Question

Correct order of irrigation terminologies with respect to amount of water involved during a given event of irrigation (WR stands for water requirement, IR for irrigation requirement, NIR for net irrigation requirement and E for evaporation)

• A. E $>$ WR $>$ NIR $>$ IR
• B. IR $>$ WR $>$ E $>$ NIR
• C. IR $>$ WR $>$ NIR $>$ E
• D. WR $>$ IR $>$ NIR $>$ E

Hint:

To solve this easily: the Water Requirement (WR) is always the largest value because it represents the total water needed from all sources. Evaporation (E) is just a single loss component, so it is the smallest. This leaves Option (D) as the only logical sequence.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Irrigation management utilizes several distinct terms to quantify water needs at different stages of calculation.
These values range from the total physical water needs of the crop to the net amount that must be supplied artificially.

Step 2: Detailed Explanation:
Let us define and compare each term to establish their correct relative magnitudes:
Water Requirement (WR): This is the total quantity of water required by a crop for its normal growth and yield under field conditions. It includes evapotranspiration, metabolic water, and any losses during application. This is the largest value.
\[ \text{WR} = \text{ET} + \text{Application Losses} + \text{Special Needs} \] Irrigation Requirement (IR): This is the portion of the water requirement that must be met artificially through irrigation. It is less than WR because some water is supplied naturally by rainfall and soil moisture:
\[ \text{IR} = \text{WR} - (\text{Effective Rainfall} + \text{Groundwater Contribution} + \text{Soil Moisture Contribution}) \] Net Irrigation Requirement (NIR): This is the actual depth of water required to bring the soil moisture in the root zone to field capacity:
\[ \text{NIR} = \text{ET}_c - \text{Effective Rainfall} \] Since it does not include application or conveyance losses (which are added to find the Gross Irrigation Requirement), NIR is less than or equal to the total Irrigation Requirement (IR).
Evaporation (E): This is a single physical component of the water balance representing water lost from the soil surface, which is only a small fraction of the total water requirement (WR).
Therefore, the correct descending order of these volumes is:
\[ \text{WR} > \text{IR} > \text{NIR} > \text{E} \]

Step 3: Final Answer:
The correct option is (D).