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Response of land accretion of the Yellow River delta to global climate change and human activity
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Quaternary International 186 (2008) 4–11
Response of land accretion of the Yellow River delta to global climate change and human activity Jiongxin Xu Key Laboratory for Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Available online 4 September 2007
Abstract In the past 50 years, influenced by global climate change, the Eastern Asian summer monsoon intensity (SMI) changed significantly, and that has led to some response in the water cycle system of the Yellow River basin and in the land accretion process of the delta. The variation in annual precipitation is synchronic with that in SMI. From 1950 to 1970, annual air temperature showed a slight decrease with large fluctuations. Since 1970, a significant increasing trend can be seen. Climate change may result in a change in sediment flux into the sea, and therefore in a change in the rate of land accretion of delta (Rla). The annual Rla and sediment flux into the sea showed an increasing trend from 1952 to 1964, but a decreasing trend after 1964, which is similar to that in the SMI. Human activity such as soil conservation measures and water division also has some effect on land accretion of the delta. A multiple regression analysis indicates that the Ral decreased with decrease in summer monsoon index (SMI), increase in annual temperature (T), the increase in the area of water and soil conservation measures (Atfg) and an increase in water diversion (Qw,div). The contribution of the variations in the variables to the variation in Rla was estimated as 34.94%, 3.80%, 53.82% and 7.44%, respectively. The contribution of the two climate factors totals 38.7%, indicating that the influence of global climate change on the variation in land accretion of Yellow River delta is significant. r 2007 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Land–ocean interaction influenced by global climate change and human activity is an important issue in global climate change study (Holigon and Boois, 1993). Global climate change is referred to as the climate change at global scale induced by greenhouse warming, which involves the changes in temperature and precipitation as well as vegetation. As the runoff- and sediment-producing processes of large rivers are closely related with the condition of precipitation, temperature and vegetation, global climate change may result in a change in water and sediment fluxes to the sea, and therefore a change in land accretion of large rivers’ delta. As a heavily sediment laden large river emptying into a sallow inner sea, land accretion of the Yellow River delta is predominated by flow and sediment from the river, and the latter have been greatly changed due to climate change and human activity in the past 50 years. Thus, the Yellow River Tel.: +86 10 64889333; fax: +86 10 64851844.
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delta provides an ideal area for a study of land accretion of delta in response to changing natural and human factors. Much research has been done with the Yellow River and its delta (Chien et al., 1987; Ye, 1994), and many results have been achieved, involving water and sediment fluxes to the sea and the temporal variation (Zeng et al., 1997; Xu, 2003, 2005), sediment transport, deposition and erosion in the estuary (Zhang, 1999), the formation and change of the mouth bar (Li, 1993; Ji and Hu, 1995), and the evolution of the delta and the variation in land accretion of the delta (Ji et al., 1994; Cao, 1997; Xu, 2002, 2006). However, so far little attention has been paid to the response of land accretion of the Yellow River delta to global climate change and changing human activity.
2. Outlines of study area The Yellow River, draining an area of 730,000 km2, crosses ‘‘the three major stairs’’ of the macroscopic landform structure of China, i.e., the Qinghai-Tibet
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Plateau, the Loess Plateau and the North China Plain, and empties into the Pacific Ocean. The whole drainage basin is controlled by the monsoon circulation system, and thus climate change is closely related with the variation in monsoon. In physical geography, the Yellow River basin is located in the transitional zone from semi-arid to subhumid climates and mantled by thick loess, and is regarded as an ecologically fragile area sensitive to disturbances by humans. The Yellow River (Fig. 1) is one of the most heavily sediment-laden rivers of the world. Annually, 1.32 billion tons of sediment were transported into the sea during the period 1950–1960, according to the hydrometric data at Lijin station, the end control of Yellow River. Downstream of the delta, the Bohai Sea is shallow with
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weak tidal and wave activity. The land accretion processes are dominated by the river’s sediment and runoff, rather than by the marine dynamics. Hence, the Yellow River delta provides an ideal site to study interaction of natural and human-induced processes. Fig. 2 shows the Yellow River delta. Since the river changed the route of its lower reaches to the Bohai Sea in 1855, a vast delta that extends for 5400 km2 has been formed. The delta has a fan shape resulting from a periodic swing of the mouth channel (Fig. 2). Between 1855 and the present day, the mouth channel has migrated nine times on a large scale. Around each mouth channel a deltaic lobe was constructed. The avulsion points or the apices of the deltaic lobes were located near Ninghai before 1934 and near Yuwa afterwards. The mouth channels have an average lifetime of about 10 years (Zeng et al., 1997; Shi and Zhang, 2003). Since 1950, shifting of the mouth channel occurred three times in 1953, 1964 and 1976, and the mouth channels that formed following each swing are known as Shenxiankou, Diaokouhe and Qingshuigou, respectively, and numbered 9, 10 and 11 in Fig. 2. 3. Data source and method 3.1. Land accretion of delta
Fig. 1. Yellow River basin.
In the present study, the net increase of the area of the delta per year is adopted for land accretion rate of the delta
Fig. 2. Yellow River delta (after Shi and Zhang, 2003).
