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Before a computer model of water soil content and runoff is constructed, a review of soil properties and the hydrologic or water cycle is appropriate. Our model will be specifically consider the water holding capacity or water content of a hypothetical piece of soil. Those terms are described below.
After reviewing water and soil, the concept of feedback loops will be
discussed. This model will contain more than one feedback loop and changes
in initial conditions may lead to non-obvious final results. In particular,
the air temperature and the amount of precipitation will be linked in a
positive feedback loop. This model was originally designed to help explain
how hotter and drier summers might develop.
Soil
Figure 1 - Soil cross-section |
A scoopful of soil looks the same throughout...or does it? Upon closer
examination, you might be able to see particles of sand or clay, tiny organisms,
plant matter, or even water. The soil is like a big sponge; numerous tiny
pores and channels wind through its structure. Some pores are so tiny that
even a bacteria can not enter. Other pores are the equivalent of superhighways
with large quantities of materials periodically rushing through them. Still
other channels meander up, down, left, and right thoughout the soil until
they are somehow blocked or collapse. Some pores will be waterlogged;
others dry.
In fact, soil is a heterogenous three-phase system, made up of solids, liquids, and gases. The solid part is composed of mineral (non-living or inorganic) material and the living (organic) matter components. Water and air make up the liquid and gas phases of the soil. Water, in its liquid or gaseous form, may occupy the pore spaces not occupied by gases. Water acts as a medium where nutrients from the minerals and decaying organic matter are released and made available to plant roots and other organisms. Water is also an effective temperature regulator of the soil, giving the living organisms a more stable environment to grow and reproduce. Given the important role of water in the soil system, its movement or quantity is a critical requirement in describing the soil system. For the plants and animals living in the soil, excessive or diminished water content will mean the difference between life and death. |
Although humans might not initially be concerned with some excessively
wet soil, consider that widespread flooding, flash floods, and mudslides
all are the ultimate result of runoff from several small sections of soil
which cannot hold any more water. The small amount of excess water
(a few mm per square meter) literally spills out in a domino effect strong
enough to reshape the land, moving (sometime destroying or killing) anything
or anybody in its way. Similarly, computer models have shown the
a few excessively dry water conditions can participate in an positive feedback
loop to produce warmer weather which in turn drys out more tracts of soil
to produce even warmer temperatures, etc. While not all instances
of excessively wet or dry soil lead to such dramatic events, they are still
potential outcomes.
The water field capacity is an indication of the maximum amount of water which can be held by a meter, while the water holding capacity is used to indicate how much more water could be held (i.e., water holding capacity varies with season). The soil water content is the amount of water which is present in the soil. The sum of the soil water content and the water holding capacity should be the waer field capacity.
The figure below shows the global distribution of soil water holding capacity, at field capacity for the top soil (0 - 30cm), with a one degree latitude/longitude spatial resolution, and was derived from information on soil type and texture (FAO Soil Map of the World).
Figure 2 - Worldwide Soil Water Holding Capacities
All of the earth's water goes through a cycle in which the water
changes its location or physical state through different processes. In
accordance with the law of conservation of matter, water is not created
or destroyed. It just changes form. Water can be found in all three
states of matter during the cycle: solid (ice caps), liquid (lakes) and
gas (water vapor).
There are five processes by which water moves through the cycle. Water in oceans and lakes evaporates into the air. Cool air in the atmosphere causes this water vapor to condense into a cloud. Precipitation from the cloud falls to the ground as rain, sleet or snow. The water on the ground percolates through the soil and some of it is absorbed by plants. As the plants go through photosynthesis (converting sunlight, water and carbon dioxide for their own food), they absorb water from the soil and release some of it back into the air through transpiration.
These patterns of change can vary, but the cycle occurs continuously. Water has been cycling by means of these processes since time began.
Figure 3 - The Hydrologic or Water Cycle
The water cycle is the continuous movement of water from oceans, lakes, and other sources: to the air: and back to the land and oceans. The cycle occurs because of the changes in states of water. Water evaporates from oceans, lakes, rivers, plants, soil, and other sources into the air where it exists as water vapor. The rate of evaporation increases with increasing temperatures, and drier air. Plants absorb water through their root hairs, and is transferred through the plant along with various nutrients to the leaves, where it evaporates and returns to the atmosphere. This process is called transpiration. The air can contain varying amounts of water vapor from near zero to over 4% by volume. When moist air cools, it reaches a saturation level called the dew point temperature, condensation occurs and clouds form. When further dynamic processes take place, these tiny cloud droplets grow by a million times to become rain drop candidates. Precipitation begins, and liquid or frozen forms of water fall to the earth to begin the process again.
Precipitation is the process where water vapor in the air becomes liquid or solid water and falls to the land. Precipitation can take the forms of rain, snow, sleet, or hail. Precipitation occurs when areas of high pressure and low pressure air meet, or when areas of hot and cold air meet.
Runoff is the process of water running off the land surface. Water flows downhill from high places like mountains and hills into streams, then rivers, and finally into the ocean.
Infiltration is the process where water seeps into the ground. Just like a sponge soaks up water, so does the soil. When it rains, some of the water will infiltrate or soak into the soil. When the soil is full of water or saturated, then the rain will become runoff. Percolation - Some of the rain soaks into the ground and travels through rocks, gravel & sand.
Groundwater Flow is the process where water in the ground comes back to the land surface. The term groundwater describes water that is stored underground in the rocks or geologic formations. When water infiltrates into the ground, it moves to the groundwater and is stored. Over time, the water stored in the groundwater flows (like a very, very slow river) back to the land surface further downhill. Usually this groundwater flows into the bottom of a creek or river or the ocean.
Discharge - Some ground water eventually travels back to the earth's surface.
Evapo-transpiration is the process where liquid water either on the land surface, in the soil, or in the ocean goes back into the air as water vapor. The term evapo-transpiration is actually two processes: evaporation and transpiration. Evaporation occurs when liquid water is heated and becomes water vapor, like boiling a pot of water. Water also evaporates when the air does not contain much water vapor. Transpiration describes how plants take liquid water from the soil and turn it into water vapor, which is released through leaves. The water is used to move nutrients from the soil into the leaves. In the leaves, sunlight and water and nutrients all play a part in photosynthesis, where plants store energy in sugars and starches. Evaporation and transpiration are grouped together, because it is hard to tell the difference between them.
Figure 4 - The sun acts as a pump, keeping water going around and around.
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| The Woodrow Wilson
National Fellowship Foundation
CN 5281, Princeton NJ 08543-5281 - Tel:(609)452-7007 - Fax:(609)452-0066 Technical contact: lpt@woodrow.org |