Transcript Water Cycle in Hawaii

Honouliuli Preserve
• Has dark fertile lands that stretch from the waters of Pearl
Harbor to the summit of the Wai`anae Mountains
• 70 rare and endangered plant and animal species
West Maui Mountains Watershed
• Biologically diverse and pristine in the islands
• Threatened by invasive species like insects and plant diseases
Acid Rain in Hawaiian Isles is caused by emissions from eruptions
Kapunakea Preserve
• Has 24 species of rare plants, including four endangered species
• Has only known kauila tree of its kind on Maui
Lana’i Forest and Watershed Partnership
• Ensures the future supply of water for the island of Lana`i,
• Protect the health of near-shore waters, fisheries and beaches
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 Effects
The 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
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.
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.
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.
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.
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
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 O`ahu and Kaua`i, 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 Ni`ihau 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.
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.
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
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.
Watershed, Catchment Area, Drainage Divide
Watershed and Preserves In Hawaii
Precipitation
average
2,000 Million Gallons
Daily MGD
Evaporation
500 MGD
Plant Transpiration and
500 MGD
Soil Moisture
Runoff to ocean
500 MGD
Percolation to Groundwater
500 MGD
Water Supply
average
371 Million Gallons
Daily MGD
Evaporation
Plant Transpiration and
MGD
MGD
Soil Moisture
Runoff to ocean
MGD
Percolation to Groundwater
MGD
Precipitation
average
325 Million Gallons
Daily MGD
Evaporation
Plant Transpiration and
MGD
MGD
Soil Moisture
Runoff to ocean
MGD
Percolation to Groundwater
145 MGD
Precipitation
average
522 Million Gallons
Daily MGD
Evaporation
274 MGD
Plant Transpiration and
MGD
Soil Moisture
Runoff to ocean
89 MGD
Percolation to Groundwater
189 MGD
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.
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.
O`ahu has two main watersheds:
1. Ko`olau Mountains Catchment Area : The Ko`olau run
perpendicular to the Northeast trades and experience the
heaviest rainfall.
2. Wai`anae Range Catchment Area.
The Wai`anae peaks, though higher, sit in the Ko`olau rain
shadow and receive less rain, even on their windward slopes
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.
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.
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