ABSTRACT: Reservoirs provide most of the water supply for Puerto Rico. Most reservoirs were constructed in the early and middle 20th century and sediment accumulation has significantly reduced their original storage capacity.  The U.S. Geological Survey (USGS) has been conducting a series of bathymetric surveys using a global positioning system (GPS) and a geographic information system (GIS) technology on the principal reservoirs in Puerto Rico, and comparing the results with previous surveys to quantify storage capacity loss.  Hurricanes Hortense and Georges substantially reduced the storage capacity of Lago Caonillas and Lago Dos Bocas.  The storage capacity losses for 14 reservoirs studied range from 12 to 81 percent, averaging 35 percent.  The sediment yields for the reservoir basins studied ranged from 483 to 4,102 cubic meters per square kilometer per year.  The rate of capacity loss of the reservoirs studied ranges from 834 to 3,937 cubic meters per square kilometer per year.  The difference between the sediment yield and capacity loss per area per year reflects the different trapping efficiencies of the various reservoirs.

KEY TERMS: Reservoir, reservoir sedimentation rate, sediment yield, bathymetric surveys, Puerto Rico.

The humid tropical environment and mountainous terrane of Puerto Rico are conducive to high rates of sedimentation.  Washed from the hillslopes and construction sites, sediments settle out in the calm waters of the reservoirs, reducing the storage capacity and eventually ending their useful life.  Major floods associated with hurricanes and tropical disturbances cause extensive land erosion and sediment transport that rapidly deplete the storage capacity of reservoirs.

The USGS, in cooperation with the Government of Puerto Rico, has conducted sedimentation surveys since 1994 in 14 reservoirs used for domestic and industrial water supply, power generation, and irrigation.  A GPS, a survey-grade echo sounder, and GIS, were used to plan, collect, and analyze hydrographic data to determine the existing storage capacity, the sedimentation rates, and the sediment yield of the reservoir drainage areas.

Bathymetric surveys were conducted by collecting data of geographic location and water depths simultaneously, and storing the data in a portable personal computer.  The standard methodology consisted of navigating the reservoirs at pre-established cross sections spaced at 50 meters (or less as needed), starting at the reservoir dam and continuing upstream along the different branches of the reservoirs.  The collected data were then edited to eliminate spurious positions and depths, and adjusted to represent water depths at spillway or normal pool elevations.  A triangulated irregular network (TIN) of reservoir bottoms, contour maps, cross sections representing the reservoir bottom depths from shore to shore, and longitudinal sections along the thalweg of the different branches of the reservoirs were generated using GIS, to graphically show the location and amount of sediment deposition.

Sediment accumulation has substantially reduced the storage capacity of the principal water-supply reservoirs in Puerto Rico over time.  Their storage losses range from 12 to 81 percent of the reservoirs’ original storage capacities.  Some of the smaller reservoirs studied are near the end of their useful life.  The individual sedimentation trends of the various reservoirs vary with land use, topography, development, and principally, rainfall magnitudes and frequencies within the basins. Table 1 summarizes the results of bathymetric studies conducted on the 14 reservoirs.

Reservoir

Original capacity, in Mm³

Const.year

Study year

Age 

Storage capacity, in Mm³

Totalvol. loss,Mm³

Long-term volume loss, in m³/yr

Loss in pecent

Long-term storage loss per year, in percent

Drainage area, in km²

Surface area, in km²

Depos-ition rate, in cm/yr

Sediment yield, in m³/km²/yr

Storage loss, in m³/km²/yr

Yahuecas

1.76

1956

1997

41

0.33

1.43

34,878

81

2.0

45.17

0.22

15.8

1,430

772

The capacity loss rates per area and basin sediment yields, increase with rainfall magnitudes and frequencies, and with human impacts.  Also, the drainage area extension and morphology influences sediment erosion processes and sediment transport rates within the basins.

