AP Environmental Science

## Learning Outcomes

- Explain why furrow irrigation often wastes water compared with drip irrigation
- Identify the main drawbacks of furrow irrigation, including runoff, deep percolation, evaporation, and erosion
- Understand how slope, soil texture, furrow length, and timing affect irrigation efficiency
- Distinguish between waterlogging and salinization as irrigation-related soil problems
- Apply application efficiency ideas to simple AP Environmental Science calculations
- Analyse how furrow irrigation can contribute to sediment, nutrient, pesticide, and salt pollution

## AP Environmental Science Syllabus

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

- Know that **furrow irrigation** is a type of **surface irrigation** in which water flows by gravity through channels between crop rows
- Identify major furrow-irrigation losses as runoff, deep percolation, and evaporation
- Explain why water distribution in a furrow is often uneven, with more infiltration near the inlet and less near the far end
- Understand how excess irrigation can contribute to **waterlogging** and **salinization**
- Recognise that furrow irrigation can increase soil erosion and sediment transport
- Understand that irrigation return flows may carry sediment, nutrients, pesticides, and salts into nearby water bodies
- Compare furrow irrigation with drip irrigation in terms of cost, water efficiency, and environmental impact
- Apply a simple water-balance or application-efficiency calculation to an irrigation scenario

## 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. Why does furrow irrigation often deliver too much water to the top of a field and too little to the bottom?
2. What is the difference between runoff loss and deep percolation loss?
3. How can furrow irrigation contribute to both waterlogging and salinization?
4. Why is furrow irrigation considered less efficient than drip irrigation?
5. Name one environmental problem caused by furrow irrigation besides water waste.

## Introduction

Irrigation is essential to food production, especially in dry climates and in places where rainfall is seasonal or unreliable. In AP Environmental Science, irrigation is not just about helping crops grow. It is also about understanding trade-offs: higher yields can come with aquifer depletion, soil degradation, erosion, and water pollution.

Furrow irrigation is a common low-cost method, but it has several serious drawbacks. The exam may test these as direct facts, as cause-and-effect chains, or in free-response questions about sustainable agriculture, water use, and pollution. To score well, you need to explain not only what the drawbacks are, but why they happen.

> **Key Term: furrow irrigation**
> A surface irrigation method in which water flows by gravity through small channels, or furrows, between crop rows.

### Why furrow irrigation is often inefficient

Furrow irrigation looks simple: farmers apply water at the upper end of a field, and gravity moves it down the furrows. The problem is that water does not reach every part of the furrow at the same time. The upper part gets wet first and stays exposed to flowing water for longer. That longer contact time means more infiltration into the soil at the top of the field.

The lower end gets water later. If the farmer shuts the flow off too early, the lower end may not receive enough water. If the farmer keeps the water on long enough to fully wet the lower end, then the upper end is often overirrigated. This is the basic design problem of furrow irrigation: equal water application is hard to achieve.

> **Key Term: nonuniform infiltration**
> Uneven soaking of water into soil along a furrow because some parts of the field are exposed to water for longer than others.

This is why students should not say only that furrow irrigation wastes water through evaporation. Evaporation matters, but in many field situations the larger losses are runoff and deep percolation. That distinction is high value on the exam.

> **Key Term: deep percolation**
> Water that infiltrates below the crop root zone and becomes unavailable to the plant.

Deep percolation is especially likely near the upper end of the furrow. The water arrives there first, so it has more time to soak in. If the soil is coarse or sandy, the problem becomes worse because water moves downward quickly through large pore spaces. The crop only benefits from water stored in the root zone. Water below that depth is effectively wasted.

At the far end of the furrow, the opposite problem occurs. Water may arrive too late or in too small a quantity. So one part of the field can be overwatered while another part is underwatered during the same irrigation event. That is why furrow systems can have poor distribution uniformity.

A simple way to think about this is as a trade-off:

- Shut water off early: less deep percolation at the top, but crops at the bottom may stay too dry
- Keep water running longer: the bottom gets enough water, but the top loses more water below the root zone and may become waterlogged

This is one reason drip irrigation is generally more efficient. Drip systems apply water directly to the root zone and reduce the distance water has to travel across the field surface.

