Freshwater Resources 

The availability of freshwater resources is already a problem for many island communities, because of their unique geography and the growth of population, tourism, and urban centers.  Many islands suffer from frequent droughts and water scarcity from not enough precipitation.  In other cases, rainfall is abundant but access to freshwater is limited by lack of adequate storage facilities and delivery systems, or a mismatch between where it rains and where the water is needed.  For example, it rains on one side of the island but the population is mostly located on the other side of the island.  For the Pacific Islands, both a present and future key issue is drought or water scarcity conditions.  For the Caribbean, drought is also a problem, but floods and landslides associated with hurricanes and heavy rainfall are an important addition to this region's list of key water-related issues.  For example, rainfall-triggered landslides are the most common type of landslide in the central mountains and foothills of Puerto Rico. 

Future climate changes in the islands regions could include: changes in naturally occurring variations in weather patterns (e.g., El Niño could occur more frequently or last longer), ocean temperature, and ocean currents; changes in the frequency, intensity, and tracks of tropical cyclones (storms called hurricanes in the Atlantic and typhoons in the Pacific) and their resulting precipitation; and/or changes in sea level.  Any of these changes would affect the amount, timing, or availability of freshwater, such that freshwater issues will be increasingly serious concerns for the US affiliated Islands.   

Many island communities rely upon groundwater resources called freshwater lenses.  The size of the groundwater lens is directly related to the size of the island.  It is also related to the normal amount and type of precipitation (e.g., heavy downpours recharge lenses, while light rain generally does not), and the leaking of fresh lens water into the ocean.  Larger islands have larger lenses and are more buffered from drought conditions.  Smaller islands have no lenses or shallow lenses that easily become depleted or contaminated with salt water.  During drought conditions, there is no recharge to the lens, and the fresh water is depleted rapidly, especially if consumption is high. 

Low sea levels associated with El Niño lower the water table even more, making it more difficult to access the water and easier to damage the fragile connection between the fresh water lens and the underlying salt water.  Low-lying atolls rely heavily on rainwater collection and are therefore least buffered against drought conditions.  Water quality is also an issue: many volcanic islands have highly permeable rock, which increases the potential for groundwater contamination.  In some Pacific Islands pollution problems reduce the ability of the system to provide clean, fresh water. 

Patterns of precipitation are important in determining whether islands have an adequate freshwater supply.  Long periods of rainfall are needed to recharge the freshwater lenses because short and light rainfall tends not to contribute to filling aquifers (ground water sources).  Land cover is also an important factor in how much water permeates into the ground or flows into rivers and streams.  If the land is covered by forest, the forest floor absorbs and holds the rainwater for drier periods, but if the forest has been removed by urban development, for example, the rain runs off faster leaving less for use during dry conditions.  On some islands, destruction of forest cover has caused many formerly year-round streams to stop flowing in the dry seasons and has contributed to landslides during periods of heavy rain.

Environmental and Societal Impacts

Pacific Islands:  Rainfall, stream flow, and groundwater are fairly abundant on the Big Island of Hawaii, but many areas are withdrawing water at rates close to the estimated renewable amount of the aquifers near the populated areas.  Guam and the island of Hawaii are examples of the distribution problem due to their “geographic mismatch -- between supply (available water) and demand (where it is needed).  For example, the "Hilo side" of the Big Island of Hawaii consistently receives abundant rainfall and is one of the wettest places in the Hawaiian Islands, while the "Kona side" (opposite coast) is where the majority of tourism development is occurring and has experienced significant periods of drought.  

Other islands that more easily access their adequate rainfall, face storage, distribution, and maintenance problems.  Pohnpei normally has an abundance of water, including waterfalls, springs, and rivers in the central areas.  Despite its abundant rainfall, the public water system in Kolonia, the largest town, cannot provide an adequate supply of quality water to residents under “normal -- circumstances.  The island lacks water storage and distribution systems and this lack causes severe impacts especially under drought conditions.  In the outer islands of the Marshall Islands and Federated States of Micronesia (FSM), residents rely on small rooftop catchment systems with only a few thousand gallons of storage capacity.  The residents of Colonia, Yap (FSM) depend on a small reservoir for water, but the reservoir, even if full at the beginning of a severe drought, dries up. 

Currently the Pacific Islands experience drought conditions every two to six years, usually associated with the El Niño/Southern Oscillation (ENSO) cycle.  The most extreme droughts appear to be associated with very strong El Niño events like those that occurred in 1982-1983 and 1997-1998.  Rainfall during the 1997-1998 event in some areas was well below normal for the period October 1997 through September 1998.  In some months, rainfall was as low as 3% of normal.  These low monthly totals meant trouble for islands relying on catchment systems. 

The water resources for many of the Pacific Islands could be most negatively affected by any increase in El Niño or El Niño-like conditions.  These increases could take many forms such as more frequent and/or severe El Niño events, medium-sized El Niños that last for multi-year periods, or other conditions that diminish precipitation over these islands, such as the “El Niño-like conditions -- that could result from increased ocean temperatures.  A secondary consequence of rainfall changes is migration.  If more frequent or more severe drought conditions occur over wide regions of the tropical Pacific, atoll populations could migrate in significant numbers to high-island population centers with more stable freshwater resources. 

The Caribbean:  In Puerto Rico and the US Virgin Islands, high populations and the conversion of tropical forest to other uses have affected the water cycle and water resources.  These changes have contributed to overuse of existing water supplies, filling of reservoirs with sediment (from erosion during heavy rainfall), and contamination of groundwater and surface water.  In the US Virgin Islands, and to a lesser extent in Puerto Rico, leaky septic tanks and inadequate sewage treatment facilities have contributed to reducing the quality of near-surface groundwater supplies. 

