Soil moisture calibration of TDR multilevel probes

Abstract

Time domain reflectometry (TDR) probes are increasingly used for field estimation of soil water content. The objective of this study was to evaluate the accuracy of the multilevel TDR probe under field conditions. For this purpose, eight such TDR probes were installed in small plots that were seeded with beans and sorghum. Data collection from the probes was such that soil moisture readings were automated and logged using a standalone field unit. Neutron probe measurements were used to calibrate the TDR probes. Soil-probe contact and soil compaction were critical to the accuracy of the TDR, especially when a number of TDR probes are combined for a single calibration curve. If each probe is calibrated individually, approximate measurement errors were between 0.005 and 0.015 m 3 m -3 . However, measurement errors doubled to approximately 0.025 to 0.03 m 3 m -3 , when TDR probes were combined to yield a single calibration curve. Key words: TDR, soil water content, neutron probe CALIBRAÇÃO DE SONDAS TDR DE NÍVEIS MÚLTIPLOS DE PROFUNDIDADE, PARA UMIDADE DO SOLORESUMO: As sondas TDR (Time Domain Reflectometry) são cada vez mais utilizadas para a estimativa da umidade do solo no campo. Este estudo teve como objetivo avaliar a exatidão das sondas TDR providas de sensores múltiplos para diversas profundidades. Para isso, oito sondas foram instaladas em pequenas parcelas que foram semeadas com feijão e sorgo. A coleta de dados com as sondas foi feita de forma automatizada e armazenada em "data logger". Para calibrar as sondas TDR foi utilizada sonda de neutrons. A exatidão do TDR foi sensível ao contato solo-sonda e à compactação do solo, especialmente quando várias sondas são utilizadas para compor uma única curva de calibração. Quando cada sonda foi calibrada individualmente, os erros de medidas foram da ordem de 0,005 a 0,015 m 3 m -3 . Entretanto, os erros dobraram para 0,025 a 0,03 m 3 m -3 quando as sondas TDR foram combinadas para produzir uma única curva de calibração. Palavras-chave: TDR, sonda, água do solo, neutron

Calibration of a water content reflectometer and soil water dynamics for an agroforestry practice

“…Therefore, soil profiles with relatively high water infiltration in surface horizons and relatively low infiltration in subsurface horizons may need individual calibration equations to better understand water dynamics. In support of this, Serrarens et al (2000) observed that the measurement error doubled when a single calibration equation was used in a TDR calibration study with six soil depths.…”

“…In the current study packing also may have contributed to sensor readings. Compaction, air gaps, porosity, and spatial variability affect travel time and thereby the measured h v (Serrarens et al 2000;Vaz and Hopmans 2001;Stangl et al 2009). Root mean square error (RMSE) for linear and quadratic equations with period was 0.038 and 0.039 m 3 m -3 , respectively, for the three soils combined as compared to 0.15 m 3 m -3 for the manufacturer-provided equation.…”

Condição de contorno para calibração de reflectômetro usado para medição de água no solo

“…Em alguns casos, há necessidade ainda de corrigir a calibração obtida em laboratório com medidas posteriores de campo (STANGL et al, 2009), procedimento que pode ser feito individualmente por guia de TDR (SERRARENS et al, 2000). O aumento no comprimento do cabo entre o emissor do pulso elétrico e a guia de medição instalada no campo pode atenuar o sinal refletido e alterar o valor de K a (COELHO et al, 2005).…”

Performance of a multi-level TDR system exposed to tropical field conditions – A time-space comparison with tensiometry

“…Good results were found in controlled laboratory conditions (Frueh and Hopmans, 1997;Gong et al, 2003;Hook et al, 1992;Sun et al, 2000), but greater uncertainties were reported from field tests (Dasberg et al, 1995;Heathman et al, 2003;Serrarens et al, 2000). For instance, Serrarens et al (2000) needed to split the data into two layers to obtain reasonable calibration. Frueh and Hopmans (1997) and Heathman et al (2003) reported that calibration specific for particular locations provided better results.…”