water in the atmosphere

1: Water in the Atmosphere

Hawaii's fresh water supply is intimately dependent upon a continuous chain of events called the water cycle. The atmosphere, ocean, and land -- as well as the sun and other elements of environment -- are linked to replenish this island's fresh water supply. A healthy water cycle is essential. Without it, no fresh water would be available and no living things, including humans, could survive on our islands.

Naturally occurring activities in the atmosphere play vital roles in our water cycle. Here in the Hawaiian Islands, where it is especially critical for water to move from ocean to land and back again, there is a finely orchestrated interaction between ocean water, evaporation, trade winds, and precipitation.

Northeast Trade Winds

Hawaii's fresh water supply and balmy climate are intimately dependent upon massive systems of moving air called trade winds, which blow across subtropical regions of the Pacific Ocean much of the year. In the Northern Hemisphere, trade winds blow from the northeast, bringing mild days and nights and an abundance of moisture to landmasses like the island of Oahu.

Trade winds traveling across the ocean pick up moisture that has evaporated from ocean water warmed by the sun. On Oahu, the moisture-laden trade winds arrive from the northeast, on the island's windward side, and are deflected upward when they reach the steep cliffs of the Koolau Mountains.

How Mountains Affect Rainfall

On older, eroded islands like Oahu and Kauai, steep mountains play a vital role in ensuring abundant rainfall. On Oahu's windward side, for example, moist trade winds arrive onshore from the northeast, then hit up against the steep slopes of the Koolau Mountains. This deflects the moist air upward. As the rising moisture nears the mountaintops, it cools, forms clouds, and then condenses and falls as rain to the ground.

If Oahu had no mountain ranges and flat plains, the island wouldn't be able to capture enough moist air from the trade winds to support the variety of living things that live here now. The semi-arid island of Niihau, for example, receives very little rainfall because it lacks high elevation slopes. Without slopes to divert air upward, winds cannot push the moisture to high elevations where it can be drawn from the air, resulting in the formation of clouds and precipitation.

But don't think that all windward peaks are lush and green. On young, extremely high mountains, such as the Big Island's, clouds drop their precipitation before they are pushed on up to the highest elevations. This leaves the upper peaks dry and desert-like.


Precipitation is water, in any form, that condenses from clouds and falls to earth. Rain is the predominate form of precipitation in our islands, although dew and fog drip also add water. During winter, snow, freezing rain, and sleet occasionally fall on the high slopes of Mauna Loa and Mauna Kea on the Big Island, and on Haleakala on Maui. Across the Hawaiian Islands, the amount of precipitation varies widely from 20 to more than 300 inches a year, depending upon location.

In a normal day, about two billion gallons of rain fall on Oahu! Almost a third replenishes the island's aquifer, another third nurtures vegetation and evaporates and a third is discharged as runoff into the ocean.

Most of Oahu's precipitation occurs on the peaks and northeast-facing slopes of the Koolau Mountains. On average, more than 250 inches of rain fall upon the peaks of this range each year. That's far more than the meager amounts that usually fall upon the arid leeward areas of the island.

The peaks of the Waianae Mountains receive comparatively less moisture than the Koolau. The Waianae range is located in the "rain shadow" of the Koolau, which blocks and captures most of the moist air that move along Oahu's windward side. Little moisture is blown farther on to areas south and west of the lush tall Koolau.

Evaporation Transpiration and Runoff

Evaporation occurs when water is heated and changes from a liquid to a vapor, or gas. In the Hawaiian Islands, where we are surrounded by water and have plentiful sunshine, most of the evaporation in our water cycle takes place offshore. Ocean water is warmed by the sun, then water molecules evaporate in the form of vapor, rising into the atmosphere. Aloft, the moisture is blown onshore by trade winds. On land, fresh water from streams, ponds, marshes -- even puddles -- also evaporates and contributes to the water cycle.

