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LUCC Impact on Sediment Loads in Subtropical Rainy Areas

Xiaoling Chen, Shuming Bao, Hui Li, Xiaobin Cai, Peng Guo, Zhongyi Wu, Weijuan Fu, and Hongmei Zhao

Abstract the world also obviously showed that the erosion rates were In this paper, we evaluate the impacts of land-use/cover sensitive to land-use and related human activities (Walling, changes (LUCC) on sediment loads at the outlets of five sub- 1999). Land-use/cover change (LUCC) can alter the velocity of watersheds of the watershed by integrating water, whether in the form of streams or overland flow, by remote sensing and GIS with statistical analysis. The inten- changing slope or gradient and the roughness encountered sively farmed watershed is characterized by a mountainous by the flow (Wardrop and Brooks, 1998), which affect and hilly topography and a rainy climate. The primary goal sediment loads, and consequently impact the downstream of this paper is to help a better understanding of land- ecosystem. However, there is limited information available use/cover change and its driving forces. We discuss spatio- regarding the effect of land-use/cover change on the sedi- temporal variations in rainfall and sediment loads and ment loads. identify factors contributing to those variations, analyze the State-of-the-art methodologies have recently been devel- comprehensive impacts of land-use/cover change on chang- oped to enhance land-use/cover change detection, including ing climate and human activities, and conclude that the remote sensing and GIS techniques. Currently, satellite changing rates of forest cover and climate regimes are imagery possesses the capacity to detect land-use/cover primary factors for sediment discharges in the Poyang Lake change on various scales and can also derive many biophysi- watershed. Our results suggest that the eco-system still have cal parameters, allowing for spatial and temporal compar- large capacities to support human activities in the area. isons (Carlson and Arthur, 2000). Computer simulation models are becoming increasingly popular in predicting soil loss for various land-use and management practices (Bhuyan Introduction et al., 2002). This study is aimed at presenting a change It is well known that there is a complex interaction among detection result of land-use/cover change in the sub-tropical climate, land-use, vegetation cover density, erosion rates, and rainy area, taking the Poyang Lake watershed occurring sediment loads in the watershed. As an important factor for around 10-year interval in the 1990s as a case. The driving water quality, sediment not only affects the optical property of forces for those changes were then identified. The relation- water but acts as a carrier of pollutants. The previous studies ship between rainfall and sediment discharges showed the suggested that land-use can affect the soil erodibility and impact of land-use/cover change on sediments in the water- sediment source (Woodward and Foster, 1997) as well as the shed. The analysis of land-use/cover change and sediment amount of sediments generated by soil erosion (Yang, 2004). discharges in the Poyang Lake watershed would be helpful Human activities were proven to make its land-use/cover to reveal the trend of land-use/cover change, and then to patterns change more rapidly, and thus brought different provide reference background information for remote sensing impacts on bio-physical processes (Chakrapani, 2005; Chen in the cloudy and rainy areas. et al., 2005; Restrepo and Syvitski, 2006). Rapid land-use/cover changes may affect both water and sediment discharges. The effect of land-use and development is vital in understanding Study Area Description the global sediment flux, and the regional variations (Syvitski, The study area, the Poyang Lake watershed, is located to the 2003). After human settlement effects, climate shifts are often south of middle reach of the River in the monsoon the major driving factor on sediment discharges. The previous zone of East Asia. The area of the watershed is 162.2 103 study showed that the land-use was probably the dominant km2, taking up 97 percent of provincial territory, control on particulate fluxes in areas of low relief and large- which lies at 113°25E to 118°29E, and 24°29N to 30°05N. scale urbanization, while the mountainous regions were likely The nearly identical boundaries of natural watershed and to be dominated by natural processes (Wasson, 1996). the provincial administrative units bring a greatly advantage The results obtained from erosion plot experiments and for integrating data analysis related to biophysical factors and experimental watershed studies in many different areas of anthropogenic driving forces collecting from both natural and administrative units. The topography in the Poyang Lake watershed mainly consists of mountainous and hilly red soil areas. The Xiaoling Chen and Shuming Bao are with the Key Lab of elevation ranges from less than 10 m to around 2,000 m Poyang Lake Ecological Environment and Resource Develop- ment, Jiangxi Normal University, , Jiangxi, , 330022 ([email protected]). Photogrammetric Engineering & Remote Sensing Vol. 73, No. 3, March 2007, pp. 319–327. Hui Li, Xiaobin Cai, Peng Guo, Zhongyi Wu, Weijuan Fu, and Hongmei Zhao are with the State Key Laboratory of 0099-1112/07/7303–0319/$3.00/0 Information Engineering in Surveying, Mapping and Remote © 2007 American Society for Photogrammetry Sensing, Wuhan University, Wuhan, China, 430079. and Remote Sensing

