Honouliuli Preserve

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Watershed
Honouliuli Preserve

• Has dark fertile lands that stretch from the
  waters of Pearl
• Harbor to the summit of the Waianae Mountains

• 70 rare and endangered plant and animal species

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West Maui Mountains Watershed

• Biologically diverse and pristine in the islands
• Threatened by invasive species like insects and
  plant diseases

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Acid Rain in Hawaiian Isles is caused by
emissions from eruptions
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Kapunakea Preserve

• Has 24 species of rare plants, including four
  endangered species

• Has only known kauila tree of its kind on Maui

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Watershed
Lanai Forest and Watershed Partnership

• Ensures the future supply of water for the
  island of Lanai,

• Protect the health of near-shore waters,
  fisheries and beaches

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Irrigation Agriculture
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Water Scarcity
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Hydrological Cycle
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The hydrologic cycle on an oceanic island is one
of constant motion and transformation. As water
changes form through evaporation, condensation,
melting and freezing, energy is released and
absorbed, linking water to the environment's
larger energy cycle The Hawaiian islands are
geologically youngest in the southeast and oldest
in the northwest. Climatic EffectsThe Hawaiian
islands are near the northern margin of the
tropics, and because of the prevailing northeast
tradewinds and the buffering effect of the
surrounding ocean, air temperature at a given
location in Hawaii is generally equable. At the
Honolulu International Airport, for example, the
warmest month of the year is August, which has a
mean temperature of 80.5 degrees Fahrenheit, and
the coolest month is February, which has a mean
temperature of 72.0 degrees Fahrenheit. Air
temperature can vary greatly from one location to
another in Hawaii. The air temperature in the
eight-island group can range from about 95
degrees Fahrenheit at sea level to below freezing
at the top of some peaks on the island of Hawaii.
In the geologic past, these peaks have been
glaciated
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Northeasterly tradewinds are present about 85 to
95 percent of the time during the summer months
(May through September), and 50 to 80 percent of
the time during the winter months (October
through April). The tradewinds are occasionally
interrupted by large-scale storm systems which
pass near the islands. The southwestern parts of
some islands receive most of their rainfall from
these severe storms, which produce a relatively
uniform spatial distribution of precipitation. In
general, the northeastern, or windward sides of
the islands are wettest (fig. 34). This pattern
is controlled by the orographic lifting of
moisture-laden northeasterly tradewinds along the
windward slopes of the islands. The winds blow
across open ocean before arriving at the islands
when the moisture-laden air mass rises over the
mountains, the moisture condenses as
precipitation. Maximum rainfall occurs between
altitudes of 2,000 and 6,000 feet above sea
level, but exact amounts vary depending on the
form, location, and topography of each island.
Above 6,000 feet, precipitation decreases and the
highest altitudes are semiarid. High mountain
areas are dry because the upslope flow of moist
air is prevented from penetrating above altitudes
of about 6,000 to 8,000 feet by a temperature
inversion. Areas that are leeward (southwest) of
mountain barriers are generally dry because air
is desiccated during its ascent over an upwind
orographic barrier. This is known as the
rain-shadow effect.
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Orographic Precipitation As this air rises up
the mountain cliff, it suddenly contacts a colder
air mass. Its moisture condenses resulting in
rainfall. This is called the Orographic
Precipitation. Though the outer margin of the
Windward Oahu receives only about 75-100 mm (3-4
in) of rain per year, the ridge crest of the
Koolau Mountains receive 380-500 mm (15-20 in)
per year. Occasional severe winter storms release
large amounts of rain that cause flooding that
can severely impact the Windward Oahu.
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Orographic Precipitation The Koolau Mountain
Chain forms a topographic ridge with sharp
cliffs, Pali, perpendicular to the prevailing NE
trades. As the trades winds encounter the
ridge-cliffs, their moisture-laden air is forced
upward suddenly and swiftly. This updraft is so
strong near the Nuuanu Pali where the highway
crosses the cliff that it sometimes creates an
"upside down waterfall". The wind blows water
from existing falls upward during many days of
the year (Carlquist, 1980). During rainfall this
updraft may even make raindrops appear to fall
upward. Liquid Sunshine on the leeward Oahu
is similar ly caused by the orographic
precipitation.
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On Kauai, the island summit receives more than
435 inches of average annual rainfall (1916-83).
West Maui has a small area where average annual
rainfall is greater than 355 inches. Average
annual rainfall is greater than 275 inches on the
northeastern parts of Maui and Oahu, and greater
than 235 inches on the northeastern part of the
island of Hawaii. Because the island of Lanai is
in the rain shadow of Maui and Molokai, it
receives much less rain than the larger islands.
Most of the southwestern coastal areas of all
islands receive less than 40 inches of rain
annually the island of Hawaii has areas at high
altitudes that receive less than 20 inches. Two
rainfall seasons are typical-a wet season during
the winter months from October through April and
a dry season during the summer months from May
through September. An exception is the western
side of the island of Hawaii, where summer months
are wettest because of a thermally driven sea
breeze.
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Evapotranspiration, which is the loss of water to
the atmosphere by the combination of
transpiration of plants and direct evaporation
from land and water surfaces, is a major
component of the hydrologic budget of the
islands. In the Honolulu area of Oahu, for
example, actual evapotranspiration was estimated
to be about 40 percent of the total water
(rainfall plus irrigation) falling on or applied
to the ground surface during 1946-75. Pan
evaporation is the main measurement used in
