Water Cycle II
In this lesson we’re going to look at the water cycle in more detail. We’ll explore the different states that water can take on, and how it moves from one state to another. Finally, we’ll talk a little more about the factors that influence the water cycle.
Author: Subject Coach
Added on: 30th Sep 2018
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In this presentation we’re going to look at the water cycle in more detail. We’ll explore the different states that water can take on, and how it moves from one state to another. Finally, we’ll talk a little more about the factors that influence the water cycle.
Let’s start off by talking about the water cycle. The water cycle describes how water moves around the Earth from one state to another. Water evaporates from the surface of the earth, rises into the atmosphere, cools and condenses into rain or snow in clouds and falls again to the surface as precipitation. The water falling on the land collects in rivers and lakes, soil and the porous layers of rock. Much of it flows back into the oceans, from where it will evaporate again, continuing the water cycle. At each step of the water cycle, water changes state.
Water has three states: solid (ice), liquid (water as we think of it) and gas (water vapour). It is the only substance that can exist on Earth in each of its three states, and it can easily change from one state to another. The water cycle is a continuous pattern that allows water to change its location. Changes in location are achieved through changes of state for the water.
The total number of water particles (called water molecules) in the water cycle remains constant, despite changes of state of the water. The water cycle involves a number of different changes of state. The possible changes of state are melting: changing state from solid to liquid, sublimation: changing state from solid to gas, directly; evaporation: changing state from liquid to gas; freezing: changing state from liquid to solid; condensation: changing state from gas to liquid and deposition: changing state directly from gas to solid.
Every change of state involves energy transfers. The behaviour of water particles changes as energy is absorbed or released. This brings about the changes of state. In condensation, deposition and freezing, the energy of the water particles is decreased. In sublimation, evaporation and melting, the energy of the water particles is increased. Let’s now look at what happens in each of these changes of state.
We’ve all seen examples of melting, for example when you put ice cubes into a cold drink. It’s a gradual process as the solid ice gains heat from the surrounding liquid, but eventually all of the solid ice turns to liquid water in the process called melting.
Sometimes, when solid ice receives heat from the surrounding air, the process called sublimation occurs. The solid ice is converted directly from its solid form to water vapour (water gas), without first becoming liquid water. You may have seen this with dry ice turning into carbon dioxide gas, but it can happen with solid water ice as well.
When liquid water absorbs enough heat energy (for example, from the sun, or a heating element on your stove), it turns into a gas called water vapour or steam. This change of state is the process called evaporation.
Let’s start losing energy now. If liquid water loses thermal energy, for example when you put an ice cube tray into the freezer, or the temperature drops below zero degrees, it is converted into the solid we call ice. We call this change of state “freezing”. It happens in your freezer, but it also happens in nature when Earth’s surface water freezes at low temperatures to form solid ice.
If you get into the car in the morning during the winter, or leave clothes out on the clothes line overnight, you’ll notice that they’re wet because dew has formed on them. Dew and rain are examples of the change of state called condensation. In condensation, water vapour loses thermal energy and becomes liquid water. This is the reason why rain and dew are wet!
Deposition is the change of state when a gas, in our case, water vapour, changes state, turning directly into a solid (in our case, ice). It is the reverse of sublimation. You’ve probably never seen this occur, but it occurs high in the atmosphere or at the top of high mountains where the temperature is extremely low. Water vapour will turn into snow without becoming a liquid first.
We can summarise the water cycle into four stages, each of which involves a change of state. Water from oceans, rivers and lakes evaporates and becomes water vapour, it condenses to form clouds, then precipitates as rain or snow, and finally melts and flows back into water bodies.
This slide shows the four steps of the water cycle, each of which involves a change of state. Evaporation is when water in the ocean, lake, pond, stream or even your glass absorbs heat and is converted into water vapour, so that it disappears from the water body. After the water evaporates, it loses heat and condenses into a liquid. These tiny droplets of water are mixed together with tiny dust particles from the atmosphere to form clouds. We’ve all heard of precipitation – it’s rain, hail, sleet or snow. It occurs when a cloud can no longer hold so much water and expels it as snow, rain, sleet or hail. Finally, transpiration is a process when water evaporates from a solid such as a plant, tree or the ground.
We’re now going to look at a number of factors that influence the water cycle. These are precipitation, evaporation, interception, transpiration, infiltration, percolation, retention, detention, overland flow, through flow and run off. These are all ways in which water can move from one location to another.
As we’ve seen before, precipitation is the process that occurs when a cloud holds enough water that it needs to release some in the form of snow, rain, sleet or hail.
Interception occurs when something gets in the way of letting the ground absorb water. For example, the Earth’s surface may be covered in dense vegetation, causing the precipitation to be held on the leaves, limbs and stems of plants. The water may be directly evaporated from these surfaces before it has a chance to reach the ground.
When precipitation reaches the ground as snow, it may stay put for a long time. However, if it falls as rain, it may evaporate, be absorbed by the soil, be caught in small catchment areas or become overland flow. This is a form of runoff.
Plants draw water form the soil through their root systems. A system of vessels in the stems or trunks of plants carries the water up to the leaves, where it is discharged through little pores as water vapour. This process is called transpiration.
When the soil absorbs water, it moves through the layers of the soil because of gravity and capillary forces. This movement is called percolation.
Another process by which water moves is called infiltration. This is when water that falls onto the ground surface and enters the soil. Soil scientists measure the infiltration rate. This is a measure of how quickly a type of soil is able to absorb rainfall or irrigation. This is important as it measures the ability of the soil to support growing vegetation.
Storm water runoff occurs when water from rain or melting snow flows over the ground. Surfaces such as driveways, roads and concrete paths prevent stormwater from naturally soaking into the ground. Nutrients, pollutants and sediment collected by the stormwater runoff can be highly concentrated and lead to pollution of our waterways. Detention basins are used to mitigate these effects by providing temporary storage areas and controlling the release of stormwater into downstream areas.
Humans are champion water cycle disruptors. We take water out of the system to irrigate our crops, to provide our drinking water and to use in industrial processes. We add chemicals and particles to the water, leading to water pollution. This may be deliberate, or it may occur as precipitation flows over the ground and runs into rivers and creeks, picking up a range of pollutants on its way.
These pollutants may include pesticides, herbicides and fertilisers in rural areas. Other pollutants come from faulty septic systems and manure that is not handled properly. Urban pollutants may include petrol, oil, waste from pets, fertilisers, pesticides, salt and treated human waste from sewage treatment plants.
Let’s summarise what we’ve talked about in this presentation by looking at a more detailed diagram of the water cycle. There are a number of places where water can be stored: the oceans, rivers and streams, groundwater storage, the atmosphere, and ice and snow. Water changes its locations in a number of ways. These include evaporation, transpiration, runoff, precipitation, groundwater discharge, infiltration, sublimation and condensation. Many of these processes involve changes of state as heat energy is gained or lost by the water.