Irrigation Fundamentals and Water Requirements

Introduction

Irrigation is the controlled application of water to land to supplement rainfall and provide crops with the moisture required for germination, growth, yield formation, and quality. It is one of the principal elements of agricultural water management and is closely related to canals, diversion structures, reservoirs, drainage, pumps, and field channels.

For a civil draftsman, irrigation knowledge is essential because drawings must communicate command boundaries, canal alignments, outlets, field channels, access roads, drainage provisions, pipe networks, and hydraulic structures. Sound drafting depends on understanding not only the geometry of a project but also the water requirement that governs its size and operation.

Irrigation design aims to deliver the required quantity of water at the required time, with minimum losses, acceptable cost, and no long-term damage to soil or groundwater. Over-irrigation can cause waterlogging, salinity, nutrient leaching, and wastage; under-irrigation reduces crop yield.

Learning Objectives

After studying this chapter, the learner should be able to:

• explain the purposes and principles of irrigation;
• distinguish crop water requirement, field requirement, and irrigation requirement;
• define duty, delta, base period, command area, and related terms;
• calculate basic water requirements using consistent units;
• identify irrigation efficiencies and major water losses;
• explain irrigation scheduling and the factors affecting it;
• compare surface, sprinkler, and drip irrigation systems; and
• apply these concepts to preliminary planning and drafting of an irrigation scheme.

1. Purposes and Principles of Irrigation

Rainfall is often seasonal, irregular, or insufficiently distributed. Irrigation provides a dependable supply during dry periods and may permit more than one crop to be grown annually. Its principal purposes are:

1. Supplementing rainfall: supplying water during rainfall deficits.
2. Increasing agricultural productivity: maintaining soil moisture near the range preferred by a crop.
3. Stabilising production: reducing dependence on uncertain rainfall.
4. Enabling multiple cropping: providing water for an additional crop season.
5. Protecting crops: applying water for frost protection, cooling, or salt leaching where appropriate.
6. Improving rural development: supporting food production, employment, and associated infrastructure.

Irrigation water should be applied uniformly and in quantities compatible with soil infiltration, crop-root depth, climate, and drainage. A successful scheme therefore requires coordination among the source, conveyance system, distribution network, field application method, and drainage system.

Irrigation and drainage

Irrigation and drainage are complementary. Irrigation adds water to the root zone; drainage removes excess surface or subsurface water. Without adequate drainage, a high water table may reduce soil aeration and bring dissolved salts toward the surface. Consequently, an irrigation layout should reserve space and hydraulic capacity for natural or artificial drainage.

2. Water Requirement Terminology

The following terms are used in planning and design.

Crop evapotranspiration

A widely used planning relationship is:

Here, ET_c is crop evapotranspiration, K_c is dimensionless, and ETo is reference evapotranspiration. Both evapotranspiration terms must use the same time unit, commonly millimetres per day or millimetres per month. The relationship is applicable when a suitable reference evapotranspiration estimate and crop coefficient are available. The value of K_c changes with crop type, growth stage, climate, and management.

A simplified root-zone water balance is:

where I is irrigation water entering the root zone, P_e is effective rainfall, C_r is capillary contribution from groundwater, R is runoff, D is deep percolation below the root zone, ET_c is crop evapotranspiration, and ΔS is the change in soil-water storage. All terms must refer to the same area and time interval. For a planned irrigation event that restores soil moisture to a target level, ΔS is determined from the desired change in root-zone storage.

For a basic scheme calculation, if capillary rise and runoff are negligible, the net irrigation requirement may be approximated by:

If effective rainfall exceeds crop evapotranspiration during the period, the calculated irrigation requirement is taken as zero for that period, although drainage may still be necessary.

Net and gross irrigation requirement

Net irrigation requirement is the water that must be stored in the crop root zone. Gross irrigation requirement is larger because water is lost in conveyance and field application. If overall irrigation efficiency is E_i, expressed as a decimal:

The equation assumes that NIR and GIR are depths over the same area and period, and that the efficiency represents the fraction of delivered water ultimately stored and useful in the root zone.

3. Duty, Delta, and Base Period

Base period

The base period, B, is the interval from the first watering of a crop to its last watering. It is normally expressed in days. It is not necessarily identical to the total biological duration of the crop because pre-sowing irrigation and final irrigation may occur outside the main growth stages.

Delta

Delta, Δ, is the total depth of water required by a crop over its base period. It is generally expressed in metres or millimetres. If the average continuous flow serving an area is known, delta can be related to duty.

