghai–Xizang Plateau upper parts of western tributaries upper parts of western tributaries mid elevations of western tributaries valleys in western tributaries eastern tributaries Sichuan Basin
D.L. Higgitt, X.X. Lu r Geomorphology 41 (2001) 143–156
ronmental data sets will enable more sophisticated land use representation and associated modelling to be attempted.
5. Conclusion The paper has described some of the aspects of the temporal and spatial variation of sediment yields in the Upper Yangtze. The apparent paradox of increasing catchment erosion without increasing sediment export has been investigated. Certain subcatchments experience marked increases in sediment yield over time, but large tributary reservoirs succeed in restricting downstream conveyance. Thus, the sediment load delivered to the Three Gorges may reflect a balance between increased sediment production counteracted by increased storage in tributary impoundments. However, a hypothetical model of sediment delivery from catchments undergoing a wave of disturbance Žsimilar to the recent land use history of southern China. indicates that in some circumstances a near constant yield may be produced. This model needs to be tested against field evidence. Work is in progress to identify key points in the basin where sediment yields have been affected by recent land use changes for more detailed sediment budget investigation, using the spatial and temporal analysis of sediment loads in the Upper Yangtze. The study has demonstrated that novel approaches to using GIS techniques to extract spatially distributed data can be applied within large catchments. The high degree of scatter has been overcome to some extent by grouping strategies that allow variations in the nature of the relationship in different parts of the Upper Yangtze or at different scales to be exhibited. Ultimately, explanation of the variation of sediment yield in the eastern portion of the catchment remains relatively poor though individual outliers can often be attributed to special conditions. Future improvement in the resolution and availability of global environmental data sets may permit within catchment modelling to be advanced. The examination of spatial and temporal variability of sediment yields indicates the main source areas of sediment within the Upper Yangtze, the areas where sediment loads are apparently increasing and the extent to which the pattern can be explained by a
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combination of topographic, hydroclimatic, population and land use variables. The detailed analysis of sediment loads within a large basin has implications for the management of potential sedimentation and for policy development at the catchment scale. Critical areas for sediment control can be identified. One of these is the immediate land area surrounding the Three Gorges, where soils are susceptible to erosion and the resettlement of the displaced population may generate further land degradation. Consideration of the local component to sediment delivery to the Three Gorges is addressed in the second paper ŽLu and Higgitt, this volume..
Acknowledgements We gratefully acknowledge the contribution of Professor Chen Zhongyuan and Professor Avijit Gupta in organising the IAG Large Rivers Conference on the Yangtze River and editing the papers. Comments from Gordon E. Grant helped to improve the manuscript. Cartographers from the University of Durham and the National University of Singapore produced the diagrams.
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