Transpiration is a process closely related to respiration. Plants absorb water from the soil, then some of that water leaves from the pores of the plant's leaves and stems and rises into the atmosphere. In areas with abundant foliage, like on the mountaintops of Oahu, transpiration is a key component in the water cycle.

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water on the ground

2. Water on the Ground

Isolated from its nearest neighbor by ocean water, each island in the Hawaiian chain has a finite amount of ground surface onto which precipitation can fall. The plants and animals on Oahu, for example, depend upon the island's mere 604 square miles to serve as catchments for all the life-giving fresh water they need for survival.

The moment rain fall makes contact with the ground, it may become surface water, a vital link in the island's water cycle. The path the surface water takes from that point -- either above ground as surface water, or below as groundwater -- depends upon a complex interplay among geologic, atmospheric, and environmental factors.

The Watershed

A watershed is an area of land, encompassing a mountain or valley, that catches and collects rainwater. The surface features, and geologic character, of that land influence whether rainwater moves toward the sea via rivers and streams or by movement underground.

The watershed is Oahu's collection basin. The watershed catches, collects, and stores water.

Oahu has two main regional watersheds: one in the Koolau Mountains and another in the Waianae Mountains. The Koolaus run perpendicular to the Northeast trade winds and experience the heaviest rainfall. The Waianae peaks, though higher, sit in the Koolau rain shadow and receive far less rain, even on their windward slopes.

The Rainforest

A rainforest is a lush forest ecosystem in which rainfall is abundant throughout the year. The plants that live in the rain forest provide our essential components of a healthy watershed.

From trees that create a broad canopy cover to the miniscule ferns and mosses that blanket the forest floor, all of the rain forest plants catch rain and dew and their roots stabilize the upper soil layers, letting rainwater gradually filter through to deeper layers of earth. The forest growth also stabilizes stream banks, and prevents erosion during surface water flow.

Roles of Trees, Shrubs and Plants

Hawaii's native forest have evolved over millions of years to become highly effective watershed covers. Within every healthy forest, there are five distinct horizontal levels, from the treetops to the ground cover. Vegetation exists within each level, soaking up rainfall like a multi-tiered, giant sponge, allowing water to drip slowing into the ground and into streams.

  • Emergent Trees: Tall trees, such as koa, are the first to intercept heavy raindrops, absorbing the energy and force of their fall. The leaves from these trees pull moisture from passing clouds via condensation or fog drip, and the water runs from leaves, down branches, to plants at lower levels.
  • Canopy Trees: Canopy trees, like ohia, catch the majority of raindrops, plus additional fog drip. Water flos along the tree's branches to its trunk, and the bark texture slows the rate of flow.
  • Subcanopy Trees and Shrubs: This level of the forest, which may contain plants such as kawau and hapuu tree ferns, absorb tree drip from the layers above. It holds much of the water in its vegetative structure and passing some flow along plant stems to the ground. Subcanopy plants keep the air near the ground water-saturated, which slows evaporation from the ground layer.
  • Understory: Ferns and shrubs like the ohelo and puahanui absorb water from higher plants and as well as the energy of dripping water.
  • Ground Cover: Mosses and small ferns such as ekaha form a spongy absorptive layer just above the soil. They inhibit evaporation from the ground and prevent soil erosion.

Negative Impact of a Degraded Rainforest

When a rain forest is degrade, there are innumberable negative consequences that affect the area's water resources.

Rain falling on bare earth causes erosion. This means that the water-retaining upper soil layers will wash away, leaving behind less permeable, or water absorbing, clays. Water runs off this dense, impermeable surface rather than filtering down to replenish the aquifer.

Streams that originate from deforested mountains usually flood during rainfalls. The loss of stabilizing tree and plant roots results in landslides. And debris carried by streams ends up in ocean coastal areas, causing siltation of coral reefs.

When a native forest is eroded and damaged, opportunitistic foreign species invade. While these new plants can stabilize bare ground, the watershed cover they create is not effective as that of the native forest.

