Integrated development of Nature-Based Solutions (NBS) for tropical urban watershed restoration

Unplanned urban growth has been a recurrent problem affecting the hydrological watershed response, especially in tropical countries with short rainfall durations and high rainfall intensities. Uncontrolled urban growth complemented with extreme rainfall conditions causes high runoff volumes that require large-scale engineering interventions for proper management. This study aims to develop integral-multifunctional naturebased solution (NBS) scenarios to aid the hydrological, ecological and social recovery of urban watersheds in tropical areas and promote the fulfillment of the 2030 milestones of the United Nations Sustainable Development Goals (SDGs). A tropical watershed (23 km²) located in the Greater Metropolitan Area of Costa Rica was investigated regarding the spatio-temporal urbanization process between 1945 and 2019, based on remote sensing data and its hydrological response. The analysis of the urbanization process of the watershed (+64% urban) allowed identifying severe alterations of the hydrological cycle resulting in runoff volume increases (80%), peak flow and peak specific discharge increase (250%), and a time to peak reduction (25 minutes), triggering flooding problems. Once the problem was identified, integral NBS implementation scenarios were developed, based on their performance, space availability, and strategic location within the watershed to provide multiple functions, evaluating the level of recovery that could be achieved. For the development of the scenarios, some of the hydrological, ecological, social, economic and political benefits provided by NBS were analyzed. The available space was evaluated, based on the current land cover classification and its urban configuration, defining the potential for NBS implementation, both in residential and industrial areas. Specific opportunities and needs for increasing social and ecological connectivity through NBS were considered. This scenario building tool aims to develop a comprehensive replicable methodology for the recovery of urban tropical watersheds and promote better progress in the pursuit of meeting the 2030 milestones of the United Nations SDGs.