【正文】 g either the diffusive wave model or the dynamic wave model. The dynamic wave model solves the full St. Venant equations. The Preissmann implicit,fourpoint, finite difference scheme is used to discretize the governing equations. Linearized iteration schemes for the discretized governing equations are established and solved using a double sweep algorithm. The influence of hydraulic structures such as culverts, measuring flumes, bridge crossings and drop structures has been considered in the CCHE1D model. Stagedischarge relations for hydraulic structures are derived so that the hydraulic structures bee an intrinsic part of the numerical algorithm. Sediment Transport Model. The CCHE1D model calculates nonuniform sediment transport in rivers using a nonequilibrium approach. The governing equation for the nonequilibrium transport of nonuniform total load is (1)where A is the flow area。 Ctk is the depthaveraged totalload concentration of size class k。 Qtk is the actual totalload transport rate。 Qt*k is the totalload transport capacity。 Ls is the adaptation length of nonequilibrium sediment transport。 and qlk is the side sediment discharge from banks or tributaries per unit channel length, with the contribution from banks being simulated by CCHE1D bank erosion and bank failure module, and the contribution from upland erosion being simulated by SWAT or AGNPS. The sediment transport capacity can be written as a general form (2)where pbk is the bed material gradation。 Q*tk is the potential sediment transport rate, which is determined with SEDTRA module (Garbrecht et al., 1995), Wu, Wang and Jia’s formula (2000), the modified Ackers and White’s 1973 formula (Proffitt and Sutherland, 1983), or the modified Engelund and Hansen’s 1967 formula (with Wu, Wang and Jia’s correction factor, 2000). The bed deformation due to size class k is determined with (3)where p′ is the bed material porosity, which is calculated with the methods of Komura and Simmons (1967), Han et al (1981), or is specified by the user according to available measurement data。 ? Abk / ? t is the bed deformation rate of size class k. The bed material is divided into several layers. The variation of bed material gradation pbk at the mixing layer (surface layer) is determined by the following equation (Wu and Li, 1992) (4)where Am is the crosssectional area of the mixing layer。 ? Ab / ? t is the total bed deformation rate, defined as ? Ab/ ? t =k=1N? Ab/ ? t。 N is the total number of size classes。 p*bk is pbk of the mixing layer when ?Am/ ? t ? ? Ab/?t≤0 , and p*bk is the percentage of the kth size class of bed material in subsurface layer (under mixing layer) when ?Am/ ? t ? ? Ab/?t 0. Eq. (1) is discretized using the Preissmann implicit scheme, with its source term being discretized by the same formulation as that for the righthand term of Eq. (3) in order to satisfy the sediment continuity. Eq. (4) is discretized by a difference scheme that satisfies mass conservation. A coupled method for the calculations of sediment transport, bed change and bed material sorting is established by implicitly treating the pbk in Eq. (2) as pbkn+1and simultaneously solving the set of algebraic equations corresponding to Eqs. (1)(4) by using the direct method proposed by Wu and Li (1992). This coupled method is more stable and can more easily eliminate the occurrence of the puted negative bed material gradation, when pared to the decoupled method, in which the pbk in Eq. (2) is treated explicitly. However, the aforementioned coupling procedure for sediment transport, bed change and bed material sorting putations is still decoupled from the flow calculation. Model Parameters to be Analyzed The parameters in numerical models of flow and sediment transport in rivers can be classified into two groups: numerical parameters and physical parameters. The numerical parameters result from the discretization and solution procedures, while the physical parameters represent the physical properties of flow and sediment, or the quantities derived from the modeling of flow and sediment transport. In the CCHE1D channel network model, the numerical parameters include putation time step and grid length, and the physical parameters are the Manning’s roughness coefficient, nonequilibrium adaptation length of sediment transport, mixing layer thickness, bed material porosity, etc. Usually, the numerical parameters can be more easily handled than the physical parameters. Some of these physical parameters, such as the Manning’s roughness coefficient and bed material porosity, have been studied by many investigators and may be determined by measurement. However, the nonequilibrium adaptation length and the mixing layer thickness are less understood and must be prescribed empirically. Therefore, the main concern in this paper is to analyze the influence of these two physical parameters on the simulation results. The nonequilibrium adaptation length Ls characterizes the distance for sediment to adjust from a nonequilibrium state to an equilibrium state. Wu, Rodi and Wenka (2000) and Wu and Vieira (2000) reviewed in detail those empirical and semiempirical methods for determining Ls published in the literature, such as Bell and Sutherland’s (1981), Armanini and di Silvio’s (1988), etc. It was found that those methods provide significantly different estimations of Ls. In CCHE1D, the adaptation length for wash load transport is set as infinitely large because the net exchange between wash load and channel bed is usually negligible. The adaptation length for suspended load transport is calculated with Ls=uh/αωs, in which u is the sectionaveraged velocity, h is the flow depth, ωs is the settling