Impacts of Irrigation (College Board AP® Environmental Science) : Study Guide

What is salinization?

Definition of salinization

• Salinization occurs when salts accumulate in the soil

  • This reduces soil fertility and harms plant growth

• This happens when water evaporates from the soil, leaving behind salt deposits

• Over time, excessive salt buildup can make soil toxic to plants

  • This can prevent crops from growing

Causes of salinization

Irrigation practices

• Over-irrigation can cause water to seep into the soil, carrying salts that accumulate over time

• Poor drainage systems allow water to stagnate and evaporate, leaving salt residues

• Use of saline groundwater for irrigation increases soil salinity

High evaporation rates

• In hot, arid regions, water evaporates quickly, leaving behind concentrated salts

  • Example: Dry climates like parts of California, Australia, and the Middle East experience high levels of salinization

Rising water tables

• In areas with high groundwater levels, salts can rise to the surface through capillary action

• This is common in low-lying agricultural lands where groundwater is heavily used

  • Agricultural lands at lower elevations tend to have higher groundwater levels

  • When farmers heavily use groundwater for irrigation, it can cause the water table to rise

  • As the groundwater moves upward, it brings dissolved salts to the surface through capillary action, contributing to soil salinization

Poor soil drainage

• Clay soils and compacted land prevent proper water infiltration

  • This traps water and increases salt deposits

  • This means that agricultural fields with heavy clay content are more prone to salinization

Awaiting image: Causes of salinization
Image caption: Salinization can be caused by irrigation and a rising water table

Effects of salinization

Impact on plant growth

• Salt stress disrupts plant metabolism, reducing water absorption and nutrient uptake

• Symptoms include stunted growth, leaf burn, and reduced crop yields

Soil degradation

• Loss of soil fertility: High salt levels kill beneficial soil microbes and reduce organic matter

• Crust formation: Salt deposits can form a hard crust on the soil, making it difficult for plant roots to penetrate

Water quality issues

• Salt runoff can enter freshwater sources, contaminating rivers, lakes, and groundwater

• Reduces availability of clean drinking water and impacts aquatic ecosystems

  • Salt buildup in water bodies can alter pH levels and reduce oxygen availability, harming aquatic life

Preventing and managing salinization

Improved irrigation techniques

• Drip irrigation delivers water directly to plant roots, reducing evaporation and salt buildup

• Proper drainage systems help flush excess salts out of the soil

Soil management

• Adding organic matter (compost, mulch) improves soil structure and increases water retention

• Crop rotation with salt-tolerant plants helps maintain soil balance

  • Salt-tolerant plants absorb excess salts, reducing soil salinity over time

Water management

• Using fresh, low-salinity water for irrigation prevents salt buildup

• Flushing soil periodically with excess water can help remove accumulated salts (through leaching)

  • The water dissolves the accumulated salts and carries them downward through the soil layers, preventing salt buildup near plant roots

Depletion of aquifers

What is aquifer depletion?

• Aquifer depletion occurs when groundwater is withdrawn faster than it can be naturally replenished

• Aquifers are underground layers of permeable rock or sediment that store and supply groundwater

• Overuse, primarily for agriculture, industry, and urban water supply, can cause serious environmental and economic consequences

Causes of aquifer depletion

Agricultural irrigation

• Irrigation accounts for around 70% of global freshwater use, with much of it drawn from aquifers

• Excessive groundwater pumping for crop production, especially in dry regions, lowers water tables

  • Many farming regions rely on aquifers for year-round irrigation, even when rainfall is low

Urban and industrial water demand

• Growing cities extract large volumes of groundwater for drinking water and industrial use

• Unregulated pumping can deplete aquifers faster than they recharge

  • Many metropolitan areas experience groundwater shortages due to high population demands

Climate change and reduced recharge rates

• Droughts and reduced precipitation slow the natural recharge of aquifers

• Increased temperatures lead to higher evaporation rates, reducing available groundwater

Consequences of aquifer depletion

Land subsidence

• Excessive groundwater withdrawal can cause land to sink (subsidence) as underground water-filled spaces collapse

  • For example, parts of California's Central Valley have experienced land sinking due to over-pumping

Reduced water availability

• Wells and springs dry up, making it harder to access clean drinking water

• Deeper drilling is required, increasing water costs

  • For example, farmers are forced to dig deeper wells, making groundwater extraction more expensive

Saltwater intrusion

• In coastal areas, depleted aquifers allow saltwater to seep into freshwater supplies, contaminating drinking water

  • For example, Florida has experienced saltwater intrusion due to over-extraction of groundwater