United States Department of Agriculture Quantifying the Role of Forested Lands in Providing Surface Drinking Water Supply for Puerto Rico Erika Cohen, Ge Sun, Liangxia Zhang, Peter Caldwell, and Suzanne Krieger Forest Service Southern Research Station General Technical Report SRS-197 Addendum September 2017 The Authors: Erika Cohen, Resource Information Specialist, Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, Raleigh, NC 27606; Ge Sun, Research Hydrologist, Eastern Forest Environmental Threat Assessment Center, Southern Research Station, U.S. Department of Agriculture Forest Service, Raleigh, NC 27606; Liangxia Zhang, Lecturer, Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD) / Jiangsu Key Laboratory of Agricultural Meteorology, Nanjing University of Information Science & Technology, Nanjing 210044, China; Peter Caldwell, Research Hydrologist, Coweeta Hydrologic Laboratory, Southern Research Station, U.S. Department of Agriculture Forest Service, Otto, NC 28763; and Suzanne Krieger, Regional Hydrology Program Manager, Region 8, U.S. Department of Agriculture Forest Service, Atlanta, GA 30309. Cover photo Unique, low-stature elfin forests are found on the tops of Puerto Rico’s highest peaks. (Photo by Thomas J. Brandeis, Southern Research Station) Any opinions, findings, conclusions, or recommendations expressed in the material are those of the authors and do not necessarily reflect the views of the USDA Forest Service. Forest Service Southern Research Station GTR-SRS-197 Addendum September 2017 Southern Research Station 200 W.T. Weaver Blvd. Asheville, NC 28804 www.srs.fs.usda.gov Quantifying the Role of Forested Lands in Providing Surface Drinking Water Supply for Puerto Rico Erika Cohen, Ge Sun, Liangxia Zhang, Peter Caldwell, and Suzanne Krieger The delivery of fresh water, held here by the Rio de la Plata dam, is one of the vital ecosystem services provided by Puerto Rico’s forests. (Photo by Dr. Humfredo Marcano, Southern Research Station) Where not disturbed by human activity, dense, highly diverse forests cover the slopes of Puerto Rico’s mountains. (Photo by Thomas J. Brandeis, Southern Research Station) Contents Abstract . .vi Introduction . 1 Study Area . 1 Climate . 2 Forest Land Cover . 4 Methods. 5 Extent and Scale of Analysis . 5 Quantifying Water Supply from Forested Lands . 5 Drinking Water Intakes and Population Served . 9 Results. 11 Water Supply from Forested Lands . 11 Population and Communities Served by Water from Forested Lands . 13 Discussion and Summary. 19 References . 20 Quantifying the Role of Forested Lands in Providing Surface Drinking Water Supply for Puerto Rico Abstract The Forest Service, U.S. Department of Agriculture published a General Technical Report (GTR-SRS-197) in 2014 that quantified the role that water originating on National Forest System lands contributed to the drinking water supply and determined what population and communities were being served in the 13 Southern States of Region 8 of the Forest Service. The Commonwealth of Puerto Rico is included in Region 8, but it was not included in the results of GTR-SRS-197 because at the time of that report the Water Supply Stress Index (WaSSI) model used for the assessment was not parameterized for Puerto Rico. The goal of this project was to implement the same methodology used in GTR-SRS-197 for the mainland of Puerto Rico. WaSSI is a monthly water balance and flow routing model that can quantify the amount of water at a watershed outlet. Water intake location data from the U.S. Geological Survey (USGS) and population served data from the Puerto Rico Aqueduct and Sewer Authority were used to quantify the amount of population being served by water originating on forested land. Puerto Rico has a total land area of 8,900 km2, and 38 percent of that land area is forested with 1.2 percent in National Forest System land and 37.1 percent in commonwealth and private forest land. We estimated that all lands in mainland Puerto Rico produced 6.5 billion m3/year of water. Of that total, National Forest System lands provided 219 million m3/year of water (3.3 percent of total land), and commonwealth and private forest lands provided 2.6 billion m3/year of water (40 percent of total land). A population of 3.6 million people was served by 113 intakes located downstream of commonwealth and private forest land, and 13 of those intakes were also located downstream of El Yunque National Forest and served a population of 780,000. National Forest System land and commonwealth and private forest land play an unequal role in providing water for the people of Puerto Rico. More than 2.1 million people receive at least 30 percent of their water from commonwealth and private forest land. In comparison, more than 139,000 people receive at least 30 percent of their water from National Forest System land. Our analysis provides quantitative information on water yield from forest lands