Puerto Rico was formed by volcanism and sedimentation characteristic of tectonic activity at a 

plate boundary, and the study watersheds were chosen to overlie volcanic 

rocks typical of such regions. Similar 

areas in the humid tropics include many 

parts of the Greater Antilles, Central 

America, Southeast Asia, and islands in 

Luquillo Mountains, Puerto Rico

U.S. Department of the Interior 

U.S. Geological Survey

Fact Sheet-163-99

June 2000

A Water, Energy, and Biogeochemical Budgets Program Site

The Luquillo study area is located in the 

eastern mountains of Puerto Rico.

the western Pacific to the Indian Ocean. 

Puerto Rico provides a glimpse into 

the future of tropical environments that 

are now undergoing deforestation. The 

island reached a maximum of deforestation during the 1940s and has been gradually reforested since then. Puerto Rico 

is a natural laboratory for analysis of 

how landscapes respond to and recover 

from massive land-cover change.

Water Budgets 

Water used for public consumption 

may be the most important product of 

the Luquillo Mountains. Estimated as 

the cost paid by the consumer, water 

extracted from the streams that drain the 

Luquillo Mountains is worth about $25 

million per year. Because of the importance of understanding where the water 

is most available and how the quantity 

changes with individual storms, season, 

and on annual to decadal time scales, 

water budgets have been developed. 

The budgets are used to show how 

much precipitation, stream runoff, and 

evapotranspiration have occurred in four 

watersheds in the study area during the

1990s. The water budgets are strongly 

controlled by elevation and orientation 

with respect to prevailing (trade) winds. 

Because of the steep gradients in these 

mountain watersheds, runoff is rapid and 

streamflow peaks are very brief.

Chemical and Physical Erosion

There are two types of erosion: chemical 

and physical. In chemical erosion, water 

dissolves and carries off elements from 

the bedrock, whereas in physical erosion, solid particles are removed by 

water, wind, and mass movement, such 

as landslides. When rocks are exposed 

to water, they decompose by reacting 

with the water and dissolved carbon 

dioxide, oxygen, nitric acid, sulfuric 

acid, and organic compounds. This is 

called “chemical weathering.” As the 

rocks weather, some of the elements 

in minerals, such as feldspars, react 

and release dissolved sodium, potassium, magnesium, calcium, and some 

silicon. The volcanic rocks of eastern 

Approximate water budgets for WEBB 

study watersheds. Inputs include rainfall, 

cloud drip, and septic tank infiltration. Outputs include stream discharge, groundwater flow, and public-supply withdrawals.

The Water, Energy, and Biogeochemical Budgets (WEBB) Program was started 

in 1991 at five small watersheds in the United States to examine water, energy, 

and biogeochemical fluxes and to determine the effects of atmospheric 

deposition, climatic variables, and human influences on watershed processes. 

The five sites are at Loch Vale, Colorado; Luquillo Experimental Forest, Puerto 

Rico; Panola Mountain, Georgia; Sleepers River, Vermont; and Trout Lake, 

Wisconsin. These sites are supported, in part, by other programs in the 

USGS, other Federal and State Agencies, and Universities.

THE WEBB PROGRAM

Luquillo Experimental Forest

Weather station

Streamflow-gaging station

Rainfall station

Canóvanas

Cayaguás

Loíza Res.

Icacos

Atlantic Ocean

Caribbean Sea 

PUERTO

RICO 0 10 km

0 10 mi

San Juan Grande de Loíza N

Mameyes

The Puerto Rico research site consists of the 113 square-kilometer Luquillo 

Experimental Forest (LEF), administered by the U.S. Forest Service, and 

the nearby Río Grande de Loíza drainage basin, an urbanized and agriculturally-developed watershed. This combined region serves as a terrestrial 

laboratory for the study of issues related to the global loss of tropical 

forest, and the associated changes in land-use practices. Findings from 

the WEBB research help scientists understand how vegetation, landscape, 

and people interact to affect the quantity and quality of water and the 

erosion of the landscape. The results of this work can be applied not only 

to Puerto Rico, but also to many other regions, where deforestation and 

rapid land-use change are issues.