Duty

Duty, D, is the area irrigated by a unit discharge flowing continuously throughout the base period. Common units are hectares per cubic metre per second, written ha/(m³/s). A high duty means that a relatively small discharge serves a large area; a low duty means that more discharge is needed per unit area.

The fundamental relationship is:

when Δ is in metres, B is in days, and D is in hectares per m³/s. The constant 8.64 follows from unit conversion:

• 1 m³/s flowing for 1 day supplies 86,400 m³;
• 1 hectare is 10,000 m²;
• depth = volume/area.

Thus, 86,400 ÷ 10,000 = 8.64. The equivalent forms are:

and

where Q is discharge in m³/s and A is area in hectares. These relationships assume a uniform equivalent duty and continuous flow over the stated base period. Actual canal design must also consider rotation, peak crop demand, losses, and operating constraints.

Worked example: duty and delta

A canal supplies 1.20 m³/s to 1,500 ha during a base period of 120 days. Find the equivalent delta.

Using Δ = 8.64B/D, first calculate duty:

D = A/Q = 1500/1.20 = 1250\ ha/(m³/s)

Therefore,

Δ = (8.64 × 120)/1250 = 0.829\ m

The equivalent seasonal depth is approximately 0.829 m, or 829 mm. This is a gross depth if the supplied discharge is measured at the canal delivery point and includes conveyance or field losses.

4. Command Area and Irrigation Planning

The gross command area (GCA) is the total area that can theoretically be commanded by gravity from a proposed canal system. It includes cultivated land, settlements, roads, drainage lines, ponds, rocky land, and other areas.

The cultivable command area (CCA) is the portion of the gross command area suitable for cultivation. A useful planning relation is:

The culturable proportion is dimensionless and must be based on a land-use survey. It should not be assumed to be 100 percent.

The intensity of irrigation expresses the irrigated area as a percentage of the cultivable command area:

For a command served in several seasons, seasonal areas must be distinguished. Gross seasonal area is the sum of areas irrigated in each season, whereas net irrigated area counts land irrigated at least once during the year. These values are important when sizing canals and assessing agricultural benefits.

In preparing drawings, the draftsman should show the command boundary, contour information, canal hierarchy, outlets, watercourses, roads, drainage paths, and excluded areas. Contours are particularly important because gravity canals require a suitable longitudinal fall and should avoid excessive cutting, filling, or unstable terrain.

5. Irrigation Efficiencies and Water Losses

Efficiency is the ratio of useful water to water supplied. The denominator must always be stated clearly because efficiency can be measured at different stages.

Conveyance efficiency

Here, W_d is water delivered into the conveyance system and W_f is water reaching the farm or field inlet. Losses include seepage, evaporation, leakage, operational spills, and unauthorized withdrawals.

Field application efficiency

Here, W_s is water stored in the crop root zone and W_f is water delivered to the field. Runoff, deep percolation, and uneven distribution reduce this efficiency.

Storage efficiency

W_n is the water needed in the root zone to bring it to the desired moisture condition. This measure indicates whether the irrigation event adequately refilled the root zone.

Distribution uniformity

Efficiency alone may conceal poor distribution. Distribution uniformity measures how evenly water is applied. In field practice, it may be estimated from collected application depths or emitter flows. A system can have a reasonable average efficiency but poor uniformity, leaving some areas dry and others over-irrigated.

Overall efficiency

If conveyance and application stages are independent for planning purposes:

Efficiencies must be expressed as decimals in this equation. If conveyance efficiency is 80% and application efficiency is 70%, E_o = 0.80 × 0.70 = 0.56, or 56%. The product is a planning approximation; measured system performance may differ because losses and operating conditions are not always independent.

Major water losses

• Seepage: movement through the bed and sides of unlined channels.
• Evaporation: water lost from exposed canal and field surfaces.
• Transpiration: beneficial crop use, although excessive plant transpiration may reflect high atmospheric demand.
• Runoff: water leaving the field without entering the root zone.
• Deep percolation: water moving below the effective root zone.
• Leakage: losses through cracks, joints, embankments, gates, or defective pipes.
• Operational losses: spills caused by poor regulation, delayed gate operation, or excess supply.

Lining canals, maintaining embankments, controlling weeds, using appropriate field lengths, regulating outlets, and adopting pressurised systems can reduce losses. However, lining may increase construction cost and can reduce groundwater recharge; design decisions should therefore consider the complete water balance.

