Laguna Grande at Las Cabezas de San Juan Natural Reserve, Fajardo

Laguna Grande at Las Cabezas de San Juan Natural Reserve, Fajardo

The island of Puerto Rico enjoys abundant water, given its size and both its permanent and transitory populations. Proportionately, its average rainfall exceeds that of most of the countries in the world. This generates abundant runoff all through the year. The aquifers are relatively large, with ample storage and production capacity, which makes them important reserves of fresh water. The amount of surface and underground fresh water that is used is less than one third of what could be used for the long-term growth of the island’s population. In addition, it can provide water supplies for agriculture, industry, commerce and tourism. Though the southern region receives less rain than the northern region, even in the former, rain and runoff are substantial in comparison with other countries and islands.

The climate of Puerto Rico is the principal factor controlling the hydrology of the island and directly affecting the availability of water resources. The climate includes temperature and winds, rain, and evapotranspiration. Rain and evapotranspiration are very important factors among these parameters, though all are related.

Figure 1. Average rainfall in Puerto Rico

Figure 1. Average rainfall in Puerto Rico

The average annual rainfall in Puerto Rico is approximately 69 inches, which varies from a minimum of 35 inches in the coastal area south of the Lajas Valley, to a maximum of 174 inches in the Caribbean National Forest in the region of El Yunque in the Sierra de Luquillo (Figure 1). These averages may vary significantly, and in some years go down as far as 29 inches in the Lajas Valley and increase to as much as 250 inches in the region of El Yunque. It is worth noting that traditionally it has been accepted that the average rainfall in Puerto Rico is from 72 to 74 inches per year. Recent statistical analyses by the Departamento de Recursos Naturales y Ambientales (DRNA, Department of Natural and Environmental Resources) of historical rainfall data dating from 1899 have established that the annual average is 69 inches.

During the year, rainfall in Puerto Rico varies significantly, with a dry period that normally runs from January to March or April, followed by intense rainfall in May and June, a second dry period in July and August, then a period of frequent rains from September to the end of the year (Figure 2). Orographic effects in the eastern, northern, and western regions produce frequent evening rains over most of the year. Low pressure systems, cold fronts, storms and hurricanes may produce rain events of up to 26 inches in 24 hours, which causes severe regional or general flooding throughout the island.

Figure 2

Descripción: Lluvia promedio mensual en Puerto Rico
Fuente: SNM,2002;DRNA,2004
Servidumbre de conservación en el barrio Rabanal, Cidra

Servidumbre de conservación en el barrio Rabanal, Cidra

Evapotranspiration is one of the most important factors in the water cycle in Puerto Rico, since it reduces the available runoff in the streams and rivers. Evapotranspiration includes evaporation of water through the action of wind (convection) on saturated surfaces such as rivers, reservoirs, wetlands and lagoons; evaporation due to solar radiation; the water that plants consume in the process of photosynthesis; and plant transpiration, which returns water to the atmosphere in the form of vapor. In Puerto Rico, there is no adequate field data to form a precise idea of the average evapotranspiration on the island. Sporadic data from studies by the US Geological Survey (the US Fish and Wildlife Service and other researchers can be used with mathematical models to calculate an average for this parameter. The average evapotranspiration for Puerto Rico that we will give here is based on a method defined by Giusti (1978). This method was adopted because it is considered the most precise one available for the geographical and climatic variations of the island. Using Giusti’s method, a map of evapotranspiration values was developed for Puerto Rico (Figure 3), and a general average was estimated for the island as a whole of 43 inches per year, which varies from a minimum of 22 inches in the Río Mameyes basin to a maximum of 48 inches along the Río Piedras.

Figure 3

Descripción: Evapotranspiración promedio de Puerto Rico.
Autor: DRNA, 2004

Average evapotranspiration in Puerto Rico is approximately 62 percent of rainfall, though it could be up to 90 percent in some coastal areas in the Southern Region. The implication of these estimates is that, though the average rainfall in Puerto Rico is abundant, approximately two-thirds of that is not available as runoff because it evaporates from the land or is transpired by the vegetation.

Lago Luchetti en el municipio de Yauco

Lago Luchetti en el municipio de Yauco

Temperature

The temperature in Puerto Rico is an important factor in the water cycle, though it varies relatively little across the island throughout the year. The average annual temperature varies from 68º to 80º Fahrenheit (Figure 4). The principal role of temperature in the water cycle is that it regulates the mechanism of evapotranspiration, which in turn reduces the availability of rainfall in the form of runoff. Once the water is moving through the rivers and streams or has infiltrated into the aquifers, temperature does not play an important role in the water cycle of the island given the relative uniformity of temperature. However, in the long term, climatic changes and increases in global temperatures will have a great impact on water resources in Puerto Rico.

