Runoff apparatuses (RA) are developed to study infiltration, runoff generation, and erosion processes. Several RA designs are available, but limited attention has been given to the effects of the equipment scale, initial, and boundary conditions on measured runoff. This paper presents a model-based evaluation of RAs using a finite element solution for Richard’s equation and a novel ground surface boundary condition designed to accommodate unsaturated soil behavior. The hydraulic properties of two tropical soils were considered, with multiple combinations of initial water contents, specimen dimensions, and sloping angle. The numerical exercises indicate that soils with lower air-entry values require an equilibrium stage for the establishment of initial conditions. Testing protocols with equilibrium times of 48 hours are recommended. Moisture flow produced by gravity when sloping the specimen was shown to potentially affect surface conditions and, consequently, runoff. Testing specifications to minimize the effects of specimen sloping are presented. The runoff mechanism in an RA was shown to have up to three stages, all with clear physical meaning. The third stage is an undesirable consequence of the influence of the RA’s impervious bottom. The establishment of the minimum specimen thickness that prevents boundary effects was shown to have major importance to testing results.
The computation of soil-atmosphere water fluxes such as infiltration, evapotranspiration, and runoff is required for the analysis of numerous problems in geotechnical, geoenvironmental engineering and hydrogeology. The soil-atmosphere interaction processes can be represented by a series of partial differential equations. This paper presents a PDE formulation that was developed for soil-atmosphere analysis and presents three cases demonstrating the application of the formulation developed to laboratory and fields conditions. Comparisons against experimental data show that evaporative fluxes can be successfully reproduced by theoretical models. The PDE solutions were used for the simulation of the fluxes through two soil cover configurations to exemplify the application of theoretical models to design. The results indicate that the manner how runoff is computed strongly affects the results. The numerical solutions appear robust and can be applied to the design of soil structures such as soil-cover systems, geo-hazard hazard quantification, and other unsaturated soil problems. 1.
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