AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/5/5-5)
Unit 5 · Topic 5.5
5.5 Irrigation Methods
About 70% of the fresh water people withdraw goes to agriculture, so the way crops are watered matters a lot. Furrow and flood irrigation are cheap but waste water, spray irrigation wastes less, and drip irrigation is the most efficient but costs the most, while overwatering can waterlog soil or leave it salty.
Key terms
- furrow irrigation
- flood irrigation
- spray irrigation
- drip irrigation
- waterlogging
- salinization
Agriculture is the biggest water user
Worldwide, about 70% of the fresh water people withdraw from rivers, lakes and aquifers is used for agriculture, mostly to irrigate crops. Industry and household use make up the rest. Because farming uses so much, making irrigation more efficient is one of the most effective ways to save water.
Irrigation methods
Drip irrigation is the most efficient method, but many farmers, especially in lower-income countries, can't afford it. Cost is the main reason inefficient methods remain common.
| Method | How it works | Approximate water lost to evaporation and runoff | Tradeoffs |
|---|---|---|---|
| Furrow | Water flows down trenches dug between crop rows | About one-third | Cheap and simple; wastes the most water |
| Flood | The whole field is covered with water | About 20% | Cheap; can waterlog soil; used for rice |
| Spray | Water is pumped through sprinklers or pivots | 25% or less | Less waste; more expensive and needs energy to pump |
| Drip | Tubes with small holes release water right at the roots | About 5% | Most efficient; most expensive to install |
Waterlogging and salinization
Waterlogging happens when too much water saturates the soil. The water table rises toward the surface, filling the spaces that normally hold air. Roots can't get the oxygen they need, so crops grow poorly or die.
Salinization is the buildup of salt in soil. All irrigation water, whether from rivers or wells, contains small amounts of dissolved salts. When the water evaporates from the field, especially in hot, dry climates, the salts are left behind. Over years, the salt builds up until the soil is too salty for most crops. Salinization is one of the main ways irrigated land is ruined. Remedies include using drip irrigation (less water, less evaporation), flushing salts with extra fresh water where drainage is good, and planting salt-tolerant crops.
Groundwater depletion
Much irrigation water comes from aquifers. When wells pump faster than precipitation recharges the aquifer, the water table drops. Around a heavily pumped well, the water table forms a funnel-shaped low area called a cone of depression, which can dry up nearby shallow wells. Near coasts, overpumping can pull salt water into the aquifer (saltwater intrusion).
The Ogallala Aquifer, under parts of eight states in the U.S. Great Plains, supplies much of the region's irrigation water. In many areas it's being pumped far faster than it recharges, so it's effectively being mined like a nonrenewable resource.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
How much water must be applied?
A field's crops need 400,000 L of water to reach their roots this season. Using the approximate losses in the table, how much water must be applied with flood irrigation (about 80% reaches the crops) and with drip irrigation (about 95% reaches the crops)? How much would switching to drip save?
Show the solutionHide the solution
- Step 1: Water applied × fraction reaching crops = water the crops get, so water applied = water needed ÷ fraction reaching crops.
- Step 2: Flood: 400,000 ÷ 0.80 = 500,000 L.
- Step 3: Drip: 400,000 ÷ 0.95 ≈ 421,000 L.
- Step 4: Savings: 500,000 − 421,000 ≈ 79,000 L.
Answer: Flood: 500,000 L; drip: about 421,000 L; drip saves about 79,000 L.
- Example 2
Explaining salinization
Farmers in a hot, dry valley have flood-irrigated their fields with river water for 40 years. Crop yields are falling and a white crust has appeared on the soil. Explain what's happening and propose one solution.
Show the solutionHide the solution
- Step 1: Identify the problem: salinization. The white crust is salt.
- Step 2: Explain: river water contains small amounts of dissolved salts. In the hot, dry climate, much of the flood water evaporates and leaves its salts behind. Over 40 years, salt has built up to levels that harm crops.
- Step 3: Solution: switch to drip irrigation, which applies less water directly to roots so less evaporates and less salt is deposited. (Other valid answers: improve drainage and flush salts, or plant salt-tolerant crops.)
Answer: Salinization from evaporating irrigation water; switching to drip irrigation would reduce the salt left behind.
Common mistakes
- Saying salinization comes from irrigating with sea water. It comes from the small amount of salt in ordinary fresh irrigation water, left behind when water evaporates.
- Calculating water needed by multiplying by the efficiency instead of dividing. If only 80% reaches the crops, you must apply more than the crops need.
- Calling drip irrigation the cheapest method. It's the most efficient but the most expensive.
On the exam
- Know the relative efficiency of the four methods and one advantage and disadvantage of each.
- Calculation questions may compare water use under different methods. Set up the equation clearly and include units (liters or gallons).
Connected topics
Videos
Check yourself
4 questions on 5.5 Irrigation Methods. Pick an answer to see if you got it, and why.
| Method | Approximate share of water lost to evaporation and runoff |
|---|---|
| Furrow | About one-third (33%) |
| Flood | About 20% |
| Spray | 25% or less |
| Drip | About 5% |
Typical approximate values for each method
A farmer applies 1,200,000 liters of water per season using furrow irrigation. If she switched to drip irrigation and applied the same amount, about how much more water would reach her crops?
Which statement best explains why drip irrigation is not used on more farms worldwide?
After decades of irrigation in a hot, dry valley, a white crust forms on some fields and crop yields fall. Which process best explains this?
A farmer floods a field with far more irrigation water than the crops need, season after season, and the water table rises close to the surface. Crop growth declines. Which problem is most likely?
0 of 4 answered