Irrigation Management: Critical Stages, Micro-Irrigation & Watershed

NABARD Grade A • NABARD Grade B • IBPS AFO • UPSC CSE

⚡ Quick Answer & AI Exam Snapshot (TL;DR)

Water Resources, Irrigation Management, and Watershed Development is a top-scoring ARD topic in NABARD Grade A. Every aspirant must know Soil Moisture Constants (Saturation = 0 bar, Field Capacity = -1/3 bar [-0.33 bar], Permanent Wilting Point = -15 bars, Hygroscopic Coefficient = -31 bars; Available Water = FC – PWP), Most Critical Irrigation Stages of Crops (Crown Root Initiation [CRI] at 21 DAS in Wheat by R.B.L. Bhardwaj), Irrigation Efficiencies (Drip = 90%-95% vs Sprinkler = 70%-85% vs Surface = 40%-50%), the Canal Engineering relationship Delta = 8.64 B / D, and the 5-Tier Size Classification of Watersheds (Macro, Sub, Milli, Micro [100-1,000 ha], Mini [1-100 ha]).

1. Soil Moisture Constants, Tension Bars & Plant Available Water (PAW)

Soil Moisture Constant	Soil Moisture Tension (in Bars / Atmospheres)	pF Value (Log₁₀ of cm of Water Column — Schofield, 1935)	Plant Availability & Exam Significance
1. Saturation Capacity (Maximum Water Holding Capacity)	0 Bar (or 0.001 Bar)	`pF = 0`	All macro and micro pores are filled with water; water between Saturation (0 bar) and Field Capacity (−0.33 bar) drains away within 48–72 hours by gravity as ‘Gravitational (Superfluous) Water’ (unavailable to non-aquatic plants!).
2. Field Capacity (FC) (Upper Limit of Available Water!)	−1/3 Bar (−0.33 Bar / −33 kPa) (−0.1 Bar for sandy soils)	`pF = 2.53 (~2.54)`	Moisture retained in micro-pores against gravity 48–72 hours after a heavy irrigation/rain. 100% of Available Water is present at Field Capacity!
3. Permanent Wilting Point (PWP) (Lower Limit of Available Water!) (Indicator Plant: Dwarf Sunflower* — Briggs & Shantz, 1912)*	−15 Bars (−1,500 kPa) (Range: −7 to −32 bars; Ultimate Wilting Point = −60 bars)	`pF = 4.18 (~4.2)`	Moisture tension at which leaves wilt permanently and do NOT recover turgidity even when placed in a 100% humid chamber overnight. `Plant Available Water (PAW) = Field Capacity (−0.33 bar) − Permanent Wilting Point (−15 bars)` (Part of Capillary Water!).
4. Hygroscopic Coefficient	−31 Bars (−3,100 kPa)	`pF = 4.50`	Held tightly as non-liquid vapour film around soil colloids (between −31 and −10,000 bars, `pF 4.5 to 7.0`); completely UNAVAILABLE to higher plants!

Soil Moisture Measuring Instruments: (1) Tensiometer (Irrometer): Measures soil matric tension directly in sandy/loamy soils, but works ONLY up to −0.85 Bar! (2) Gypsum Block (Bouyoucos Electrical Resistance Block): Works from −1 to −15 Bars (not suitable for saline soils!). (3) Neutron Moisture Meter: Fastest, non-destructive in-situ field measurement using Americium-Beryllium source (not accurate for top 15 cm surface layer). (4) Pressure Plate & Pressure Membrane Apparatus (Richards): Lab determination of FC (−0.33 bar) and PWP (−15 bars). (5) Lysimeter: Direct field measurement of Evapotranspiration (ET) and Percolation Losses!

