Day 3: Intro Biogeochemical Cycles- Water Cycle

Matter, in the form of nutrients, cycles within and among ecosystems and and the biosphere, and human activities are altering these chemical cycles. 

Nutrients Cycle Within and Among Ecosystems

The elements and compounds that make up nutrients move continually through air, water, soil, rock, and living organisms within ecosystems, in cycles called nutrient cycles, or biogeochemical cycles (life-earth-chemical cycles). They represent the chemical cycling principle of sustainability in action. These cycles are driven directly or indirectly by incoming solar energy and by the earth's gravity and include the hydrologic (water), carbon, nitrogen, and phosphorus cycles. Human activities are altering these important components of the earth's natural capital. 

Nutrients cycles connect past, present, and future forms of life. Some of the carbon atoms in your skin may once have been part of an oak leaf, a dinosaurs's skin, or a layer of limestone rock. Your grandmother, George Washington, or a hunter-gatherer who lived 25,000 years ago may have inhaled some of the nitrogen (N2) molecules you just inhaled. 

The Water Cycle Sustains All Life 

Water (H2O) is an amazing substance that is necessary for life on the earth. The hydrologic cycle, also called the water cycle, collects, purifies, and distributes the earth's fixed supply of water. 

The sun powers the water cycle. Incoming solar energy causes evaporation- the conversion of some of the liquid water in the earth's oceans, lakes, rivers, soil, and plants to vapor. This water vapor rises into the atmosphere, where it condenses into droplets. Gravity then draws the water back to the earth's surface as precipitation (rain, snow, sleet, and dew). 

Most precipitation falling on terrestrial ecosystems becomes surface runoff. This water flows into streams, rivers, lakes, wetlands, and oceans, from which it can evaporate to repeat the cycle. Some precipitation seeps into the upper layers of soils and is used by plants, and some evaporates from the soils back into the atmosphere. Some precipitation also sinks through soil into underground layers of rock, sand, and gravel called aquifers. This water stored underground is called groundwater. Some precipitation is converted to ice that is stored in glaciers. 

Because water is good at dissolving many different compounds, it can easily be polluted. However, natural processes in the water cycle can purify water-an important and free ecosystem service. 

Only about 0.024% of the earth's huge water supply is available to humans and other species as liquid freshwater in accessible groundwater deposits and in lakes, rivers, and streams. The rest of the planet's water is too salty, is too deep underground to extract at affordable prices, or is stored as ice. 

Humans alter the water cycle in three major ways. First, sometimes we withdraw freshwater from rivers, lakes, and aquifers at rates faster than natural processes can replace it. As a result, some aquifers are being depleted and some rivers no longer flow to the ocean. 

Second, we clear vegetation from land for agriculture, mining, road building, and other activities, and cover much of the land with buildings, concrete, and asphalt. This increases water runoff and reduces infiltration that would normally recharge groundwater supplies. 

Third, we drain and fill wetlands for farming and urban development. Left undisturbed, wetlands provide the ecosystem service of flood control. They act like sponges to absorb and hold overflows of water from drenching rains or rapidly melting snow. 

Go to 1.7 Page to Complete All Other Work and Activities!