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Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin
Quaternary International xxx (2013) 1e10
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Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin Yunxia Yan a, *, Suiji Wang a, Ming Yan a, Li He a, Lan Zhang b a Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (CAS), Beijing 100101, China b Department of Civil Engineering, University of Akron, Akron, OH 444325, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The scale effects of Area-Specific sediment yield (YS) are studied for Zhujiang basin and its four subbasins. Considering the entire Zhujiang basin, there is no apparent relation between Ys and contributing area (Ad) for stations when Ad is less than 100,000 km2, while there is an upper envelope outline of Ys. For the stations with Ad larger than 100,000 km2 mainly located on the middle and lower reaches, there is a negative relation of Ys to Ad. Considering the four sub-basins of Zhujiang basin, three kinds of relations between YS and Ad are established: (1) Ys increases with Ad for the sub-basin of the upper reach; (2) Ys increases with Ad then declines for the two sub-basins of the middle reaches; (3) Ys decreases with Ad for the sub-basin of the lower reaches. The YseAd relation is further investigated using the spatial variation of hill slopes, the Land Use and Land Cover (LULC), and the bed slopes. Finally, to remove the effect of Ad and to make Ys comparable across scales, the scaled Ys calculation equations are given for both linear and non-linear relations between Ys and Ad first. And then the Ys are scaled to the standard units of 1000 km2and 10,000 km2. Using Kriging method, the Ys maps are created under GIS platform where high Ys centers are outlined, and the spatial patterns of Ys are compared for the two standard unit Ys maps. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
steep slope); whereas, the sediment may have more chances to be deposited in the middle and lower reaches (i.e., stations with large contributing areas located at low elevation and/or gentle slope). With this in mind, the negative correlated relations between YS and Ad has been applied for many river basins around the world (Walling, 1983; Morris and Fan, 1997; Church et al., 1999; Syvitski et al., 2005; de Vente et al., 2005a; Vanmaercke et al., 2011, and among others). However, this assertion has been challenged in many studies, both positive linear and non-linear relations between YS and Ad have also been found for many river basins around world (Ashmore and Day, 1988; Church and Slaymaker, 1989; Church et al., 1999; Dedkov and Moszherin, 1992; Dedkov, 2004; de Vente and Poesen, 2005b; Walling, 2005; Xu and Yan, 2005; Yan and Xu, 2007; Yan et al., 2011; Vanmaercke et al., 2011; as a few examples). The above studies clearly indicated that the YS e Ad relation may be significantly impacted by the local factors, such as topography, lithology, landscape history, LULC, human impacts, and the scaledependent contribution of different erosion processes (e.g. Church and Slaymaker, 1989; Walling and Webb, 1996; Church et al., 1999; Xu and Yan, 2005; Syvitski and Milliman, 2007; de Vente et al., 2007; Vanmaercke et al., 2011).
Area-specific sediment yield (Ys, t km2 yr1, i.e. the quantity of sediment reaching the catchment outlet per unit of time and per unit of area) at the catchment level has received considerable attention. It is crucial for many issues, such as capacity loss of reservoirs, soil erosion prevention, sedimentation of estuaries and harbors, coastal erosion, river delta development, transportation of pollutants and damage to ecological habitats (e.g. Syvitski et al., 2005; Owens et al., 2005; Rekolainen et al., 2006; ; Verstraeten et al., 2006). Ignoring the influence of scale on YS, YS is usually computed and compared using total sediment load divided by the contributing area (Ad). However, sediment mobilization and deposition effects are often systematically distributed in the landscape and impose a scaling characteristic on sediment yield (Church et al., 1999). Conventionally, it has been understood that sediment may have more chances to be transported in the upper reach (i.e., stations with small contributing areas located at high elevation and/or with * Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Yan). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.08.002
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Y. Yan et al. / Quaternary International xxx (2013) 1e10
Throughout the years, many studies have been carried out on Zhujiang basin. Dai et al. (2008) studied the impact of dams on the sediment flux. Zhang et al. (2008) studied the spatial changes of discharge and sediment load. Zhang et al. (2011) revealed the trends of annual and seasonal variation of suspended sediment load and analyzed the influence of human activities on the variation of the suspended sediment load. However, little research has been focused on the spatial variation of Ys in Zhujiang basin. Thus it is much needed to investigate as many river basins as possible to establish the general and specific YS e Ad relation. To this end, the objective of this study is to investigate the YS e Ad relation as following: (1) establish the relation of YS to Ad; (2) analyze the factors that may be responsible for the spatially varied relation between YS and Ad; (3) create scaled Ys maps with different standard units; and (4) study the spatial variations of Ys maps. 2. Study area Originating in Wumeng Mountain in Yunnan Province in the west, Zhujiang flows east and empties into the South China Sea. Zhujiang basin is the third largest in China in terms of drainage area, (i.e., A ¼ 453:7 103 km2 ; Zhang et al., 2008). Zhujiang is a compound river system composed by the first order tributaries of Xijiang, Beijiang and Dongjiang, and some small tributaries draining into Zhujiang delta (Fig. 1). Xijiang is the largest tributary located in the upper and middle reaches of the river covering 78% of Zhujiang basin (Sun et al., 2010). The upper reach includes two big tributaries, Nanpanjiang and Beipanjiang (first order tributaries of Xijiang). The middle reach includes big tributaries of Liujang, Guijiang and Yujiang (first order tributaries of Xijiang). The lower reach includes Beijiang and Dongjiang (Fig. 1). In climatic sense, Zhujiang basin is dominated by subtropical to tropical monsoon climate with the Tropic of cancer passing through the basin. Decreasing from east to west, the mean annual temperature and precipitation across the basin ranges from 14 C to 22 C, and 1200 mm to 2200 mm respectively. Precipitation during Aprile September accounts for 72e88% of the annual total. The interannual variation of the precipitation is around 14%e16% (Zhang et al., 2008; Zhang et al., 2009).
In hydrological sense, the annual average runoff and sediment load from 1954 to 2010 of whole basin are 283 109 m3, 72.4 106 t respectively (Ministry of Water Resource, PRC, 1954e 2010). From April to September, the accumulative runoff and sediment load account for 84% and 94% of the annual totals. Runoff and sediment load during the flood season (June to August) account for 50% and 72% of the annual totals (He et al., 2003). In geographical and geological senses, surrounded by the Yunnan-Guizhou Plateau in the west and the Five Ridges in the south, 85% of Zhujiang basin is mainly covered by hills (elevation ranging from 50 to 500 m) and mountains (elevation ranging from 500 to 2000 m) shown in Fig. 2. Karst is very common in Zhujiang basin due to the widely distributed carbonate rocks mainly located in the upper and middle reaches (about 45% of the basin). Clastic rock is mainly located in the upper reach (about 27% of the basin). Granite is mainly located in the lower reach (about 16% of the basin) (Cheng, 1990). Bounded by 24 N latitude, the lateritic red soil is mainly distributed in the south of the basin, red soil is mainly distributed in the north. Yellow soil is usually distributed on mountains with elevation higher than 700 m and limestone soil is mainly distributed in limestone mountains of Xijiang basin (Institute of Soil Science, 1990; Likens, 2010). In the sense of LULC, Fig. 3 shows LULC of Zhujiang basin obtained from Institute of Geographical Science and Natural Resources Research (1991). Timber forest, bush, and non-irrigated field are mainly distributed in the upper reach and Yujiang tributary in the middle reach. Irrigated field is mainly distributed in Liujiang and Guijiang of the middle reach and Beijiang and Dongjiang of the lower reach. Sparse wood, grassland and paddy are distributed across the entire Zhujiang basin. Fig. 4 shows the percentage of coverage regarding to each classified LULC. 3. Data, methods and sub-basin division 3.1. Data The suspended sediment load (SSL) of Zhujiang basin are requested from Ministry of Water Conservancy and Electric Power (MWCEP)dunpublished data provided only for internal use to
Fig. 1. Distribution of sub-basin and hydro-stations in Zhujiang basin.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Fig. 2. DEM of Zhujiang basin (http://srtm.csi.cgiar.org/).
further the engineering design and scientific research which the measurement (including river stage, discharge, suspended sediment concentration (SSC), grain size distribution, and SSL) are strictly following the national standards issued by MWCEP (1962, 1975) as: (1) water flow velocity is measured at vertical profiles at different depths (normally the surface, 0.2H, 0.6H, 0.8H and the bottom, where H is the depth of the water column); (2) SSC is measured by bottle samples of water at the depths where the flow velocities are measured; (3) water discharge is calculated using the cross-section area and the mean velocity of the cross-section; (4) SSL is calculated using water discharge and SSC. In the dry seasons, water flow velocity and SSC are measured every 3e5 days. In the flood seasons, water flow velocity and SSC are measured every 1e3 days. During the flooding events, water flow velocity and SSC are measured several times a day. Limited by data available, the annual SSL data from 1954 to 1987 are used for the study. To further minimize the impacts of
extreme flood, drought, and disturbance at any given station (e.g., dams, mining that might obscure the regional trends), the stations applied in the study are determined with the following two criteria: (1) the stations with at least four years of data available; (2) removing the stations with disturbance. Therefore, 90 stations are identified for the analyses with the time scales extending from 4 years to 33 years with an average of 18.5 years as shown in Fig. 1. The following maps are applied to reveal the mechanism of the scale effects of Ys: maps of geology (1:5,000,000) (Cheng, 1990), soil (1:4,000,000) (Institute of Geographical Science and Natural Resources Research, Chinese Academy of Science (IGSNRR-CAS), 1991), LULC (Institute of Soil Science, 1990). All the maps are digitalized using the Data Sharing Infrastructure of Earth System Science (DSIESS) in IGSNRR-CAS. The STRM 90 m DEM data of Zhujing basin are obtained from CGIARCSI website (http://srtm.csi.cgiar. org). The bed slope are calculated from DEM. Areas of rocks and
Fig. 3. LULC of Zhujiang basin.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Fig. 4. Percents of different LULC types of Zhujaing Basin.
