# The Impact of Spatial Resolutions on Nature-Based Solution Suitability Mapping for Europe
## 2 Study Area and NBS  
The NBS Toolkit aims to recommend Nature-Based Solutions at the local scale across Europe and, therefore, is based on open-source input layers at the European scale. Yet, it is aimed to derive local recommendations, which can be of different sizes. In order to gain a better understanding on the appropriateness of three different LULC datasets for this purpose, the testing is performed in the following selected test area: the federal state Rhineland Palatinate in Germany (Figure 1). This area was selected due to its rural and urban land uses, as well as the recent flooding event in July 2021. The results of this test site are then used to compute the suitability maps at the European scale. The test study area (Rhineland Palatinate) encompasses 19,846 km\({}^{2}\) and is located in the west of Germany, where the riverbanks of the rivers Rhine, Moselle, and Ahr are primarily covered by wine yards, roads, and small urban areas; thus, riverbanks are largely occupied by agricultural and built infrastructure. The area was severely affected by flooding in 2021. The event highlighted the challenges of the densely built river sides, which were largely flooded; hence, recovery actions have also been striving to enhance the water retention and storage capacities [22; 23; 24]. With the focus of the test study area on flooding, one solution for future flood reduction is to make room for the river in the upstream of built-up areas. For the test study, riparian forest buffers were selected due to their ability to slow water runoff with their rooting system, filter sediments through greater friction, and increase infiltration and storage of water by enhancing soil conditions [25; 26; 27]. Hence, slowing runoff and enlarging water storage areas along German streams and rivers can help to compensate densely built-up riverbanks. Yet, the effectiveness of riparian buffers for flood reduction is dependent on the width and length of them; thus, their planting should be targeted to extend existing buffer habitats to counteract habitat fragmentation [28; 29]. Riparian Forest Buffer Suitability CriteriaThe following suitability criteria for riparian forest buffer zones were identified from the literature:  
* _Land use and land cover._ Riparian forest buffers are commonly implemented on agricultural and pastural land, but also on unvegetated areas, which are often associated with low infiltration rates and high runoff potential compared to forest areas [32; 33]. In accordance, the following land use and land covers can be defined as suitable: agricultural areas, pastures, natural grassland, sparsely vegetated areas, and burnt  
Figure 1: The test study area, Rhineland Palatinate in Germany, depicted with the water bodies [30] and flood hazard areas [31]. areas. Furthermore, forests with low cover density can be suitable for riparian forest buffer restoration (discussed in the next sub-section: forest density). Land cover types such as dense urban areas and water bodies were deemed not suitable. * _Forest density._ Forests can intercept and store water in a manner comparable to sponges, and on average, forest covers with densities of 30% or more are found to have higher water retention abilities. Yet, this retention potential threshold is varying between forest types (e.g., coniferous forests have higher runoff retention potentials), biogeographical zones (e.g., alpine, boreal, continental), and seasons [34]. Rhineland Palatinate is located in the continental region; thus, according to the EEA [34], the threshold for a medium retention potential of broad-leaved tree cover density threshold is 20%, while for coniferous and mixed forests it is even lower. High retention potential thresholds are above 65% for coniferous forests and 80% or more for broad-leaved forests. * _Soil._ Different aspects of soil can be favourable or limiting for planting trees.