Spatially explicit reconstruction of cropland cover for China over the past millennium

Reconstructing historical land use and land cover change using explicit temporal,quantitative,and spatial information is not only the prerequisite for simulating long-term climate change and ecological effects but is also a scientific basis for comprehensively understanding the process and mechanism of anthropogenic land use alterations.Considering changes in historical borders and administrative divisions,a provincial cropland area dataset for China over the past millennium was created on the basis of existing estimations since the Song Dynasty.Land suitability for cultivation was then assessed by incorporating altitude,slope,soil texture,and the maximum potential productivity of climate.Subsequently,a gridding allocation model for cropland was constructed,and the provincial cropland area for 24 years over the past millennium was allocated into grids with a resolution of 10 km.The cropland area in China increased from 3.71×10^(7)ha in AD 1000 to 12.92×10^(7)ha in AD 1999,with a peak of 13.50×10^(7)ha in AD 1980.The total cropland area in China showed fluctuating increasing trends that can be divided into three phases:fluctuation without notable net change for AD 1000–1290,slow increase for AD 1290–1661,and rapid increase for AD 1661–1999.Spatially,cropland intensified in the middle-lower reaches of the Yellow and Yangtze Rivers and expanded to mountainous and frontier areas over the last millennium.Specifically,over the entire period,the fractional cropland areas(FCAs)in the middle-lower reaches of the Yellow and Yangtze Rivers increased by 1.4 and 0.8 times,respectively.Since the mid-Qing Dynasty,large-scale land reclamation expanded to areas of low reclamation in southwest and northeast China.The FCAs for southwest and northeast China increased from 2.13%and 0.55%in AD 1661 to 18.00%and 26.61%in AD 1999,respectively.For AD 1661–1999,the proportion of cropland increased by 55%in the hills and low mountains and 27%in the middle and high mountains.The comparison with remote sensing cropland data shows that the grid cells with absolute differences of 0–10%accounted for 70.35%of all grid cells,while grids with differences exceeding 60%accounted for only 0.83%.This finding indicates that the reconstruction method is feasible,and the reconstruction results objectively reveal the historical spatiotemporal changes in cropland.

Reduced aliasing artifacts using shaking projection k-space sampling trajectory

Radial imaging techniques, such as projection-reconstruction （PR）, are used in magnetic resonance imaging （MRI） for dynamic imaging, angiography, and short-T2 imaging. They are less sensitive to flow and motion artifacts, and support fast imaging with short echo times. However, aliasing and streaking artifacts are two main sources which degrade radial imaging quality. For a given fixed number of k-space projections, data distributions along radial and angular directions will influence the level of aliasing and streaking artifacts. Conventional radial k-space sampling trajectory introduces an aliasing artifact at the first principal ring of point spread function （PSF）. In this paper, a shaking projection （SP） k-space sampling trajectory was proposed to reduce aliasing artifacts in MR images. SP sampling trajectory shifts the projection alternately along the k-space center, which separates k-space data in the azimuthal direction. Simulations based on conventional and SP sampling trajectories were compared with the same number projections. A significant reduction of aliasing artifacts was observed using the SP sampling trajectory. These two trajectories were also compared with different sampling frequencies. ASP trajectory has the same aliasing character when using half sampling frequency （or half data） for reconstruction. SNR comparisons with different white noise levels show that these two trajectories have the same SNR character. In conclusion, the SP trajectory can reduce the aliasing artifact without decreasing SNR and also provide a way for undersampling recon- struction. Furthermore, this method can be applied to three-dimensional （3D） hybrid or spherical radial k-space sampling for a more efficient reduction of aliasing artifacts.

Reconstruction of cropland area and spatial distribution in the mid-Northern Song Dynasty （AD1004-1085）

To understand historical human-induced land cover change and its climatic effects, it is necessary to create historical land use datasets with explicit spatial information. Using the taxes-cropland area and number of families compiled from historical documents, we estimated the real cropland area and populations within each Lu （a province-level political region in the Northern Song Dynasty） in the mid-Northern Song Dynasty （AD1004-1085）. The estimations were accomplished through analyzing the contemporary policies of tax, population and agricultural development. Then, we converted the political region-based cropland area to geographically explicit grid cell-based fractional cropland at the cell size of 60 km by 60 km. The conversion was based on calculating cultivation suitability of each grid cell using the topographic slope, altitude and population density as the independent variables. As a result, the total area of cropland within the Northern Song territory in the 1070s was estimated to be about 720 million mu （Chinese area unit, 1 mu = 666.7 m2）, of which 40.1% and 59.9% oc- curred in the north and south respectively. The population was estimated to be about 87.2 million, of which 38.7% and 61.3% were in the north and south respectively, and per capita cropland area was about 8.2 mu. The national mean reclamation ratio （i.e. ratio of cropland area to total land area; RRA hereafter for short） was bout 16.6%. The plain areas, such as the North China Plain, the middle and lower reaches of the Yangtze River, Guanzhong Plain, plains surrounding the Dongting Lake and Poyang Lake and Sichuan Basin, had a higher RRA, being mostly over 40%; while the hilly and mountainous areas, such as south of Nanling Mountains, the southwest regions （excluding the Chengdu Plain）, Loess Plateau and south- east coastal regions, had a lower RRA, being less than 20%. Moreover, RRA varied with topographic slope and altitude. In the areas of low altitude （~〈250 m）, middle altitude （250-100 m） and high altitude （1000-3500 m）, there were 443 million, 215 million and 64 million mu of cropland respectively and their regional mean RRAs were 27.5%, 12.6% and 7.2% respectively. In the areas of flat slope, gentle slope, medium slope and steep slope, there were 116 million, 456 million, 144 million and 2 million mu of cropland respectively and their regional mean RRAs were 34.6%, 20.7%, 8.5% and 2.3% respectively.

