A socioecological approach using watershed health and social vulnerability in a headwater sub-basin of the Río Grande de Arecibo

Abstract

The Río Grande de Arecibo (RGA) Watershed is a crucial source of water through the North Coast Super Aqueduct for the residents of the Island of Puerto Rico, including those living in the San Juan metropolitan area. It is also significant for forest conservation, with its five state forests providing around 10% of the watershed's protected forest area. However, land cover changes in the region are putting the watershed's sustainability at risk, as is the case in many places worldwide.<br /> <br /> This study takes an integrated socioecological approach to examine changes in the watershed health as well as the social vulnerability in a Headwater Sub-basin of the RGA Watershed over a 20-year period (2001-2021). The socioecological approach was based on the use of the EPA Watershed Health Index and the CDC Social Vulnerability Index. For the construction of the EPA Watershed Health Index, geospatial analysis (land cover changes analysis), and modelling methods (Soil and Water Assessment Tool (SWAT) were combined, to study changes in the quantity and quality of the surface water generated in the watershed as a function of land cover and climate parameters. Landsat 8 Operational Land Imager (OLI) was used to assess land cover and land use changes (LCLUC) along the sub basin. SWAT model provided the metrics to conduct the hydrological assessment based on the Hydrological Component of the EPA Watershed Health Index, which are namely: Water Yield (WYLD) for the General Metric, Surface Runoff (SURQ) and Lateral Flow (LATQ) for Surface Process Metric, Soil Water Content (SW) for Soil Water Dynamics; and Percolation (PERCOL) for Groundwater Dynamics.<br /> <br /> Our findings indicated that based on the Watershed Health Index (WHI), the Headwater Subbasin of the RGA showed slightly less healthier conditions (2021) than 20 years ago. We linked this slight deterioration to a less healthy condition on Landscape Condition, Habitat Condition, Geomorphological Condition, and Hydrological Condition. The Hydrological Condition was less "healthy" than 20 years later, due to the increase on the surface runoff metric and decrease in lateral flow metric in 2021.<br /> <br /> In terms of land cover changes, the Headwater Sub-basin showed a decrease in both Forest Land and Range Land and an increase on Urban Built-up Land cover 20 years later. In addition, several socially vulnerable populations were identified along the Headwater Subbasin that varies spatially, in the 20-year time frame. The identification of those socially vulnerable populations brings information about the social disparities occurring on the sub-basin that represents the whole watershed. The CDC Social Vulnerability Index presented limitations because some variables such as Minority Status and Housing Types and Transportation do not represent the realities of Puerto Rico.<br /> <br /> The study provides a baseline to conduct future socioecological watershed studies and sustainable management actions. Also, this research is a novel approach, which combines geospatial analysis with hydrological modeling, that could be applied to other watersheds, particularly in the tropics, where few studies were done.