Which way of watering crops wastes the least water, and why does irrigation sometimes ruin the soil it feeds?
Topic 5.5 Irrigation Methods: compare the main irrigation methods and explain the problems of salinisation, waterlogging and aquifer depletion.
A focused answer to APES Topic 5.5, covering flood (furrow), spray, drip and other irrigation methods, their water efficiency, and the problems of salinisation, waterlogging and aquifer depletion, with a worked irrigation-efficiency calculation.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
The College Board (Topic 5.5) wants you to compare irrigation methods by water efficiency and explain the problems they cause: salinisation, waterlogging and aquifer depletion.
The main methods
Salinisation and waterlogging
Salinisation and waterlogging are why irrigation can degrade the very land it feeds, especially where drainage is poor.
Aquifer depletion
Many aquifers are effectively non-renewable on a human timescale, because they recharge extremely slowly (sometimes over thousands of years), so the water pumped today is not replaced within a lifetime. This makes groundwater-fed irrigation a clear example of using a resource faster than it regenerates, the core problem that sustainability (Topic 5.12) addresses, and it is why efficient methods and limits on withdrawal matter so much in dry farming regions.
Why this matters
Irrigation ties agriculture to the water cycle (Topic 1.7) and watersheds (Topic 4.6), and its problems (salinisation, depletion) are major forms of land degradation. Choosing efficient methods such as drip irrigation is a key sustainable agriculture (Topic 5.15) strategy, especially as water scarcity grows.
Try this
Q1. Identify the irrigation method with the highest water-use efficiency. [1 point]
- Cue. Drip irrigation.
Q2. Explain how irrigation can cause salinisation of soil. [2 points]
- Cue. Irrigation water carries dissolved salts; as the water evaporates from the soil surface, the salts are left behind and accumulate over repeated cycles, eventually raising soil salinity to levels that damage crops.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Identify the most water-efficient common irrigation method. (b) Explain why flood (furrow) irrigation wastes more water than drip irrigation. (c) Describe how repeated irrigation can cause salinisation. (d) Explain what happens when groundwater is withdrawn from an aquifer faster than it is recharged.Show worked answer →
A 4-point FRQ on irrigation.
(a) Identify (1 point): drip irrigation, which delivers water directly to plant roots and loses the least to evaporation and runoff.
(b) Explain (1 point): flood (furrow) irrigation spreads water across the field surface, where much is lost to evaporation and runoff and only some reaches the roots, while drip targets the roots directly.
(c) Describe (1 point): irrigation water contains dissolved salts; as the water evaporates from the soil surface, the salts are left behind and accumulate, raising soil salinity until it harms crops (salinisation).
(d) Explain (1 point): the water table drops (aquifer depletion); wells can run dry, the land may subside, and along coasts saltwater can intrude into the aquifer.
Markers reward drip as most efficient, surface losses for flood irrigation, salt left by evaporation for salinisation, and falling water table or saltwater intrusion for over-pumping.
AP 2018 (style)1 marksSection I (multiple choice). Which irrigation method generally has the highest water-use efficiency (least water lost to evaporation and runoff)? (A) Flood irrigation (B) Furrow irrigation (C) Spray irrigation (D) Drip irrigation. Justify your choice.Show worked answer →
A 1-point MCQ on irrigation methods. The answer is (D).
Drip irrigation delivers water slowly and directly to the base of each plant, so very little is lost to evaporation or runoff, making it the most efficient. Flood (A) and furrow (B) irrigation lose large amounts to evaporation and runoff across the soil surface; spray (C) loses water to evaporation and wind drift. The trap is choosing spray as "modern"; drip is the most efficient.
Related dot points
- Topic 5.4 The Impact of Agricultural Practices: explain how tillage, fertilizer use, overgrazing and other farming practices degrade soil and water.
A focused answer to APES Topic 5.4, covering how tillage, fertilizer use, overgrazing, and confined animal feeding degrade soil and water through erosion, nutrient runoff, salinisation, desertification and waste, with a worked nutrient-loading calculation.
- Topic 5.3 The Green Revolution: describe the methods and benefits of the Green Revolution and explain its environmental costs.
A focused answer to APES Topic 5.3, covering the Green Revolution, high-yield crop varieties, fertilizers, pesticides, irrigation and mechanisation, its benefits for food supply, and its environmental costs, with a worked yield-increase calculation.
- Topic 5.15 Sustainable Agriculture: describe sustainable farming practices that conserve soil and water and maintain long-term productivity.
A focused answer to APES Topic 5.15, covering sustainable agriculture practices (crop rotation, contour ploughing, terracing, no-till, cover crops, strip cropping, agroforestry, rotational grazing) and how each conserves soil and water and maintains productivity, with a worked erosion-reduction calculation.
- Topic 4.6 Watersheds: define a watershed, describe the factors that affect its characteristics, and explain how land use changes runoff and water quality.
A focused answer to APES Topic 4.6, covering the definition of a watershed, divides, the factors that shape watershed behavior (area, slope, vegetation, soil), runoff versus infiltration, and how land use affects flooding and water quality, with a worked runoff comparison.
- Topic 1.7 The Hydrologic (Water) Cycle: describe the processes of the water cycle and explain how human activities alter the storage and movement of water.
A focused answer to APES Topic 1.7, covering evaporation, transpiration, condensation, precipitation, runoff, infiltration and groundwater, and how deforestation, paving and irrigation alter the cycle, with a worked water-budget calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)