0

200

400

Icacos Mameyes Canóvanas Cayaguás

Water (centimeters per year)

Inputs

Output

Evapotranspiration
Puerto Rico also contain abundant silicon, iron, and aluminum. The weathering of minerals in the rocks forms the 

clays and reddish iron minerals that typify regional soils and stream sediments.

Water from streams in the LEF is 

sweet tasting and refreshing. This flavor 

comes from the substances dissolved in 

the water called solutes. These solutes 

include inorganic ions, which are the 

electrically charged components of dissolved salts, natural organic compounds 

and ions, and neutral constituents including, silica and gases, such as oxygen, 

nitrogen, and carbon dioxide.

The composition of water that falls as 

rain on the forest changes dramatically 

as it passes over and through the soil into 

streams. The composition of rain water 

falling on the LEF is controlled primarily by salts 

from the ocean. 

When waves break 

and bubbles burst, 

droplets of seawater enter the 

atmosphere. In 

addition, marine 

algae release sulfur 

gases to the atmosphere. The gases 

become sulfuric acid. Lesser contributions of sulfuric acid, nitric acid, and 

ammonia come from air pollution from 

North America and Europe, and Sahara 

dust contributes calcium and sulfate 

from Africa. The composition of the 

water changes immediately upon falling 

through the leafy canopy of forest trees, 

picking up leaf exudates and feeding 

communities of plants and bacteria growing on leaf surfaces, branches, and trunks. 

The composition of the water again 

changes as it falls on and soaks through 

soil. Roots and associated fungi remove 

plant nutrients from the water. 

Plants promote chemical weathering 

through release of root exudates and 

through respiration and decay, which add 

carbonic, nitric, and organic acids to soil 

water. These acids react with bedrock 

and release various ions and silica from 

the rock into the 

soil water, while 

also forming the 

clay minerals that 

become soil and 

the fine sediment 

carried by the 

streams.

The composition of water that enters 

from the rain and dust, and streamflow 

that leaves the Icacos watershed is shown 

in the figure to the left. All the chloride 

in the Icacos water was assumed to come 

from sea salt. In acquiring these ions, 

the water has nourished the forest and 

has carried a small part of the Luquillo 

Mountains to the ocean. The Luquillo 

Mountains have eroded about 5 centimeters since Columbus first visited the 

island 500 years ago. 

Chemical weathering is important in 

understanding nutrient cycling and forest 

dynamics, but also in making abundant 

regolith (loose material, including soil) 

available for physical erosion. In eastern 

Puerto Rico, two classes of physical 

erosion are identified: surficial erosion, 

where particles are transported from the 

ground surface, and landsliding, where 

particles are excavated deeply as a moving mass of rock and soil. A variety of 

chemical techniques are used to understand the rates of physical and chemical 

erosion. Many of these rely on natural 

radioactive isotopes to estimate rates or

relative importance of chemical or physiIn the Luquillo Mountains, chemical 

weathering of the bedrock, which is 

promoted by plants, contributes to 

most of the dissolved load of rivers 

and forms soils, which in turn, when 

physically eroded, become the suspended and bed loads in rivers.

Discharge weighted average 

concentrations of solutes in 

the Icacos River ranked 

according to concentration. 

Inorganic nitrogen combines 

nitrate and ammonia. Nonmarine sulfur and nitrogen 

gases are presumably 

derived mainly from human 

activities. The negative values correspond to ecosystem 

losses of nitrogen and for 

bicarbonate to the acid 

brought in by rain. Sodium, 

calcium, magnesium, potassium, and ammonia are 

positive ions (cations). Bicarbonate, chloride, sulfate, and 

nitrate are negative ions 

(anions). Organic carbon is a 

mix of ions and molecules. 

Silica is a molecule.

This representation of hydrologic and climatologic characteristics of eastern Puerto Rico 

shows how elevation increases precipitation 

and enhances runoff.

