Chapter 6 — Water-Use Efficiency & Irrigation Scheduling

6.1 Water Use Efficiency — Definition and Types

• Water Use Efficiency (WUE) — Definition (PYQ 2020 Q5a): In general agronomy, WUE is defined as the crop production (economic yield) generated per unit of water consumed by the crop. It is typically expressed as kilograms of yield per hectare per millimeter of water (kg/ha-mm) or kilograms per cubic meter (kg/m³).
• The WUE Formula: It is calculated as WUE = Y / ET, where 'Y' is the yield in kg/ha and 'ET' is the Evapotranspiration in millimeters. Alternatively, it can be calculated as Yield divided by the total water applied (irrigation plus effective rainfall).
• Understanding Evapotranspiration (ET): ET is the combined water loss from direct soil evaporation (E) and productive plant transpiration (T). Because only transpiration is biologically "useful" for crop production (while evaporation is pure waste), any management practice that increases the T:E ratio directly improves the overall WUE.
• Water Productivity (WP) (PYQ 2020 Q5a): This is a broader, basin-scale concept defined as Yield divided by the total water input (which includes rainfall, irrigation, and stored soil water). Physical WP measures kilograms of crop per cubic meter of water (e.g., wheat yields 0.5–1.5 kg/m³, while rice yields only 0.2–0.4 kg/m³). Economic WP translates this into financial terms, measuring the Rupee value generated per cubic meter of water, allowing planners to compare low-value cereals against high-value vegetables and fruits.
• Crops with High and Low WUE (PYQ 2020 Q5a): * High WUE Crops: Sorghum, pearl millet, finger millet, maize, wheat, cotton, and sunflower. The millets and maize utilize the C4 photosynthetic pathway, allowing them to produce 3 to 4 grams of dry matter per kilogram of water, effectively giving them a 30 to 40% higher WUE than C3 crops.
• Low WUE Crops: Flooded rice (paddy) has an enormous water requirement (1000 to 2000 mm per season), even though its actual biological ET is only 500 to 600 mm; the rest is lost to deep percolation and seepage. Sugarcane and plantation crops also possess inherently low WUE due to their massive, long-term water requirements.

6.2 Major Aims of Irrigation (PYQ 2023 Q5b)

• Maintain Soil Moisture: To keep the root zone moisture strictly between Field Capacity (the maximum water held against gravity) and the Permanent Wilting Point, ensuring the crop never faces severe stress during critical physiological stages.
• Leach Excess Salts: In arid regions with poor-quality groundwater, heavy irrigation is periodically required to flush accumulated, toxic salts deep below the active root zone to prevent soil salinization.
• Cool the Soil and Plant: Applying water physically reduces extreme summer soil temperatures. Foliar overhead irrigation can lower canopy temperatures, which is critical for protecting pollen viability during blistering heatwaves.
• Reduce Frost Damage: In northern India's potato and mustard belts, applying a pre-dawn irrigation releases the latent heat of water into the air, raising near-surface temperatures by 2 to 3°C to protect sensitive crops from sudden frost.
• Improve Fertilizer Efficiency (Fertigation): Applying water-soluble fertilizers directly through the irrigation system ensures precision nutrient delivery, drastically reducing nitrogen leaching and improving phosphorus uptake.
• Facilitate Tillage and Sowing: Applying a pre-sowing irrigation (known traditionally as palewa) wets rock-hard, dry soil, enabling proper tractor tillage and ensuring uniform germination for Rabi crops following a dry October.
• Support Crop Establishment: Providing immediate "life-saving" seedling irrigation for transplanted crops (like rice, cotton, and vegetables) whose tiny roots cannot yet reach deep soil moisture.

6.3 Types of Irrigation Efficiencies (PYQ 2023 Q5b)

• Conveyance Efficiency (Ec): The percentage of water actually delivered to the farmer's field compared to the total volume diverted from the reservoir or river source. In India, canal Ec is historically poor (60 to 70%), meaning 30 to 40% of the water is lost to seepage and evaporation before it even reaches the farm.
• Field Application Efficiency (Ea): The percentage of water physically stored in the crop's root zone compared to the total volume applied to the field. Losses here occur via deep percolation and surface runoff. Traditional flood irrigation has an Ea of 40 to 60%, while modern drip irrigation boasts an Ea of 90 to 95%.
• Water Use Efficiency (WUE): The total crop production achieved per unit of water biologically consumed (Evapotranspiration).
• Project Efficiency (Ep): The ultimate, cumulative efficiency of the entire irrigation system from the dam to the plant. It is the product of Conveyance, Application, and Water Use efficiencies (Ec × Ea × WUE). Indian project efficiency generally hovers around a dismal 35 to 40%.
• Distribution Efficiency (Ed): A measure of how uniformly the water is distributed across the physical field area. A high Ed means every plant receives equal water. Drip systems achieve 90 to 95% Ed, whereas poor flood irrigation leaves sections of the field flooded and others bone dry.

