Error Analysis And Improvement Of The Combination Method For Measuring Soil-Surface Heat Flux

Soil-surface heat flux is an important component of surface energy balance.Combination method is used to determine soil-surface heat flux,which combines subsurface soil heat flux?using soil heat flux plate method or gradient method?and the soil heat storage in upper soil layer.However,several potential errors or limitations are suggested to associate with the combination method:1)plate method is prone to suffer from several errors under field conditions,but few people make efforts to identify extent of errors or find ways to minimize them;2)thermal conductivity models provide a potential for obtaining soil thermal conductivity dynamics,but there is a lack of information about its field performance and how model works with gradient method for determining near-surface soil heat flux;and 3)combination method commonly requires several parameters that determined based on several measurement procedures both in the lab and field.There is no single compound sensor available for determining soil-surface heat flux directly.To address these problems,we made quantitative analysis to identify main measurement errors associated with plate method and thermal conductivity model-based gradient method,investigated methods for minimizing errors,and simplified the application of combination method by using a novel type of heat-pulse probe sensor.Main findings of this study are as follows:First,field tests data showed that main error source of plate method(underestimate soil heat flux by 4.3-30.9 W m^-2 in 2-10 cm soil)is heat flow distortion,which is due to the great mismatch between soil thermal conductivity and that of the plate in the field.Philip correction?1963?reduced the errors to 3.3^-20.8 W m^-2;by using measured plate parameters instead of manufacturer-specified values,effectiveness of Phillip correction was improved,with plate errors reduced to 3.2-8.2 W m^-2.Self-calibrating plate worked well at 6 and 10 cm(errors were 2.3-6.4 W m^-2)by mitigating heat flow distortion effectively.At 2 cm,however,the self-calibrating plate?having the largest size among commonly-used plates?blocked water movement and the coupled heat transfer through the plate body,and thus,produced erroneous plate data(overestimate soil heat flux by 17.3 W m^-2).The heat flow generated during the self-calibrating procedure biased plate signals significantly and resulted in plate errors up to 150 W m^-2.It is suggested to place the self-calibrating plate at depths greater that 6 cm,and discard plate signals that record during and shortly?5-10 min?after self-calibrating.Second,soil thermal conductivity showed a high temporal variability near the surface.Using a fixed thermal conductivity led to large errors(16.9-43.6 W m^-2)in gradient method soil heat flux measurements at 2 cm depth.Thermal conductivity model?Lu et al.,2014?performed well at near-surface 2-10 cm soil layer?errors were 0.05-0.08 W m-1 ?-1?,and work well with the gradient method for obtaining reliable soil heat flux data(errors<8 W m^-2).Many long-term experiments provide information of model parameters?water content,bulk density,and soil texture?,which make the model-based method of high availability.Error analysis showed that ignoring temporal changes in 0-5 cm soil bulk density led to larger errors in the model?0.08 W m-1 ?-1?and gradient method(7.6 W m^-2).Using variable bulk density instead of fixed value,errors in the model and gradient method were reduced by 38%and 22%,respectively.Besides,ignoring soil water content temporal changes in short-time scale resulted in erroneous thermal conductivity and soil heat flux estimates(e.g.,led to an underestimate of soil heat flux by 6 W m^-2 or 0.54 MJ m^-2 shortly after a rainfall event).During drying-wetting cycles near the surface,it is recommended to use in situ measuring technique?e.g.,the time domain reflectometry technique?to better capture the temporal variations of soil water content,and thus further improve performance of model and model-based gradient method.Using sand content as a proxy of quartz content brought uncertainties to model accuracy.It is suggested to conduct one-point calibration to minimize effect from this potential error source.Third,based on one multi-needle heat-pulse probe?HPP?,dynamics of soil-surface heat flux were determined with high accuracy.This HPP-based method produced measurements that were in good agreement with those from independent methods(mean relative difference<5 W m^-2),and had the advantage of determining required parameters concurrently and colocated.By reducing the number of needles from 11 to 5,the datalogging requirement was reduced by half without losing accuracy in soil-surface heat flux measurement.Additional analysis emphasized the importance of soil heat storage in combination method Neglecting soil heat storage led to an underestimate(by 32.3 W m^-2 or 2.6 MJ m^-2)and a phase lag?2 h?of soil-surface heat flux dynamics.In surface energy balance observations,it is suggested to measure subsurface soil heat flux at depths ? 50 mm to minimize likelihood of double-recording latent heat flux.The results and findings from this study provide guidelines and reference to accurately determine soil-surface heat flux with the combination method,and are useful for facilitating studies in the disciplines of agronomy,forestry,meteorology and soil science that rely on soil-surface heat flux measurements.