OPS 2-7
What the flux is up with soil heat flux? Errors and uncertainties associated with self-calibrating soil heat plate
The National Ecological Observatory Network (NEON) plans to deploy self-calibrating soil heat flux plates at NEON’s 60 terrestrial research sites. Unlike other types of soil heat flux plates, self-calibrating versions have the ability to correct for errors caused by differences in thermal conductivity between the plates and the surrounding soil. This is done with integrated heaters, well defined heat pulses, and subsequent dynamic adjustments the calibration coefficients of each individual plate. This should provide a more accurate estimation of soil heat flux though field studies implementing self-calibrating units and routines are limited and continued investigation is needed. Here, a set of self-calibrating soil heat flux plates are evaluated in-situ in Nunn clay loam soil at NEON’s calibration and validation outdoor laboratory.
Results/Conclusions
The soil heat flux plates were installed 8 cm below the ground surface and the sensor output was recorded at 1 sec intervals. The manufacturer recommended self-calibration procedures were followed, and a short (3 min) heat pulse from the integrated heater was activated every 3.25 hours. Preliminary results show that the in-situ calibration coefficients are 30% lower on average that the original manufacturer supplied calibration coefficient. This was particularly evident after major rain events when soil moisture increased by up to 190% of the pre-rain event soil moisture. During these times, the calibration coefficients were nearly 32% lower than prior to the rain event. Using only the manufacturer supplied calibration coefficients, the soil heat flux is underestimated by up to 100%. We also found that during the calibration cycle, the heat pulse from the integrated heater influences the readings for about 30 min following the calibration cycle. This causes erroneous soil heat flux measurements that should be discarded. These preliminary results are used by NEON in designing configurations which mitigate soil heat flux uncertainties.