In addition, both Puerto Rico and the US Virgin Islands experience flash flooding and landslides that result from the combination of steep slopes and heavy rainfall typical of this region.  In the US Virgin Islands, buildings and roads have been built that cover many of the water channels that allowed water to be absorbed into the ground.  This not only diminishes water supply, but also greatly increases the flash flood hazard.  Poor drainage on floodplains has increased the potential for damage to these areas from a variety of flooding events.  Building in undesirable locations with inadequate construction techniques increases the risk to the population.  

In the Caribbean, tropical storms/hurricanes are significant natural hazards, mostly because of storm waves and high winds.  In coastal locations of Puerto Rico and the US Virgin Islands, saltwater intrusion (from rising sea levels and from storm waves) threatens the continued use of fresh groundwater and limits groundwater withdrawals.  

Puerto Rico has abundant ground-and surface-water resources due to relatively heavy rainfall over the mountainous portions of the island and porous rocks around the island's northern edge that forms a large aquifer system.  Reservoirs on major waterways (rivers and streams) collect runoff and are used for water supply, flood control, and limited hydroelectric power generation.  Ground water accounts for ~30% of the total amount of water used in Puerto Rico and surface water accounts for ~70%.  

The US Virgin Islands are much smaller, lower, and receive less rainfall and retain less fresh water.  The US Virgin Islands have no year-round streams and only limited ground water resources.  In the US Virgin Islands, 65% of drinking water supplies are provided by desalinating, removing the salt, from seawater.  Another approximately 22% of the drinking water supply is from groundwater and 13% from rooftop catchments. 

Economic Impacts

The Pacific

The 1997-1998 El Niño event offers a vivid example of the impacts -- both social and economic - that water shortages can cause.  In September 1997, the Pacific ENSO Applications Center (PEAC) predicted a near-record drought for Micronesia, beginning in the November-December timeframe and ending in the May-June (1998) timeframe.  As a result of the warning, the government of the Commonwealth of the Northern Marianas implemented a tight water-rationing schedule where each business and household was assigned “water hours -- of 2-4 hours each day.  By February, the island's rainfall level dropped to 25%, and the shortage was still projected to continue for many months. 

At the height of the drought, residents of Majuro were getting 7 hours of water every 15 days.  In the late winter/early spring of 1998, the Japanese and American governments provided numerous reverse osmosis (RO) units so the residents could desalt seawater.  In June rainfall increased and the RO units were no longer needed.

Agriculture throughout the US-affiliated Pacific Islands during 1997-1998 suffered everywhere from the droughts, except on Guam.  There, farmers used public water for irrigation, and the delay of heavy rains toward the end of the drought resulted in one of the most productive harvests in recent history.  In the Commonwealth of the Northern Mariana Islands (CNMI), citrus and garden crops were most affected by the drought, and the hospital had to buy imported fruits and vegetables rather than rely on local suppliers.  In Pohnpei serious losses of both food and cash crops were sustained.  Over half the banana trees had died or were considered seriously stressed.  The loss of sakau (kava) was probably the most serious economic loss because it had become a major cash crop for Pohnpei.  On Yap, taro losses were estimated at 50-65%, and betel nut prices increased more than 500%, although only 15-20% of the betel nut trees was lost.  In Palau, imported food shipments increased from twice a month to once a week.  During the drought wildfires were significant.  Guam, for example, experienced some 1400 fires and grass fire burned 20% of Pohnpei. 

The Caribbean

Droughts are frequent and severe in the US Virgin Islands.  Any minor depletions in rainfall dramatically affect agriculture and require water rationing.  Droughts are infrequent in Puerto Rico.  However, mandatory water rationing was implemented six times during the 1990s resulting in significant agricultural and other economic losses.  A drought in 1994-95 affected more than one million people who endured mandatory water rationing for more than a year.  Although Puerto Rico had seen lower yearly averages in rainfall in previous decades, the population growth changed the needed amount of freshwater.  Public works projects were initiated including a $60 million project to dredge the sediment from the Loiza reservoir and $350 million for a new aqueduct system to interconnect north coast water supply systems. 

Because the US Virgin Islands never has enough freshwater and 65% of the drinking water supplies are provided by desalination (removing the salt, from seawater) it is already the most expensive publicly supplied water in the United States.  Any reductions in the amount or type of precipitation will only increase those costs. 

Strategies to Address Potential Impacts on Freshwater

Strategies for providing adequate water resources under changing climate conditions for island communities vary in focus and in cost.  Some options, listed below, relate generally to water resource issues that are felt today as well as are likely in the future.  This list is not exhaustive and should be used as a starting point. 

• Improve rainfall catchment systems;

• Improve storage and distribution systems;

• Develop under-utilized or alternative water sources;

• Implement better management of supply and infrastructure;

• Increase water conservation programs;

• Construct groundwater recharge basins for runoff;

• Make more effective use of ENSO forecast information; and

• Apply the use of new/improved technology, such as desalinization where appropriate. 

For key sectors, integration of climate considerations and projections of sea-level rise into community planning, water management decision making, and tourism development would be important to long-term coping strategies.

Accurate assessments of current water budgets are essential for effective management of water resources, especially on small, densely populated islands with limited storage capacity.  Therefore, it would be useful for island communities to prepare water needs assessments for the future and prepare, test, and improve emergency/contingency plans for periods of water shortage before they occur.

Strategies for potential flood conditions are considered in the next section.