Restoring the Watersheds

By the beginnings of the 1900s, thousands of acres of Oahu's watershed forest had been destroyed. Sugar planters, who used most of the island's water, were the first to act to protect the watershed. In 1903, the Hawaii Sugar Planter's Association supported the creation of a Territorial Board of Agriculture and Forestry. Mandated to save the watersheds, Forestry staff fenced forest reserves, eliminated feral animals, and reforested large tracts of land. Initially, the planters used native trees like ohia and koa, but these grew too slowly to reverse the erosion and water loss that were prevalent on Oahu. Foresters soon started planting faster-growing nonnative species such as eucalyptus, ironwood, silk oak, and Norflow pine.

Emergency conservation workers planted trees for the Forestry Board through the 1920s, but during the 1930s, federal funds paid for Civilian Conservation Corps workers to do the job. In 1945, a government act directed the Board of Water Supply to safeguard Honolulu's watersheds, and by 1946, the Board had acquired over 240 acres of reserves in Kalihi, Pauoa, and Manoa.

The conservation ethic continues today. Oahu now has 32,500 acres of forest reserve. The Board of Water Supply recently joined six major landowners, who collectively own 80 percent of the Koolau watershed, and formed the Koolau Mountains Watershed Partnership. All partners have agreed to work together to protect this precious resource.


High on the lush mountain slopes of the Hawaiian Islands, rainwater falls from the sky and begins a long and winding journey toward the ocean. Much percolates into the ground, but nearly one-third of rainwater at these elevations collects on the surface and travels downward toward the ocean. These flowing bodies of water are called streams.

Most of the natural streams in Hawaii occur on the windward sides of the islands, where rainfall is most plentiful due to the moisture brought by trade winds. On Oahu, streams can blanket the northeastern slopes of the Koolau and Waianae ranges, but they are rare on the arid southern and leeward slopes.

Stream water follow the path of least resistance as it flows from higher to lower elevations. It's no wonder streams from and travel in canyons, crevices, and other V-shaped land formations. After a heavy downpour, new streams-called intermittent streams-sometimes take shape almost instantly. You can easily spot this type of stream crashing over the steep cliffs of the Koolau Mountains in a long, needle-like formation.

At lower elevations, swiftly rushing intermittent streams sometimes meet up with established streams and cause overflowing of banks and dangerous flash floods. Once they've made their way down from the mountains, streams take various routes to reach the ocean. Some are channeled into manmade canals or ditches.

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water below the ground

3: Water Below the Ground

Once water beings to make its way below ground, it joins a vast and complex subterranean network of geological structures. These formations and the cracks, crevices, and pockets between them channel water, store water, purify water, and ultimately allow water to once again emerge from below groun as it continues its never-ending journey in the water cycle.


Fresh water travels down into the earth through a process called percolation. On the Hawaiian Islands, water first percolates through soil, if present, then through porous volcanic rock to the water table within the lavas.

Water's journey is a long one, with many twists and turns through the maze of underground rock structures. One raindrop's trip from the top of the Koolau Mountains to the aquifer below can take 25 years!


Fresh water travels down into the earth through a process called percolation. On the Hawaiian Islands, water first percolates through soil, if present, then through porous volcanic rock to aquifers, which are deep reservoirs within porous rock.

Perched Water

Sometimes percolating water becomes trapped when it meets layers of fine volcanic ash or clay-like soil that occur between the remnants of Hawaii's ancient underground lava flows. This perched water can no longer seep downward, so it collects and moved sideways, sometimes appearing as a spring. At its relatively high elevation above the water table, perched water can be a locally valuable source.

Dike Impounded Water

Groundwater in the Hawaiian islands often becomes trapped in massive vertical compartments formed by volcanic dikes.

During the volcanic eruptions that created the Hawaiian Islands, molten rock beneath the surface flowed up from the center of the volcanoes, then spewed out as lava that oozed its way down slopes toward the ocean. Both lava and magma eventually hardened, forming the foundation of our islands.

