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Picture of Guajataca Lake, 1994.

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Questions and Answers about Droughts


This page offers some questions and answers about the hydrology of droughts. This information is taken from the web site of the U.S. Geological Survey Water Science Center in Maryland, Delaware, and Washington, D.C. Some content was modified and adapted to Puerto Rico.
http://water.usgs.gov/edu/qadroughts.html.


Questions

Answers

What is a drought?

While it is relatively easy to define what a hurricane or earthquake is, defining a drought is more subjective. Droughts do not have the immediate effects of floods, but sustained droughts can cause economic stress throughout an area. The word "drought" has various meanings, depending on a person's perspective. To a farmer, a drought is a period of moisture deficiency that affects the crops under cultivation—even two weeks without rainfall can stress many crops during certain periods of the growing cycle. To a meteorologist, a drought is a prolonged period when precipitation is less than normal. To a water manager, a drought is a deficiency in water supply that affects water availability and water quality. To a hydrologist, a drought is an extended period of decreased precipitation and streamflow. Droughts in Puerto Rico have severely affected municipal and industrial water supplies, agriculture, stream water quality, recreation at major reservoirs, hydropower generation, and forest resources.


What causes droughts?

A drought is a period of drier-than-normal conditions that results in water-related problems. Precipitation falls in uneven patterns across the country. The amount of precipitation at a particular location varies from year to year, but over a period of years, the average amount is fairly constant. The spatial distribution of rainfall in Puerto Rico is variable. Rainfall is greatest in the Sierra de Luquillo rainforest in the eastern part of Puerto Rico. The mean annual total rainfall in Sierra de Luquillo is 169 inches per year (in/yr). The least amount of rainfall occurs in the vicinity of Guánica at Ensenada in southwestern Puerto Rico. In this area, the mean annual total rainfall is 30 in/yr.

Air temperatures fluctuate little throughout the year as a result of relatively constant insolation (that is, solar radiation) and seawater temperatures. The rate of delivery of solar radiation is nearly constant because the difference in daylight hours varies little throughout the year. Between the longest day of the year (13 hours, 13 minutes) and the shortest day (11 hours, 2 minutes), the amount of daylight differs by only slightly more than 2 hours. Mean monthly seawater temperatures vary by only about 4 °C; the mean maximum water temperature of 28 °C occurs in October, and the mean minimum water temperature of 24 °C occurs in January.

Major rainfall events producing substantial volumes of rain in Puerto Rico and the outlying islands are caused by one of two climate mechanisms—the passage of an easterly wave or the passage of a cold front. Easterly waves generally occur during May to November with some having sufficient intensity to evolve into tropical storms and (or) hurricanes. Cold fronts generally occur during November to April and may produce sufficient rainfall to cause flooding even during the period from December to March, which is a relatively dry period. The number of easterly waves or cold fronts passing over the region in any given year ultimately determines whether the region experiences relatively dry conditions or wet conditions. As a result, localized droughts occur yearly within many of the geographic areas of Puerto Rico.

Reference: Hydrogeology of Puerto Rico and the Outlying Islands of Vieques, Culebra, and Mona.


When does a drought begin?

The beginning of a drought is difficult to determine. Several weeks, months, or even years may pass before people know that a drought is occurring. The end of a drought can occur as gradually as it began. Dry periods can last for 10 years or more. During the 1930's, most of the United States was much drier than normal. In Puerto Rico, for the 50-year period (1961 to 2010) there were six (6) years considered as drought years (1964, 1967, 1973, 1991, 1994, and 1997).

Climatological drought conditions in the Eastern Interior Climatic area of Puerto Rico typically occur when the annual rainfall amount is less than 70-80 percent of normal rainfall (a deficit of 20 to 30 percent below normal).

Reference: Hydrogeology of Puerto Rico and the Outlying Islands of Vieques, Culebra, and Mona.


Does a shortage of rain mean that a drought will occur?