Gridded reconstruction of the population in the traditional cultivated region of China from 1776 to 1953

Using modern census and environmental factor data,this study first identified the environmental factors that significantly affect the population distribution through Geodetector analysis and then constructed a population spatial distribution model based on the random forest regression algorithm.Finally,with this model and historical population data that were examined and corrected by historians,gridded population distributions with a spatial resolution of 10 km by 10 km in the traditional cultivated region of China(TCRC,hereafter)were reconstructed for six time slices from 1776 to 1953.Using the reconstruction dataset,the spatiotemporal characteristics of the population distribution were depicted.The results showed that(1)the environmental factors that significantly affected the population density differences among counties in the TCRC mainly consisted of elevation,slope,relief amplitude,distances to the nearest prefectural and provincial capitals,distance to the nearest river and the climatology moisture index.(2)Using the census data of 1934 counties in the TCRC in 2000 and the abovementioned environmental factor data,a random forest regression algorithm-based population spatial distribution model was constructed.Its determination coefficient(R^(2))is 0.81.In 88.4%of the counties(districts),the relative errors of the model predictions were less than 50%.(3)From 1776 to 1953,the total population in the study area showed an uptrend.Prior to 1851,the population increased mainly in the Yangtze River Delta.During this period,the number of grid cells in which the population densities were greater than 500 persons per km^(2) increased from 292 to 683.From 1851 to 1953,the population increased extensively across the study area.In the North China Plain and the Pearl River Delta,the number of grid cells in which the population densities were greater than 500 persons per km^(2) increased from 36 to 88 and from 4 to 35,respectively.The spatial clustering pattern of the population distribution varied temporally.The potential reasons included the shifts in economic development hot spots,traditional beliefs,wars,famine,and immigration policies.(4)Between our reconstructions and the HYDE dataset,there are large differences in the data sources,selected environmental factors and modeling methods.As a consequence,in comparison to our reconstructions,there were fewer populations in the eastern area and more populations in the western area from 1776 to 1851 and more populations in urban areas and fewer populations in rural areas after 1851 in the HYDE dataset.

Reconstructing the historical spatial land use pattern for Jiangsu Province in mid-Qing Dynasty

This study is proposed to reconstruct a high-resolution spatial distribution of historical land use pattern with all land use types to overcome low-accuracy and/or the monotonic land use type in current historical land use reconstruction studies. The year of 1820 is set as the temporal section and the administrative area of Jiangsu Province is the study area. Land use types being reconstructed include farmland, residential land （including both urban land and rural residential land）, water body, and other land （including forest land, grassland, and unused land）. Data sources mainly refer to historical documents, historical geographic research outcomes, contemporary statistics, and natural environmental data. With great considerations over regional natural resources and social and economic conditions, a few theoretical assumptions have been proposed to facilitate the adjustment on prefecture farmland, urban land, and rural residential land. Upholding the idea that the contemporary land use pattern has been inherently in sequence with the historical land use pattern as well as the land use pattern shall be consistent to its accessibility, this study reconstructs the land use pattern in Jiangsu Province in 1820 with 100 m＊100 m grids based on accessibility analysis and comprehensive evaluation. The outcome has been tested as valid by regionalization and correlation analysis. The resulted spatial distribution shows that back in 1820 in Jiangsu Province： （1） farmland, urban land, rural residential land, water body, and other land take about 48.49%, 4.46%, 0.16%, 15.03%, and 31.86% of the total land area respectively; （2） the land use pattern features high proportion of land in farming while low-proportion land in non-farming uses while population, topography, and the density of water body lead to great spatial variations; and （3） the reconstruction methodology has been tested as reasonable based on significant positive correlations between 1820 data and 1985 for both farmland and rural residential land at the prefecture level.