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above sea level, and the watershed consists of 36 percent TABLE 1. SUBCATCHMENT CHARACTERISTICS OF POYANG LAKE WATERSHED mountainous area, 42 percent hilly area, 14.7 percent flat Sub- Gauge Area Average and upland area, and 7.3 percent water. The mountainous 2 3 area is mostly covered by forest, brush and grass, and the catchment River Station (km ) SSC (kg/m ) hilly area consists of natural vegetation, barren land, and Gan WaiZhou 80948 0.0978 cropland, respectively. The mountainous and hilly areas Xin MeiGang 15535 0.0693 contribute most sediment loads due to the soil erosion in Xiu WanJiaBu 3548 0.0693 the watershed. The flat and upland area is mostly used as Rao Chang River DuFengKeng 5013 0.0372 cropland and built-up area where much more human AnLe River HuShan 6374 0.0389 activities have been concentrated. The water mainly consists Fu FuRiver LiJiaDu 15811 0.0673 of the Poyang Lake and its five rivers, and Poyang Lake is the largest fresh water lake in China with an important impact on Jiangxi Province and the lower Yangtze River sub-watershed is the largest, and its area is almost as twofold regions, and has been designated by WWF as one of the as the sum of other four sub-watersheds (Table 1). The globally important ecological areas. Poyang Lake is charac- Gan River has the greatest amount of suspended sediment terized by its dramatic seasonal fluctuation of water level, concentration (SSC) and contributed the most water dis- which creates a vast area of wetland surrounding this big charges and sediment loads in the Poyang Lake watershed. lake (Figure 1). The area of the lake greatly varies with the The flooding period for the rivers in the Poyang Lake fluctuation of its water level. The water level rises during watershed mainly lasts from April to July, and the highest the flood period and then the water surface suddenly 2 water discharge and water level typically appear in May or expands, with a water surface area of around 3,210 km . June (Zhang et al., 2004). And, it drops at low water with its bottomland becoming The Poyang lake watershed belongs to a warm, humid exposed out; only several wandering watercourses remain subtropical climate which extends along the south to north during the dry period with a water surface area of about 2 direction, with 500 km for its widest west to east direction 146 km . and 620 km for its longest north to south direction. The In this watershed, the annual mean runoff of the 3 annual rainfall ranges from 1,341 mm to 1,940 mm, with 50 watershed is 152.5 billion m , accounting for 16.3 percent percent of it concentrated from April to June, and May and of the Yangtze River watershed. Surface runoff mainly runs June usually contributing the most monthly precipitation in a to the Poyang Lake, and the Poyang Lake holds water from year, ranging from 200 mm to 350 mm per month; even more five rivers: Gan River, , Xin River, Rao River, and than 700 mm in some extreme months. The spatial distribu- Xiu River which empty into the Yangtze River. Five sub- tion of rainfall often varies, and more precipitation usually watersheds consisting of the above five rivers constitute falls in the south, east, and mountainous area in the water- the Poyang Lake watershed. Among them, the Gan River shed. The mild climate, abundant rainfall and almost synchronism of water and heat are suitable to vegetation growth, which encourages agricultural productivity and other related human activities.

