ABSTRACT Soil water retention is among the soil hydraulic properties most routinely measured in studies of soil physics and related areas. This property is used in dynamic simulations of vadose zone processes such as soil water availability, surface boundary processes of evaporation and infiltration, and the fate of soil pollutants. The most common measurement technique consists in establishing a hydrostatic equilibrium between an initially saturated soil sample and a porous medium at a certain tension on a tension table or pressure plate. However, there is reasonable doubt about the assumed hydrostatic equilibrium, especially in the dry range at low pressure heads. In this study we compared the traditional pressure plate apparatus protocol to an inverse parameter estimation protocol based on a transient evaporation experiment. Independent pressure head measurements using a dewpoint device were also performed. We sampled a variety of soil textures typical of the Brazilian subtropical humid zones, aiming to show differences between textures in their subjection to hydraulic nonequilibrium. The performed experiments allow to conclude that the two compared protocols showed real pressure heads in samples on a pressure plate to be less negative than the assumed ones, leading to an overestimation of the soil water content in the dry range at low pressure heads, especially in fine-textured soil samples. This affects the reliability of most soil hydraulic databases, derived PTFs in the dry range, as well as the wilting point estimation. Water availability predictions based on total and readily available water are more sensitive to the water retention measurement method when the chosen lower limits of available water are closer to the wilting point. In this sense, irrigation timing criteria based on readily available water should be preferred over total available water, especially for fine-textured soils. Finally, given the low reliability of the pressure plate apparatus for low pressure heads, possibly biasing hydrological simulations and their interpretation, alternative measurement methods for the drier part of the soil water retention curve should be preferred, e.g., the proposed inverse modeling of evaporation experiments.

ABSTRACT The soil water retention curve (SWRC) is essential for vadose zone hydrological modeling and related applications. In 2004, Groenevelt and Grant (GRT) presented a mathematical model for describing the SWRC and reported its mathematical versatility and good fit to soils from a Dutch database. In order to evaluate the application of GRT to SWRCs of Brazilian soils, we aimed to analyze the performance of GRT for 72 soils from Brazil. Besides that, the obtained results with GRT for these soils were compared to the fitting performance of the most frequently used models: Brooks and Corey (BC) and van Genuchten-Mualem (VGM). The three models were fitted to available soil water retention data by minimizing the sum of square errors. The Pearson correlation coefficient (r) and the Root Mean Square Error (RMSE) were used to assess the goodness-of-fit. Results showed high correlation coefficients (r≥0.95) and small values of RMSE (RMSE ≤0.03 cm3 cm-3) for all fits. The goodness-of-fit was of similar performance for the three models with a positively correlation between them. The major difference in shape among GRT, BC, and VGM occurred in the near saturated range, while they were almost identical for low matric potentials. The exponent of GRT showed to be highly correlated with exponents of BC and VGM, but the correlation between the other shape parameters is not well defined, making a direct conversion still difficult.

ABSTRACT Among the equations available to describe the relation between matric potential and soil water content, the soil water retention function, the most commonly used is the equation proposed by Van Genuchten in his 1980 landmark paper. In soil physics literature, especially in Brazil, several authors relate the inverse of the Van Genuchten parameter α to the air-entry pressure. This study aimed to show this common interpretation to be erroneous, as 1/α corresponds to water contents lower than saturation. The deviation depends on the m parameter. In fact, α is merely a scaling parameter relative to the matric potential axis. Recognizing this mathematical fact may improve the interpretation of soil hydraulic properties based on water retention parameters.

ABSTRACT Solute transport parameters can be determined in miscible displacement experiments, usually performed in columns with disturbed (sieved) soil samples. Experiments with undisturbed samples are uncommon, due to the difficulty of taking undisturbed samples at the size required for these experiments. Structural alteration of the disturbed material implies modifications in the pore geometry that determines hydraulic properties, including hydraulic conductivity and retention and properties related to miscible displacement. An existing model for prediction of breakthrough curves based on retention properties was tested using material from a medium-textured Ferralsol, and alterations caused by sample disturbance were investigated. Soil water retention curves and miscible displacement parameters were determined in breakthrough experiments with nitrate salts in columns filled with undisturbed and disturbed soil samples. Data obtained from the undisturbed samples showed a higher dispersion, suggesting homogenization of pore geometry and a reduction in the representative elementary volume by the disaggregation and sieving of the soil material. The transport parameters for nitrate determined in disturbed and undisturbed samples were significantly different and the model was able to simulate the observed breakthrough curves after fitting the pore connectivity parameter.

