AP Environmental Science

## Learning Outcomes

- Understand why irrigation is the largest human use of freshwater
- Identify the difference between **freshwater withdrawal**, **consumptive use**, and **return flow**
- Explain how surface water and groundwater are used for irrigation
- Compare major irrigation methods by water efficiency and environmental impact
- Work through a simple irrigation efficiency calculation
- Understand how irrigation can cause aquifer depletion, salinization, and waterlogging
- Apply these ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand freshwater use for irrigation, with a focus on the following syllabus points:

- Know that irrigation is the artificial application of water to crops and that agriculture accounts for about **70% of global freshwater withdrawals**
- Identify the difference between **freshwater withdrawal** and **consumptive use**
- Explain how irrigation water can come from **surface water** sources or **groundwater** aquifers
- Compare irrigation methods, especially **surface/flood**, **furrow**, **sprinkler**, and **drip** irrigation
- Understand that water withdrawn for irrigation is usually greater than crop water need because of **conveyance loss** and low **application efficiency**
- Explain how over-irrigation can cause **salinization** and **waterlogging**
- Understand how heavy groundwater pumping for irrigation can lower water tables and contribute to **aquifer depletion**
- Interpret simple irrigation efficiency calculations and method comparisons

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between freshwater withdrawal and consumptive use in irrigation?
2. Why does drip irrigation usually conserve more freshwater than flood irrigation?
3. How can irrigation lead to salinization of soil?
4. What is one major difference between surface-water irrigation and groundwater irrigation?
5. Why are irrigation withdrawals usually larger than the actual amount of water crops need?

## Introduction

Freshwater use for irrigation sits at the center of AP Environmental Science Unit 5 because it connects agriculture, water resources, soil quality, and sustainability. Irrigation has allowed crops to be grown in dry regions and has raised food production, but it also places very heavy pressure on rivers, reservoirs, and aquifers.

For the exam, students often lose marks not because they do not know what irrigation is, but because they mix up related terms. They may confuse water **withdrawal** with water actually **consumed** by crops, or assume all “lost” irrigation water is permanently gone. This topic rewards precise language and clear cause-and-effect thinking.

> **Key Term: irrigation**
> The artificial application of water to crops.

### Why irrigation uses so much freshwater

Irrigation is the largest human use of freshwater worldwide. A common APES figure is that agriculture accounts for about **70% of global freshwater withdrawals**. In the United States, irrigation also represents a very large share of freshwater use, especially in dry western states.

Why is this number so high? The basic reason is biological. Plants need water for photosynthesis, nutrient transport, cell structure, and cooling through transpiration. In humid climates, rainfall may supply much of this need. In dry climates, or during drought, farmers must supply that water themselves.

The key point is that crops do not just need water once. They need a continuing supply through the growing season. A field of corn, rice, vegetables, or fruit trees may require repeated watering, and large farms cover huge areas. That turns individual plant needs into very large regional withdrawals.

Another reason irrigation demand is so high is inefficiency. Not all the water taken from a river, reservoir, or aquifer reaches plant roots. Some is lost while moving to the field. Some evaporates. Some infiltrates too deeply below the root zone. Some runs off the field.

> **Key Term: freshwater withdrawal**
> Water removed from a river, lake, reservoir, or aquifer for use.
>
> **Key Term: consumptive use**
> The portion of withdrawn water that is evaporated, transpired, incorporated into crops, or otherwise not immediately available for reuse.

A very common exam mistake is to treat these terms as identical. They are not. Withdrawal is the total amount taken. Consumptive use is the amount effectively used up from the local system.

For example, if a farmer pumps 100 units of water from a source and only 40 units are lost through evapotranspiration, then the withdrawal is 100 units but consumptive use is 40 units. The rest may be runoff, seepage, or recharge.

> **Test Tip:** If a question asks how much water is “taken from” a source, think **withdrawal**. If it asks how much water is “not returned” because crops use it or it evaporates, think **consumptive use**.

### Surface water, groundwater, and where irrigation water comes from

Irrigation water comes from two main source types: **surface water** and **groundwater**.

Surface-water irrigation uses rivers, lakes, reservoirs, canals, or aqueducts. A major example is irrigation using water diverted from the Colorado River system. This kind of irrigation can reduce streamflow downstream, shrink reservoir storage, and increase competition among cities, farms, and wildlife habitatss.

Groundwater irrigation uses wells that pump water from aquifers. A major APES example is the **Ogallala Aquifer** beneath the Great Plains. This aquifer supports a large share of U.S. irrigation, but heavy pumping has lowered water tables in many places.