Data and Methods Data and Analysis The study was carried out with data from 21 years of daily rainfall at 22 meteorological stations, water discharge, and sediment loads at five gauge stations in the Poyang Lake watershed. Water flow in the river is a major factor influenc- ing sediment loads in the river, which had a significant correlation with rainfall. The monsoon season, which accounts for 85 percent to 95 percent of total annual rainfall in the watershed, is the main source of water flow in the river. Almost 85 to 98 percent of annual sediment loads in the river were transported during and preprocessed for further analysis the monsoon season (April to July). Some data are available for the Poyang Lake watershed. They consist of a time series of rainfall, suspended sediment concentrations (SSC), flow discharge, and water level which were measured at the outlets of five distinctive sub-watersheds for a 46-year period (1956 to 2001). A digital elevation model (DEM) (1:250 000 scale) was obtained to delineate the water- shed into five sub-watersheds in ESRI’s ArcHydro® module. Daily rainfall records of the years 1992 and 2000 from 22 meteorological stations within Jiangxi Province were collected to account for the rainfall of each sub-watershed (Figure 1). The data of each meteorological station were processed to 10-day period data by adding each continuous ten-day precipitation together. Thus, three ten-day period rainfall data were obtained each month and 36 ten-day- Figure 1. Location of study area. A color version of this period rainfall data for each year. The discrete data from figure is available at the ASPRS website: www.asprs.org. 22 rainfall stations were then interpolated into spatially continuous data for the entire study watershed using spline

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interpolation method. As a result, 36 continuous data layers patterns in such a large watershed do not vary much in the were obtained for each year. The mean ten-day-period short period, and the selected images can still capture the rainfall data for each sub-watershed was calculated using nature of the land-use/cover patterns. Zonal Statistical function in ArcGIS® spatial analysis module based on the continuous data layers. As a consequence, each Land-use/Cover Classification sub-watershed has 36 mean rainfall data at basis of ten-day- Remote sensing techniques have been used to acquire the period each year. land-use/cover data of the area with the aid of the software The daily sediment discharge data obtained from packages ERDAS Imagine® v. 8.6 and ArcGIS® v. 8.3. Land- the gauge station located at the outlet of each sub-watershed use/cover patterns for the study area were determined by were also processed to 10-day-period data using the method interpreting digital imagery. Landsat TM and ETM images for mentioned above. However, the interpolation did not the study watershed round about 1990 and 2000, respec- apply on sediment discharge data because the gauge station tively, were obtained. A digital line graph (DLG) (1:250 000 measured all the sediment discharged from the upstream scale) of the watershed was acquired and used in the rectifi- sub-watershed. Similarly, 36 ten-day-period sediment dis- cation of the images. The images were registered and re-sampled charge data were obtained for each sub-watershed each year. to UTM (North 50) coordinates by using the nearest neighbor After all the data were prepared, the rainfall data were method. The output images composed of 30 m 30 m correlated against the sediment discharge data at 10-day- pixels with an RMS error of less than one pixel. period basis for the years of 1992 and 2000, respectively, to In classification, masks were first used to remove most study the relationship between precipitation and sediment of water body pixels from the images to reduce spectral discharge and to study the changes during the study period. complexity of the images. In order to get a better classifica- The time series of rainfall and sediment discharge data tion result, the NDVI was used to classify the study area as was conducted on the five sub-watersheds to clarify the non-vegetation area, mixed area and vegetation area, which effect of human activities and climate change on sediment was roughly corresponded as NDVI0.1, 0.1NDVI0.3, loads over a long period. Fifteen rainfall stations within the NDVI0.3, respectively, although in this case the spectral watershed were selected. The daily rainfall data from the characteristics of vegetation had some variation in different rainfall stations from 1980 to 2000 were converted to annual images under study. The images without water pixels were data (see in Figure 1, stations from 1 to 15). Twenty-one further divided into three separated layers based on NDVI annual data sets for each rainfall station were then interpo- values. Then, an unsupervised classification was conducted lated using spline interpolation, and the mean annual to arrive at 20 clusters for three layers (Yin et al., 2005). rainfall data from 1980 to 2000 for the five sub-watersheds Each cluster was then examined and assigned as one of the were acquired using the procedure mentioned above. Twenty- following land-use/cover types: water, wetland, grassland, one years of sediment discharge data were plotted against forest, rice paddy, irrigated field, built-up, and exposed the corresponding rainfall data (Figures 4, 5, and 6). rock. The least significant difference method (F-test) was Considering the spatial resolution of Landsat sensors used to evaluate the results of the measurements in terms of is suitable for the study at the watershed scale, and its significant differences between land-use/cover types. Land- long time series is good for LUCC detection, Landsat images use/cover types were merged to physical units if no signifi- were selected for the LUCC detection in the Poyang Lake cant differences were found. In this process, additional watershed. The whole watershed is covered by 14 Landsat Landsat images from different dates were used to assist in TM/ETM scenes, and the main area lies in seven scenes correcting classification results. In order to collect ground (Table 2). Due to the wide area of the watershed plus its truth information, a 10-day intensive field checking effort cloudy and rainy climate, images acquired at the same time was made in the early-July 2005. Some errors in prior for a certain year are not easy to be collected. In this study, classification were then corrected. the Landsat images around 10-year interval were selected to account for the land-use/cover data source of two examined periods (Table 2). It is true that the images acquired at Results and Discussion different time may not represent the same land-use condi- tions of the given time, but we believe land-use/cover Land-use/Cover Change Detection The classification and detection of the satellite imagery for the two study periods provided the land-use/cover cate- gories, their areas and land-use/cover change. The spatial TABLE 2. LANDSAT TM/ETM IMAGES USED IN THE STUDY** distribution of various land-use/cover types and dynamics between the two time periods are well represented in Figure 2, Date Date in which Figure 2a is a result from images in the end of Image No. Path/Row (Landsat TM) (Landsat ETM ) 1980s and early of 1990s (briefly described as around 1990 1 120–39 Oct. 15, 1990 Nov. 3, 2000 in this study), and Figure 2b is a result from images in the 2 120–40** Oct. 20, 1992** Nov. 3, 2000** end of 1990s and early of 2000 (briefly described as around 3 120–41 Oct. 20, 1992 Oct. 21, 2001 2000 in this study). It is clearly shown that the land-use/ 4 121–39 July. 15, 1989 Dec. 10, 1999 cover types were strongly dependent on landforms. Agricul- 5 121–40** July. 15, 1989** Sept. 23, 2000** tural and built-up land-use types are confined to the flat 6 121–41** Oct. 16, 1988** Dec. 10, 1999** valleys, while the forest is distributed in mountains area, 7 121–42** Oct. 9, 1991** Dec. 26, 1999** and the grassland is mostly the new cover of mountains 8 121–43 Oct. 9, 1991 Dec. 26, 1999 area as result of deforestation or the hilly areas with thin 9 122–40** Oct. 18, 1992** Sept. 14, 2000** soil layer. 10 122–41** Nov. 9, 1994** Sept. 14, 2000** 11 122–42** Dec. 24, 1993** Nov. 20, 2001** After classification, accuracy assessment was conducted 12 122–43 Oct. 5, 1993 Nov. 20, 2001 by randomly selecting points on the images (Table 3); 13 123–40 Oct. 12, 1993 Sept. 24, 2001 the accuracy was 79.27 percent for the entire watershed. 14 123–41 Aug. 25, 1993.08.25 Dec. 29, 2001 The major error source appears to be exposed rock being confused with build-up and grassland with rice paddies **Images with a large proportion of study area coverage. due to their very similar spectral signatures. Some error

