Long-term stability and vertical displacement of soil sensors at Cabauw site

Accurate observations of surface soil heat flux are an integral part of better understanding land surface-boundary layer interactions. Surface soil heat flux is typically derived from a combination of soil temperature and soil heat flux sensors at various levels below the surface. Over time, the action of soil fauna, weathering (rain and frost), and gravity, shifts the plant material, soil, and ultimately, the sensors. If not accounted for, this displacement causes uncertainty in surface soil heat flux estimations, becoming especially relevant for long-term observations. Yet these same long-term observations can also be used to evaluate the reliability and stability of soil thermal measurements. Aptly, a variety of soil observations are available at a mid-latitude grassland site since 1986 as a part of the Cabauw in-situ program. We analyse the timeseries from 2003 – 2019, investigating the history, vertical movements, and positional problems of the soil heat flux sensor system as well as the complex application of these measurements for Surface Energy Balance (SEB). We examine the sensor movement in the soil by comparing the phase of the topsoil temperature and the soil heat flux at 0.05-m depth to the phases of the shortwave downward radiation and 2-m air temperature. Naturally, we observe gradual sinking and displacement of the soil sensors. In fact, some components were found approximately 0.05 m lower, 15 years after burial. In 2015, a new soil thermal system was reconstructed with sensors positioned as precisely as possible, and after one year, the grass conditions were comparable to the surrounding terrain. Therefore, we assume that the top sensors are still in the correct vertical position during the one year of observations taken after this point and compare the soil heat flux to the old sensor system. We observe large discrepancies in the contribution of the soil heat flux to the SEB for the new and old system, highlighting the importance of accurate soil observations.