LULC (if not specified) are estimated from corresponding digitalized maps in ArcGIS platform. 3.2. Method The Ys scaling relations are studied in the following fashion: (1) applying the logarithmic transformation with base 10 for Ys and Ad; (2) establishing the relation by fitting linear/nonlinear equations to the transformed Ys and Ad; (3) calculating the scaled Ys; (4) mapping Ys with different standard units using Kriging method; and (5) analyzing Ys e Ad relation based on topography, geology, lithology, and LULC. Following Church et al. (1999), and Yan and Xu (2007), the scaled Ys is calculated in the following manner: (I) Plotting the transformed data of Ys and Ad in log-10 space which is shown as the scatter plot in Fig. 5 where, Y ¼ Log(YS), X ¼ Log(Ad), and (X1, Y1) is the logarithmic transformed data of (Ys, Ad) of a given station; (II) Calculating the scaled YS (SY) using Yr (scaled Y1 under a standard unit of Xr in log-10 space based on either linear or non-linear relation as: ① SY calculation for linear relation (Fig. 5a) Given the fitted linear regression equation as, Yr is obtained by moving the point of (X1, Y1) to the new position of (Xr, Yr) along the line of
Y ¼ AX þ B0
(1)
Where:Y ¼ AX þ B0 is parallel to the original fitted equation; A is the slope, B, B0 are the intercepts. From Eq. (1), the intercept B0 is computated as:
B0 ¼ Y1 AX1
(2)
② SY calculation for non-linear relation (Fig. 5b): Given the fitted quadratic equation as Y ¼ AX2þBX þ C, the equation to calculate Yr is given as:
Y ¼ AX 2 þ BX þ C 0
(5)
Where: Y ¼ AX 2 þ BX þ C 0 is parallel to the original fitted equation; A, B, C, and C0 are parameters. From Eq. (5), C0 is calculated as:
C 0 ¼ Y1 AX12 þ BX1
(6)
Substitute Eq. (6) into Eq. (5), one obtains
Y ¼ AX 2 þ BX þ Y1 AX12 þ BX1
(7)
Finally, SY is calculated by Eq. (7) as:
Substitute Eq. (2) into Eq. (1), one obtains
Y ¼ AX þ Y1 AX1
Fig. 5. Illustration of the scaling process: (a): Linear equation; (b): Non-linear eqution.
(3)
SY ¼ 10Yr ¼ 10AXr þBXr þY1 ðAX1 þBX1 Þ 2
2
(8)
Finally, SY is calculated by Eq. (3) as:
SY ¼ 10Yr ¼ 10ðAXr þY1 AX1 Þ
3.3. Sub-basins division
(4)
Using the standard area unit of 100 km2 as an example, the Xr ¼ Log(100) ¼ 2, and the corresponding SY is
SY ¼ 10ð2AþY1 AX1 Þ
According to the similarities of topography, geology, and LULC, Zhujiang basin is divided into four sub-basins: the upper reach subbasin, Liujiang-Guijiang sub-basin, Yujiang sub-basin and BeijiangDongjiang sub-basin listed in Table 1 and further discussed in what follows.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Table 1 The sizes and biophysical characteristics of the four sub-basins Sub-basin
Area (103 km2)
LULC (percent of the sub-basin)
Slopes (percent of the sub-basin)
Lithology
Dem
The upper reach sub-basin
114
<6 (15%) 6 e15 (35%) 15 e25 (22%) >25 (28%)
Carbonate rock:24% Clastic rock:44% Metamorphic rock:25%
<50 m (0%) 50e500 m (10%) 500e1000 m(23%) >1000 m(66%)
Liujiang- Guijiang sub-basin
109
Timber forest (24%) Grassland (22%) Bush(18%) Non-irrigated field (14%) Paddy (10%) Irrigated field (32%) Grassland (18%) Paddy (17%) Sparse wood (12%) bush (10%) Timber forest (19%) Bush(18%) Grassland (16%) Non-irrigated field(12%) Sparse wood (11%) Irrigated field (36%) Paddy (28%) Grassland (15%) Sparse wood (12%)
<6 (37%) 6 e15 (32%) 15 e25 (21%) >25 (10%)
Carbonate rock:49% Granite: 5% Clastic rock:39%
<50 m (2.3%) 50e500 m (63%) 500e1000 m(26%) >1000 m(5%)
<6 (45%) 6 e15 (29%) 15 e25 (18%) >25 (7%)
Carbonate rock:68% Granite: 8% Clastic rock:5% Metamorphic rock:10%
<50 m (3%) 50e500 m (67%) 500e1000 m (23%) >1000 m (12%)
<6 (49%) 6 e15 (29%) 15 e25 (16%) >25 (6%)
Carbonate rock:30% Granite: 32% Clastic rock:20% Metamorphic rock:8%
<50 m (24%) 50e500 m (59%) 500e1000 m (15%) >1000 m (2%)
Yujiang sub-basin
82
Beijiangedongjiang sub-basin
96
The upper reach sub-basin: It is mainly composed by YunnanGuizhou Plateau and Hengduan mountains. Around 90% of the sub-basin has elevation higher than 500 m (Fig. 2, Table 1). Clastic rock, metamorphic rock and carbonate rock are the dominate rocks. Timber forest, bush, and grassland are the major LULC types. Around 50% of non-irrigated field of the entire Zhujiang basin is concentrated in this sub-basin (Fig. 3, Table 1). Beipanjiang in this sub-basin is a tributary with highest elevations and more hills and mountains. More than 88% of Beipanjiang area has elevation higher than 2000 m (An and Zhou, 1994). Along with limited plains areas, soil erosion is much more intensive in Beipanjiang area, comparing to the rest of this sub-basin (An and Zhou, 1994; Lin et al., 2002). Thus Beipanjiang area (covering 13% of the upper reach sub-basin) is further divided as Beipanjiang sub-area (Fig. 1). Yujiang sub-basin: It is mainly composed by hills with elevation ranging from 50 to 500 m (Fig. 2). Carbonate rock and clastic rock are the dominate rocks (Table 1). The LULC types are similar to that of the upper reach sub-basin. This is another sub-basin where nonirrigated field is concentrated (Fig. 3, Table 1). Liujiang-Guijiang sub-basin: The topography of this sub-basin is similar to that of Yujiang sub-basin (Fig. 2). Carbonate rock is the dominate rock (Table 1). Irrigated field is concentrated in this subbasin which accounts about 50% of the total irrigated field in Zhujiang basin (Fig. 3, Table 1). Longjiang area in this sub-basin is
more mountainous (54% of area with elevation higher than 500 m) and with more hill grass and bush (covering 61% of the area), comparing to rest of the sub-basin. Thus, Longjiang area is also divided as Longjiang sub-area. Beijiang-Dongjiang sub-basin: It is mainly composed by hills with the elevation ranging from 50 to 500 m and plains with elevation lower than 50 m (Fig. 2). Granite and Carbonate rocks are the dominate rocks (Table 1). Irrigated field and paddy are the major LULC types. This is another sub-basin where irrigated field is concentrated (48% of the total irrigated field) (Fig. 3, Table 1). 4. Results 4.1. Scale effects of Ys of entire Zhujiang basin The Ys and Ad in log-10 space for entire Zhujiang basion is graphed in Fig. 6. It indicates log(Ad) ¼ 5 (i.e., Ad ¼ 100,000 km2) as the changing point for the entire Zhujiang basin. For stations with Ad <100,000 km2, there is no apparent relation between Ys to Ad. However, after removing the extreme YS (i.e., data of Beipanjiang sub-area, the soil erosion center of Zhujiang basin, An and Zhou, 1994; Lin et al., 2002), there is an upper envelope outline, i.e., Ys increases with Ad to around 600 t km2 yr1, then decreases with the changing point of Ad about 5000 km2 with the lowest Ys around 40 t km2 yr1. The stations with Ad >100,000 km2 are all located on the main channel of the middle and lower reaches. With the decrease of the bed slope in the main stem, there is a decreasing trend of the Ys vs. Ad. 4.2. Scale effects of Ys of each sub-basin (1) The upper reach sub-basin (Fig. 7a, Tab, 2)
Fig. 6. Relation of YS to Ad for Zhujiang basin.
