# Integrating XAJ Model with GIUH Based on Nash Model for Rainfall-Runoff Modelling

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State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, No. 1 Xikang Road, Gulou District, Nanjing 210098, China

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College of Hydrology and Water Resources, Hohai University, No.1 Xikang Road, Gulou District, Nanjing 210098, China

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Jiangsu Province Hydrology and Water Resources Investigation Bureau, No. 5 Shanghai Road, Gulou District, Nanjing 220098, China

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Bureau of Hydrology, Ministry of Water Resources of the People’s Republic of China, Beijing 10053, China

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Author to whom correspondence should be addressed.

Received: 25 January 2019 / Revised: 28 March 2019 / Accepted: 7 April 2019 / Published: 13 April 2019

(This article belongs to the Special Issue Water Related Disaster and Water Environment Management)

The geomorphologic instantaneous unit hydrograph (GIUH) is an applicable approach that simulates the runoff for the ungauged basins. The nash model is an efficient tool to derive the unit hydrograph (UH), which only requires two items, including the indices n and k. Theoretically, the GIUH method describes the process of a droplet flowing from which it falls on to the basin outlet, only covering the flow concentration process. The traditional technique for flood estimation using GIUH method always uses the effective rainfall, which is empirically obtained and scant of accuracy, and then calculates the convolution of the effective rainfall and GIUH. To improve the predictive capability of the GIUH model, the Xin’anjiang (XAJ) model, which is a conceptual model with clear physical meaning, is applied to simulate the runoff yielding and the slope flow concentration, integrating with the GIUH derived based on Nash model to compute the river network flow convergence, forming a modified GIUH model for flood simulation. The average flow velocity is the key to obtain the indices k, and two methods to calculate the flow velocity were compared in this study. 10 flood events in three catchments in Fujian, China are selected to calibrate the model, and six for validation. Four criteria, including the time-to-peak error, the relative peak flow error, the relative runoff depth error, and the Nash–Sutcliff efficiency coefficient are computed for the model performance evaluation. The observed runoff value and simulated series in validation stage is also presented in the scatter plots to analyze the fitting degree. The analysis results show the modified model with a convenient calculation and a high fitting and illustrates that the model is reliable for the flood estimation and has potential for practical flood forecasting.

*Keywords:*flood forecasting; GIUH; Nash model; Xin’anjiang model; flow velocity