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A sound measurement of splash detachment rates for erosion and eluviation modelling
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Splash is well recognised as a key process of interrill erosion and soil surface degradation. Splash has more recently been pointedout as a mechanism of colloid mobilisation suggesting the role of this process in controlling colloid-facilitated transfer andlessivage. Measured splash detachment rates are used to parameterise interrill erodibility in soil erosion and landscape evolutionmodels as well as eluviation in pedogenesis models. However, the available measurements of splash detachment rate are biased andexperiment-specific. Firstly, splash detachment is often considered as a single process whereas, depending on the measurementconditions (e.g. soil aggregation, surface water content, rainfall kinetic energy), raindrop impact on soil can trigger differentphysical mechanisms. Secondly, the interaction between the measurement device and the redistribution pattern of splashedparticles produces bias that are generally not taken into account when interpreting the experimental data. Thirdly, the sizeselectivity is mostly disregarded whereas several authors have already pointed to its preeminence. The objectives of thiscommunication are thus 1) to propose a measurement framework that is physically and experimentally sound to determine splashdetachment rate, 2) to quantify reliable detachment rates for different soils and size fractions, 3) to analyse the way splashdetachment is represented and parametrised in different soil erosion and pedogenesis models, and 4) to suggest a new approach tomodel splash detachment.The experimental framework we designed enabled to: 1) distinguish the consequences on soil aggregates of aggregate breakdown,and transport by splash; 2) propose an invertible mathematical model for a non-biased interpretation of the measurements. Thismodel was obtained from a numerically integrated point-equation by first conducting a sensitivity analysis to determine thesignificant parameters and then finding a simplified form by meta-modeling.Two laboratory devices were set up to sample the soil fragments produced by aggregate breakdown and transported by splash,respectively. Each device has a central circular 18-cm2 source. Sieved air-dried soil (3-to-5-mm size fraction) was exposed tosimulated rainfall at 29 mm.h-1 and with a time specific kinetic energy of 250 J.m-2.h-1. Those conditions are representative of astorm on a dry seedbed. Three cultivated soil surface materials, with various sensitivities to erosion, were tested: a silt loam, a clayloam and a silty clay loam. The size distributions (16 size fractions from 0.05 to > 2000 μm) of the broken down and splashed soilfragments were determined by combining sieving and laser diffraction. These size measurements were performed in ethanol inorder to preserve aggregation.The results show that the splash rate depends mainly on the size fraction of the aggregates available at the soil surface. The ratesare higher for the middle-sized fractions (50 to 1000 μm) and, contrarily to what is generally thought, splash of colloidal-sized soilfragments is not significant. The physical mechanism generating colloidal-sized soil fragments under rainfall impact is breakdownand it occurs only on aggregated soils. In the literature, splash detachment refers to various physical processes: in soil erosion andlandscape models, it encompasses both aggregate breakdown and splash sensu stricto, and, in eluviation and colloidal-particlestransport models, it actually relates to aggregate breakdown only. Thus splash detachment in soil erosion and landscape modelsshould be parametrised by combining aggregate stability tests (representative of aggregate breakdown) and splash cupmeasurements. Eluvation and colloidal-particles transport models should be parametrised by aggregate stability tests only.
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