A positive linear relation is found between Ys and Ad in log-10 space in the sub-basin excluding the Beipanjiang sub-area. Ys of Beipanjiang sub-area is found to be much higher than that of the rest of the sub-basin. Table 2 lists the ranges of Ys, Ad, mean and standard deviation of Ys, the fitted linear equation, R2, F-test results and corresponding significance level.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Table 2 Relation of YS to Ad for all sub-basins Sub-basin
Sub-area
The upper reach Beipanjiang Longjiang-Guijiang Longjiang Yujiang Beijiang-Dongjiang
Stations
Range of Ys(t km2 yr1)
Range of Ad(km2)
Mean of Ys (t km2 yr1)
Standard deviation of Ys
Fitting equation
R2
F
Sig。
19 5 29 4 15 15
94e425 591e1678 42e551 43e88 53e230 152e248
500e120000 5000e9000 200e46000 1900e16000 1600e90000 800e39000
244 920 187 71 116 180
105 451 110 16 55 36
y ¼ 0.1722x þ 1.7051 No relation y ¼ 0.414x2 þ 2.9695x2.9833 y ¼ 1.0141x2 þ 7.8213x13.092 y ¼ 0.4071x2 þ 3.4292x5.0336 y ¼ 0.13x þ 2.7384
0.50
12.279
0.001
0.37 0.96 0.45 0.58
7.702 10.993 4.932 18.239
0.002 0.209 0.027 0.001
Where : x ¼ Log(Ad); y ¼ Log(Ys).
(2) Liujiang-Guijiang sub-basin (Fig. 7b, Tab, 2)
4.3. Scaled Ys maps
Non-linear relation is found for this sub-basin (excluding the Longjiang sub-area) as well as for Longjiang sub-area. In LiujiangGuijiang sub-basin, Ys increases with Ad to the maximum, then declines in the log-10 space. The changing point (Ad) is around 3000 km2. The fitted curve of the sub-basin (excluding the LongJiang sub-area) lies above that of Longjiang sub-area, that is, given the same Ad, the Ys for the sub-basin (excluding Longjiang sub-area) is higher than that of Longjiang sub-area.
To build the scaled YS map, the standard units need to be chosen. Since the areas of all stations are greater than 100 km2, and only 6 out of the 90 stations with areas larger than 100,000 km2, 1000 km2 and 10,000 km2 are selected as the standard units. The observed Ys is then scaled into the standard units using Eqs. (1)e(8). Fig. 8 graphs the scaled YS maps created with Kriging method under the ArcGIS platform. It shows that more than 80% of Zhujiang basin has YS less than 300 t km2 yr1 under both standard units and is further discussed in what follows: Standard unit of 10,000 km2 (Fig. 8a): (1) 53% of the basin has YS between 100 and 200 t km2 yr1, which are mainly distributed in the middle and lower reach of the basin, i.e., LiujiangGuijiang, Yujiang, and Beijiang-Dongjiang sub-basins. (2) The areas with YS between 200 and 300 t km2 yr1 account for around 28% of the entire Zhujiang basin and are mainly located in the upper reach sub-basin and near the main stem in LiujiangGuijiang sub-basin. (3) There are two high YS centers with YS > 300 t km2 yr-1 (19% of the entire Zhuajiang basin): Beipanjiang sub-area in the upper reach sub-basin (larger one) and Luodingjiang basin (smaller one).
(3) Yujiang sub-basin (Fig. 7c, Table 2) The YseAd relation is found to be similar to that of the LiujiangGuijiang sub-basin, that is, Ys increases with Ad to the maximum then declines in log-10 space. The changing point is around 10,000 km2. (4) Beijiang-Dongjiang sub-basin (Fig. 7d, Table 2) A negative linear relation was found between Ys and Ad in log-10 space.
Fig. 7. Relation of YS to Ad for: (a) the upper reach sub-basin; (b) Liujiang-Guijiang sub-basin; (c) Yujiang sub-basin; (d) Dongjiang-Beijiang sub-basin.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Fig. 8. Ys map on standard unit of (a) 10,000 km2; (b) 1000 km2.
Standard unit of 1000 km2 (Fig. 8b): The areas with YS < 100 t km2 yr-1 (19% of the entire Zhuajiang basin) are mainly distributed in Yujiang sub-basin and Longjiang sub-area. The areas with YS between 100 and 200 t km2 yr1 (40% of the basin) are mainly distributed in the Liujiang-Guijiang sub-basin. The areas with YS between 200 and 300 t km2 yr-1 (about 28% of the entire Zhujiang basin) are mainly distributed in Liujiang-Guijiang subbasin and part of Bejiang-Dongjiang sub-basin. (3) Similar to that under the standard unit of 10,000 km2, there are two high YS centers with YS > 300 t km2 yr-1 (12% of the entire Zhuajiang basin): Beipanjiang sub-area in the upper reach sub-basin and Luodingjiang basin. 5. Discussion 5.1. Scale effect of Ys of entire Zhujiang basin There is no apparent relation between Ys and Ad for the entire Zhujiang basin (Fig. 6). For stations with Ad < 100,000 km2, the points are very scattered which is caused by the change of the topography, geology and LULC from the upper reach to the lower
reach. For the same Ad, Ys is usually higher for area with high elevation, steep slope and non-irrigated field than that with low elevation, gentle slope and irrigated field. The non-linear upper envelope curve of the Ys e Ad with the changing point of Ad ¼ 5000 km2 (Fig. 6) (i.e., after removing the extreme YS) may be explained by: slope croplands, including irrigated/non-irrigated field on slope of 5 -25 (Zeng, 2008), which are the main sediment resources of Zhujiang basin contributing more than 50% of the soil erosion volumes (An and Zhou, 1994; Zeng, 2008). For stations with Ad < 5000 km2 which are mainly located in upper reaches of tributaries with relatively steep hill slopes (Fig. 1), the increase of slope cropland results in the increase of the YS with the increase of Ad (reflected by the left part of the envelop curve in Fig. 6). For stations with Ad > 5000 km2 which are mainly located in the main channels of middle and lower reaches of large tributaries (Fig. 1), Ys decreases with Ad since sediment has more chances to deposit with the decrease of bed slopes and increase of flood plain areas (reflected by the right part of the envelope curve in Fig. 6). Stations with Ad >100,000 km2 are all located in the middle and lower part of the main channel, Ys decreases with Ad, since sediment has more chance to deposit with the decrease of bed slope.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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5.2. Scale effects of Ys for sub-basins (1) The upper reach sub-basin This sub-basin is located in the transition zone from YunnanGuizhou Plateau to Guangxi-Guangdong middle-low mountains and basins, where positive linear relation is found between Ys and Ad (Fig. 7a, Table 2). Steep slopes dominate this sub-basin (Table 1). Fig. 9 plots the four major LULC types (accounting for about 80%) with corresponding average elevations of this sub-basin. For the head water area which is dominated by timber forest (Figs. 3 and 9), soil erosion is limited and Ys is relatively low. With the increase of Ad, the LULC changes from timber forest to non-irrigated field and results in the increase of the soil erosion intensity, and positive relation between YS and Ad (Figs. 3 and 9). With the further increase of Ad, the steep bed slopes of the upper reach become the dominating factor to influence the change of Ys. Sediment transport is benefitted by the steep bed slopes with high elevations, limited floodplains (Table 1), and the good connectivity between slopes and channels such that YS increases further with the increase of Ad. Ys of Beipanjiang sub-area are higher than that of the rest of this sub-basin (Fig. 7a) that may be explained by: 1) higher elevation, almost 90% of this sub-area with elevation higher than 1000 m; 2) high percentage of slope cropland [80% with slope of >6 , and 50% with slope of >15 (Lin et al., 2002)]; 3) less percentage of timber forest (10% of the sub-area); 4) high frequency of landslide and debris flows (An and Zhou, 1994). (2) Liujiang-Guijiang sub-basin and Yujiang sub-basin in the middle reach
be explained as following. For Liujiang-Guijiang sub-basin, stations with areas < 3000 km2 are mainly located in the tributaries of Liujiang and Guijiang with steep slopes; however, stations with areas > 3000 km2 are mainly located in the main river channel with much gentler slopes. For Yujiang sub-basin, the changing point from tributaries to the main river channel is around 10,000 km2. The lower Ys of Longjiang sub-area comparing with that of the rest of Liujiang-Guijiang sub-basin (Fig. 7b) may be contributed to the more forest, bush and grass (70%) and less irrigated/nonirrigated field (15%), which are 41% and 42% respectively for the rest of Liujiang-Guijiang sub-basin respectively (Fig. 3, Table 1). (3) Beijiang-Dongjiang sub-basin in the lower reach Beijiang-Dongjiang sub-basin is located in the lower reach of Zhujiang basin. The sub-basin is dominated by hills and plains with elevation lower than 500 m. 64% of the sub-basin is covered by croplands (the highest among all sub-basins). The negative relation is found between Ys and Ad (Fig. 7d, Table 2) for the sub-basin. The negative relation between Ys and Ad may be attributed to the intensive soil erosion on the hill slopes caused by the dense population density. The population density is 278 people per km2, which is about twice of the other two sub-basins of Liujiang-Guijiang and Yujiang (Han and Chen, 1999). Deforestation and waste reclamation have been intensified and has been mainly occurred on slope land. As the result, the hill region suffering soil erosion (with soil erosion modulus > 200 t km2 yr1) increased from 7444 km2 in the 1950s e 1960s to 17,070 km2 in the 1980s (Dai et al., 2008). With the increase of Ad, Ys decreases for the gentle bed slopes. 5.3. Scaled Ys maps under different standard units
These two sub-basins, in the middle reach of Zhujiang basin, are all located in Guangxi Zhuang Autonomous Region (Guangxi) and covering most of Guangxi (Fig. 1), where non-linear relations are found between Ys and Ad (Fig. 7b, c, Table 2). Guangxi is basin-like with 60% of mountains, 17% of hills and tablelands, and 21% of plains and basins (Fig. 2). YS is relatively low for Mountain regions with elevation higher than 400 m which are mainly covered by forest and grass (accounting for 40% of Guangxi, Fig. 3). YS is high for the regions with elevations between 200 and 400 m which are mainly composed of low mountains and hills (accounting for 29% of Guangxi, Figs. 2e3). These regions have relatively intensive soil erosion caused by intensive human activities (Tan et al., 2002). In these regions, area with soil erosion modulus more than 200 t km2 yr1 increased from 12,000 km2 in the 1950s e 1960s to 30,600 km2 in the 1980s (Dai et al., 2008). YS starts to decrease again with Ad for the regions with elevation lower than 200 m which is mainly covered by plains and basins. In these regions, sediment is buffered from the slopes to channels (Fig. 2). There exist differences on the changing points of YS e Ad relations between the two sub-basins (Fig.7bec). The differences may
The scaled Ys maps with different standard units are useful to determine the key areas for soil loss control. Two high Ys centers (Ys > 300 t km2 yr1) appear at the similar location with the two standard units (Fig. 8). The first center around the Beipanjiang subarea is the compound results of the high elevations, steep slopes, slope croplands, and high frequency of landslides and debris flows (An and Zhou, 1994; Lin et al., 2002). The second center around Luodingjiang has been also called “the little Yellow River in the Guangdong province” for its high Ys (Zhang and Lu, 2008). Besides the steep slopes and intensive human activities, the rich sediment source from the weathered granite and sandstone is another reason for the high Ys of Luodingjiang (Zhang and Lu, 2008). Comparing Ys with different standard units, one may obtain the information about the area where soil erosion mainly occurs. For a river basin where soil erosion is mainly occured in the upper part, there usually exists a negative relation between Ys and Ad. The scaled Ys on the map with the smaller standard unit of Ad may be larger than that with the larger unit. This finding may be demonstrated by the scaled Ys maps of Beijiang-Dongjiang sub-basin (Fig. 8). However, for a river basin where soil erosion is mainly occurred in the middle and lower part, Ys with the smaller standard unit of Ad may be lower than that with the larger unit. This finding may be demonstrated by the three sub-basins on the upper and middle reaches of Zhujiang basin: upper reach sub-basin, LiujiangGuijiang sub-basin, and Yujiang sub-basin. (Fig. 8aeb). 5.4. References for the other drainage basins
Fig. 9. Average elevation of the major land covers of the upper reach sub-basin.