> **Key Term: aquifer depletion**
> The lowering of groundwater reserves when water is removed faster than natural recharge replaces it.

The distinction matters because the environmental risks differ.

With surface water:

- rivers may lose flow
- wetlands and downstream habitats may shrink
- reservoirs may fall
- water conflicts may increase between upstream and downstream users

With groundwater:

- water tables may drop
- wells may go dry
- pumping costs may rise because water must be lifted from greater depth
- in some areas, land subsidence can occur
- recharge may take a very long time

The Ogallala Aquifer is a strong case study because it shows how irrigation can support major food production in dry regions, but also how that success can depend on water that is being depleted faster than nature can replace it.

Students should also remember that not all groundwater is replenished quickly. Some aquifers recharge slowly, especially in semi-arid areas. That means irrigation based on groundwater can appear successful for decades, while quietly creating a long-term water crisis.

### Why withdrawals exceed crop water need

A crop’s water need is often described by evapotranspiration minus effective rainfall. In simple terms, crops need enough added water to replace what they lose and what rainfall does not already supply.

But the actual withdrawal from a source must be larger than that crop need because irrigation systems are imperfect.

There are two major reasons:

1. **Conveyance loss**  
   Water can be lost before it even reaches the field.

2. **Application inefficiency**  
    Water reaching the field is not always stored in the crop root zone.
   > **Key Term: conveyance loss**
   > Water lost while moving from the source to the field, usually by seepage, leaks, or evaporation.
   >
   > **Key Term: application efficiency**
   > The fraction of water delivered to the field that is beneficially stored in the crop root zone for plant use.

Some water that is not used by plants becomes **return flow**.

> **Key Term: return flow**
> Irrigation water that reenters surface water or groundwater and may become available again later.

This is another subtle point. Water that leaves the field is not always gone forever. From the farm’s point of view, it may be wasted. From a watershed point of view, some of it may recharge groundwater or reenter streams. That is why withdrawals are usually greater than consumptive use.

A useful APES-style relationship is:

- withdrawals = consumptive use + return flow + losses

This does not mean the system is harmless. Even if some water returns later, high withdrawals can still reduce immediate water availability and damage natural habitatss.

#### Worked example: gross irrigation requirement

Suppose a crop needs **60 units** of water in the root zone after accounting for rainfall.

If irrigation efficiency is **60%**, how much water must be withdrawn?

Step 1: Write the relationship

Gross withdrawal = net crop water need ÷ efficiency

Step 2: Convert efficiency to decimal

60% = 0.60

Step 3: Calculate

Gross withdrawal = 60 ÷ 0.60 = 100 units

So the farmer must withdraw **100 units** of water to meet a **60-unit** crop need.

If the same crop were irrigated with **90%** efficiency:

Gross withdrawal = 60 ÷ 0.90 = 66.7 units

This shows why efficient irrigation matters. The crop need is unchanged, but the amount withdrawn from the environment falls sharply.

> **Test Tip:** On calculation questions, do not subtract efficiency from the net need. Divide by efficiency. That is the most common student error.

### Comparing irrigation methods and freshwater efficiency

APES expects you to compare irrigation methods by efficiency and impact. A good ranking from least to most efficient is:

surface/flood < furrow < sprinkler/spray < drip

Typical approximate efficiencies often used in introductory environmental science are:

- surface or furrow: about **60%**
- sprinkler: about **75%**
- drip: about **90–95%**

These are approximate, not fixed constants. Real efficiency depends on slope, climate, maintenance, soil, and management.

#### Surface or flood irrigation

In flood irrigation, water is pumped or diverted into a field and allowed to flow across the surface.

Advantages:

- simple
- cheap to install
- widely used in less-developed regions

Disadvantages:

- high water use
- large evaporative and runoff losses
- can cause waterlogging
- can contribute to salinization

Flood irrigation often applies more water than plants actually need. In hot climates, some evaporates before being useful. In flat fields, water may sit too long. In sloped fields, runoff increases.

#### Furrow irrigation

In furrow irrigation, water flows through channels between crop rows.

Advantages:

- cheaper than high-tech systems
- can work for row crops

Disadvantages:

- uneven distribution
- more infiltration near the top of the furrow
- less water reaches plants near the far end
- losses to evaporation and runoff

This unevenness is a classic APES detail. Students should know that furrow systems often overwater one part of the field and underwater another.

