The surface heat flux density of a bare soil

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Abstract
Half‐hourly measurements of soil surface heat flux density (G0 ), solar irradiance (S), and the surface energy balance components were made at Agassiz, b.c., in the spring and early summer of 1978 at two adjacent bare‐soil sites, one of which was culti‐packed while the other was disc‐harrowed. G0 was calculated using the null‐alignment procedure from half‐hourly measurements of soil temperature at 30 depths down to 1 m, and volumetric soil heat capacity calculated from measurements of bulk density, organic matter fraction, and moisture content. The latent and sensible heat flux densities were measured using the energy balance/Bowen ratio technique. It was found that both the daily averages and diurnal variations of Go at each site were not affected as the soil surface dried, despite reductions in evaporation rate of as much as 50% at the culti‐packed site and 75% at the disc‐harrowed site on the clear dry‐soil days. Diurnal variations of G0 at the disc‐harrowed site were about 25% less than at the culti‐packed site, although daily averages were similar at both sites. Daily and daytime averages of G0 at each site were linear functions of S alone, or functions of net radiation and some measure of near‐surface soil water content. Night‐time averages of G0 at each site were linear functions of a cloudiness ratio equal to the fraction received of the clear‐day S.