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(Rla, km2/a). Here the net increase means the difference between the land gain by deposition and the land loss by erosion. The 0 m isobath is taken as the lower boundary of the delta; the area between the boundary lines of the delta in two periods is the land gain or land loss area, and the sum of land gain and land loss is the net land accretion area. The measurement of the area is carried out by using the sea charts surveyed in the river mouth and the neighboring sea area. If the time interval between the two maps is longer or shorter than 1 year, the area is distributed among all months involved by considering the monthly sediment load into the sea as a weight, and the annual land accretion area for each complete year can be obtained. The land accretion area in the period 1955–1980 was calculated following the above procedures by Wang (1985), and used in the present study. Data for the period 1981–1996 were measured from the remote sensing images of the Yellow River delta. Similarly, if the time interval between two images is longer or less than 1 year, the land accretion area is distributed to each month of the period, and the area for each complete year can be obtained. 3.2. Index of global climate change In the study of global climate change, temperature and precipitation are two commonly used indices. The climate in the Yellow River basin is under the control of the continental monsoon, and the summer monsoon provides the majority of vapor source for rainfall, which concentrated in summer. When the summer monsoon is strong, the vapor from the ocean can be transported deep in the Yellow River drainage basin, the upper and middle drainage basin may get abundant vapor to form rain, and the runoff and the water flux to the sea may increase. In contrast, when the summer monsoon is weak, the transport of vapor cannot reach the inland area of China, and the rain band remains in the Yangtze and Huaihe River basins, resulting in more rainfall in south China and less in the north. In these years, both the Yellow River’s runoff and water flux to the sea are at low levels. The Eastern Asian summer monsoon is driven by the thermal difference and the resulting dynamic effect that are caused by the great difference between the Qinghai-Tibet Plateau and the Pacific and Indian Oceans. Since warming shows some regional differences, the thermal difference and the resulting dynamic effect may vary inter-annually, and thus the intensity of the summer monsoon also varies. In this consideration, the change in the intensity of the summer monsoon may be regarded as a reflection of global climate change at regional scales. Hence, besides temperature, the intensity of summer monsoon is also used in the present study for expressing global climate change. To describe the Eastern Asian Monsoon, some indices have been proposed (Guo, 1990). Among these indices, the summer monsoon index (SMI) by Guo (1990) is based on the formative mechanism of the Eastern Asian Monsoon and defined in terms of the gradient of air pressure between
the Asian Continent and Pacific Ocean. The SMI is defined as the difference in the monthly average air pressure in July at the sea level between the continent and the ocean in the range of 10–501N. The procedures for the calculation are as follows: (1) For a given year, along a given latitude line, calculating the difference between the monthly average air pressure of the continent and that of the ocean. The former is represented by the longitude of 1101E, and the latter by the longitude of 1601E. The difference is expressed by the former minus the latter. (2) Calculate this difference for all latitudes in the range 10–501N. (3) Select the values of the above difference that are less than 5 102 Pa. (4) Calculate the sum of these values and then get the absolute value, the continent–ocean air pressure difference of the given year. (5) Do the above procedures for all the years studied and establish the time series. (6) Standardize the time series by dividing each value by the mean of all values. The final results are a time series of the SMI for the given period. Following the above procedures, the SMI for the period from 1951 to 2000 has been calculated in this study. Change in SMI may lead to a change in precipitation. There is a network of rain gauges in the Yellow River drainage basin, totaling more than 900 stations. The annual precipitation from 913 stations has been used to calculate the area-average annual precipitation over the whole drainage basin above Huayuankou hydrometric station in the period from 1950 to 1997. 3.3. Indices of human activity In the past 50 years, the most important two aspects of human activities in the Yellow River basin are basinwide erosion control practice and water diversion for irrigation and other human use. Affected by this, the runoff and sediment fluxes into the sea have significantly decreased. Soil erosion control measures can be classified as terrace building, and tree- and grass-planting. The effect of soil erosion control measures may be indexed by the area of the measurement. The Yellow River Water Conservancy Commission has detailed statistics for the area of all kinds of soil erosion control measures, and also year-to-year statistics for the quantity of water diverted from the Yellow River and its tributaries for irrigation, industrial and domestic uses. All these data are used in the present study, including the total area of terrace land building and treeand grass-planting (Atfg, 104 ha) and the quantity of ‘‘net’’ water diversion (Qw,div, 108 m3/yr), which is defined the annual quantity of water diverted from the Yellow River minus the annual quantity of the water returning to the river after human use. With the diversion of water,
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sediment is also diverted, which may decrease sediment fluxes to the sea. To express the effect of erosion control measures for each year, the cumulative area of the erosion control measures from 1950 to that year is adopted. The data for the cumulative areas of erosion control measures are available for 1959, 1969, 1979, 1989 and 1996; data for the years between are determined by linear interpolation. Based on the above data, time series analysis was conducted to reveal the trend in all variables, and then multiple regression was performed to elucidate the response of land accretion of the Yellow River delta to SMI and the indices of human activity. 4. Results and discussion
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change; (3) from 1965 to 2000, SMI remains at low levels and showed a slow decreasing trend. The inter-annual fluctuation was large, roughly with a period of 3–5 years. However, in an overall trend the variation in precipitation is roughly synchronic with that in SMI. From 1950 to the mid-1960s, the annual precipitation increased; afterwards, it decreased (Fig. 3a). Fig. 3b shows that from 1950 to1970, annual air temperature has a mild decreasing trend, but the interannual fluctuation is marked. However, afterwards, air temperature shows an increasing trend of fluctuations. Thus, the year 1970 may be regarded as a point with abrupt change in T, 7 years behind the point of abrupt change in SMI, which was roughly in 1963. 4.2. Response of the land accretion of the Yellow River delta to climate change
4.1. Temporal variation in summer monsoon index, air temperature and precipitation Fig. 3 shows the temporal variation in annual SMI, annual air temperature (T) and annual precipitation (P). For comparison, the temporal variation of SMI and P was plotted in the one ordinate, SMI and T in another. The variation in SMI shows three stages: (1) from 1951 to 1963, SMI increased; (2) from 1963 to 1965, SMI declined sharply, a feature that may be regarded as an abrupt
As an estuary with weak tidal activity, the land accretion of the Yellow River delta is mainly controlled by sediment flux from the river to the sea. A close positive correlation exists between the annual land accretion rate (Rla) of the delta and sediment flux to the sea represented by annual suspended load (Qs,Lijin) at Lijin station (Fig. 4a), and the
1.5
600
1.4 1.3
550
1.2
500
1.1
450
1 0.9
400
0.8 350
0.7
150 100 50 0 0 -50
1.3 18
1.2
17.5
1.1 1
17
0.9
16.5
0.8 16
Summer monsoon index
Annual temperature (˚C)
1.4
0.7
0.6 15.5 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 3. Comparison of the temporal variations between annual summer monsoon index and annual precipitation (a) and between annual summer monsoon index and annual precipitation (b). The thick lines are the 3-year moving averages.
Annual land accretion rate of the delta (km2/yr)
1.5
18.5
15
20
25
y= 5.482x - 8.477 r 2 = 0.7944 Sediment flux into the sea (10t8/yr)
Year
Annual temperature Summer monsoon index
10
-100
0.6 300 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
19
5
200 150
25 Land accretion rate Sediment flux into the sea
20
100 15 50 10 0 -50
5
Sediment flux into the sea (10t8/yr)
Annual temperature Summer monsoon index
Summer monsoon index
Annual precipitation (mm)
650
Annual land accretion rate of the delta (km2/yr)
200
0 -100 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 4. Relationship between land accretion rate of the delta and sediment flux into the sea (a) and temporal variations in these two variables (b). The thick lines are for 3-year moving average. Considering that the Sanmenxia Reservoir trapped huge quantities of sediment from 1960 to 1963, these 4 years are excluded from the Qs, Lijin data. The thick lines are the 3-year moving averages.
