Sampling and TDR probe insertion in the determination of the volumetric soil water content

Abstract

Volumetric soil water content (theta) can be evaluated in the field by direct or indirect methods. Among the direct, the gravimetric method is regarded as highly reliable and thus often preferred. Its main disadvantages are that sampling and laboratory procedures are labor intensive, and that the method is destructive, which makes resampling of a same point impossible. Recently, the time domain reflectometry (TDR) technique has become a widely used indirect, non-destructive method to evaluate theta. In this study, evaluations of the apparent dielectric number of soils (epsilon) and samplings for the gravimetrical determination of the volumetric soil water content (thetaGrav) were carried out at four sites of a Xanthic Ferralsol in Manaus - Brazil. With the obtained epsilon values, theta was estimated using empirical equations (thetaTDR), and compared with thetaGrav derived from disturbed and undisturbed samples. The main objective of this study was the comparison of thetaTDR estimates of horizontally as well as vertically inserted probes with the thetaGrav values determined by disturbed and undisturbed samples. Results showed that thetaTDR estimates of vertically inserted probes and the average of horizontally measured layers were only slightly and insignificantly different. However, significant differences were found between the thetaTDR estimates of different equations and between disturbed and undisturbed samples in the thetaGrav determinations. The use of the theoretical Knight et al. model, which permits an evaluation of the soil volume assessed by TDR probes, is also discussed. It was concluded that the TDR technique, when properly calibrated, permits in situ, nondestructive measurements of q in Xanthic Ferralsols of similar accuracy as the gravimetric method.

What drives the seasonality of photosynthesis across the Amazon basin? A cross-site analysis of eddy flux tower measurements from the Brasil flux network

“…2). Soil moisture (Â in m 3 m −3 ) was measured using frequency-domain reflectometry (TDR) at K34 (Teixeira et al, 2003), K83 and at the control site of the Tapajos Throughfall Exclusion Experiment (TEE control ∼ 4.5 km from the K67 tower) (Brando et al, 2006). We used consistency of relationships between similar sensors to identify and remove faulty periods and obtain a more complete meteorological time series (e.g.…”

Dielectric constant obtained from TDR and volumetric moisture of soils in southern Brazil

“…Empirical models that relate soil volumetric moisture with the soil dielectric constant are practical and useful for establishing calibration curves of TDR and have been investigated by different authors (Otto & Alcaide, 2001;Tommaselli & Bacchi, 2001;Teixeira et al, 2003;Coelho et al, 2006). Although the models proposed by Topp et al (1980) and Ledieu et al (1986) estimate soil moisture in a satisfactory manner, the adjusted models have their applicability limited to local conditions in terms of differences that exist in electrical conductivity, clay and quartz content (Tommaselli & Bacchi, 2001) and soil organic matter (Pumpanem & Iivesniemi, 2005).…”

“…The use of a single general empirical model adjusted for different soil types may compromise accuracy of soil moisture determinations when applied to each soil individually (Tommaselli & Bacchi, 2001;Teixeira et al, 2003). Specific models originate from calibration curves for each soil condition and type (Coelho et al, 2006).…”

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

“…Essa diferença já era esperada, pois os parâmetros desses modelos foram obtidos em condições diferentes das estabelecidas neste experimento, sobretudo quanto ao tipo de solo e granulometria, sendo usados na maioria dos experimentos solos de textura argilosa a muito argilosa. Existe ainda o problema do menor contato das guias de onda com as partículas de solo nos solos arenosos, devido à sua baixa coesão, que, segundo Teixeira et al (2003), causa erros na leitura da constante dielétrica do solo. Esse resultado corrobora a afirmação de Villwock et al (2004), de que para cada tipo de solo existe a necessidade de obtenção de um modelo de calibração empírico para obtenção de bons resultados com o uso da técnica TDR.…”