| Facing the world food crisis, the requirements of population growth constantly improve on rice yield and quality, however, a huge population and limited acreage are basic national conditions of China. In this situation, it is particularly important to study the rice planting. Qijiang is located in the joint part of southeast of Sichuan basin and the Yunnan-Guizhou plateau, known as the "south gate of Chongqing" with complex terrain (mostly are mountains and hills) which belongs to karst landforms. With the largest planting area, rice cultivation has important status in food crops planting of Qijiang. Under the complex topography condition, researching the internal and external effects of Qijiang terrain on rice cultivation is a kind of supplement for choosing the high quality rice varieties and the improved planting technology. The landform has big difference in Qijiang, three-dimensional climate is obvious, and varieties of farming system and layout can also preliminary judge its ecological adaptability according to the terrain characteristics.In this paper, we use the DEM with 30 meters resolution of the study area to extract the main topographic index of Qijiang, combining the rice planting area extracted from land use map of Qijiang and village-level rice yield data investigated in 2012.Based on the series of software platforms(ARCGIS9.3, Sim-DTA、SPSS and MF-Office), we studied the influence of the topographic index for the regional distribution of rice cultivation in Qijiang by overlay analysis and statistical analysis; Based on K-means clustering analysis, correlation analysis and CART model analysis, we analysis the influences of the topographic index on rice yield, relative importance, and the correlation. The main conclusions are as follows:(1) The influence of terrain factors on regional distribution of rice cultivation in Qijiang:for elevation, rice planting area is mainly distributed in the range of 300m-750m with the distribution ratio reached 68.24%. For slope,87.26% of the rice cultivation area distributed in the places where slope is less than 20°, and the most concentrated distribution area is between 5°-15°. For aspect, the distributions of rice planting area in all kinds have some differences, but the differences are not too big. For relief degree, the distribution ratio reached 86.09% in the scope of relief less than 75m, which can be the main rice planting area of Qijiang. For terrain ruggedness index, rice planting area is mainly distributed in the places where terrain ruggedness index is less than 6, within the distribution proportion reached 71.91%. For topographic wetness index, rice planting area are mainly distributed in the range of 5-9, and rice planting area has the most concentrated distribution in the scope of 6-7. For specific catchment area, it presents the trend of reverse changes between rice planting area and catchment area in the mass; Rice planting area are mainly distributed in the scope where specific catchment area is less than 100 and 100 to 200, While the distribution under 500 accounting for 76.35% can be the main area for rice planting. For aspect, rice cultivation area of Qijiang has wide regional distribution in the middle slope with a distribution ratio of 57.82%.(2) The correlation of topographic index and rice yield in Qijiang:elevation, slope, relief, terrain ruggedness index, topographic wetness index significantly correlate with rice yield, while elevation, slope, relief, terrain ruggedness index negatively correlate with rice yield, and topographic wetness index and rice yield positively correlated with the correlation coefficient of 0.273; The correlation between elevation and rice yield is the strongest with a coefficient of-0.519; Differences in the relevant between slope, relief, terrain ruggedness index and rice yield is small with the correlation coefficient of -0.229,-0.219 and -0.232 respectively. However, aspect, specific catchment area and rice yield didn’t show obvious correlation.(3) The clustering relations of rice yield and topographic index:According to the DB index, the rice yield data was assembled into three classes, and corresponded to the low, middle, high yield, with low yield region clustering center of the rice yield is about 344.51 kg/mu, the clustering center of the middle yield of rice yield is about 484.61 kg/mu, the clustering center of the high yield of rice yield is about 561.37 kg/mu; rice yield is still concentrated in the middle area. From low yield to middle and to high yield, elevation, slope, relief, terrain ruggedness index and specific catchment area gradually decreases, and the average of topographic wetness index gradually increases, the average of aspect presented no regular change. It present the trend of inverse between the rice yield in the study area and the elevation, slope, relief, terrain ruggedness index, specific catchment area; the relationship with the topographic wetness index was positively to changes, and although the slope have certain effects on rice yield, but no significant correlation.For elevation, low production areas are mainly distributed in the range of 600m-1000m, with middle production areas are mainly distributed in the range of 400m-800m, and high production areas almost distributed in elevation below 600 m. For slope, low production areas are mainly distributed in the places where slope is more than 15° slope, while middle and high production areas are mainly distributed under 15°, but the distribution difference is not too big; while slope is under 15°, the effect on rice yield in the study area is abate, and can’t be the main factors influencing the yield of rice. Aspect has small influence on the rice yield in the study area. For relief, low yield region are mainly distributed in the places where relief is more than 50m, while middle and high production areas are mainly distributed below 60m (the difference of middle and high yield distribution is smaller). For terrain ruggedness index, low production areas are mainly distributed in areas that terrain ruggedness index is more than 6, and high production areas are mainly distributed in the area where terrain ruggedness index is less than 7; It present a trend of reverse change between rice yield and the terrain roughness index. For topographic wetness index, it presents a trend of reverse change between rice yield and topographic wetness index; Low yield are mainly distributed in the range of 6.5-7, while the medium and high yield are mainly distributed in the range of more than 6.75. For specific catchment area, rice yield and the change of the flow area have no significant relationship.(4) The relative importance of the topographic index on rice yield is as the follow:elevation> topographic wetness index>terrain ruggedness index>slope>relief>aspect>specific catchment area; The relative importance of elevation for rice yield is 100%, the relative importance of topographic wetness index on rice yield is about 60%, and the relative importance of the rest several index for rice yield nearly all of around 20%. Generally speaking, elevation is the most important factors influencing the yield of rice, and the relative importance of aspect and specific catchment area for rice yield is smaller. The importance of different slope positions on rice yield of Qijiang is as follows:middle slope>upper slope>flat slope>lower slope>valley>ridge; The relative importance of middle slope positions on rice yield is 100%; The relative importance of upper slope and lower slope on rice yield is about 50%, with the minimal relative importance of ridges on rice production is about 25%. For the Qijiang district, the middle slope area has the largest contribution rate to rice production in whole area, and it is optimization of rice planting area currently. |
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