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temporal variations in Rla and Qs,Lijin are synchronous (Fig. 4b). The curves showing 3-year moving averages in Rla and Qs,Lijin are also given in Fig. 4b, which indicates that the Rla and Qs,Lijin increased to a peak in 1964, followed by a decline. Global climate change would result in a change in sediment flux to the sea, and the latter would in turn result in a change in land accretion of the delta. Fig. 5a shows the temporal variations in sediment flux (Qs, Lijin) to the sea and SMI in the Yellow River basin. The figure indicates that before 1964, Qs, Lijin increased mildly, the correlation coefficient between Qs, Lijin and time is 0.5229 (N ¼ 11, with the years 1960–1963 excluded because of water
1.5 Sediment flux into the sea Summermon soon index
20
1.4 1.3 1.2
15
1.1 1
10
0.9 0.8
5
Summer monsoon index
Sediment flux into the sea (108t/yr)
25
0.7 0.6 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
200
25 Landaccretion rate Sediment flux into the sea
150
20
100 15 50 10 0 5
-50
Sediment flux into the sea (102t / yr)
Annual land accretion rate of the delta (km2/yr)
Year
-100 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
200 150
Land accretion rate Annual temperature
18 17.5
100 17 50 16.5 0 16
-50
Annualtemperature(˚C)
Annual land accretion rate of the delta (km2/yr)
Year
-100 15.5 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 5. Comparisons between annual sediment flux into the sea and summer monsoon index (a); between land accretion rate of the delta and summer monsoon index (b); and between land accretion rate of the delta and air temperature (c).
storage and sediment trapping of the Sanmeanxia Reservoir), which is significant at a level of po0.10, but afterwards, it decreased, the correlation coefficient between Qs, Lijin and time is 0.5907 (N ¼ 33), which is significant at a level of po0.01. This trend is similar to that in SMI. Before 1964, SMI increased, the correlation coefficient between SMI and time is 0.7164 (N ¼ 14), which is significant at a level of po0.01, but afterwards, it decreased, the correlation coefficient between SMI and time is 0.5907 (N ¼ 36), which is significant at a level of po0.01. The calculation also shows that before 1964, land accretion rate of the delta (Rla) increased mildly, the correlation coefficient between Rla and time is 0.6173 (N ¼ 9), which is significant at a level of o0.08, but afterwards, it decreased, the correlation coefficient between Rla and time is 0.5345 (N ¼ 33), which is significant at a level of po0.01. Thus, it can be thought that the year 1964 was a turning point, at which the trend of both SMI and Qs, Lijin abruptly changed. This is because the decreased summer monsoon intensity after that year may have reduced the vapor flux to the upper and middle Yellow River basin, and thus precipitation decreased and therefore runoff also decreased. This may further result in a reduction in erosivity of rainfall and runoff, and the sediment carrying ability of the river may also be reduced. As a result, sediment flux to the sea decreased. This would further result in a decrease in land accretion rate of the delta, as demonstrated by Fig. 5b, where the temporal variations in Rla and SMI show some synchronous trend. The temporal variation in basin-averaged annual air temperature (T) is plotted in Fig. 5c. Before 1970, there is no trending variation in T. However, from 1970, the T shows an increasing trend, the correlation coefficient between T and time is 0.7453 (N ¼ 19), which is significant at a level of po0.01. In the meantime, the land accretion rate of the delta decreased. To reveal the influence of SMI and air temperature on land accretion rate of the delta, the Rla is plotted against SMI and T in Fig. 6. Although the points are scattered, the correlation coefficients between Rla and SMI and between Rla and T are 0.54 and 0.45, respectively. The critical correlation coefficient is 0.39 when the number of the samples N ¼ 41, and thus the correlation for both plots is significant at a level of o0.01. The cause for the positive correlation between Rla and SMI has been discussed above. The negative correlation between SMI and T may be explained by considering the effect of a higher air temperature on runoff generation. The increased temperature may result in stronger evapotranspiration, which may lower the level of soil moisture and reduce runoff generation and soil erosion intensity. Furthermore, the increased temperature may increase the water diversion for irrigation, which would reduce the river flow and water flux to the sea. All these factors may reduce sediment flux to the sea and the land accretion rate of the delta.
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Annual land accretion rate of the delta (km2/yr)
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200 150 100 50 0 y = 125.35x - 86.508 r = 0.5520
-50 -100
Annual land accretion rate of the delta (km2/yr)
0.6
0.81
1.2 Summer monsoon index
1.4
1.6
200 y = -29.454x + 536.01 r = -0.4511
150 100 50 0 -50 -100 15
15.5
16 16.5 17 17.5 Annual temperature (˚C)
18
18.5
19
1600 500 Net water diversion 450 Area of soil erosion control measures 1400 400 1200 350 1000 300 800 250 200 600 150 400 100 200 50 0 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Area of soil erosion congtrol measures (km2)
Net water diversion (108m3/yr)
Fig. 6. Plots of land accretion rate of the delta against annual summer monsoon index (a) and annual air temperature (b).
Fig. 7. Temporal variations in annual water division and the area of soil and water conservation measures.
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of population increase, and can also be related with the influence of climate change, namely, the decreased precipitation and increased temperature. The decreased precipitation cannot meet the water demand of crops, and the increased temperature enhances evapotranspiration, both leading to an increase in water diversion. The change in land accretion rate of the delta occurred as a result of both climate change and the change in human activity. How to differentiate the contributions of the two factors to the variation in Rla is an important issue needed to be addressed. For this purpose, multiple regression analysis is conducted. The data in the periods 1955–1959 and 1961–1996 are available, totaling 41 years. Table 1 shows the correlation matrix between land accretion rate of the delta (Ral) and four influencing factors, including SMI, annual mean air temperature in the Yellow River drainage basin (T), annual water diversion (Qw,div) and the area (Atfg) of land terracing and tree- and grass-planting. The critical correlation coefficients at 0.01 and 0.05 levels are 0.3932 and 0.3044, respectively. Thus, the correlation of Ral with SMI, Qw,div and Atfg is significant at a level of 0.01, and the correlation of Ral with T is significant at a level of 0.05. The multiple regression equation is established as follows: Rd ¼ 45:418 þ 54:915SMI 2:2948T 0:0288Atfg 0:02371Qw;div .
