每個遠離大陸的海島上都有淡水嗎？島嶼上淡水的水循環是怎樣的？

11-27

世界上所有自然島嶼的淡水源都來自地下水。

夏威夷的淡水消耗有80%來自地下水。而地下水的源頭來自被土壤吸收的降雨。這就是為什麼很多海島都可以看見植物，如下圖的熱帶島嶼。任何陸地植物都仰賴淡水。而棕櫚樹之所以可以生長在海邊是因為它有極強的耐鹽水性，而不是他們可以靠海水生存。它們紮根的地下就是有淡水的。

根據蓋本-赫茲伯格（Ghyben-Herziberg）模型，一個小島的地下含水層（aquifer）可以吸收降雨，並且把這些淡水長期儲存。由於淡水的密度小于海水，因此淡水可以浮在海水上方，並向下擠壓鹽水層，使其遠離陸地表面。兩者可以分離的原因除了密度不同外，還有島嶼的存在防止了海浪對淡水和鹹水的混合。
如下圖可以看見圖中島嶼下顏色較淺的含水層是地下淡水，而較深顏色的是地下鹽水。兩者之間還有一層過渡層（brackish water，鹹淡混合層）。

根據這個模型，我們可以估計地下淡水層的深度大約是地下淡水水平面到海平面的距離的四十倍。由於這層淡水層的現狀很像一塊凸透鏡，這種地理現象又被稱為」蓋本-赫茲伯格透鏡「（Ghyben-Herziberg Lens）

所以只要島上居民挖一口深度處於淡水層的井就可以使用淡水了。當然，由於雨水是島上唯一的淡水來源，若是淡水消耗速度高於雨水的補充，地下水就有被抽乾的危險。如下圖

過量吸收淡水會使得地下鹽水層的水平面上升，井裡打上來的水就會變成鹽水。若地下鹽水層的水平面持續上升，就有可能產生海水倒灌的現象。

除此之外，更嚴重的是地陷。由於大部分沙土或黏土的體積和含水量成正比，若地下含水層的水被抽離，地平面就會下陷。這種現象很常見於濫用井水的城市地區，比如墨西哥城的地表相對於一百年前有很大程度的下陷。

所以對於島嶼城市來說，若是用水量超過了降水量，那麼依賴地下水就不是一個好主意。

參考資料拓展閱讀
What is the Current State of Fresh Water Supplies in Honolulu and Oahu: Will We Have Enough Water for the Future? a€」 Hawaii First Water
Saltwater intrusion
Posts about Ghyben Herzberg lens on Groundwater
新加坡最大海水淡化廠落成兩廠共可滿足1/4淡水需求

小島海水淡化居多，稍微大點有水。

舉個例子，馬代多數酒店島都是海水淡化，居民島也有很多是淡化，但是處理不好一股味。喝的多是瓶裝水

對比帛琉，因為主島有淡水，所以水費並不貴。

最後一句話，有電就行，有電就有水

不是的
外伶仃島，也就十來平方公里的大小，離珠海市區70分鐘船程，島上的淡水都是靠補給船供給的，因為島小也沒什麼循環系統（估計廢水是直接排海），所以島上的水也很貴（據說是十五元一方），連沖馬桶也是用海水。

不是！
相反，大部分島嶼奇缺淡水

啊哪裡來的數據？
我去過的大多數海島沒有淡水。海水淡化居多。

具體可以refer連接
What is the Current State of Fresh Water Supplies in Honolulu and Oahu: Will We Have Enough Water for the Future? a€」 Hawaii First Water

夏威夷是島吧？夠經典夠著名吧？
來這邊是栗子……

Executive Summary;
Within our grandkids』 lifetimes, Hawaii』s fresh water supply will likely not be able to handle the water demands of Hawaii』s increasing population due to decreasing aquifer levels and increased drought conditions due to long term climate changes.

Oahu』s aquifers and streams have already begun to decline over the past decade. Expanding paved urban areas and decreasing agricultural production particularly on the island of Oahu–where a majority of the island』s population resides–will also further reduce fresh water resupply to the aquifers.

Despite Hawaii』s growing population, the demand for fresh water to consumers has remained stable or reduced for reasons that are yet unknown to water officials.

Future scarcities can be forestalled by increasing conservation measures, more gray water reuse and eventually ocean water desalination which will come at an increased cost to future Hawaii economic growth and prosperity. Impending fresh water scarcities in Hawaii will favor those businesses and individuals who are taking a proactive approach to providing for their future fresh water needs.

