Water is vital for human survival and deeply integrated into our everyday life's, yet it is easy to take for granted. Up to 60% of the human body is compose of water and every system in your body depends on water. Water flushes toxins out of vital organs, carries nutrients to your cells, helps regulate body temperature, and much more.
Each day humans must replenish water lost by drinking fresh water. Although water covers more than 70% of the earth, only 1% of that is available as a source of drinking water. Most of the water on earth is salt water: 96.5% makes up our oceans, 0.93% is saline groundwater, and 0.07% is in saline lakes. Freshwater only makes up 2.5% of the water on earth and of that small percentage only a small amount of glaciers, groundwater, rivers, and reservoirs are accessible for drinking water.
Ample freshwater is not only important for drinking water but also for electric generation, industrial uses, and agricultural activities. According to the National Atlas water withdrawals in the United States in 2000 were 85% freshwater and 15% saline water. Of these withdrawals thermoelectric power made up 48%, industrial made up 5%, and irrigation made up 34%. Irrigation is the single largest user of freshwater and usage for this purpose continues to increase, albeit at a slowing pace.
Water is continuously moving among these places and the atmosphere. This is known as the hydrological cycle. The balance of water in the hydrological cycle remains fairly constant over time. Water can be locked up in living organisms or temporarily broken down into it's constituent particles, Hydrogen and Oxygen (H2O), but this is generally balanced out by the reentry of water into the system from the break down of living organisms and binding of hydrogen and oxygen into H2O.
The continuous movement of water through the hydrological cycle brings it through different phases: liquid, solid, and gas. Processes such as evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow move water through the cycle. This movement purifies the water, replenishes the land with freshwater, and influences regional climates and day-to-day weather that we experience. The movement of water across the land also affects the shape of the land through processes called erosion and sedimentation.
Water has several unique properties that drastically shape our earth.
- Water is the only natural substance that found in all three physical states - liquid, solid, and gas - at the temperatures normally found on earth.
- Water is unusual in that the solid form, ice, is less dense than the liquid form. Meaning that ice floats.
- Water is referred to as the "universal solvent" because it dissolves more substances than any other liquid.
- Water has high surface tension, meaning water is sticky and elastic, and tends to clump together in drops rather than spread out.
- Water has a high specific heat index, meaning it absorbs a lot of heat before it begins to get hot.
So what does this all mean?
- If water did not naturally occur in all three states at the temperatures found on earth the water cycle would not function as it does currently; if at all. If for example water could not become a gas at temperatures found on earth then there would be no evaporation or evapotranspiration, breaking a vital link in the cycle of water. Without the water cycle life, could not survive.
- When water freezes and floats on top of a large water body it insulates the unfrozen water and allows organisms to survive in the warmer water below. If ice sank instead of floating then the oceans of the arctic would be frozen solid and after several winters in northern regions the rivers and lakes would also freeze solid only thawing at the top during the summer. The earth would be a very different place, and able to sustain much less life.
- Water is the "universal solvent" capable of dissolving a variety of different substances. This is important for maintaining natural chemical processes on earth. As water moves through the ground or through our bodies, it collects valuable chemicals, minerals, and nutrients. In our bodies, water serves as the solvent for electrolytes, oxygen, and nutrients needed by cells and carries waste material away from the cells. In the hydrological cycle, water has a similar role as a solvent but at a much larger scale, carrying nutrients from one place to another, holding dissolved gases like oxygen needed by aquatic species, and diluting and carrying waste materials away.
- The high surface tension of water may only seem to help water striders walk on water but there are other reasons why this is important. First, surface tension affects waters gaseous diffusion rate, allowing water to hold higher levels of dissolved oxygen and carbon dioxide needed by aquatic animals and plants. Second, surface tension allows what is known as capillary action. Capillary action is the ability of a liquid to flow through narrow spaces, seemingly defying gravity. Capillary action along with other properties such as diffusion/osmosis and adhesion/cohesion allows plants to uptake water from the ground and allows water to spread through the soil rather than just flowing down with the force of gravity.
The high specific heat index of water has a great deal to do with the regulation of extremes in our environment. Being that many organisms are made up of large percentages of water, the high specific heat index of water serves to buffer the internal temperatures of organisms. In water bodies, the high specific heat index buffers against extreme temperature shifts as the seasons change, protecting aquatic life. Land has a lower specific heat capacity than water and benefits from the climate moderating effects
of large water bodies. This moderating effect is why many coastal areas experience less extreme temperatures than inland areas.