6.4 Irrigation Scheduling — Methods (PYQ 2020 Q6a, 2018 Q7b)

• Definition and Objective: Irrigation scheduling is the scientific process of determining exactly when to irrigate (timing) and exactly how much water to apply (depth). The objective is to maximize crop yield per unit of water while preventing waterlogging, halting deep percolation, and preventing drought stress at critical growth stages.
• Soil Moisture Approach: This involves directly measuring the water in the soil and triggering irrigation when moisture falls to a pre-defined threshold (known as the Management Allowed Deficit, typically 50 to 60% of available water). Methods include the destructive Gravimetric method, Tensiometers (which measure soil suction, triggering irrigation when tension exceeds 0.5 bar), high-tech Neutron probes, and Time-Domain Reflectometry (TDR).
• Climatological Approach (ET-Based): Calculating the crop's water requirement dynamically based on daily meteorological data (sunlight, wind, temperature). The IW/CPE ratio is the most widely adopted climatological method in India.
• Plant-Based Approach: Relying on the physical responses of the crop to indicate stress. This includes visual indicators like leaf rolling in maize (which is often too late to prevent yield loss), or high-tech tools like pressure bombs (measuring leaf water potential), porometers (measuring stomatal closure), and infrared thermometry (measuring canopy temperature spikes).
• Critical Growth Stage Approach: A highly pragmatic method for resource-limited farmers. Water is applied exclusively during the crop's most biologically sensitive physiological stages, regardless of the current soil moisture reading.
• Crop-Wise Critical Growth Stages:
• Wheat: Crown Root Initiation (CRI at 21 Days After Sowing) and Heading are the absolute most critical stages. Missing irrigation here guarantees a 30 to 40% yield loss.
• Rice: Tillering, Panicle Initiation, and Heading.
• Maize: The Silking stage is exceptionally sensitive; even a 2-day drought during silking slashes yields by 30 to 50%.
• Sugarcane: The Formative and Grand Growth phases.
• Groundnut: Pegging and Pod filling. Drought during pegging causes the pegs to fail to enter the hard soil.
• Soybean: The Pod filling stage.

6.5 The IW/CPE Ratio — Concept and Execution (PYQ 2024 Q5b)

• The Concept: IW stands for Irrigation Water applied (measured in mm depth). CPE stands for Cumulative Pan Evaporation (measured in mm from a standard open Class A pan evaporimeter). Pan evaporation acts as a highly accurate physical proxy for the atmosphere's evaporative demand (ET). As the CPE number accumulates day by day, soil moisture is proportionally depleted.
• The Rule: An irrigation event is triggered when the accumulated CPE reaches a specific threshold calculated by the designated IW/CPE ratio for that crop.
• Common Indian Ratios: Wheat is heavily irrigated at a ratio of 0.6 to 1.0. Groundnut operates at 0.6 to 0.8. Drought-hardy Sorghum operates at a conservative 0.4 to 0.6.
• Merits of the System: It is incredibly simple and practical, requiring only a cheap evaporation pan rather than expensive soil sensors. It automatically integrates all complex weather variables (temperature, humidity, wind, solar radiation) into one simple number, removing farmer guesswork.
• Demerits of the System: It completely fails to account for mid-cycle rainfall (which replenishes soil moisture but does not reset the pan). It uses a static threshold and ignores the changing sensitivity of the crop across different growth stages. Finally, the pan must be perfectly sited in an open field, as nearby trees or buildings will ruin the evaporation readings.

6.6 Irrigation Scheduling Numerical Problems

(Note: Numericals are a recurring feature in this subtopic. Let's explore the mechanics before utilizing the interactive calculator below).

