🚰Irrigation: Principles, Soil-Water Concepts, and Scientific Management

From Rainfall Uncertainty to Reliable Harvests

In the previous lesson, we established how water is distributed on Earth — the hydrologic cycle, aquifer types, and India’s water budget. Now we move from understanding where water is to understanding how to apply it to crops — the science of irrigation.

A wheat farmer in Uttar Pradesh depends on timely winter rains for his crop. In a good year, the rains arrive perfectly during crown root initiation and tillering. But in a bad year, a three-week dry spell at the critical CRI stage destroys the entire harvest — months of labour and investment lost to a single rainless fortnight. Irrigation solves this problem by giving the farmer control over water supply, transforming uncertain rainfed farming into reliable, high-yielding agriculture. This chapter covers what irrigation is, why India needs it, and the scientific concepts behind managing it effectively.

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What is Irrigation?

Irrigation is the artificial application of water to the soil for crop growth and crop production, supplementing rainfall and groundwater contribution. It is one of the most critical inputs in modern agriculture.

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Purpose of Irrigation

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Importance of Irrigation in India

India’s agriculture is deeply dependent on water — yet water availability is highly uneven across regions and seasons.

• Agriculture is the biggest user of water — 78% of India’s freshwater consumption (CWC, 2014).
• Only about 49% of cultivated area has irrigation access; more than half still depends on rainfall.
• About 86% of irrigated agriculture comes from two sources:
  • Groundwater (62%)
  • Canals (24%)

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Advantages of Irrigation

• Increases food production to feed the expanding population.
• Ensures stable production in traditional dryland farming systems.
• Prolongs the growing period, enabling multiple cropping and employment generation.
• Makes additional inputs (fertilizers, tillage, plant protection) economically feasible.
• Reduces the risk of expensive inputs being wasted by drought.

Agricultural example: In Haryana, the introduction of canal irrigation in the 1960s transformed semi-arid land into one of India’s most productive wheat-rice belts, enabling two reliable harvests per year instead of one uncertain monsoon crop.

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Adverse Effects of Excess Irrigation

While irrigation is essential, over-irrigation causes serious problems:

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Life-Saving Irrigation

• Also called contingency irrigation — an emergency measure for severe moisture stress.
• Supplemental irrigation applied to dryland crops.
• Land is not prepared for irrigation.
• Fields divided into plots of 20—25 m width with small bunds to guide water.

Agricultural example: During a mid-season drought in sorghum, a single life-saving irrigation of 5 cm at the flowering stage can prevent 40—60% yield loss. Farmers in dryland Maharashtra keep a farm pond specifically for such emergencies.

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Irrigation Management

Irrigation water management is the act of timing and regulating water applications to satisfy crop water requirement without wasting water, soil, plant nutrients, or energy.

It requires understanding four key factors:

The scientific management of all these factors is called Irrigation Agronomy.

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Importance of Irrigation Management

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Why India Needs Irrigation

• India receives most rainfall in the monsoon season (June—September).
• There is huge spatial and temporal variation in rainfall patterns — Cherrapunji receives over 11,000 mm while Jaisalmer gets less than 200 mm.
• Climate change has increased this unpredictability.
• Irrigation provides a reliable, controlled water supply regardless of rainfall.

India’s rainfall follows a distinct seasonal pattern: the southwest monsoon (June—September) delivers about 75% of annual rainfall, followed by the northeast monsoon (October—December) in peninsular India, a dry winter (January—February), and the pre-monsoon hot weather period (March—May). This uneven distribution makes irrigation essential for year-round cropping.

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Important Irrigation Concepts

These concepts form the scientific foundation of irrigation management. They progress logically from how water is held in soil, to how it moves, and finally to how we measure its energy state.

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Soil Water Relationships — From Saturation to Wilting

Understanding how soil holds and releases water is critical for deciding when and how much to irrigate. The following concepts describe a continuum — from soil completely full of water (saturation) to soil so dry that plants die (ultimate wilting point).

Saturation Capacity

The maximum water holding capacity of soil where all pores (macropores and micropores) are completely filled with water. No more water can be absorbed; additional water ponds on surface or runs off.

Agricultural example: Immediately after heavy monsoon rain, paddy fields reach saturation — ideal for rice (which tolerates standing water) but harmful for wheat (whose roots suffocate without air in the pore spaces).

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Field Capacity (FC)

• Soil moisture content 2—3 days after irrigation, after gravitational water has drained and moisture is relatively stable.
• Large pores filled with air; micropores filled with water.
• The upper limit of water availability to plants.