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Figure 2. Land-use/cover classification of two study periods: (a) 1990, and (b) 2000. A color version of this figure is available at the ASPRS website: www.asprs.org.

TABLE 3. ERROR MATRIX OF CLASSIFICATION ACCURACY ASSESSMENT ing of forest and grass in each sub-watershed which ranged from around 60 percent to 70 percent; the forest Reference Classified Number Producer’s User’s showed an obvious increase in a rate of about 5 percent. It Class Name Totals Totals Correct Accuracy Accuracy indicated that natural processes would play a very impor- Grassland 101 89 68 67.33% 76.40% tant role in the study sub-watersheds, and the ecosystem Rice paddy 125 123 102 81.60% 82.93% was still in a good state. The second largest cover rate in Exposed rock 23 56 17 73.91% 30.36% the five sub-watersheds was cropland, which consisted of Forest 258 251 234 90.70% 93.23% rice paddy fields and irrigated fields and ranged from Irrigated field 92 80 63 68.48% 78.75% 17 percent to 31 percent. The vegetation and cropland Built-up 17 55 17 100.00% 30.91% covered 90 percent to 96 percent area in each sub-water- Unused land 35 25 24 68.57% 96.00% shed, constituting the main land-use/cover type. Although Water body 92 64 64 69.57% 100.00% the built-up area increased by a rate of more than 70 Totals 743 743 589 percent in the Gan River sub-watershed and Fu River sub- Overall classification accuracy 79.27% watershed, it just covered less than 3 percent area for their sub-watersheds, and the agricultural activities could be considered as the dominant anthropogenic influence in the also comes from the overlap area of two images because five sub-watersheds. the external conditions such as sun elevation angle and Land-use/cover change is a combined result of interaction atmospheric condition at image acquisition time are of both natural process and social economic development. different. But in the short run, it is the social economy that accounts In order to identify the land-use/cover change in the much for the changes (Han et al., 2004). The growth in the study period, a change percentage of each land-use/cover urban population will increase the land used for roads, type, taking the early time as a basic comparison, was schools, hospitals, and other infrastructure, which may come calculated. Different levels of land-use/cover change from the other land-use types by replacing other land-use occurred in the two study times in the Poyang Lake types or by making full use of urban land. People in rural watershed. areas have to reclaim more land for food production by Table 4 shows the detailed information for the land- means of deforestation, filling in lakes, or harvesting wood for use/cover change in five defined sub-watersheds in this fuel, which results in land-use/cover change. According to the study. The largest cover occurrence was vegetation consist- population census data, the urbanization level was around

