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GLOBE Ireland Classroom Resource Introduction to Rainfall and Flooding

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Note to Teachers

This presentation is for you to download and use however you wish.

Please feel free to use what you like, remove what you like, edit it and do whatever best facilitates your teaching of this information!

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Contents

Section 1: The Water Cycle

Section 2: Soil

Section 3: Flooding

Section 4: Nature Based Solutions

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The Water Cycle

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What is precipitation?

Precipitation is any liquid or frozen water that forms in the atmosphere and falls to Earth.

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Did you know

that when you drink a glass of water, you are drinking the same water that wooly mammoths, Egyptian pharaohs and the first humans drank?

Earth has been recycling water for over 4 billion years through the water cycle.

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Why does rainfall matter?

About 71% of Earth’s surface is covered with water. However, less than 1% of this is found in the surface water of rivers, lakes and swamps.

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2. Soil

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What is soil?

Minerals are bits of rock
Organic material is the remains of living things that have died.
Without soil, most life on Earth could not survive. Soil allows plants to grow by holding water in place for their roots and providing nutrients needed for growth.
Soil also provides a home for many animals and other living things.

Soil is a mixture of minerals and organic material that covers much of Earth’s surface. In some places on Earth, only 15cm of soil lies on top of rock – in others, it is hundreds of metres deep.

Soil is a mixture of minerals and organic material that covers much of Earth’s surface.

Minerals are bits of rock, and organic material is the remains of living things that have died. Soil is not as solid as rock. It has many small spaces, called pores, that hold water and air. In some places on Earth, a thin layer of soil only 6 inches (15 centimeters) thick lies on top of rock. In other places, though, soil may be hundreds of feet deep

Without soil, most life on Earth could not survive. Soil provides a place for plants to grow. It holds water in place for their roots. It contains nutrients, or food substances, needed for their growth. Soil also provides a home for many animals and other living things.

Soil forms slowly. A layer of soil that is 1 inch (2.5 centimeters) thick may take 500 to 1,000 years to form. For this reason it is important to protect soil from erosion—being blown or washed away.

The mineral part of soil forms from rocks. Such forces as wind, water, and temperature changes break rocks down into tiny particles, or bits. The smallest particles are called clay. Medium-sized particles are called silt. The largest particles are called sand. Different types of soil contain different mixtures of clay, silt, and sand.

The organic part of soil forms when plants and animals die. Their bodies decay, or break down, and mix with the rock particles. The organic material, called humus, helps to form the pores in the soil. It keeps the soil soft and loose. Humus also provides the nutrients used by plants.

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Types of soil

The mineral part of soil forms from rocks broken down into tiny particles by wind, water and temperature changes.

The smallest particles are called clay, medium-sized are called silt and the largest are called sand.

Clay soils are heavy, high in nutrients, wet and cold in winter and baked dry in summer

Silt soils are fertile, light but moisture-retentive, and easily compacted

Sandy soils are light, dry, warm, low in nutrients and often acidic

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Why do soil types retain water differently?

Soil is not as solid as rock - it has many small spaces, called pores, that hold water and air.

The amount of water that soil can retain depends on:

How much water is already in the soil (soil moisture)
How big the pore spaces are.
How connected the pore spaces are

Trees and plants are also crucial for soil’s ability to absorb water, as the roots create channels.

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Soil Saturation

Between the soil particles are larger pores that can be filled with water – the level of this water (the water table) fluctuates with weather conditions
The process of water entering the soil is called infiltration. When the soil cannot absorb any more water, it is saturated.
Some of this infiltrated water continues down to underground reservoirs and rivers, and some is absorbed by plants and evaporates.

Soil moisture refers to the water WITHIN the soil particle, whereas soil saturation refers to the water BETWEEN soil particles.

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What is the water table?

It is an underground boundary between the soil surface and the area where groundwater saturates spaces between sediments and cracks in rock.

Unsaturated zone = area above water table where both oxygen and water fill the spaces between sediments.

Saturated zone = Area below water table where water fills all spaces between sediments

The water table is an underground boundary between the soil surface and the area where groundwater saturates spaces between sediments and cracks in rock.
The soil surface above the water table is called the unsaturated zone, where both oxygen and water fill the spaces between sediments. The unsaturated zone is also called the zone of aeration due to the presence of oxygen in the soil.
Underneath the water table is the saturated zone, where water fills all spaces between sediments. The saturated zone is bounded at the bottom by impenetrable rock.
The groundwater found below the water table comes from precipitation that has seeped through surface soil. Springs are formed where the water table naturally meets the land surface, causing groundwater to flow from the surface and eventually into a stream, river, or lake.

