The Hydrosphere: Properties, Distribution, and Dynamics
The Hydrosphere
Origin
This form of liquid water, solid, and gaseous Earth. Water is a relatively abundant molecule in the universe, especially in rocky material. Earth is the only one of the four rocky planets (Mercury, Venus, Mars, and Earth) that have huge amounts of liquid water on its surface. The hydrosphere was formed while the atmosphere experienced mantle degassing.
Hydrologic Cycle
The water cycle is a dissipative system that is still in operation. It receives constant energy, responsible for the evaporation of water. The tropospheric temperature gradient determines precipitation, condensation of water, and gravity causes the flow from the continents into the oceans. The flow causes a wash of soluble salts, which are accumulating in the ocean, resulting in a higher salinity than other bodies of water.
Properties of Water
Water Molecule Dipole
Water molecules have a positive pole in H atoms and a negative pole located at the O atoms.
Hydrogen Bridge
Examples of positive and negative poles of two different molecules show great attraction. By molecular weight, water should be a gas, but at a molecular level, H tends to bind to O at each other, building bridges of H. This allows water to behave like a liquid.
Universal Solvent
Water effectively dissolves salts and gases, allowing ionic dissociation.
Density
In its solid state, water crystals form that occupy a larger volume than liquid water, resulting in ice having less density.
Elevated CE
The CE of the water is unusually high. It is necessary to provide heat to raise its temperature, meaning it admits a large amount of thermal energy while its temperature changes very slowly. For this reason, it amends air temperature, which has a much lower Ce.
Maximum Density of Water is at 4°C
The density of water varies abnormally with temperature, presenting a maximum value at 4°C.
Distribution of the Hydrosphere
The main reservoir is the oceans, which contain 97% of the volume of water on the Earth’s surface. Continental water is only 3% of the hydrosphere; the majority of this water is in solid form in glaciers, and more than half of the liquid freshwater is groundwater.
Water Balance
Water balance is a calculation used to determine the net flow of incoming or outgoing water present in a system over a long time. This calculation can make predictions about the volume of water that the system contains at any given time, its saturation, incoming flows, and so on.
Water Balance Calculation
Calculate the water inlets to the system and the exits, and then subtract the two results:
Net flow system = sum of inflows – sum of outgoing flows.
If the result is positive, it means the system has a net contribution of stored water, meaning the volume tends to increase over time. A negative result indicates the system has a net outflow of water, and the stored water columns tend to decrease. When considering long periods, most systems have a zero balance (outputs = inputs).
It is normal for a river basin to have a zero annual water balance. The flow of rivers and streams that drain the basin are an outflow whose value may increase or decrease spontaneously, adapting to various actions in the input fluid. However, some aquifers have no natural outflow but can provide a record for long periods. If more water is extracted from the aquifer than its recharge, it will have a negative water balance. The water balance calculation can be done in any container but is usually applied to water catchment areas.
Residence Time
The residence time of a system is the average time it takes for a water molecule to enter and leave. It is used to calculate the system’s ability to cleanse itself. It is calculated by dividing the total water volume of the system by the incoming flow:
Residence time = Volume of water in the system / System inflow
Renewal Rate
This indicates the rate at which water flows through the system. In general, the shorter the residence time or the higher the turnover rate, the greater the facility a system has to cleanse itself. It is also more likely that there will be a mixture of water both inside and outside, presenting a higher concentration of dissolved oxygen. In systems with long residence time and low turnover rate, such as some aquifers, pollution occurs slowly and requires purification for a long time, making the system very difficult to recover.
Ocean Dynamics
Changes in Density of Seawater Originate Flows
There are two types of variables that influence the density of water:
- Salinity: Water with more salt tends to sink below less saline water. Ocean water has an average salinity of 3.6%, but concentrations vary across the sea. The more closed a sea basin is and the more arid the climate, the higher the water salinity.
- Temperature: Due to solar radiation absorption, a body of water’s surface region temperature is usually greater than at depth. It is common for a narrow transition zone with a sharp temperature change (thermocline) to exist between two bodies of water.
Ocean Currents
They originate mainly due to:
- Prevailing Winds: These push the water surface and trigger surface currents in a specific direction.
- Density Differences: These produce the sinking of cold and salty water. This causes vertical mixing of the water body and leads to deep currents.