The traditional methods to create Ys map follow three steps as: first, select hydrometric stations with long-records of Ys; second, determine the Ad represented by the stations, either above a station or between two adjacent stations; third, draw the isolines of Ys using certain interpolation method. The traditional methods can be
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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applied only under the hypothesis that the variation of Ad has no effect on Ys, or this effect can be omitted. However, some functions have been found in the relation between Ys and Ad, as pointed above, which demonstrated that the foregoing hypothesis is not valid. Hence, to create Ys map, scale effect must be taken into consideration. And the correction procedure must be adopted to convert the Ys of each drainage area to a new reference Ys value with a standard unit. Using the methods in this paper, one may create any scaled Ys maps on any drainage river as to compare Ys among different tributaries and to outline the high Ys areas for protection, without considering the scale of the tributaries. Meanwhile, based on the aim of the paper, one may also select the proper standard unit to create scaled Ys map, as to make the management more reasonable and economical. Additionally, under natural condition, scale effects of Ys may be influenced by many factors, which make the YseAd relations different among different drainage river basins. As the result, the mechanism of different YseAd relations may be better revealed by analyzing as many river basins as possible, which may be applied to create the relation for the river basin with limited data. 6. Conclusion The scale effects of Ys are studied for the entire Zhujiang basin and its four sub-basins. For the entire Zhujiang basin, there is no apparent relation between Ys and Ad when Ad is < 100,000 km2. However there is an upper envelope outline of Ys, i.e., Ys increases with Ad, and then declines. The increase of the Ys of the envelope outline may be attributed to the hill slope variations and the spatial distribution of LULC. The decrease of Ys of the envelope outline may be attributed to the dramatic decrease of the bed slopes. Ys decreases with Ad when Ad is > 100,000 km2, which may be attributed to the gentle bed slope (<0.015&) of the stations mainly located in the middle and lower reaches of Zhujiang basin. Study of four sub-basins of Zhujiang basin indicates there are three Ys- Ad relations: 1) For the upper reach sub-basin, Ys increases with the increase of Ad. The positive relationp between Ys and Ad may be attributed to the spatial distribution of LULC on the hill slopes and the steep bed slopes of the main river channel in the upper reach. 2) For Liujiang-Guijiang and Yujiang sub-basins in the middle reach, Ys increases with Ad to a maximum then declines. The increase of Ys with the increase of Ad may be attributed to the spatial distribution of LULC of the hill slopes, i.e., with the decrease of the hill slopes, LULC changes from forest, bush and grass dominated to the irrigation/non-irrigation field dominated. The decreases of Ys with Ad may again be attributed to the dramatic decrease of the bed slopes. 3) For Beijiang-Dongjiang sub-basin in the lower reach, Ys decreases with Ad. Soil erosion is mainly occurred on hill slopes with thick granitic weathering crust, intensive deforestation and slope cultivation. With the increase of Ad, YS decreases for the gentle bed slopes. Two scaled Ys maps with the standard units of 1000 km2 and 10,000 km2 are created using Kriging method. More than 80% of the basin has Ys < 300 t km2 yr1. There are two high Ys centers with Ys > 300 t km2 yr1: Beipanjiang sub-area in the upper reach and Luodingjiang basin in the middle reach. The rest area has Ys between 100 and 300 t km2 yr1 with the standard units of 10,000 km2. About 19% of the basin has Ys between 0 and 100 t km2 yr1 with the standard units of 1000 km2. The comparison of the scaled Ys map with standard unit of 1000 km2 to that with the standard unit of 10,000 km2 indicates that the increase of the Ys mainly occurs on the three sub-basins of the upper and the middle reaches of Zhujiang basin, while the decreases of the Ys are mainly occurs on the sub-basin in the low reach. The changes of the
9
Ys with different standard units could reveal the soil erosion distribution of Zhujiang basin. Acknowledgements Financial support from the National Natural Science Foundation of China (40701018) is gratefully acknowledged. All hydrometrical data used in this study is requested from the Ministry of Water Conservancy and Electric Power (MWCEP), and we wish to express our deep gratitude to all workers at the gauging stations, whose hard work made this study possible. The Data Sharing Infrastructure of Earth System Science of the Institute of Geographic Sciences and Natural Resources Research is also greatly appreciated for providing the digital maps of geology, soil and LULC of China. And we also want to express our deeply gratitude Dr. Lan Zhang of the University of Akron for her careful proof-reading. References An, H.P., Zhou, J.W., 1994. Soil erosion and lts preventive strategies in south-Pan river and North-Pan river of Guizhou province. Journal of Soil Erosion and Water Conservation 3, 36e45 (in Chinese). Ashmore, P.E., Day, T.J., 1988. Spatial and temporal patterns of suspended-sediment yield in the Saskatchewan River basin. Canadian Journal of Earth Sciences 25, 1450e1463. Cheng, Y.Q., 1990. Geological atlas of China (1: 5,000,000). China Cartographic Pressing House, Beijing (in Chinese). Church, M., Ham, D., Hassan, M., Slaymaker, O., 1999. Fluvial clastic sediment yield in Canada: scale analysis. Canadian Journal of Earth Sciences 36 (1), 1267e1280. Church, M., Slaymaker, O., 1989. Disequilibrium of Holocene sediment yield in glaciated British Columbia. Nature 337, 452e454. Dai, S.B., Yang, S.L., Cai, A.M., 2008. Impacts of dams on the sediment flux of the Pearl River, southern China. Catena 76, 36e43. de Vente, J., Poesen, J., Verstraeten, G., 2005a. The application of semi-quantitative methods and reservoir sedimentation rates for the prediction of basin sediment yield in Spain. Journal of Hydrology 305 (1e4), 63e86. de Vente, J., Poesen, J., 2005b. Predicting soil erosion and sediment yield at the basin scale: scale issues and semi-quantitative models. Earth-Science Reviews 71, 95e125. de Vente, J., Poesen, J., Arabkhedri, M., Verstraeten, G., 2007. The sediment delivery problem revisited. Progress in Physical Geography 31, 155e178. Dedkov, A.P., Moszherin, V.T., 1992. Erosion and Sediment Yield in Mountain Areas of the World, vol. 209. IAHS Publ, pp. 29e36. Dedkov, A.P., 2004. The Relationship between Sediment Yield and Drainage Basin Area. In: Sediment Transfer through the Fluvial System, vol. 288. IAHS Publ, Wallingford, UK, pp. 197e204. Han, G.H., Chen, X.B., 1999. Spatial structure and the changing trends of Chinese population resources in recent thirty years. Population & Economics 6, 56e58 (in Chinese). He, L., Yang, Q., Ou, S., 2003. Temporal variation of runoff and sediment on the main channels of Xijiang River and Beijiang River. Guangdong Water Resources and Hydropower 1, 57e62 (in Chinese). Institute of Geographical Science and Natural Resources Research, CAS (IGSNRRCAS), 1991. Land Use Maps in China (1: 4,000,000). SurveyingandMapping Press, Beijing (in Chinese). Institute of Soil Science, CAS, 1990. Soil Atlas of China (1: 4,000,000). China Cartographic Pressing House, Beijing (in Chinese). Likens, G.E., 2010. River Ecosystem Ecology: a Global Perspective. Academic Press/ Elsevier, San Diego, CA. Lin, C.H., Zhang, X.M., Zhang, H.L., 2002. Study on distribution of dry slopeland in Guizhou mountain Region. Journal of Soil and Water Conservation 16 (5), 90e 93 (in Chinese). Ministry of Water Conservancy and Electric Power (MWCEP), PRC, 1962e1975. National Standards for Hydrological Survey, vol. 1e7. China Industry Press, Beijing (in Chinese). Ministry of Water Resource, PRC, 1954e2010. China River Sediment Bulletin. China Water Power Press, Beijing (in Chinese). Morris, G.L., Fan, J., 1997. Reservoir Sedimentation Handbook. McGraw-Hill, New York. Owens, P.N., Batalla, R.J., Collins, A.J., et al., 2005. Fine-grained sediment in river systems: environmental significance and management issues. River Research and Applications 21, 693e717. Rekolainen, S., Ekholm, P., Heathwaite, L., Lehtoranta, J., Uusitalo, R., 2006. Off-site impacts of erosion: eutrophication as an example. In: Boardman, J., Poesen, J. (Eds.), Soil Erosion in Europe. JohnWiley and Sons Ltd., Chichester, United Kingdom, pp. 775e789. Sun, H.G., Han, J., Li, D., Zhang, S.R., Lu, X.X., 2010. Chemical weathering inferred from riverine water chemistry in the lower Xijiang basin, South China. Science of the Total Environment 408, 4749e4760.