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Why does soil absorption matter?

Healthy, water retaining soils can:

Absorb water during heavy rain events and store it, reducing flood risk
Enhance drought resilience by storing water that can be used in dry periods
Provide conditions for healthy forest ecosystems, enhancing the environment’s resilience to landslides and erosion
Improve agricultural activity and food security

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Why does flooding occur?

When it rains too much:

Flooding often occurs when soil becomes completely saturated with water. There is nowhere else for the water to go, so it starts to wash over the ground.
This can happen when it rains very hard for a short time, or when it rains softly for a long time. Both the amount of rain and the time for infiltration plays a role in flooding.

After long periods without rain:

Droughts are often followed by floods. Why do you think this is?

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Why do floods often follow droughts?

Compacted Soil - During extended droughts, soil becomes hard, acting like concrete. This reduces its ability to absorb water when it rains.

Vegetation Loss - Drought stress can kill or weaken plants and trees, which play a critical role in stabilizing the soil and absorbing water.

Compacted Soil - During extended droughts, soil becomes hard, acting like concrete. This reduces its ability to absorb water when it rains, increasing runoff and the likelihood of flooding.

Vegetation Loss - Drought stress can kill or weaken plants and trees, which play a critical role in stabilizing the soil and absorbing water. Without this vegetation, there is less rainwater absorption and therefore more runoff.

Earth’s natural process of moving, distributing, and managing water is quite an incredible phenomena. In the hydrologic spectrum, droughts and floods represent two extreme ends - however, they are frequently interlinked. Many are often surprised to learn that floods follow periods of drought.

This is because when the ground is too dry, it can’t absorb rain fast enough and so the water runs over land, flooding the surrounding area

As climate change alters precipitation patterns, some regions may experience prolonged droughts punctuated by heavy rainstorms, creating ideal conditions for flooding. California, for example, was facing its worst drought in a millennium in 2022. Then, during the first three months of 2023, heavy rains caused record flooding

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Healthy soil retains more water.

So what makes soil healthy?

Soil is one of the main reservoirs of biodiversity on the planet – a teaspoon of soil contains approximately 50 billion microbes and tens of thousands of species.

Fungal mycelium networks transport nutrients, water and carbon, and create a stable structure which prevents erosion.

Soil is also an important carbon store, absorbed by plants during photosynthesis. Scientists believe more carbon resides in soil than in the atmosphere and all plant life combined!

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Unfortunately, intensive agriculture practices (such as industrial fertilizer, monocropping and repeated ploughing) have eliminated the life and biodiversity from soil, leaving nothing but lifeless dirt.

This then accelerates erosion, as lifeless dust is blown away and washed downstream – carbon is also released as soil washes into waterways.

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How can we improve soil health?

1. Increase Organic Matter

(such as manure and compost)

2. Plant Diverse Species

to provide a range of benefits to your soil

3. Grazing Management

to allow soil to recover

4. Reduce Pesticide Use

to allow earthworms to flourish

5. Plant Vegetation

to increase water infiltration and minimise soil erosion

Increase organic matter inputs - As organic matter such as manure and compost decays into humus, it improves soil structure and drainage, holds moisture, and provides nutrients to the soil. Perennial, grass-dominant pastures are effective in increasing organic matter. Crops are also a valuable source, and residues should be returned to the soil.
Plant Diverse species - A variety of plants means a range of benefits for your soil. Crops with taproots can ease compaction, while fibrous roots add stability. Species that host beneficial mycorrhizal fungi increase its presence in the soil, while legumes add nitrogen. Multi-species pasture cropping, sowing crops into dormant pasture, is one proven technique to improve soil health through plant diversity
Grazing Management - Strategically manage grazing and pastures to maintain ground cover and encourage root growth, both of which are important to protecting the soil and maintaining its health. Time control or rotational grazing allows for the physical and chemical recovery of the soil after each grazing period
Reduce Pesticide Use - Earthworms increase nutrient availability, improve drainage and create more stable soil as they feed on organic matter and burrow underground. These invertebrates can be harmed by the pesticides, so reducing reliance on these chemicals will allow earthworms to flourish.
Control Water Flow - Planting vegetation and adding landscape features can slow the flow of water across your property. This increases the infiltration of water and minimises the soil erosion that can follow heavy rainfall

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3. Flooding

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Types of flooding

Fluvial (river floods) – occurs when the water level of a river overflows onto neighbouring land due to excessive rain or snowmelt

Coastal – the inundation of land along the coast by seawater, usually caused by storm events coinciding with a high tide

Pluvial - occurs when an extreme rainfall event creates a flood independent of an overflowing water body - pluvial flooding can happen in any location, urban or rural, even in areas with no nearby bodies of water

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Pluvial Flooding in Ireland

In north-western Europe, pluvial floods are increasing because:

Flood-generating rainfall is becoming more frequent
Soils are becoming wetter, resulting in a higher water table.