• PYQ 2018 Q7(c) — Groundnut Depletion Scheduling:
• The Problem: Given a Field Capacity (FC) of 16% and a Permanent Wilting Point (PWP) of 6%, calculate the exact soil moisture triggers for irrigating at 25%, 50%, and 75% depletion.
• Solution:
• Step 1: Identify the Given Data
• Field Capacity (FC): 16%
• Permanent Wilting Point (PWP): 6%
• Target Depletion Levels: 25%, 50%, 75%
• Step 2: Calculate the Available Water Capacity (AWC)
• The Available Water Capacity is the total amount of water physically available to the plant, sitting between the upper limit (FC) and the lower limit (PWP).
• AWC = FC - PWP
• AWC = 16% - 6% = 10%
• Step 3: State the General Formula for Moisture Triggers
• To find the exact soil moisture percentage at which irrigation should be triggered, we must subtract the allowed depleted water from the Field Capacity.
• Trigger Moisture = FC - (Depletion Fraction × AWC)
• Step 4: Calculate Triggers for Each Depletion Level
• A) For 25% Depletion:
• Trigger = 16% - (0.25 × 10%)
• Trigger = 16% - 2.5% = 13.5%
• Answer: Irrigate when soil moisture drops to 13.5%.
• B) For 50% Depletion:
• Trigger = 16% - (0.50 × 10%)
• Trigger = 16% - 5.0% = 11.0%
• Answer: Irrigate when soil moisture drops to 11.0%.
• C) For 75% Depletion:
• Trigger = 16% - (0.75 × 10%)
• Trigger = 16% - 7.5% = 8.5%
• Answer: Irrigate when soil moisture drops to 8.5%.
• Step 5: Agronomic Interpretation (Groundnut Application)
• Different growth stages require different depletion thresholds.
• For groundnut, agronomists recommend irrigating at 50% depletion (11.0% moisture) during the less-sensitive vegetative growth stage.
• However, during the highly critical pegging and pod-filling stages, moisture stress must be avoided at all costs, requiring a shift to a highly conservative 25% depletion trigger (13.5% moisture).

📝 Exam Tip for Students: Always write down the AWC calculation first. Many students accidentally calculate the depletion percentage directly against the Field Capacity (e.g., taking 25% of 16%). You must always calculate depletion as a percentage of the Available Water Capacity (AWC).

PYQ 2024 Q5(e) — Pump Operation Duration:

• The Problem: A farmer applies 4 irrigations to a 6-hectare wheat field at a depth of 60 mm each using a pump that discharges 5 liters per second. Calculate the total operation duration.
• Solution:

• Step 1: Identify the Given Data and Standardize Units
• The most common mistake in hydrology numericals is mismatched units. We must convert all values to meters (m), square meters (m²), cubic meters (m³), and seconds.
• Area (A): 6 hectares
• A = 6 × 10,000 = 60,000 m²
• Depth per irrigation (D): 60 mm
• D = 60 / 1000 = 0.06 m
• Number of irrigations (N): 4
• Pump Discharge (Q): 5 Liters/second (Recall that 1,000 Liters = 1 cubic meter)
• Q = 5 / 1000 = 0.005 m³/second
• 
  
• Step 2: Calculate the Total Volume of Water Required (V)
• The total volume is the area of the field multiplied by the depth of water applied, multiplied by the number of times it is applied.
• Volume = Area × Depth × Number of Irrigations
• Volume = 60,000 m² × 0.06 m × 4
• Volume = 3,600 m³ × 4 = 14,400 m³

• Step 3: Calculate Total Time Required in Seconds (T)
• Time is equal to the total volume divided by the rate of discharge.
• Time = Volume / Discharge
• Time = 14,400 m³ / 0.005 m³/second
• Time = 2,880,000 seconds

• Step 4: Convert Seconds into Standard Time Units (Hours & Days)
• Convert to Hours: Hours = 2,880,000 / 3,600 = 800 hours
• Convert to Days (Assuming continuous 24-hour operation): Days = 800 / 24 = 33.33 days

• Final Answer: The total pump operation duration required to complete all 4 irrigations is 800 hours, which equates to 33.33 days of continuous 24-hour operation.

6.7 Techniques to Improve Water Use Efficiency (PYQ 2017 Q6b)

A. Agronomic Techniques

• Timely Sowing: Sowing at the exact optimal window aligns the crop's peak water demand with the monsoon's peak availability. A late-sown crop inevitably faces blistering summer heat and water stress simultaneously, causing WUE to plummet by 20 to 40%.
• Optimum Plant Population: Planting too densely creates vicious competition for limited water. Planting too sparsely wastes applied water on bare soil.
• Mulching and Weed Management: Spreading a heavy surface mulch slashes unproductive soil evaporation by 30 to 50%, ensuring all water goes toward crop transpiration. Furthermore, aggressive early weeding prevents invasive plants from stealing 20 to 40% of the applied water.
• Balanced Fertilization: Supplying optimal nitrogen and phosphorus forces the crop to develop a massive, deep root system capable of mining deep subsoil moisture. Supplying adequate potassium chemically controls the stomatal guard cells, preventing unnecessary transpiration leaks.