2. Most Critical Stages of Irrigation Across Major Field Crops (Guaranteed MCQ!)

If a farmer has limited water for only ONE or TWO irrigations, irrigation MUST be applied at the crop’s Most Critical Physiological Moisture-Sensitive Stage:

Crop	#1 Most Critical Stage for Irrigation	Other Critical Stages & Total Water Requirement (mm)
1. Wheat (6 Irrigations total)	Crown Root Initiation (CRI Stage — 21 Days After Sowing [DAS]!) (Discovered by R.B.L. Bhardwaj)	6 Stages (Mnemonic: C-T-J-F-M-D): 1. CRI (21 DAS) → 2. Tillering (40–45 DAS) → 3. Jointing (60–65 DAS) → 4. Flowering/Booting (80–85 DAS) → 5. Milking (100–105 DAS) → 6. Dough (115–120 DAS). Water Requirement: 450–650 mm.
2. Rice (Paddy)	Panicle Initiation (PI) to Flowering / Booting Stage	Tillering and Milky stage. Water Requirement: 1,100–1,500 mm (Highest total water volume; Sugarcane has highest durational water requirement of 1,500–2,500 mm!).
3. Maize	Tasseling and Silking Stage	Most sensitive to BOTH moisture stress and waterlogging at Tasseling/Silking! (Water Req: 500–800 mm).
4. Sorghum (Jowar) & Pearl Millet (Bajra)	Booting / Earhead Emergence & Flowering Stage	Seedling and Flowering stages (Water Req: Bajra = 250–350 mm; Jowar = 450–650 mm).
5. Groundnut	Pegging (Peg Penetration) to Pod Formation Stage	Flowering, Pegging, and Pod Development (Water Req: 500–700 mm).
6. Chickpea (Gram) & Pulses (Arhar/Pea)	Pre-Flowering (Branching) and Pod Development Stage	Crucial Rule for Gram: NEVER irrigate Chickpea during peak 50% Flowering (causes flower drop and vegetative overgrowth!).
7. Cotton	Flowering and Boll Formation / Square Formation Stage	Sympodial branching, Flowering, and Boll development (Water Req: 700–1,300 mm).
8. Sugarcane	Formative (Tillering / Grand Growth) Stage (60 to 130 DAS)	Germination and Formative phases (Water Req: 1,500–2,500 mm).
9. Potato	Stolonization to Tuber Initiation & Tuber Enlargement Stage	Requires frequent light irrigations (Water Req: 500–700 mm).

3. Surface vs Sprinkler vs Drip Irrigation, Duty-Delta Formula & Watershed Classification

Irrigation Method	Water Application Efficiency (%)	Discharge Rate & Operating Pressure	Best Suited Soils, Slopes & Crops
1. Surface Irrigation (Check Basin, Border Strip, Furrow, Surge, Cablegation)	40% to 50% (Highest conveyance & evaporation loss!)	Gravity flow	• Check Basin: Most common in India; level fields (Rice/Wheat). • Furrow / Corrugation: Row crops & root/tuber crops (Potato, Sugarcane, Maize, Cotton, Vegetables). • Ring Basin: Fruit orchard trees.
2. Sprinkler (Overhead) Irrigation	70% to 85% (Saves 30%–50% water)	> 1,000 Litres/hour Pressure: 2.5 to 4.5 kg/cm²	Best for Undulating / Rolling topography (sandy/shallow soils where leveling is costly) and closely spaced crops (Wheat, Mustard, Pulses, Tea/Coffee). Not suitable under high wind (>15 km/h) or saline water (causes leaf scorch)!
3. Drip (Trickle) Irrigation (Developed by Simcha Blass* in Israel!)*	90% to 95% (Saves 50%–70% water; Fertigation Efficiency: 80%–90%!)	1 to 8 Litres/hour per emitter (dripper) Pressure: 1.5 to 2.5 kg/cm² (Typhoon/Micro-tube: <1 kg/cm²)	Best for Widely-spaced Orchard Fruits, Vegetables, Sugarcane, Cotton, Coconut, Banana; CAN USE SALINE WATER safely because salts are pushed to the periphery of the wetting bulb while roots remain at Field Capacity!

1. Canal Irrigation Formula Linking Duty (D), Delta (Δ), and Base Period (B):
Δ (in Metres) = (8.64 × B) / D OR Δ (in Centimetres) = (864 × B) / D
• Base Period (B): Number of days between the first watering at sowing and the last watering before harvesting.
• Delta (Δ): Total depth of water (in metres or cm) required by a crop during its entire Base Period.
• Duty of Water (D): Number of Hectares of land irrigated by a continuous flow of 1 Cumec (1 m³/second) of water throughout the Base Period!