Hawaii to assess the amount of water loss by
evapotranspiration. Over the open ocean, the
estimated annual pan-evaporation rate is 65
inches. As with precipitation, pan-evaporation
rates in Hawaii are related to topography. At
altitudes between 2,000 and 4,000 feet, where
humidity is high and sunlight intensity is
reduced because of clouds, pan-evaporation rates
are reduced to as low as 25 percent of the
open-ocean rate. In the leeward coastal areas,
wind carrying dry, warm air increases annual
pan-evaporation rates to as much as 100 inches.
At the summits of Mauna Kea and Mauna Loa on the
island of Hawaii, annual pan-evaporation rates
exceed 70 inches because of clear skies and dry
air. The amount of recharge available to enter
the aquifers on an annual basis is about equal to
average annual precipitation minus water losses
(average annual runoff and evapotranspiration).
Runoff is directly related to rainfall,
topography, soil type, and land use, and ranges
from less than 5 to as much as 200 inches per
year. Runoff typically averages about 10 to 40
percent of the average annual precipitation, but
is greater than average where precipitation is
high and slopes are steep and where precipitation
falls on less-permeable land surfaces. Runoff is
less than average where low amounts of
precipitation fall on gentle slopes or where
precipitation falls on highly permeable soils or
rocks. Streams generally are small and have steep
gradients, and many flow only immediately after
periods of rainfall. Some streams, however,
receive water from aquifers and have perennial
flow.
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On young, high mountains such as the Big
Island's, clouds drop their precipitation before
they are pushed to the highest elevations,
leaving the upper reaches dry and
desert-like. On older, eroded islands such as
Oahu and Kauai, rainfall is heaviest on the
windward slopes and mountain peaks, allowing lush
vegetation to cover even the highest ridges. A
relatively flat island such as Niihau has very
little rainfall because it lacks the high
elevation slopes. Without the slopes, winds
cannot push moist air upwards to produce clouds
and precipitation.
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Hydrological Cycle
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Geological History of Windward Oahu The
difference between the leeward and windward sides
of the Koolau Range is striking. The long, gentle
slopes of the leeward Honolulu side terminate in
vertical cliffs 0.8 km (0.5 mi) high on the
windward side. This cliffline, or Pali, extends
for 32 km (20 mi) along the windward side of
Oahu. The character of the cliff changes
northward along the Pali due to two different
agents of erosion. Massive sea cliffs in some
places are formed by wave erosion. Vertical walls
in other places are formed by stream erosion.
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Sources of Surface Water - Streams The landforms
of surface water drainage basins reflect the
geologic age and stream erosion in different
parts of the Hawaiian Islands. Watersheds are
typically small. Of the streams gauged on Oahu,
80 percent have drainage areas smaller than 13
sq. km (5 sq. mi. or 3200 acres). The relief of
the watershed land and the stream channel is
steep, especially in the headwaters. Stream
channels are short and lack storage, and at times
of heavy rain they are liable to flash
floods. Many streams are nearly perennial in
mountainous areas where orographic rain occurs
daily. Streams that traverse dry lowland and
coastal plains tend to lose water as it seeps
into the porous soil. Stream water is usually
tapped by diversion in rainy headwater areas and
transported long distances from the watershed.
Stream water quality is generally lower than that
of groundwater in terms of turbidity, nutrients,
and coliform, especially during wet weather
periods. Outside of the forest reserve headwater
areas, urban and agricultural activities
introduce pollutants such as traces of pesticides
and heavy
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Large volumes of runoff can also transport very
large quantities of sediment. Sediment load
varies with the amount and rate of rainfall. The
seven streams were estimated to average 1.8 x 105
kg (200 tons) of terrigenous sediment per day,
with an average annual yield of 6.6 x 107 kg
(73,000 tons) (Jones et al., 1971). Fan (1973)
estimated that one of the larger streams alone in
flood carried 8.9 x 106 kg (9800 tons) of
sediment per day. Channelization of streams and
removal of vegetation for urban and agricultural
development have increased freshwater runoff and
erosion in the bay.
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Watershed, Catchment Area, Drainage Divide
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Watershed and Preserves In Hawaii
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Precipitation 2,000 Million Gallons
average Daily MGD Evaporation 500
MGD Plant Transpiration and 500 MGD Soil
Moisture Runoff to ocean 500 MGD Percolation
to Groundwater 500 MGD
Oahu Water Budget
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Water Supply 371 Million Gallons
average Daily MGD Evaporation MGD Plant
Transpiration and MGD Soil Moisture Runoff
to ocean MGD Percolation to Groundwater
MGD
Kauai Water Budget
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Precipitation 325 Million Gallons
average Daily MGD Evaporation MGD Plant
Transpiration and MGD Soil Moisture Runoff
to ocean MGD Percolation to Groundwater
145 MGD
Lahiana Water Budget
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Precipitation 522 Million Gallons
average Daily MGD Evaporation 274
MGD Plant Transpiration and MGD Soil
Moisture Runoff to ocean 89 MGD Percolation
to Groundwater 189 MGD
Molokai Water Budget
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Water Cycle A part of water cycle and water
budget can be described for Oahu. The water
budget pattern varies seasonally, at different
times and places according to differences in
atmospheric conditions, landforms, soils, and
rainfall. The cycle and water budget is also
modified by human activities, such as diversion
of stream water for irrigation, loss of
groundwater from wells, alteration of
infiltration by resurfacing the land, altering
evapotranspiration and runoff patterns by
agricultural and urban development, and disposal
of sewage effluent into the ocean.
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Watershed
watershed is an area of land, such as a mountain
or a valley, that catches and collects rainwater.
Topography influences whether rainwater moves
toward the sea via rivers and streams or movement
underground.
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Watershed

