Soil water content (SWC) sensors are widely used for scientific studies or for the management of agricultural practices. The most common sensing techniques provide an estimate of volumetric soil water content based on sensing of dielectric permittivity. These techniques include frequency domain reflectometry (FDR), time domain reflectometry (TDR), capacitance and even remote-sensing techniques such as ground-penetrating radar (GPR) and microwave-based techniques. Here, we will focus on frequency domain reflectometry (FDR) sensors and more specifically on the questioning of their factory calibration, which does not take into account soil-specific features and therefore possibly leads to inconsistent SWC estimates. We conducted the present study in the southwest of France on two plots that are part of the ICOS ERIC network (Integrated Carbon Observation System, European Research and Infrastructure Consortium), FR-Lam and FR-Aur. We propose a simple protocol for soil-specific calibration, particularly suitable for clayey soil, to improve the accuracy of SWC determination when using commercial FDR sensors. We compared the sensing accuracy after soil-specific calibration versus factory calibration. Our results stress the necessity of performing a thorough soil-specific calibration for very clayey soils. Hence, locally, we found that factory calibration results in a strong overestimation of the actual soil water content. Indeed, we report relative errors as large as +115 % with a factory-calibrated sensor based on the real part of dielectric permittivity and up to + 245 % with a factory-calibrated sensor based on the modulus of dielectric permittivity.

Soil evaporation concerns water and our life support sources, which are important for agriculture or for climate change prediction science. A simple instrument based on the nonsteady-state (NSS) technique for soil evaporation measurement appears suitable. However, because the NSS chamber technique is highly invasive, special care should be provided to correct the wind speed influence on the evaporation process. Soil evaporation is a complex process that involves many soil and air characteristics. Measurement chamber installation on the soil and its head deployment may perturb these characteristics. We therefore had to minimize differences or to correct the measurements. Most of the differences between bare soil and soil with a deployed chamber head can be minimized, except for the wind speed influences that are not reproducible inside a chamber head. Meanwhile, as the wind influences depend on numerous variables that are not monitorable in real time, a self-calibrating chamber with a corresponding protocol called the Autocalibrated Soil Evapo-respiration Chamber (ASERC) was developed to make the measurements easily corrigible on bare soil with a unique variable (wind speed, W_S), regardless of the soil composition, soil texture, and other soil or air meteorological variables. A simple protocol followed by this chamber allows us to determine the soil evaporation wind speed susceptibility (Z) and to correct the measurements achieving 0.95 as the coefficient of determination. Some interesting findings on sandy and clayey soil evaporation measured during laboratory calibration and “slow” sensor simulation will also be reported in the two appendices.

Soil respiration measurement is important to assess natural carbon dioxide production. The closed-chamber technique allows relatively easy soil respiration monitoring. A planned spatially large-scale campaign incites us to implement our ultra-low-cost portative chamber. The chamber itself is entirely built from commercial parts with little, easy-to-perform, quick machining work. The resulting setup is an easy-to-operate, standalone, robust device. The used sensors are cost-effective yet accurate digital sensors that were successfully checked against some reference sensors. All of these characteristics made the described chamber accessible to build and use for a wide scientific and educational community. In this short note, we describe this simple device along with its sensors and apparent respiration quotient tip.

Soil heat flux is an important component of the surface energy balance (SEB) equation. Measuring it requires an indirect measurement. Every used technique may present some possible errors tied with the utilized specific technique, soil inhomogeneities, or physical phenomena such as latent heat conversion beneath the plates, especially in desiccation cracking soil or vertisol. The installation place may also induce imbalances. Finally, some errors resulting from the physical sensor presence, vegetation presence, or soil inhomogeneities may occur and are not avoidable. For all these reasons it is important to check the validity of the measurements. A quick and easy way is to integrate results over 1 year. By consideration of the inert core internal energy conservation law, it is shown that the corresponding integration should be close to zero after a necessary geothermal heat efflux subtraction. However, below-plate evaporation and vegetation-absorbed water or rainwater infiltration may also contribute to the observed short-scale and/or long-scale imbalance generating convective heat fluxes not sensed by the heat flux sensors. Another energy source is usually not included in the SEB equation: rainfall or irrigation. Yet its importance for short- and long-term integration is notable. As an example, the most used sensor, soil heat flux plates (SHFPs), is given.

Soil heat flux is an important component of the Surface Energy Balance (SEB) equation. Measuring it require an indirect measurement. Every used technique may present some possible errors tied with each specific technique, soil inhomogeneities or physicals phenomenon such as latent heat conversion beneath the plates especially in a desiccation cracking soil or vertisol. The installation place may also induce imbalances. Finally, some errors resulting from the physical sensor presence, vegetation presence or soil inhomogeneities may occur and are not avoidable. For all these reasons it is important to check the validity of the measurements. One quick and easy way is to integrate results during one year. The corresponding integration should be close to zero after a necessary geothermal heat efflux subtraction which should be included into the SEB equation for long term integrations. However, below plate evaporation and vegetation absorbed water or rainfall water the infiltration may also contribute to the observed short scale or/and long scale imbalance. Another energy source is usually not included in the SEB equation: the rainfall or irrigation. Yet its importance for a short- and long-term integration is notable. As an example, the most used sensors: Soil Heat Flux Plates (SHFP), is given.