2. 5-Tier Size Classification of Watersheds in India (AISLUS / Soil & Water Conservation Society):
• 1. Macro Watershed: > 50,000 Hectares
• 2. Sub-Watershed: 10,000 to 50,000 Hectares
• 3. Milli-Watershed: 1,000 to 10,000 Hectares
• 4. Micro-Watershed: 100 to 1,000 Hectares
• 5. Mini-Watershed: 1 to 100 Hectares (10 to 100 ha)!

🎯 High-Yield Exam One-Liners (Last-Minute Revision Box)

Field Capacity (FC) occurs at a soil moisture tension of −1/3 Bar (−0.33 Bar or pF = 2.54) and represents the Upper Limit of Plant Available Water.
Permanent Wilting Point (PWP) occurs at −15 Bars (pF = 4.2) and represents the Lower Limit of Plant Available Water; its biological indicator plant is Dwarf Sunflower (Helianthus annuus).
Plant Available Water (PAW) is the Capillary Water held between Field Capacity (−0.33 bar) and Permanent Wilting Point (−15 bars).
A Tensiometer (Irrometer) is effective ONLY in moist sandy/loamy soils up to a tension of −0.85 Bar.
A Lysimeter is used for accurate in-situ field measurement of Evapotranspiration (ET) and percolation losses.
The most critical stage of irrigation in Wheat is the Crown Root Initiation (CRI) stage, occurring at 21 Days After Sowing (21 DAS).
If only 1 irrigation is available for Wheat, apply it at CRI (21 DAS); if 2 irrigations are available, apply at CRI (21 DAS) and Flowering/Late Jointing!
The most critical stage of irrigation is Tasseling and Silking in Maize, Panicle Initiation to Flowering in Rice, and Pegging to Pod Formation in Groundnut.
Drip (Trickle) Irrigation was invented by Simcha Blass in Israel and achieves the highest Water Application Efficiency of 90% to 95% (and Fertigation efficiency of 80%–90%).
Discharge rate of emitters/drippers in Drip Irrigation is 1 to 8 Litres per hour at an operating pressure of 1.5 to 2.5 kg/cm².
Sprinkler Irrigation operates at a pressure of 2.5 to 4.5 kg/cm² with a discharge rate >1,000 Litres/hour and is ideal for undulating sandy/hilly terrains.
When irrigation water has high salinity, DRIP Irrigation is best (and Sprinkler is worst, as salty droplets cause leaf scorching).
Surge Irrigation is the intermittent (On-Off cycle) application of water to furrows/borders through gated pipes to reduce deep percolation loss.
In Canal Engineering, Delta (in metres) = (8.64 × Base Period in days) / Duty (in hectares per cumec).
1 Cusec (cubic foot per second) = 28.32 Litres per second; 1 Cumec (cubic metre per second) = 1,000 Litres per second = 35.315 Cusecs; 1 Hectare-cm of water = 1,00,000 (1 Lakh) Litres!
Under PMKSY (Pradhan Mantri Krishi Sinchayee Yojana, launched July 2015), ‘Per Drop More Crop’ (Micro-Irrigation) is implemented by the Ministry of Agriculture & Farmers Welfare (subsidy: 55% for Small & Marginal Farmers, 45% for Other Farmers).
In India’s 5-tier Watershed classification: Mini-Watershed = 1–100 ha; Micro-Watershed = 100–1,000 ha; Milli-Watershed = 1,000–10,000 ha; Sub-Watershed = 10,000–50,000 ha; Macro-Watershed = >50,000 ha.

📝 UPSC & Regulatory Level Practice MCQs (With Detailed Solutions)

Q1. Plant Available Water (PAW) in the root zone is defined as the soil moisture held between ‘Field Capacity’ (FC) and ‘Permanent Wilting Point’ (PWP). What are the standard soil moisture tension values (in Bars) at Field Capacity and Permanent Wilting Point respectively?

A. Field Capacity = 0 Bar; Permanent Wilting Point = −0.33 Bar
B. Field Capacity = −1/3 Bar (−0.33 Bar); Permanent Wilting Point = −15 Bars
C. Field Capacity = −15 Bars; Permanent Wilting Point = −31 Bars
D. Field Capacity = −1 Bar; Permanent Wilting Point = −60 Bars

Correct Answer: (B) — Field Capacity is held at −1/3 Bar (−0.33 Bar, pF 2.54) and Permanent Wilting Point is held at −15 Bars (pF 4.2). Water between −0.33 Bar and −15 Bars is Plant Available Capillary Water (whereas −31 Bars is the Hygroscopic Coefficient).