MEAN ANNUAL

RAINFALL

5,000 millimeters

(1,000 m elevation)

1,500 millimeters

(sea level)

Slow ground-water 

flow (meters 

per year)

Rapid runoff and 

near-surface interflow (centimeters 

per second)

Rapid infiltration

(millimeters to

centimeters per

second)

Flashy streamflow

(meters per second)

EVAPOTRANSPIRATION LOSSES

10 to 20 percent of rainfall

(1,000 m elevation)

40 to 60 percent of

rainfall (sea level)

DAILY SOLAR RADIATION

80 to 180 W/m2*

Mean annual temperature 19°C

(1,000 meters elevation)

160 to 300 W/m2

Mean annual temperature 27°C

(Sea level)

*W/m2 = watts 

per square meter

0 100 200

Concentrations

(micromoles per liter)

Silica

Bicarbonate 

Organic Carbon

Sodium

Chloride

Calcium

Magnesium

Sulfate

Potassium

Inorganic Nitrogen 

Ocean sources delivered by 

wet and dry deposition

Soil and ecosystem processes

River water composition 

Non-marine sulfur and

nitrogen gas sources
cal processes. The concentration of the 

radioactive cosmogenic isotope, beryllium-10, in quartz grains that occur in 

sediment in river channels and on hillslopes has been measured. The beryllium-10 forms as the slow, steady rain 

of cosmic-ray neutrons strikes atoms in 

quartz and other minerals in soil and 

bedrock within a meter or so of the 

ground surface. The concentration of 

beryllium-10 is related directly to erosion rate. The results of this work indicates that in the Icacos watershed the 

fine-grained sediment appears to be 

derived from surficial erosion, whereas 

much of the coarse-grained sediment 

comes from landslides. Beryllium-10 

was also used to estimate what the erosion rate of an area was before it was 

deforested.

Fluvial Sediment and the Effects of 

Humans on Landscape

Intense agricultural practices of the 19th 

and early 20th century have resulted in 

high suspended-sediment yields in the 

Loíza basin many decades after farming 

had been abandoned. Soil was eroded 

from the hillslopes, mainly by landsliding, while they were being farmed 

and was then deposited at the base 

of the hillslopes near stream channels. 

Since then, stormflows have episodically 

moved the sediment from the base of 

the hillslopes downstream where it is 

deposited in reservoirs or in estuaries 

and in coastal waters where coral reefs 

are located. The sediment and associated contaminants degrade the water 

quality and are harmful to aquatic 

organisms in these areas.

To understand these processes, the four 

Luquillo WEBB study watersheds were 

paired to compare and contrast the 

effects of land use as well as bedrock 

geology on fluvial sediment yield (combined suspended and bedload sediment). 

During the period 1991 to 1995, sediment concentration, which was calculated as sediment yield normalized to 

runoff, was about 3.5 times greater in the 

two watersheds in secondary forest and 

pasture compared to sediment concentration in the watersheds in primary forest. 

However, the influence of lithology was 

almost as great: sediment concentration 

in intrusive-bedrock (quartz diorite and 

granodiorite) watersheds was about 3.2 

times higher than sediment concentration

in volcaniclastic-bedrock (volcanic sandstone, mudstone, and breccia) watersheds. These contrasts highlight the 

well-known impact of land use on sediment concentration and yield but stress 

the strong control of bedrock geology 

as well. Most importantly, the high 

sediment yields in 

the watersheds that 

were converted to 

pasture and secondary forest, even 

after 60 years of 

forest reestablishment, provide a 

glimpse into the future of tropical watersheds elsewhere that are now undergoing 

deforestation.

Landslides in a Tropical Setting

Landslides are common in steep mountainous areas of Puerto Rico, where 

mean annual rainfall is high and intense 

storms are frequent. In general, when 

rainfall exceeds 200 millimeters in 24 

hours, landslides occur. Each year, landslides cause extensive damage to property and occasional loss of life. The

increasing population of Puerto Rico 

(3.9 million in 1999) increases stress 

on the natural environment and physical 

infrastructure; this makes human populations more vulnerable to landslide 

hazards. Maps of recent landslides, 

which were developed from 1:20,000 

scale aerial photographs in combination 

with a geographic 

information system, were used to 

evaluate the frequency and distribution of shallow 

landslides. Several types of landslides were documented. Rainfall 

triggered debris flows, shallow soil slips, 

and slumps were most abundant. Hillslopes that are most prone to landsliding 

have been modified by humans. The hillslopes exceed 12 degrees in gradient, are 

greater than 300 meters in elevation, and 

face the east-northeast. Areas along road 

corridors are also particularly vulnerable. The rate of landsliding disturbance 

is increased from 5 to 8 times in a 170 

meter wide swath along highways in the 

Luquillo Mountains.

