Understanding the Hydrological Cycle

The Hydrological Cycle

• Evaporation is when the sun heats water from bodies of water and turns it into water vapor.
• Evaporation is important because it is a key factor in controlling weather patterns and climate.
• Transpiration occurs when plants release water vapor into the air through stomata in their leaves.
• Transpiration is essential in forests and other ecosystems for regulating microclimates and supporting plant hydration.
• Condensation is the process of water vapor cooling and turning back into liquid droplets to form clouds.
• Condensation leads to precipitation and influences climate by reflecting sunlight and affecting air temperature.
• Precipitation is when water falls from clouds to Earth as rain, snow, sleet, or hail.
• Precipitation replenishes freshwater sources, supports plant growth, and sustains ecosystems.
• Infiltration is when water from precipitation soaks into the ground and enters the soil.
• Infiltration maintains soil moisture, providing water for plants and improving soil health.
• Percolation is when water moves deeper through the soil and rock layers to reach underground aquifiers.
• Percolation is important because groundwater is a key source for drinking water, irrigation, and maintaining ecosystems.
• Runoff is the flow of excess water across the land into rivers, lakes, and oceans when the ground is saturated.
• Runoff helps maintain river flow and surface water levels, but can cause soil erosion and pollution if uncontrolled.
• Groundwater flow occurs through underground aquifers, which are layers of rock or sediment that store water.
• Groundwater is a stable water source during dry periods and sustains rivers and lakes, especially in arid regions.

Water’s Unique Physical Properties

• Specific heat is the energy needed to raise the temperature of one gram of water by one degree Celsius.
• Water's high specific heat helps stabilize temperatures in ecosystems, preventing sudden harmful shifts.
• Cohesion is when water molecules stick to each other due to hydrogen bonds, forming droplets.
• Adhesion is when water molecules stick to other surfaces.
• Cohesion helps water travel through plants via capillary action.
• Adhesion helps water stick to soil, keeping plants hydrated.
• Water is most dense at 4°C and becomes less dense when it freezes, causing ice to float.
• Floating ice acts as an insulator, allowing aquatic organisms to survive in cold environments.
• Water can dissolve many substances due to its polarity.
• Water's solvent property is crucial for nutrient cycling and supports metabolic functions in organisms.
• Sunlight can penetrate water due to its transparency.
• Sunlight reaching underwater plants is essential for photosynthesis.
• Latent heat is the energy required for water to change states, such as from liquid to vapor or liquid to ice.
• Water absorbs heat when it evaporates and releases heat when it condenses or freezes, moderating Earth’s temperature.
• Viscosity is the measure of how resistant water is to flow.
• Water's low viscosity allows organisms to move easily through it and helps nutrients and gases circulate efficiently.
• Surface tension is caused by cohesive forces between water molecules at the surface.
• Surface tension allows insects to walk on water and helps water move through plant vascular systems.
• Buoyancy is the upward force water exerts on objects, helping them float.
• Buoyancy allows aquatic organisms to float with less effort and aids in the movement of gases and nutrients.

Temperature Profiles of Water Bodies

• Water becomes denser as it cools until it reaches its maximum density at 4°C, then sinks.
• Ice is less dense than liquid water because its molecules are more spread out.
• Bodies of water heat from the surface due to sunlight or contact with warmer air.
• Lentic systems are still water bodies like ponds and lakes.
• Lotic systems are flowing water bodies like streams and rivers.
• Wetlands can exhibit characteristics of both lentic and lotic systems.
• Eutrophication is when water bodies become overloaded with nutrients, causing excessive growth of algae and plants.
• Eutrophication can disrupt ecosystems, reduce water quality, and harm aquatic life.
• Human activities like fertilizer runoff can cause eutrophication, but it can also occur naturally.
• Oligotrophic lakes have low nutrient levels.
• Eutrophic lakes have high nutrient levels.
• Over-fertilization can lead to hypereutrophic or hypertrophic conditions.

Thermal Stratification of Water Bodies

• Thermal stratification occurs in still bodies of water during warmer months, forming layers of different temperatures due to density differences.
• The epilimnion is the upper, warmer water layer that mixes with wind.
• The thermocline is the boundary layer between the epilimnion and hypolimnion where temperature changes rapidly.
• The hypolimnion is the deeper, colder water layer (around 4°C) that does not mix with the upper layers.
• The thermocline limits the exchange of materials between the upper and lower layers, affecting the distribution of aquatic organisms.
• Haloclines form where water layers of different salt concentrations separate, typically with fresher water above saltier water.

Eutrophication

• Eutrophication is the enrichment of water by excess nutrients, leading to algae and plant overgrowth.
• Natural eutrophication occurs in pristine habitats due to natural factors.
• Anthropogenic eutrophication is caused by human activities like nutrient runoff.
• Eutrophication is a gradual process of ecosystem change due to nutrient input.
• Oligotrophic systems have low nutrient levels, clear water, and minimal algae growth.
• Eutrophic systems have high nutrient levels and excessive plant and algae growth.
• Hypereutrophic/Hypertrophic systems have extreme nutrient levels.
• Stratification occurs when temperature or salinity differences create layers in water bodies.
• The hypolimnion (bottom layer) is often low in oxygen.
• Decomposition reduces oxygen in the deeper layers of stratified water bodies.
• Oxygen is replenished in the ocean’s deeper layers through upwelling, where surface water cools and sinks.
• The oxygen minimum zone is between 200 m and 2,000 m deep, where oxygen is depleted due to respiration and decomposition.