1. Where does Honolulu and Oahu gets most of its water?
Much of the population and economy of the islands is centered in Honolulu on the island of Oahu. Most of Honolulu』s consumers get their fresh water from the island』s extensive aquifer systems. Despite having the 「wettest spot」 of the world in the Hawaiian islands in Kauai, streams and other fresh water bodies are not reliable sources for fresh water in Hawaii. The Honolulu Board of Water Supply uses four shafts, 12 tunnels and 84 well stations around the island of Oahu to tap water from the aquifers. Water from these dike tunnels and wells is fed into nearly 2,000 miles of transmission pipes to household users.[1]
What are Fresh Water Aquifers?Aquifers are permeable rock formations from which fresh water can be drawn. In some cases–in Hawaii–some of the wells are artesian which means water is naturally under pressure and flows to the surface without pumps. This water is also sometimes called 「fossil water」 because it has taken hundreds or millions of years to filter down to the aquifer rock. This water is not easily replaced if overdrawn. Additionally in Hawaii, salt water intrusion caused by rising sea levels is also a potential concern for adequate future fresh water supplies. In the early 90』s, most of the islands』 aquifers had their boundaries defined and their 「sustainable yields」 (SY) were characterized to build baselines on their sustainability.
2. What are the population trends for the islands?
The population of the State of Hawaii will continue to grow into the future placing greater demands on the state』s freshwater supplies. The population of city and county of Honolulu will increase from about 876,000 (in 2000) to about 1,117,000 in 2030 increasing at about .8%, according to the State of Hawaii government.[2] These projections are on the conservative side anticipating that the number of military personnel will probably decrease due to budgetary pressures; however many of the military bases draw their water separately from the municipal systems although from the same aquifers.
3. What are the rainfall trends for the Hawaiian islands?
The best long term climate models for rainfall predict that the Hawaiian islands will generally be get dryer over the next decades, according to a recently released study from the US Government. For Hawaii, downscaled statistical models predict a 5%-10% reduction for the Hawaii wet season and a 5% increase in the dry season by the end of the century.[3] The reduction in rain overall will increase the demand for fresh water from current rain fed irrigation systems placing increased strains on the aquifers. Additionally people using rain fed household water systems such as those on the Big Island of Hawaii may seek alternative water systems such as wells and aquifers.
4. What do the records say about the amount of water in the aquifers feeding Honolulu?
Generally freshwater aquifers on Oahu are in a state of very gradual decline, according figures from the Honolulu Board of Water Supply and US Geological Survey.[4] This is especially troubling since the Honolulu』s public demand for fresh water also leveled off during this time despite the addition of more consumers. This suggests that the aquifers are incapable of maintaining this level of draw, a concern with the .8% annual population increase suggested for future Oahu.
An examination of US Geological Survey well data on Oahu also suggests fresh water well head declines in about half of the 15 sample USGS wells. The accuracy of this examination is also significantly hampered by the large reduction of monitored wells due to budgetary constraints in the federal government. Online records only suggest only two of the 21 wells are currently being monitored, although some records may be held locally according to the USGS.[5]
A similar picture of significant declines in water flow emerges when 20 of the main steams on Oahu are compared. With the exception of the Manoa stream, all streams have declined by about half from 2004 to 2010.[6] All together streams on Oahu carried 259 million gallons a day (mgd) in 2004 and this was reduced to 121 mgd in 2010. In addition to carrying storm water overflow after storms, the rivers function as a natural outlet for the island』s artesian springs and aquifers.

5. What is the rate of water use in the City and County of Honolulu?
Municipal household water consumption has leveled off over the past decade on the island of Oahu since 2001 despite the nearly 10% increase in water users.[7] Potable water use from 1990 through 2009 averaged about 155 mgd.[8] Since these figures do not include the large agricultural users which have reduced over the past decades, most officials are at a loss to explain why per person water use has declined especially because water rates and costs have been stable for most of this period. One possible answer for the decreased use of consumer water is the prevalent use of low flow showers and toilets.
6. So will Oahu have enough water for its future needs?
The island of Oahu has sufficient fresh water to supply its near term needs but will begin to strain its fresh water supplies within 100 years. The Hawaii State government estimates that the population on Oahu will increase to about 1,130,000 by 2030 which will demand approximately 206 mgd. According to the Board of Water Supply, municipal users use about 76% of Oahu』s ground water compared to 24% for agriculture, military and other private users.

If this 24% of military and private users is added to the future 2030 predictions of 206 mgd for municipal users, the total would be 274 mgd for the island of Oahu. Not including brackish water supplies, the Board of Water Supply estimates that it should be able to sustain 407 mgd from its fresh water aquifers, according to its best data.

Using the State of Hawaii』s own population 「high growth」 scenario for Oahu, the island could begin to reach this 407 mgd maximum within a century.[9]

The estimate of 100 years is probably on the high side. There is a large degree of uncertainty about the real sustainability of Oahu』s aquifers.

