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Abstract Measurements made in the Columbia River Basin (Oregon) in an area of irregular terrain during the second Wind Forecast Improvement Project (WFIP 2) field campaign are used to develop an optimized hybrid bulk algorithm to predict the surface turbulent fluxes from readily measured or modelled quantities over dry and wet bare or lightly vegetated soil surfaces. The hybrid (synthetic) algorithm combines (i) an aerodynamic method for turbulent flow which is based on the transfer coefficients (drag coefficient and Stanton number), roughness lengths, and Monin-Obukhov similarity and (ii) a modified Priestley-Taylor (P-T) algorithm with physically based ecophysiological constraints which is essentially based on the surface energy budget (SEB) equation. Soil heat flux in the latter case was estimated from measurements of soil temperature and soil moisture. In the framework of the hybrid algorithm, bulk estimates of the momentum flux and the sensible heat flux are derived from a traditional aerodynamic approach, whereas the latent heat flux (or moisture flux) is evaluated from a modified P-T model. Direct measurements of the surface fluxes (turbulent and radiative) and other ancillary atmospheric/soil parameters made during WFIP 2 for different soil conditions (dry and wet) are used to optimize and tune the hybrid bulk algorithm. The bulk flux estimates are validated against the measured eddy-covariance fluxes. We also discuss the SEB closure over dry and wet surfaces at various timescales based on the modelled and measured fluxes. Although this bulk flux algorithm is optimized for the data collected during the WFIP 2, a hybrid approach can be used for similar flux-tower sites and field campaigns.
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A field calibration method that improves soil heat flux accuracy
Abstract Soil heat flux plates (SHFPs) are widely used to measure soil heat flux (Gs). Gs is often underestimated by SHFPs (Gp). Although calibration methods are used, they are not always effective. The objective of this study is to evaluate the effectiveness of a field calibration method applied to various SHFPs installed in a full canopy maize field. A 5‐day measurement period with wet and dry soil conditions was used for calibration, while 80‐day and 60‐day measurement periods were used for evaluation. Uncorrected SHFP measured values (Gp) underestimated the actual reference Gs determined by the gradient method (Gs_grad) by 42%–64%. Gp values in the evaluation period were corrected (Gp_corr) by dividing them by the ratio of Gp/Gs_grad determined over the calibration period. After the correction, the Gp_corr agreed well with the Gs_grad, with Gp_corr/Gs_grad of four of six SHFPs being 0.90–1.01, improving to 74%–98%. The field calibration performed approximately the same with the wet and dry calibration periods, whether the calibration and evaluation periods were consecutive in time or had relatively long time intervals, indicating that this method accounted for almost all errors with SHFP. This is largely due to the slight variation in soil thermal conductivity and the linearity between soil temperature gradients from SHFP and the gradient method under relatively stable soil moisture conditions. This study deepens our understanding and improves the accuracy of soil heat flux measurements. Calibration of SHFPs under various land covers and weather conditions is warranted in future studies.
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- Award ID(s):
- 2037504
- PAR ID:
- 10610005
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Vadose Zone Journal
- Volume:
- 24
- Issue:
- 3
- ISSN:
- 1539-1663
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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