Hydraulic properties estimated by numerical inversion of tension disc infiltrometer data and by laboratory methods

T.B. Ramos¹, M.C. Gonçalves¹, G. Jacinto², J.C. Martins¹ & F.P. Pires¹

RESUMO

A modelação inversa de dados da infiltração de água no solo, juntamente com os teores de água inicial (?i) e final do solo (?f), fornece uma estimativa da curva de retenção de água ? (h) e da curva da condutividade hidráulica no solo K(h), permitindo determinar os parâmetros que definem as propriedades hidráulicas. Relativamente a 3 locais do Alentejo, com solos de textura mediana e grosseira, comparam-se ?(h) e K(h) obtidas por modelação inversa a partir das leituras recolhidas no campo com testes de infiltração, com as determinadas por métodos laboratoriais. Os testes de infiltração foram realizados com infiltrómetros de tensão, com placas porosas de 20 cm de diâmetro, e a tensões de humidade de 0, 3, 6 e 15 cm, numa sequência descendente. Os parâmetros ?i e ?f foram determinadas por gravimetria. Os métodos laboratoriais utilizados foram as caixas de sucção, placas de pressão e evaporação, realizados em amostras de solo não perturbadas. O modelo hidráulico escolhido para representar ambas as curvas foi o de Mualem-van Genuchten (M-vG), tendo as curvas ?(h) e K(h) sido comparadas por regressão linear simples. As curvas ?(h) e respectivos parâmetros do modelo M-vG obtidos por modelação inversa mostram que os resultados são concordantes com os determinados pelos métodos laboratoriais, resultando em coeficientes de determinação (R²) superiores a 0.9798 e coeficientes de regressão (b) próximos da bissectriz. Já nas curvas K(h) observaram-se algumas diferenças entre os métodos, variando o R² entre 0,7772 e 0,9997 e o b entre 0,3865 e 3,6381.

ABSTRACT

Numerical inversion of cumulative infiltration data during transient water flow, complemented with initial (?i) and final water content data (?f) of the infiltration process, is a new tool to define soil water retention ? (h) and hydraulic conductivity K(h) curves and to estimate the unknown parameters in Mualem-Van Genuchten`s model (M-vG) of the unsaturated soil hydraulic properties. In 3 different soils in Alentejo, with coarse to medium texture, ?(h) e K(h) obtained from numerical inversion of tension infiltration data and from laboratory methods were compared. Field measurements were taken using a tension disc infiltrometer, with a diameter of 20 cm, with application of consecutive tensions of 0, 3, 6 and 15 cm, in a descendent sequence. ?i e ?f were determined by gravimetry. The laboratory methods used were suction tables with sand and kaolin, pressure plates and evaporation to measure hydraulic properties in undisturbed soil samples. ?(h) curves and respective parameters estimated by numerical inversion and from laboratory data reproduced closely, in such a way that determination coefficients (R²) were always above 0.9798 and regression coefficients (b) close to the bissectriz. K(h) curves showed more differences between both methods, with R² varying between 0.7772 and 0.9997 while b changed between 0.3865 and 3.6381.

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¹ Estação Agronómica Nacional, Quinta do Marquês, 2784-505 Oeiras, Portugal – Tel: (+351) 214 403 500 – Fax: (+351) 214 416 011 – E-mail: Tiago_Ramos@netcabo.pt;

² Universidade de Évora, Departamento de Matemática e CIMA-UE, Rua Romão, 59, 7000-671 Évora, Portugal – Tel: (+351) 266 745 370 – Fax: (+351) 266 745 393