Field experiments have been done on simulated flows over different rough pre-rill and rill bed surfaces. The purpose of the experiments was to evaluate the hydraulic roughness properties of these bed surfaces. Conventional equations describing the relation of the resistance coefficient to the Reynolds number for flow in closed conduits or open channels can also be applied to shallow flows provided the boundary values of the Reynolds number, which discriminate between the various flow regimes, are adapted. When the Reynolds number is less than 100 the flow regime is laminar. Above this figure the flow is either transitional or turbulent, depending on the roughness of the bed surface. The degree of turbulence can be deduced from a morphological description of the pre-rill and rill bed surface roughness.
The paper is prompted by apparent deficiencies in the design of plot studies in regional erosion surveys. The principal shortcomings of observational erosion research have been poor sampling design and inadequate analyses of data. The paper identifies various sources of bias which must be taken into account before plot datacan be ext'rapolated to land units in a regional survey. Judging from soil loss data of a case-study in the Ardbhe rangelands one may conclude that even accurate plot measurements can still be rather a rough basis for regional erosion assessment. Finally, the paper highlights strategies that might be used to improve erosion sampling.
This paper presents a case study of runoff and sediment generation under Submediterranean rangeland conditions (Ardkhe drainage basin, France). Measurements indicate that on a rough hillslope interrill runoff and sediment are not produced uniformly over the slope surface. It is observed that runoff concentrates immediately in non-permanent interrill flow paths, which under average storm conditions vary in length from 1.0 to 125m. Long interrill flow paths may eventually become permanent. These permanent flow paths, called pre-rills, are introduced as a new source area, and are considered to be the initial stage in the development of rills. Along pre-rills considerable quantities of runoff and sediment are carried away.This study also shows that calculations based on interrill, pre-rill, and rill runoff will only have significance if storm and soil conditions are specified in detail. It is concluded from a correlation analysis between the runoff volume and the amount of soil loss on a storm-by-storm basis that the runoff volume alone cannot explain the amount of sediment that is generated in each source area; soil availability is an additional factor that must be taken into account.
Data on the lateral variability of soil loss from Submediterranean rangelands are presented. Modified Gerlach troughs were used to measure both rill and interrill erosion. Random, systematic, stratified random, and cluster sampling methods were tested for various numbers of plots and plot widths in order to find the method that gives the most accurate estimate of soil loss. If both the rill and interrill subpopulations are sampled, random sampling and stratified random sampling give almost identical standard errors. Cluster sampling yields larger errors and is therefore less effective. Systematic sampling, however, allows one to choose more accurate samples of an economical size, but its efficacy depends largely on the number of rills in the study area. The smallest standard errors are obtained by measuring the total rill erosion and by sampling only the interrill subpopulation. When this procedure is applied, the four sampling methods turn out to be equally accurate.
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