Surface Currents
These currents arise as a consequence of the prevailing winds on the surface:
- When the prevailing wind blows obliquely towards the coast, it produces a parallel current (littoral drift current).
- When the prevailing wind blows from the land towards the sea, there is the rise of nutrient-rich deep waters (upwelling).
- Stable anticyclones originate circular currents, such as the Canary Current, which is due to winds associated with the Azores anticyclone.
General Circulation of the Ocean
Deep currents originate when changes in the density of water masses produce subsidence or ascent. The water mass sinks to the ocean bottom in the Norwegian Sea. Displacement along the bottom then causes a large current that flows through all ocean basins (thermohaline current). As the thermohaline current travels through all ocean basins, it mixes water worldwide, providing oxygen to the ocean floor. The deep cold current running east through the southern Atlantic and Indian oceans is coupled to the Antarctic Circumpolar Current: an extremely cold stream that rotates clockwise around Antarctica.
Formation of Seas
Seas are formed by the action of wind on the sea surface. The three factors that determine the height of waves are wind strength, duration, and the ocean surface area affected. When a large ocean surface is exposed to an intense and constant wind for many hours, well-organized waves develop with great height and long wavelength. These organized waves can travel hundreds of kilometers, slowly decreasing in size, so they can reach areas where the wind no longer blows or even reach the coast where the sea was formed. This wave is known as “the sea swell.” It conducts an intense mixture of the top 10 to 15 meters of the water, producing a very homogeneous distribution of nutrients, temperature, and the concentration of dissolved gases. In areas of the continental shelf, the seabed is under the influence of waves, which causes the mobilization of sediments. This can increase water turbidity and facilitate the oxygenation of the sediment.
River Dynamics
Rivers
Rivers are permanent streams. Creeks and streams are intermittent courses that can stay dry at times with limited water supply. All the rivers and streams that collect water from an area form a drainage network, which can be:
- Exorheic: If they pour water into the sea.
- Endorheic: If they pour water into an inner zone of the continent isolated from the sea, which may or may not be a lake, such as the Dead Sea and the Aral Sea basin.
A drainage system generally has a main river and some tributaries that pour their waters into it, separated by watersheds. The flow of a river at a point is the volume of water that passes through that point per unit of time. The environmental flow is the minimum flow rate that a river must have to sustain the structure, composition, and functioning of its ecosystem.
Longitudinal Profile and Profile
Balance Profile
The elevation of the bed of a river plotted against the distance from its source to its mouth is its longitudinal profile. The balance profile is a theoretical curve that represents the longitudinal profile that the river would have if it eroded its banks by reducing the slope until it lost its erosion capacity.
Hydrograph Interpretation
Hydrographs are plots that represent the flow of a river or stream over time.
- Yearly Hydrograph: Represents the evolution of the flow of a river over a year.
- Storm Hydrograph: Represents the flow of a stream in response to a downpour.
- Response Time: The time interval from the time precipitation starts until half the total precipitation is reached at the peak flow. The shorter the response time, the more likely a catastrophic flood event.
Groundwater
Aquifers
Water infiltrates the ground, occupying the pores of soil and rocks, as well as other possible fissures and cavities. The water descends by gravity until it reaches an impermeable surface that prevents it from continuing and then accumulates. Aquifers are porous geological formations that have hollows, broken by dissolution or other causes, where water can accumulate. Permeable formations allow groundwater to circulate easily inside. There are two types of aquifers:
- Confined Aquifer: An aquifer that is between two impermeable formations. Its recharge zone (where the aquifer is free) does not occupy the entire extent of the aquifer.
- Free Aquifer: An aquifer that can be recharged from the surface along its entire length. A perched aquifer is a free aquifer that is above the regional water table and disconnected from it, meaning it can be located at higher elevations.
Recharge Zone
The area of a confined aquifer where materials are permeable and allow water to pass.
Piezometric Level
The level to which water pressure will rise in a well to reach atmospheric pressure.
Artesian Well
A well drilled into a confined aquifer where water rises spontaneously. If the level of groundwater is above the surface, the water overflows to the outside, forming an artesian well.
Water Table
This is the boundary between the zone of saturation and the zone of aeration in a free aquifer. This is generally not a horizontal surface.
Water Surface Maps
These usually arise in areas where the ground surface cuts the water table.
Zone of Aeration
The zone located between the water table and the ground surface. The rocks are not saturated with water.
Karst
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