As any model of real-world phenomena, soil erosion models must be tested against empirical evidence to have their performance evaluated. This is critical to develop knowledge and confidence in model predictions. However, evaluating soil erosion models is complicated due to the uncertainties involved in the estimation of model parameters and measurements of system responses. Here, we undertake a term co-occurrence analysis to investigate how model evaluation is approached in soil erosion research. The analysis illustrates how model testing is often neglected, and how model evaluation topics are segregated from current research interests. We perform a meta-analysis of model performance to understand the mechanisms that influence model predictive accuracy. Results indicate that different models do not systematically outperform each other, and that calibration seems to be the main mechanism of model improvement. We review how soil erosion models have been evaluated at different temporal and spatial scales, focusing on the methods, assumptions, and data used for model testing. We discuss the implications of uncertainty and equifinality in soil erosion models, and implement a case study of uncertainty assessment that enables models to be tested as hypotheses. A comment on the way forward for the evaluation of erosion models is presented, discussing philosophical aspects of hypothesis testing in environmental modelling. We refute the notion that soil erosion models can be validated, and emphasize the necessity of defining fit-for-purpose tests, based on multiple sources of data, that allow for a broad investigation of model usefulness and consistency.

R ESU M OPlantas com alto índice de cobertura e elevada produtividade de fitomassa propiciam melhor proteção ao solo, amortecendo o impacto das gotas de chuva sobre a superfície do solo reduzindo, consequentemente, os processos erosivos. Objetivou-se avaliar a influência das plantas de cobertura e dos espaçamentos de plantio no controle das perdas de solo, água e nutrientes por erosão hídrica, visando à conservação do solo. O experimento foi conduzido em um Argissolo Vermelho-Amarelo, cujos tratamentos foram definidos por três espécies de plantas de cobertura: crotalária júncea, feijão-deporco e milheto e com dois espaçamentos entrelinhas (0,25 e 0,50 m). Para a quantificação das perdas de solo, água e nutrientes, utilizaram-se parcelas de 4 x 12 m em que a maior dimensão obedeceu ao sentido do declive. Nos meses de dezembro e janeiro ocorrem os maiores valores de erosividade requerendo, neste período, melhor cobertura da superfície do solo. O feijão-de-porco foi a espécie que apresentou maior potencial no controle dos processos erosivos; quanto ao espaçamento a melhor proteção do solo para todas as culturas foi obtida no espaçamento de 0,25 m, propiciando menores perdas de solo, água e nutrientes. Palavras-chave: crotalária júncea, feijão-de-porco e milhetoCover crops to control soil, water and nutrient losses by water erosion A B ST R A C T Cover plants with high coverage index and biomass productivity provide a better soil protection, cushioning the impact of the raindrops on the soil surface, thus reducing the erosion process. The objective of this study was to evaluate the influence of cover crops and plant spacing in the control of soil, water and nutrient of this study losses by water erosion, aiming the soil conservation. The experiment was conducted in the Red-Yellow Argisol, with three treatments of cover crops; sunn hemp, jackbean and millet, in two row spacing (0.25 and 0.50 m). To quantify the loss of soil, water and nutrients, plots of 4 x 12 m were used, and the larger dimension followed up and down slope. During the months of December and January there were the highest erosivity values, requiring in this period a better coverage of the soil surface. The jackbean was the species with the highest potential in the erosion control; and the spacing that promoted the best soil protection for all crops was 0.25 m, providing lesser losses of soil, water and nutrients.

