Crop Water Requirement

Definition

• Crop Water Requirement (CWR): The total amount of water required by a crop for its normal growth and development, including losses, during the entire crop season, from sowing to harvest. It includes:
  • Water for evapotranspiration (ET)
  • Water lost in deep percolation, seepage, runoff
  • Water required for special operations (puddling in rice, leaching of salts, germination, land preparation).

Formula:

WR = IR + ER + S + GW

Where:

• WR = Water Requirement
• IR = Irrigation Requirement
• ER = Effective Rainfall
• S = Contribution from soil profile
• GW = Contribution from groundwater table

Components of Crop Water Requirement

• Consumptive Use (CU) / Crop Evapotranspiration (ETc): Water lost by crop transpiration + soil evaporation. Measured by lysimeter.
• Special Needs: Land preparation (puddling in rice). Leaching requirement (in saline soils). Crop operations (like interculture).
• Field Application Losses: Conveyance losses in canals, distributaries. Field losses: seepage, percolation, runoff.

Irrigation Requirement (IR)

IR = WR – ER –SM – GW

Where:

• ER = Effective rainfall (usable rainfall during crop season).
• SM = Contribution from soil moisture.
• GW = Contribution from groundwater.

Thus, IR is the net irrigation water needed from external sources.

Factors Affecting Crop Water Requirement

• Climatic Factors: Temperature, relative humidity, wind velocity, sunshine hours, rainfall. Hot & dry climates → high ET → higher CWR.
• Crop Factors: Type of crop (rice > wheat > pulses). Growth duration (long duration crops need more water). Critical growth stages.
• Soil Factors: Texture (sandy soils → higher CWR due to low WHC). Structure, depth, infiltration rate.
• Management Factors: Method of irrigation (surface < sprinkler < drip). Mulching, tillage, land leveling. Cropping pattern and intensity.

Methods of Estimating CWR

• Direct Methods; Lysimeter: Most accurate, measures ET directly.
• Soil Moisture Depletion Method; Measures changes in soil moisture before & after irrigation.
• Climatic Methods
  • Blaney–Criddle method (based on temp & day length).
  • Penman–Monteith method (FAO recommended).
  • Pan Evaporation method:
  • ETc = Kc × ETo
    • ETo: reference ET (pan evaporation).
    • Kc: crop coefficient.
• IW/CPE Ratio Method; Scheduling based on ratio of irrigation depth (IW) to cumulative pan evaporation (CPE).

Typical Water Requirements of Major Crops in India

(values vary with soil & climate, but average ranges are important for exams)

Critical Growth Stages of Crops (Most Important for Exams)

• Rice: Panicle initiation & flowering
• Wheat: CRI (21 DAS), booting, flowering, milking
• Maize: Knee-high, tasseling, silking, grain filling
• Sorghum: Flowering, grain filling
• Pulses: Flowering, pod filling
• Groundnut: Pegging, pod formation
• Cotton: Flowering, boll development
• Sugarcane: Tillering, grand growth, maturity
• Potato: Tuber initiation, tuber bulking

Missing irrigation at these stages causes maximum yield reduction.

Water Use Efficiency (WUE)

• Defined as crop yield per unit of water used.
• WUE = Grain yield (kg/ha) / ET (mm)
• To improve WUE:
  • Micro-irrigation (drip, sprinkler).
  • Mulching.
  • Short-duration & drought-resistant varieties.
  • Proper irrigation scheduling.

Indian Concerns in CWR

• Agriculture consumes ~80–85% of India’s total water use.
• Average irrigation efficiency is low (~38%).
• Crops like rice & sugarcane (water guzzlers) dominate irrigated area.
• Need to shift towards water-efficient crops + micro-irrigation.
• PMKSY (Har Khet Ko Pani, More Crop per Drop) promotes efficiency.

Summary for

• Definition: CWR = ET + losses + special needs.
• Factors: Climate, crop, soil, management.
• Methods: Lysimeter, soil moisture, pan evaporation, formulas.
• Crop-wise needs: Rice (highest), pulses (lowest).
• Critical stages: CRI in wheat, tasseling in maize, panicle initiation in rice.
• Efficiency: WUE must be improved through micro-irrigation & scheduling.