Daily Dynamics of Soil Heat Flux and Its Relationship with Net Radiation in Different Urban Riparian Woodlands

Soil heat flux (G) not only affects the Earth’s surface energy balance but also models of calculating soil evaporation. A better understanding on the effect of timing, soil and vegetation on riparian G helps to improve energy balance closure and G simulation in riparian areas with various woodlands. This paper examined diurnal and seasonal variation patterns of soil heat flux in urban riparian areas, together with its relationship with net radiation (Rn) including midday G/Rn and the hysteresis phenomenon under the mutual influence of the timing, soil wetness and vegetation conditions. Study sites lie in the riparian areas of Shanghai with seven vegetation-covered conditions—grassland (CH), broadleaf evergreen woodlands with shrubs (CCO), broadleaf evergreen woodlands (CCH), broadleaf deciduous woodlands with shrubs (CUO), broadleaf deciduous woodlands (CUH), conifer with shrubs (CMO) and conifer (CMH). Hourly data of Rn and G on typical days in four seasons starting from 11/2020 to 10/2021 were obtained with automated data-logging sensors. Diurnal variations in soil heat flux were characterized as two patterns depending on leaf area index (LAI)—unimodal curves followed cycles of Rn in woodlands with low LAI (CCH, CCO, CH and CUO) and sinusoidal ones in woodlands with high LAI (CMO, CMH and CUH). Midday G/Rn was generally no more than 10% with slight variations in most woodlands across the four seasons, but upward trends in the grass and CUO were observed in the afternoon. They were found significantly correlated with SWC. For sparse-canopied riparian sites, hourly G was found to be significantly correlated with Rn and SWC in summer, whereas, for dense sites, the role of canopy characteristics overwhelmed soil properties. Equations were derived to estimate diurnal G from Rn, SWC and LAI. The G of all riparian sites was subject to hysteresis problems to Rn. Phase shifts ranged from one to eight hours in riparian sites and were positively related with LAI and SWC, mainly accounting for the second diurnal pattern of G.