Especially since the aquifers have been gradually declining even though consumers are using fresh water at well below max sustainable rates indicating they are not replenishing even at with this moderate rate of use.

This may be due to the gradual urbanization of the island which is increasing the area of non porous surfaces; such as roads and cityscape and this trend will only increase in the future. The increase of non porous surfaces causes rain to more rapidly run into the oceans instead of seeping into the ground.

Finally the impact of climate changes will almost certainly bring worsening conditions to the aquifers in the form of less rain and higher sea levels which will gradually increase the salinity levels of the aquifers making more of the water unusable without treatment.[10]
7. What if the estimates prove wrong and the aquifers begin to run dry earlier, what can be done to meet the future fresh water needs of Oahu?
Because of the location of the Hawaiian islands in the middle of Pacific Ocean, the islands do not have the backup reserve of water supplies that say Los Angeles or other mainland areas have. Additionally the supply of water has been deemed a consumer 「life」 service which no household consumer can be deprived of due to their inability to pay which means that a delicate balance of use and conservation must be maintained.
Conservation measures is the best way to begin to control the use of fresh water by island consumers. One of primary means is by conducting household water use surveys and ensuring that all households have meters to understand their water use baselines.

According to a California water saving survey report, metering will generally save about 20-30% overall and residential awareness surveys will generally achieve 32.2 gallons per day (gpd) of savings.[11]

Low flow toilets will generally save 21.2-27.2 gpd per single family household.

High efficiency washing machines will save about 85-109 gallons per week per household.

Low flow shower heads (5.5 gpd), toilet dams (4.2 gpd) and aerators (1.5 gpd) making up the difference.

Water reuse and the use of brackish (somewhat saline) water is another means to increase the 「effectiveness」 of water. Hawaii has begun to reuse some of its water on golf courses and for power plant cooling. Increasing the use of 「gray water」 from showers and washers will require some minor though not insignificant modifications of households to reuse the water to flush toilets and water gardens. Some municipalities have begun to filter their gray and even toilet sewer water for reuse, but such uses involve extensive public education programs to overcome the 「ick」 factor.

Because of Hawaii』s extensive rainfall in some parts of the islands, increased use of stormwater mitigation strategies can effectively control 「brown water」 run off and potentially become a source of increased fresh water on the islands. Stormwater run off can be funneled into rain barrels for household use and may be used to help reinvigorate aquifers in some limited circumstances. Finally, Hawaii might have to increase the historical use of cross island pipes and troughs to move water from the eastern wet sides to the western drier areas of the islands to help with distribution of water when it is present.
Desalination is increasingly becoming more technically and economically viable. Some municipalities in southern California are using economically viable 「reverse osmosis」 filtration methods to create fresh drinking water. Because of its very arid conditions and the wealth of the its government coffers, Middle Eastern countries such as Saudi Arabia have been using desalination technologies for drinking water for a number of years. On some Hawaii islands wind energy creates power when demand is low, Hawaii could begin to use this surplus energy to desalinate sea water in the future when supplies are scarce.
[1] Honolulu Board of Water Supply one pager on 「How Does Water Get To Your Tap?」 from BWS website.
[2] Honolulu Board of Water, 「Ko』olau Poko Watershed Management Plan,」 19 December 2011.
[3] National Climate Assessment 2014, Chapter 23, Hawaii and US Affiliated Pacific Islands, 2014, page 542.
[4] Minutes for the Honolulu Board of Water Supply from 2005 to 2014.
[5] USGS National Water Information System, USGS Water Resources Database, as of 7/2014.
[6] Honolulu Board of Water Supply, 「Ko』olau Poko Watershed Management Plan,」 12/19/2011, pg 1-35.
[7] Honolulu Advertiser, Oahu Drinking Water Use Down 7% since 2001, 9 Aug 2009.
[8] Honolulu Board of Water Supply, 12/19/2011, 1-20

[9] State of Hawaii estimates total aquifer capacity at 407 mgd minus 68 mgd (ag/mil use) minus 212 mgd (2030 est) =127 which is divided by SOH estimate of 1.6 mgd increased use per year gives 80 years from 2030 or 2110 till demand equals projected sustainable yield
[10] National Climate Assessment 2014, Chapter 23, Hawaii and US Affiliated Pacific Islands, 2014, page 542.
[11] A and N Technical Services, Analysis of Urban Water Conservation Best Management Practices prepared for The California Urban Water Conservation Council, March 2005.

小海島也就指望降水了，地下水鹽化很快的。

大部分島嶼很小，小到沒有溪流湖泊存在

不是的。

我家這邊淡水大部分是海水淡化的。