## 2 Materials and Methods  
### Study Site  
The selected study area is located in the southwest of Europe within the upper Guadalquivir Basin (Spain). The basin has a small size of \(A_{\text{drain}}=6.9\) km\({}^{2}\), a mean slope of 19% and a maximum (minimum) height greater (lower) than 750 m MSL (360); see Figure 1. The catchment mimics an ideal laboratory with a homogeneous surface but at a much larger, real scale. Olive groves dominate the soil use, occupying 93.4% of the drainage area. There are approximately 46,500 olive trees (green dots in Figure 1a), with a density of 70 trees-hectare\({}^{-1}\). The combination of steep slopes (Figure 1b) and land use of olive groves under conventional tillage prevent infiltration [18]. Additionally, the lack of natural cover exposes sediment to erosion and causes a high development of laminar and gully processes. Figure 1: General characteristics of the Manillas basin in Arquillos (Jaén, Southeast Spain). (**a**) Catchment overview (perimeter highlighted with the black solid line) with three regions of interest: the urban areas (black), the man-made channel that should protect the town from inundation (thick line in brown), and olive trees (green dots). The drainage network is composed of rills, gullies, and streams. (**b**) Hyposometric map and drainage network. For completeness, main traffic roads are coloured in orange. Maps created from scratch with Matlab and GlobalMapper based on a Light Detection and Ranging (LiDAR) dataset acquired on May 2021 with 1.5 points per square meter ([[https://www.ign.es/](https://www.ign.es/)]([https://www.ign.es/](https://www.ign.es/)), accessed on 1 July 2022). Rills, gullies, and streams were obtained from the distributed hydrological simulations presented in this paper. We identified the following sediment sources according to the classification by [PERSON] [50], see Figure 1: rill (light blue), gully (red), and stream (dark blue and greenish blue). Rills are small micro-channels where overland flow exerts a shear stress on the soil close to the threshold value for erosion with flow depths of up to 30 mm. Gullies are relatively permanent ephemeral channels in semi-arid environments without vegetation cover, eroded during highly seasonal precipitation. The gully channel is narrow, with a steep bed slope and a characteristic flow depth larger than 0.5 m. Smaller features, intermediate in size between gullies and rills, are referred to as ephemeral gullies in Soil Science Society of America [51]. Stream channels collect and transfer the sediment budget produced in rills and gullies [52]. In the Manillas basin, streams develop in regions with much smaller bottom slopes than for ephemeral gullies. All these elements of the drainage network are connected to the primary river system, referred to as the Manillas stream. In general, the channel form of the streams can adapt to altered environmental conditions [53]. The absence of civil infrastructure in most of the watershed ensures that runoff responds naturally to torrential rainfall. Hence, overland flows erodes many gullies and shallow streams that drain transverse to the Manillas stream. The Manillas stream catches overland flows from the gullies and shallow streams along 4.6 km with a mean slope of 4.2% and two nick points. The stepper reach is located in the headwater (i.e., first 500 m with a 6.7% slope). Later, the bed slope decreases to 3.3% for 1000 m. Further downstream, it reaches a constant value of 1.8%, which remains for 3 km. During its fluvial course, the main channel exhibits non-uniform bankfull dimensions. On occasion, the water has sculpted a much wider and deeper channel than other river stretches. Therefore, the channel-reach morphology may correspond to a single confined flow or a floodplain channel, depending on the river stretch [52]. The wash load is the dominant transport process due to the small sediment size and straight planform [54].