Pedotransfer functions to estimate water retention parameters of soils in northeastern Brazil.

Abstract: Pedotransfer functions (PTF) were developed to estimate the parameters of the van Genuchten model (1980) to describe soil water retention curves. The data came from various sources, mainly from studies conducted by universities in Northeast Brazil, by the Brazilian Agricultural Research Corporation (Embrapa) and by a corporation for the development of the São Francisco and Parnaíba river basins (Codevasf), totaling 786 retention curves, which were divided into two data sets: 85 % for the development of PTFs, and 15 % for testing and validation, considered independent data. Aside from the development of general PTFs for all soils together, specific PTFs were developed for the soil classes Ultisols, Oxisols, Entisols, and Alfisols by multiple regression techniques, using a stepwise procedure (forward and backward) to select the best predictors. Two types of PTFs were developed: the first included all predictors (soil density, proportions of sand, silt, clay, and organic matter), and the second only the proportions of sand, silt and clay. The evaluation of adequacy of the PTFs was based on the correlation coefficient (R) and Willmott index (d). To evaluate the PTF for the moisture content at specific pressure heads, we used the root mean square error (RMSE). The PTF- predicted retention curve is relatively poor, except for the residual water content. The inclusion of organic matter as a PTF predictor improved the prediction of parameter (alfa) of van Genuchten. The performance of soil-class-specific PTFs was not better than of the general PTF. Except for the water content of saturated soil estimated by particle size distribution, the tested models for water content prediction at specific pressure heads proved satisfactory. Predictions of water content at pressure heads more negative than -0.6 m, using a PTF considering particle size distribution, are only slightly lower than those obtained by PTFs including bulk density and organic matter content. Resumo: Foram desenvolvidas funções de pedotransferência (PTFs) para estimar os parâmetros do modelo de van Genuchten (1980), utilizados para descrever curvas de retenção de água no solo. Os dados usados foram provenientes de diversas fontes, principalmente de estudos realizados na Região Nordeste pelas universidades, pela Embrapa e Codevasf, totalizando 786 curvas de retenção, que foram divididas em dois conjuntos de dados: 85 %, para desenvolvimento das PTFs, e 15 %, para teste e validação, considerados como dados independentes. Além do desenvolvimento das PTFs de caráter geral para todos os solos conjuntamente, desenvolveram-se PTFs específicas para as classes Argissolos, Latossolos, Neossolos e Planossolos, utilizando técnicas de regressão múltipla, com o uso do procedimento stepwise (forward e backward), para selecionar os melhores preditores. Dois tipos de PTFs foram desenvolvidos: o primeiro inclui todos os preditores, densidade do solo, teores de areia, silte, argila e de matéria orgânica, e o segundo, apenas com os teores de areia, silte e argila. A avaliação da adequação das PTFs foi com base no coeficiente de correlação (R) e índice de Willmott (d). Para avaliar as PTFs, para o teor de água em potenciais matriciais específicos, utilizou-se a raiz do erro médio quadrado (RMSE). A predição da curva de retenção por PTF é relativamente fraca, exceto para o teor de água residual. A inclusão do teor de matéria orgânica como preditor da PTF melhora a predição do parâmetro (alfa) de van Genuchten. Não houve melhora de desempenho das PTFs específicas por classe de solo, em comparação com uma PTF geral. Exceto no caso do teor de água do solo saturado, estimado pela distribuição granulométrica, modelos para a predição do teor de água em potenciais matriciais específicos são bons. Predições do teor de água em potenciais matriciais mais negativos do que -0,6 m, usando uma PTF contendo a distribuição granulométrica, são somente um pouco inferiores àquelas obtidas por PTFs, que incluem densidade do solo e teor de matéria orgânica

Determinação da lâmina média de irrigação em pivô central

With the purpose of improving the accuracy of the average irrigation depth calculation, an expression is presented assuming linear variation of the depth between two following collectors, for the determination of the average depth of center pivot irrigation systems. It is concluded that the proposed expression should be used for the determination of the mean weighed depth for being conceptually more correct, although, in practice the values calculated by the proposed methodology are very close to those obtained with the traditional calculation method.Com o objetivo de melhorar a exatidão do cálculo da lâmina média d'água, por faixa representativa do coletor, apresentou-se uma expressão, considerando a variação linear de lâmina entre dois coletores subseqüentes, para fins de avaliação do sistema de irrigação do tipo pivô central. Concluiu-se que a expressão proposta deve ser utilizada para determinação da lâmina média ponderada por estar conceitualmente mais correta, apesar de na prática os valores calculados pela metodologia proposta serem muito próximos daqueles obtidos pelo cálculo tradicional

Mechanistic framework to link root growth models with weather and soil physical properties, including example applications to soybean growth in Brazil

Background and aimsRoot elongation is generally limited by a combination of mechanical impedance and water stress in most arable soils. However, dynamic changes of soil penetration resistance with soil water content are rarely included in models for predicting root growth. Better modelling frameworks are needed to understand root growth interactions between plant genotype, soil management, and climate. Aim of paper is to describe a new model of root elongation in relation to soil physical characteristics like penetration resistance, matric potential, and hypoxia.MethodsA new diagrammatic framework is proposed to illustrate the interaction between root elongation, soil management, and climatic conditions. The new model was written in Matlab®, using the root architecture model RootBox and a model that solves the 1D Richards equations for water flux in soil. Inputs: root architectural parameters for Soybean; soil hydraulic properties; root water uptake function in relation to matric flux potential; root elongation rate as a function of soil physical characteristics. Simulation scenarios: (a) compact soil layer at 16 to 20 cm; (b) test against a field experiment in Brazil during contrasting drought and normal rainfall seasons.Results(a) Soil compaction substantially slowed root growth into and below the compact layer. (b) Simulated root length density was very similar to field measurements, which was influenced greatly by drought. The main factor slowing root elongation in the simulations was evaluated using a stress reduction function.ConclusionThe proposed framework offers a way to explore the interaction between soil physical properties, weather and root growth. It may be applied to most root elongation models, and offers the potential to evaluate likely factors limiting root growth in different soils and tillage regimes

Mathematical and computational modelling of vegetated soil incorporating hydraulically-driven finite strain deformation

In this paper a new model for the hydro-mechanical behaviour of rooted soils is developed. It is a physically-based model that couples finite strain soil deformation with unsaturated water and air flow, while improving on existing cohesion-based approaches to mechanical root reinforcement and empirical soil water-uptake approaches typically used to deal with rooted slopes. The model is used to show that the dynamics of soil-water pressure and soil deformation depend strongly on the physics of the root-water uptake and the elasto-plastic soil mechanics. Root water uptake can cause suctions and corresponding soil shrinkage sufficiently large to necessitate a finite-strain approach. Although this deformation can change the intrinsic permeability, hydraulic conductivity remains dominated by the water content. The model incorporates simultaneous air-flow, but this is shown to be unimportant for soil-water dynamics under the conditions assumed in example simulations. The mechanical action of roots is incorporated via a root stress tensor and a simulation is used to show how root tension is mobilised within a swelling soil. The developed model may be used to simulate both laboratory experiments and full-scale vegetated slopes