Purpose Soils are important regulators of Critical Zone processes that influence the development of geochemical signals used for sediment fingerprinting. In this study, pedological knowledge of tropical soils was incorporated into sediment source stratification and tracer selection in a large Brazilian catchment. Materials and methods In the Ingaí River basin (~1200 km 2), Brazil, three source end-members were defined according to the interpretation of soil and geological maps: the upper, mid, and lower catchment. A tributary sampling design was employed, and sediment geochemistry of three different size fractions was analyzed (2-0.2 mm; 0.2-0.062 mm, and < 0.062 mm). A commonly used statistical methodology to element selection was compared to a knowledge-based approach. The mass balance un-mixing models were solved by a Monte Carlo simulation. Modeled source contributions were evaluated against a set of artificial mixtures with known source proportions. Results and discussion For the coarse fraction (2-0.2 mm), both approaches to element selection yielded high errors compared to the artificial mixtures (23.8% and 17.8% for the statistical and the knowledge-based approach, respectively). The knowledgebased approach provided the lowest errors for the intermediate (0.2-0.062 mm) (10.9%) and fine (< 0.062 mm) (11.8%) fractions. Model predictions for catchment outlet target samples were highly uncertain for the coarse and intermediate fractions. This is likely the result of the spatial scale of the source stratification not being able to represent sediment dynamics for these fractions. Both approaches to element selection show that most of the fine sediments (median > 90%) reaching the catchment outlet were derived from Ustorthents in the lower catchment. Conclusions The different element selection methods and the artificial mixtures provide multiple lines of evidence for evaluating the fingerprint approaches. Our findings highlight the importance of considering pedogenetic processes in source stratification, and demonstrate that different sampling strategies might be necessary to model specific sediment fractions.

R E S U M OO solo é um dos recursos naturais de maior importância cuja heterogeneidade reflete na variabilidade de seus atributos químicos e físicos. Esta heterogeneidade é decorrente do processo de formação natural e do uso e manejo realizados pelo homem. Assim, o objetivo deste estudo foi avaliar a variabilidade espacial dos atributos químicos e físicos do solo e elaborar seus mapas de distribuição espacial na Sub-Bacia das Posses, Extrema. Para isto foram coletadas amostras em 150 pontos nas profundidades de 0-0,20 e determinados os atributos físicos e químicos. As variáveis: Ca, Mg, K, P, pH, matéria orgânica (MO), diâmetro médio geométrico dos agregados (DMG cp ), diâmetro médio ponderado dos agregados (DMP cp ) e índice de floculação (IF). IF apresentaram estrutura de dependência espacial, o que permitiu seu mapeamento a partir de técnicas geoestatísticas. O P, DMG sp e DMP sp apresentaram estrutura com efeito pepita puro, possivelmente decorrente de uma distância insuficiente para captar dependência espacial na amostragem para esses atributos e interferências antrópicas. A variação verificada para o Ca pode ser explicada pela aplicação localizada de calcário, a qual foi realizada em algumas pequenas propriedades localizadas nas subbacias. O DMG p , DMP p , e IF avaliados foram ajustados em conformidade entre si indicando boa agregação do solo. Variability of soil attributes under pasturing and Atlantic Forest on the watershed basin scale A B S T R A C TSoil is one of the most important natural resource and its heterogeneity reflects the variability of its chemical and physical attributes. Soil heterogeneity occurs due to the natural process of formation and use by men and management. The objectives of this study were to evaluate the spatial variability of physical and chemical soil attributes and map the spatial distribution of these in Posses sub-basin, Extrema, representative of the Atlantic Forest biome present in southern Minas Gerais. For this, samples (0-0.20 m depth) were collected at 150 points, and their physical and chemical attributes were determined. Variables: Ca, Mg, K, P, pH, organic matter (MO), geometric mean diameter of aggregates (DMG cp) , weighted mean diameter of aggregates (DMP cp ) and flocculation index (IF) showed spatial dependence structure, which allowed mapping from geostatistical techniques. P, DMG sp and DMP sp presented structure with nugget effect, possibly due to an insufficient distance to capture spatial dependence in the sample for these attributes, and human interferences. The variation for Ca can be explained due to lime application, which occurred in some small properties located in the sub-basin. The DMG cp , DMP cp , and IF evaluated were adjusted in line with each other, indicating good soil aggregation. ISSN 1807 -1929 v.18, n.5, p.517-526, 2014 Introdução O Brasil é um país rico em recursos naturais distribuídos em seus diversos biomas. A Mata Atlântica é considerada, atualmente, seu bioma mais importante em razão do seu estado crítico -nela se concent...