ð1Þ
The residual standard deviation of the regression line is S.E. ¼ 28.765, multiple correlation coefficient R ¼ 0.62, the result of F-test F ¼ 5.81. The multiple correlation coefficient is relatively low, but it is still significant at a level of p ¼ 0.001031. Some characteristics of the regression are given in Table 2. It can be seen from the above equation that the land accretion rate of the delta (Rla) increases with the SMI, but Table 1 Correlation coefficient matrix among all variables
SMI T Qw, div Atfg Ral
SMI
T
Qw,div
Atfg
Ral
1.00 0.28 0.68 0.66 0.54
0.28 1.00 0.62 0.54 0.35
0.68 0.62 1.00 0.75 0.60
0.66 0.54 0.75 1.00 0.51
0.54 0.35 0.60 0.51 1.00
4.3. Influence of human activity In the past 40 years, in addition to the climate change, human activity also has shown marked changes, of which water diversion for irrigation and domestic uses and largescale soil erosion control measures are the two main aspects. The temporal variations in net water diversion and the total area of land terracing and tree- and grass-planting are shown in Fig. 7, both indicating a marked increasing trend. The increase in water diversion occurred as a result
Table 2 Some characteristics of the multiple regression Sum of squares
Degree of freedom
Mean squares
F
p-level
Regression Residual
19227.16 29787.09
4 36
4806.79 827.41
5.809
0.001031
Total
49014.25
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decreases with the air temperature (T), the area of terrace land, tree- and grass-planting (Atfg), and the net water diversion (Qw,div). By using Eq. (1), the change of the land accretion rate of the delta in the future can be estimated, according to the planned increase in the areas of erosion control measures, the predicted increase in the water diversion and the estimated variation in precipitation and air temperature. The mechanism for the above equation can be explained as follows. The relatively strong summer monsoon means abundant vapor transported to the drainage basin and therefore more opportunity for rainstorms, and thus erosion may be strong. The increased runoff has higher sediment transporting ability and can carry more sediment to the sea, and thus, the land accretion rate of the delta becomes higher. The increased air temperature may enhance evapo-transpiration, reduce runoff, and therefore may reduce the sediment transporting ability and increase sediment deposition in river channels, leading to a decrease in sediment flux to the sea and the land accretion rate of the delta. Land terracing reduces slope steepness of hillslopes, enhances infiltration, and reduces surface runoff and soil erosion. The increased area of trees and grasses enhances the protection of vegetation on land surface, and also reduces soil erosion. Hence, the sediment flux to the sea decreases. The sediment flux to the sea during low-flow seasons is mainly produced by channel scour because of low sediment concentrations. Water diversion during lowflow seasons reduces river flow and also reduces channel scour. When water diversion occurs, some sediment is diverted with water, especially when the water diversion occurs during high-flow seasons when sediment concentration is high. This factor also reduces sediment flux to the sea and the land accretion rate of the delta. As the range of the absolute values of the variables differs from each other, the regression coefficients of the influencing variables do not reflect their contribution to the dependent variable. Therefore, the data have been standardized to the range of (0–1), and then the multiple regression equation is re-established as follows: Rd ¼ 0:2439SMI 0:02649T 0:3757Atfg 0:05195Qw;div . (2) By comparing the regression coefficients of the four independent variables, they can be ranked in terms of their contribution to the variation in the land accretion rate of the Yellow River delta. Assuming the total contribution of the four influencing variables to the variation in Ral is 1.0, contributions of SMI, T, Atfg and Qw, div were calculated as 34.94%, 3.80%, 53.82% and 7.44%, respectively. It has been further calculated that the contribution from two climate variables is 38.7%, and that from two variables of human activity is 61.3%, indicating that the contribution from climate change is less than that from human activity.
5. Conclusions The variation in SMI shows three stages: (1) from 1951 to 1963, SMI increased; (2) from 1963 to 1965, SMI declined sharply, a feature that may be regarded as an abrupt change; (3) from 1965 to 2000, SMI remained at low levels and showed a decreasing trend. Climate change may result is a change in sediment flux into the sea, and therefore a change in the rate of land accretion of delta (Rla). The annual Ral and sediment flux into the sea tended to increase from 1952 to 1964, but decreased after 1964, a trend similar to that in the SMI. Apart from climate factor, human activity such as water and soil conservation measures and water division in the drainage basin also has some effect on land accretion of the Yellow River delta. A multiple regression equation has been established, which indicates that the Rla decreased with decrease in SMI, increase in annual temperature (T), increase in the area of water and soil conservation measures (Atfg) and increase in water diversion (Qw,div). The contributions of the variations of the above influencing variables to the variation in Rla were calculated as 34.94%, 3.80%, 53.82% and 7.44%, respectively. The contributions of the two climate factors sums to 38.7%, indicating that the influence of global climate change on the variation in land accretion of Yellow River delta is significant and should not be ignored. Acknowledgments Financial supports from the National Natural Science Foundation of China (40671019) and from the Third-phase Knowledge Innovation Program of the Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, are gratefully acknowledged. Thanks are expressed to the Water Conservancy Commission of the Yellow River, for access to the internally issued hydrometric data, the data of erosion control measures and the data of water diversion; to Professor Guo Qiyun, for providing the data of SMI. References Cao, W.H., 1997. Study on evolution of the Yellow River delta and its reactions. Journal of Sediment Research 4, 1–6. Chien, N., Zhang, R., Zhou, Z.D., 1987. River Channel Processes. Science Press, Beijing. Guo, Q.Y., 1990. A study of Eastern Asian summer monsoon. In: Zuo, D.K. (Ed.), Progress in Geography. Science Press, Beijing, pp. 63–67 (in Chinese). Holigon, P.M., Boois, H., 1993. Land-ocean interactions in the coastal zone (LOICZ) science plan [R]. Global Change Report No. 25. IGBP Secretariat, Stockholm, pp. 1–150. Ji, Z.W., Hu, C.H., 1995. Analysis on recent evolution for mouth bar of the Yellow River. Journal of Sediment Research 3, 1–10. Ji, Z.W., Hu, C.H., Zeng, Q.H., Yan, Y., 1994. Analysis of recent evolution of the Yellow River Estuary by landsat image. Journal of Sediment Research 3, 12–22.
ARTICLE IN PRESS J. Xu / Quaternary International 186 (2008) 4–11 Li, Z.G., 1993. Mechanism of the mouth bar formation in the Yellow River estuary. Acta Oceanologica Sinica 15 (1), 84–91. Shi, C.X., Zhang, D., 2003. Processes and mechanisms of dynamic channel adjustment to delta progradation: the case of the mouth channel of the Yellow River, China. Earth Surface Processes and Landforms 28, 609–624. Wang, K.C., 1985. The prediction of the tendency of Yellow River mouth evolution. Research Report of the Yellow River Institute of Hydraulic Engineering, No. 8529 (in Chinese). Xu, J.X., 2002. A study of thresholds of runoff and sediment for the land accretion of the Yellow River Delta. Geographical Research 21, 163–170 (in Chinese). Xu, J., 2003. Sediment flux into the sea as influenced by the changing human activities and precipitation: example of the Yellow River, China. Environmental Management 31, 328–341.
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Xu, J., 2005. The water fluxes of the Yellow River to the sea in the past 50 years, in response to climate change and human activities. Environmental Management 35, 620–631. Xu, J.X., 2006. Land-accretion of the Yellow River Delta influenced by drainage basin factors. Geografisca Annaler 88A (1), 31–42. Ye, Q.C., 1994. Researches on Environmental Changes of the Yellow River Basin and Laws of Water and Sediment Transportation. Shandong Science and Technology Press, Jinan (in Chinese). Zeng, Q.H., Zhang, S.Q., Hu, C.H., Yin, X.L., Li, Z.G., Jiao, Y.L., 1997. Evolution of the Yellow River Mouth. Publishing House for Yellow River Water Conservancy, Zhengzhou (in Chinese). Zhang, S.Q., 1999. One-D and two-D combined model for estuary sedimentation. International Journal of Sediment Research 14 (1), 37–45 (in Chinese).