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TABLE 4. LAND USE/COVER CHANGE DETECTION FOR THE SUB-WATERSHEDS OF THE POYANG LAKE WATERSHED FROM THE EARLY 1990STOTHE EARLY 2000S

Rao River Xiu River Sub-watershed Sub-watershed

Gan River Fu River Xin River Sub-watershed Sub-watershed Sub-watershed Le’an River Region Liao River Region

Area (%) Area (%) Area (%) Area (%) Area (%)

Land Use/ Change Change Change Change Change Cover Type 1992 2000 (%) 1992 2000 (%) 1992 2000 (%) 1992 2000 (%) 1992 2000 (%)

Grassland 11.43 11.54 0.97 9.34 10.60 13.48 10.44 8.25 20.90 10.02 9.44 5.71 8.93 7.67 14.19 Paddy 18.22 15.17 16.75 19.74 14.70 25.53 21.12 19.89 5.81 12.97 13.56 4.55 22.07 18.05 18.21 Exposed 1.97 1.85 6.16 0.70 2.33 235.05 3.81 3.40 10.67 1.96 2.27 15.84 2.25 0.89 60.49 rock Forest 57.01 56.88 0.22 55.06 56.97 3.51 49.94 54.68 9.54 67.97 66.51 2.15 55.68 61.69 10.80 Irrigated 7.87 9.61 22.10 11.79 10.76 8.68 9.76 8.97 8.08 4.28 5.20 21.58 7.9 8.61 8.97 field Built-up 1.76 3.01 70.91 1.41 2.47 75.48 2.76 2.41 12.72 1.41 1.37 2.73 0.84 0.93 10.87 Unused 0.35 0.42 21.79 0.49 0.23 53.51 0.82 0.79 4.06 0.14 0.42 192.34 1.00 0.70 29.61 land Water body 1.27 1.49 18.05 1.32 1.94 46.54 0.89 1.58 77.95 0.65 1.23 90.78 0.69 1.34 94.31 Wetland 0.12 0.02 85.21 0.15 0.00 100.00 0.45 0.02 95.06 0.61 0.00 100.00 0.64 0.13 80.59

