Where to locate large-scale nature-based solutions? Finding suitable locations for rainwater harvesting

Nature-Based Solutions (NBS) are suitable responses for hydrometeorological hazard reduction, integrating the hydrology, geomorphology, hydraulic and ecological dynamics of a catchment. Large-scale NBS are implemented on a catchment scale and may provide more co-benefits than small-scale NBS. Literature shows a methodological gap in finding suitable locations to implement large-scale NBS. Developing spatial analysis tools in a GIS environment is essential to generate information for decision support. In this research, we developed a method for finding suitable rainwater harvesting locations (RWH) and applied it to the Municipality of Santiago de Machaca, part of the La Paz Department in Bolivia. Large-scale RWH is the collection of rainwater from ground surfaces and streams and its storage in depressions made for that purpose. RWH is implemented to store and provide water supply in stressed regions and mitigate the impacts of floods by diverting and storing runoff. The raster datasets required to map suitable locations for RWH implementation are annual precipitation depth, coefficient of variation of the monthly precipitation, runoff coefficient, aridity index, population density and slope. The datasets are normalized to generate a standard scale between 0 and 100, and the analytic hierarchy process (AHP) was used to weigh, prioritize and rank alternatives. Built-up areas and buffered roads and rivers should be removed from the raster file generated after the AHP analysis. For the Santiago de Machaca case study, we categorized the final raster cells as no, low, medium and high suitability for implementing RWH. The areas highly suitable for RWH are located in the valleys, but some are small patches fragmented by the road network. It should be noted that the final result is dependent on the matrix built to obtain the raster weights in the AHP analysis and the final suitability ranking categories. However, the developed method is a generic one that can be applied in any site and is a step forward in general for planning the implementation of large-scale RWH as an NBS.