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Soil heat flux (<i>G</i>) 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 <i>G</i> helps to improve energy balance closure and <i>G</i> 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 (<i>Rn</i>) including midday <i>G/Rn</i> 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 (C_H), broadleaf evergreen woodlands with shrubs (C_CO), broadleaf evergreen woodlands (C_CH), broadleaf deciduous woodlands with shrubs (C_UO), broadleaf deciduous woodlands (C_UH), conifer with shrubs (C_MO) and conifer (C_MH). Hourly data of <i>Rn</i> and <i>G</i> 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 (<i>LAI</i>)—unimodal curves followed cycles of <i>Rn</i> in woodlands with low <i>LAI</i> (C_CH, C_CO, C_H and C_UO) and sinusoidal ones in woodlands with high <i>LAI</i> (C_MO, C_MH and C_UH). Midday <i>G</i>/<i>Rn</i> was generally no more than 10% with slight variations in most woodlands across the four seasons, but upward trends in the grass and C_UO were observed in the afternoon. They were found significantly correlated with <i>SWC</i>. For sparse-canopied riparian sites, hourly <i>G</i> was found to be significantly correlated with <i>Rn</i> and <i>SWC</i> in summer, whereas, for dense sites, the role of canopy characteristics overwhelmed soil properties. Equations were derived to estimate diurnal <i>G</i> from <i>Rn</i>, <i>SWC</i> and <i>LAI</i>. The <i>G</i> of all riparian sites was subject to hysteresis problems to <i>Rn</i>. Phase shifts ranged from one to eight hours in riparian sites and were positively related with <i>LAI</i> and <i>SWC</i>, mainly accounting for the second diurnal pattern of <i>G</i>.