#### Sprinkler irrigation

Sprinklers spray water over crops through overhead systems.

Advantages:

- more even than flood or furrow
- useful on varied terrain
- can cover large fields

Disadvantages:

- expensive
- needs energy to operate
- water can evaporate during spraying
- wind can blow spray off target

Sprinkler systems improve efficiency, but they do not eliminate losses. In hot, dry, or windy places, much sprayed water never reaches the root zone.

#### Drip irrigation

Drip irrigation delivers water slowly and directly to plant roots through tubes or emitters.

Advantages:

- highest efficiency
- minimal evaporation
- low runoff
- low deep percolation loss
- can reduce weed growth because non-target soil stays drier

Disadvantages:

- high setup cost
- maintenance needs
- emitters can clog with debris or mineral buildup
- often used mainly on high-value crops because of cost

Drip irrigation is the most efficient method because it targets the root zone directly rather than wetting the whole field surface or spraying water into the air.

This is why APES frequently treats drip irrigation as the best answer for reducing freshwater withdrawals.

### Environmental impacts of irrigation: why more water is not always better

Students often assume irrigation is simply beneficial because it raises crop yield. That is only half the story. Excess irrigation can damage both soil and water systems.

#### Salinization

In dry climates, irrigation water often contains dissolved salts. When water evaporates from soil, those salts remain behind. Over time, salt accumulates in topsoil and harms plant growth.

> **Key Term: salinization**
> The buildup of salts in soil as irrigation water evaporates and leaves dissolved salts behind.

This is especially serious in arid and semi-arid regions because evaporation is high and rainfall is too low to flush salts away naturally. Crop yields fall, plants become stunted, and land may eventually become unproductive.

#### Waterlogging

If too much irrigation water remains in the soil, the root zone becomes saturated.

> **Key Term: waterlogging**
> Saturation of soil with excess water, which reduces oxygen available to roots.

Plant roots need oxygen. In waterlogged soils, oxygen diffusion drops sharply, and roots can suffocate. Waterlogging can also raise the water table and worsen salinization later when the soil dries out.

#### Aquifer depletion and water table decline

Overpumping groundwater for irrigation lowers the water table. Wells may need deeper pumps, which raises energy costs. Nearby streams, ponds, or wetlands may also lose water if they are connected to the aquifer.

#### Runoff and nutrient transport

Excess irrigation water can carry fertilizers into nearby water bodies. That links Unit 5 to Unit 8, because irrigation runoff can contribute to eutrophication.

Irrigation is therefore not just a water issue. It is also a soil issue, an energy issue, and a pollution issue.

### Evaluating trade-offs on the AP exam

APES questions often ask you to judge a method, not just define it. The best answers show trade-offs.

For example:

- **Drip irrigation** conserves water and can increase yield, but costs more and needs maintenance.
- **Flood irrigation** is cheap and simple, but wastes water and can damage soil.
- **Groundwater irrigation** provides reliable supply in dry years, but may deplete aquifers.
- **Surface-water irrigation** can support large farms, but may reduce downstream river flow.

A high-scoring answer should avoid extreme claims. Drip irrigation does not reduce crop water demand itself. Crops still need similar biological amounts of water. What drip reduces is the amount that must be withdrawn to meet that need.

Likewise, not every irrigation loss is a total loss at the watershed scale. Some becomes return flow. But return flow does not erase the local environmental impacts of overwithdrawal.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Irrigation is the artificial application of water to crops
- Agriculture accounts for about 70% of global freshwater withdrawals in many APES sources
- Freshwater withdrawal means water taken from a source, while consumptive use means water not immediately returned
- Irrigation water can come from surface water or groundwater
- Surface-water irrigation can reduce river flow and reservoir levels
- Groundwater irrigation can cause aquifer depletion and falling water tables
- Withdrawals are usually greater than crop water need because of conveyance loss and application inefficiency
- Surface and flood irrigation are less efficient than sprinkler irrigation
- Drip irrigation is the most efficient major irrigation method because it delivers water directly to roots
- Irrigation can cause salinization when evaporation leaves salts in topsoil
- Irrigation can cause waterlogging when soil becomes saturated and roots lose oxygen
- Efficient irrigation reduces the amount of freshwater that must be withdrawn for the same crop need

## Key Terms and Concepts

- irrigation
- freshwater withdrawal
- consumptive use
- aquifer depletion
- conveyance loss
- application efficiency
- return flow
- salinization
- waterlogging


Irrigation Methods - Freshwater use for irrigation