A period of below-normal rainfall does not necessarily result in drought conditions. Some rain returns to the air as water vapor when water evaporates from water surfaces and from moist soil. Plant roots draw some of the moisture from the soil and return it to the air through a process called transpiration. The total amount of water returned to the air by these processes is called evapotranspiration. Sunlight, humidity, temperature, and wind affect the rate of evapotranspiration. When evapotranspiration rates are high, soils can lose moisture and dry conditions can develop. During cool, cloudy weather, evapotranspiration rates may be low enough to offset periods of below-normal precipitation and a drought may be less severe or may not develop at all.

Reference: Moreland, 1993.


Why doesn't a drought go away when it rains?

Rainfall in any form will provide some drought relief. A good analogy might be how medicine and illness relate to each other. A single dose of medicine can alleviate symptoms of illness, but it usually takes a sustained program of medication to cure an illness. Likewise, a single rainstorm will not break the drought, but it may provide temporary relief.

A light to moderate shower will probably only provide cosmetic relief. It might make folks feel better for awhile, provide cooling, and make the vegetation perk up. During the growing season, most of the rain that falls will be quickly evaporated or used by plants. Its impact is short term.

A thunderstorm will provide some of the same benefits as the shower, but it also may cause loss of life and property if it is severe. Thunderstorms often produce large amounts of precipitation in a very short time, and most of the rain will run off into drainage channels and streams rather than soak into the ground. If the rain happens to fall upstream of a reservoir, much of the runoff will be captured by the reservoir and add to the available water supply. No matter where the rain falls, stream levels will rise quickly and flooding may result. Also, because the rainfall and runoff can be intense, the resulting runoff can carry significant loads of sediment and pollutants that are washed from the land surface.

Soaking rains are the best medicine to alleviate drought. Water that enters the soil recharges ground water, which in turn sustains vegetation and feeds streams during periods when it is not raining. A single soaking rain will provide lasting relief from drought conditions, but multiple such rains over several months may be required to break a drought and return conditions to within the normal range.

Tropical storm rains are usually of the soaking variety, although they may also be intense such as during a thunderstorm and lead to some of the same problems. Tropical storms often produce more total rainfall than a "regular" soaking rain and can provide longer relief than a single soaking rain. However, tropical rains may also be of such intensity that they exceed the capacity of soil to absorb water and often result in significant runoff and flooding. Tropical rains can help to fill water-supply reservoirs and provide long-term drought insurance. If significant rainfall does not occur upstream of reservoirs, the drought relief aspects of tropical storms may be of only little consequence. All things considered, a single tropical storm at the right place, at the right time, and with the right amount of rainfall can break a drought.

Considering all of the above, even when a drought has been broken it may not be truly over. The benefits of substantial rainfall such as from a tropical storm may last for months, but a return to normal rainfall patterns and amounts is necessary for conditions in streams, reservoirs, and ground water to also return to normal.

Reference: Moreland, 1993.


Can drought affect the water level in wells?

Ground water, which is found in aquifers below the surface of the Earth, is one of the Nation's most important natural resources. Ground water is used to provide a large portion of the Nation's population with drinking water, it provides business and industries water for their purposes, and is used extensively for irrigation.

The water level in the aquifer that supplies a well does not always stay the same. Droughts, seasonal variations in rainfall, and pumping affect the height of the underground water levels. If a well is pumped at a faster rate than the aquifer around it is recharged by precipitation or other underground flow, then water levels in the well can be lowered. This can happen during drought, due to the extreme deficit of rain. The water level in a well can also be lowered if other wells near it are withdrawing too much water.


References

Gómez-Gómez and others, 2014, Drought: Hydrogeology of Puerto Rico and the Outlying Islands of Vieques, Culebra, and Mona , Scientific Investigations Map 3296., http://dx.doi.org/10.3133/sim3296

Moreland, J.A., 1993, Drought: U.S. Geological Survey Water Fact Sheet, Open-File Report 93-642, 2p.

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