PHOTOGRAMMETRIC AND RILLMETER TECHNIQUES FOR HYDRAULIC MEASUREMENT INSOIL-EROSION STUDIES

To estimate the rill erodibility of a soil, it is necessary to relate observed erosion rates to the hydraulic shear and width of the rill ch annel. The hydraulic shear can be calculated given the hydraulic radiu s for a given flow condition. To find eroding rill channel hydraulic r adii, photogrammetric and rillmeter methods were used in the Water Ero sion Prediction Project cropland erodibility study conducted by the Ag ricultural Research Service, U.S. Department of Agriculture. The objec tives of this document are to describe two methods of measuring the ri ll shape on experimental sites and to compare the resulting rill shape parameter values. The photogrammetric method showed larger measuremen ts for hydraulic radii, with greater scatter but allowed for the colle ction of more topographic data, and had greater flexibility than the r illmeter method For the rillmeter method, the mean hydraulic radii var ied from 6.2 to 14.3 mm with a 95% confidence interval of +/-4.0 mm. F or the photogrammetric method, the hydraulic radii varied from 10.2 to 25.6 mm with a 95% confidence limit of +/-9.4 mm. We concluded that f or a study of this scope, the photogrammetric technology with the capa bility of providing considerably more site data is the preferred metho d. We also concluded that a rillmeter backup is desirable, rather than relying on a single measurement technology.