Em oposição a definições objetivas da física do solo, o termo subjetivo “qualidade física do solo” aparece com frequência cada vez maior em trabalhos publicados da área. Um suposto indicador de qualidade física do solo que recebe muita atenção, especialmente na literatura brasileira, é o Intervalo Hídrico Ótimo (IHO), definido por quatro teores-limite de água. Nesse texto, discutem-se seus quatro teores-limite à luz de propriedades físicas do solo determináveis objetivamente, apontando-se incoerências na definição e no cálculo do IHO. Discute-se a interpretação do IHO como indicador da produtividade das culturas ou da qualidade física do solo, demonstrando-se sua incapacidade de abranger condições de contorno fenológicas e pedológicas comuns. Demonstra-se, também, que as densidades críticas encontradas pelo IHO por método comumente empregado são questionáveis. Considerando a disponibilidade de modelos comprovadamente robustos para as áreas de agronomia, ecologia, hidrologia, meteorologia e outras conexas, a popularidade do IHO como indicador na física do solo brasileira não pode ser entendido pela sua eficácia, pois essa nunca foi comprovada, mas deve ser explicado pela simplicidade com que ele é tratado. A determinação dos respectivos teores-limite de forma simplificada, deixando o estudo ou a preocupação com o real funcionamento do sistema para o segundo plano, vai à contramão da construção científica e do entendimento sistêmico. Sugere-se um realinhamento da pesquisa em física do solo no Brasil com os preceitos da ciência, na direção da física do solo mecanística em detrimento da busca por correlações empíricas aquém do estado da arte em física do solo.

As opposed to objective definitions in soil physics, the subjective term “soil physical quality” is increasingly found in publications in the soil physics area. A supposed indicator of soil physical quality that has been the focus of attention, especially in the Brazilian literature, is the Least Limiting Water Range (RLL), translated in Portuguese as "Intervalo Hídrico Ótimo" or IHO. In this paper the four limiting water contents that define RLLare discussed in the light of objectively determinable soil physical properties, pointing to inconsistencies in the RLLdefinition and calculation. It also discusses the interpretation of RLL as an indicator of crop productivity or soil physical quality, showing its inability to consider common phenological and pedological boundary conditions. It is shown that so-called “critical densities” found by the RLL through a commonly applied calculation method are questionable. Considering the availability of robust models for agronomy, ecology, hydrology, meteorology and other related areas, the attractiveness of RLL as an indicator to Brazilian soil physicists is not related to its (never proven) effectiveness, but rather to the simplicity with which it is dealt. Determining the respective limiting contents in a simplified manner, relegating the study or concern on the actual functioning of the system to a lower priority, goes against scientific construction and systemic understanding. This study suggests a realignment of the research in soil physics in Brazil with scientific precepts, towards mechanistic soil physics, to replace the currently predominant search for empirical correlations below the state of the art of soil physics.

A avaliação da capacidade de raízes de plantas em extrair água do solo é de grande importância na modelagem da taxa de transpiração e, para entender o crescimento e rendimento vegetal e o balanço de água e de solutos no solo. Para testar um modelo de extração radicular macroscópico baseado no processo em escala microscópica, descreveram-se os resultados de um experimento com plantas cujo sistema radicular foi dividido entre camadas de solo com propriedades hidráulicas contrastantes. Um experimento de lisímetro dividido com plantas de sorgo foi realizado em Piracicaba-SP. Quatro lisímetros com dois compartimentos separados fisicamente (split-pot) foram construídos e preenchidos com material de dois tipos de solo de diferentes classes texturais (um solo de textura média - AR e outro de textura argilosa - AG). Durante um mês e meio foi imposto um regime hídrico, alternando a irrigação entre os compartimentos. O teor de água nos compartimentos dos lisímetros foi monitorado com TDR e tensiômetros. O material dos dois solos foi analisado conforme método-padrão quanto às suas propriedades de retenção e condução da água. A densidade radicular foi determinada por pesagem no fim do experimento, tendo ficado em torno de duas vezes maior no solo AR do que no AG. Observou-se que a extração de água ocorreu preferencialmente do compartimento do lisímetro com maior potencial de fluxo matricial. Em certas ocasiões houve transferência de água do lado de maior para o de menor potencial de fluxo matricial, com a liberação da água ao solo pelo sistema radicular (hydraulic lift). Para compensar o efeito da heterogeneidade da distribuição radicular e da atividade radicular, incluiu-se, no modelo, um fator empírico f de correção. O modelo testado descreveu bem 80 % das observações com a utilização de valores de f de 0,01506 e 0,003713, para os solos AR e AG, respectivamente. O modelo simulou a liberação de água ao solo mais frequentemente do que ela ocorreu no experimento. Esse fato pode indicar que a resistência interna do sistema radicular, não contabilizada pelo modelo, pode ter papel importante nas relações hídricas na rizosfera.