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Response of land accretion of the Yellow River delta to global climate change and human activity Jiongxin Xu Key Laboratory for Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China Available online 4 September 2007
Abstract In the past 50 years, influenced by global climate change, the Eastern Asian summer monsoon intensity (SMI) changed significantly, and that has led to some response in the water cycle system of the Yellow River basin and in the land accretion process of the delta. The variation in annual precipitation is synchronic with that in SMI. From 1950 to 1970, annual air temperature showed a slight decrease with large fluctuations. Since 1970, a significant increasing trend can be seen. Climate change may result in a change in sediment flux into the sea, and therefore in a change in the rate of land accretion of delta (Rla). The annual Rla and sediment flux into the sea showed an increasing trend from 1952 to 1964, but a decreasing trend after 1964, which is similar to that in the SMI. Human activity such as soil conservation measures and water division also has some effect on land accretion of the delta. A multiple regression analysis indicates that the Ral decreased with decrease in summer monsoon index (SMI), increase in annual temperature (T), the increase in the area of water and soil conservation measures (Atfg) and an increase in water diversion (Qw,div). The contribution of the variations in the variables to the variation in Rla was estimated as 34.94%, 3.80%, 53.82% and 7.44%, respectively. The contribution of the two climate factors totals 38.7%, indicating that the influence of global climate change on the variation in land accretion of Yellow River delta is significant. r 2007 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction Land–ocean interaction influenced by global climate change and human activity is an important issue in global climate change study (Holigon and Boois, 1993). Global climate change is referred to as the climate change at global scale induced by greenhouse warming, which involves the changes in temperature and precipitation as well as vegetation. As the runoff- and sediment-producing processes of large rivers are closely related with the condition of precipitation, temperature and vegetation, global climate change may result in a change in water and sediment fluxes to the sea, and therefore a change in land accretion of large rivers’ delta. As a heavily sediment laden large river emptying into a sallow inner sea, land accretion of the Yellow River delta is predominated by flow and sediment from the river, and the latter have been greatly changed due to climate change and human activity in the past 50 years. Thus, the Yellow River Tel.: +86 10 64889333; fax: +86 10 64851844.
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delta provides an ideal area for a study of land accretion of delta in response to changing natural and human factors. Much research has been done with the Yellow River and its delta (Chien et al., 1987; Ye, 1994), and many results have been achieved, involving water and sediment fluxes to the sea and the temporal variation (Zeng et al., 1997; Xu, 2003, 2005), sediment transport, deposition and erosion in the estuary (Zhang, 1999), the formation and change of the mouth bar (Li, 1993; Ji and Hu, 1995), and the evolution of the delta and the variation in land accretion of the delta (Ji et al., 1994; Cao, 1997; Xu, 2002, 2006). However, so far little attention has been paid to the response of land accretion of the Yellow River delta to global climate change and changing human activity.
2. Outlines of study area The Yellow River, draining an area of 730,000 km2, crosses ‘‘the three major stairs’’ of the macroscopic landform structure of China, i.e., the Qinghai-Tibet
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Plateau, the Loess Plateau and the North China Plain, and empties into the Pacific Ocean. The whole drainage basin is controlled by the monsoon circulation system, and thus climate change is closely related with the variation in monsoon. In physical geography, the Yellow River basin is located in the transitional zone from semi-arid to subhumid climates and mantled by thick loess, and is regarded as an ecologically fragile area sensitive to disturbances by humans. The Yellow River (Fig. 1) is one of the most heavily sediment-laden rivers of the world. Annually, 1.32 billion tons of sediment were transported into the sea during the period 1950–1960, according to the hydrometric data at Lijin station, the end control of Yellow River. Downstream of the delta, the Bohai Sea is shallow with
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weak tidal and wave activity. The land accretion processes are dominated by the river’s sediment and runoff, rather than by the marine dynamics. Hence, the Yellow River delta provides an ideal site to study interaction of natural and human-induced processes. Fig. 2 shows the Yellow River delta. Since the river changed the route of its lower reaches to the Bohai Sea in 1855, a vast delta that extends for 5400 km2 has been formed. The delta has a fan shape resulting from a periodic swing of the mouth channel (Fig. 2). Between 1855 and the present day, the mouth channel has migrated nine times on a large scale. Around each mouth channel a deltaic lobe was constructed. The avulsion points or the apices of the deltaic lobes were located near Ninghai before 1934 and near Yuwa afterwards. The mouth channels have an average lifetime of about 10 years (Zeng et al., 1997; Shi and Zhang, 2003). Since 1950, shifting of the mouth channel occurred three times in 1953, 1964 and 1976, and the mouth channels that formed following each swing are known as Shenxiankou, Diaokouhe and Qingshuigou, respectively, and numbered 9, 10 and 11 in Fig. 2. 3. Data source and method 3.1. Land accretion of delta
Fig. 1. Yellow River basin.
In the present study, the net increase of the area of the delta per year is adopted for land accretion rate of the delta
Fig. 2. Yellow River delta (after Shi and Zhang, 2003).
