Concept:
The IW/CPE ratio approach schedules irrigation by comparing how much water is applied against how much water the pan has lost through evaporation. A ratio of 0.5 means for every 1 cm of pan evaporation, only 0.5 cm of irrigation water gets applied over the same period, or equivalently the irrigation depth is half the cumulative evaporation depth.
Step 1: Set up the relation.
\[ IW = 0.5 \times CPE \]
Step 2: Substitute the known irrigation depth.
We are told \(IW = 5\) cm, so:
\[ 5 = 0.5 \times CPE \]
Step 3: Solve for CPE.
\[ CPE = \frac{5}{0.5} = 10 \text{ cm} \]
Checking it back: half of 10 cm is 5 cm, which matches the given irrigation depth, so the number is consistent.
Step 4: Final answer.
\[ \boxed{CPE = 10 \text{ cm}} \]