Study notes

Qwestrum Engineering360 · Civil Engineering · Water Resources Engineering

Irrigation Water Requirement

Compute the crop water requirement from the reference evapotranspiration and crop coefficient (ET_c = K_c·ET_0), and relate duty, delta and base period through Δ = 8.64 B/D to size the canal supply.

Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

Base period, kor watering, critical growth stages
Field irrigation efficiency = water stored / water supplied
Conveyance and application losses in canal command

Topic details

Introduction

The irrigation water requirement is the depth and rate of water a crop needs, adjusted for the efficiency of delivery. It starts from the crop evapotranspiration, the water lost to the atmosphere by transpiration and soil evaporation.

Scope in B.Tech and GATE syllabus

Reference evapotranspiration ET_0 (for a standard grass) is computed from climate data (Penman-Monteith), and the crop coefficient K_c scales it to the actual crop and growth stage, giving the crop water requirement ET_c.

Why this topic matters in practice

The practical design links three quantities — duty (area a unit discharge can irrigate), delta (total depth of water for the crop) and base period (days the crop needs water) — through the relation Δ = 8.64B/D, which sizes the canal capacity for a command area.

Key relations & formulas

Crop water need ET_{c} = K_{c} \times ET_{0}

(Penman-Monteith ET_0)

Duty D = area \frac{irrigated}{discharge}

(ha/cumec)

Delta \Delta = depth of water applied per irrigation

(cm)

Notation and sign conventions

Relation 1 —

Crop water need ET_{c} = K_{c} \times ET_{0}

Crop water need ET_{c} = K_{c} \times ET_{0}

(Penman-Monteith ET_0)

Write this relation with symbols exactly as in Irrigation & Water Power Engineering — BC Punmia before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Duty D = area \frac{irrigated}{discharge}

Duty D = area \frac{irrigated}{discharge}

(ha/cumec)

Relation 3 —

Delta \Delta = depth of water applied per irrigation

Delta \Delta = depth of water applied per irrigation

(cm)

Fundamentals and definitions

Evapotranspiration combines transpiration through the crop and evaporation from the soil; it varies with climate (temperature, humidity, wind, radiation) captured in ET_0 and with the crop’s stage captured in K_c, which is low at planting, peaks at full canopy and declines at maturity.

Governing relations in practice

Duty is the area (in hectares) that one cumec of continuously supplied water can irrigate through the base period; a higher duty means water is used more efficiently. Delta is the total depth of water (in cm or m) supplied to the crop over its base period.

Design and analysis considerations

The relation Δ = 8.64B/D (with B in days, Δ in metres, D in ha/cumec) ties the three together; it lets a designer convert a crop’s water depth and growing period into the canal discharge needed for a command area.

Advanced theory and extensions

Efficiencies reduce the field requirement to a larger supply requirement: conveyance losses in the canal network and application losses in the field mean the water diverted at the head exceeds the net crop need, so overall project efficiency (often only 40–60%) is applied.

Assumptions and validity limits

State assumptions explicitly before using any relation for irrigation water requirement — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Water Resources viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Water Resources papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to irrigation water requirement.
4. Use equation 1:

Crop water need ET_{c} = K_{c} \times ET_{0}

.
5. Use equation 2:

Duty D = area \frac{irrigated}{discharge}

.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Irrigation Water Requirement appears in agricultural and municipal water supply. In Indian civil curricula this topic is tested because it connects theory to canals, reservoirs, and irrigation.

GATE and semester exams often combine irrigation water requirement with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use irrigation water requirement?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

• Confusing reference ET_0 with crop ET_c (forgetting the crop coefficient).
• Mixing units in the duty-delta-base-period relation.
• Applying net crop requirement without accounting for conveyance and field losses.
• Using a single K_c for the whole season instead of stage-wise values.

Quick revision checklist

Before attempting irrigation water requirement problems, confirm you can:

1. Base period, kor watering, critical growth stages
2. Field irrigation efficiency = water stored / water supplied
3. Conveyance and application losses in canal command

Revise the solved examples in Irrigation & Water Power Engineering — BC Punmia and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Delta from duty and base period

Problem

A crop has a base period of 120 days and the duty of water is 1800 ha/cumec. Find the delta (depth of water) for the crop.

Solution

Using Δ = 8.64 B/D (Δ in metres, B in days, D in ha/cumec): Δ = 8.64 × 120 / 1800 = 1036.8/1800 = 0.576 m = 57.6 cm. This delta represents the total depth of water delivered over the base period; dividing by the number of waterings gives the depth per irrigation.

Conceptual check — Irrigation Water Requirement