**2. Materials and Methods**

#### *2.1. Research Sites and Data Sources*

C and Q data from 13 catchments (ranging from 2.2 to 21,000 km2) were used to test the robustness of the model under different environmental characteristics (Figure 1; Table 1; Table S1 in Supplementary Materials). Note that we consider environmental characteristics, as mainly, the catchment area and climatic condition of catchments.

**Figure 1.** Location of the monitoring sites (see Table 1 and Table S1 for details of the catchments).



#### *Water* **2019**, *11*, 878

These catchments are classified into 5 categories according to their climatic characteristics or geographical regions: (i) temperate oceanic, (ii) snowmelt, (iii) Mediterranean mountainous, (iv) Mediterranean, and (v) European-continental (Table 1).

The temperate oceanic climate is represented by two catchments located in Northwestern France, Violettes and Moulinet, where the mean longitudinal channel slope is about 1.8% in both sites [33,41]. The land in this region is used extensively for dairy cattle farming, including pastures. Cattle disturbance has been associated with bank erosion and increased suspended sediment concentration in the stream [42]. Turbidity sensors were installed at the outlet of each catchment and C was estimated from the relationship between turbidity and C [41]. The Q and C parameters were measured every 30 s, and 10-min average values were reported. Similar climatic and land use conditions are found in the Owenabue and Bandon catchments in Southern Ireland, where 90% of the land is used for pasture and tillage [16]. The Q and C values of these two catchments were provided by Ireland's National Office of Public Works for Q and the Cork Institute of Technology for C.

Snowmelt- and glacial melt-dominated streams have periodic flow rates and corresponding fine sediment concentration fluctuations [29]. One of such streams was the Incline Creek in Nevada, which drains into Lake Tahoe, CA, USA. For this stream, Q and C were measured at 15-min intervals between 4 April and 24 May 2000. This snowmelt-dominated catchment provides an extreme test of the model, given that there are only 24 h between flood events. The gauge elevation is 2100 m a.s.l., therefore, winter precipitation falls mainly in the form of snow. The flow regime of the United States Geological Survey (USGS) station (site number 103366993) shows daily cycles reflecting snowmelt conditions in spring. The C was continuously estimated from turbidity measurements, based on the turbidity-C relationship developed by Langlois et al. [34]. The earlier arrival of suspended sediment concentration peaks compared with peak water discharge shows clockwise hysteresis loops for almost all flood events.

The Mediterranean mountainous climate was assessed in four different catchments located in two different regions. Firstly, the two tributaries of the Bléone catchment, the Galabre and Bes rivers, located in the subalpine region of southeastern France, were evaluated. The climate of the catchment is characterized by a pronounced seasonality with the occurrence of frost in winter and high-intensity rainfall in summer. The peak water discharge during the spring season is affected by snowmelt in the Bès catchment where the median grain size of bed surface materials is 70 mm [35,43]. The main types of land cover found are forests, scrubland, sparse vegetation, and grassland. Continuous Q and C values were monitored at two gauging stations located at the outlet. Depending on its magnitude, the Q was regularly gauged with the salt (NaCl) dilution method and a current flow meter. The concentration C was estimated from turbidity data based on the method developed by Navratil et al. [44].

The Ribera Salada stream, located in the southern Pyrenees, and the river Isábena (which presents frequent flooding that causes relatively high sediment transport rates) located in the southern central Pyrenees, are others representative of the Mediterranean mountainous climate. Mean annual precipitation at both catchments is around 800 mm, being the monitoring period selected for the present study representative of the long-term hydrological regime. Predominant land uses are forest in headwaters and forest mixed with agriculture at the lowlands. Q and C (estimated from turbidity sensors by establishing rating curves between turbidity and C) were continuously measured at 15-min intervals, at the Inglabaga monitoring station (channel slope at around 1%) in the case of the former [36,45], and at the Capella gauging station (channel slope at around 0.4%) for the latter, and the median grain sizes of bed surface materials are 49.0 mm and 69.5 mm in the Ribera Salada and Isábena catchments, respectively [37,46].

Another different Mediterranean environment is that of the Carapelle catchment, located in the Puglia region of Southern Italy. It presents yearly precipitation that ranges from 450 to 800 mm, and land use is mostly agricultural where the mean slope of main channel is 1.8% [38,47]. Continuous Q and C values have been measured from 1 January 2007 to 31 December 2011 in this catchment, which is characterized by long periods with low flows and a prevalence of counter-clockwise hysteresis [48]. For this reason, a shorter period, between 3 March and 31 April 2009, was utilized for calibration in this study where 2009 was quite humid year (annual rainfall 786 mm) [38,47].

The Hopland and Guerneville catchments, both located in the Russian River, California, CA, USA, analyzed in a previous study [32] were included in the catchment list; the climate in this region is also Mediterranean-type, with warmer, drier summers and cooler, wetter winters, where the median grain sizes of bed surface materials are 7.9 mm and 7.1 mm in the Hopland and Guerneville catchments, respectively.

Finally, the wet European-continental climate was considered with the inclusion of the Meuse River catchment, located at the Belgian–Dutch border, with a length of 935 km and a catchment area of 36,000 km2, where wet season is between October and April, and dry season is between May and September [40]. The main channel has steep slopes and land is dominantly used for both agriculture and forest [40]. Daily Q and C data reported over 15 years by the Dutch Institute for Inland Water Management and Waste Water Treatment (RIZA) are available from the upstream of the Eijsden gauging station, where the river length is approximately 700 km and the catchment area is ~21,000 km2. Because the identification of model parameters requires sufficient resolution to discern the hysteresis of C to Q in rising and falling limbs during a flood event, daily data were applicable for model simulation in this relatively large catchment.