Water as a Solvent

• Water can dissolve many substances, like salts, sugars, and gases.
• Water's solvent property allows it to transport nutrients and gases, which is essential for ecosystems.
• Substances like salt accumulate in oceans over time due to water's solvent properties and evaporation.

Acidity

• Acidity refers to the concentration of hydrogen ions (H+) in a solution, measured by pH.
• A pH less than 7 indicates an acidic solution.
• A pH greater than 7 indicates a basic or alkaline solution.
• A pH of 7 indicates a neutral solution.
• Acidity affects chemical reactions, biological processes, and the stability of food products.
• Acid strength is determined by the dissociation constant (Ka) and concentration (C) of the acid.
• Pure water has a pH of 7, but natural waters typically range from pH 2 to 12 due to dissolved ions.
• Ocean water is slightly alkaline, with a pH range of 7.5 to 8.4.
• Pollutants from burning oil and coal mix with water in the atmosphere to form acids, creating acid rain.
• Acid rain can acidify surface waters and cause harm to ecosystems.
• Acid rain can cause normally insoluble ions, like aluminium, to dissolve into water, leading to damage.
• Acid rain causes fish kills in lakes and tree deaths in forests.

Movement of Surface Water

• Waves are created when wind blows across the water, transferring energy.
• Waves carry energy across large distances, affecting water circulation and mixing.
• Seiches are standing waves that form in partially enclosed water bodies due to wind or pressure changes.
• Seiches can disrupt water distribution, sediment movement, and aquatic habitats.

Tides

• Tides are the rise and fall of ocean water.
• There are two high tides and two low tides every day.
• The Moon's gravity pulls on the Earth and the ocean, causing tides.
• .The Sun also pulls on the Earth and the ocean, but not as much as the Moon.
• Gravity is a force that pulls things toward each other.
• The Moon’s gravity pulls the water toward it, making the water rise.
• On the opposite side of the Earth, there’s also a rise because the Earth is pulled more than the water.
• Spring tides occur when the Moon and Sun are aligned, resulting in bigger high tides and lower low tides.
• Neap tides occur when the Moon is at a right angle to the Sun, resulting in smaller high tides and higher low tides.
• The Earth spins, and the Moon orbits around the Earth, causing tides to change continuously.

Factors Causing Variation in Tides

• The Moon's gravity is the primary force causing tides. The Sun also exerts a gravitational force.
• Spring tides are caused when the Earth, Moon, and Sun align, creating higher high tides and lower low tides.
• Neap tides occur when the Moon is at a 90° angle to the Earth-Sun line, resulting in lower high tides and higher low tides.
• Earth's rotation creates a centrifugal force that counteracts the gravitational pull from the Moon and Sun
• The shape of the coastline determines the size and timing of tides.
• Amplification of tidal range can occur in narrow bays or river estuaries, while wide, shallow coastlines may experience smaller tidal ranges.
• Ocean currents move water across vast distances and can influence local tidal patterns.
• Atmospheric pressure affects the water surface, with low pressure causing higher tides and high pressure causing lower tides.

Estuaries

• Estuaries are coastal areas where freshwater from rivers and streams meets saltwater from the ocean.
• Estuaries are transitional zones between land and sea.
• Estuaries are among the most productive ecosystems in the world.
• Estuaries provide essential habitats for the early development of fish and shellfish.
• Estuaries support a variety of life due to the mixture of fresh and saltwater.
• Mixing of freshwater and seawater in estuaries is influenced by tides and river flow.
• Freshwater is less dense than seawater, so it typically stays on the surface.
• Seawater is denser than freshwater and tends to flow beneath it.
• During incoming tides, denser saltwater can push upriver, causing tidal overmixing.
• Salt concentration is highest at the mouth of the estuary and decreases as you move upriver.
• Organisms in estuaries must adapt to changing salinity levels.
• Fluctuating salinity makes estuaries stressful habitats.
• Estuaries are highly productive due to the constant influx of nutrients from rivers.
• Estuaries are important for commerce, transportation, recreation, and waste management.
• Estuaries are vulnerable to pollution and climate change due to their unique physical and chemical characteristics.
• Protecting estuaries is crucial for maintaining biodiversity, supporting commercial and recreational activities, and ensuring the overall health of ecosystems.
• Degradation of estuaries can result in loss of biodiversity, reduced fishing opportunities, and risks to human health.
• Estuaries are highly productive ecosystems, rich in nutrients, and support diverse habitats like mudflats, salt marshes, and eelgrass beds.
• Estuaries serve as nursery grounds for many fish and shellfish species.

Explore the hydrological cycle's key stages: evaporation, transpiration, condensation and precipitation. Learn how water moves between water bodies, plants, and the atmosphere. Understand the crucial role of these processes in weather patterns, climate regulation, and ecosystem sustainability.