6. Irrigation Scheduling

Irrigation scheduling determines when to irrigate and how much water to apply. A sound schedule balances crop demand, soil storage, rainfall, system capacity, and labour or energy availability.

Important scheduling factors include:

• crop type and growth stage;
• effective rooting depth;
• soil texture and available water capacity;
• evapotranspiration and weather forecasts;
• rainfall amount and reliability;
• irrigation method and application rate;
• canal rotation and available discharge; and
• groundwater level and drainage condition.

The available water in the root zone can be represented by the difference between field capacity and permanent wilting point. If θ_FC and θ_PWP are volumetric water contents, and Z_r is root-zone depth, total available water is:

TAW is in millimetres when Z_r is in metres. The factor 1000 converts metres of water to millimetres. Allowable depletion is commonly represented by:

where p is the allowable depletion fraction. Irrigation is scheduled when soil depletion approaches RAW, subject to local crop and soil recommendations.

The approximate irrigation interval is:

provided RAW and ET_c use compatible units, such as millimetres and millimetres per day. This is a planning estimate; rainfall, changing crop coefficients, and application constraints must be incorporated in operational scheduling.

Worked example: scheduling interval

A crop has θ_FC=0.30, θ_PWP=0.15, and effective root depth Z_r=0.60\ m. If allowable depletion is 50% and daily crop evapotranspiration is 5 mm/day:

TAW = 1000(0.30-0.15)(0.60)=90\ mm

RAW = 0.50×90=45\ mm

Interval = 45/5=9\ days

Thus, irrigation may be planned at approximately nine-day intervals, provided rainfall and field observations do not materially change the water balance.

7. Irrigation Methods

Surface irrigation

Surface irrigation distributes water over the soil surface by gravity. Common forms are basin, border, furrow, and uncontrolled flooding.

• Basins are level or nearly level areas surrounded by low bunds. They are suitable for rice and some orchard systems.
• Borders are long, relatively narrow strips separated by small ridges. Water flows as a shallow sheet.
• Furrows are small channels between crop rows. They reduce wetting of the entire surface and are useful for row crops.

Advantages include low energy demand, simple operation, and compatibility with gravity canals. Limitations include relatively lower uniformity on poorly graded land, labour requirements, runoff, deep percolation, and the need for suitable land levelling. Proper design requires field slope, infiltration rate, furrow or border dimensions, inflow rate, and cut-off time.

Sprinkler irrigation

Sprinkler systems apply water through pressurised pipes and nozzles in the form of droplets resembling rainfall. Components commonly include a pump, mainline, submain, laterals, risers, sprinklers, valves, filters, and pressure controls.

Sprinklers are useful on undulating land, light soils, and areas where land levelling is costly. They can achieve good control and reduce conveyance losses, but they require energy, maintenance, suitable pressure, and protection from excessive wind. Droplet impact may cause soil sealing or erosion if the application rate exceeds the soil infiltration rate.

The application rate must not exceed the soil infiltration capacity for prolonged periods. Uniform spacing, operating pressure, nozzle selection, and wind conditions strongly affect performance.

Drip irrigation

Drip irrigation applies water slowly and frequently near the plant root zone through emitters. A typical system contains a pump or elevated source, filter, pressure regulator, mainline, submains, laterals, emitters, and fertigation equipment where permitted.

Advantages include high application efficiency, reduced evaporation and runoff, reduced weed growth between rows, and precise delivery to high-value crops. Limitations include high initial cost, filter and emitter clogging, sensitivity to poor water quality, pressure variation, and the need for skilled maintenance.

A drip system must include filtration, flushing arrangements, pressure control, and accessible valves. Drawings should show pipe diameters, emitter spacing, operating zones, isolation valves, flushing ends, and protection from vehicles or cultivation equipment.

Comparison of methods

No method is universally best. Selection should consider water availability, soil, topography, crop value, labour, energy price, water quality, capital cost, and technical capacity.

8. Applied Case: Preliminary Planning of a Small Command

Illustrative case. A proposed minor canal supplies 0.75 m³/s to a cultivable command of 900 ha. The principal crop has a 100-day base period, and the estimated duty at the canal outlet is 1,500 ha/(m³/s). The area contains medium-textured soils, gently sloping land, and irregular rainfall.

Problem

The planning team must determine whether the stated discharge can serve the command during the crop base period and recommend suitable field application methods.

