## 1 Introduction  
The frequency and intensity of sub-daily floods have increased in the Upper Guadalquivir Basin (southern Spain) during the last twenty years due to the actual scenario of climate change [1]. This kind of extreme hydrological event has become more and more relevant in many Mediterranean areas of Europe [2], corroborating the predictions of the Intergovernmental Panel on Climate Change (IPCC) [3] and the European Environmental Agency (EEA) [4]. High precipitation rates combined with anthropogenic effects of soil use and management have provoked unsustainable soil-loss rates and large-scale gully erosion in some European agricultural catchments [5]. Simultaneously, floods have developed at the catchment scale because of deep runoff [6]. Both problems are linked because they depend, in leading order, on the dynamics of overland flows [7]. The Mediterranean Experts on Climate and Environmental Change (MedECC) [8] recommend the implementation of NbS and adaptive practices of sustainable land management because climate change is expected to increase soil erosion in a complex, non-linear way. Mitigating flood risk and preventing soil erosion at large scales cannot be achieved with isolated civil engineering actions; the economic costs and environmental impacts would be prohibitive. Alternatively,we adopt Nature-Based Solutions (NbSs) [9; 10; 11] that seek to delay the flooding, reduce the peak water discharge and erosion capacity of overland flows, and promote infiltration at the basin scale. We considered a pilot study site of the Upper Guadalquivir Basin where the current land management practice and the increase in extreme rainfall rates (up to approximately 30 mm\(\cdot\)h\({}^{-1}\)) have caused soil loss rates of 20-50 t ha\({}^{-1}\) yr\({}^{-1}\) (see Figure 2 in [12]). Such values are nearly a maximum in Europe, so the Guadalquivir watershed has attracted the scientific community's interest in gully erosion processes [13; 14]. More than 80% of the catchment is dedicated to traditional olive groves (i.e., the vegetation surrounding the crops was removed), a practice extended across the entire Mediterranean basin [15]. In such environments, we have shown the potential of the two-dimensional Saint-Venant Equations (2D-SVEs) to map the extent of the inundations and describe fluvial geomorphological processes at the river-stretch scale [16; 17]. High-spatial resolution numerical simulations using 2D-SVE-distributed hydrological models yield further details of the rainfall-runoff-inundation processes at the watershed [6]. The output products for a given rainfall event are the flow depth and velocity not only in the main river but also in smaller spatial-scale elements of the whole drainage network. If the intensity of the precipitation is high enough to provoke water discharges above the drainage capacity of the streams and rivers, the 2D-SVE-distributed numerical simulation gives all the flooded areas across the whole watershed. Furthermore, the interpretation of the flow-velocity map serves to predict the locations where fine sediments transported as wash load settle and provoke slackwater deposits [1]. The main objective of this article is to find the optimal NbS (including its location) which provides the most benefits in terms of hydraulic effectiveness to prevent flooding and soil erosion in steep-slope olive groves. We address both problems at the same time instead of independently. Subsidiary--but no less important--objectives are the increase in the retention capacity of overland flows and the soil infiltration capacity to avoid the undesirable phenomenon of soil-water repellence and higher surface runoff volumes in olive groves [18]. All of them contribute to mitigating street-scale urban floods in the site study. So, this work extends the capabilities and applicability of our former model, initially calibrated and planned only for flood-risk mapping in ungauged olive-orchard basins [1; 6].