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Drought Conditions in Puerto Rico


Drought Defined

A drought is a period of drier-than-normal conditions that results in water-related problems. When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall, and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.

The term "drought" can have different meanings. The World Meteorological Organization identified five general types of drought (Subrahmanyam, 1967), and Wilhite, D.A. et al (1985) identified a sixth type:

• Meteorologic
precipitation deficiencies, in absolute amounts, for a given period
• Climatological
precipitation deficiencies, in percentages of normal values
• Atmospheric
precipitation deficiencies, also possibly in terms of temperature, humidity, or wind speed
• Agricultural
soil moisture deficiencies and effects on plant growth and behavior
• Hydrologic
streamflow, lake or reservoir storage reduction, and lowering of ground-water levels
• Socio-economic
water shortages in time and space associated with supply and demand

Documented history of droughts in Puerto Rico

Drought conditions in Puerto Rico are perceived initially following a deficiency in precipitation, in absolute amounts, for a given period (figure 1). The National Oceanographic and Atmospheric Administration (NOAA) expresses the deficiency in precipitation, usually within a 12-month period, using three distinct statistical methods (U.S. Drought Monitor webpage, 2015):


  1. Percentages of Normal precipitation (figure 2),
  2. Standardized Precipitation Index (figure 3), and
  3. Palmer Drought Severity Index.

The normal precipitation and the two statistical indexes are used to better understand the departure of the precipitation from normal. When these departures are appropriately weighted, the resulting index numbers (SPI, and PDI) appear to be reasonably comparable in space and time (Palmer, W.C., 1965).

Climatic subdivisions of Puerto Rico map

Figure 1. Climatic subdivisions of Puerto Rico. [Long-term National Weather Station rainfall observation site locations and names shown].



Drought – Relation to Normal Precipitation Distribution

A 30-year Normal is defined as the arithmetic average of a climate element, such as rainfall, using a data record that extends over a period of 30 years. A 30-year period is used because it is long enough to incorporate any annual seasonal variations or anomalies, but also short enough to reveal longer climatic trends.

The most recent 30-year Normal precipitation for Puerto Rico was determined for the period January 1, 1981 to December 31, 2010. The Normals are computed every ten years during a 30-year period. The previous computation period was January 1, 1971 to December 31, 2000. The spatial distributions of the 30-year Normal precipitation on the main island of Puerto Rico for the periods 1971 – 2000 and 1981 – 2010 (figure 2).

30-year Normal Precipitation distribution maps

Figure 2. 30-year Normal Precipitation distribution maps for Puerto Rico (A) 1971–2000, and (B) 1981–2010 [dots indicate National Weather Service rainfall observation stations and numbers indicate Normal precipitation in inches; ellipsoid is location of the Gurabo Agricultural Experimental Station rain gage; source: http://www.srh.noaa.gov/sju/?n=mean_annual_precipitation].


(A)

Standardized Precipitation Index for PR

(B)

Standardized Precipitation Index for Easter Interior of PR

Figure 3. Standardized Precipitation Index for: A. Puerto Rico, and B. Eastern Interior, period of 12-months (values shown are subject to revision, Martinez-Sánchez, O., National Weather Service, written communication, 2015).


Throughout Puerto Rico, the drought of 1993-1996 was the most intense drought on record (figure 3A), whereas the same precipitation record indicates that the 1966-1969 drought was the most intense drought in the Eastern Interior area (Odalys Martinez-Sánchez, National Weather Service, written communication, 2015) (figure 3B)



Compounding Effects of Diminishing Reservoir Storage on Droughts in Puerto Rico

One very prominent effect of drought is on the supply of potable water. About 1.3 million inhabitants (about 35 percent of the population of the island) live in the San Juan Metropolitan area and are highly dependent on the sustainable yield of water from several reservoirs. A deficiency of rainfall can be compounded by a reduction in reservoir storage capacity, generally the result of sedimentation within the reservoir. Water yields from the reservoirs are highly dependent on their storage capacity. Recurring droughts and sedimentation-induced reductions in reservoir storage present a compounded challenge to potable water supply in Puerto Rico.

The Lago Loíza reservoir (figure 4) is an example of the impact of reservoir sedimentation on reservoir storage capacity. The capacity of this reservoir has changed over the years from its original design capacity in 1953, generally decreasing with sedimentation. In an attempt to mitigate the deleterious impact of sedimentation on storage capacity, the reservoir storage capacity was increased in 1977 by installing flashboards above the Carraizo dam crest (Webb and Soler-López, 1997), and between 1997 and 1999 dredging was conducted to increase storage capacity (Soler-López and Gómez-Gómez, 2005). However, the reservoir has lost nearly 40 percent of its storage capacity over the 1953 to 2009 period (figure 5). This loss in storage capacity reduces the ability of the reservoir to store water during wetter times for use in drier times.

Drainage basin map of Rio Grande de Loiza

Figure 4. Drainage basin map: Upper Río Grande de Loíza drainage basin [long-term streamflow gaging stations are the Río Grande de Loíza at Caguas (50055000) and Río Gurabo at Gurabo (50057000) shown as red triangles; other gaging stations since 1990 shown in white triangles; blue diamond is the Gurabo Agricultural Experimental Station at Gurabo; and, the Caguas Regional Wastewater Treatment plant, shown as orange square].



Storage capacity chart

Figure 5. Storage Capacity Long-term sedimentation rates at Lago Loíza showing the effects of sedimentation from Hurricane David and Tropical storm Frederic in 1979, and sediment dredging in 1997. [Source: Soler-López, L.R. and Gómez-Gómez, F., 2005, 0.31 million cubic meters equals 251.4 acre-feet].



References

National Climatic Data Center, Definition of drought, available at http://www.ncdc.noaa.gov/monitoring-references/dyk/drought-definition

National Oceanic and Atmospheric Administration National Weather Service, Glossary, available at http://w1.weather.gov/glossary/index.php?word=NORMAL

Subrahmanyam, V.P., 1967, Incidence and spread of continental drought: World Meteorological Organization, International Hydrological Decade, Reports on WMO/IHD Projects, no. 2, Geneva, Switzerland

Soler-Lopez, L.R. and Gomez-Gomez, F., 2005, Sedimentation survey of Lago Loíza, Puerto Rico, January 2004: U.S. Geological Survey Water-Resources Investigations Report 2005-5239, 26 p., 2 pls.

Soler-Lopez, L.R. and Licha-Soler,N.A., 2012, Sedimentation survey of Lago Loíza, Puerto Rico, July 2009: U.S. Geological Survey Scientific Investigations Map 3219.

Webb, R.T. and Soler-López, L.R., 1997, Sedimentation history of Lago Loíza, Puerto Rico, 1953 – 1994: U.S. Geological Survey water Resources Investigations Report-97-4108, 18 p., 9 pls.

Wilhite, D.A.; and M.H. Glantz. 1985. Understanding the Drought Phenomenon: The Role of Definitions. Water International 10(3):111–120.

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