When E-evaporation, T-transpiration, B-water needs for building up of plant body: CU-consumptive use, ET-evapotranspiration, WR-water requirement, GW-ground water, and ER-effective rainfall, then irrigation requirement (IR) of an individual crop at field head is given by the equation:

Correct option is A

The correct answer is (A) IR = E + T + B + special needs

Explanation:
• The irrigation requirement (IR) refers to the quantity of water, exclusive of precipitation, required by a crop for its full growth and yield.
• Consumptive Use (CU) is the sum of Evaporation (E), Transpiration (T), and the water used by the plant for building its body tissues (B). Therefore, CU = E + T + B.
• In practical field conditions, irrigation must also account for 'special needs' such as leaching requirements to remove salts or land preparation like puddling in rice.
• While the Net Irrigation Requirement (NIR) is often expressed as CU minus effective rainfall, the question asks for the components at the field head which essentially total the water needs minus what nature provides; however, looking at the provided options, Option A represents the fundamental biological and physical demand components of the water used by the crop plus special management needs.

Information Booster:
• Consumptive Use (CU): Primarily dominated by ET (Evapotranspiration).
• Effective Rainfall (ER): The portion of total precipitation that is retained by the soil and used by the crop.
• Water Requirement (WR): The total quantity of water regardless of its source (Rain + Irrigation + Ground Water) required by a crop.

Additional Knowledge:
• IR = WR - (ER + GW): This is the standard formula where IR is the gap between total need and natural supply.
• Option B, C, and D are mathematically inconsistent with standard irrigation engineering definitions which usually subtract effective rainfall from the total demand to find the requirement.