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Xu, J.X., Yan, Y.X., 2005. Scale effects on specific sediment yield in the Yellow River basin and geomorphological explanations. Journal of Hydrology 307 (1e4), 219e232. Yan, Y.X., Wang, S.J., Chen, J.F., 2011. Spatial patterns of scale effect on specific sediment yield in the Yangtze River Basin. Geomorphology 130 (1e2), 29e39. Yan, Y.X., Xu, J.X., 2007. A study of scale effect on specific sediment yield in the Loess plateau, China. Science in China D: Earth Science 50 (1), 102e112. Zeng, L.Y., 2008. Study on Soil Erosion in Karst Area Based on RUSLE Modelda Case Study of the Catchment of Hongfenghu Lake in Guizhou Province[D]. Peking University, Beijing (in Chinese). Zhang, Q., Xu, C., Gemmer, M., Chen, Y., Liu, C., 2009. Changing properties of precipitation concentration in the Pearl River basin, China. Stochastic Environmental Research and Risk Assessment 23 (3), 377e385. Zhang, S.R., Lu, X.X., Higgitt, David L., et al., 2008. Recent changes of water discharge and sediment load in the Zhujiang (Pearl River) basin, China. Global and Planetary Change 60, 365e380. Zhang, S.R., Lu, X.X., 2008. Chemical and physical weathering in a mountainous tributary of the Zhujiang (Pearl River), China. In: Schmidt, Jochen, et al. (Eds.), Sediment Dynamics in Changing Environments, 325. IAHS Publ, pp. 359e366. Zhang, W., Mu, S., Hang, Y., et al., 2011. Temporal and spatial variability of annual extreme water level in the Pearl River Delta, China. Global and Planetary Change 69, 35e47.
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Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin Yunxia Yan a, *, Suiji Wang a, Ming Yan a, Li He a, Lan Zhang b a Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences (CAS), Beijing 100101, China b Department of Civil Engineering, University of Akron, Akron, OH 444325, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
The scale effects of Area-Specific sediment yield (YS) are studied for Zhujiang basin and its four subbasins. Considering the entire Zhujiang basin, there is no apparent relation between Ys and contributing area (Ad) for stations when Ad is less than 100,000 km2, while there is an upper envelope outline of Ys. For the stations with Ad larger than 100,000 km2 mainly located on the middle and lower reaches, there is a negative relation of Ys to Ad. Considering the four sub-basins of Zhujiang basin, three kinds of relations between YS and Ad are established: (1) Ys increases with Ad for the sub-basin of the upper reach; (2) Ys increases with Ad then declines for the two sub-basins of the middle reaches; (3) Ys decreases with Ad for the sub-basin of the lower reaches. The YseAd relation is further investigated using the spatial variation of hill slopes, the Land Use and Land Cover (LULC), and the bed slopes. Finally, to remove the effect of Ad and to make Ys comparable across scales, the scaled Ys calculation equations are given for both linear and non-linear relations between Ys and Ad first. And then the Ys are scaled to the standard units of 1000 km2and 10,000 km2. Using Kriging method, the Ys maps are created under GIS platform where high Ys centers are outlined, and the spatial patterns of Ys are compared for the two standard unit Ys maps. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
steep slope); whereas, the sediment may have more chances to be deposited in the middle and lower reaches (i.e., stations with large contributing areas located at low elevation and/or gentle slope). With this in mind, the negative correlated relations between YS and Ad has been applied for many river basins around the world (Walling, 1983; Morris and Fan, 1997; Church et al., 1999; Syvitski et al., 2005; de Vente et al., 2005a; Vanmaercke et al., 2011, and among others). However, this assertion has been challenged in many studies, both positive linear and non-linear relations between YS and Ad have also been found for many river basins around world (Ashmore and Day, 1988; Church and Slaymaker, 1989; Church et al., 1999; Dedkov and Moszherin, 1992; Dedkov, 2004; de Vente and Poesen, 2005b; Walling, 2005; Xu and Yan, 2005; Yan and Xu, 2007; Yan et al., 2011; Vanmaercke et al., 2011; as a few examples). The above studies clearly indicated that the YS e Ad relation may be significantly impacted by the local factors, such as topography, lithology, landscape history, LULC, human impacts, and the scaledependent contribution of different erosion processes (e.g. Church and Slaymaker, 1989; Walling and Webb, 1996; Church et al., 1999; Xu and Yan, 2005; Syvitski and Milliman, 2007; de Vente et al., 2007; Vanmaercke et al., 2011).
Area-specific sediment yield (Ys, t km2 yr1, i.e. the quantity of sediment reaching the catchment outlet per unit of time and per unit of area) at the catchment level has received considerable attention. It is crucial for many issues, such as capacity loss of reservoirs, soil erosion prevention, sedimentation of estuaries and harbors, coastal erosion, river delta development, transportation of pollutants and damage to ecological habitats (e.g. Syvitski et al., 2005; Owens et al., 2005; Rekolainen et al., 2006; ; Verstraeten et al., 2006). Ignoring the influence of scale on YS, YS is usually computed and compared using total sediment load divided by the contributing area (Ad). However, sediment mobilization and deposition effects are often systematically distributed in the landscape and impose a scaling characteristic on sediment yield (Church et al., 1999). Conventionally, it has been understood that sediment may have more chances to be transported in the upper reach (i.e., stations with small contributing areas located at high elevation and/or with * Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Yan). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.08.002
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Throughout the years, many studies have been carried out on Zhujiang basin. Dai et al. (2008) studied the impact of dams on the sediment flux. Zhang et al. (2008) studied the spatial changes of discharge and sediment load. Zhang et al. (2011) revealed the trends of annual and seasonal variation of suspended sediment load and analyzed the influence of human activities on the variation of the suspended sediment load. However, little research has been focused on the spatial variation of Ys in Zhujiang basin. Thus it is much needed to investigate as many river basins as possible to establish the general and specific YS e Ad relation. To this end, the objective of this study is to investigate the YS e Ad relation as following: (1) establish the relation of YS to Ad; (2) analyze the factors that may be responsible for the spatially varied relation between YS and Ad; (3) create scaled Ys maps with different standard units; and (4) study the spatial variations of Ys maps. 2. Study area Originating in Wumeng Mountain in Yunnan Province in the west, Zhujiang flows east and empties into the South China Sea. Zhujiang basin is the third largest in China in terms of drainage area, (i.e., A ¼ 453:7 103 km2 ; Zhang et al., 2008). Zhujiang is a compound river system composed by the first order tributaries of Xijiang, Beijiang and Dongjiang, and some small tributaries draining into Zhujiang delta (Fig. 1). Xijiang is the largest tributary located in the upper and middle reaches of the river covering 78% of Zhujiang basin (Sun et al., 2010). The upper reach includes two big tributaries, Nanpanjiang and Beipanjiang (first order tributaries of Xijiang). The middle reach includes big tributaries of Liujang, Guijiang and Yujiang (first order tributaries of Xijiang). The lower reach includes Beijiang and Dongjiang (Fig. 1). In climatic sense, Zhujiang basin is dominated by subtropical to tropical monsoon climate with the Tropic of cancer passing through the basin. Decreasing from east to west, the mean annual temperature and precipitation across the basin ranges from 14 C to 22 C, and 1200 mm to 2200 mm respectively. Precipitation during Aprile September accounts for 72e88% of the annual total. The interannual variation of the precipitation is around 14%e16% (Zhang et al., 2008; Zhang et al., 2009).
