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Professional Paper 1789

Water Quality and Landscape Processes of Four Watersheds in Eastern Puerto Rico

Edited by Sheila F. Murphy and Robert F. Stallard

Contributions by Heather L. Buss, William A. Gould, Matthew C. Larsen, Zhigang Liu, Sebastián Martinuzzi, Sheila F. Murphy, Robert F. Stallard,
Isabel K. Parés-Ramos, Arthur F. White, and Xiaoming Zou

Thumbnail of and link to report PDF (41.9 MB)Abstract

Humid tropical regions occupy about a quarter of Earth’s land surface, yet they contribute a substantially higher fraction of the water, solutes, and sediment discharged to the world’s oceans. Nearly half of Earth’s population lives in the tropics, and development stresses can potentially harm soil resources, water quality, and water supply and in addition increase landslide and flood hazards. Owing to Puerto Rico’s steep topography, low water storage capacity, and dependence on trade-wind precipitation, the island’s people, ecosystems, and water supply are vulnerable to extreme weather such as hurricanes, floods, and droughts. Eastern Puerto Rico offers a natural laboratory for separating geologic and land-cover influences from regional- and global-scale influences because of its various bedrock types and the changing land cover surrounding intact, mature forest of the Luquillo Experimental Forest. Accordingly, a multiyear assessment of hydrological and biogeochemical processes was designed to develop an understanding of the effects of these differences on local climate, streamflow, water quality, and ecosystems, and to form the basis for a long-term and event-based program of climate and hydrologic monitoring.

Because infrequent, large storms play a major role in this landscape, we focused on high-runoff events, sampling 263 storms, including all major hurricanes from 1991 through 2005. The largest storms have profound geomorphic consequences, such as landslides, debris flows, deep gullying on deforested lands, excavation and suspension of sediment in stream channels, and delivery of a substantial fraction of annual stream sediment load. Large storms sometimes entrain ocean foam and spray causing high concentrations of seasalt-derived constituents in stream waters during the storm. Past deforestation and agricultural activities in the Cayaguás and Canóvanas watersheds accelerated erosion and soil loss, and this material continues to be remobilized during large storms.

Nearly 5,000 routine and event samples were analyzed for parameters that allow determination of denudation rates based on suspended and dissolved loads; 860 of these samples were analyzed for a comprehensive suite of chemical constituents. The rivers studied are generally similar in water-quality characteristics, and windward or leeward aspect appears to exert a stronger influence on water quality than geology or land cover. Of samples analyzed for comprehensive chemistry and for sediment, 543 were collected at runoff rates greater than 1 millimeter per hour, 256 at rates exceeding 10 millimeters per hour, and 3 at rates exceeding 90 millimeters per hour. Streams have rarely been sampled during events with such high runoff rates.

Rates of physical and chemical weathering are especially high, and physical denudation rates, forested watersheds included, are considerably greater than is expected for a steady-state system. The elevated physical erosion drives an increased particulate organic carbon flux, one that is large, important to the carbon cycle, and sustainable, because soil-carbon regeneration is rapid.

The 15-year Water, Energy, and Biogeochemical Budget dataset, which includes discharge, field parameters, suspended sediment, major cations and anions, and nutrients, is available from the U.S. Geological Survey’s National Water Information System (http://waterdata.usgs.gov/nwis). The dataset provides a baseline for characterizing future environmental change and will improve our understanding of the interdependencies of land, water, and biological resources and their responses to changes in climate and land use. Because eastern Puerto Rico resembles many tropical regions in terms of geology and patterns of development, implications from this study are transferable to other tropical regions facing deforestation, rapid land-use change, and climate change.

First posted July 30, 2012

The larger PDF for Professional Paper 1789 combines all chapters.

To immediately access smaller PDFs of the individual chapters, use the links listed at the bottom of this webpage.

For additional information contact:
Chief Scientist for Hydrology
U.S. Geological Survey
436 National Center
Reston, Virginia 20192
http://water.usgs.gov/nrp/

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Suggested citation:

Murphy, S.F., and Stallard, R.F., eds., 2012, Water quality and landscape processes of four watersheds in eastern Puerto Rico: U.S. Geological Survey Professional Paper 1789, 292 p.



Contents

Front Matter

Foreword By Matthew C. Larsen

Executive Summary By Sheila F. Murphy and Robert F. Stallard

A. Physiography, Geology, and Land Cover of Four Watersheds in Eastern
Puerto Rico
By Sheila F. Murphy, Robert F. Stallard, Matthew C. Larsen, and William A. Gould

B. Land Use, Population Dynamics, and Land-Cover Change in Eastern
Puerto Rico
By William A. Gould, Sebastián Martinuzzi, and Isabel K. Parés-Ramos

C. Hydrology and Climate of Four Watersheds in Eastern Puerto Rico By Sheila F. Murphy and Robert F. Stallard

D. Atmospheric Inputs to Watersheds of the Luquillo Mountains in Eastern Puerto Rico By Robert F. Stallard

E. Water Quality and Mass Transport in Four Watersheds in Eastern Puerto Rico By Robert F. Stallard and Sheila F. Murphy

F. Landslides and Sediment Budgets in Four Watersheds in Eastern Puerto Rico By Matthew C. Larsen

G. Effects of Earthworms on Slopewash, Surface Runoff, and Fine-Litter Transport on a Humid-Tropical Forested Hillslope in Eastern Puerto Rico By Matthew C. Larsen, Zhigang Liu, and Xiaoming Zou

H. Weathering, Landscape Equilibrium, and Carbon in Four Watersheds in Eastern Puerto Rico By Robert F. Stallard

I. Weathering Processes in the Icacos and Mameyes Watersheds in Eastern Puerto Rico By Heather L. Buss and Arthur F. White

Appendix 1. Data Processing and Computation to Characterize Hydrology and Compare Water Quality of Four Watersheds in Eastern Puerto Rico

Appendix 2. Methods Used to Analyze Water Quality of Four Watersheds in Eastern Puerto Rico


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