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(Rla, km2/a). Here the net increase means the difference between the land gain by deposition and the land loss by erosion. The 0 m isobath is taken as the lower boundary of the delta; the area between the boundary lines of the delta in two periods is the land gain or land loss area, and the sum of land gain and land loss is the net land accretion area. The measurement of the area is carried out by using the sea charts surveyed in the river mouth and the neighboring sea area. If the time interval between the two maps is longer or shorter than 1 year, the area is distributed among all months involved by considering the monthly sediment load into the sea as a weight, and the annual land accretion area for each complete year can be obtained. The land accretion area in the period 1955–1980 was calculated following the above procedures by Wang (1985), and used in the present study. Data for the period 1981–1996 were measured from the remote sensing images of the Yellow River delta. Similarly, if the time interval between two images is longer or less than 1 year, the land accretion area is distributed to each month of the period, and the area for each complete year can be obtained. 3.2. Index of global climate change In the study of global climate change, temperature and precipitation are two commonly used indices. The climate in the Yellow River basin is under the control of the continental monsoon, and the summer monsoon provides the majority of vapor source for rainfall, which concentrated in summer. When the summer monsoon is strong, the vapor from the ocean can be transported deep in the Yellow River drainage basin, the upper and middle drainage basin may get abundant vapor to form rain, and the runoff and the water flux to the sea may increase. In contrast, when the summer monsoon is weak, the transport of vapor cannot reach the inland area of China, and the rain band remains in the Yangtze and Huaihe River basins, resulting in more rainfall in south China and less in the north. In these years, both the Yellow River’s runoff and water flux to the sea are at low levels. The Eastern Asian summer monsoon is driven by the thermal difference and the resulting dynamic effect that are caused by the great difference between the Qinghai-Tibet Plateau and the Pacific and Indian Oceans. Since warming shows some regional differences, the thermal difference and the resulting dynamic effect may vary inter-annually, and thus the intensity of the summer monsoon also varies. In this consideration, the change in the intensity of the summer monsoon may be regarded as a reflection of global climate change at regional scales. Hence, besides temperature, the intensity of summer monsoon is also used in the present study for expressing global climate change. To describe the Eastern Asian Monsoon, some indices have been proposed (Guo, 1990). Among these indices, the summer monsoon index (SMI) by Guo (1990) is based on the formative mechanism of the Eastern Asian Monsoon and defined in terms of the gradient of air pressure between
the Asian Continent and Pacific Ocean. The SMI is defined as the difference in the monthly average air pressure in July at the sea level between the continent and the ocean in the range of 10–501N. The procedures for the calculation are as follows: (1) For a given year, along a given latitude line, calculating the difference between the monthly average air pressure of the continent and that of the ocean. The former is represented by the longitude of 1101E, and the latter by the longitude of 1601E. The difference is expressed by the former minus the latter. (2) Calculate this difference for all latitudes in the range 10–501N. (3) Select the values of the above difference that are less than 5 102 Pa. (4) Calculate the sum of these values and then get the absolute value, the continent–ocean air pressure difference of the given year. (5) Do the above procedures for all the years studied and establish the time series. (6) Standardize the time series by dividing each value by the mean of all values. The final results are a time series of the SMI for the given period. Following the above procedures, the SMI for the period from 1951 to 2000 has been calculated in this study. Change in SMI may lead to a change in precipitation. There is a network of rain gauges in the Yellow River drainage basin, totaling more than 900 stations. The annual precipitation from 913 stations has been used to calculate the area-average annual precipitation over the whole drainage basin above Huayuankou hydrometric station in the period from 1950 to 1997. 3.3. Indices of human activity In the past 50 years, the most important two aspects of human activities in the Yellow River basin are basinwide erosion control practice and water diversion for irrigation and other human use. Affected by this, the runoff and sediment fluxes into the sea have significantly decreased. Soil erosion control measures can be classified as terrace building, and tree- and grass-planting. The effect of soil erosion control measures may be indexed by the area of the measurement. The Yellow River Water Conservancy Commission has detailed statistics for the area of all kinds of soil erosion control measures, and also year-to-year statistics for the quantity of water diverted from the Yellow River and its tributaries for irrigation, industrial and domestic uses. All these data are used in the present study, including the total area of terrace land building and treeand grass-planting (Atfg, 104 ha) and the quantity of ‘‘net’’ water diversion (Qw,div, 108 m3/yr), which is defined the annual quantity of water diverted from the Yellow River minus the annual quantity of the water returning to the river after human use. With the diversion of water,
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sediment is also diverted, which may decrease sediment fluxes to the sea. To express the effect of erosion control measures for each year, the cumulative area of the erosion control measures from 1950 to that year is adopted. The data for the cumulative areas of erosion control measures are available for 1959, 1969, 1979, 1989 and 1996; data for the years between are determined by linear interpolation. Based on the above data, time series analysis was conducted to reveal the trend in all variables, and then multiple regression was performed to elucidate the response of land accretion of the Yellow River delta to SMI and the indices of human activity. 4. Results and discussion
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change; (3) from 1965 to 2000, SMI remains at low levels and showed a slow decreasing trend. The inter-annual fluctuation was large, roughly with a period of 3–5 years. However, in an overall trend the variation in precipitation is roughly synchronic with that in SMI. From 1950 to the mid-1960s, the annual precipitation increased; afterwards, it decreased (Fig. 3a). Fig. 3b shows that from 1950 to1970, annual air temperature has a mild decreasing trend, but the interannual fluctuation is marked. However, afterwards, air temperature shows an increasing trend of fluctuations. Thus, the year 1970 may be regarded as a point with abrupt change in T, 7 years behind the point of abrupt change in SMI, which was roughly in 1963. 4.2. Response of the land accretion of the Yellow River delta to climate change
4.1. Temporal variation in summer monsoon index, air temperature and precipitation Fig. 3 shows the temporal variation in annual SMI, annual air temperature (T) and annual precipitation (P). For comparison, the temporal variation of SMI and P was plotted in the one ordinate, SMI and T in another. The variation in SMI shows three stages: (1) from 1951 to 1963, SMI increased; (2) from 1963 to 1965, SMI declined sharply, a feature that may be regarded as an abrupt
As an estuary with weak tidal activity, the land accretion of the Yellow River delta is mainly controlled by sediment flux from the river to the sea. A close positive correlation exists between the annual land accretion rate (Rla) of the delta and sediment flux to the sea represented by annual suspended load (Qs,Lijin) at Lijin station (Fig. 4a), and the
1.5
600
1.4 1.3
550
1.2
500
1.1
450
1 0.9
400
0.8 350
0.7
150 100 50 0 0 -50
1.3 18
1.2
17.5
1.1 1
17
0.9
16.5
0.8 16
Summer monsoon index
Annual temperature (˚C)
1.4
0.7
0.6 15.5 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 3. Comparison of the temporal variations between annual summer monsoon index and annual precipitation (a) and between annual summer monsoon index and annual precipitation (b). The thick lines are the 3-year moving averages.
Annual land accretion rate of the delta (km2/yr)
1.5
18.5
15
20
25
y= 5.482x - 8.477 r 2 = 0.7944 Sediment flux into the sea (10t8/yr)
Year
Annual temperature Summer monsoon index
10
-100
0.6 300 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
19
5
200 150
25 Land accretion rate Sediment flux into the sea
20
100 15 50 10 0 -50
5
Sediment flux into the sea (10t8/yr)
Annual temperature Summer monsoon index
Summer monsoon index
Annual precipitation (mm)
650
Annual land accretion rate of the delta (km2/yr)
200
0 -100 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 4. Relationship between land accretion rate of the delta and sediment flux into the sea (a) and temporal variations in these two variables (b). The thick lines are for 3-year moving average. Considering that the Sanmenxia Reservoir trapped huge quantities of sediment from 1960 to 1963, these 4 years are excluded from the Qs, Lijin data. The thick lines are the 3-year moving averages.
