晋西黄土区蔡家川小流域切沟的空间分布及形态特征

Spatial distribution and characteristics of the gullies in Caijiachuan watershed in loess region of Western Shanxi Province,China

为了掌握切沟的空间分布及发育特点,实现小流域综合治理中切沟治理措施的精准对位配置,该研究以晋西黄土区吉县蔡家川小流域为对象,利用Quickbird影像和数字高程模型(DEM)对流域内370条切沟的形态特征和空间分布进行了提取和解译,并对其中40条切沟进行实地调查验证,采用核密度指标探讨切沟在小流域内的空间分布特征,分析坡度、坡向、高程等地形因子对切沟空间分异特征的影响。结果表明:1)蔡家川小流域内农地、果园的主要分布区(东北部和东南部)是切沟的高聚集区,而在人工林为主的中上游区属于中密集区;2)切沟数量、密度、频度和规模均随坡度增加呈现先增大后减小的趋势,坡度阈值为25°。5°~25°的坡面切沟广泛发育,数量占研究区的71.5%,密度、频度也远高于其他坡度的坡面;3)切沟密度和频度的坡向分异与流域走向有关。切沟密度北坡最大(3.03 km/km^(2)),东坡最小(0.86 km/km^(2)),切沟频度西北坡最大(12.7条/km^(2)),东坡最小(6.1条/km^(2))。南北向的切沟规模明显大于东西向;4)切沟数量、密度、频度随海拔增加呈现先增大后减小的趋势,在海拔为1000~1200 m的坡面上切沟数量占77%,海拔为1100~1200 m的坡面上切沟密度最大(2.17 km/km^(2)),海拔为1000~1100 m的坡面上切沟频度最高(15.5条/km^(2))。随海拔升高切沟逐渐向大型化发展,沟道越发平缓,下切侵蚀逐渐减弱。研究结果可揭示流域切沟发育的重点地形以及在地形上的共性特点,为黄土区小流域切沟治理提供依据,可为科学认知切沟空间分布提供参考。

A gully is one of the erosion geomorphic types in the Loess Plateau.Taking the Caijiachuan watershed in Ji County,the loess region of Western Shanxi in China as the object,this study aims to determine the spatial distribution and characteristics of cutting ditches using the QuickBird images and topographic mapping.A field investigation of 40 gullies was conducted to translate the 370 gullies in the watershed.An analysis was also made on the effects of topographic factors(such as slope,aspect,and elevation)on the spatial differentiation of the gully.The number,density,and frequency of erosion gullies were then calculated on the different topographic factors.The spatial variations of morphological and development indicators were determined,such as the erosion gullies area,length,morphological elements,gradient,Horizontal Proximity Index(HPI),Vertical Proximity Index(VPI),and Proximity Index of Variation(PIV).The main results are as follows:1)The kernel density of the gully reached more than 8.23 in the northeast and southeast of the watershed,indicating a high concentration area.The kernel density of the gully was 4.11-8.23 in the middle dense area,which was located in the upper-middle reaches of the watershed;2)The largest erosion was found in the gullies at the slope of 15-25°.The erosion gullies presented the maximum density and frequency when the slope range was between 5-15°.As such,the slope range of 5-25°was the most serious erosion for the drainage channel.According to the average of PIV,the vertical proximity index of the erosion gullies on the slope was greater than the horizontal proximity index as the slope level between 0-5°,15-25°,25-35°,35-45°and more than 45°.The main type of erosion was the undercutting in these slope levels.Moreover,there were similar morphological characteristics of gullies on the slope of 25°-45°.3)There were 66 erosion gullies in the northeast,which was the largest number in the study area.A minimum,34 erosion gullies were located in the south.The maximum density of erosion gullies was 3.03 km/km^(2)in the north,whereas,the minimum density was 0.87 km/km^(2)in the east.The frequency of erosion gullies was significantly greater on the west side of the boundary line of 67.5°-247.50°in the slope direction than that on the east side.There were similar morphological characteristics of each gully on the west slope.4)The altitude between 1000 to 1200 m was the most serious erosion of gully,indicating the highest density and frequency of erosion gullies.The PIV gradually approached zero,as the altitude increased from the altitude level of 1000-1100 m,but there were the negative average PIVs of all the altitude levels,where the main erosion type was undercutting.There were also similar morphological characteristics of the trenches below 1100 m.Therefore,the key features were revealed in the mutual topography for the erosion gullies in the watershed.The finding can also provide a strong basis to manage the spatial distribution of erosion gullies in the small watersheds of the Loess areas.