The highest drainage area sediment yields and storage capacity loss rates per area occur in Lago Lucchetti, Lago Guayo, and Lago Caonillas.  These reservoirs are located in areas where average rainfall is about 1,800 to 2,300 millimeters per year (Calvesbert, 1970).  The reservoirs were constructed about 50 years ago and have been affected by three hurricanes over their history (Santa Clara, 1956; Hortense, 1996; and Georges, 1998).  Moreover, these reservoirs are connected to small, upper basin reservoirs via underground tunnels, to artificially increase the natural drainage areas.  As these small upper reservoirs have filled with sediment, however, their bottoms have reached the elevation of the outlet structures, making it likely that sediment passes through the underground tunnels during flood events, accelerating the storage capacity losses of the lower reservoirs (Soler-López, 1997, 2000).  Also, these reservoirs are located in areas where agriculture was widespread during the early part of dam operation, which probably contributed to the storage loss.

The high drainage area sediment yields and storage loss rates per area of Lago Carite, Lago Guajataca, and Lago Patillas could be the result of high average rainfall rates (about 2,000 millimeters per year, Calvesbert, 1970) combined with the passage of five hurricanes (San Felipe, 1938, San Ciprian, 1932; Santa Clara, 1956; Hortense, 1996; and Georges, 1998) during their history.  Land use within these drainage areas (cultivation of tobacco) may have contributed substantially to the high sedimentation rates in the early life of these reservoirs.  Lago Caonillas, Lago Dos Bocas, Lago Guayo, Lago Loíza, and Lago Lucchetti have been affected by three hurricanes (Santa Clara, Hortense, and Georges). In addition, the reservoirs are located in areas with an average rainfall of about 2,000 millimeters per year (Calvesbert, 1970).  These high annual rainfall rates combined with the high relief and steep slopes of the drainage basins, can result in loosen soils being flushed into the reservoirs by storms, thus depleting reservoir storage capacities rapidly.

More than half of the reservoirs studied exceed the design storage loss rates per area of about 1,000 (the average for the 14 reservoirs is 1,389 cubic meters per square kilometer per year).  Furthermore, storage loss rates per unit of area for the three reservoirs with supplementary input via underground tunnels (Lucchetti, Guayo, and Caonillas) are more than twice the design standards.  Most of the depositional rates presented in table 1 exceed the design rate of about 5 centimeters per year.   Although Lago Dos Bocas receives water from Lago Caonillas through an underground tunnel from Lago Caonillas, sediment bypass does not occur because the Lago Caonillas bottom has not reached the elevation of the power outlet structure; however, the bottom is close to reaching the structure and the capacity loss of Lago Dos Bocas could be exacerbated in the near future (Soler-López, 2000).

Major floods caused by Hurricanes Hortense and Georges in September 1996 and 1998, respectively, generated massive runoff that induced land erosion.  Consequently, large amounts of sediment were deposited in the reservoirs, substantially depleting their storage capacity.  Sedimentation surveys conducted in Lago Caonillas and Lago Dos Bocas soon before and after these hurricanes (1995 and 2000 for Caonillas and 1994, 1997, and 1999 for Dos Bocas) indicate that the storage capacity of Lago Caonillas was reduced about 12 percent in 5 years. This represents a loss rate of about 2.4 percent per year for that period, which is about five times higher than the long-term percent loss of 0.5 percent.  The storage capacity of Lago Dos Bocas was reduced about 9 percent in 5 years, giving a loss rate of about 1.8 percent per year for that period, which is about two times higher than the long-term storage percent loss per year of Lago Dos Bocas (Soler-López and Webb, 1997; Soler-López, 2000).

In 1994 and 1995, there were no major tropical disturbances and rainfall was below average, resulting in water rationing in the San Juan metropolitan area in 1994.  In 1996, Hurricane Hortense made landfall with rains that exceeded 400 millimeters in the central Puerto Rico area during the event (Torres-Sierra, 1997).  During 1997, there were no major tropical disturbances, resulting in implementing water rationing in western Puerto Rico.  In 1998, Hurricane Georges passed over Puerto Rico and produced rainfall exceeding 550 millimeters across much of Puerto Rico (USGS, 1999).  Based on the timing of the bathymetric surveys, and the periods of drought and hurricane activity, it is likely that most of the storage capacity losses of both reservoirs between the 1994 and 2000 surveys actually occurred between 1996 and 1998, demonstrating the severe impact of these major storms on the reservoirs.