• Oahu has two main watersheds
• Koolau Mountains Catchment Area The Koolau
  run perpendicular to the Northeast trades and
  experience the heaviest rainfall.
• Waianae Range Catchment Area.
• The Waianae peaks, though higher, sit in the
  Koolau rain shadow and receive less rain, even
  on their windward slopes

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What Happens When the Rain Forest is Degraded?
When a forest is degraded, rain falling on bare
earth causes rill-erosion. The water-retaining
upper soil layers are washed away, leaving behind
less permeable bedrock. Water runs off this
impermeable surface rather than filtering down to
replenish the aquifer. When a native forest is
eroded and damaged, opportunistic alein species
invade. While these new plants can stabilize bare
ground, the watershed cover they create is not as
effective as that of the native forest.
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This eroded, barren tract used to be a healthy
native rainforest. The thinned vegetation now
offers few layers to intercept rainfall and the
remaining root systems are insufficient to hold
the soil, so erosion is worsened. Runoff is
greater and more water is now lost to evaporation
due to the lack of shade and wind protection.
Weedy grasses move in to take advantage of
exposed soil.
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What Happens When the Rain Forest is Degraded?
Streams that emanate from deforested mountains
flood during rains. When the rains stop, these
streams run dry. The loss of stabilizing tree and
plant roots results in landslides. Debris carried
by streams ends up in ocean coastal areas,
causing siltation of reefs
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Ancient Hawaiians Lived in Harmony With Water
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