Analysis

The area that can theoretically be served by the discharge is:

A = DQ = 1500×0.75 = 1125\ ha

The nominal capacity therefore exceeds the 900-ha command. The equivalent delta for 900 ha is found from the actual duty:

D_actual=900/0.75=1200\ ha/(m³/s)

Δ=(8.64×100)/1200=0.72\ m

The supplied seasonal depth is approximately 720 mm at the delivery point. This does not prove that the crop will receive sufficient water, because crop demand, effective rainfall, conveyance efficiency, rotational operation, and field application efficiency must also be checked.

Approach and solution

The design team should:

1. verify crop evapotranspiration using local climate data and crop coefficients;
2. estimate effective rainfall and net irrigation requirement;
3. apply measured or conservative conveyance and application efficiencies;
4. divide the command into rotational blocks if continuous supply is unavailable;
5. provide field channels and drainage lines along suitable contours;
6. use surface irrigation on properly graded field blocks; and
7. consider sprinkler or drip systems for irregular plots, vegetables, or water-scarce zones.

For drafting, the plan should distinguish the canal, distributaries, minors, outlets, farm channels, drainage, roads, and command boundary. Longitudinal sections should show bed levels, banks, design water levels, and falls. Cross-sections should show bed width, side slopes, freeboard, lining, and access provisions where specified.

Outcome and learning points

The initial discharge appears adequate in gross area terms, but final adequacy depends on the complete water balance and operating schedule. A capacity calculation alone cannot guarantee crop production. The case demonstrates the importance of distinguishing gross supply from net root-zone use and of integrating hydraulic, agricultural, and drafting information.

9. Professional Practice, Sustainability, and Current Developments

Irrigation works should comply with applicable water-allocation laws, environmental requirements, construction standards, land-acquisition procedures, and occupational safety rules. Water rights and downstream users must be respected. Designers should avoid promoting excessive abstraction that depletes rivers or aquifers.

Sustainable practice includes measuring flows, repairing leaks, scheduling according to crop need, protecting drainage paths, controlling salinity, and selecting methods according to life-cycle cost rather than initial cost alone. Reuse of suitably treated water may be possible under local regulation, but salinity, pathogens, heavy metals, and crop restrictions must be assessed.

Modern systems increasingly use soil-moisture sensors, weather stations, remote sensing, telemetry, automated gates, variable-speed pumps, and geographic information systems. Artificial intelligence can assist with demand forecasting, anomaly detection, and irrigation recommendations, but its outputs require field validation, reliable data, transparent assumptions, and responsible professional oversight. Technology should support—not replace—hydrologic measurement and engineering judgement.

Common design and operation mistakes include confusing net and gross depths, using percentages as whole numbers in efficiency calculations, ignoring effective rainfall, omitting drainage, drawing outlets without considering contours, and specifying drip or sprinkler systems without filtration and pressure control. A competent civil draftsman checks units, datum, scale, north direction, chainages, labels, right-of-way, and consistency between plan, profile, and detail drawings.

Chapter Summary

Irrigation supplements rainfall and supplies crops with water at the required time and rate. Crop water requirement is commonly estimated from reference evapotranspiration and a crop coefficient, while irrigation requirement is obtained after considering effective rainfall and other water contributions. Duty describes the area served by a unit discharge over a base period; delta is the seasonal depth of water; and their relationship depends on consistent units.

Command-area planning distinguishes gross command area from cultivable command area and must account for topography, land use, canals, outlets, and drainage. Conveyance, application, storage, and overall efficiencies describe different stages of water delivery. Seepage, evaporation, runoff, deep percolation, leakage, and operational spills are major losses.

Scheduling depends on root-zone storage, allowable depletion, evapotranspiration, rainfall, soil, crop stage, and system operation. Surface irrigation is simple and energy-efficient but depends on land and water management. Sprinkler irrigation provides pressurised rainfall-like application, while drip irrigation provides precise root-zone delivery with higher equipment and maintenance requirements. Effective irrigation design integrates agricultural demand, hydraulic capacity, environmental protection, operation, and accurate technical drawings.

Key Terms

Irrigation: Controlled application of water to land for crop production.

Crop water requirement: Water consumed by a crop through evapotranspiration and associated growth needs.

Evapotranspiration: Combined loss of water by soil or plant-surface evaporation and plant transpiration.

Reference evapotranspiration: Atmospheric evaporative demand from a standard reference surface.

Crop coefficient: Factor used to relate crop evapotranspiration to reference evapotranspiration.

Effective rainfall: Rainfall stored in the root zone and available to the crop.

