A Time-Varying Distributed Unit Hydrograph considering soil moisture content
Abstract. The distributed unit hydrograph (DUH) method has been widely used for flood routing simulation, because it can well characterize the underlying surface characteristics and various rainfall intensities. The core of the DUH is the calculation of flow velocity. However, the current velocity formula assumed a global equilibrium of the watershed and ignored the impact of time-varying soil moisture content on flow velocity, which leads to a larger flow velocity value. The goal of this study is to identify a soil moisture content factor, which was derived based on the water storage capacity curve, to explore the responses of DUH to soil moisture content in unsaturated areas. Thus, an improved distributed unit hydrograph based on time-varying soil moisture content was proposed in this paper. The proposed method considered the impact of both the time-varying rainfall intensity and soil moisture content on the flow velocity, and the watershed is assumed not to be equilibrium but vary with the soil moisture. The Qin River Basin was selected as a case study, and results of the time-varying distributed unit hydrograph (TDUH) and current DUH methods were used as comparisons with that of proposed method. Influence mechanism of time-varying soil moisture content on the flow velocity and flood forecasts were explored. Results show that the proposed method performs the best among the three methods. The shape and duration of the unit hydrograph can be mainly related to the soil moisture content at initial stage of a storm. When the watershed is approximately saturated, the grid flow velocity is majorly dominated by the excess rainfall.
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