Dikes formed when magma stopped flowing to the surface, then cooled over time to form dense, nonporous rock in vast, miles-long sheets aligned vertically in the rift zones. Intruded into the porous lava flows, dikes form walls of compartments. In some places, water rises up to 1,000 feet in elevation.

Fresh water percolating down between the dikes compartment becomes trapped between the nearly impenetrable walls of the dikes. The water can only escape when its level rises and overflows the walls of the dike, or when great internal pressure causes leakage. Sometimes a freshwater spring will form above ground when such water spews from a dike.

Fresh Water

The lens-shaped body of fresh water that exists within Oahu's porous volcanic rock is called an aquifer, or fresh water lens. This water is among the cleanest anywhere, having been purified through years of percolating downward through soil and volcanic rock. It is the source of water for many wells and springs.

The fresh water lens is held in place by the island's outlying, underwater caprock that reaches inland from the shoreline. Because of the lower density of this fresh water, it floats on a broader lens of denser salt water beneath it much like an ice cube in a glass of water. Occasionally, fresh water is released from the aquifer in the form of a spring that breaks through the caprock barrier, or through the rock crust, and flows onto the surface.

Historical Over-Pumping and Fixes

Although the quantitative information is unavailable because water use was essentially unregulated and unmeasured until formation of the Board of Water Supply in 1929, the ensuring years after about 1890 was a period of waste of the resource. During this period water levels fell drastically, artesian flow diminished, and wells were abandoned due to salt content. Leaky wells were required to be repaired or sealed, as were abandoned wells. Meters were installed to record water use from wells as well as water use by residents and businesses from the municipal system. These measures permitted a stabilization of the resource until the jet age brought about prosperity and growth that required increased water supply. Water use in agriculture, the largest user of water until the 90's, and from the municipal system increases when rainfall is sparse. In virtually every decade of record, there has been up to three consecutive years of lower than normal rainfall with high water use. These years probably experienced greater than normal pumping. Overpumping, however, for a period of time is not necessarily bad if this can be offset by sufficient reduced pumping during wet years when recharge is greater and water use lessened. Whether ground water recharge from the other years have been sufficient to offset the overuse was not clear until data from deep monitor wells penetrating into salt water has become available. The wells indicated the balance of the fresh water lens in response to pumping of ground water over time. For four areas where deep monitor wells were located, three of the areas show shrinkage of the fresh water lens and confirm overpumping over time. With the recent expansion of the monitoring system to the remainder of the island, we will be able to better determine overpumping on an island-wide basis.

Transition Zone

Brackish water -- a mixture of fresh and salt water -- occupies the transition zone where fresh water meets the underlying saltwater. The thickness and concentration of this zone is continually in flux. The transition zone is affected by natural seasonal influences and pumping or drafting of fresh water to meet the population's needs.


Dense salt water from the ocean occurs beneath the brackish transition zone. This salt water saturates the porous volcanic rock below the transition zone and the fresh water in the aquifer.

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water to your home

4. Water to Your Home

How We Deliver Water

Water to your home is extracted from the ground from a network of shafts, water tunnels, and many wells located all over Oahu. Except for the North Shore area that is only connected from Waialee to Waialua, an interconnected system from Maakua to Makaha services nearly 98% of the island residents. Mains, booster systems, and reservoirs convey and store water for home use. From the mains, service connections are made to homes through a water meter. Water quality meets all federal and state standards.

When you turn on your tap, you become part of the extraordinary process that beings deep beneath the island of Oahu. Water is pumped from the aquifer through wells, shafts, and tunnels. Once it reaches the surface, the water enters an island-wide transmission system. With this massive system, water is moved from pumping stations -- sometimes via booster stations -- to mains and reservoirs, where it is stored until needed by homes and businesses.

water transmission diagram

The BWS pumps an average of 150 million gallons of water a day through its extensive system. It maintains over 1,900 miles of pipes, 6 shafts, 18 tunnels, 82 well stations, 192 wells, and 169 reservoirs.

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For More Information

Water for Life
Oahu's Water Cycle and You