Understanding Earth’s Water Cycle and Its Properties
The Hydrosphere
Formation
Around 4.6 billion years ago, during Earth’s formation, high temperatures kept water in vapor form. As the Earth cooled below water’s boiling point, immense precipitation filled the lower surface areas, creating oceans. The majority of Earth’s water resides in these oceans as saltwater. Most freshwater exists as ice and groundwater. The remaining water is distributed across continents and in the atmosphere.
The Water Cycle
Water is in constant motion. Atmospheric water vapor condenses and falls as rain or snow on continents and oceans. Water falling on land flows down mountains into rivers or infiltrates the ground to become groundwater. Much of this inland water eventually reaches the ocean, evaporates, or is transpired by plants back into the atmosphere. Ocean water also constantly evaporates. Solar energy drives this continuous cycle.
Annually, 500,000 km3 of water evaporates, averaging 980 L/m2. This is equivalent to a 980 mm (three feet) layer of water evaporating from Earth’s entire surface each year. Since the atmosphere holds only 12,000 km3, the evaporated 500,000 km3 also falls as precipitation. While the global average for evaporation and precipitation is 980 mm, distribution is uneven, particularly on continents. Deserts receive less than 200 mm, while some mountainous regions receive over 6,000 mm.
A water molecule’s average time in various cycle stages is: 9-10 days in the atmosphere, 12-20 days in rivers, 1-100 years in lakes, 300 years in underground shafts, and 3,000 years in oceans. This residence time significantly influences pollutant persistence in aquatic ecosystems. For example, a polluted river can cleanse itself in days or weeks as pollutants flow to the ocean and dilute. However, groundwater contamination can persist for decades or centuries.
Characteristics of Water
Water’s unique properties make it essential for life. Its high polarity is particularly important, influencing other key properties.
a. Polarity
Water molecules are polar, making water an excellent solvent for salts and other polar substances, but a poor solvent for nonpolar gases, fats, and oils.
b. Specific Heat of Vaporization and Fusion
The heat required to vaporize and melt water is high compared to similar-sized substances. This is due to the strong electrical forces between water molecules. This property makes water a good heat buffer, helping regulate Earth’s and organisms’ temperatures.
c. Cohesion
Cohesion, a product of water’s polarity, arises from the attraction between water molecules. It explains phenomena like water movement through soil.
d. Density and Stratification
Water’s density is 1 kg/L, but varies slightly with temperature and dissolved substances, which has ecological implications. Density increases as temperature decreases, reaching a maximum at 4°C. Below 4°C, density decreases, allowing ice to float. This insulation prevents complete freezing of lakes and oceans, allowing life to persist beneath the ice. Water layers with different densities stratify, functioning independently. This can lead to nutrient imbalances, with some layers depleted while others are abundant.
e. Salinity
Salinity arises from ions originating from continental runoff and mid-ocean ridge magmas. One liter of seawater typically contains 35 g of salt, two-thirds of which is sodium chloride. Salinity varies geographically (e.g., higher in the Mediterranean, lower in the Baltic), but ion ratios remain relatively constant. Some inland seas, like the Dead Sea (226 g/L), have extremely high salinity.
Freshwater contains far fewer ions. Calcium bicarbonate (around decigrams per liter) is the main component, indicating water hardness.
Dissolved Gases
: Oxygen dissolved in water is a major constraint to the organisms that live in this environment. While in one liter of air is 209 ml of oxygen, water quantity is reached to dissolve is 25 times smaller. The diffusion of oxygen in water is very slow. The turbulence of the waters, to agitate and mix, accelerates the diffusion process thousands of times.
The temperature affects the solubility. While the solids dissolve better at higher temperatures, the opposite is true gases. The cold water dissolves more oxygen and other gases that warm water because higher temperatures mean more agitation in the molecules making it easier for the gas out of the liquid. Inland Waters: Rivers. The rivers originate in springs which rise to the surface ground or in places where the glaciers melt. Since its inception following the slope to reach the sea. A river with its tributaries drains an area known as a watershed. The separation between river is the boundary of aguas.Desde his birth in a mountainous area and high up the estuary the river is usually decreasing its slope. The longitudinal profile shows clearly the passage of the river until it reaches the sea. Typically the slope is strong in the first section of the river, as they travel over the mountains (upper reaches), and becomes very small, almost horizontal, as it approaches the mouth (lower section). The mouth marks the base of the river level. The river suffers from variations in its flow. In the rainy season increases and decreases in the dry, although some rivers are the maximum flow at the time of thaw. Floods can be gradual or very abrupt. Lagos lakes form when the water collected in an area not directly out to sea but passes or ends in a depression. In many cases the lake a river that goes out to sea, but in others there is no drainage, but water evaporates into the atmosphere directly from the lake. Groundwater. Some of the water that falls on the ground sliding down to rivers and lakes (water runoff), but another part is infiltrated either directly when it rains, or from rivers and lakes. From the floor of the water leaves by evaporation, or by springs or rivers and lakes fed by his bed. The water that penetrates the pores of a permeable rock ends up in a waterproof area stops. Thus the permeable fills with water (saturated zone). The area above it, in which the water goes down, but there is still air pores, calledzone of aeration and contact between the two, the water table. The water level goes above the surface after heavy rains when the soil is waterlogged. The porous and permeable rocks that store and transmit water are called aquifers, are an important source of water for human use. The rocks and soils that let water through permeable called as opposed to waterproof. T he main types of aquifer are : Detrital Aquifer, are composed of masses of broken rock, like sand or gravel, which store water in the interstitial spaces. karst aquifers: Some rocks are dissolved by water and are called typical geological structures capable of storing large Karst amounts of water. The limestones are rocks that are most commonly Karst, but also the dolomite, gypsum and salts can be trained. Carbonate rocks occupy more than 100 000 km2 in the Iberian peninsula, so the landscapes and karst aquifers are common.