| Rill erosion is one of the main processes of soil erosion in the Loess Plateau of China.Monitoring the evolution of morphology and quantifying the shape of rill on the slope is of great significance to the study of soil erosion mechanism.However,none of the previous technology observes the soil erosion process at fine spatial and temporal scale under continuous rainfall conditions.In order to solve this issue,a digital close range photogrammetric observation system based on wireless networking technique was explored and established in this study.The evolution of soil surface topography was dynamically monitored by instantaneous image acquisition at different time intervals during ongoing rainfall.Noises on the images such as raindrops was removed by K-means clustering,digital point clouds were calculated and digital elevation model(DEM)was then generated.Based on the established observation system,the evolution process of rill erosion on slopes with different soil types were studied through artificial simulation experiment in laboratory.The amount of soil erosion,sediment transport rate and erosion rate were calculated,the rill network of slope erosion was extracted,the morphological characteristics of slope rill were quantified,the temporal and spatial variation characteristics of the rill were analyzed,and the digital expression of soil erosion was realized.This study provides a new way for the study of soil erosion process,and promotes the development of soil erosion process,mechanism and modeling research.The main conclusions of this study are as follows:(1)A digital close-range photogrammetry system for dynamic monitoring of soil erosion slopes during continuous rainfall was developed.The measurement precision of the established system could reach a sub-millimeter level,and the minimum measurement error was 0.0069 mm;the maximum relative error between the measured value and the actual measured value was-2.9683%.Compared with the traditional runoff and sediment measurement method,the average error of the estimated soil loss of the system was-7.06% and 1.05%,and the accuracy of single observation was up to 99.26%.(2)Raindrop was removed from the images during data acquisition,and soil morphology evolution was monitored under continuous rainfall.The digital images collected by each group of sensors in a single shot were sorted by time,and they were processed by dichotomy according to their grayscale value pixel by pixel,and the noise formed by raindrops on the digital images was then removed by k-means algorithm.The results showed that the variance of the original image was between 28.96 ~ 29.95,while the variance of the image after removing the raindrops was 24.14,indicating it can effectively remove the raindrop noise.(3)The synchronous acquisition of images was realized and the temporal resolution of the system was improved.The wireless networking technology enabled multiple cameras to shoot simultaneously.The time interval of image collection depended on the exposure time and shutter speed of the collector camera.The time observation resolution of this system could reach a min-level,and the spatial resolution reached to 2.0 mm.The soil erosion process was more accurately described at a fine spatial and temporal scale,and the issue of time-space inconsistency in erosion observation was solved,which provided a new method and technology for the study of the mechanism of soil erosion processes.(4)The development of eroded rill on the loess slope was monitored.The evolution process of rill erosion morphology was obtained by calculating the digital image of slope erosion morphology at different time intervals.The rill erosion process showed obvious stage changes,and under different experimental conditions,slope erosion morphology evolution process was different.Under the condition of 90 mm/h rain intensity and 15°slope,the rill erosion process of loess soil could be divided into several stages: raindrop splashing-eclipse-drop pit-erosion cave-rill cutting head-intermittent rill-continuous rill-rill erosion-rill network.Under the condition of 120 mm/h rain intensity and 20°slope,the rill erosion process of loess soil was: raindrop splashing-eclipse-drop pit-erosion cave-rill cutting head-rill erosion-rill network.(5)The morphological characteristics of rill on loess slope were quantified.With the extension of rainfall duration,the erosion intensity increased,rill density,degree of splitting and maximum trench depth increased rapidly.After the 80 min of the rainfall,the maximum width and depth increased initially and then decreased with the increase of unit slope length.The lowest inflection point appeared at the position of 7.5 m from the top of the slope,and the trend of the maximum rill width increasing first and then decreasing before the inflection point was not obvious.The area of 7.5 m to 9.5 m away from the top of the surface was the most active area of rill erosion.The change process of runoff and sediment yield and the development of erosion morphology on slopes of two different soil textures were compared and analyzed.Rills of the clay loam soil slope developed earlier than that of loess soil slope,however,once the rills on the slope of the loess soil began to develop,the speed of the rills changed faster.After the end of the rainfall,rill erosion patterns on the clay loam soil and loess soil slopes were parallel and like trees,respectively. |
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