> **Test Tip:** In an FRQ, do not write that furrow irrigation is inefficient “because of evaporation” and stop there. Add runoff and deep percolation, because those are often the larger losses and show stronger understanding.

### The main water losses: runoff, tailwater, evaporation, and a worked example

Water applied to furrow systems can be divided into useful water and losses. Useful water is the amount stored in the crop root zone. The main losses are:

- **runoff** from the field surface
- **deep percolation** below the root zone
- **evaporation** from exposed water and wet soil

When runoff leaves the lower end of the field, it is often called tailwater.

> **Key Term: tailwater**
> Runoff water leaving the downstream end of a furrow-irrigated field.

Tailwater is easy to see, which makes runoff one of the most visible furrow-irrigation losses. If water flows off the field instead of infiltrating into the root zone, the system is wasting freshwater and may also carry pollutants away.

The textbook material for AP Environmental Science notes that furrow irrigation can lose about one-third of its water to evaporation and runoff. Even where the exact percentage varies by field conditions, the bigger point is clear: furrow irrigation often has relatively low water-use efficiency.

A useful exam calculation is **application efficiency**, the percentage of applied water actually stored in the root zone.

> **Key Term: application efficiency**
> The percentage of irrigation water applied to a field that is stored in the crop root zone and is available to plants.

The formula is:

Application efficiency = (water stored in root zone / water applied) × 100

Worked example:

A farmer applies **1,200 L** of water to a furrow-irrigated plot.  
Of that total:

- **700 L** is stored in the root zone
- **300 L** runs off as tailwater
- **200 L** is lost to deep percolation

Step 1: Identify useful water  
Useful water = **700 L**

Step 2: Divide by total applied water  
700 ÷ 1,200 = **0.5833**

Step 3: Convert to a percentage  
0.5833 × 100 = **58.3%**

So, the **application efficiency = 58.3%**

This means **41.7%** of the applied water was lost.

You can also check the water balance:

- Stored water: 700 L
- Runoff: 300 L
- Deep percolation: 200 L
- Total = 1,200 L

The numbers add up, so the calculation is consistent.

A student might now compare this with a drip system, where a much larger proportion would usually reach the root zone.

> **Test Tip:** In calculation questions, always label useful water clearly. Students often use total infiltrated water instead of water stored in the root zone, which overstates efficiency.

### Soil and pollution impacts: erosion, waterlogging, salinization, and return flow

Furrow irrigation does more than waste water. It can also damage soil and spread pollution.

One major issue is erosion. Water moving along open furrows can detach and carry away soil particles. This is especially serious on steeper slopes or when inflow is too strong. Because the process depends on moving water across bare soil channels, furrow irrigation can be inherently erosive.

Erosion matters for two reasons. First, the field loses topsoil, reducing fertility and long-term productivity. Second, the eroded sediment enters streams, canals, ponds, or rivers.

Sediment often carries other pollutants with it, including:

- nitrogen from fertilizers
- phosphorus attached to soil particles
- pesticides
- salts

This means runoff from furrow irrigation can become a form of nonpoint source pollution.

> **Key Term: irrigation return flow**
> Water leaving an irrigated field that may carry sediment, nutrients, pesticides, and salts into nearby water bodies.

This connects Unit 5 to Unit 8. A question could start with irrigation method choice and end with eutrophication or aquatic pollution. If runoff carries nitrogen and phosphorus into a lake or stream, algal growth may increase. That can raise biological oxygen demand and lower dissolved oxygen.

Another set of furrow-irrigation problems affects the soil itself: **waterlogging** and **salinization**.

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

Waterlogging happens when excess water remains in the soil and raises the water table. Roots need oxygen for respiration. In waterlogged soils, pore spaces fill with water instead of air, so roots struggle to obtain oxygen. This can reduce growth or kill crops.