20.35 percent in Jiangxi province in 1990; although it was Sediment Discharges believed to be over-estimated, it was still lower than the The sediment transportation is a major process, and it is average level, 26.41 percent, in China. The level reached directly or indirectly caused by and leads to land-use/cover 36.22 percent in Jiangxi province in 2000, but it was much change through soil erosion, sedimentation, and other lower than the Country’s average level of 44.77 percent (Ye hydraulics. The water and sediment discharge usually reflect et al., 2003). It showed that in the study period, the urbaniza- the landscape, rainfall, land-use and cover change, soil tion level was in a relatively low stage compared with that erosion, and geomorphic character of rivers in the water- level in China. In addition, the provincial government shed. Land-use and cover change due to human activities has encouraged economic development in three basic fields and/or climate is the key to gaining an understanding of related to eco-agriculture, green food production, and eco- anthropogenic versus climate-change impacts on the hydro- tourism since 1998, which would be helpful for the soil logical process. Understanding the sediment discharge across preservation and thus reduce the sediment discharge in the a broad time scale will allow us to make better predictions watershed scale. for the future. The land-use/cover change is strongly influenced by the There is an obvious hydrographic cycle with two periods past and current conditions related to climate, hydrology, in the study area: flood period and dry period. The dry landforms, soil, and human activities in the watershed. The period is characterized by consistently low sediment discharge anthropogenic influences and changing climate can affect in the rivers of the Poyang Lake watershed, and the flood the sediment supply and discharge along the hydrological period has suspended sediment flux about five times greater pathway. than the dry period. From these facts, we may conclude that the spatial distribution of rainfall may coincide with an Rainfall in 1992 and 2000 identical contribution to sediment discharge if the land-use/ According to the study, the rainfall and sediment discharge cover pattern is unchanging in the same watershed. So after in the 10-day-period had showed a similar relationship; we excluding the rainfall impact, we may consider the land- choose the years of 1992 and 2000 to analyze the quantita- use/cover change to be a major reason for the sediment tive relationship and spatial distribution. It could be seen in discharge. Figure 3 that the annual total rainfall had a great variation The sediment discharge at the outlets of gauge stations in amount and spatial distribution in 1992 and 2000. The give an idea of the order of magnitude of erosion in the rainfall varied considerably between the meteorological upstream area of each sub-watershed controlled by the stations. The interpolation results in Figure 3 showed that gauge stations. Table 1 shows that the area of the five the rainfall in 1992 was higher than that in 2000, and the studied sub-watersheds has an order in their size: Gan River Liao River sub-watershed and Gan River sub-watershed in sub-watershed (80,948 km2), Fu River sub-watershed (15,811 the west part of the Poyang Lake watershed had received km2), Xin River sub-watershed (15,535 km2), LeAn River more precipitation. The precipitation was high in the sub- sub-watershed (6,374 km2), and Liao River sub-watershed watersheds of Fu River and parts of LeAn River and Xin (3,548 km2). The Gan River sub-watershed is almost twice as River in the east part of the Poyang Lake watershed, and large as the sum of other four sub-watersheds. According to low in other two sub-watersheds in the west part of the the analysis of water and sediment discharge in the time study area. The waters inflowing into the Poyang Lake period from 1956 to 2001, the greatest sediment discharge in through the rivers originated mainly from the rainfall-runoff the time series for all the gauge stations appeared in June for process in the five sub-watersheds; the sediment loads all five sub-watersheds, which suggested the rainfall should reaching Poyang Lake would heavily depend on rainfall and be a very important factor for sediment discharge. therefore have a significant correlation with it if the land- Figure 4 shows some trends of rainfall and sediment use and land-cover in the watershed has not changed. discharge in the time period from 1980 to 2000 for the five

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Figure 4. Annual total precipitation and sediment discharge in the five sub-watersheds of the Poyang Lake watershed during the time period from 1980 to 2000: (a) mean annual rainfall, (b) annual sediment discharge of outlets at gaging stations, and (c) annual sediment discharge for the Gan River sub-watershed. A color version of this figure is available at the ASPRS website: www.asprs.org. Figure 3. Spatial Distribution of Total annual Rainfall in (a) 1992 and (b) 2000. A color version of this figure is available at the ASPRS website: www.asprs.org. watersheds fluctuated and did not show an obvious trend in the time series (Figure 4b). It suggested that the sediment discharge in the Gan River sub-watershed had an important sub-watersheds of the Poyang Lake watershed. The rainfall anthropogenic influence, while other four sub-watersheds in the five sub-watersheds was found to have a similar did not make such a conclusion in the period from 1980 to fluctuation in the time series (Figure 4a). The sediment 2000. Figure 4c presents information on trends of sediment discharge was mainly concentrated in the Gan River sub- discharge at Wai Zhou gauge station and the average annual watershed and showed an obvious declined trend in the total rainfall for the Gan River sub-watershed in the time time series, while the sediment discharge in other four sub- period from 1980 to 2000. Although the two curves generally