Evaluating plant root capacity in extrating water from the soil is important for transpiration modeling and to understand crop growth and yield and soil water and nutrient balance. Aiming to test a macroscopic root water extraction model based on the microscopic process description, an experiment was described in which the root system of plants penetrated different soil layers with contrasting hydraulic properties. Four lysimeters containing two physically divided compartments were built and filled with material of two soils with different texture (a medium textured soil - AR and a clayey soil - AG). During a month and a half a water regime was imposed alternating the irrigation among the compartments. The soil water content in the compartments was measured with TDR and tensiometers. Soil hydraulic properties - retention and conductivity - were analyzed by standard methods. Root density was determined by weighing at the end of the experiment, resulting in values twice as high in AR than in AG soil. It was observed that water extraction occurred preferentially from the lysimeter compartments with the highest matric flux potential. Occasionally, water transfer from the compartment with higher matric flux potential to the lower one was observed, releasing water from root to soil (hydraulic lift). To compensate for the effect of heterogeneity of root distribution and root activity and soil-root contact, an empirical factor f was added to the model. Its value was determined by a numerical fitting procedure aiming at the highest correlation between model and observation in the four lysimeters. The model described 80% of the observations satisfactorily by using these f values, which were 0.01506 and 0.003713, respectively, for AR and AG. Model predictions indicated a much more frequent water release from roots to soil than observed in the experiment. This may suggest internal root resistance, not considered by the model, may play an important role in root-water relations.

A condutividade hidráulica do solo é uma propriedade cuja quantificação é essencial para qualquer estudo que envolva o movimento da água no solo. Os métodos para sua determinação baseados na drenagem interna, como o do perfil instantâneo - MPI, são os mais empregados, restringindo-se à determinação da condutividade hidráulica entre a condição de solo saturado e a umidade na capacidade de campo. Com a finalidade de obter a condutividade hidráulica para menores valores de umidade, propôs-se um método de emprego do MPI com evaporação, apresentando-se o procedimento para a determinação do plano de fluxo zero e estimativa da densidade de fluxo. Demonstrou-se a aplicação do método, obtendo-se resultados de condutividade hidráulica para a camada superficial de um perfil monitorado a partir de um experimento de campo realizado durante um período de 12 dias. A profundidade máxima do plano de fluxo zero estimado foi de aproximadamente 0,50 m. A faixa de umidade obtida variou de 0,25 a 0,21 m³ m-3, valores complementares aos obtidos em experimentos sem evaporação no mesmo local. Os resultados indicaram a viabilidade do método, útil especialmente para se estender à faixa de umidade obtida na camada superficial de tais experimentos.

Soil hydraulic conductivity is an essential property and its quantification is essential for any study that involves soil water movement. Methods of determination based on internal drainage, such as the instantaneous profile method (IPM) are among the most commonly used, but their determination is restricted to the soil water range between saturation and field capacity. In order to obtain hydraulic conductivity for lower water contents, we propose an IPM that takes into account evaporation as well as a calculation routines to determine the zero flux depth and to estimate flux densities. The application of the method is demonstrated by obtaining results for hydraulic conductivity of the surface layer of a monitored profile in a 12 -day field experiment. The estimated maximum zero flux depth was about 0.50 m. Soil water content varied from 0.25 to 0.21 m³ m-3, complementing the range obtained in experiments without evaporation at the same site. Results confirm the applicability of the method, which is especially useful to extend the range of soil water contents in the surface layer obtained in this kind of experiment.