Assessment of Erosion, Deposition and Rill Development On Irregular Soil Surfaces Using Close Range Digital Photogrammetry

Understanding the spatial pattern of erosion in inter-rill and rill areas can be useful in locating sources of sediment and in determining down-slope sediment delivery. However, spatial information about erosion processes in the inter-rill and rill areas under field conditions is often limited owing to the non-availability of a detailed microtopography. This paper aims to present a method for the survey of microtopographic soil surface changes caused by erosion and to investigate the development of sediment transport and rill erosion in the inter-rill and rill areas for a given rainfall event. The approach is based on the application of close range digital photogrammetry to produce differences of digital elevation models (DEMs), automatically measured and generated but with manual 3D point editing. Quality analysis of the DEM measurements has shown that erosion and deposition can be detected with a relative error of +/- 2 center dot 8 to +/- 5 center dot 3 mm. The minimum average sediment yield from plots with an area of 53 m2after the occurrence of 46 to 70 mm of erosive rainfall was estimated to be 1 center dot 24 to 3 center dot 5 kg m(-2). The spatial pattern of erosion and deposition on inter-rill and rill areas has proved the relative efficiency of rills to transport sediments from inter-rill areas with a sediment delivery ratio greater than one. Thus at erosive rainfall events, the transport capacity of the rill flow was not a limiting condition. The rill network was controlled by abrupt change in surface microtopography where rough surfaces produced many smaller rills but fewer contributing rills than relatively smooth surfaces. Tillage-induced surface roughness, the presence of stones and sparse vegetation, and the borders of individual plots were found to present challenges to the photogrammetric techniques, and to introduce errors which inhibit fully automatic DEM generation. The precision of the DEMs was constrained by the precision of ground control points (GCPs), camera lens distortion towards format edges and DEM extraction parameters. Despite this limitation, however, the application of digital photogrammetry enabled the measurement of high-frequency microtopographic surfaces and characterisation of the spatial distribution of erosion and deposition and of rill erosion development during highly erosive rainfall events.