Irrigation and Drainage

Irrigation supplies water to soil where rainfall is insufficient, while drainage removes excess water; together they manage the soil water balance to sustain crops and protect the soil resource.

Definition

Irrigation is the artificial application of water to soil to meet crop water requirements; drainage is the removal of excess surface or subsurface water; both manage the soil water balance to keep water content within the range favourable for plant growth.

Scope

This topic covers methods of applying irrigation water and scheduling its timing and amount, the removal of excess water by surface and subsurface drainage, the soil water balance and crop water requirements, and the management of salinity and waterlogging that poorly managed water can cause. It applies soil physics to the practical management of soil water.

Core questions

• How are irrigation amount and timing matched to crop water needs?
• What methods deliver irrigation water and how efficient are they?
• Why and how is excess water removed by drainage?
• How do irrigation and drainage interact to control salinity and waterlogging?

Key concepts

• Soil water balance
• Crop water requirement and evapotranspiration
• Irrigation methods and efficiency
• Irrigation scheduling
• Surface and subsurface drainage
• Leaching requirement and salinity control

Key theories

Soil water balance and irrigation scheduling

Irrigation is scheduled by tracking the soil water balance of inputs and losses and replenishing plant-available water before crop stress, using field capacity, wilting point, and crop water demand as the controlling reference points.

Leaching requirement and salinity control

Because irrigation water carries dissolved salts, a fraction of applied water must drain through the profile to leach salts below the root zone, so adequate drainage is essential to prevent the salinization that has degraded many irrigated lands.

Mechanisms

Irrigation replenishes the plant-available water depleted by evapotranspiration, with the amount and timing set by the soil water balance and the soil's storage capacity between field capacity and wilting point. Surface, sprinkler, and drip methods differ in how uniformly and efficiently they wet the soil. Drainage removes water that would otherwise saturate the profile, restoring aeration and lowering water tables; because irrigation adds salts, a planned leaching fraction and functioning drainage are needed to flush salts below the roots and prevent salinization and waterlogging.

Clinical relevance

Sound irrigation and drainage sustain productivity in dry and humid regions alike, use scarce water efficiently, and prevent the twin degradations of waterlogging and salinization that have damaged large areas of irrigated land worldwide; they are central to food production and to conserving water and soil.

History

Irrigation is among the oldest agricultural technologies, but its scientific management developed in the 20th century alongside soil physics, with quantitative treatment of the soil water balance, crop water requirements, and the leaching needed to control salinity, the last codified in mid-century work on saline and alkali soils.

Key figures

• Daniel Hillel
• Nyle C. Brady
• Ray R. Weil

Seminal works

• hillel1998
• usslab1954
• brady2016

Frequently asked questions

Why can irrigation lead to salty soils?

All irrigation water contains some dissolved salts, and as the water evaporates or is taken up by plants the salts are left behind and accumulate; without enough drainage and deliberate leaching to flush salts below the root zone, they build up and can degrade the soil.

Why do some soils need drainage?

When excess water from rainfall, irrigation, or a high water table fills the soil pores, it excludes air and starves roots of oxygen; drainage removes that excess water, restoring aeration and allowing normal root growth and microbial activity.