Question

Calculate the cumulative pan evaporation (CPE) required for scheduling irrigation at an \(IW/CPE\) ratio of \(0.5\), given \(5\) cm of irrigation water (IW).

• A. 5 cm
• B. 10 cm
• C. 15 cm
• D. 20 cm

Hint:

Use \(CPE = IW / \text{ratio}\) to find the pan evaporation depth.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Irrigation scheduling using the IW/CPE ratio means we apply a fixed depth of irrigation water (IW) once the cumulative pan evaporation (CPE) since the last irrigation reaches a certain value, set by the chosen ratio.

Step 2: Key Formula or Approach:
The scheduling rule is written as:
\[ \frac{IW}{CPE} = \text{ratio} \]
so
\[ CPE = \frac{IW}{\text{ratio}} \]

Step 3: Detailed Explanation:
Here the ratio is \(0.5\) and the depth of irrigation water is \(IW = 5\) cm.
Putting these numbers into the formula:
\[ CPE = \frac{5}{0.5} = 10 \text{ cm} \]
So the farmer needs to wait until 10 cm of cumulative pan evaporation has taken place before applying the next 5 cm irrigation, keeping the IW/CPE ratio at 0.5.

Step 4: Final Answer:
The cumulative evaporation required is 10 cm.
\[ \boxed{10 \text{ cm}} \]