In hydrological sense, the annual average runoff and sediment load from 1954 to 2010 of whole basin are 283 109 m3, 72.4 106 t respectively (Ministry of Water Resource, PRC, 1954e 2010). From April to September, the accumulative runoff and sediment load account for 84% and 94% of the annual totals. Runoff and sediment load during the flood season (June to August) account for 50% and 72% of the annual totals (He et al., 2003). In geographical and geological senses, surrounded by the Yunnan-Guizhou Plateau in the west and the Five Ridges in the south, 85% of Zhujiang basin is mainly covered by hills (elevation ranging from 50 to 500 m) and mountains (elevation ranging from 500 to 2000 m) shown in Fig. 2. Karst is very common in Zhujiang basin due to the widely distributed carbonate rocks mainly located in the upper and middle reaches (about 45% of the basin). Clastic rock is mainly located in the upper reach (about 27% of the basin). Granite is mainly located in the lower reach (about 16% of the basin) (Cheng, 1990). Bounded by 24 N latitude, the lateritic red soil is mainly distributed in the south of the basin, red soil is mainly distributed in the north. Yellow soil is usually distributed on mountains with elevation higher than 700 m and limestone soil is mainly distributed in limestone mountains of Xijiang basin (Institute of Soil Science, 1990; Likens, 2010). In the sense of LULC, Fig. 3 shows LULC of Zhujiang basin obtained from Institute of Geographical Science and Natural Resources Research (1991). Timber forest, bush, and non-irrigated field are mainly distributed in the upper reach and Yujiang tributary in the middle reach. Irrigated field is mainly distributed in Liujiang and Guijiang of the middle reach and Beijiang and Dongjiang of the lower reach. Sparse wood, grassland and paddy are distributed across the entire Zhujiang basin. Fig. 4 shows the percentage of coverage regarding to each classified LULC. 3. Data, methods and sub-basin division 3.1. Data The suspended sediment load (SSL) of Zhujiang basin are requested from Ministry of Water Conservancy and Electric Power (MWCEP)dunpublished data provided only for internal use to
Fig. 1. Distribution of sub-basin and hydro-stations in Zhujiang basin.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Fig. 2. DEM of Zhujiang basin (http://srtm.csi.cgiar.org/).
further the engineering design and scientific research which the measurement (including river stage, discharge, suspended sediment concentration (SSC), grain size distribution, and SSL) are strictly following the national standards issued by MWCEP (1962, 1975) as: (1) water flow velocity is measured at vertical profiles at different depths (normally the surface, 0.2H, 0.6H, 0.8H and the bottom, where H is the depth of the water column); (2) SSC is measured by bottle samples of water at the depths where the flow velocities are measured; (3) water discharge is calculated using the cross-section area and the mean velocity of the cross-section; (4) SSL is calculated using water discharge and SSC. In the dry seasons, water flow velocity and SSC are measured every 3e5 days. In the flood seasons, water flow velocity and SSC are measured every 1e3 days. During the flooding events, water flow velocity and SSC are measured several times a day. Limited by data available, the annual SSL data from 1954 to 1987 are used for the study. To further minimize the impacts of
extreme flood, drought, and disturbance at any given station (e.g., dams, mining that might obscure the regional trends), the stations applied in the study are determined with the following two criteria: (1) the stations with at least four years of data available; (2) removing the stations with disturbance. Therefore, 90 stations are identified for the analyses with the time scales extending from 4 years to 33 years with an average of 18.5 years as shown in Fig. 1. The following maps are applied to reveal the mechanism of the scale effects of Ys: maps of geology (1:5,000,000) (Cheng, 1990), soil (1:4,000,000) (Institute of Geographical Science and Natural Resources Research, Chinese Academy of Science (IGSNRR-CAS), 1991), LULC (Institute of Soil Science, 1990). All the maps are digitalized using the Data Sharing Infrastructure of Earth System Science (DSIESS) in IGSNRR-CAS. The STRM 90 m DEM data of Zhujing basin are obtained from CGIARCSI website (http://srtm.csi.cgiar. org). The bed slope are calculated from DEM. Areas of rocks and
Fig. 3. LULC of Zhujiang basin.
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Fig. 4. Percents of different LULC types of Zhujaing Basin.
LULC (if not specified) are estimated from corresponding digitalized maps in ArcGIS platform. 3.2. Method The Ys scaling relations are studied in the following fashion: (1) applying the logarithmic transformation with base 10 for Ys and Ad; (2) establishing the relation by fitting linear/nonlinear equations to the transformed Ys and Ad; (3) calculating the scaled Ys; (4) mapping Ys with different standard units using Kriging method; and (5) analyzing Ys e Ad relation based on topography, geology, lithology, and LULC. Following Church et al. (1999), and Yan and Xu (2007), the scaled Ys is calculated in the following manner: (I) Plotting the transformed data of Ys and Ad in log-10 space which is shown as the scatter plot in Fig. 5 where, Y ¼ Log(YS), X ¼ Log(Ad), and (X1, Y1) is the logarithmic transformed data of (Ys, Ad) of a given station; (II) Calculating the scaled YS (SY) using Yr (scaled Y1 under a standard unit of Xr in log-10 space based on either linear or non-linear relation as: ① SY calculation for linear relation (Fig. 5a) Given the fitted linear regression equation as, Yr is obtained by moving the point of (X1, Y1) to the new position of (Xr, Yr) along the line of
Y ¼ AX þ B0
(1)
Where:Y ¼ AX þ B0 is parallel to the original fitted equation; A is the slope, B, B0 are the intercepts. From Eq. (1), the intercept B0 is computated as:
B0 ¼ Y1 AX1
(2)
② SY calculation for non-linear relation (Fig. 5b): Given the fitted quadratic equation as Y ¼ AX2þBX þ C, the equation to calculate Yr is given as:
Y ¼ AX 2 þ BX þ C 0
(5)
Where: Y ¼ AX 2 þ BX þ C 0 is parallel to the original fitted equation; A, B, C, and C0 are parameters. From Eq. (5), C0 is calculated as:
C 0 ¼ Y1 AX12 þ BX1
(6)
Substitute Eq. (6) into Eq. (5), one obtains
Y ¼ AX 2 þ BX þ Y1 AX12 þ BX1
(7)
Finally, SY is calculated by Eq. (7) as:
Substitute Eq. (2) into Eq. (1), one obtains
Y ¼ AX þ Y1 AX1
Fig. 5. Illustration of the scaling process: (a): Linear equation; (b): Non-linear eqution.
(3)
SY ¼ 10Yr ¼ 10AXr þBXr þY1 ðAX1 þBX1 Þ 2
2
(8)
Finally, SY is calculated by Eq. (3) as:
SY ¼ 10Yr ¼ 10ðAXr þY1 AX1 Þ
3.3. Sub-basins division
(4)
Using the standard area unit of 100 km2 as an example, the Xr ¼ Log(100) ¼ 2, and the corresponding SY is
SY ¼ 10ð2AþY1 AX1 Þ
According to the similarities of topography, geology, and LULC, Zhujiang basin is divided into four sub-basins: the upper reach subbasin, Liujiang-Guijiang sub-basin, Yujiang sub-basin and BeijiangDongjiang sub-basin listed in Table 1 and further discussed in what follows.
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5
Table 1 The sizes and biophysical characteristics of the four sub-basins Sub-basin
Area (103 km2)
LULC (percent of the sub-basin)
Slopes (percent of the sub-basin)
Lithology
Dem
The upper reach sub-basin
114
<6 (15%) 6 e15 (35%) 15 e25 (22%) >25 (28%)
Carbonate rock:24% Clastic rock:44% Metamorphic rock:25%
<50 m (0%) 50e500 m (10%) 500e1000 m(23%) >1000 m(66%)
Liujiang- Guijiang sub-basin
109
Timber forest (24%) Grassland (22%) Bush(18%) Non-irrigated field (14%) Paddy (10%) Irrigated field (32%) Grassland (18%) Paddy (17%) Sparse wood (12%) bush (10%) Timber forest (19%) Bush(18%) Grassland (16%) Non-irrigated field(12%) Sparse wood (11%) Irrigated field (36%) Paddy (28%) Grassland (15%) Sparse wood (12%)
<6 (37%) 6 e15 (32%) 15 e25 (21%) >25 (10%)
Carbonate rock:49% Granite: 5% Clastic rock:39%
<50 m (2.3%) 50e500 m (63%) 500e1000 m(26%) >1000 m(5%)
<6 (45%) 6 e15 (29%) 15 e25 (18%) >25 (7%)
Carbonate rock:68% Granite: 8% Clastic rock:5% Metamorphic rock:10%
<50 m (3%) 50e500 m (67%) 500e1000 m (23%) >1000 m (12%)
<6 (49%) 6 e15 (29%) 15 e25 (16%) >25 (6%)
Carbonate rock:30% Granite: 32% Clastic rock:20% Metamorphic rock:8%
<50 m (24%) 50e500 m (59%) 500e1000 m (15%) >1000 m (2%)
Yujiang sub-basin
82
Beijiangedongjiang sub-basin
96
The upper reach sub-basin: It is mainly composed by YunnanGuizhou Plateau and Hengduan mountains. Around 90% of the sub-basin has elevation higher than 500 m (Fig. 2, Table 1). Clastic rock, metamorphic rock and carbonate rock are the dominate rocks. Timber forest, bush, and grassland are the major LULC types. Around 50% of non-irrigated field of the entire Zhujiang basin is concentrated in this sub-basin (Fig. 3, Table 1). Beipanjiang in this sub-basin is a tributary with highest elevations and more hills and mountains. More than 88% of Beipanjiang area has elevation higher than 2000 m (An and Zhou, 1994). Along with limited plains areas, soil erosion is much more intensive in Beipanjiang area, comparing to the rest of this sub-basin (An and Zhou, 1994; Lin et al., 2002). Thus Beipanjiang area (covering 13% of the upper reach sub-basin) is further divided as Beipanjiang sub-area (Fig. 1). Yujiang sub-basin: It is mainly composed by hills with elevation ranging from 50 to 500 m (Fig. 2). Carbonate rock and clastic rock are the dominate rocks (Table 1). The LULC types are similar to that of the upper reach sub-basin. This is another sub-basin where nonirrigated field is concentrated (Fig. 3, Table 1). Liujiang-Guijiang sub-basin: The topography of this sub-basin is similar to that of Yujiang sub-basin (Fig. 2). Carbonate rock is the dominate rock (Table 1). Irrigated field is concentrated in this subbasin which accounts about 50% of the total irrigated field in Zhujiang basin (Fig. 3, Table 1). Longjiang area in this sub-basin is
more mountainous (54% of area with elevation higher than 500 m) and with more hill grass and bush (covering 61% of the area), comparing to rest of the sub-basin. Thus, Longjiang area is also divided as Longjiang sub-area. Beijiang-Dongjiang sub-basin: It is mainly composed by hills with the elevation ranging from 50 to 500 m and plains with elevation lower than 50 m (Fig. 2). Granite and Carbonate rocks are the dominate rocks (Table 1). Irrigated field and paddy are the major LULC types. This is another sub-basin where irrigated field is concentrated (48% of the total irrigated field) (Fig. 3, Table 1). 4. Results 4.1. Scale effects of Ys of entire Zhujiang basin The Ys and Ad in log-10 space for entire Zhujiang basion is graphed in Fig. 6. It indicates log(Ad) ¼ 5 (i.e., Ad ¼ 100,000 km2) as the changing point for the entire Zhujiang basin. For stations with Ad <100,000 km2, there is no apparent relation between Ys to Ad. However, after removing the extreme YS (i.e., data of Beipanjiang sub-area, the soil erosion center of Zhujiang basin, An and Zhou, 1994; Lin et al., 2002), there is an upper envelope outline, i.e., Ys increases with Ad to around 600 t km2 yr1, then decreases with the changing point of Ad about 5000 km2 with the lowest Ys around 40 t km2 yr1. The stations with Ad >100,000 km2 are all located on the main channel of the middle and lower reaches. With the decrease of the bed slope in the main stem, there is a decreasing trend of the Ys vs. Ad. 4.2. Scale effects of Ys of each sub-basin (1) The upper reach sub-basin (Fig. 7a, Tab, 2)
Fig. 6. Relation of YS to Ad for Zhujiang basin.
