*3.2. Sediment Transport in a Limnologically Rich Setting*

Sediment transport in the Claise was simulated using the SWAT model. The model was calibrated using the SUFI-2 algorithm of SWATCUP following Jalowska and Yuan (2019) [38] proposed sequential calibration for settings characterized by the presence of water impoundments. As mentioned previously, due to the unavailability of sediment measurements for this study, only a hydrologic calibration was performed with the most related sensitive parameters (Table 8). Results of the hydrologic calibration yielded an R<sup>2</sup> of 0.7 during calibration and 0.67 during validation.


**Table 8.** Sensitivity analysis for calibration of the SWAT model.

As a result of an exhaustive hydrologic calibration, the average sediment yield of the Claise basin for the studied period under current and alternative conditions is presented in Figure 9.

**Figure 9.** Sediment yields of the Claise basin under (**a**) current conditions and (**b**) the pondless scenario.

As can be seen from Figure 9, low sediment yields correspond to ponded sub–basins in contrast to pondless sub-basins which present lower soil loss rates. However, this gradient varies according to the setting of the ponds in these sub–basins. Despite that sub-basins 1 and 9 contain a small number of ponds, these correspond to areas of high sediment yield as a result of their dominant agricultural cover and their location in the basin's steepest areas. This shows that the presence of ponds alone is not sufficient to alter the sediment yields. In fact, the setting of ponds under the form of a collective dense network is the main modifier of sediment transport patterns. In order to further highlight the impact of ponds, alternative scenario testing was performed where a pondless land occupation setting was re-inputted to the calibrated SWAT model in order to determine subsequent sediment transport changes. Results of sediment yields, highlighting the impact of ponds, are presented in Table 9.


**Table 9.** Shifts of sediment yields as a result of pond presence or absence.

In the low sediment yield sub-basins, where the vast majority of ponds lies, occupying a large portion of the sub-basins they are located in, the land cover change from pond to grasslands is far too great and the effect of ponds presence/absence is quite obvious. Accordingly, the role of ponds as integral features of the landscape processes is solidified. Having determined their effect, an increased exposure for their integration into management plans is recommended, while an understanding of basin scale sediment dynamics offers insights regarding the role of ponds in water resources. Such implications could matter not only for the Claise and similar basins, but also for expectations regarding the landscape role of sediment retention basins, and for the general understanding of sediment transport in rain-dominated contexts. This, in turn, could be considered as a contribution to hydrologic modelling efforts, where small waterbodies, especially artificial ones, are often neglected, or even smoothed out, from digital elevation models.
