What is the primary difference between waterlogging and salinization in terms of plant health?

Waterlogging kills plants by depriving roots of oxygen needed for respiration, while salinization kills plants through osmotic stress and salt toxicity that prevents water absorption.

How does aquifer depletion lead to saltwater intrusion in coastal areas?

As freshwater is pumped out, the pressure of the freshwater lens decreases, allowing denser saltwater from the ocean to move inland and contaminate the groundwater supply.

Why is drip irrigation considered a solution for soil salinization?

Drip irrigation reduces the total volume of water used and minimizes evaporation at the surface, which prevents the rapid accumulation of salts compared to flood or furrow methods.

True or False: Salinization only occurs if farmers use salt water for their crops.

False. Even freshwater contains small amounts of dissolved salts which become concentrated in the soil as the water evaporates or is used by the plants.

What is a common misconception about the cause of land subsidence?

Many believe it is caused by the weight of the soil, but it is actually caused by the loss of water pressure in an aquifer, which allows the soil and rock particles to collapse into the empty spaces.

Why is 'flushing' soil not always a viable solution for salinization?

Flushing requires massive amounts of clean freshwater, which is usually scarce in the arid regions where salinization occurs, and it can lead to waterlogging if drainage is poor.

Define 'Capillary Action' in the context of soil science.

Capillary action is the upward movement of water through tiny soil pores against gravity, which can bring dissolved salts from high water tables up to the surface.

An aquifer is an underground layer of permeable rock, gravel, or sand that stores and transmits groundwater, serving as a primary source for irrigation.

What is land subsidence?

Land subsidence is the gradual settling or sudden sinking of the Earth's surface due to the removal of subsurface support, such as the over-pumping of groundwater.

How does a high water table contribute to soil salinization?

A high water table brings water closer to the surface where it can evaporate more easily, leaving behind salts that were dissolved in the groundwater.

Impacts of Irrigation | College Board AP Environmental Science

Revision Notes

College Board

Environmental Science

Unit 5: Land & Water Use

Impacts of Irrigation

Summary

While irrigation is vital for global food security, it can lead to severe environmental consequences including soil degradation through waterlogging and salinization, as well as the long-term depletion of groundwater resources. Understanding these impacts is essential for developing sustainable agricultural practices that balance high crop yields with ecosystem health.

1. Soil Waterlogging

Waterlogging occurs when excessive irrigation or poor drainage causes the soil to become completely saturated with water, raising the local water table. This saturation fills the pore spaces in the soil that are normally occupied by air, effectively cutting off the supply of oxygen to plant roots.

Without adequate oxygen, roots cannot perform cellular respiration, leading to nutrient uptake issues, yellowing of leaves (chlorosis), and eventual root rot. This condition is most common in heavy clay soils or low-lying areas where water cannot easily infiltrate or drain away.

Diagram showing a cross-section of soil with a raised water table, capillary action moving water upward, and salt accumulation at the surface due to evaporation.

2. Soil Salinization

Salinization is the process by which water-soluble salts accumulate in the soil layers, often reaching levels that are toxic to most plants. This typically happens in arid regions where high evaporation rates pull water toward the surface, leaving behind concentrated mineral deposits.

Even freshwater used for irrigation contains small amounts of dissolved salts; as the water evaporates or is transpired by plants, these salts remain in the soil. Over time, this creates a hard salt crust on the surface that prevents water infiltration and disrupts the osmotic balance of plant cells, making it harder for them to absorb water.

3. Aquifer Depletion

Aquifer depletion occurs when groundwater is pumped out for irrigation at a rate faster than the natural recharge from precipitation and infiltration can replace it. Because agriculture accounts for approximately 70% of global freshwater use, heavy reliance on groundwater can lead to a permanent drop in the water table.

As aquifers are emptied, the structural integrity of the underground rock layers can fail, leading to land subsidence, where the ground surface actually sinks. In coastal regions, the reduction in freshwater pressure can allow saltwater to seep into the aquifer, a process known as saltwater intrusion, which ruins the well water for both farming and domestic use.

4. Key Distinctions

5. Exam Strategy & Tips