and sources of water supply in Puerto Rico. This study is an addendum to GTR-SRS-197. Keywords: commonwealth and private forest lands, drinking water, hydrologic modeling, National Forest System, Puerto Rico, State and private forest lands, WaSSI, water supply. vi GTR-SRS-197 ADDENDUM Introduction The Southern Region (Region 8) of the Forest System Land. At the time of the project, the Service, U.S. Department of Agriculture manages WaSSI model was not parameterized for Puerto National Forest System (NFS) lands in 13 Southern Rico, and Puerto Rico was not included in the States and in the Commonwealth of Puerto Rico. results of the General Technical Report-SRS-197, The 13 States include Texas, Oklahoma, Arkansas, “Quantifying the Role of National Forest System Louisiana, Mississippi, Tennessee, Kentucky, Lands in Providing Surface Drinking Water Alabama, Georgia, Florida, South Carolina, North Supply for the Southern United States” (Caldwell Carolina, and Virginia. Region 8 conducted a project and others 2014). In 2015, the WaSSI model was from 2012 to 2014 to determine what quantity of refined and expanded to study Puerto Rico at water originating on NFS lands contributed to the sixth-level Hydrologic Unit Code (HUC12) the drinking water supply, and to determine the scale using improved hydrological algorithms total population and communities being served by unique to the forest ecosystems of the tropical this water in the 13 Southern States of Region 8 rainforest. With this expansion, the capability (Caldwell and others 2014). exists to quantify the amount of water from various land cover types with a focus on forested The project used the Water Supply Stress Index lands. The overall goal of this project was to (WaSSI) model (Caldwell and others 2012; Sun implement the same methodology described in and others 2008, 2011a, 2011b) developed by the GTR-SRS-197 to determine what role forests play Forest Service to quantify the amount of water in providing drinking water to the populations and originating from various land cover types in the communities within Puerto Rico. This report is an South with particular focus on National Forest addendum to GTR-SRS-197. The overall goal of this project was to implement the same methodology described in GTR-SRS-197 to determine what role forests play in providing drinking water to the populations and communities within Puerto Rico. Study Area The Commonwealth of Puerto Rico is an archipelago located in the northeastern Caribbean Sea. Mainland Puerto Rico is the smallest of the Greater Antilles, and it is composed of multiple islands, with the largest having a total land area of 8,900 km2 (fig. 1). Puerto Rico is densely populated with 3.7 million people living primarily on the mainland island (U.S. Census Bureau 2012). The study area for this project was mainland Puerto Rico, a very mountainous region with four mountain ranges dominating the landscape. The two most notable of these mountain ranges are the Cordillera Central, which runs the full, east-west length of the island and contains the highest elevation on the island of 1330 m, and the Sierra de Luquillo, which is located in the Figure 1—The Commonwealth of Puerto Rico is located northeastern portion of the island and contains the in the northeastern Caribbean Sea (top); it comprises multiple islands, the largest of which has a total land area largest protected area on the island, El Yunque of 8,900 km2 (bottom). National Forest (NF) (fig. 2). 1 Quantifying the Role of Forested Lands in Providing Surface Drinking Water Supply for Puerto Rico Cordillera Central Sierra de Cayey Sierra de Luquillo 0 20 40 80 km Elevation (m) < 0 200 – 400 600 – 800 > 1,000 0 – 200 400 – 600 800 – 1,000 Figure 2—The topography of mainland Puerto Rico is very mountainous, including three notable ranges: the Cordillera Central, Sierra de Luquillo, and Sierra de Cayey. As of December 2016, the Commonwealth of forest types including tabonuco (Dacryodes excelsa), Puerto Rico has 164 protected areas, and 137 of palo Colorado (Cyrilla racemiflora), sierra palm those protected areas are terrestrial protected areas, (Prestoea montana), and dwarf forest. The tabonuco which equates to 16 percent (143 590 ha) of the forests are found on foothills and slopes below 600 total land area (Caribbean Landscape Conservation m. The palo Colorado forests are found on saturated Cooperative 2016). These protected areas are soils, gentle slopes, and valleys between 600 m and managed by Federal and Commonwealth agencies, 900 m. The sierra palm forests are found on steep as well as nonprofit and private organizations (Gould slopes in the same elevation range as tabonuco and and others 2011, Quinones and others 2013). El palo Colorado forests. The dwarf forests, also known Yunque National Forest spans nearly 11 310 ha and as elkin forests, are found on the peaks above 900 is the only tropical rainforest in the U.S.