The sediment yields of the reservoir basins vary also with the drainage area size and the length of the river channels upstream from the reservoirs.  Figure 1 shows the correlation between the drainage area size and capacity losses per area per year for the 14 reservoirs studied.

FIGURE 1.  Correlation between reservoir drainage area and capacity loss rates per drainage for all the reservoirs studied.

In general, the higher rates of capacity losses per drainage area per year (that reflect erosion and transport rates) occur in the reservoirs with smaller and somewhat disrupted basins.  Sediment eroded from the basins has to travel through the river channels before reaching the calm waters of reservoirs.  In small drainage areas the distance traveled is short, thus, requiring less travel time and runoff to be deposited.  Also, sheet erosion (eroded material that is transported across the hill slopes instead of the river channels) affects smaller basins at a larger scale (for example Lago Carite, Lago Patillas, Lago Guajataca, and Lago Loco).  The exacerbated storage loss rates per area for Lago Guayo and Lago Lucchetti  probably reflects the sediment bypass from the upper reservoirs, as mentioned previously.

On the other hand, in large drainage areas (for example Lago La Plata, Lago Loíza, and Lago Dos Bocas), sediment travels longer distances, and therefore, requires more travel time and runoff to reach the reservoir.  Large drainage areas have the capacity to temporarily store large amounts of eroded material.  This temporarily stored material, however, is metastable and has the potential to be suddenly flushed downstream during high magnitude events, such as tropical disturbances and hurricanes.  This temporarily stored material appears to have been a major factor in the rapid infilling of Lago Caonillas and Lago Dos Bocas during Hurricanes Hortense and Georges (Soler-López, 1999, 2000).

Although Lago Garzas and Lago Cidra have small drainage areas, the storage capacity loss rates per area are low (table 1). This is probably because the Lago Garzas drainage basin is practically undisturbed and densely vegetated, which protects the soil from erosion processes.  Although some portions of the Lago Cidra drainage basin are currently being developed, historically, development has been limited in the basin.  Additionally, the low rates of storage capacity loss per area for Lago Prieto and Lago Yahuecas probably reflects the sediment bypass process that occurs, meaning that the sediment transported into the reservoirs is bypassed into Lago Guayo and Lago Lucchetti, respectively.  Even though Lago Caonillas falls into the large drainage area-reservoirs, the storage capacity loss rate per area is high because of the sediment bypass from upstream reservoirs (Lago Viví and Lago Jordán).

Despite the efforts made to substantially increase the overall water resources in Puerto Rico by constructing additional dams in the last 60 years, the outcome has not been sufficient at a long-term scale (fig. 2).  The high existing sediment yields and sedimentation rates are depleting the resources faster than predicted.  The net water gain is practically lost in about 15 to 20 years, and the difference between the desired volume and the volume obtained is increasing.  Although Lago Cerrillos was not studied, it is included in figure 2 predicting the storage loss using the traditional design rate loss (1,000 cubic meters per square kilometer per year) to illustrate the trends.

FIGURE 2. Cumulative water resources and sedimentation trends for all of the 14 reservoirs studied, and for Lago Cerrillos.

The reservoirs used as water supply for human consumption have high water renewal rates (i.e. low water residence times). Lago Caonillas has water renewal about six times per year.  Lago Cidra has water renewals about three times per year.  Lago Dos Bocas has water renewal about 22 times per year. Lago Guajataca has water renewal about three times per year. Lago Loíza has water renewal about 26 times per year, and Lago La Plata has water renewal about 8 times per year.  Water renewals can be beneficial in terms of water quality.

