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Unformatted text preview: Julie Vu
October 20, 2009 Earth’s Water Systems
Although desertification can occur naturally, there has been increased evidence by scientists that human use of
semi-arid lands has led to increased desert-like conditions of the soil. Land degradation is occurring in some
semiagricultural regions, which leads to a reduction in the fertility of the soil and requires a greater investment in
agricultural methods for crop production.
The Earth’s fresh water supply is critical to the process of photosynthesis for plant growth and vitality. In fact,
in a cyclical pattern, plants and animals help convert rocks into soil the dynamic medium that stores water and
nutrients and supports all of the Earth’s life. The flow of water between the lithosphere, biosphere, and
atmosphere is an example of a biogeochemical cycle. It is the movement of water throughout these spheres
that connects all living matter.
The earth’s temperature allows water to exist in all three states—solid liquid and gas and allows for rapid
change from one state to another. Large amounts of heat energy are utilized to necessitate the changes of state.
Water is the Earth’s solvent It dissolves most substances, and is essential for natural chemical reactions
Most importantly, all living things consist primarily of water. Water cycles throughout the atmosphere,
lithosphere, and hydrosphere by means of evaporation, condensation, precipitation, and runoff. This flow is called
the hydrologic cycle.
The oceans are the Earth’s largest reservoirs of water. They contain almost 97% of all the Earth’s water. About
2% of the Earth’s water is frozen as glacial ice and the remaining is groundwater with only about .014% as
rivers and lakes. The total water quantities change very little from year to year, but there are drastic changes
over very long periods of time. The arguments for and against global warming/climate change hinge on this
very question: How much change can the Earth take, and to what extent is the change in climate the result
of human interaction?
Water budgets are the accounting for all of the inflows and outflows of water in a given system over a time
period. These budgets are measurements that are used as tools for water resource managers to determine
water available for human use Methods such as measuring precipitation and determining evapotranspiration
allow us to predict average river flow Water budgets help us understand how some rivers can have much
more water flow even though their drainage basins may be relatively small in comparison to rivers with less
flow and larger basins.
soil. wellPlants play an important role in removing water from the soil In well- vegetated areas, much of the conversion
of water from liquid to vapor takes place in the leaves of plants. Water for the plants is then replaced by water
drawn from the soil through plant roots This plant mechanism is called transpiration and when combined with
evaporation we call the process evapotranspiration This natural hydrologic cycle can be drastically affected
from human interaction with the water source.
Evapotranspiration is low in winter, when many plants go dormant and high in summer, reflecting active
transpiration in plants. In the winter, precipitation exceeds evapotranspiration. The surplus either runs off into
drainage basins, or becomes stored as groundwater. In the warmer months, plants grow leaves and water use
becomes necessary, which oftentimes exceeds precipitation. Initially, the water is resupplied by the soil and
later, through groundwater sources. If precipitation lags demand, drought conditions can occur over prolonged
periods. Julie Vu
October 20, 2009 Earth’s Water Systems
A local water budget compares precipitation and evapotranspiration. Water budgets vary tremendously from
one climate to another. Human use must be carefully considered for local water budgets. Conditions can spiral
downward quickly in warmer climates due to increased water use (ex. Atlantás drought this past summer).
Many municipal resource managers may have to enforce strict water use regulations so that the water source
does not become exhausted
Most of the Earth’s land area is covered with a layer of rock debris and organic matter known as soil Soil is
critical in the hydrologic cycle because it acts as a “water ba nk How effectively soil is able to store water
depends upon its infiltration capacity (for storage in the root zone the local climatic conditions, and the ability
for local vegetation to transpire water into the atmosphere. Soil texture is also important to how much water
soil can hold. Soils with higher contents of finer particles drain slowly (sand vs. clay). The local vegetation plays
a large role in that shallow-rooted vegetation (grass) leads to greater runoff than plants with deep roots (trees),
which provide greater ground coverage.
Forests occur where there is an abundance of moisture available throughout the year. Trees demand large
volumes of water, and a single large tree can transpire 1000 liters of water on a warm day. Because of their
deep roots, trees are able to tap into the groundwater sources, and are the dominant medium in returning
rainwater to the atmosphere. Large forested regions like the Amazon River basin play a major role in shaping
the glo bal weather patterns
Forest management has major water budget implications In the Southeastern U.S., areas formerly covered
with deciduous trees (shed leaves in autumn) have been replanted with faster-growing evergreens This has
largely been done by the timber industry in their efforts to replace lumber sources. There is a positive effect to
this practice. However, there is a negative effect as well. Positive evergreens grow year-round, which means
that they absorb carbon dioxide and produce o xygen at greater levels than deciduous trees. While our air
quality may benefit from this, our watersheds are harmed. Negative: evergreen needle leaves catch more rain
than deciduous trees, which leads to a greater loss of water to evaporation. Over time, this loss of potential
groundwater can greatly decrease the amount of stream- flow and water- table levels
Deforestation for agricultural purposes in the Amazon River basin has led to the clear-cutting of an accumulated
area equal to the size of Texas. This loss of forest is critical to the water balance of the Amazon Region. Though
the Amazon is still operating at a surplus of precipitation to runoff, most of the rainfall is in areas near the
coastline The excess water saturates the ground and runs off into the Atlantic, only to be recycled into rainfall
for the Amazon. The Amazon basin’s interior is very dependent upon the evapotranspiration of eastern trees to
receive its precipitation. Easterly winds deliver the moisture that falls as rain upon the interior.
An extreme loss of trees in one region can have major effects upon the entire Amazon Basin. Grasses that grow
in the clear-cut areas (for farming or ranching) have the ability to contribute moisture through
evapotranspiration, but their roots are very short, which leaves them very susceptible to climate fluctuations. A
reduction in moisture due to the inefficient nature of grasses can harm the entire forest. At this time, scientists
are only able to make predictions with computer models and theories (see Al Gore), however, the consequences
of abuse to our hydrologic cycle could potentially have far-reaching results upon the Earth’s ecosystems
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- Spring '08