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showed a similar fluctuation through the time series, which implied an important influence of climate change to sedi- ment discharge, the sediment discharge obviously showed a decline trend in the time series. The Gan River was well known for its very high sediment loads and high erosion rate, which contributed the most sediment discharge in the Poyang Lake watershed (Figure 5). According to the two curves in Gan River sub- watershed, same precipitation created less sediments in 2000 compared with that in 1992, which implied that the soil erosion should be less in 2000. This situation might be linked to some human activities. It also suggested that when the land was mostly covered by vegetation including forest, grass, and crops in such a rainy climate suitable to vegetation growth, a small amount of land-use/cover change would not change the relationship between rainfall and sediment discharge although the same amount rainfall may create less sediment. It showed a significant reduction in sediment loads as a result of erosion-control measures taken, which includes increase of forest coverage and some engineering measures. Figure 5 shows some relationship between rainfall and sediment discharge from data of 10-day period in both 1992 and 2000. All the relationships indicated that the rainfall and sediment discharge in 1992 were greater than that in 2000, and the same amount of rainfall corresponded to higher sediment discharge in 1992 compared with the situation in 2000. It suggested that the erodible area or soil erosion intensity should mitigate in 2000. The examined period witnessed a stable economic growth in Jiangxi province. The GDP in 1992 was 57.26 billion Yuan (7.3 billion USD) and reached 200.31 billion Yuan (25.7 billion USD) in 2000, which was 3.5 times of that in 1992. Although such rapid economic growth was surely a powerful force to drive the land-use/cover change from one category to some more profitable uses, the sediment discharge in total still decreased, and showed a high agreement with the annual total rainfall (Figure 5). It suggested that the ecosystem was not over-burdened by human activities in this rainy sub- tropical area. For other four sub-watersheds, the sediment discharge decreased orderly from Xin River sub-watershed, Fu River sub-watershed, LeAn River sub-watershed, and Liao River sub-watershed compared with the same rainfall level, which had a significant agreement with the rainfall. Their orders were almost identical to the cover rate of forest in every sub-watershed. It indicated that vegetation percentage of cover played a key role in controlling the sediment dis- charge. A total decrease trend of sediment discharge in 2000 compared with the situation in 1992 may imply the less soil erosion associated with a lower precipitation. From Table 5 we can see that the sediment discharge had a greater decrease rate compared with the rainfall decrease from 1992 to 2000. It may indicate some anthropogenic influences on sediment discharge except for the changing climate. Since the generation of sediment is a complicated process, it is not easy to explain all the facts. However, Figure 5. Relationship between rainfall and sediment there is no doubt that land-use/cover change should be a discharge in 10-day period for five sub-watersheds in major factor to reduce soil erosion, thus lessening sediment 1992 and 2000: (a) the Gan River, (b) 1992, and (c) yield in the investigated watershed. 2000. A color version of this figure is available at the The Poyang Lake watershed is located in monsoon zone ASPRS website: www.asprs.org. of south China and characterized by adequate rainfall. Red soil dominates in the area, which is easily eroded by water in the rainy season, and the watershed is characterized by mountains, which result in steep slopes in a large area polices were practiced (Xie, 1999). From then on, soil and provide opportunity for erosion. The soil erosion could erosion became more and more serious until the 1980s. be explained by both natural processes and extensive At the decision-making level, there was a need to raise human activities. Deforestation occurred frequently in recent awareness of the importance of appropriate policies for history, especially in 1950s when unfavorable government successful watershed management. Fortunately, the government

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TABLE 5. CHANGES OF RAINFALL AND SEDIMENT DISCHARGE FOR THE FIVE RIVERS IN THE POYANG LAKE WATERSHED IN BOTH 1992 AND 2000 (UNIT: 104 ton)

Precipitation (mm) Sediment Discharge (104 ton)

River/Gauge Station 1992 2000 Changes % 1992 2000 Changes %

Gan River/WaiZhou 17909.1 14644.8 326.43 18.23 1192.06 256.36 935.7 78.49 Xin River/MeiGang 2078.45 2115.68 37.23 1.79 288.76 139.11 149.65 51.83 AnLe River/HuShan 1995.10 1759.78 235.32 11.79 87.20 23.96 63.24 72.523 Liao River/WanJiaBu 1483.47 1265.29 218.19 14.71 35.40 26.73 8.67 24.49 Fu River/LiJiaDu 1886.59 1693.83 192.76 10.22 211.34 79.52 131.82 62.37 Total 9151.19 8427.82 723.37 7.90 1814.76 525.68 1289.08 71.03

