THE NATURAL ENVIRONMENT

Geography 101

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WATER

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Soil Water Balance: Water Out

Runoff

Surface runoff provides the water for some of Hawai'i most spectacular natural features: the high, hanging waterfalls that appear after rainstorms in the mountains.  It also produces some of the worst environmental problems in Hawai'i and elsewhere, including soil erosion and flash flooding.

As noted earlier, surface runoff is determined by the properties of the surface itself. If it is compacted, saturated with water, or has an impermeable coating, like concrete, runoff will be high. If it is highly porous, like bare lava rock or loose soil, runoff will be low or non-existent.

Overall, the Hawaiian Islands are quite porous at the surface, which encourages infiltration and limits runoff. Because of this, Hawai'i has very few perennial streams. Those that do exist, such as Wailua River on Kaua'i and Waihe'e Stream on Maui, are sustained by groundwater sources. During a heavy rain, however, the ground becomes saturated and thousands of small ephemeral streams appear, producing those tall, streaming mountain waterfalls.

In other areas of the world, the surface is often much less porous and dependent on vegetation cover to slow runoff and promote infiltration. Trees and other plants slow the overland flow of water and their roots open up tiny channels in the soil, which encourages infiltration. Organisms that live in the soil and feed on plant litter bore even more holes, which also helps aerate the soil and improve infiltration. When forest is cleared from an area, termed deforestation, the soil becomes compacted and surface runoff greatly increases. The bare soil is then exposed directly to falling raindrops, which both compact the soil and dislodge soil grains. This can lead to severe erosion of topsoil, as shown in the deforested area in the image. When this happens, soil that has been stripped from upland areas is carried downstream in muddy rivers, where it can clog lowland navigational channels. Also, because increased runoff means reduced recharge, lowland rivers may run dry during low-rainfall months because they lack groundwater to sustain them.

The clogging of rivers with silt from erosion and loss of navigability during dry seasons is a very common problem in tropical countries where upland forests have been cleared. An additional hazard occurs during heavy rain seasons: flash flooding. Trees and other vegetation slow runoff and promote infiltration. When forest is cleared, however, heavy rainfall can produce high runoff that quickly fills river channels accustomed to carrying much lower water volumes. These pulses of water, which normally would have soaked into the ground, surge downstream carrying devastating loads of mud and debris. Every year in tropical countries, hundreds of people die in flash floods directly attributable to reduced infiltration rate caused by deforestation.

Evaporation and Transpiration

Evaporation and transpiration were discussed earlier (see Chapter 5 -> Evap). Transpiration through plant stomata is the main pathway for water entering the atmosphere over land. Roots suck water out of the soil, xylem tissue transports it to plant leaves, and leaf stomata provide openings for water to evaporate directly into the atmosphere. To a water resources person, transpiration is considered a loss to the watershed. If plants could somehow use less water, the amount saved would remain on the ground to increase recharge.

For example, California has experimented with converting forest to grassland to decrease transpiration and thus increase recharge to groundwater aquifers. This works because grasses have similar ability to slow runoff, but shallower root systems than trees. In other words, a forest root zone 10 meters deep will suck a lot more water out of the ground than a grass root zone of less than 2 meters depth, with the difference changing the recharge. The opposite is also true; a study in Nebraska showed that conversion of native grassland to forest significantly decreased groundwater recharge.

Recharge

Water that manages to evade plant roots and percolate below the root zone becomes recharge to groundwater aquifers. Many types of aquifers exist around the world and they constitute another subject of intense research because humans often depend on them for fresh water. That is certainly the case in Hawai'i, where residents rely on hundreds of wells drilled on all of the major islands for their water supply.

The Water Use table shows sources of fresh water for Hawai'i residents. This includes residential, military, government and all commercial use. Notice that Maui and Kaua'i have the most extensive exploitation of surface water.

Notice, too, that O'ahu pumps very near the maximum capacity of the groundwater aquifers. During dry periods, the amount extracted from aquifers has reached the maximum possible and water restrictions have had to be enforced. This was the case in the early 1980's and again in 2003.

It is extremely important that Hawaiian aquifers not be over-exploited because they are vulnerable to long-term contamination by saltwater. This results from a unique aquifer type that exists on islands called a freshwater lens, or Ghyben-Hertzberg lens. Fresh water has a lower density than salt water and forms a floating lens-shaped aquifer, similar to an iceberg floating on the ocean. Because it is 1/40 less dense, for each meter above sea level that the water table rises, the aquifer is an additional 40 meters deep below sea level. This property causes the aquifer to form a convex shape due to higher water table and recharge of interior areas. For example, if the water table is 3 meters (ten feet) above sea level in the center of an island, the lens will be 123 meters (410 feet) deep at that location (3 meters above sea level + (3 x 40 =) 120 meters below sea level). This relationship is not exact, of course, because a transition zone of brackish water generally exists and impermeable layers may modify the lens shape.

We tap the freshwater lens in a variety of ways. The first successful attempts to find fresh groundwater, on the Ewa Plain, O'ahu in 1879, drilled into artesian wells that fountained 5 meters (16 feet) into the air. Artesian wells develop when water that is confined and under pressure is given an outlet to the surface. They require no pumping as water flows to the surface naturally.

Some wells are drilled vertically for up to about thousand meters. The well at Waiki'i, Big Island, for example, is over 1200 meters (4000 feet) deep. Another common type of well has an inclined shaft, with a horizontal infiltration gallery that reduces the "coning" effect (discussed below) and skims water from the upper surface of the lens where the water is freshest. Overall, more than 1000 wells have been drilled in the Islands and most are still operating using pumping equipment such as this station on central Maui.

Other wells are horizontal shafts drilled into mountain slopes to tap water confined by vertical, impermeable, volcanic dikes. Dikes are simply walls of dense basalt created when magma was forced to infuse cracks in the Island's volcanic base.

In some areas of Hawai'i, notably Pearl Harbor/Honolulu on O'ahu and in central Maui, groundwater pumping is reaching its absolute maximum for sustainable yields. Over-pumping depletes the entire freshwater lens system, but the problem becomes particularly acute near the well shaft. As pumps withdraw water, a cone of depression forms in the water table around the well. Directly below, a cone of ascension forms in which salt water rises to fill the void left by the removal of fresh water. This causes long-term contamination of the lens as it may take many years, even decades, to flush out salt water and restore the fresh water balance.

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