Agricultural example: When a well-drained loamy soil in a mango orchard reaches field capacity, both water and air are available in the root zone — ideal conditions for root growth and nutrient uptake.

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Permanent Wilting Point (PWP)

• Concept proposed by Briggs and Shantz in 1912 using dwarf sunflower as indicator plant.
• Soil moisture at which plants can no longer obtain enough water and remain permanently wilted unless water is added.
• The lower limit of available water to plants.
• Plants are not dead but are in a permanently wilted condition.

Agricultural example: A cotton crop in Vidarbha reaching PWP shows drooping leaves that do not recover even in the evening. If irrigation is provided within 1—2 days, the crop can still recover.

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Available Water

• Concept given by Veihmayer and Hendrickson (1981).
• Available Water = Field Capacity - Permanent Wilting Point
• This is the range of soil moisture actually useful for crop growth.

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Ultimate Wilting Point (UWP)

• Moisture content at which wilting is complete and plants die (irreversible).
• Soil moisture tension reaches -60 bars.

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Wilting Coefficient

The percentage of moisture in the root zone at permanent wilting — numerically equivalent to PWP expressed as a percentage. Also called critical moisture point.

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Comparison of Soil Moisture Constants

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How Water Enters and Moves Through Soil

Water movement in soil follows a logical sequence: it first enters the surface (infiltration), then moves downward through the profile (percolation), may move sideways (seepage), carries substances out of the root zone (leaching), or flows off the surface entirely (runoff).

Infiltration

• Entry of water from the surface into the upper soil layers.
• Occurs in unsaturated soil.
• Infiltration rate is highest when soil is dry and shows exponential decay as soil wets.
• The nearly constant rate after prolonged irrigation is the basic infiltration rate.
• Infiltration rate = maximum rate water can enter soil; Infiltration velocity = actual rate at any time.

Key factors affecting infiltration:

Agricultural example: A farmer adding farmyard manure to clay soil before sowing groundnut improves infiltration, reducing waterlogging risk during monsoon showers.

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Percolation

• Downward movement of water through saturated or nearly saturated soil due to gravity. ^{AFO 2017}
• Water moves from the unsaturated zone to the saturated zone, eventually reaching the water table and replenishing groundwater.

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Infiltration vs Percolation

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Seepage

• Horizontal flow of water from irrigation channels or canals.
• Main cause of water loss from irrigation conveyance systems.
• Affected by: soil texture, water temperature, siltation, bank storage, water velocity, and water table fluctuations.
• Can cause waterlogging and salinization of adjacent land.
• Solution: Lining canals with concrete or impervious materials.

Agricultural example: In the Indira Gandhi Nahar Project (Rajasthan), unlined canal sections lose up to 40% of water through seepage, causing waterlogging in nearby fields. Lined sections reduce losses to under 10%.

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Leaching

Downward movement of nutrients and salts from the root zone with water. While it causes loss of valuable nutrients (especially nitrogen), it is also used intentionally in reclamation leaching to remove excess salts from saline soils.

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Runoff

Flow of excess water from the field after soil saturation. Represents water not utilized by crops — minimizing runoff improves water use efficiency.

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Water Movement Summary

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Soil Water Energy Concepts

Water in soil is not only held in different amounts but also with different amounts of energy. Understanding these energy concepts explains why water moves from one place to another in the soil.

Matric Potential

The water potential attributed to the solid colloidal matrix of the soil system — measures how tightly water is held by adsorption and capillarity. The drier the soil, the more negative the matric potential.

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Capillary Potential

The energy with which water is held by soil through surface tension in tiny pore spaces. Capillary forces pull and hold water against gravity.

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Soil Moisture Tension

A measure of how tenaciously water is retained in soil — the force per unit area needed to remove water. Higher tension = drier soil = harder for plants to absorb water. Critical parameter for irrigation scheduling.

Agricultural example: Tensiometers installed in a drip-irrigated tomato field measure soil moisture tension. When tension exceeds 30 centibars, the farmer knows it is time to irrigate — this prevents both under- and over-watering.

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Soil Water Potential (Psi)

• Water in soil does not move rapidly, so kinetic energy is negligible.
• Water moves from higher potential to lower potential (wet to dry soil).
• Soil Water Potential (Psi) = work water can do moving from its present state to a reference state (pool of pure water at zero elevation).
• At saturation, Psi = 0 (zero).
• As soil dries, Psi becomes more negative.
• When Psi is high (close to 0), water is held loosely, highly available, and ready to move.