Q2. Match the crops in List-I with their single MOST CRITICAL physiological stage for irrigation in List-II:
List-I: (a) Wheat, (b) Maize, (c) Groundnut, (d) Sugarcane
List-II: (1) Pegging (Peg Penetration) Stage, (2) Formative / Tillering Stage, (3) Crown Root Initiation (CRI — 21 DAS), (4) Tasseling and Silking Stage

A. a-3, b-4, c-1, d-2
B. a-4, b-3, c-2, d-1
C. a-3, b-1, c-4, d-2
D. a-2, b-4, c-1, d-3

Correct Answer: (A) — Wheat’s most critical stage is Crown Root Initiation at 21 DAS (3); Maize is Tasseling and Silking (4); Groundnut is Pegging to Pod Formation (1); and Sugarcane is the Formative/Tillering stage (2).

Q3. A crop has a Base Period (B) of 120 days and requires a total depth of water (Delta, Δ) of 86.4 cm (i.e., 0.864 metres) during its growing season. What is the ‘Duty of Water’ (D) for this crop in hectares per cumec?

A. 864 hectares/cumec
B. 1,000 hectares/cumec
C. 1,200 hectares/cumec
D. 1,440 hectares/cumec

Correct Answer: (C) — Using the formula Δ (in metres) = (8.64 × B) / D => D = (8.64 × B) / Δ = (8.64 × 120) / 0.864 = 10 × 120 = 1,200 hectares per cumec.

Q4. In the official 5-tier hydrological classification of Watersheds in India, what is the geographical area coverage of a ‘Micro-Watershed’ and a ‘Milli-Watershed’ respectively?

A. Micro-Watershed: 1 to 100 ha; Milli-Watershed: 100 to 1,000 ha
B. Micro-Watershed: 100 to 1,000 Hectares; Milli-Watershed: 1,000 to 10,000 Hectares
C. Micro-Watershed: 10,000 to 50,000 ha; Milli-Watershed: > 50,000 ha
D. Micro-Watershed: 500 to 5,000 ha; Milli-Watershed: 5,000 to 25,000 ha

Correct Answer: (B) — The 5 tiers are: Mini-Watershed (1–100 ha) < Micro-Watershed (100–1,000 ha) < Milli-Watershed (1,000–10,000 ha) < Sub-Watershed (10,000–50,000 ha) < Macro-Watershed (>50,000 ha).

Q5. How many Litres of water are equivalent to ‘1 Hectare-Centimetre’ (1 ha-cm) of irrigation depth applied over a 1-hectare field?

A. 10,000 Litres
B. 1,00,000 (1 Lakh) Litres
C. 10,00,000 (10 Lakh) Litres
D. 1,000 Litres

Correct Answer: (B) — 1 Hectare = 10,000 m². Depth = 1 cm = 0.01 m. Volume = 10,000 m² × 0.01 m = 100 m³. Since 1 m³ = 1,000 Litres, 1 ha-cm = 100 × 1,000 = 1,00,000 (1 Lakh) Litres (and 1 ha-mm = 10,000 Litres).

❓ Frequently Asked Questions (Exam FAQs)

Q: Why can saline groundwater be safely used in Drip Irrigation but not in Sprinkler Irrigation?

In Sprinkler Irrigation, saline water droplets fall directly on plant leaves, and as water evaporates, concentrated salts burn the foliage (leaf scorch). In Drip Irrigation, water is applied directly to the root zone at low tension (near Field Capacity), pushing dissolved salts outward to the perimeter of the wetted soil bulb away from active feeder roots.

Q: What is the IW/CPE Ratio method of scheduling irrigation?

Proposed by Parihar et al. (1974), the IW/CPE Ratio schedules irrigation by comparing a fixed depth of Irrigation Water (IW, e.g., 5 cm or 6 cm) to Cumulative Pan Evaporation (CPE) measured from a USWB Class-A Open Pan Evaporimeter. For example, Wheat has an optimal IW/CPE ratio of 0.90 (whereas Rice has 1.20 and Safflower/Mustard has 0.40–0.60).

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