Built-up and agricultural areas are more vulnerable to major flooding events because of a lack of trees and plants that absorb water.

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Rain in an Urban Environment

During periods of heavy rain in urban areas, absorption is low due to impermeable surfaces like concrete
This causes rainwater run-off into stormwater drains, which can overflow.
Overflowing drains means that untreated polluted water (from oil and toxins on the streets, or fertilizer) is washed into rivers, lakes and coastal waters
This causes flood damage and ecosystem pollution.

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So what do we mean by slowing the flow?

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Rising global temperatures is leading to more frequent and severe weather events, intensifying both droughts and floods.

What can we do about this?

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What creates vulnerability to climate impacts such as flooding? Which of these can we do something about?

Infrastructure

Eg buildings, roads

Water, Waste and Energy Systems

Geography

Socioeconomics and Education

History

Land Cover

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Building Climate Resilience

Resilience refers to the ability of communities and ecosystems to prepare for, withstand, and recover from the impacts of climate change

Mitigation means reducing our greenhouse gas emissions to prevent further warming

Adaptation means changing our systems and behaviour to protect ourselves from the impacts of climate change already occurring.

We need to do both. In many cases, mitigation and adaptation actions are linked, such as restoring ecosystems.

Climate mitigation means switching from fossil fuels to renewable energy sources to reduce our greenhouse gas emissions. It also means protecting and enhancing carbon sinks that store these harmful gases, such as oceans, forests and soil.

Examples of climate adaptation include:

Improved water infrastructure -repairing and improving infrastructure in domestic and commercial settings
Water recycling – treating rainwater and wastewater to use in irrigation, industry and for replenishing groundwater supply
Halting deforestation – retaining precious areas of wildlife to reduce flooding and the impacts of climate change.

Resilient soils, wetlands and forests have a higher capacity for water storage. Improved water retention can mitigate the impact of floods and droughts while enhancing the environment’s resilience to landslides and soil erosion.

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4. Nature Based Solutions

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What are Nature Based Solutions?

Nature-based solutions harness the power of nature to address societal challenges through actions to protect, sustainably manage, and restore natural and modified ecosystems, benefiting people and nature at the same time.

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Nature Based Solutions (NbS)

They target major challenges like climate change, disaster risk reduction, food and water security, biodiversity loss and human health.

Examples include:

Restoring ecosystems such as wetlands, woodlands and hedgerows
Restoring forest habitats to protect against erosion and flooding.
Sustainable drainage systems
“Greening” up our cities by bringing nature into them.�

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What Are Nature-based Solutions?

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Urban Nature Based Solutions

Benefits include:

Increased natural habitat for biodiversity and natural species migration
Open green spaces for recreation
Carbon sequestration
Water regulation, provision and purification for water security and disaster risk reduction
Greener and healthier transportation, including bicycle lanes and more walkable cities
Cleaner air, through natural filters
Reduced temperatures

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Urban Nature Based Solutions

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So what does a climate and flood resilient “Spongy City” look like?

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Why should you have a Spongy School?

Global average temperatures are rising. Scientists say that this is destabilizing our weather and climate systems. In Ireland this means there will be more frequent and intense rainfall events in the summer and dryer winters.

When a lot of rain falls in a short amount of time, we can see more flooding in places with a lot of tarmac such as cities, towns, and villages.

In many school neighbourhoods and cities, the ground is covered in tarmac, concrete and stone.

This can make us even more at risk to rainfall related flooding as the water doesn’t have enough space to be absorbed and drains and sewage systems cannot process so much rain in one go.

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What does a Spongy School look like?

Rain Garden

Rainwater planter

Permeable Pavement

Tree Cover and Hedges

Rain Barrel

Green Roofs

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Watch: Living with Flooding

10 Things to Know About - RTÉ Player

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THANK YOU!

GLOBE Ireland

globe@eeu.antaisce.org