**Ling Zeng, Lihua Xiong \*, Dedi Liu, Jie Chen and Jong-Suk Kim**

State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China; zengling@whu.edu.cn (L.Z.); dediliu@whu.edu.cn (D.L.); jiechen@whu.edu.cn (J.C.); jongsuk@whu.edu.cn (J.-S.K.)

**\*** Correspondence: xionglh@whu.edu.cn; Tel.: +86-13871078660; Fax: +86-27-68773568

Received: 13 August 2019; Accepted: 25 September 2019; Published: 28 September 2019

**Abstract:** Hydrological nonstationarity has brought great challenges to the reliable application of conceptual hydrological models with time-invariant parameters. To cope with this, approaches have been proposed to consider time-varying model parameters, which can evolve in accordance with climate and watershed conditions. However, the temporal transferability of the time-varying parameter was rarely investigated. This paper aims to investigate the predictive ability and robustness of a hydrological model with time-varying parameter under changing environments. The conceptual hydrological model GR4J (Génie Rural à 4 paramètres Journalier) with only four parameters was chosen and the sensitive parameters were treated as functions of several external covariates that represent the variation of climate and watershed conditions. The investigation was carried out in Weihe Basin and Tuojiang Basin of Western China in the period from 1981 to 2010. Several sub-periods with different climate and watershed conditions were set up to test the temporal parameter transferability of the original GR4J model and the GR4J model with time-varying parameters. The results showed that the performance of streamflow simulation was improved when applying the time-varying parameters. Furthermore, in a series of split-sample tests, the GR4J model with time-varying parameters outperformed the original GR4J model by improving the model robustness. Further studies focus on more diversified model structures and watersheds conditions are necessary to verify the superiority of applying time-varying parameters.

**Keywords:** time-varying parameter; GR4J model; changing environments; temporal transferability; western China
