Abstract Ephemeral gully erosion is widespread on the Loess Plateau of China. To provide theoretical support for constructing numerical models and soil-water conservation planning, the hydrodynamic characteristics of ephemeral gully flows were studied by laboratory experiments with a physical model. We firstly concluded that, with the influence of “step-slope-pool” (or “step-pool”) terrain on the bottom of gully channels, no obvious or consistent pattern changes were evident in the hydrodynamic parameters along the flow direction. Secondly the change of hydrodynamic parameters during erosion could be classified into an initially fast changing interval followed by a stable period, and the initial adjustment period would become shorter with increasing slope and rainfall intensity. Thirdly ephemeral gully erosion flows were turbulent and changed frequently in space and time between supercritical and subcritical flows. Throughout the experiment, the Reynolds number fluctuated and increased with time. With increasing rainfall intensity and gully slope, the frequency of larger Reynolds numbers increased. In addition, the Froude number also fluctuated, but decreased with time, and the mean Froude number eventually stabilized at ~ 0.5 at different slope angles and rainfall intensities. Fourthly the distribution of erosional energy was influenced mainly by topography in loess slopes. In the case of 20° slopes, the inverse ratios of dissipation as soil erosion, sediment transport, and flow kinetic energy were relatively stable in time and space during ephemeral gully erosion. Moreover, flow velocity, shear stress, the Darcy–Weisbach friction factor, and the Manning coefficient were influenced by rainfall intensity to a lesser degree during erosion. In the stable interval, for 15° and 25° slopes, shear stress, the Darcy–Weisbach friction factor and Manning coefficient of ephemeral gully flow both increased with increasing rainfall intensity. Flow velocity on the 15° slope decreased initially then increased with increasing rainfall intensity, but flow velocity on the 25° slope decreased with increasing rainfall intensity. Finally the regularity seen in hydrodynamic parameters of ephemeral gully flows occurred despite disturbance from lateral confluences, gravity erosion of the channel bank, and changes in terrain. The sediment transport capacity of ephemeral gully flows increased with USP by a linear function (r = 0.64, n = 99, P