A positive linear relation is found between Ys and Ad in log-10 space in the sub-basin excluding the Beipanjiang sub-area. Ys of Beipanjiang sub-area is found to be much higher than that of the rest of the sub-basin. Table 2 lists the ranges of Ys, Ad, mean and standard deviation of Ys, the fitted linear equation, R2, F-test results and corresponding significance level.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Y. Yan et al. / Quaternary International xxx (2013) 1e10
Table 2 Relation of YS to Ad for all sub-basins Sub-basin
Sub-area
The upper reach Beipanjiang Longjiang-Guijiang Longjiang Yujiang Beijiang-Dongjiang
Stations
Range of Ys(t km2 yr1)
Range of Ad(km2)
Mean of Ys (t km2 yr1)
Standard deviation of Ys
Fitting equation
R2
F
Sig。
19 5 29 4 15 15
94e425 591e1678 42e551 43e88 53e230 152e248
500e120000 5000e9000 200e46000 1900e16000 1600e90000 800e39000
244 920 187 71 116 180
105 451 110 16 55 36
y ¼ 0.1722x þ 1.7051 No relation y ¼ 0.414x2 þ 2.9695x2.9833 y ¼ 1.0141x2 þ 7.8213x13.092 y ¼ 0.4071x2 þ 3.4292x5.0336 y ¼ 0.13x þ 2.7384
0.50
12.279
0.001
0.37 0.96 0.45 0.58
7.702 10.993 4.932 18.239
0.002 0.209 0.027 0.001
Where : x ¼ Log(Ad); y ¼ Log(Ys).
(2) Liujiang-Guijiang sub-basin (Fig. 7b, Tab, 2)
4.3. Scaled Ys maps
Non-linear relation is found for this sub-basin (excluding the Longjiang sub-area) as well as for Longjiang sub-area. In LiujiangGuijiang sub-basin, Ys increases with Ad to the maximum, then declines in the log-10 space. The changing point (Ad) is around 3000 km2. The fitted curve of the sub-basin (excluding the LongJiang sub-area) lies above that of Longjiang sub-area, that is, given the same Ad, the Ys for the sub-basin (excluding Longjiang sub-area) is higher than that of Longjiang sub-area.
To build the scaled YS map, the standard units need to be chosen. Since the areas of all stations are greater than 100 km2, and only 6 out of the 90 stations with areas larger than 100,000 km2, 1000 km2 and 10,000 km2 are selected as the standard units. The observed Ys is then scaled into the standard units using Eqs. (1)e(8). Fig. 8 graphs the scaled YS maps created with Kriging method under the ArcGIS platform. It shows that more than 80% of Zhujiang basin has YS less than 300 t km2 yr1 under both standard units and is further discussed in what follows: Standard unit of 10,000 km2 (Fig. 8a): (1) 53% of the basin has YS between 100 and 200 t km2 yr1, which are mainly distributed in the middle and lower reach of the basin, i.e., LiujiangGuijiang, Yujiang, and Beijiang-Dongjiang sub-basins. (2) The areas with YS between 200 and 300 t km2 yr1 account for around 28% of the entire Zhujiang basin and are mainly located in the upper reach sub-basin and near the main stem in LiujiangGuijiang sub-basin. (3) There are two high YS centers with YS > 300 t km2 yr-1 (19% of the entire Zhuajiang basin): Beipanjiang sub-area in the upper reach sub-basin (larger one) and Luodingjiang basin (smaller one).
(3) Yujiang sub-basin (Fig. 7c, Table 2) The YseAd relation is found to be similar to that of the LiujiangGuijiang sub-basin, that is, Ys increases with Ad to the maximum then declines in log-10 space. The changing point is around 10,000 km2. (4) Beijiang-Dongjiang sub-basin (Fig. 7d, Table 2) A negative linear relation was found between Ys and Ad in log-10 space.
Fig. 7. Relation of YS to Ad for: (a) the upper reach sub-basin; (b) Liujiang-Guijiang sub-basin; (c) Yujiang sub-basin; (d) Dongjiang-Beijiang sub-basin.
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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Fig. 8. Ys map on standard unit of (a) 10,000 km2; (b) 1000 km2.
Standard unit of 1000 km2 (Fig. 8b): The areas with YS < 100 t km2 yr-1 (19% of the entire Zhuajiang basin) are mainly distributed in Yujiang sub-basin and Longjiang sub-area. The areas with YS between 100 and 200 t km2 yr1 (40% of the basin) are mainly distributed in the Liujiang-Guijiang sub-basin. The areas with YS between 200 and 300 t km2 yr-1 (about 28% of the entire Zhujiang basin) are mainly distributed in Liujiang-Guijiang subbasin and part of Bejiang-Dongjiang sub-basin. (3) Similar to that under the standard unit of 10,000 km2, there are two high YS centers with YS > 300 t km2 yr-1 (12% of the entire Zhuajiang basin): Beipanjiang sub-area in the upper reach sub-basin and Luodingjiang basin. 5. Discussion 5.1. Scale effect of Ys of entire Zhujiang basin There is no apparent relation between Ys and Ad for the entire Zhujiang basin (Fig. 6). For stations with Ad < 100,000 km2, the points are very scattered which is caused by the change of the topography, geology and LULC from the upper reach to the lower
reach. For the same Ad, Ys is usually higher for area with high elevation, steep slope and non-irrigated field than that with low elevation, gentle slope and irrigated field. The non-linear upper envelope curve of the Ys e Ad with the changing point of Ad ¼ 5000 km2 (Fig. 6) (i.e., after removing the extreme YS) may be explained by: slope croplands, including irrigated/non-irrigated field on slope of 5 -25 (Zeng, 2008), which are the main sediment resources of Zhujiang basin contributing more than 50% of the soil erosion volumes (An and Zhou, 1994; Zeng, 2008). For stations with Ad < 5000 km2 which are mainly located in upper reaches of tributaries with relatively steep hill slopes (Fig. 1), the increase of slope cropland results in the increase of the YS with the increase of Ad (reflected by the left part of the envelop curve in Fig. 6). For stations with Ad > 5000 km2 which are mainly located in the main channels of middle and lower reaches of large tributaries (Fig. 1), Ys decreases with Ad since sediment has more chances to deposit with the decrease of bed slopes and increase of flood plain areas (reflected by the right part of the envelope curve in Fig. 6). Stations with Ad >100,000 km2 are all located in the middle and lower part of the main channel, Ys decreases with Ad, since sediment has more chance to deposit with the decrease of bed slope.
