| The land-use in developed area of China has changed dramatically in recent 20 years,of which,agricultural land area has decreased substantially due to rapid industrialization and urbanization.Changes in land-use may have significant effects on enviroment,soil quality and may limit the increasing of economic development,so monitoring the changes of land-use has been the case.But the traditional statistical method for acquiring the land-use data is time-consuming and hard sledding;besides, calculation mistakes easily occur and could result in errors.Satellite images for land and crop classification are in common use and remote sensing has become a powerful tool in the real-time identification of rand-use classes.A new generation of advanced optical sensor-the Moderate Resolution Imaging Spectral-radiometer(MODIS) provide dally global imagery at spatial resolutions of 250 m(red and NIR1) and 500 m(blue,green,NIR2,SWIRl,SWIR2) and hay obvious advantages for identifying paddy fields pixels compared to NOAA AVHRR 1 km spatial resolutions.The Landsat TM and ETM++ imagery offer much better quality data sets but at infrequent intervals to capture the seasonal dynamics of crops.Therefore,the combined TM /ETM+ data and MODIS data may be more suitable for timely monitoring the land-use changes of paddy fields.In this study,we test the performance of combining MODIS and TM/ETM+ analysis to extract yearly land-use informations of Wujiang County during 2000-2005.Land-use classmaps derived from the TM/ETM+ and MODIS data were further used to estimate the topsoil SOC pool and the changes of sequestration potential of paddy soils in Wujiang County(one of the earliest open districts in theYangtze River Delta of China).The soil organic carbon(SOC) data were obtained from the 2nd State Soil Survey(2SSS),the nationwide arable soil monitoring system(NASMC) and the county level soil reconnaissance data.The major results are as follows:â—According to the TM/ETM+ images analysis:1) The rice-plant area of 1984 paddy fields area has decreased 85.62 km2 during 1984-1991.The construction land,woodland(including orchard land,mulberry field,nursery land),upland and water body occupied 35.67 km2,13.79 km2,19.51 km2 and 16.65 km2, respectively.2) The rice-plant area of 1984 paddy fields was 542.95 km2 in 1998, having reduced 86.05 km2 during 1991-1998.The construction land,woodland, upland and water body occupied 47.31 km2,15.80 km2,13.01 km2 and 9.93 km2, respectively.The loss of the rice area during 1991-1998 is near to 1984-1991.But areas of the new-built of construction land and woodland during 1991-1998 were greater than 1984-1991,account for 54.98%and 18.36%of the total loss of paddy area,respectively.The areas of the new-built upland and water body during 1991-1998 were less than 1984-1991,account for 15.12%and 11.54%of the total loss of rice area,respectively;3) The land-use changes have speeded up during 1998-2005,the construction land and woodland occupied 253.28 km2 and 62.63 km2 of rice-plant area,respectively.The areas distribution of different land-use types of 1984 paddy fields in 2005 were as follows:rice 254.79km2,construction land 336.26 km2,woodland 92.22 km2,upland 10.25km2 and water-logged land 21.10km2,respectively.â—Classification feature derived from MODIS such as the vegetation indices,water indices could improve the accuracy in land-use classification.Among various classifiers,the new developed classifier by using the time series of vegetation indices and land surface water indices has the greatest potiential in monitoring the changes of land-use,and its accuracy achieved 66.60%.The decision tree classification,an algorithm based on human-computer interaction is applied to remote sensing image classification with better flexibility and robust compared with classic automatic classification technology,Classification accuracy based on neural network is not improved compared to Maximum Likelihood Classifier (MLC).In all,using a good classifier and choosing suitable features are important in MODIS land-use classification.â—Our newly-built land use classification method by combining MODIS and ETM+ data is favorable and feasible for land-use mapping.The total accuracy of 2000 MODIS land-use map is 85.25%when compare to the results of 2000 TM land-use classification.A significant linear relationship between 2000-2005 MODIS derived paddy fields areas and statistical data(R2 is 0.9873) demonstrated the potential of the newly-built MODIS classification method. According to the results of 2000-2005 MODIS images land-use classification,the total paddy fields area has decreased by 30%(113.57 km2) during 2000-2005. The area of upland was estimated to be nearly doubled during 2000-2003 but dropped dramatically during 2004-2005.This is probably because the fallow land was forbidden by the government and the fallow land was returned to paddy fields again.The waterlogged area was relatively stable in recently six years.â—The SOC contents for the plow layers in the paddy fields in 2003 follow a normal distribution.About 40%,30.6%and 12.5%of the soil samples of paddy soils had SOC contents ranging from 17.5 to 20 g·kg-1,from 15 to 17.5 g·kg-1 and 12.5 to 15 g·kg-1 in 2003.In 1984,in contrast,24.06%,39.89%and 18.97%of the soil samples had SOC contents ranging from 17.5 to 20 g·kg-1,from 15 to 17.5 g·kg-1 and from 12.5 to 15 g·kg-1,respectively.The percentage of samples with SOC contents greater than 20 g·kg-1 or less than 12.5 g.kg-1 in 2003 was relatively similar to that in 1984.However,the percentage of samples with SOC contents of 17.5-20 g·kg-1 in 2003 was greater than that in 1984.During 1980 to 2003,the SOC contents of paddy soils have been increased by 0.32 g·kg-1.This suggests that irrigation-based rice cultivation in Wujiang County has significantly increased SOC content compared to the data obtained by the 2nd SSS.â—The average SOC content of paddy soils exhibited an increase(ASOC) of 0.01g·kg-1 per year during the past 20 years.On the contrary,obvious decreases in the SOC content were found in uplands and woodlands.The annual decrease rate(â–³SOC) in uplands and woodlands were 0.03 g·kg-1 and 0.06 g·kg-1, respectively.The changes of SOC content under different land-uses conversions reflects that land-use changes may play an important role in C sequestration.â—The C stock in 2005 has increased by 0.49%during 22 years relative to the total of 2672884t C stock in topsoil of paddy fields in 1984.Of which,the C stocks have increased by 9454.9t during 1984.-1991,5834.1t during 1991-1998,2125.9t during 1998-2005.The increase rate of SOC stocks has reduced as a consequence of decreasing cultivation area of paddy rice since 1984.The total SOC loss has exceeded the total SOC sequestration since 2001 in 1984 paddy fields.Given that the SOC content of buried soils in construction land and newly converted fish-breeding water body keeps stable,the SOC was observed to decrease after paddy fields had been converted to plant nursery land during 1984 to 2005. Moreover,conversion of rice paddy to upland had been also shown decline in the SOC.In all,the method using MODIS and TM/ETM+ images for monitoring the changes of land-use is favorable in the study area during 1984 to 2005.Especially,the yearly land-use changes monitoring by using MODIS in addition to ETM+ images has obtained a reasonable results.The method used in this research could be regarded as a reference for land-use monitoring in other region or more large spatial scale in other developed area in China.According to the SOC stocks calculating results,the currently land-use changes in Wujiang County are not favorable for the increasing of the soil C stocks.Further more,the calculation of carbon pool based on data from 1980s soil survey could result in errors because dramatic change of land-use in China during the recent 20 years may has a great influence upon soil C stocks.To accurately estimate soil carbon pool in these areas,the timely informations about land-use changes based on the remote sensing are very useful.
|
PDF Full Text Request