Question

During agricultural water balances, the quantitative relativity of values for 'Consumptive Use (CU)' and 'Evapo-Transpiration (ET)' could be accurately expressed by which one of the following statements?

• A. \(\text{CU} = \text{ET}\)
• B. \(\text{CU} > \text{ET}\)
• C. \(\text{CU} < \text{ET}\)
• D. \(CU < {ET}{ only in temperate climates}

Hint:

CU = ET + Metabolic water (plant tissue building $\approx 1\%$). Therefore, theoretically $\text{CU} > \text{ET}$.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Agricultural water balance terminology distinguishes between total crop consumptive use and field evapotranspiration.
Key Formula or Approach:
\[ \text{Consumptive Use (CU)} = \text{Evapotranspiration (ET)} + \text{Water used in plant tissue/metabolism} \]

Step 2: Detailed Explanation:
1. Evapotranspiration (ET): The sum of evaporation from the soil surface and plant transpiration.
2. Consumptive Use (CU): The total volume of water consumed by crop plants, including both evapotranspiration and the water chemically retained in building plant vegetative and reproductive tissue (metabolic water, typically \(< 1\%\) of total).
Because consumptive use incorporates metabolic water in addition to evapotranspiration:
\[ \text{CU} = \text{ET} + \text{Metabolic Water} \implies \text{CU} > \text{ET} \]
(In practical field engineering, they are numerically close, but conceptually \(\text{CU} > \text{ET}\)).

Step 3: Final Answer:
Therefore, the relationship is \(\text{CU} > \text{ET}\), matching option (B).