Watershed characteristics and mean fluvial sediment yield (combined suspended and 

bedload sediment) for water years 1991 to 1995.

Landslides are the dominant cause 

of hillslope erosion in forested and 

deforested landscapes and generally occur when rainfall exceeds 200 

millimeters in a day. Road corridors 

are especially landslide prone.

17.8

25.5

3.3

26.4

Volcaniclastic

Volcaniclastic

Intrusive

Intrusive

Primary forest

Secondary forest,

pasture

Primary forest

Pasture,

secondary forest

2,441

672

3,193

1,111

227

225

954

1,163

93

335

299

1,047

Watershed

drainage

area

(km2)

Mameyes

Canóvanas

Icacos

Cayaguás

Watershed Type of

bedrock

Principal

land use

Mean

annual

runoff

(mm)

Mean annual

fluvial sediment yield

(tonnes per km2)

Mean annual

sediment

concentration

(kg/mm

of runoff)

0.1 1 1 0

Runoff

(millimeters per hour)

Suspended sediment

(tonnes per kilometer)

0.0001

1

10

100

0.1

0.01

0.001

Cayaguás: developed-intrusive

Canóvanas: developed-volcaniclastic

Icacos: forest-intrusive

Increasing yield Mameyes: forest-volcaniclastic

Suspended-sediment yield and runoff for 

four watersheds in eastern Puerto Rico 

showing variation in yield with land use 

and lithology.
For more information about the Luquillo WEBB 

study visit:

http://pr.water.usgs.gov/public/webb

For more information or for reprints, please contact:

WEBB Site Coordinator

U.S. Geological Survey

651 Federal Drive, Suite 400-15

Guaynabo, Puerto Rico 00965-5703

(787) 749-4346 or mclarsen@usgs.gov.

REFERENCES

Brown, E. T., Stallard, R. F., Larsen, M. C., Raiseck, 

G. M., and Yiou, F. 1995, Denudation rates determined 

from the accumulation of in situ produced 10Be in the 

Luquillo Experimental Forest: Puerto Rico: Earth and 

Planetary Science Letters, v. 129, p. 193–202. 

Brown, E. T., Stallard, R. F., Larsen, M. C., Bourlès, 

D. L., Raisbeck, G. M., and Yiou, F., 1998, Determination of pre-development denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using 

in situ produced 10Be in riverborne quartz: Earth and 

Planetary Sciences Letters, v. 160, p. 723–728. 

Larsen, M.C., and Simon, Andrew, 1993, Rainfall 

threshold conditions for landslides in a humid tropical 

system, Puerto Rico: Geografiska Annaler, v. 75A (12), 

p. 13–23. 

Larsen, M. C. and Parks, J. E., 1997, How wide is 

a road? The association of roads and mass wasting 

disturbance in a forested montane environment: Earth 

Surface Processes and Landforms, v. 22, p. 835–848. 

Larsen, M. C. and Concepción, I. M., 1998, Water 

budgets of forested and agriculturally developed watersheds in Puerto Rico : Proceedings, Tropical Hydrology 

and Caribbean Water Resources, R. I. Segarra García, 

ed., American Water Resources Association, San Juan, 

Puerto Rico, July 12–16, 1998, p. 199–204 

Larsen, M. C., and Torres Sánchez, A. J., 1998, The 

frequency and distribution of recent landslides in three 

montane tropical regions of Puerto Rico: Geomorphology, v. 24, no. 4, p. 309–331.

Larsen, M. C., Torres Sánchez, A. J., and Concepción, 

I. M., 1999, Slopewash, surface runoff, and fine litter 

transport in forest and landslide scars in humid tropical

steeplands, Luquillo Experimental Forest, Puerto Rico: 

Earth Surface Processes and Landforms, v. 24, p. 

481–502.

Scatena, F. N., and Larsen, M. C., 1991, Physical 

aspects of Hurricane Hugo in Puerto Rico: Biotropica, 

v. 23, no. 4A, p. 317–323. 

Stallard, R. F., 1995, Relating chemical and physical 

erosion in White, A.F., and Brantley, S.L., eds., 

Reviews in Mineralogy, Mineralogical Society of 

America, Washington, D.C., v. 31, p. 543–564.