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Y. Yan et al. / Quaternary International xxx (2013) 1e10
5.2. Scale effects of Ys for sub-basins (1) The upper reach sub-basin This sub-basin is located in the transition zone from YunnanGuizhou Plateau to Guangxi-Guangdong middle-low mountains and basins, where positive linear relation is found between Ys and Ad (Fig. 7a, Table 2). Steep slopes dominate this sub-basin (Table 1). Fig. 9 plots the four major LULC types (accounting for about 80%) with corresponding average elevations of this sub-basin. For the head water area which is dominated by timber forest (Figs. 3 and 9), soil erosion is limited and Ys is relatively low. With the increase of Ad, the LULC changes from timber forest to non-irrigated field and results in the increase of the soil erosion intensity, and positive relation between YS and Ad (Figs. 3 and 9). With the further increase of Ad, the steep bed slopes of the upper reach become the dominating factor to influence the change of Ys. Sediment transport is benefitted by the steep bed slopes with high elevations, limited floodplains (Table 1), and the good connectivity between slopes and channels such that YS increases further with the increase of Ad. Ys of Beipanjiang sub-area are higher than that of the rest of this sub-basin (Fig. 7a) that may be explained by: 1) higher elevation, almost 90% of this sub-area with elevation higher than 1000 m; 2) high percentage of slope cropland [80% with slope of >6 , and 50% with slope of >15 (Lin et al., 2002)]; 3) less percentage of timber forest (10% of the sub-area); 4) high frequency of landslide and debris flows (An and Zhou, 1994). (2) Liujiang-Guijiang sub-basin and Yujiang sub-basin in the middle reach
be explained as following. For Liujiang-Guijiang sub-basin, stations with areas < 3000 km2 are mainly located in the tributaries of Liujiang and Guijiang with steep slopes; however, stations with areas > 3000 km2 are mainly located in the main river channel with much gentler slopes. For Yujiang sub-basin, the changing point from tributaries to the main river channel is around 10,000 km2. The lower Ys of Longjiang sub-area comparing with that of the rest of Liujiang-Guijiang sub-basin (Fig. 7b) may be contributed to the more forest, bush and grass (70%) and less irrigated/nonirrigated field (15%), which are 41% and 42% respectively for the rest of Liujiang-Guijiang sub-basin respectively (Fig. 3, Table 1). (3) Beijiang-Dongjiang sub-basin in the lower reach Beijiang-Dongjiang sub-basin is located in the lower reach of Zhujiang basin. The sub-basin is dominated by hills and plains with elevation lower than 500 m. 64% of the sub-basin is covered by croplands (the highest among all sub-basins). The negative relation is found between Ys and Ad (Fig. 7d, Table 2) for the sub-basin. The negative relation between Ys and Ad may be attributed to the intensive soil erosion on the hill slopes caused by the dense population density. The population density is 278 people per km2, which is about twice of the other two sub-basins of Liujiang-Guijiang and Yujiang (Han and Chen, 1999). Deforestation and waste reclamation have been intensified and has been mainly occurred on slope land. As the result, the hill region suffering soil erosion (with soil erosion modulus > 200 t km2 yr1) increased from 7444 km2 in the 1950s e 1960s to 17,070 km2 in the 1980s (Dai et al., 2008). With the increase of Ad, Ys decreases for the gentle bed slopes. 5.3. Scaled Ys maps under different standard units
These two sub-basins, in the middle reach of Zhujiang basin, are all located in Guangxi Zhuang Autonomous Region (Guangxi) and covering most of Guangxi (Fig. 1), where non-linear relations are found between Ys and Ad (Fig. 7b, c, Table 2). Guangxi is basin-like with 60% of mountains, 17% of hills and tablelands, and 21% of plains and basins (Fig. 2). YS is relatively low for Mountain regions with elevation higher than 400 m which are mainly covered by forest and grass (accounting for 40% of Guangxi, Fig. 3). YS is high for the regions with elevations between 200 and 400 m which are mainly composed of low mountains and hills (accounting for 29% of Guangxi, Figs. 2e3). These regions have relatively intensive soil erosion caused by intensive human activities (Tan et al., 2002). In these regions, area with soil erosion modulus more than 200 t km2 yr1 increased from 12,000 km2 in the 1950s e 1960s to 30,600 km2 in the 1980s (Dai et al., 2008). YS starts to decrease again with Ad for the regions with elevation lower than 200 m which is mainly covered by plains and basins. In these regions, sediment is buffered from the slopes to channels (Fig. 2). There exist differences on the changing points of YS e Ad relations between the two sub-basins (Fig.7bec). The differences may
The scaled Ys maps with different standard units are useful to determine the key areas for soil loss control. Two high Ys centers (Ys > 300 t km2 yr1) appear at the similar location with the two standard units (Fig. 8). The first center around the Beipanjiang subarea is the compound results of the high elevations, steep slopes, slope croplands, and high frequency of landslides and debris flows (An and Zhou, 1994; Lin et al., 2002). The second center around Luodingjiang has been also called “the little Yellow River in the Guangdong province” for its high Ys (Zhang and Lu, 2008). Besides the steep slopes and intensive human activities, the rich sediment source from the weathered granite and sandstone is another reason for the high Ys of Luodingjiang (Zhang and Lu, 2008). Comparing Ys with different standard units, one may obtain the information about the area where soil erosion mainly occurs. For a river basin where soil erosion is mainly occured in the upper part, there usually exists a negative relation between Ys and Ad. The scaled Ys on the map with the smaller standard unit of Ad may be larger than that with the larger unit. This finding may be demonstrated by the scaled Ys maps of Beijiang-Dongjiang sub-basin (Fig. 8). However, for a river basin where soil erosion is mainly occurred in the middle and lower part, Ys with the smaller standard unit of Ad may be lower than that with the larger unit. This finding may be demonstrated by the three sub-basins on the upper and middle reaches of Zhujiang basin: upper reach sub-basin, LiujiangGuijiang sub-basin, and Yujiang sub-basin. (Fig. 8aeb). 5.4. References for the other drainage basins
Fig. 9. Average elevation of the major land covers of the upper reach sub-basin.
The traditional methods to create Ys map follow three steps as: first, select hydrometric stations with long-records of Ys; second, determine the Ad represented by the stations, either above a station or between two adjacent stations; third, draw the isolines of Ys using certain interpolation method. The traditional methods can be
Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002
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applied only under the hypothesis that the variation of Ad has no effect on Ys, or this effect can be omitted. However, some functions have been found in the relation between Ys and Ad, as pointed above, which demonstrated that the foregoing hypothesis is not valid. Hence, to create Ys map, scale effect must be taken into consideration. And the correction procedure must be adopted to convert the Ys of each drainage area to a new reference Ys value with a standard unit. Using the methods in this paper, one may create any scaled Ys maps on any drainage river as to compare Ys among different tributaries and to outline the high Ys areas for protection, without considering the scale of the tributaries. Meanwhile, based on the aim of the paper, one may also select the proper standard unit to create scaled Ys map, as to make the management more reasonable and economical. Additionally, under natural condition, scale effects of Ys may be influenced by many factors, which make the YseAd relations different among different drainage river basins. As the result, the mechanism of different YseAd relations may be better revealed by analyzing as many river basins as possible, which may be applied to create the relation for the river basin with limited data. 6. Conclusion The scale effects of Ys are studied for the entire Zhujiang basin and its four sub-basins. For the entire Zhujiang basin, there is no apparent relation between Ys and Ad when Ad is < 100,000 km2. However there is an upper envelope outline of Ys, i.e., Ys increases with Ad, and then declines. The increase of the Ys of the envelope outline may be attributed to the hill slope variations and the spatial distribution of LULC. The decrease of Ys of the envelope outline may be attributed to the dramatic decrease of the bed slopes. Ys decreases with Ad when Ad is > 100,000 km2, which may be attributed to the gentle bed slope (<0.015&) of the stations mainly located in the middle and lower reaches of Zhujiang basin. Study of four sub-basins of Zhujiang basin indicates there are three Ys- Ad relations: 1) For the upper reach sub-basin, Ys increases with the increase of Ad. The positive relationp between Ys and Ad may be attributed to the spatial distribution of LULC on the hill slopes and the steep bed slopes of the main river channel in the upper reach. 2) For Liujiang-Guijiang and Yujiang sub-basins in the middle reach, Ys increases with Ad to a maximum then declines. The increase of Ys with the increase of Ad may be attributed to the spatial distribution of LULC of the hill slopes, i.e., with the decrease of the hill slopes, LULC changes from forest, bush and grass dominated to the irrigation/non-irrigation field dominated. The decreases of Ys with Ad may again be attributed to the dramatic decrease of the bed slopes. 3) For Beijiang-Dongjiang sub-basin in the lower reach, Ys decreases with Ad. Soil erosion is mainly occurred on hill slopes with thick granitic weathering crust, intensive deforestation and slope cultivation. With the increase of Ad, YS decreases for the gentle bed slopes. Two scaled Ys maps with the standard units of 1000 km2 and 10,000 km2 are created using Kriging method. More than 80% of the basin has Ys < 300 t km2 yr1. There are two high Ys centers with Ys > 300 t km2 yr1: Beipanjiang sub-area in the upper reach and Luodingjiang basin in the middle reach. The rest area has Ys between 100 and 300 t km2 yr1 with the standard units of 10,000 km2. About 19% of the basin has Ys between 0 and 100 t km2 yr1 with the standard units of 1000 km2. The comparison of the scaled Ys map with standard unit of 1000 km2 to that with the standard unit of 10,000 km2 indicates that the increase of the Ys mainly occurs on the three sub-basins of the upper and the middle reaches of Zhujiang basin, while the decreases of the Ys are mainly occurs on the sub-basin in the low reach. The changes of the
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Please cite this article in press as: Yan, Y., et al., Spatial variation of scale effects of specific sediment yield in Zhujiang (Pearl River) basin, Quaternary International (2013), http://dx.doi.org/10.1016/j.quaint.2013.08.002