Influência do tipo de amostragem na constante dielétrica do solo e na calibração de sondas de TDR

A técnica da TDR é uma importante ferramenta para o estudo do teor de água no solo. Para o correto emprego dessa técnica, é necessário calibrar modelos que relacionam o conteúdo volumétrico de água no solo com a constante dielétrica deste, considerando as características de cada solo. Assim, objetivou-se avaliar a influência do tipo de solo e da forma de amostragem (amostra deformada e não deformada) na constante dielétrica (Ka) do solo e no desempenho de modelos para a estimativa do conteúdo volumétrico de água no solo (), usando a técnica TDR. Os solos utilizados foram um Cambissolo Háplico Tb distrófico (CX), um Latossolo Vermelho ácrico típico (LV) e um Neossolo Quartzarênico (RQ), que apresentam diferenças quanto à textura. As amostras de solo referentes a CX e LV foram coletadas com estruturas deformadas e não deformadas, e as de RQ, apenas deformadas, devido à baixa agregação deste. A calibração foi realizada com sondas compostas de três hastes com 0,10 m de comprimento efetivo e 0,05 m de resina, com espaçamento entre hastes de 0,017 m e sem resistor na haste central, conectadas a um equipamento TDR 100 da Campbell Cientific. Os valores de Ka, para um mesmo valor de , foram alterados somente pela variação do tipo de solo, não sendo influenciados pelo tipo de amostragem do solo (deformada e não deformada). Em relação aos modelos testados para estimativa de em função de Ka, o polinomial cúbico foi o que apresentou melhor ajuste aos dados de determinados por gravimetria para o Neossolo Quartzarênico e para o Cambissolo Háplico, enquanto para o Latossolo Vermelho um modelo linear apresentou melhor ajuste.The technique of TDR (time-domain reflectometry) is an important tool for the study of soil water content. For a correct use of this technique, models that relate the volumetric water content in soil with the soil dielectric constant must be calibrated, considering the characteristics of each soil. The purpose of this study was to evaluate the influence of soil type and sampling form (disturbed and undisturbed samples) on the dielectric constant (Ka) of the soil and the performance of models to estimate the volumetric water content in soil (θ) by TDR. The soils were a Haplic Cambisol, an Oxisol and a Typic Quartzipsamment, with different textures. Disturbed and undisturbed Haplic Cambisol and Oxisol samples were collected, but only disturbed Typic Quartzipsamment samples, due to its low aggregation. Calibration was performed with probes consisting of three rods (effective length 0.10 m and resin length 0.05 m), spaced 0.017 apart and without resistor on the center rod, connected to a TDR 100 (Campbell Scientific) equipment. The Ka values, for a same è value, were affected by variation of the soil type, but not influenced by the type of soil sampling (disturbed or undisturbed). For the models tested to estimate è as a function of Ka, the cubic polynomial model fit the θ data determined by gravimetry best for the Typic Quartzipsamment and Haplic Cambisol, whereas for the Oxisol a linear model fit better

Criteria for the estimation of field capacity and their implications for the bucket type model

Field capacity (FC) is crucial for modelling soil–plant–water dynamics with bucket‐type models, supporting prevention of environmental problems (e.g. chemical leaching to deeper layers and groundwater and waste of water resources). As FC measurement in situ is labour intensive, approaches for its estimation have been proposed. However, because they differ conceptually and are based on different assumptions, the response to an application (e.g. crop yield (CY) modelling) can be rather different. This study evaluated frequently used FC approaches in scenarios quantifying differences in behaviour of soil water and air retention, evapotranspiration (ET) and CY. Also, a soil texture‐based pedotransfer function (PTF) was compared with other methods. Six sites with different soil types and management practices from tropical and temperate climates were investigated. Field capacity was estimated based on a static criterion using the water content (θ) at pressure heads (h) of 0.6, 1 and 3.3 m and at the inflection point (θi) of all water retention curves (SWRCs) and the Assouline and Or model (AO). Moreover, four equations found in the literature based on a dynamic criterion were also evaluated. For all soils, the largest FC results were obtained when θi was set as FC. The smallest FC values were obtained with θ (3.3 m) (tropical) and AO (temperate). The coefficient of variation (CV) between FC estimates, based on nine approaches, ranged from 7 to 54%. Available water storage, air capacity (AC) and ET results were sensitive to FC and showed more variation for the Brazilian sites. The PTF used estimated FC within the range of results obtained by the other nine approaches. In addition, AquaCrop was used to study the effect of FC on CY by fixing all model parameters, whereas FC was the only flexible parameter. Crop yield was sensitive to the variation in FC under low and medium rainfall, but increased with larger FC. For high rainfall, the yield was small in a scenario with large θfc because AC became yield limiting