Stallard, R. F., 1995, Tectonic, environmental, and 

human aspects of weathering and erosion: A global 

review using a steady state perspective: Annual 

Review of Earth and Planetary Sciences, v. 12, p. 

11–39. 

Stallard, R. F., 1998, Terrestrial sedimentation and 

the carbon cycle: Coupling weathering and erosion to 

carbon burial: Global Biogeochemical Cycles, v. 12, 

no. 2, p. 231–252.

Stonestrom, D. A., White, A. F., and Akstin, K. C., 

1998, Determining rates of chemical weathering in 

soils: Solute transport versus profile evolution: Journal 

of Hydrology, v. 209, p. 331–345.

White, A. F, Blum, A. E., Schulz, M. S., Vivit, D. 

V., Stonestrom, D. A., Larsen, M. C., Murphy, S. F., 

and Eberl, D., 1998, Chemical weathering in a tropical 

watershed, Luquillo Mountains, Puerto Rico: I. Long 

term versus short term weathering fluxes: Geochimica 

et Cosmochimica Acta, v. 62, no. 2, p. 209–226.

—Matthew C. Larsen and Robert F. Stallard

COLLABORATORS

USGS Luquillo WEBB Project researchers work 

closely with scientists involved with the National 

Science Foundation-funded Long Term Ecological 

Research (LTER) program in the Luquillo Experimental Forest. Many of these scientists are based at the 

U.S. Forest Service International Institute of Tropical 

Forestry, and the University of Puerto Rico Institute 

for Tropical Ecosystem Studies, located in Río Piedras, Puerto Rico. USGS and LTER scientists cooperate by sharing data and research equipment and publishing research papers together.

Students and volunteers from a number of universities 

in Puerto Rico and in the United States have made 

many important contributions towards the success of 

the USGS Luquillo WEBB project by helping in the 

collection, processing, and archiving of data and samples. Graduate students have advanced our scientific 

understanding of important geochemical and hydrological processes and budgets by conducting research 

and publishing their results.

Despite being a hazard, landslides, under 

natural conditions, are important because 

of their role in controlling the sediment 

loads and chemistry of the pristine 

streams that drain the Luquillo Mountains. Landslides have this effect 

because of the periodic introduction of 

a large quantity of soil and weathered 

rock directly into stream channels where 

it often remains for weeks to months as 

streamflow gradually erodes the material.

Under natural conditions, landslide erosion exceeds erosion of soil by other processes, such as slopewash, which is the 

surficial mobilization of individual soil 

particles. For example, the Mameyes and 

Icacos rivers annually export from 227 to 

954 tonnes of sediment per square kilometer, respectively, based on data collected 

by manual and automatic water sampling at USGS streamflow-gaging stations. According to recent data published 

by the Luquillo WEBB Project, slopewash ranges from 10 to 50 tonnes per 

square kilometer per year on steep slopes 

in the LEF. This erosion rate is far less 

than the total amount of sediment carried 

by rivers that drain the study area. Much 

of the above-mentioned fluvial sediment 

is instead mobilized by landslides.

Landslide scar and deposit in the Icacos 

River near the USGS gaging station, 

Luquillo Mountains.

SUMMARY

• Puerto Rico provides a glimpse into the future of tropical environments 

that are now undergoing deforestation. The island reached a maximum 

of deforestation during the 1940’s and has been partially reforested 

since then. Puerto Rico is a natural laboratory for analysis of how 

landscapes respond to and recover from massive land-cover change.

• Landscapes that have been deforested have accelerated physical erosion rates; they have a 3.5 times higher rate of sediment yield when 

compared to their forested counterparts.

• Landslides are the dominant cause of hillslope erosion in forested and 

deforested landscapes and generally occur when rainfall exceeds 200 

millimeters in a day.

• Runoff varies from 49 to 82 percent of inputs (mainly precipitation and 

cloud drip) and is strongly controlled by elevation and location with 

respect to prevailing (trade) winds.

• Streamflow in the Luquillo Mountains is chemically dilute and transport of mass is predominately physical, as sediment particles.

• The Luquillo Mountains have been eroded by 5 centimeters since 

Columbus first visited the island because of combined chemical and 

physical weathering of bedrock.