The proportion of average annual inflow that these reservoirs are capable of storing, however, is low. In general, the water storage capacity to sustain the yearly water demands from these reservoirs is low (table 2).  As a result, these reservoirs are rapidly depleted of stored water during periods of below-normal rainfall.

TABLE 2.  Percent of annual inflow storage and water demand storage for Lago Caonillas, Lago Cidra, Lago Dos Bocas, Lago Guajataca, Lago Loíza and Lago La Plata [Mm³, mega cubic meter]

The principal reservoirs of Puerto Rico are rapidly losing their water storage capacity because of high rates of sediment influx and accumulation.  The factors that promote sedimentation in the reservoirs include land use, steep drainage basin slopes and relief, and most importantly, high rainfall magnitudes and frequencies, like the ones generated from major tropical disturbances.

Connecting reservoirs at high elevations with those at lower elevations, results in an increased drainage area and available water.  As the upstream reservoirs fill with sediment, however, the bottoms reach the outlet structures and sediment is bypassed into the lower reservoirs, accelerating the sedimentation rates in these impoundments.

In general, reservoirs with small drainage areas lose storage capacity at a faster rate than reservoirs with larger basins, especially in developed basins.  The drainage area size and river channel length above the reservoirs influence sediment transport rates, and hence, storage capacity loss rates.  Even in reservoirs with large basins, however, intense rainfall in a short period can transport large amounts of sediment into the reservoirs, rapidly depleting their storage capacity.

Efforts to significantly increase the overall water resources by simply building more dams over time, is proving to be ineffective on a long-term basis.  Erosion mitigation, as well as reservoir maintenance plans are essential to preserve the existing and future reservoirs.

Water-renewal rates are generally high in the reservoirs used for human supply, which is generally beneficial for water quality.  These reservoirs, however, are only able to retain a small portion of the annual surface-water inflow, and therefore the principal water-supply reservoirs in Puerto Rico cannot sustain normal water demands in periods of low rainfall.

• Calvesbert, R. J., 1970, "Climate of Puerto Rico and U.S. Virgin Islands, revised"; U.S. Environmental Science Services Administration, Climatology of the United States 60-52, 29 pp.
  
  
• Soler-López, L. R., and R. M. T. Webb, 1998, "Sedimentation Survey of Lago Dos Bocas, Puerto Rico, June 1997"; U. S. Geological Survey Water-Resources Investigation Report 98-4188, 14 pp., 1 pl.
  
  
• Soler-López, L. R., 1999, "Sedimentation Survey of Lago Guayo, Puerto Rico, October 1997"; U.S. Geological Survey Water-Resources Investigations Report 99-4053, 20 pp. 1 pl.
  
  
• Soler-López, L. R., 1999, "Sedimentation Survey of Lago Dos Bocas, Puerto Rico, October 1999"; U. S. Geological Survey Water-Resources Investigations Report 00-4234, 18 pp. 1 pl.
  
  
• Soler-López, L. R., 2001, "Sedimentation Survey of Lago Caonillas, Puerto Rico, February 2000";U. S. Geological Survey Water-Resources Investigation Report 01-4043, 18 pp. 1 pl.
  
  
• Torres-Sierra, H., 1997, "Hurricane Hortense, Impact on Surface Water in Puerto Rico"; U. S. Geological Survey Fact Sheet FS-014-97, 4 pp.
  
  
• U. S. Geological Survey, 1999, "Puerto Rico – Hurricane Georges"; U.S. Geological Survey Fact Sheet 040-99, 4 pp.

¹ U.S. Geological Survey, Caribbean District, GSA Center, 651 Federal Drive, Suite 400-15, Guaynabo, PR  00965   (phone 787/749-4346, fax 787/749-4462) lssoler@usgs.gov

Soler-López, L.R., 2001, "Sedimentation survey results of the principal water supply reservoirs of Puerto Rico"; in W.F. Sylva (ed.), Proceedings of the Sixth Caribbean Islands Water Resources Congress, Mayagüez, Puerto Rico, February 22 and 23, 2001, unpaginated CD