Net irrigation requirement: Water needed in the root zone from irrigation after other contributions.

Gross irrigation requirement: Water supplied before accounting for delivery and application losses.

Base period: Period between the first and last irrigation of a crop.

Delta: Total depth of water required during the base period.

Duty: Area served by a unit discharge during the base period.

Command area: Land that can be served by an irrigation system.

Gross command area: Total theoretically serviceable area.

Cultivable command area: Serviceable area suitable for cultivation.

Conveyance efficiency: Ratio of water reaching the farm to water entering conveyance.

Application efficiency: Ratio of water stored in the root zone to water delivered to the field.

Surface irrigation: Gravity application of water over the soil surface.

Sprinkler irrigation: Pressurised application of water through spray nozzles.

Drip irrigation: Low-rate application near plant roots through emitters.

Irrigation scheduling: Determination of irrigation timing and application quantity.

Review Questions

Short Answer Questions

1. State four purposes of irrigation.
2. Define duty, delta, and base period.
3. Distinguish gross command area from cultivable command area.
4. Name four major irrigation water losses.
5. What is the principal function of a filter in a drip irrigation system?

Long Answer Questions

1. Explain crop water requirement, effective rainfall, net irrigation requirement, and gross irrigation requirement.
2. Derive and explain the relationship between duty, delta, and base period, including units.
3. Describe conveyance, application, storage, and overall irrigation efficiencies.
4. Compare surface, sprinkler, and drip irrigation with respect to operation, cost, energy, and suitability.
5. Explain the factors that should be considered when preparing an irrigation schedule.

Analytical / Application Questions

1. A canal supplies 1.5 m³/s for 90 days to 1,200 ha. Calculate the equivalent delta in metres and comment on the unit assumptions.
2. A soil has field capacity 0.28, wilting-point water content 0.14, root depth 0.50 m, and allowable depletion of 40%. If daily crop evapotranspiration is 4 mm, estimate the irrigation interval.
3. A command has uneven land, limited water, expensive electricity, and high-value vegetable crops. Recommend an irrigation method and justify the choice, including the essential components that should appear on the layout drawing.

Multiple Choice Questions with Answers

Question 1

Which expression relates crop evapotranspiration to reference evapotranspiration?

A. ET_c=K_cETo

B. ET_c=ETo/K_c

C. ET_c=K_c+ETo

D. ET_c=ETo-K_c

Correct Answer: A. ET_c=K_cETo

Question 2

A duty of 1,000 ha/(m³/s) applies for a base period of 100 days. What is the equivalent delta?

A. 0.0864 m

B. 0.864 m

C. 8.64 m

D. 86.4 m

Correct Answer: B. 0.864 m

Question 3

Which area includes roads, settlements, and uncultivable land within the theoretical service boundary?

A. Net irrigated area

B. Cultivable command area

C. Gross command area

D. Effective root-zone area

Correct Answer: C. Gross command area

Question 4

If conveyance efficiency is 80% and application efficiency is 75%, the approximate overall efficiency is:

A. 5%

B. 55%

C. 60%

D. 155%

Correct Answer: C. 60%

Question 5

Which loss occurs when water moves below the effective crop-root zone?

A. Runoff

B. Deep percolation

C. Evaporation from a canal

D. Operational spill

Correct Answer: B. Deep percolation

Question 6

A soil has θ_FC=0.30, θ_PWP=0.15, and root depth 0.40 m. What is its total available water?

A. 6 mm

B. 30 mm

C. 60 mm

D. 600 mm

Correct Answer: C. 60 mm

Question 7

Which irrigation method is most directly dependent on filtration to prevent emitter clogging?

A. Basin irrigation

B. Border irrigation

C. Drip irrigation

D. Uncontrolled flooding

Correct Answer: C. Drip irrigation

Question 8

The main purpose of irrigation scheduling is to determine:

A. Only the canal lining thickness

B. When and how much water to apply

C. Only the crop selling price

D. The legal ownership of farmland

Correct Answer: B. When and how much water to apply

Question 9

For a fixed discharge and base period, increasing the irrigated area generally causes the equivalent delta to:

A. Increase

B. Remain exactly constant

C. Decrease

D. Become dimensionless

Correct Answer: C. Decrease

Question 10

Which condition is especially important for sprinkler performance?

A. Wind and operating pressure

B. Only the colour of the pipe

C. The crop market price alone

D. Absence of all soil infiltration

Correct Answer: A. Wind and operating pressure