2.3.2 Xinanjiang Model
The Xinanjiang model, which is a conceptual rainfall–runoff model developed by Zhao et al. (1980), has been widely used in many regions of the world (Lin et al., 2014; Liu et al., 2009; Yao et al., 2014). In the model, actual evapotranspiration is computed from potential evapotranspiration while the soil storage deficit is represented in three layers, i.e., upper, lower and deep soil layers. The total runoff of the basin is divided into surface runoff, interflow and groundwater runoff using a free water capacity distribution curve and is estimated using a soil moisture storage capacity distribution curve based on the concept of runoff formation on repletion of storage. The surface runoff is routed by the lag–and–route method, while interflow and groundwater are routed through linear reservoirs representing interflow and groundwater storage, respectively (Zhao., 1992; Zhao and Liu., 1995). The determination of the sensitive parameters was adopted to the results of a local sensitivity analysis. We observed the change in the outputs of discharge and runoff by disturbing all parameters. The sensitive parameters in the Xinanjiang model mainly include the evapotranspiration coefficient (K C), areal mean free water capacity of the surface soil layer (SM ) and the recession constant of surface water storage (CS ) (Table 2). KCcontrols the total water balance, which is important for water calculations. SM plays a decisive role in the amount of surface runoff, which reflects the ability of surface soil to store water and determines the proportion of surface runoff and groundwater. CSreflects the geomorphology of the river network and determines the final flow process. The ranges of parameters were chosen based on the work of Ren et al. (1989), Wang & Zhao (1989) and Zhao et al. (1992). In this research, 20 samples were generated following LHS method for these uncertain parameters. We assumed that uncertain parameters follow uniform or normal distributions.