*2.3. Determination of the Model Parameters*

The input parameters for the model simulation are Qc, Mmax, Qmax, and background suspended sediment concentration (Cb). These parameters were determined for the 13 catchments using Equations (6)–(8) following Park et al. [32] as described below.

Park et al. [31] observed the transition in the relationship between suspended sediment load (Qs) and Q, which occurs when the flow initiates mobilization of bed materials. The value of Q at the transition corresponds to the Q that initiates bed mobilization, defined as Qc. The suspended sediments in the water column above the sediment bed are separated according to the source of the sediments as "background suspended sediments from the catchment (Cb)" and "fine sediments released from the sediment bed during the rising limb of a flood event", where Cb is site-specific and represented by Equation (6):

$$\mathbf{C}\_{\Phi}(\mathbf{t}) = \mathbf{y} \mathbf{Q}(\mathbf{t}),\tag{6}$$

in which Cb is expressed in (mg/L), Q is expressed in (m3/s), and <sup>γ</sup> is expressed in (mg·s/L/m3).

The observed mass released from the sediment bed into the water column by a flood event (Mf, obs) is calculated from observed data using Equation (7), whereas Mf, model, the modeled mass of fine sediments released from the sediment bed into the water column by a flood event, is estimated by Equation (2):

$$\mathbf{M}\_{\rm fi,obs} = \int\_{t\_{\rm s}}^{t\_{\rm vj}} \mathbf{Q}(\mathbf{t}) [\mathbf{C}(\mathbf{t}) - \mathbf{C}\_{\rm b}(\mathbf{t})] \, \mathrm{d}\mathbf{t}, \tag{7}$$

The lower limit of integration ts,i is the beginning of flood event i, which is either the first occurrence when Q(ts) > Qc or when dQ(t)/dt transitions from negative to positive, while Q > Qc during multiple high-flow events. The upper limit of integration te,i represents the time of the end of flood i, either when Q(te,i) < Qc or when dQ/dt transitions from negative to positive. The maximum value of Mf, obs during the model calibration period is defined as Mmax in the sediment bed, assuming that all fine sediments stored in the sediment bed are re-suspended during the maximum flood event when the highest peak flow rate (Qmax) is observed.