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temporal variations in Rla and Qs,Lijin are synchronous (Fig. 4b). The curves showing 3-year moving averages in Rla and Qs,Lijin are also given in Fig. 4b, which indicates that the Rla and Qs,Lijin increased to a peak in 1964, followed by a decline. Global climate change would result in a change in sediment flux to the sea, and the latter would in turn result in a change in land accretion of the delta. Fig. 5a shows the temporal variations in sediment flux (Qs, Lijin) to the sea and SMI in the Yellow River basin. The figure indicates that before 1964, Qs, Lijin increased mildly, the correlation coefficient between Qs, Lijin and time is 0.5229 (N ¼ 11, with the years 1960–1963 excluded because of water
1.5 Sediment flux into the sea Summermon soon index
20
1.4 1.3 1.2
15
1.1 1
10
0.9 0.8
5
Summer monsoon index
Sediment flux into the sea (108t/yr)
25
0.7 0.6 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
200
25 Landaccretion rate Sediment flux into the sea
150
20
100 15 50 10 0 5
-50
Sediment flux into the sea (102t / yr)
Annual land accretion rate of the delta (km2/yr)
Year
-100 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
200 150
Land accretion rate Annual temperature
18 17.5
100 17 50 16.5 0 16
-50
Annualtemperature(˚C)
Annual land accretion rate of the delta (km2/yr)
Year
-100 15.5 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Fig. 5. Comparisons between annual sediment flux into the sea and summer monsoon index (a); between land accretion rate of the delta and summer monsoon index (b); and between land accretion rate of the delta and air temperature (c).
storage and sediment trapping of the Sanmeanxia Reservoir), which is significant at a level of po0.10, but afterwards, it decreased, the correlation coefficient between Qs, Lijin and time is 0.5907 (N ¼ 33), which is significant at a level of po0.01. This trend is similar to that in SMI. Before 1964, SMI increased, the correlation coefficient between SMI and time is 0.7164 (N ¼ 14), which is significant at a level of po0.01, but afterwards, it decreased, the correlation coefficient between SMI and time is 0.5907 (N ¼ 36), which is significant at a level of po0.01. The calculation also shows that before 1964, land accretion rate of the delta (Rla) increased mildly, the correlation coefficient between Rla and time is 0.6173 (N ¼ 9), which is significant at a level of o0.08, but afterwards, it decreased, the correlation coefficient between Rla and time is 0.5345 (N ¼ 33), which is significant at a level of po0.01. Thus, it can be thought that the year 1964 was a turning point, at which the trend of both SMI and Qs, Lijin abruptly changed. This is because the decreased summer monsoon intensity after that year may have reduced the vapor flux to the upper and middle Yellow River basin, and thus precipitation decreased and therefore runoff also decreased. This may further result in a reduction in erosivity of rainfall and runoff, and the sediment carrying ability of the river may also be reduced. As a result, sediment flux to the sea decreased. This would further result in a decrease in land accretion rate of the delta, as demonstrated by Fig. 5b, where the temporal variations in Rla and SMI show some synchronous trend. The temporal variation in basin-averaged annual air temperature (T) is plotted in Fig. 5c. Before 1970, there is no trending variation in T. However, from 1970, the T shows an increasing trend, the correlation coefficient between T and time is 0.7453 (N ¼ 19), which is significant at a level of po0.01. In the meantime, the land accretion rate of the delta decreased. To reveal the influence of SMI and air temperature on land accretion rate of the delta, the Rla is plotted against SMI and T in Fig. 6. Although the points are scattered, the correlation coefficients between Rla and SMI and between Rla and T are 0.54 and 0.45, respectively. The critical correlation coefficient is 0.39 when the number of the samples N ¼ 41, and thus the correlation for both plots is significant at a level of o0.01. The cause for the positive correlation between Rla and SMI has been discussed above. The negative correlation between SMI and T may be explained by considering the effect of a higher air temperature on runoff generation. The increased temperature may result in stronger evapotranspiration, which may lower the level of soil moisture and reduce runoff generation and soil erosion intensity. Furthermore, the increased temperature may increase the water diversion for irrigation, which would reduce the river flow and water flux to the sea. All these factors may reduce sediment flux to the sea and the land accretion rate of the delta.
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Annual land accretion rate of the delta (km2/yr)
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200 150 100 50 0 y = 125.35x - 86.508 r = 0.5520
-50 -100
Annual land accretion rate of the delta (km2/yr)
0.6
0.81
1.2 Summer monsoon index
1.4
1.6
200 y = -29.454x + 536.01 r = -0.4511
150 100 50 0 -50 -100 15
15.5
16 16.5 17 17.5 Annual temperature (˚C)
18
18.5
19
1600 500 Net water diversion 450 Area of soil erosion control measures 1400 400 1200 350 1000 300 800 250 200 600 150 400 100 200 50 0 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year
Area of soil erosion congtrol measures (km2)
Net water diversion (108m3/yr)
Fig. 6. Plots of land accretion rate of the delta against annual summer monsoon index (a) and annual air temperature (b).
Fig. 7. Temporal variations in annual water division and the area of soil and water conservation measures.
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of population increase, and can also be related with the influence of climate change, namely, the decreased precipitation and increased temperature. The decreased precipitation cannot meet the water demand of crops, and the increased temperature enhances evapotranspiration, both leading to an increase in water diversion. The change in land accretion rate of the delta occurred as a result of both climate change and the change in human activity. How to differentiate the contributions of the two factors to the variation in Rla is an important issue needed to be addressed. For this purpose, multiple regression analysis is conducted. The data in the periods 1955–1959 and 1961–1996 are available, totaling 41 years. Table 1 shows the correlation matrix between land accretion rate of the delta (Ral) and four influencing factors, including SMI, annual mean air temperature in the Yellow River drainage basin (T), annual water diversion (Qw,div) and the area (Atfg) of land terracing and tree- and grass-planting. The critical correlation coefficients at 0.01 and 0.05 levels are 0.3932 and 0.3044, respectively. Thus, the correlation of Ral with SMI, Qw,div and Atfg is significant at a level of 0.01, and the correlation of Ral with T is significant at a level of 0.05. The multiple regression equation is established as follows: Rd ¼ 45:418 þ 54:915SMI 2:2948T 0:0288Atfg 0:02371Qw;div .