B. Irrigation Method Techniques

• Alternate Furrow Irrigation: The farmer irrigates only every second furrow, effectively cutting water application by 30 to 50%. The deep crop roots easily reach sideways to drink from the wetted furrow while the dry furrow prevents evaporation.
• Deficit Irrigation: A highly advanced technique where the farmer deliberately applies less water than the full ET demand, allowing the crop to experience mild, controlled stress during non-critical growth stages. This slashes water usage while causing almost zero penalty to the final yield.
• Precision Land Levelling: Utilizing laser-guided tractors to perfectly flatten a field. This completely eliminates high spots (which remain dry) and low spots (which suffer waterlogging), cutting water waste by 20 to 30%.

C. Pressurised Irrigation for 'More Crop Per Drop' (PYQ 2024 Q8b)

• Drip Irrigation: Water is delivered drop-by-drop via emitters directly to the plant's root zone, pushing application efficiency to 95%. It accommodates direct liquid fertigation, eliminates erosion, and routinely increases yields in sugarcane and cotton by 25 to 40%.
• Sprinkler and Micro-Sprinkler Irrigation: Sprinklers act as artificial rainfall, making them ideal for undulating, sloped terrain where flood irrigation is impossible. Low-pressure micro-sprinklers are deployed specifically under the canopy of dense orchards (like mango and citrus) to minimize evaporation.
• The PMKSY-PDMC Initiative: The 'Per Drop More Crop' component of the government's flagship scheme provides massive financial subsidies (ranging from 45% for general farmers to 75% for smallholders and women) to install pressurized systems, driving India's total micro-irrigation coverage to over 12 million hectares.

6.8 Quality of Irrigation Water — Parameters and Management (PYQ 2022 Q8a)

A. Key Parameters for Assessing Water Quality

• Electrical Conductivity (EC): The primary measure of total dissolved salts. An EC < 0.75 dS/m is excellent. Water with an EC > 3.0 is poor, and > 7.5 is generally toxic to most crops.
• Sodium Adsorption Ratio (SAR): Calculated using the concentration of Sodium against Calcium and Magnesium (SAR = Na⁺ / √((Ca²⁺ + Mg²⁺)/2)). An SAR < 10 is excellent. High SAR water forces sodium onto the soil's clay exchange sites, completely destroying soil structure and rendering it impermeable.
• Residual Sodium Carbonate (RSC): Measures the hazardous accumulation of carbonate and bicarbonate ions. An RSC > 2.5 is unsuitable for agriculture. High RSC water causes calcium and magnesium to precipitate out of the soil as solid rock, leaving sodium to totally dominate and poison the clay.
• Boron and Specific Ions: Boron is highly toxic to sensitive fruit crops (like citrus and walnuts) at concentrations above 2.0 mg/L. WHO limits mandate that safe irrigation water must contain Chlorides < 10 meq/L and Arsenic < 0.1 mg/L.

B. Management of Poor Quality Irrigation Water (PYQ 2023 Q6b, 2025 Q7a)

• Blending: Physically mixing poor-quality, highly saline groundwater with fresh canal or rainwater in a surface tank to dilute the EC and SAR down to safe, tolerable limits before applying it to the field.
• Cyclic Use: Alternating the water sources. A farmer applies poor-quality water for one irrigation cycle, followed by high-quality fresh water in the next cycle to instantly leach away the accumulated salts.
• Leaching Irrigation: Calculating and applying a deliberate excess volume of water (the Leaching Fraction) to physically flush the harsh salts deep below the active root zone.
• Soil Amendments and Acid Treatment: When forced to irrigate with high-SAR sodic water, the farmer must broadcast gypsum (CaSO₄) across the field to provide a massive dose of calcium to protect the clay structure. If the water suffers from extremely high bicarbonate (high RSC), injecting dilute sulfuric acid directly into the drip irrigation lines neutralizes the alkalinity and prevents the emitters from clogging with calcium rock.

📝 Exam Focus / Past Year Question (PYQ) Hooks

• PYQ 2024 Q5(b) 10M: Explain irrigation scheduling; elaborate IW/CPE ratio with its merits and demerits. → Define the primary objectives from Section 6.4, then smoothly transition into Section 6.5 to explicitly define IW and CPE. List 3 robust merits (simplicity, integration of weather) and 3 strict demerits (ignores rainfall, ignores growth stages) to claim the full 10 marks.

• PYQ 2024 Q8(b) 20M: WUE in relation to crop production; role of pressurised irrigation for 'more crop per drop' programme. → Start by defining the mathematical formula for WUE and explaining Evapotranspiration from Section 6.1. Dedicate the second half of the essay to Section 6.7C, detailing the mechanical efficiencies of drip and sprinkler systems and explicitly tying them to the government's PMKSY-PDMC subsidy framework.