Winds

Winds affect the water cycle and water resources both directly and indirectly. East-northeasterly winds predominate in Puerto Rico most of the year (Figure 5). These wind systems, called alisios (the trade winds), originate in zones of high pressure in the Azores, to the west of the continent of Africa. They move great masses of air that are partially saturated with humidity from the sea. When they approach the central mountain range of Puerto Rico, the Cordillera Central, these large masses of air rise and move more quickly to cross the mountain barrier. The changes in height and speed of the air masses exploit their internal energy, and the result is a cooling of the masses of air, which causes condensation and precipitation of part of that humidity. The effect is most evident after midday on the eastern and northern slopes, though it is notable in the western region as well. There, winds in the Mona Passage have the same effect, particularly near Mayagüez. In the southern region, except for the portion of the year in which the prevailing winds are from the south-southeast, the effect is not so great. In the coastal valleys, these orographic effects (an influence of topography on the climate) are not directly observable except when the clouds saturated with water reach the coast and the showers that fall on the slopes also reach the valleys.

Other important factors in the water cycle of Puerto Rico are the cyclonic winds caused by tropical storms and hurricanes. These wind systems, of up to 150 mph, move enormous masses of humid air over the island producing intense rainfall of up to 26 inches in 24 hours, as occurred during Hurricane Hortense in 1996 (Figure 6).

These general patterns of the east-northeast winds vary with several factors. One of these is the daytime sea breeze on the coasts induced by differences in the rates of heating and cooling between the earth and the sea during the day and night. This phenomenon produces a breeze from sea to land during the day and in the opposite direction during the night.

The difference between the amounts of rainfall and evapotranspiration is the net amount of water available on the island. This amount, equivalent to 4,190 mgd per year (or 4.69 million acre-feet per year), appears in Figure 7, where the average water cycle of the island is summarized. However, this amount of water is not entirely available for use. Of the island’s annual total amount of water, fractions of the runoff are stored in the reservoirs and an also a relatively minor part infiltrates into the aquifers. Though the aquifers store approximately 60,300 million gallons (185,000 acre-feet) per year, the amount that can be extracted is limited by the geology of the coasts and problems of quality of the induced and natural waters, including chemical contamination and saline intrusion. After subtracting infiltrations into aquifers and storage in reservoirs, the net amount of runoff available for potential uses is 4,112 mgd (4.61 million acre-feet per year). By way of comparison, the total use of water in 2002 for domestic purposes (drinking water), agricultural irrigation, industrial and commercial use was approximately 722 mgd (2,215 acre-feet per day). These extractions come to only 19 percent of the available runoff. The rest of the water, approximately 3,000 mgd (3.36 million acre-feet per year), runs into the sea. Though a substantial part of the runoff generally takes place from September to December of each year, the data indicate that, even so, the available water resources in Puerto Rico for future use are abundant.

Laguna Guaniquilla, lagoon in the municipality of Cabo Rojo

Laguna Guaniquilla, lagoon in the municipality of Cabo Rojo

The water resources of the island are also abundant during extreme droughts. Historical data indicates that the year in which the least rain fell in Puerto Rico in the 20th century was 1994, with an average of 43.2 inches, equivalent to 63 percent of the average annual rainfall. Supposing that evapotranspiration during a drought is equal to that of normal years (a conservative assumption), the amount of rainfall would produce a runoff of approximately 1,277 mgd (1.43 million acre-feet per year). In this analysis, it is important to take into account that the reservoirs in Puerto Rico provide storage to supply most of the water needed during a drought for 60 to 90 days, the water normally being accumulated before the amount of rainfall declines. If the extractions of water during a drought remain equal to those of 2002 (estimated at 702 mgd, or 0.79 million acre-feet per year), water resources would continue to be adequate for present and future needs of the island even during droughts. The reservoirs play a crucial role in this.

There are several fundamental reasons why Puerto Rico and the outlying islands could experience a shortage of water during moderate droughts in several of the basins and service areas of the Autoridad de Acueductos y Alcantarillados (AAA, Aqueducts and Sewer Authority). These include the following:

1. A lack of adequate storage in the basins to capture a large part of the runoff for use during droughts. The reservoirs in most of the basins of the island are relatively small when the available annual runoff is taken into account.

2. The capacity of the water filtration plants in some regions is smaller than that which is needed for the production of water. To satisfy the need, the plants are run beyond their production capacity. This requires that the filters be cleaned more frequently, which increases the amount of water that is returned to the receiving body of water. The water returned is considered part of the losses in the production process. The result is that the net production is reduced.

The water cycle of Puerto Rico, presented in Figure 7, reflects the conditions that existed in 2002. Included in this illustration are elements of water use that are not traditionally part of water cycle analyses. These include sources of freshwater and discharges of treated waste waters from the plants that are operated by the AAA in Puerto Rico.

 

Author: Sigfredo Torres González
Published: August 27, 2014.

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