ð1Þ
The residual standard deviation of the regression line is S.E. ¼ 28.765, multiple correlation coefficient R ¼ 0.62, the result of F-test F ¼ 5.81. The multiple correlation coefficient is relatively low, but it is still significant at a level of p ¼ 0.001031. Some characteristics of the regression are given in Table 2. It can be seen from the above equation that the land accretion rate of the delta (Rla) increases with the SMI, but Table 1 Correlation coefficient matrix among all variables
SMI T Qw, div Atfg Ral
SMI
T
Qw,div
Atfg
Ral
1.00 0.28 0.68 0.66 0.54
0.28 1.00 0.62 0.54 0.35
0.68 0.62 1.00 0.75 0.60
0.66 0.54 0.75 1.00 0.51
0.54 0.35 0.60 0.51 1.00
4.3. Influence of human activity In the past 40 years, in addition to the climate change, human activity also has shown marked changes, of which water diversion for irrigation and domestic uses and largescale soil erosion control measures are the two main aspects. The temporal variations in net water diversion and the total area of land terracing and tree- and grass-planting are shown in Fig. 7, both indicating a marked increasing trend. The increase in water diversion occurred as a result
Table 2 Some characteristics of the multiple regression Sum of squares
Degree of freedom
Mean squares
F
p-level
Regression Residual
19227.16 29787.09
4 36
4806.79 827.41
5.809
0.001031
Total
49014.25
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decreases with the air temperature (T), the area of terrace land, tree- and grass-planting (Atfg), and the net water diversion (Qw,div). By using Eq. (1), the change of the land accretion rate of the delta in the future can be estimated, according to the planned increase in the areas of erosion control measures, the predicted increase in the water diversion and the estimated variation in precipitation and air temperature. The mechanism for the above equation can be explained as follows. The relatively strong summer monsoon means abundant vapor transported to the drainage basin and therefore more opportunity for rainstorms, and thus erosion may be strong. The increased runoff has higher sediment transporting ability and can carry more sediment to the sea, and thus, the land accretion rate of the delta becomes higher. The increased air temperature may enhance evapo-transpiration, reduce runoff, and therefore may reduce the sediment transporting ability and increase sediment deposition in river channels, leading to a decrease in sediment flux to the sea and the land accretion rate of the delta. Land terracing reduces slope steepness of hillslopes, enhances infiltration, and reduces surface runoff and soil erosion. The increased area of trees and grasses enhances the protection of vegetation on land surface, and also reduces soil erosion. Hence, the sediment flux to the sea decreases. The sediment flux to the sea during low-flow seasons is mainly produced by channel scour because of low sediment concentrations. Water diversion during lowflow seasons reduces river flow and also reduces channel scour. When water diversion occurs, some sediment is diverted with water, especially when the water diversion occurs during high-flow seasons when sediment concentration is high. This factor also reduces sediment flux to the sea and the land accretion rate of the delta. As the range of the absolute values of the variables differs from each other, the regression coefficients of the influencing variables do not reflect their contribution to the dependent variable. Therefore, the data have been standardized to the range of (0–1), and then the multiple regression equation is re-established as follows: Rd ¼ 0:2439SMI 0:02649T 0:3757Atfg 0:05195Qw;div . (2) By comparing the regression coefficients of the four independent variables, they can be ranked in terms of their contribution to the variation in the land accretion rate of the Yellow River delta. Assuming the total contribution of the four influencing variables to the variation in Ral is 1.0, contributions of SMI, T, Atfg and Qw, div were calculated as 34.94%, 3.80%, 53.82% and 7.44%, respectively. It has been further calculated that the contribution from two climate variables is 38.7%, and that from two variables of human activity is 61.3%, indicating that the contribution from climate change is less than that from human activity.
5. Conclusions The variation in SMI shows three stages: (1) from 1951 to 1963, SMI increased; (2) from 1963 to 1965, SMI declined sharply, a feature that may be regarded as an abrupt change; (3) from 1965 to 2000, SMI remained at low levels and showed a decreasing trend. Climate change may result is a change in sediment flux into the sea, and therefore a change in the rate of land accretion of delta (Rla). The annual Ral and sediment flux into the sea tended to increase from 1952 to 1964, but decreased after 1964, a trend similar to that in the SMI. Apart from climate factor, human activity such as water and soil conservation measures and water division in the drainage basin also has some effect on land accretion of the Yellow River delta. A multiple regression equation has been established, which indicates that the Rla decreased with decrease in SMI, increase in annual temperature (T), increase in the area of water and soil conservation measures (Atfg) and increase in water diversion (Qw,div). The contributions of the variations of the above influencing variables to the variation in Rla were calculated as 34.94%, 3.80%, 53.82% and 7.44%, respectively. The contributions of the two climate factors sums to 38.7%, indicating that the influence of global climate change on the variation in land accretion of Yellow River delta is significant and should not be ignored. Acknowledgments Financial supports from the National Natural Science Foundation of China (40671019) and from the Third-phase Knowledge Innovation Program of the Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, are gratefully acknowledged. Thanks are expressed to the Water Conservancy Commission of the Yellow River, for access to the internally issued hydrometric data, the data of erosion control measures and the data of water diversion; to Professor Guo Qiyun, for providing the data of SMI. References Cao, W.H., 1997. Study on evolution of the Yellow River delta and its reactions. Journal of Sediment Research 4, 1–6. Chien, N., Zhang, R., Zhou, Z.D., 1987. River Channel Processes. Science Press, Beijing. Guo, Q.Y., 1990. A study of Eastern Asian summer monsoon. In: Zuo, D.K. (Ed.), Progress in Geography. Science Press, Beijing, pp. 63–67 (in Chinese). Holigon, P.M., Boois, H., 1993. Land-ocean interactions in the coastal zone (LOICZ) science plan [R]. Global Change Report No. 25. IGBP Secretariat, Stockholm, pp. 1–150. Ji, Z.W., Hu, C.H., 1995. Analysis on recent evolution for mouth bar of the Yellow River. Journal of Sediment Research 3, 1–10. Ji, Z.W., Hu, C.H., Zeng, Q.H., Yan, Y., 1994. Analysis of recent evolution of the Yellow River Estuary by landsat image. Journal of Sediment Research 3, 12–22.
ARTICLE IN PRESS J. Xu / Quaternary International 186 (2008) 4–11 Li, Z.G., 1993. Mechanism of the mouth bar formation in the Yellow River estuary. Acta Oceanologica Sinica 15 (1), 84–91. Shi, C.X., Zhang, D., 2003. Processes and mechanisms of dynamic channel adjustment to delta progradation: the case of the mouth channel of the Yellow River, China. Earth Surface Processes and Landforms 28, 609–624. Wang, K.C., 1985. The prediction of the tendency of Yellow River mouth evolution. Research Report of the Yellow River Institute of Hydraulic Engineering, No. 8529 (in Chinese). Xu, J.X., 2002. A study of thresholds of runoff and sediment for the land accretion of the Yellow River Delta. Geographical Research 21, 163–170 (in Chinese). Xu, J., 2003. Sediment flux into the sea as influenced by the changing human activities and precipitation: example of the Yellow River, China. Environmental Management 31, 328–341.
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Xu, J., 2005. The water fluxes of the Yellow River to the sea in the past 50 years, in response to climate change and human activities. Environmental Management 35, 620–631. Xu, J.X., 2006. Land-accretion of the Yellow River Delta influenced by drainage basin factors. Geografisca Annaler 88A (1), 31–42. Ye, Q.C., 1994. Researches on Environmental Changes of the Yellow River Basin and Laws of Water and Sediment Transportation. Shandong Science and Technology Press, Jinan (in Chinese). Zeng, Q.H., Zhang, S.Q., Hu, C.H., Yin, X.L., Li, Z.G., Jiao, Y.L., 1997. Evolution of the Yellow River Mouth. Publishing House for Yellow River Water Conservancy, Zhengzhou (in Chinese). Zhang, S.Q., 1999. One-D and two-D combined model for estuary sedimentation. International Journal of Sediment Research 14 (1), 37–45 (in Chinese).