PHOTOGRAMMETRIC AND RILLMETER TECHNIQUES FOR HYDRAULIC MEASUREMENT IN SOIL EROSION STUDIES

Abstract

To estimate the rill erodibility of a soil, it is necessary to relate observed erosion rates to the hydraulic shearand width of the rill channel. The hydraulic shear can be calculated given the hydraulic radius for a given flow condition.To find eroding rill channel hydraulic radii, photogrammetric and rillmeter methods were used in the Water ErosionPrediction Project cropland erodibility study conducted by the Agricultural Research Service, U.S. Department ofAgriculture. The objectives of this document are to describe two methods of measuring the rill shape on experimental sitesand to compare the resulting rill shape parameter values. The photogrammetric method showed larger measurements forhydraulic radii, with greater scatter, but allowed for the collection of more topographic data, and had greater flexibilitythan the rillmeter method. For the rillmeter method, the mean hydraulic radii varied from 6.2 to 14.3 mm with a 95%confidence interval of 4.0 mm. For the photogrammetric method, the hydraulic radii varied from 10.2 to 25.6 mm with a95% confidence limit of 9.4 mm. We concluded that for a study of this scope, the photogrammetric technology with thecapability of providing considerably more site data is the preferred method. We also concluded that a rillmeter backup isdesirable, rather than relying on a single measurement technology.