沟蚀发生的地貌临界理论计算中数据获取方法及应用

沟蚀发生是一种地貌临界现象,与沟头处局地坡度及上方汇水面积有关,而沟蚀发生地貌临界理论能够预测沟头可能发生的位置。该文从沟蚀发生地貌临界理论起源、数据获取方式、参数计算方法、影响因素及应用等方面综合评述了该理论的发展及近年来国内外的有关研究。数据获取方式主要包括野外实测、高清遥感影像及地形图测量。参数计算方法包括目视(下限值)法、正交回归(95%置信区间下限)、正交回归(下限值)及分位数回归等。相对剪切力指数值反映区域主要的沟蚀发生机制,临界常数值反映当前特定外界环境下的沟蚀发生临界条件。将相对剪切力指数固定后,临界常数的时间序列变化能够表征外界环境改变对沟蚀发生的影响。人类活动改变了沟头上方汇流环境,进而影响临界条件。沟蚀发生地貌临界理论可获取沟道侵蚀风险较大的区域,为沟道侵蚀防治措施布设提供参考。结合高分辨率地形图,增加表征人类活动影响汇流过程的参数能够丰富沟蚀发生地貌临界理论。该理论与已有沟道侵蚀发展模型结合,可将沟头发生位置和沟道发展过程统一,促进沟道侵蚀全过程的模拟。

Data obtained method and application for topographic threshold theory calculation of gully initiation

Gully initiation topographic threshold theory describes gully initiation condition, and is represented by the size of catchment that controls discharge, and local slope at the channel head that controls the velocity of runoff. The main cause of gully formation is excessive (sub) surface runoff, a condition that might be brought about by either climate change or alternations in land use. In this study, this theory was reviewed from the following aspects: theory development, data sources, threshold value calculating methods, influencing factors and applications. The gully initiation threshold concept was originally developed to explain the onset of instability in 1 gully while its neighbours remained stable. The relative area (or shear stress) exponent was generally interpreted in relation to the gully erosion process in the catchment. Values higher than 0.2 were associated with erosion by surface runoff and those lower than 0.2 indicated subsurface processes or mass movement. The threshold coefficient reflected the resistance of the site to gully head development, affected by rainfall, land use, etc. The threshold values variation also depended on the methodology, including field reconnaissance survey and high-resolution remote sensing images as well as digital elevation model. The latter were more convenient for data acquisition, although field reconnaissance survey data would be more accurate. With fast development of unmanned aerial vehicles, high spatial resolution orthophotos derived from structure-from-motion photography could be used to identify the location of gully heads and corresponding catchment size and local slope values. In the early research, the topographic threshold straight line was eye-fitted through the "lower-most" points in a log-log scatter plot. The negative slope of that line was equal to relative area exponent value. Then the threshold value could be obtained as the intercept. Since this threshold line was manually drawn, it did not have statistical meaning. This method might also be problematic as multiple thresholds could exist, and the threshold line was very sensitive to extreme values. Based on orthogonal regression, the mean threshold line was fitted through the data-points. Then the minimum threshold line was defined either by the lower limit of the 95% prediction confidence interval around the mean threshold line, or parallel line below the lower limit of the scatter of the data. Quantile regression was recommended because it was statistically-based and robust to outliers. Since the domination mechanisms of gully initiation would not change within decades in a certain region, the relative area exponent could be fixed as a constant value. According to this hypothesis, the threshold coefficient of muti-periods could be used to investigate human effect on gully initiation. In China, about 70% of the research was carried out in the Loess Plateau region. The 1:10 000 topographic map was widely used to obtain local slope and catchment size, since this was the most extensive and detailed topographic map currently available. Most studies extracted the threshold conditions by using the eye-fitted line through the "lower-most" points, and few consideration was carried out for the potential errors between different calculation methods. Road construction altered the surface hydrology, and the road surface condition reduced the critical slope for a given drainage area required for gullying. Agricultural reclamation was the main reason for gully development in the Northeastern China, where ridge tillage was widely applied. Contour ridge changed runoff pathways and rearranged drainage networks, and longitudinal ridge accelerated flow concentration. Consideration of ridge-direction effect was important for gully initiation topographic threshold theory applications in this region. Using high-resolution topographic maps and adding the parameters that characterized the human activities effect on concentrated surface runoff could enrich the gully initiation topographic threshold theory. Current gully erosion model could simulate gully development while gully head needed to be mannually located. Hence gully initiation topographic threshold theory could be promoted by combining with such models, since this theory could predict where gully initiated.