Three Components of Total Soil Water Potential

Total Soil Water Potential: Psi-t = Psi-g + Psi-o + Psi-m

• Gravitational potential causes downward flow after heavy rain or irrigation.
• Osmotic potential — high soluble salts reduce water availability, causing physiological drought (plants wilt in saline soils even when water is present).
• Matric potential — in saturated soil, Psi-m = 0 (not a factor); in dry soil, matric forces dominate.

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Methods of Expressing Suctions

Atmospheric Pressure or Bars

• Common unit for expressing suction.
• Wet soil at maximum water content: suction ~ 0.01 atmospheres or 1 pF (10 cm water column).

pF Scale

• Introduced by Schofield.
• pF = logarithm of the height of water column (cm) needed to produce the necessary suction.
• The log scale conveniently expresses the wide range of soil moisture tensions.

Key pF values: wet soil at maximum water content has suction of about 0.01 atmospheres or 1 pF (10 cm water column). At field capacity, pF is approximately 2.5 (about 1/3 atmosphere). At permanent wilting point, pF reaches 4.2 (about 15 atmospheres). At the ultimate wilting point, pF is approximately 4.8 (-60 bars).

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Other Important Concepts

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Classification of Irrigation Projects

Irrigation projects in India are classified by the culturable command area (CCA) they serve: Major projects irrigate more than 10,000 hectares, medium projects irrigate 2,000—10,000 hectares, and minor projects irrigate less than 2,000 hectares. Minor irrigation includes individual wells, tube-wells, and small tanks — covering the majority of India’s irrigated area.

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The Water Man of India

• Rajendra Singh is known as Water Man of India.
• Acclaimed water conservationist from Alwar, Rajasthan.
• Uses traditional johads (earthen dams) to restore groundwater and revive rivers.
• Rejuvenated five rivers: Arvari, Ruparel, Sarsa, Bhagani, and Jahajwali.
• Awards: Ramon Magsaysay Award (2001) and Stockholm Water Prize (2015).
• Emphasizes community involvement in sustainable water management.

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Water Content in Different Plant Parts

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Soil Moisture Constants by Soil Type

Agricultural example: A groundnut farmer on sandy loam soil in Gujarat needs to irrigate every 7—10 days because the soil holds only 4—11 cm of available water per metre. A wheat farmer on clay loam in Punjab can wait 15—20 days between irrigations because the soil holds 10—18 cm.

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Reference Evapotranspiration (ET0) by Climatic Zone

Reference ET (mm/day) varies with climate and temperature:

ET0 classification: 4—5 mm/day = Low, 6—7 mm/day = Medium, 8—9 mm/day = High.

Agricultural example: In desert regions of Rajasthan (ET0 = 9—10 mm/day during summer), crops lose water very rapidly. Drip irrigation with mulching is essential to keep up with this high evaporative demand.

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Water Sensitivity of Crops

Crops grown for fresh leaves or fruits are more sensitive to water shortage than those grown for dry seeds or fruits.

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Net Irrigation Rooting Depth (mm)

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Root Depth Classification of Crops

Agricultural example: Shallow-rooted rice (30—60 cm) needs frequent light irrigation because its roots cannot access deeper moisture. Deep-rooted cotton (90—150 cm) can survive longer between irrigations by drawing water from deeper soil layers.

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Irrigation Efficiency by Method

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Effect of Waterlogging on Soil Elements

Waterlogging creates anaerobic (reduced) conditions:

• Waterlogging reduces availability of N, Mn, Fe, Cu, Zn.
• Surface drainage is suitable for land with < 1.5% slope.

Agricultural example: In waterlogged rice paddies, nitrogen applied as urea is rapidly lost through denitrification (NO3 to N2 gas). This is why neem-coated urea and deep placement of urea super granules (USG) are recommended for rice — they slow down nitrification and reduce losses.

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Plastic Mulching — Selection Guide

Mulching covers the soil surface to conserve moisture, regulate temperature, and suppress weeds. Most plastic mulch is based on LLDPE (Linear Low Density Polyethylene).

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Soil Moisture Characteristic Curve

Shows the relationship between soil water tension and moisture content:

Agricultural relevance: Sandy soil loses most of its water at low tension (drains fast), so irrigation must be frequent and in small amounts. Clay soil releases water slowly across a wide range, allowing longer intervals between irrigations.

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Summary Table

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Summary Cheat Sheet