acknowledged the erosion problem and promoted compre- soil erosion to some degree, which implied soil erosion due hensive erosion control. In this watershed, a management to different rainfall amount and intensity change and human module implemented since the 1980s named “integrated activities. This indicates that the sediment discharge meas- watershed management by taking mountain-river-lake as a ured at the river gauge station could be taken as an index to whole system” has become a successful model in China, determine the change rate of some land-use/cover types. which has been proven to be a series of scientific regula- From this study we may conclude that in a mountain- tions to maintain the sustainable land-use and sustainable ous rainy area with a vegetation of more than 60 percent development of both the economy and the ecosystem. The and a relatively low level of urbanization, vegetation, watershed has been becoming greener and greener by especially forest, would result in significantly lower sedi- increasing the vegetation cover, that is, the lands for forest ment, which strongly supports the implementation of the or grassland have been increasing. This may have a positive reforestation policy. As the urbanization in the Poyang effect on the adoption of conservation practices, especially Lake watershed is at a relative low level compared with for such a large contribution like Gan River sub-watershed the average level in China for the study period; it is not in the Poyang Lake watershed. conclusive how urbanization influences the sediment discharge. And meanwhile, a robust remote sensing classifi- cation for improving the classification accuracy needs to be Conclusions further studied, so that the results of land-use/cover change There is an increasing demand for quantitative information detection would be more reliable. Another useful conclu- at the watershed scale that would help decision makers sion that may be extracted from this study is that sediment to make appropriate decisions. Effects of land-use/cover discharge might be viewed as a good indicator to quantita- changes in sub-tropical rainy climate, especially in areas tively separate the biophysical and anthropogenic influ- with a long cropping history are much less discernible. In ences and identify critical thresholds leading to dramatic this paper, a sub-tropical rainy cropping area, the Poyang consequences upon watershed eco-system. It should be Lake watershed in China, was selected to study the land- quite helpful for a reasonable watershed development and use/cover change (LUCC) impact on sediment loads. In order management plan. to better understand how the LUCC affect the sediment loads in the Poyang Lake watershed, remote sensing, GIS tech- niques, and mathematical analysis were employed to study Acknowledgments the land-use/cover changes and sediment loads of Poyang This work was supported by the National Key Basic Research Lake watershed and their driving forces. and Development Program (Grant No. 2003CB415205), The results showed that sediment discharge in the Program for Changjiang Scholars and Innovative Research five sub-watersheds of the Poyang Lake watershed were a Team in University (Grant No. IRT0438) and the Opening combination of rainfall phenomenon and human induced Foundation of Key Lab of Poyang Lake Ecological Environ- land-use/cover change. The satellite image classification of ment and Resource Development (Jiangxi Normal University) two time periods illustrated the land-use/cover change in (Grant No. 200401006(1)). the Poyang Lake watershed. Due to the vast coverage of the study area (14 Landsat TM image scenes), it is not an easy task to classify all the images with a relatively high accu- References racy. In this study, field efforts were made to collect ground Carlson, T.N., and S.T. Arthur, 2000. The impact of land-use – land information to validate and correct classification results. cover changes due to urbanization on surface microclimate Population growth, economic development, and urbanization and hydrology: A satellite perspective, Global and Planetary are the social problems faced in the study area, and also Change, 25:49–65. identified as the driving forces that led land-use/cover Chakrapani, G.J., 2005. Factors controlling variations in river change in the watershed. sediment loads, Current Science, 88(4):569–575. Land-use/cover change has been well known influenced Chen, X.Q., E.F. Zhang, H.Q. Mu, and Y. Zong, 2005. A preliminary by some biophysical and anthropogenic factors. How to make analysis of human impacts on sediment discharges from the an index to quantify their different impacts is an interesting Yangtze, China, into the sea, Journal of Coastal Research, issue. This topic may be useful for suggesting mitigation in 21(3):515–521. rainy watersheds where similar situations are found. LUCC Han, F., J. Liu, and W. Wang, 2004. Study on land-use change and must look into studies that link natural processes and driving force in Hebei Province the past ten years, Territory & anthropogenic influences that up to now have been far from Natural Resources Study, 2:30–32. a deep understanding. The present study showed that Restrepo, J.D., and J.P.M. Syvitski, 2006. Assessing the effect of sediment discharge could reflect land-use/cover change and natural controls and land-use change on sediment yield in a

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