Waterlogging is especially likely at the upper end of a furrow because that part gets watered for the longest time. Poor drainage makes the problem worse.

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

Salinization often follows repeated irrigation in dry climates. Irrigation water always contains some dissolved salts. Plants use the water, and surface evaporation removes the water, but the salts remain in the soil. Over time, these salts accumulate, especially where drainage is poor.

Students sometimes confuse waterlogging and salinization, but they are not the same thing.

- **Waterlogging** = too much water in the soil
- **Salinization** = too much salt left in the soil

They are linked, though. Poor drainage can cause waterlogging, and when that wet soil later dries, salts may be left near the surface. Both problems reduce crop yields and contribute to land degradation.

Furrow irrigation can also shift salts unevenly within a field. Depending on flow pattern, soil type, and evaporation, salts may collect on ridges or in parts of the root zone. That means one part of a field may become more saline than another.

### Why field conditions matter and how farmers can reduce the drawbacks

Furrow irrigation is not equally poor in every situation. Its performance depends strongly on field design and management.

Important factors include:

- **Slope**: Furrows work best on gentle, uniform slopes. If the slope is too steep, runoff and erosion increase.
- **Soil texture**: Sandy soils allow rapid infiltration, increasing deep percolation losses. Heavy clay soils may increase runoff and waterlogging if infiltration is slow.
- **Furrow length**: Long furrows make uniform water delivery harder because the time difference between upper and lower ends increases.
- **Inflow rate**: If water enters too quickly, runoff and erosion rise. If it enters too slowly, the lower end may stay dry.
- **Cutoff timing**: If water is shut off too early, the lower field is underirrigated. Too late, and the upper field is overirrigated.

These variables explain why furrow irrigation often requires more management skill than students assume. It is simple in design, but not simple to optimize.

Farmers can improve furrow irrigation in several ways:

- **Land leveling or grading** so water moves more evenly
- **Improved cutoff timing** to reduce excess application
- **Tailwater recovery** systems that capture runoff for reuse
- **Surge irrigation** or cutback flow methods that improve infiltration patterns
- **Better drainage** to reduce waterlogging and salinity buildup

Still, even with improvements, furrow irrigation usually remains less efficient than drip irrigation. Drip systems reduce runoff, reduce deep percolation, and apply water directly to roots. Their main drawback is cost, especially for lower-value crops and poorer farmers.

This is an important APES evaluation point: the “best” method depends on context.

- In water-scarce areas, drip irrigation is usually more sustainable
- In low-income settings, furrow irrigation may persist because it is cheap and easy to set up
- In flat row-crop fields with good management, furrow systems can perform better than badly designed surface systems

So the exam answer should not simply say “furrow irrigation is bad.” A stronger answer says it is inexpensive and widely used, but often has lower efficiency and greater environmental impact than drip irrigation.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Furrow irrigation is a surface irrigation method in which water flows by gravity through channels between crop rows
- Water reaches the upper end of a furrow first, so infiltration there is usually greater than at the lower end
- The main furrow-irrigation losses are runoff, deep percolation, and evaporation
- Tailwater is runoff leaving the downstream end of the field
- Deep percolation is water that moves below the crop root zone and cannot be used by plants
- Furrow irrigation often has lower application efficiency than drip irrigation
- Furrow irrigation can cause soil erosion, especially on steeper slopes or with poorly controlled flow
- Irrigation return flow can carry sediment, nutrients, pesticides, and salts into nearby water bodies
- Overirrigation can lead to waterlogging, which reduces oxygen available to roots
- Repeated irrigation in dry areas can cause salinization as evaporating water leaves salts behind
- Soil texture, slope, furrow length, and cutoff timing all affect furrow irrigation performance
- Land leveling, better timing, and tailwater recovery can reduce some furrow-irrigation drawbacks

## Key Terms and Concepts

- furrow irrigation
- nonuniform infiltration
- deep percolation
- tailwater
- application efficiency
- irrigation return flow
- waterlogging
- salinization


Irrigation Methods - Furrow irrigation drawbacks