Field measurements of the water content in the top soil using a new capacitance sensor with a flat sensitive volume

Abstract

Water content directly near the soil surface plays an essential role for degradation of natural organic material and agrochemicals by soil microbes. Furthermore, the water losses by evaporation depend sensitively on the top‐soil water content. Rain, irrigation, evaporation, and the water flow between the soil horizons together with the natural inhomogeneity of soils cause a high spatial gradient and a pronounced time dependence of the water content in the top soil. To understand processes in top soil such as redox gradients, the knowledge on ecological conditions in the top soil, which is subject to rapid changes, is essential. In order to meet the requirements for such field measurements, a capacitance sensor with a depth resolution of 1 cm and an active area of 7.5 cm × 14 cm was constructed and operated by a special electronic circuit. Field measurements using these sensors at 1 cm depth showed the high dynamics when measurements were carried out every 10 min. As simultaneous measurements of the soil temperature at 1 cm depth exhibit large temperature variations during the day, its influence on the measurements must be compensated for. As the data, measured during drying periods, allow the assessment of the temperature coefficient, the water content at a reference temperature can be calculated. The course of the water content reflects precipitation events and quantifies the drying of the soil, providing these parameters for process evaluation. Furthermore, the diurnal variation exhibits the drying during the day and the possible rewetting from deeper horizons during the night.

Quantification of mycorrhizal water uptake via high-resolution on-line water content sensors

“…Thus, any differences in the decline of water content can be observed with a higher significance level. Further detailed information about the technique of the water content measurement is provided elsewhere (Ruth 1999;Ruth and Munch 2005;Ruth et al 2008). …”

Comparison of different methods for the in situ measurement of forest litter moisture content

“…The first sensor (sensor 1, group a) was a special FD sensor with a flat sensitive volume (approx. (height × breadth × length) 1 × 7.5 × 14 cm 3 ; Ruth and Munch, 2005). Further eight non-commercial two-rod FD sensors (sensors 2-9, group b) with a sensitive volume of approximately 2 × 3 × 10.5 cm 3 shared the same capacitance (C, [pF]) calculation shown in Eq.…”

“…Instrumental precision for this type of sensor is usually reported for calibrated soil moisture only (in this case: mean relative accuracy 0.14, according to Ruth and Munch, 2005). However, intrinsic instrument precision depends solely on the accuracy of frequency measurements, which is very high (∼ 10 −4 ).…”

Influence of temperature on soil water content measured by ECH2O-TE sensors

“…Due to the fact that the dielectric of water is about 80, while other soil constituents are between 1 and 5, changes in soil dielectric permittivity are highly correlated with soil water content. However, the measurements may also be impacted by the other soil environmental factors, such as soil water salinity (Kodešová et al, 2011b) and temperature (Bogena et al, 2007;Evett et al, 2006;Kizito et al, 2008;Ruth and Munch, 2005;Verhoef et al, 2006), soil texture (Chandler et al, 2004;Guber et al, 2010;Kodešová et al, 2011b;Seyfried and Murdock, 2004;Wraith and Or, 1999), soil specific surface area (Skierucha, 2009), soil chemical composition and soil bulk density (Kodešová et al, 2011b;Verhoef et al, 2006) as well as other factors, such as measurement frequency which can range from 5-500 MHz (Chen and Or, 2006;Kizito et al, 2008;Seyfried and Murdock, 2004). Chandler et al (2004) refer to the need for calibration of each sensor to correct the inter-sensor variability.…”